Application Guide For High Voltage Accessories.pdf

Application Guide for High Voltage Accessories 2nd edition

Application Guide for High Voltage Accessories Creation:

Brugg Cables

Authors:

Hansjörg Gramespacher, Ruben Vogelsang, Matthias Freilinger

Copyright:

All Rights Reserved. The use, reproduction and distribution of these information is authorized only for a non-commercial purpose on condition that the source is explicitly quoted.

A publication of the Brugg Cables Academy 2nd edition, 2015 Printed in Switzerland by Effingerhof, Brugg ISBN: 978-3-033-04936-9

Preface For modern societies all over the world, electric

High voltage cable accessories have to connect

energy is one of the key factors for industrial

two cable lengths together or, often, one cable

growth and individual prosperity. This admit-

to other high voltage interfaces, such as gas-

tedly simplified but true statement nevertheless

insulated switchgears, oil-insulated transform-

assumes a high level of availability of electric

ers or outdoor overhead lines. Illustratively

energy, ensuring uninterruptible manufacturing

speaking, high voltage accessories have to con-

processes for heavy industries, as well as a reli-

nect two worlds, which can be quite different in

able power supply for the IT infrastructure for

terms of their geometry, material and behav-

commercial business services, which both rely

iour. From the material point of view, electro-

on electric energy.

chemical aspects have to be considered, when

Besides single and spatially concentrated high

joining an aluminium conductor to a copper

voltage equipment of major importance, such

conductor, for instance. Also physical aspects

as turbo generators or large power transform-

have to be taken into account, such as different

ers, the high voltage network represents the most important asset for a utility and its customers in terms of size, value and pertinence. For many reasons, for example their environmental impact, route consumption and the resulting acceptance of the affected population,

conductor diameters or materials with different thermal coefficients of expansion. With regard to the insulation materials, commonly silicone rubber, polyethylene and oil-impregnated paper, interactions also have to be considered. And finally, pointing out the major challenge of

high voltage lines are increasingly making use

accessories, the ability to intransigently restore

of high voltage cable technology. Already very

the electrical insulation of two separate parts,

common in urban regions for the medium volt-

electrical field distribution has to be actively

age range, high voltage cable systems are also

controlled using proper materials and designs,

used successfully in the power transmission

this being the most important secret of every

sector, particularly as the knowledge and expe-

manufacturer of high voltage accessories.

rience of the major cable manufacturers is con-

All these technical aspects have to be taken into

stantly increasing, and as a result reliable prod-

account to design a reliable high voltage cable

ucts are available on the market.

accessory that meets the specified require-

As AC high voltage cables with production

ments. Economic aspects have to be considered

lengths of some hundred metres up to several

as well. To ensure a sufficient level of practical

kilometres, with a symmetrical cylindrical de-

usability, the design of the accessories also has

sign, and well-known electrical field stress appear to be state-of-the-art for system voltages of several hundred kilovolts, and therefore uncritical, cable accessories such as terminations and joints have to be discussed in more detail.

to tolerate a minimum level of faulty assembly works, making them practicable for the “real world” for a long operating period. Taking all these factors into consideration, a customer will of course choose high- quality high voltage accessories, as the accessories used ultimately

define the quality of a complete cable system, and even major parts of a cable network. This “Application Guide for High Voltage Accessories” will enable the reader to choose, and define in advance the requirements for the accessories needed for his specific task, by understanding design principles, manufacturing processes and test criteria in accordance with international standards for quality assurance.

The City of Kiel, Germany, April 2015

Professor Kay Rethmeier

Director of the Institute of Electrical Power Engineering and High Voltage Laboratories of the University of Applied Science Kiel

Foreword After the appreciation from our customers and our industry for the 1st edition of our “Application Guide for High Voltage Accessories”, we are tremendously proud to present this 2nd edition. It has been completely reworked and summarises the complex topics relevant to the reliable operation of high voltage accessories in a fundamental way.

Working in the field of high voltage accessories, we are constantly being asked a variety of questions concerning the application of accessories for high voltage cable systems. These vary from simple questions, such as the difference between composite and porcelain insulators for terminations to the more complex, such as how to design an appropriate earthing layout for cable systems. Although diverse, most of the questions have one thing in common: They are related to the application of accessories for high voltage cable systems.

High voltage cable systems are only as good as the installed high voltage accessories. Therefore we aim to supply high voltage accessories that are safe and reliable. We at Brugg Cables constantly expand our know-how and expertise on the application of high voltage accessories and are pleased to share our knowledge with you.

We wish you happy reading and a lot of useful benefits in your practical work!

Roger Braun

Aman Sapra

Head of Business Unit Power Accessories & Cables

Head of Marketing Power Accessories & Cables

Contents PREFACE FOREWORD

CONTENTS 1

FUNDAMENTALS .................................................................................................................................... 1

1.1 Fundamental electric relations ...................................................................................3 1.1.1 Electric charges, current, electric field, voltage and potential ....................................................... 3 1.1.2 Ohm’s law and Kirchhoff’s laws....................................................................................................... 5 1.1.3 Terms of electric power .................................................................................................................... 5 1.1.4 Electric resistivity, conductivity, insulators and semiconductivity................................................ 7 1.1.5 Magnetic field .................................................................................................................................... 8 1.1.6 Electromagnetic induction ................................................................................................................ 9 1.2 Electric field..............................................................................................................10 1.2.1 Electric field and field lines ............................................................................................................. 10 1.2.2 Electric field in technical objects .................................................................................................... 11 1.2.3 Capacity ............................................................................................................................................ 13 1.3 Insulating materials in high voltage technology.......................................................14 1.3.1 Solid materials ................................................................................................................................. 14 1.3.2 Liquids .............................................................................................................................................. 15 1.3.3 Gases ................................................................................................................................................ 15 1.4 Power transmission ..................................................................................................15 1.4.1 Basics of electric power transmission systems ............................................................................ 15 1.4.2 Overhead lines ................................................................................................................................. 17 1.4.3 Power cables .................................................................................................................................... 17 1.4.4 Transmission capability .................................................................................................................. 18 1.4.5 Power transmission and environment........................................................................................... 19 1.5 Terms and definitions ...............................................................................................21 1.5.1 Definition of voltage values for cable systems ............................................................................. 21 1.5.2 Definition of terms for terminations and cable systems .............................................................. 22 2

AGEING AND LIFE EXPECTANCY....................................................................................................... 23

2.1 Ageing in polymers ...................................................................................................25 2.1.1 Theory of electric lifetime law ........................................................................................................ 25 2.1.2 Practical experiences with electric lifetime law and electric ageing ........................................... 26 2.1.3 Thermal ageing................................................................................................................................ 27 2.2 Volume effect of real polymeric arrangements ........................................................27 2.3 Life expectancy .........................................................................................................28 2.3.1 Basic failure behaviour.................................................................................................................... 28 2.3.2 Short-term failures .......................................................................................................................... 28 2.3.3 Occasional failures .......................................................................................................................... 29 2.3.4 Failures due to ageing..................................................................................................................... 30

3

TESTS AND STANDARDS .................................................................................................................... 31

3.1 Tests .........................................................................................................................33 3.1.1 Basic idea of testing ........................................................................................................................ 33 3.1.2 Development tests........................................................................................................................... 33 3.1.3 Type tests ......................................................................................................................................... 33 3.1.4 Prequalification tests ....................................................................................................................... 37 3.1.5 Requalification tests ........................................................................................................................ 38 3.1.6 Routine tests .................................................................................................................................... 39 3.1.7 After installation tests ..................................................................................................................... 40 3.1.8 Alternative methods for after installation tests............................................................................. 42 3.2 Standards ..................................................................................................................43 3.2.1 Introduction...................................................................................................................................... 43 3.2.2 Main differences between IEC and IEEE standards ...................................................................... 43 3.2.3 Relevant IEC standards ................................................................................................................... 45 3.2.4 Relevant IEEE, AEIC, ANSI and ICEA standards............................................................................ 46 4

HIGH VOLTAGE XLPE CABLES .......................................................................................................... 47

4.1 Design and types of high voltage XLPE cables.........................................................49 4.1.1 Cable design..................................................................................................................................... 49 4.1.2 Types of high voltage XLPE cables ................................................................................................ 51 4.2 Cable layout and system design ...............................................................................53 4.2.1 General ............................................................................................................................................. 53 4.2.2 Electric field, capacity and charging current ................................................................................. 54 4.2.3 Inductive values of the cable .......................................................................................................... 55 4.2.4 Cable losses ..................................................................................................................................... 56 4.2.5 Dynamic forces ................................................................................................................................ 58 4.3 Laying of high voltage cables ...................................................................................58 4.3.1 Laying arrangements ...................................................................................................................... 58 4.3.2 Current carrying capacity and temperature calculation ............................................................... 61 4.3.3 Reduction of magnetic field............................................................................................................ 62 4.4 Cable selection process ............................................................................................62 5

HIGH VOLTAGE ACCESSORIES FOR POLYMER CABLES .............................................................. 63

5.1 Introduction ..............................................................................................................65 5.2 Technologies for slip-on elements ............................................................................66 5.2.1 Control of the electric field.............................................................................................................. 66 5.2.2 Semiconducting parts ..................................................................................................................... 68 5.2.3 Comparison of main materials ....................................................................................................... 68 5.2.4 Cold shrink elements....................................................................................................................... 70 5.2.5 Three-piece silicone rubber slip-on elements ............................................................................... 71 5.2.6 One-piece silicone rubber slip-on elements.................................................................................. 72 5.2.7 One-piece EPDM/EPR slip-on elements ......................................................................................... 73 5.2.8 Lapped technology .......................................................................................................................... 74 5.2.9 Final comparisons and conclusions............................................................................................... 74 5.3 Terminations .............................................................................................................75 5.3.1 Basic design ..................................................................................................................................... 75 5.3.2 Outdoor terminations...................................................................................................................... 76 5.3.3 Explosion resistant terminations.................................................................................................... 78 5.3.4 Classic SF6 and transformer terminations ..................................................................................... 79 5.3.5 Dry-type plug-in terminations ........................................................................................................ 80

5.4 Joints ........................................................................................................................82 5.4.1 Basic design ..................................................................................................................................... 82 5.4.2 Conductor connections ................................................................................................................... 82 5.4.3 Moisture and mechanical protection of joints .............................................................................. 84 5.4.4 Application of joints with different protection degrees................................................................ 85 5.4.5 Grounding connections................................................................................................................... 86 6

ADDITIONAL ACCESSORIES............................................................................................................... 89

6.1 Cable clamps.............................................................................................................91 6.1.1 Main requirements .......................................................................................................................... 91 6.1.2 Forces in a cable system ................................................................................................................. 91 6.1.3 Types of cable clamps..................................................................................................................... 93 6.1.4 Cable clamps at joints ..................................................................................................................... 94 6.1.5 Cable clamps at terminations ......................................................................................................... 94 6.1.6 Cable clamps for cable laying......................................................................................................... 95 6.2 Surge arresters .........................................................................................................98 6.2.1 Fundamentals .................................................................................................................................. 98 6.2.2 Application of sheath voltage limiters in cable systems.............................................................. 98 6.2.3 Dimensioning of sheath voltage limiters....................................................................................... 99 6.3 Earthing devices for joints and terminations..........................................................100 6.3.1 Fundamentals ................................................................................................................................ 100 6.3.2 IP and NEMA protection classes .................................................................................................. 100 6.3.3 Earthing boxes ............................................................................................................................... 102 6.3.4 Cross-bonding boxes .................................................................................................................... 103 6.3.5 Earthing clamps for terminations................................................................................................. 103 7

INSTALLATION AND OPERATION .................................................................................................... 105

7.1 Installation of accessories ......................................................................................107 7.1.1 Basics.............................................................................................................................................. 107 7.1.2 Installation of terminations........................................................................................................... 108 7.1.3 Installation of joints ....................................................................................................................... 114 7.2 Earthing ..................................................................................................................120 7.2.1 Background of earthing................................................................................................................. 120 7.2.2 Induced voltages at cable screen ................................................................................................. 120 7.2.3 Principles of earthing systems ..................................................................................................... 120 7.2.4 Earthing of terminations ............................................................................................................... 122 7.2.5 Earthing of joints ........................................................................................................................... 123 7.3 Operation ................................................................................................................123 7.3.1 Terminations in non-vertical position.......................................................................................... 123 7.3.2 Terminations on high voltage towers.......................................................................................... 124 7.3.3 Wind load for terminations........................................................................................................... 124 7.3.4 Seismic calculations ...................................................................................................................... 125 8

MEASUREMENTS, MONITORING AND DIAGNOSTICS ................................................................... 127

8.1 Introduction and basic definitions ..........................................................................129 8.1.1 Introduction.................................................................................................................................... 129 8.1.2 Basic definitions............................................................................................................................. 129 8.2 Possible PD phenomena in high voltage cables, terminations or joints.................130 8.3 Measurements of PD...............................................................................................131 8.3.1 Introduction.................................................................................................................................... 131

8.3.2 8.3.3 8.3.4 8.3.5 8.3.6

Challenges of on-site PD measurements..................................................................................... 132 Measurement methods ................................................................................................................. 132 Sensor types established for on-site measurements ................................................................. 136 PD pattern recognition .................................................................................................................. 138 PD measurement and monitoring system design ...................................................................... 139

8.4 Temperature measurements and monitoring of cables ..........................................141 8.4.1 Basics.............................................................................................................................................. 141 8.4.2 Applications ................................................................................................................................... 142 8.5 Other measurement and monitoring methods ........................................................143 8.5.1 Infrared temperature measurements for terminations .............................................................. 143 8.5.2 Water monitoring for cables......................................................................................................... 143 8.6 Other diagnostic methods ......................................................................................144 8.6.1 Oil analysis for terminations......................................................................................................... 144 8.6.2 Diagnostics based on loss-factor and polarisation-depolarisation measurements................. 144 9

TENDENCIES AND FUTURE DEVELOPMENTS................................................................................ 145

10 REFERENCES ..................................................................................................................................... 151 11 SYMBOLS AND ABBREVIATIONS..................................................................................................... 157 12 APPENDIX............................................................................................................................................ 161 12.1 SI units and SI prefixes ..........................................................................................163 12.2 Conversion table to the metric system ..................................................................164 BRUGG CABLES

1. Fundamentals

Chapter 1

Fundamentals

1

2

1. Fundamentals

1.1 Fundamental electric relations 1.1.1 Electric charges, current, elec-

3 is always proportional to the charge q. Therefore an electric field 𝐸 can be defined between the charges. Like the force 𝐹 the electric field 𝐸 is a vector. Consequently, 𝐸 is defined by:

tric field, voltage and potential Electric charges All electric phenomena are based on the flow or

𝐸=

accumulation of electric charges. The electric charges can be positive or negative and are always related to atoms. The atoms themselves consist of three different types of particles: protons, electrons and neutrons. Together, the heavy neutrons and protons form the atomic nucleus of the atoms. The much lighter electrons form the atomic shell.

𝐹 𝑞

(Eq. 1-1)

The basic unit of the electric field is newton per coulomb (N/C). In practice, however, the unit volt per metre (V/m) is much more frequently used. Common units for 𝐸 are:

𝐸 =

𝑁 𝑉 𝑘𝑉 = = 10!! 𝐶 𝑚 𝑚𝑚

While neutrons are electrically neutral particles, hence the name “neutron”, the protons and electrons possess an electric charge. The charge “e” of these particles is a fundamental parameter in our universe and has a value of e = 1.602⋅10-19 C. Protons possess the positive charge e and electrons

Since in most technical expressions, only the absolute value of the electric field is considered, in the following the electric field is given as “E”, without the vector.

the negative charge e. The electric charge is given as “q” or “Q”. The unit of electric charge q is coulomb (C), named after the French engineer and physicist August Coulomb. An attractive force occurs between positively and negatively charged particles. As atoms in general have the same number of electrons and protons, the forces are usually not noticed in our daily life. However, if an object A has more electrons (negatively charged) than protons, whilst another object

Electric field and force of two charged objects; Left: Objects have charges with different polarity and hence attracting force; Right: Both objects have same positive charges and hence repellent force

B has fewer electrons than protons (positively charged), then an attractive force occurs between the two objects. This force is given by the expres-

Electric current

sion 𝐹. The arrow above the letter indicates that the

If there is an electric conductor between the pairs

force is a vector, which means that both the abso-

of charged objects, a movement of the charged

lute value and the direction of the force have to be

particles takes place and an electric current flows.

considered.

The electric current “I” is defined as “the amount of charge that moves through a cross section of an object per unit of time” [Lindner 93]. Therefore, the

Electric field

definition of the current I is given as:

The force 𝐹, which accelerates the positive charge q towards the negative object, is proportional to the absolute value of the charge q. In general, this force is different at each point. However, the force

𝐼=

𝑐ℎ𝑎𝑟𝑔𝑒 𝑄 = 𝑡 𝑡𝑖𝑚𝑒

(Eq. 1-2)

4 The basic unit of the current is coulomb per second

Here, the difference between two potentials Φ1 and

(C/s). A much more frequently used unit for the cur-

Φ2 is called voltage “U” (sometimes written as

rent is ampere (A), named after the French physicist André Marie Ampère. It is described as:

𝐶 𝐼 = =𝐴 𝑠

voltage V). Consequently, the voltage U is described as the difference between two potentials. The unit of the voltage is Volt (V), named after the Italian physicist Alessandro Volta.

Electric potential and voltage

DC and AC current, frequency

If a charge Q in an electric field is moved from a

The term “DC” (sometimes given as “d.c.” or “dc”)

reference point P0 to another point P1, then a cer-

stands for “direct current”. The term “AC” (some-

tain amount of energy has to be applied. According

times given as a.c. or ac) stands for “alternating

to equation 1-1, this energy not only depends on

current”.

the position of point P1 but also on the charge Q. If the same charge is moved to a second point P2, this energy changes. Similar to the definition of the electrical field, these energies can be divided by the charge Q. The result of this mathematical relation is a parameter, which is related to the electric field. This parameter is called potential “Φ“ of the elec-

A current is called DC if it does not change its amplitude and direction over time. A current is called AC if it changes direction and if its value (magnitude) changes on a regular basis. The sum of the positive and negative values integrated over time is zero.

tric field 𝐸 . All points on a line that have the same

1

potential are placed on a so-called equipotential line. To move a charge along an equipotential line, However, energy is necessary for moving a charge Q from a point P1 with potential Φ1 to a point P2 with potential Φ2. This energy Welectric is [Lindner 93]:

Sinusoidal current (voltage)

0.6

dc current (voltage)

0.4

Amplitude

no energy is necessary.

0.8

0.2 0 -0.2 -0.4 -0.6 -0.8 -1

Time t

Amplitude of DC and AC currents

𝑊%#%&!"$& = 𝑄 ∙ Φ! − Φ! = 𝑄 ∙ 𝑈

(Eq. 1-3) Although the terms DC and AC refer to “currents”, they are also used to describe voltages. Consequently, DC voltage stands for a non-changing voltage and ac voltage stands for a voltage that changes its value (magnitude) on a regular basis. The number of turns of an AC current per unit of time is referred to as the frequency “f” of that current. The unit of the frequency f is given in 1/s or “Hz” (Hertz), named after the German physicist Heinrich Hertz. In ac power supply systems, the standard frequency is 50 Hz for most European and Asian states and 60 Hz for most American states. In the case of railway systems, lower frequency

Electric field lines (black) and equipotential lines (violet) in between differently charged objects

values, such as 16 ⅔ Hz, are often used.

1. Fundamentals

5

1.1.2 Ohm’s law and Kirchhoff’s

connection of resistors, the currents split in each line of the circuit.

laws Ohm’s law When a conductor is present between two potentials in an electrical field, an electric current will flow. The current can be calculated as follows:

Second Kirchhoff’s law: Mesh rule The second law of Kirchhoff says that the sum of all voltages around any closed loop (electric circuit) is zero. The second law of Kirchhoff is defined as [Lindner 93]:

𝐼=

𝑈 𝑅

(Eq. 1-4) 𝑈=0

Consequently, the resistance “R” of the conductor is given as:

(Eq. 1-7)

This law states that the sum of all voltage sources is equal to the sum of all voltage users – in other words: the electric charges in an electric circuit re-

𝑅=

𝑈 = 𝑐𝑜𝑛𝑠𝑡 𝐼

(Eq. 1-5)

main in the circuit itself. This means that in an electric circuit with a series connection of resistors, the current is the same in each part of the circuit.

This equation is called Ohm’s law, named after the German physicist, Georg Simon Ohm. The unit of

Both laws are named after the German physicist Gustav Robert Kirchhoff.

the resistance is “Ω” (Ohm) and is given as:

𝑅 =

𝑉 =Ω 𝐴

1.1.3 Terms of electric power Effective power P

Ohm’s law reveals a distinct linear relation between voltage and current. This means that with a constant electric resistance at a voltage level of 50%, only 50% of the current flows.

The effective electric power P is the product of electric current and voltage and is given as:

𝑃 =𝑈∙𝐼

(Eq. 1-8)

First Kirchhoff’s law: Nodal rule The first law of Kirchhoff considers a node in an electric circuit. It describes the principle of the con-

When applying Ohm’s law (Eq. 1-5), the electric power can also be given as:

servation of electric charge. The law says that at any node (or electric junction) the sum of all currents flowing into that node is equal to the sum of

𝑃 = 𝐼! ∙ 𝑅

(Eq. 1-9)

all currents flowing out of that node. The first law of Kirchhoff can be expressed as [Lindner 93]:

or:

𝑃= 𝐼"! =

𝐼#!"

(Eq. 1-6)

𝑈! 𝑅

(Eq. 1-10)

Equations 1-9 and 1-10 show that the electric power has a quadratic relation to voltage and current. This If incoming currents are taken as positive and outgoing currents are taken as negative, the first law describes the sum of all currents in a knot as zero. This means that in an electric circuit with a parallel

means that doubling the current (or voltage) and keeping the resistance constant means increasing the power by a factor of four.

6 The effective power P is the electric power that can

Power factor

be directly transferred into other forms of power,

The presence of reactive power leads to a phase

such as light, mechanical, thermal or chemical power. A useful example of such a transfer is electric heating. An unwanted example is the ohmic losses in the conductor of a cable during current flow. The unit of the effective power is Watt (W), named after the Scottish inventor James Watt, and is defined as:

shift between the current I and the voltage V. The angle of this phase shift is called “ϕ“ (phi). Usually this phase shift is described by the cosines of the corresponding angle ϕ and is named as the “power factor“. The greater the reactive power, the higher is the phase shift. In most electrical power equipment, the power factor cos ϕ is between 0.82 and 0.93.

𝑃 =𝑉∙𝐴 =𝑊

Considering Equation 1-11 and taking the power factor into account, the effective and reactive power in a three-phase system with voltage U can be

Reactive power Q In contrast to the effective power P, the reactive

given as:

power “Q” cannot be transferred directly into any other terms of power. Reactive power is needed to create electric and/or magnetic fields. When transferring electric energy in a cable or an overhead line, not only the effective power P (in the

𝑃 = 𝑈 ∙ 𝐼 ∙ 3 ∙ cos 𝜑 𝑄 = 𝑈 ∙ 𝐼 ∙ 3 ∙ sin 𝜑

(Eq. 1-12)

(Eq. 1-13)

conductor), but also the reactive power Q (in the insulation and in the magnetic field around the line) is transferred. Since reactive power cannot be di-

Example 1-1:

rectly used, it usually contributes to losses and

Æ A 30 MVA transformer and a 132 kV polymer cable connecting it

must therefore be limited as much as possible (as

 Electric current in the cable

far as practical application for cable systems are

P According to equation 1-11 together with 1-12 and 1-13 the current can be calculated as:

concerned). To differentiate the types of power from each other, the unit of reactive power is given in “var” (volt

𝐼=

𝑆 𝑈∙ 3

=

30 000 𝑘𝑉𝐴 = 131 𝐴 132 𝑘𝑉 ∙ 1.73

ampere reactive). Example 1-2:

Apparent power S The geometric sum of effective power P and reactive power Q is the apparent power “S”. The apparent power can be calculated by using the law of Pythagoras and is given as:

𝑆=

𝑃! + 𝑄!

Æ A 30 MW generator, a power factor of cos ϕ = 0.85 and a 132 kV polymer cable connecting it  Electric current in the cable P According to equation 1-12 the current can be calculated as:

𝐼=

𝑃 𝑈 ∙ 3 ∙ cos 𝜑

=

30 𝑀𝑊 = 155 𝐴 132 𝑘𝑉 ∙ 1.73 ∙ 0.85

(Eq. 1-11) Electric energy

To differentiate between the different types of

Electric energy is defined as the product of (usable)

power, the unit of apparent power S is given in

effective power in a certain unit of time. It is there-

“VA” (Volt Ampere). Since in almost all electric

fore given as:

equipment effective and reactive power are consumed (e.g. in a cable: effective power in the con-

𝑊%#%&!"$& = 𝑃 ∙ 𝑡

(Eq. 1-14)

ductor, reactive power in the magnetic and the electric field), apparent power is usually given to

The unit of energy is joule (J or Ws), named after

describe the power capacity of the equipment.

the British physicist James Prescott Joule.

1. Fundamentals

7

1.1.4 Electric resistivity, conductivity, insulators and semi-

conductor decreases and with conductors of a greater length, the resistance increases.

conductivity Change of resistance with temperature

Resistivity and resistance The current flow through a metallic conductor is a transport of electrons. These electrons interact with the lattice structure of the metals. The electrons lose kinetic energy when interacting with the lattice structure. On a macroscopic scale this loss of kinetic energy is described by the electrical resistivity (ρ). As the degree of interaction of electrons with the lattice structure differs according to the type of

A rise of temperature causes the lattice in the material to oscillate at higher amplitude, resulting in a stronger interaction of the electrons with the lattice. This process leads to a higher loss of energy and, ultimately, to a rise in temperature of the conductor. Most metals including all common conductor materials, such as copper or aluminium, show an increase in the resistance when subjected to a rise in temperature. Such materials are called positive

metal, the degree of electric resistivity also varies.

temperature coefficient materials. The relation of

The electric resistivity of each material can be

resistance and temperature is given by [Lindner

found in appropriate literature. Typical values of

93]:

metals are given in the table below. Electric resistivity of common materials at 20 °C [Lide 03], [Friedrich 93] Material

Electric resistivity

Silver

0.015 ⋅ 10-6 Ωm

Copper

0.017 ⋅ 10-6 Ωm

Gold

0.021 ⋅ 10-6 Ωm

Aluminium

0.027 ⋅ 10-6 Ωm

Brass

0.064 ⋅ 10-6 Ωm

Iron

0.10 ⋅ 10-6 Ωm

Lead

0.21 ⋅ 10-6 Ωm

Stainless steel

0.71 ⋅ 10-6 Ωm

𝑅 𝑇 = 𝑅!" ∙ (1 + 𝛼!" 𝑇 − 𝑇!" )

(Eq. 1-16)

In which: R (T):

Resistance at a certain temperature T

R20:

Resistance at 20°C

α20:

Temperature coefficient of the material at 20°C; αAl = 16.8⋅10-6 1/K, αCu = 23.9⋅10-6

T:

Temperature

T20:

Temperature of 20°C

Conductivity The opposite of electric resistivity is electric conductivity. The electric conductivity “κ” (Kappa) is defined as:

The resistance of a conductor is given by the material specific values and its geometry. It is defined as [Lindner 93]:

𝑅 =𝜌∙

𝑙 𝐴

(Eq. 1-15)

𝜅=

1 𝜌

(Eq. 1-17)

The unit for κ is siemens per metre (S/m). The unit S (Siemens) is named after the German engineer and inventor Werner v. Siemens.

In which: R: Resistance of the conductor ρ:

Resistivity of the conductor

l:

Length of the conductor

A:

Cross-section area of the conductor

Equation 1-15 shows that with higher cross-section values the resistance (and therefore losses) of a

Electric conduction As stated in Section 1.4, electric current is the flow of electrons. As all metals allow a very high flow of electrons, they are referred to as “electric conductors” or merely “conductors”.

8 Ionic conduction In addition to the flow of electrons, entire atoms can also move through a material. The charged atoms (negatively or positively) are called ions. Ions typically move through liquids. Positively charged ions are called “cations”, while negatively charged

1.1.5 Magnetic field Magnetic field and magnetic field strength A magnetic field occurs between the poles of permanent magnets and in the surroundings of current carrying conductors [Lindner 93].

ions are called “anions”. Whilst the transport of matter does not occur during electron flow, it does occur during the flow of ions – a phenomenon that can be seen in batteries.

N Electric insulators As a general rule, electric insulators are materials that do not allow a significant amount of electric current to flow. Electric insulators are air, oil, ce-

S

ramics, epoxy resin, glass, polymers, rubber or wood. Since they are very important for the equipment of high voltage cable systems, more details are given in a separate section (Chapter 5). Magnetic field lines of a permanent magnet

Electric semiconductivity (as applied to cable systems) Although the application of semiconducting materials in electronic components, such as doped silicon, is not covered in this book, it is worth mentioning that the expression “semiconductive (or semiconducting) material” frequently occurs in the field of high voltage technology. This expression is usually used for materials with a level of resistivity that is between the high conductivity of common metals and the low conductivity of insulators. In most cases, these semiconducting materials consist of carbon black filled polymers. The polymers can be made of thermoplastic materials, such as polyethylene, or elastomers, such as silicone rubber or EPR (ethylene propylene rubber). Carbon black filled polyethylene material is used for the inner and outer semiconducting layer of polymer cables, while carbon black filled elastomers are used for the deflectors and middle electrodes of high voltage accessories. Typical resistivities of these materials are around 10 Ωcm.

Magnetic field around a current carrying conductor

Like electric field lines, magnetic field lines represent the magnetic field of a material. In contrast to electric field lines, magnetic field lines are always “closed”, having neither a beginning nor an end. In a permanent magnet, the magnetic field lines travel outside the magnet from north- to south pole. Since magnetic field lines are always closed, they go inside the magnetic material from south- to north pole. In a current carrying conductor, mag-

1. Fundamentals

9

netic field lines occur concentrically around the conductor. The strength and direction of the magnetic field is given by the magnetic field strength “H”. Similar to the electric field E, the magnetic field strength H is a vectorial parameter and always rectangular to the

Φ=𝐵∙𝐴

(Eq. 1-19)

The unit of the magnetic flux is volt multiplied by second (Vs) or Weber (Wb), named after the German physicist Wilhelm Eduard Weber.

causing current I [Lindner 93]. For the sake of simplicity, the vector is not used in the following. The

Inductivity

unit of the magnetic field strength is ampere per

The inductivity L of a coil is given as [Lindner 93]:

metre (A/m):

𝐻 =

𝐴 𝑚

𝐿=

𝑁∙Φ 𝐼

Magnetic flux density

In which:

The magnetic flux density (sometimes referred to

N:

(Eq. 1-20)

Number of turns in a coil

as magnetic induction) “B” provides the link be-

Φ:

Magnetic flux

tween the magnetic field strength H and material

I:

Current

properties. The magnetic flux density B is given as [Lindner 93]: The unit for the inductivity is Henry (H), named af-

𝐵 = 𝜇! ∙ 𝜇! ∙ 𝐻

(Eq. 1-18)

ter the US-American physicist Joseph Henry, and can be given as:

𝐿 =𝐻=

In which: µ0 :

Absolute permeability = 1.257⋅10-6 H/m

µr:

Relative permeability

H:

Magnetic field strength

For most conductors the relative permeability µr is

𝑉∙𝑠 𝐴

1.1.6 Electromagnetic induction Electromagnetic induction is one of the fundamen-

close to one, only for strong magnetic materials -

tal findings within the field of electricity. All kinetic

the so called ferrite materials - µr is much larger

energy generated by electric energy (and vice ver-

than one. The unit of the magnetic flux density is

sa), such as that found in electric motors and gen-

tesla (T), named after the Serbian engineer and in-

erators, is based on this relation.

ventor Nikola Tesla.

Faraday found that the electromotive force (EMF)

𝐵 =𝑇=

𝑉∙𝑠 𝑚!

Magnetic flux

produced around a closed path is proportional to the rate of change of the magnetic flux through any surface bounded by that path. The electromagnetic induction (or simply “induction”) is defined as the creation of an electric volt-

The magnetic flux “Φ” is the magnetic equivalent

age in an electric conductor caused by the change

of the electric current I. It is driven by a magnetic

of the magnetic flux in the area of that conductor

field (comparable to the current I which is driven by

[Lindner 93]. The induced voltage according to the

the voltage) and describes the magnetic flux

law of magnetic induction by Faraday can be ex-

through a material. For a homogeneous field with

pressed as:

the magnetic flux density B and the area A, the magnetic flux is given as [Lindner 93]:

10

𝑉"!# = −𝑁

𝑑Φ 𝑑𝑡

(Eq. 1-21)

ture in order to explain the basic relation between the expressions. In technical objects, such as cable insulations, a very large number of electric charges occur, creat-

In which: Vind:

Induced voltage

ing the electric field in the insulation materials. In

N:

Number of turns of a coil

order to visualise the electric field, electric field

dΦ/dt:

Rate of change of magnetic flux

lines are applied. Electric field lines describe the way positive charg-

The sign “–“ indicates that the induced voltage Vind has the opposite direction to that of the change of the flux that produces this voltage. The relation becomes clearer when applying equation 1-18 and 119 to equation 1-21:

es move in the insulating material. They only occur in insulating materials, not in conductors. In contrast to magnetic field lines, electric field lines always have a start and an end point, which are always on the surface of a conductor. To prevent confusion between the terms “conductor” and “current carrying conductor” (such as the copper

𝑉"!#

𝑑𝐻 = −𝑁 ∙ 𝐴 ∙ 𝜇! ∙ 𝜇! ∙ 𝑑𝑡

(Eq. 1-22)

conductor of a cable), conductors hosting an electric field are referred to as “electrodes”. Electric field lines always enter and leave the electrodes

An electric voltage is induced in any conductor in a

perpendicular to their surfaces.

changing magnetic field, whether the field itself

If electrodes are metallic plates, lying parallel to

changes or the conductor is moved within the

each other, the direction of all field lines between

magnetic field. When the conductor in a changing

the plates is the same and the absolute values are

magnetic field is a closed loop, a current can flow

equal. In this case the electric field is homogenous.

in that loop. By applying Ohm’s law, the current flow in that loop can be determined. It is worth mentioning that only voltage can be in-

+Q

-Q

+ + + + + + + +

-

duced, not current. Current flow is always a result of the induction of voltage together with the resistivity of the material. Electromagnetic induction is particularly relevant to cables. The current flow through the (inner) conductor of a cable induces voltage into the outer ground wires of the cable. This induced voltage is the reason why the outer ground wires of the cable have to be earthed and/or cross-bonded. Since this

d

topic is highly relevant to cable design and accessory application, it is described in detail in Chapter 4.

1.2 Electric field 1.2.1 Electric field and field lines Electric field lines Section 1.1.1 showed that an electric force acts between two charges, resulting in the creation of an electric field. This explanation is of a physical na-

Schematic drawing of the homogenous electric field between two parallel plates, charged with positive and negative charge Q

Electric field strength When two parallel plates are charged with the voltage U, the correlation between the electric field and the voltage is given by the following expressions [Küchler 96]:

1. Fundamentals

𝐸=

𝑈 𝑑

11 (Eq. 1-23)

In which: E:

Electric field

V:

Voltage between the two plates

d:

Distance between the two plates

The electric field between two plates is a simple arrangement and can be expressed easily. More complex arrangements, such as the electrical field in joint bodies, are calculated with the help of computer simulations. These calculations are done with finite element programs.

1.2.2 Electric field in technical objects Electric field in insulation bodies of joints When designing electric equipment, knowledge of

FEM calculation of the electric field in the stress cone of a termination and in the basement of the insulator (red = highest electric field; dark blue = no electric field)

Electric field in cylindrical cable insulations The electric field in a cable can be calculated with an approximation of two coaxial cylinders. Thus, the electric field E(x) at the position x in the cable insulations is given as [Küchler 96]:

the electric field in and around the equipment is essential. The shape of technical equipment, such as electrodes in an insulation body of a polymer joint, are usually so complex that modern computer simulation methods (so called Finite Element Method –

𝐸 𝑥 =

𝑈 𝑥 ∙ ln

(Eq. 1-24)

𝑅 𝑟

In which:

or just FEM) are necessary to calculate the electrical

E(x):

Electric field at point x

field at each point of the arrangement. The different

U:

Applied voltage

colours represent different values of the electric

R

Radius of the outer conductive layer

field strength.

r

Radius of the inner conductive layer

x:

Position

R r FEM calculation of the electric field in the insulation body of a polymer joint (red = highest electric field; dark blue = no electric field)

E Electric field in terminations Similar to the electric field in joint bodies, the electric field in stress cones for terminations as well as in the whole arrangement of the terminations themselves is also calculated with the help of numerical FEM tools.

E(r) x Schematic drawing of the insulation between the inner and outer semiconducting screen of a cable and the radial electric field in the insulation

12 Equation 1-24 shows that the highest electrical field

is considered. The smaller the ratio, the smaller the

in the insulation of a cable occurs at the inner sem-

factor by which the electric field is increased.

iconducting layer. The design of the cable and the manufacturing process must guarantee that the maximum electric field during the different tests remains below the dielectric strength of the insulating polymer. While the maximum electric field of medium voltage cables ranges from 2 to 4 kV/mm, those of high

Ellipsoidal shaped protrusion

voltage cables are much larger and range from 6 to 14 kV/mm. Inclusions with different permittivity in the Electric field in cable insulations with inclusions at the semiconducting layer Equation 1-24 also shows that the shape of the electrodes determines the distribution of the electric field. In order to use the insulation material as effectively as possible, the electric field should re-

insulating material If two different materials A and B with a relative permittivity of εA and εB are between two parallel electrodes, the electric field EA and EB in the two materials is given by the following expression [Küchler 96]:

main uniform, otherwise considerable insulation material is needed, making the cable expensive and heavy.

𝐸! 𝜀! = 𝐸! 𝜀!

(Eq. 1-25)

To avoid unnecessary high electric fields, it is essential that the electrodes are plane and rounded.

Equation 1-25 shows that in the material with the

Sharp edges or peaks increase the electric field. For

higher relative permittivity, the electric field is low-

this reason, it is very important that high voltage

er.

cables and cable accessories are manufactured in clean surroundings. Even very small conductive

+Q

particles of 50 to 100 µm on the electrode surface will increase the electric field dramatically [Weis-

+ + + + + + + +

senberg 86], [Weissenberg 09].

Electric field lines

High voltage

Ground

-Q

ε1=1

ε2=2

Ε1=2

Ε2=1

d/2

d/2

-

Influence of a sharp edged particle on the electric field distribution

Schematic drawing of electric fields in two different materials with different relative permittivity (the density in the field lines reflects the electrical field strength)

The factor by which the electric field is locally in-

If an air or gas inclusion occurs in the insulating

creased depends on the exact shape of the conduc-

polymeric material, the electric field in that bubble

tive particles or protrusions. Therefore, the ratio of

can be calculated by using Equation 1-25 as fol-

height to width of an ellipsoidal shaped protrusion

lows:

1. Fundamentals

13 be seen to cause electric discharges and, hence,

𝐸#"! =

𝜀%#!'$&(%"# ∙ 𝐸%#!'$&(%"# 𝜀#"!

(Eq. 1-26)

insulation failure. Although the above calculations were applied to inclusions of air in the insulation material of XLPE,

Equation 1-26 shows that the electric field in air in-

they are also applicable and relevant to all insula-

clusions is higher than in the surrounding polymer.

tion materials.

Taking into account that the dielectric strength of air is also much lower than that of the polymer, the danger of air inclusions in polymeric materials becomes obvious.

1.2.3 Capacity Every

Polymer material

arrangement

of

a

“conductor-insulator-

conductor” is able to store electrical charges. The capability of such an object to store charges is given by the capacity “C”. The capacity of such an object depends only on the insulating material and the geometrical arrangement. The capacity of two

Gas

parallel plates can be calculated as [Küchler 96]:

𝐶 = 𝜀! ∙ 𝜀! ∙ Schematic drawing of the electric field in an air-filled void within a polymeric insulation (the density in the field lines reflects the electric field strength) (idealised situation)

𝐴 𝑑

(Eq. 1-27)

In which: C:

Capacity

ε0 :

Absolute permittivity or electric field constant; ε0= 8.854 ⋅ 10-12 F/m

εr :

Relative permittivity

Example 1-3:

A:

Area of the plates

Æ The relative permittivity εr of XLPE = 2.3, the electric field in the insulation of Einsulation = 10 kV/mm and the dielectric strength of air is: Ebreakd. air = 2.5 kV/mm

d:

Distance between the plates

 Electric field of air inclusion compared to dielectric strength of air P Taking the relative permittivity of air as εair = 1 and calculating according to Equation 1-26, the electric field in the air bubble can be calculated as:

𝐸#"! =

!.! !

∙ 10

!" !!

= 23

!" !!

.

9 The electric field strength of the air bubble is about 10 times higher than the dielectric strength of air. If such an air bubble were in the insulation, a local breakdown (partial discharges) in the bubble would occur, thus damaging the cable insulation.

The unit of the capacity is farad (F), named after the English physicist Michael Faraday. It is defined as:

𝐶 =𝐹=

𝐴∙𝑠 𝑉

The relation between the stored charge Q and the capacity C is given by:

𝐶=

𝑄 𝑈

(Eq. 1-28)

Example 1-3 illustrates why small air or gas filled

The relative permittivity εr of the material reflects

voids are so dangerous in insulating materials. In

the polarity of the atoms or molecules in the mate-

these voids, the electric field is higher than in the

rial and is expressed with a dimensionless number.

surrounding insulating material and the dielectric

In other words, this material constant describes

strength of air is much lower than that of the solid

how well electrical charges can be held in an insu-

insulation. Thus, voids in an insulation material can

lating material. The higher the relative permittivity,

14 the higher the polarity of the material. The higher the polarity of the material, the more charges can

1.3 Insulating materials in high voltage technology

be stored in the insulation – hence the capacity of the material is higher. Typical values of the relative permittivity are given in the table below.

1.3.1 Solid materials There are two general types of solid insulating ma-

Relative permittivity εr of different insulating materials, used in high voltage technology (at 20°C and 50 Hz); [ABB 92], [Küchler 96]

terials in high voltage technology, ceramic materials and polymers. The most common type of ceramic material is porcelain, used for insulators and the supporting bodies of high voltage terminations.

Material

Relative permittivity

Cross-linked polyethylene (XLPE)

2.3

Polyvinylchloride (PVC)

3.3 – 7.0

Polyurethane (PUR)

3.0

Silicone Rubber (SIR)

2.7

Epoxy Resin

3.0 – 4.2

Air

1

Silicone Oil

2.8

Transformer Oil

2.8

Porcelain

2–6

prevents the polymer from becoming liquid when

SF6

1

temperatures exceed melting point. High density

Water

80

For polymers, a considerable number of different materials occur. In general, these fall into the following groups: - Thermoplastic polymers - Thermoset materials - Elastomer materials

The most common thermoplastic material is crosslinked polyethylene (XLPE), which is used for cable insulation. The cross linking of the polyethylene

polyethylene (HDPE) and polyvinylchloride (PVC) two other thermoplastic materials - are used for the cable sheaths of high voltage cables. Thermoset materials are used as insulating bodies

Example 1-4:

for high voltage cable terminations, particularly for

Æ Area of capacitor = 1 m2, thickness of the capaci-

GIS and transformer terminations. The most com-

tor = 1 mm by considering the two materials εXLPE =

mon thermoset material for such applications is

2.3 for XLPE and εWater = 80 for water

epoxy resin.

 Capacity of the arrangement

Elastomer materials, such as silicone rubber (SiR)

P Calculating the capacity according to equation 127; the results are:

or ethylene propylene rubber (EPR), are widely

𝐹 1 𝑚! ∙ 2.3 ∙ = 0.02 𝜇𝐹 𝑚 0.001 𝑚

advantage of elastomers is their flexibility and the

𝐶 = 8.85 ∙ 10

!"!

𝐹 1 𝑚! 𝐶 = 8.85 ∙ 10!"! ∙ 80 ∙ = 0.71 𝜇𝐹 𝑚 0.001 𝑚

used for stress cones and joint bodies. The main fact that they can be elongated considerably, especially the material of silicone rubber. Thus, these properties enable certain sizes of stress cones to be used with a wide range of different cable diameters. Silicone rubber compounds are also used as material for outdoor terminations. Particularly in heavily polluted environments, to which their hydrophobicity is excellently suited.

1. Fundamentals

1.3.2 Liquids The majority of liquids used in high voltage technology are mineral and synthetic oils. These oils are used in oil-filled cables and transformers.

15

1.4 Power transmission 1.4.1 Basics of electric power transmission systems

Another insulating liquid is silicone oil, which is

According to a general definition, electric power

used in terminations for polymer cables. For these

transmission is the bulk transfer of electrical ener-

applications it is important that the silicone oils ful-

gy, a process involving the delivery of electricity to

fil certain electric requirements. The most im-

consumers [Wikipedia 09-2].

portant ones are the dielectric strength and the loss

A power transmission network typically connects

factor – both of which are influenced by the humidi-

power plants to multiple substations near a popu-

ty of the oil.

lated or industrial area. The wiring from the power plants to the substations is referred to as electrical

1.3.3 Gases The most important gas in high voltage technology – aside from air – is sulphur hexafluoride (SF6). This gas is used in gas insulated switchgears (GIS) and has a good dielectric strength. In addition, SF6 is a gas with electro negativity properties, which makes it very suitable for use in switching chambers of circuit breakers [ABB 92]. In general, the breakdown voltage of gases also depends on the pressure and distances between the electrodes. This relation is described by the “Paschen law”, named after the German physicist Friedrich Paschen. The following diagram shows the “Paschen law” for air. The breakdown voltage is a function of the air pressure times the distance between the electrodes.

transmission. The wiring from substations to consumers is referred to as electrical distribution. The electric power transmission allows distant energy sources to be connected to consumers in population centres. The energy sources can be traditional energy sources, such as coal or hydroelectric power plants, or renewable energy sources, such as wind farms or solar plants. The power transmission network is referred to as the “grid”. Multiple and redundant lines between points of the network are made in order that the power can be routed from any power plant in the grid to any load centre through a variety of lines. This is done to provide redundancy in order to enhance security of energy supplies. Recently, power transmission has come to be greatly influenced by the economics of the transmission path. Thus, the cost of power has become just as important as the redundancy of the system.

Relation between the breakdown voltage and the product of the gas pressure and the distance between the electrodes for air.

16

Coal plants

Nuclear plants Extra high voltage

275 kV – 1000 kV (mostly AC, some HVDC)

Large wind farms

High voltage 110 kV and higher Medium sized power plants

Transmission Grid Industrial Power plants Distribution Grid

City power plants

Medium voltage

Industrial customers

Private solar farms

Low voltage

Private consumers

Wind farms Extra high High Medium Low voltage

Transformer

Solar farms

Structure of a power transmission and distribution system

Transmission lines usually use a three-phase alter-

enables a maximum transmission capacity with a

nating current (ac). In order to reduce losses during

minimum of losses.

transmission, electricity is transmitted at high volt-

High voltage direct current (HVDC) is used for long

ages. In terms of transmission lines, “high voltage” means voltage levels above or equal 110 kV. In general, the higher the transmission distance, the higher the transmission voltage should be. This

distance transmission or in long oversea cables.

1. Fundamentals

17

1.4.2 Overhead lines Overhead lines are metal-wired conductors with no insulation cover. The conductor material is usually aluminium alloy consisting of several strands, often reinforced with steel strands. Copper in overhead lines is only used for very specific applications, such as railway lines. Conductor sizes range from those of 10 mm2 to those of 1000 mm2, with varying resistance and current-carrying capacity. Thicker wires are more expensive, leading only to a relatively small increase in current carrying capacity due to the skin

Optical ground wire – OPGW

effect (for more information, see Chapter 4). Overhead lines contain a tower and insulators to separate the voltage-carrying conductors from the

1.4.3 Power cables

ground. Most high voltage overhead lines have an

Power cables are metal-wired conductors covered

earth wire on top, which conducts earthing current

by a solid insulation. Their conductors consist of

at an asymmetrical phase shift of the three phases

either copper or aluminium. Their solid insulation

and protects the high voltage overhead line from

consists of either a polymer or of an oil-filled or

lightning strokes. Such earth wires are usually

mass impregnated paper.

made with integrated fibre optics, the “optical ground wire” (OPGW). OPGW are used for additional data transmission. Since overhead transmission lines are insulated by air, their design requires a minimum of clearances to the ground for maintaining the required safety.

Advantages of underground power cables in comparison to overhead lines are: - Far less subject to damage from severe weather conditions

Adverse weather conditions – such as heavy winds

- Less required space for transmission path

or storms, ice loads, or even extremely high tem-

- Can be used to cross large lakes or seas

peratures – can affect the performance and have to

- Invisible to the public

be considered in their design and operation.

- Less required material for insulators and towers - Lower probability of external failures, such as tree-falling or bird collision - No danger to flying aircrafts

Disadvantages of cables in comparison to overhead lines are: - Higher costs - Lower transmission capacity due to lower heat dissipation - More difficult to repair failures in the system Tower of an overhead line with wires, insulators and OPGW on top of the tower

- High capacitive charging current for the operation of the system

18 Example 1-5: Depending on the voltage level, cables only require approximately 1 – 10 metres for installation, whereas overhead lines require a surrounding strip of approximately 20 – 200 metres, which must be kept permanently clear for safety, maintenance and repair.

tem, detailed information on these components is given in Chapter 5. For an efficient cable system further accessories are necessary; these are: cross-link boxes, earthlinking boxes, surge voltage limiters or cable clamps. More information about these additional accessories is given in Chapter 6.

Electric power cables have their ingredient part in the electric power transmission. They are used for the transmission of power in and through: - Densely populated areas - Areas where land is unavailable for overhead lines because the consent for planning of overhead lines may not be given

1.4.4 Transmission capability The amount of power that can be transmitted over transmission lines, be it overhead lines or cables, is limited. The reasons for such limits are related to the length of the line. In the case of short lines,

- Rivers and other natural obstacles

losses caused by current flow through the conduc-

- Territory with natural or environmental herit-

tors produce a thermal limit. If too much current

age - Areas of significant or prestigious infrastructural development - Territory which needs to maintain its value for future urban expansion or other developments

flows through a cable, the insulation of the equipment may be damaged irreversibly. The insulation of cables makes the thermal limits lower than those of overhead lines. For lines of intermediate or long lengths, the voltage drop of the lines sets the limit. As a general rule, the economic distance of an ac overhead line

Before the 1970s, underground power cables were

in km is approx. equal to the voltage level in kV. For

insulated with oil-paper. The oil was held under

example; when transmitting power over a distance

pressure in order to prevent formation of voids,

of 350 km, a voltage level of 380 kV is typically

which would lead to partial discharges in the insu-

used.

lation and finally cause a breakdown of the system. Throughout the world, many of today’s power grids still make use of oil-paper insulated cables. However, since that technology is not state of the art and several literatures are available on the market covering that topic, high voltage oil-filled cables

Despite urban areas, higher voltages are usually transmitted through overhead lines, whilst cables usually transmit lower voltages. The ratio of AC overhead lines to cables in the German grid is shown below [Kirchner 09], [Henningsen 09].

in this book. Nowadays most of the high voltage underground cables are insulated by cross-linked polyethylene (more information is given in Chapter 4). To connect the cable to other electrical equipment, such as substations, transformers or overhead lines, terminations are used. To achieve greater transmission length, joints are used to connect two cable segments together. Terminations and joints for high voltage cables are referred to as “high voltage accessories”. Since high voltage accessories are vital for the functioning of the cable sys-

% of overhead lines and cables

and their associated accessories are not considered 100

Overhead lines

90

79.7

80

99.7

93.8

Cables

70

64.4

60 50 40 30 20

35.6 20.3 6.2

10 0

≤ 1 kV

>1 - 60 kV

110 kV

0.3

220 kV

Voltage level

Rate of overhead lines to cables in the German grid [Kirchner 09], [Henningsen 09]

1. Fundamentals Until a few years ago, it was difficult to predict temperature distributions along a cable route. As a

19 Typical electric fields for different situations

result, the maximum applicable current load was

Description

Electric field

usually a compromise between an estimation of

Natural electric field on earth (without thunderstorm) [Wikipedia 10-1]

About 130 V/m = 0.13 kV/m

Typical value in houses (due to electric home equipment and home power supply) [TU-Graz 10]

5 – 40 V/m = 0.005 – 0.04 kV/m

todays and future operation conditions and also reflected the desire to reduce the risk of thermal failures to a minimum. Today, with the availability of industrial distributed temperature sensing systems, the monitoring of cables is easier. Together with intelligent software, it enables the operator to predict the thermal load of the system. For more

50/60 Hz 220 kV overhead line [TU-Graz 10]

detailed information on the distributed temperature

- directly under the line

2.5 – 6 kV/m

system of cables, see Chapter 8.

- with 20 m distance of the line

1 – 2 kV/m

During use of an electric blanket (50/60 Hz) [TU-Graz 10]

4.5 kV/m

1.4.5 Power transmission and environment Electric field

The values of electric field limits are different in certain countries or regions. A selection of limits for the ac electric field is given in the table below.

The topic of power transmission and environment, especially the issue of the influence of the electro-

Selected limits for electric ac fields at 50 Hz

magnetic field on surrounding individuals, has become increasingly prominent over the past years.

Source

As this topic is also related to cable systems, the most important issues are discussed here. However, due to the complexity of this topic, additional details from the relevant literature should be taken into account. When generating, transmitting or using electrical power, electromagnetic fields (EMF) occur. In the case of overhead lines, EMF passes in an unfiltered manner out to the external environment. In contrast, EMF in cables is considerably lower due to shielding by the outer metallic shield of the cable, as well as the soil around the cables themselves.

Limits of the electric field

26th BImSchV (German regulation for electromagnetic pollution) [BImSchV 97]

5000 V/m = 5 kV/m

International Commission on Non-Ionizing Radiation Protection (ICNIRP) World Health Organisation (WHO) [ICNIRP 10]

5000 V/m = 5 kV/m

DIN/VDE 0848 (German Electrotechnical Standard) for average populated area [DIN/VDE 0848]

7000 V/m = 7 kV/m

DIN/VDE 0848 (German Electrotechnical Standard) for working space area [DIN/VDE 0848]

20000 V/m = 20 kV/m

Although there are no shields for the electromagnetic field of overhead lines, this must not necessarily be a cause for concern, as they are usually located at a greater distance from the ground or

The values in the above tables are for 50 Hz. However, they can also be assumed for 60 Hz. The val-

surroundings. The highest value for the electric

ues for 16 ⅔ Hz, typically occurring in railway

field of high voltage overhead lines occurs directly

overhead lines, may slightly differ. In the 26th BIm-

under the line. The electric field in terms of envi-

SchV (German regulation for electromagnetic pol-

ronmental influence is usually given in V/m or

lution), the limit for a 16 ⅔ Hz electric field is

kV/m. Typical electric field levels in overhead lines

10 kV/m (instead of 5 kV/mm for 50 Hz fields) [BIm-

located in various environments are shown in the

SchV 97]. A similar IEC standard to that of the

table below.

DIN/VDE 0848 is the IEC 62226 [IEC 62226].

20 Magnetic field Overhead lines and cables not only emit electric fields, but also magnetic fields. It is hardly possible to give a number for the value for underground cables, as this is very much depends on the laying conditions of the cable system and the current flow. When laying cables close together in the ground, the magnetic field on the ground surface is

Vacuum cleaner at a distance of 30 cm [BfS 10]

2 – 20 µT

Drill machine at a distance of 30 cm [BfS 10]

2 – 3.5 µT

Electric cooking oven at a distance of 30 cm [BfS 10]

0.15 – 0.5 µT

Fluorescent lamp at a distance of 30 cm [BfS 10]

0.5 – 2 µT

almost degraded and is thus very low. When laying cables somewhat apart (e.g. at a distance of 60 – 80

* It should be mentioned that the natural magnetic field

cm, and at a depth of 1 – 4 m), similar values to

on the surface of the earth is approximately 30 µT at the

those occurring in high voltage overhead lines can

equator, and 60 µT at the poles. While the earth’s magnet-

be seen. Typical magnetic field levels of overhead

ic field is static, the magnetic field emitted by overhead

lines and cables in various environments are

lines varies according to line frequency, and is thus

shown in the table below.

16 ⅔ Hz, 50 or 60 Hz.

Typical magnetic fields for low and intermediate frequency (similar to 50/60 Hz) for different situations

e.g. by means of a faraday cage, it requires special

Description Natural magnetic field [Wikipedia 10-2] Typical house values due to domestic electric equipment and power supply) [TU-Graz 10] High voltage overhead line with a current flow of 1000 A [TUGraz 10] - directly under the line - with 50 m distance of the line At certain working space areas (close to transformer stations, switching stations or inductive heating ovens) [TU-Graz 10]

Magnetic field

While electric fields are relatively easy to shield, engineering techniques or designs to reduce their magnetic fields. The values for limits of the magnetic field differ be-

about 50 µT*

tween certain countries or regions. A selection of limits for alternating magnetic fields of low to in-

0.05 – 0.1 µT

termediate frequency - such as those caused by electric power supply equipment is given in the table below.

8 – 16 µT 1 – 3 µT

Selected limits for magnetic alternating fields at 50 Hz Source

up to few 1000 µT

Medium value in a German city (due to electric equipment in the city) [BfS 10]

0.06 µT

Medium value in a German family home (due to electric home equipment) [BfS 10]

0.06 µT

Medium value in a German highrise building (due to electric equipment in the building) [BfS 10]

0.076 µT

Medium value in a German office (due to electric office equipment) [BfS 10]

0.05 µT

Hairdryer at a distance of 30 cm [BfS 10]

0.01 – 7 µT

Limits of the magnetic field

26th BImSchV (German regulation for electromagnetic pollution) [BImSchV 97]

100 µT

International Commission on Non-Ionizing Radiation Protection (ICNIRP) World Health Organisation (WHO) [ICNIRP 10]

100 µT

DIN/VDE 0848 (German Electrotechnical Standard) exposition area 2, areas, such as living areas, sport or leisure areas [DIN/VDE 0848]

424 µT

Depending on the country or region as well as on certain areas where people can be exposed to magnetic radiation, the limits may vary significantly.

1. Fundamentals

21

The values given in the tables above are for 50 Hz. However, they can also be assumed for 60 Hz.

1.5 Terms and definitions 1.5.1 Definition of voltage values for cable systems

The values for 16 ⅔ Hz, typically occurring in railway overhead lines, may slightly differ. In the 26th

To understand and participate in discussions on

BImSchV (German regulation for electromagnetic

technical issues, certain knowledge of the terms

pollution), the limit for a 16 ⅔ Hz electric field is

used in the particular field is necessary. The follow-

300 µT instead of 100 µT for 50 Hz fields [BIm-

ing section provides definitions of the main terms,

SchV 97].

sion, state-of-the-art and standard terms are used

A comment on the effects of EMF on health and the environment The effects of EMF on the environment, particularly in terms of human health, have been the topic of a large number of studies. The studies can be classified as laboratory studies, such as studies on cells and

epidemiological

which occur in this book. To facilitate comprehen-

investigations.

Laboratory

as much as possible.

The voltages for cable systems are given in different values. The most common are U0, U, Um and Up. According to [IEC 60183], these values are defined as follows: U0

studies focus on changes at the molecular or cellu-

sheath for which cables and accessories

lar level after exposure of material to different val-

are designed

ues of EMF. Epidemiological investigations focus on the occurrence and distribution of diseases,

U

The rated r.m.s. power-frequency voltage between any two conductors for which ca-

such as cancer in human populations.

bles and accessories are designed

In general, the results of the studies and investigations showed that the effect of EMF on human

The rated r.m.s. power-frequency voltage between each conductor and screen or

Um

The

maximum

r.m.s.

power-frequency

health depends on the frequency of the EMF, on

voltage between any two conductors for

the length of exposure to the EMF and on the

which cables and accessories are designed. It is the highest voltage that can be

strength of the electric and magnetic fields.

sustained under normal operating condi-

Further results of these studies show considerable

tions at any time and at any point in a sys-

variation and are too complex to be taken up in the

tem. It excludes temporary voltage varia-

framework of this book.

tions due to fault conditions and the sud-

The World Health Organization (WHO), as well as an independent scientific organisation, The International Commission on Non-Ionizing Radiation Protection (ICNIRP), have both published guidelines for limiting exposure to EMF up to 300 GHz [ICNIRP 10].

den disconnection of large loads Up

The peak value of the lightning impulse withstand voltage between each conductor and screen or sheath for which cables and accessories are designed

U0

U0 U

U0 U

Definition of voltage values in a cable system according to [IEC 60183]

22 Relation between U0, U and Um according to [IEC 60183] Rated voltage of cables and accessories

1.5.2 Definition of terms for terminations and cable systems

Nominal system voltage

Highest voltage for the equipment

U / kV

Um / kV

20

24

Cable-termination Equipment fitted to the end of a cable to ensure

U0 / kV 12 18

30

33

36

26

45

47

52

36

60

64

66

The main terms used for terminations in cable systems are defined in [IEC 62271].

electric connection with other parts of the system and to maintain the insulation up to the point of

69

72.5

110

115

123

76

132

138

145

87

150

161

170

127

220

230

245

Cable termination, comprised of a separating insulating barrier between the cable insulation and the

160

275

287

300

190

330

345

362

220

380

400

420

290

500

525

550

430

700

750

765

connection. Two types are described in this standard [IEC 62271].

Fluid-filled cable-termination

gas insulation of switchgear. The cable-termination includes an insulating fluid as part of the cable connection assembly [IEC 62271].

Dry-type cable-termination Cable termination, comprised of an elastomeric

According to [IEC 62271] another type of voltage is

electric stress control component in intimate con-

defined as:

tact with a separating insulating barrier (insulator)

Ur

The rated voltage for the equipment of the

between the cable insulation and the gas insulation

cable connection is equal to the lowest of

of the switchgear. The cable-termination does not

the values for the cable and the gas-

require any insulating fluid [IEC 62271].

insulated metal-enclosed switchgear Fluid and insulating fluid The rated voltage Ur shall be selected from the fol-

The term “fluid” means a liquid or a gas for insula-

lowing standard values:

tion purposes [IEC 62271].

72.5 kV – 100 kV – 123 kV – 145 kV – 170 kV – 245 kV – 300 kV – 362 kV – 420 kV – 550 kV

Cable system

In the case of cables, the rated voltage Ur corre-

accessories”

sponds to the highest voltage for equipment Um

[IEC 62067].

A cable system is defined as a “cable with installed

[IEC 62271]. According to [IEC 62271], “the equipment” is the gas-insulated metal-enclosed switchgear. However, when a cable is connected to other power equipment, the terms related to the cable can considered to be similar.

[IEC

62271],

[IEC

60840]

and

Ageing and Life Expectancy

Chapter 2

Ageing and Life Expectancy

23

24

2. Ageing and Life Expectancy

25

2.1 Ageing in polymers 2.1.1 Theory of electric lifetime law

quire polyethylene materials of maximum purity and an extremely clean manufacturing process. Electric field ageing, ageing without the presence of partial discharges, is described by the lifetime

Basics When operational loads are applied, the electric

law as follows:

lifetime of polymers, such as cross-linked polyethylene (XLPE) or silicone rubber (SiR), is determined by internal and external influences. With applied voltage, two electric ageing processes tend to occur: partial discharge (PD) ageing and field ageing [Weissenberg 86],

[Peschke 99],

[Olshausen 01],

[Weissenberg 04-1], [Weissenberg 09].

PD ageing in polymers PD ageing due to discharge processes in cavities of the polymeric material or at any interface of the insulation system, such as the interface from the cable to the silicone slip-on element, leads to a rapid breakdown of the insulation. It is therefore vital that

𝐸 ! ∙ 𝑡 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡

(Eq. 2-1)

In which: E:

Electric field

n:

Lifetime exponent

t:

Time

The lifetime law provides a mathematical relation for the physical fact that material properties, the electric field and the time to breakdown are related. The lifetime law means that the lower the electrical field, the longer the time to breakdown of a material and vice versa.

both the cable and accessories of high voltage cacharges. Because of this, PD measurements are a standard in routine testing as a post-production quality control for cables and silicone parts of accessories [Weissenberg 04-1], [Weissenberg 09], [IEC 60840], [IEC 62067]. In addition, PD tests have become increasingly popular for additional measurements during after

Log. el. field strength E

ble systems are free of any internal partial disEbd1

n>40 (Silicone) n=20 (EPR)

Ebd2 Emax tbd1

tbd2

tbdmax

Log. time to breakdown tbd

Basic relation of lifetime law for polymeric materials

installation tests (for more information see Chapter 8).

In principle, the electric lifetime law is valid for a constant temperature level only. The materials usually used for high voltage cable insulation –

Electric field ageing for polymers Even in the absence of PD, polymeric insulation is subjected to ageing once an electric field is present. This process is called electric field ageing. Microscopic spurs and occlusions in the polymeric material cause the electric field ageing. At these areas, the electric field is elevated and the electric ageing process takes place more intensively. These spurs and occlusions are normal for polymeric materials and do not indicate a poor quality. However, the ageing process at these occlusions means that high and extra high voltage cable insulations re-

XLPE or ethylene propylene rubber (EPR) for cables and SiR or EPR for slip-on bodies of accessories – are very stable in the temperature range of their application. The influence of temperature on the electric field ageing is therefore negligible. Pure thermal ageing is, however, quite a different issue. This process is described in Section 2.1.3. The lifetime law is used to determine the expected lifetime of the insulation. For this purpose, breakdown tests with different levels of the electric field are carried out. The advantage of that is that the lifetime law is well-known and the estimated values are reliable. The disadvantage is that breakdown

26

2.1.2 Practical experiences with

values for real arrangements, such as polymeric cables, require a considerable effort to be deter-

electric lifetime law and electric

mined. If reliable values of the lifetime law are to

ageing

be obtained, breakdown values with a low electric field must be taken. This may take a long time and

In detailed investigations on breakdown of XLPE

is very expensive to carry out.

cables, it was found that XLPE has a lifetime expo-

Log. el. field strength E

nent of around 12. These investigations were made Short-term tests Ebd1 Ebd2

on cable samples, representing the real arrangement of a high voltage cable [Weissenberg 86],

Long-term tests

[Peschke 99], [Olshausen 01], [Weissenberg 04-1], [Weissenberg 09].

Designed lifetime

Ebd3 Ebd4 Emax

Max. electric field in insulation

Minutes

Months Hours Log. time to breakdown

Breakdown

50 years

b = 21 - 49 mm d = 12 - 31 mm

Different types of tests to determine expected lifetime of polymeric insulations

b

The design of the insulation is constructed in such

d

a way that the maximum electric field in the obCable dimensions used for ageing investigations

jects, such as cables or accessories, is lower than the residual dielectric strength after a period of about 50 years.

In detailed investigations on breakdown of SiR, it

Although not covered in this book, it should be

was found that SiR has a lifetime exponent above

mentioned that the ageing of oil-filled cables (and

that of 40. For SiR, investigations were made on

their accessories) can not be described with this

material samples as well as on real arrangements

lifetime law.

[Oesterheld 96].

100

n

Elektrical field strenght [kV/mm] Electric field strength [kV/mm]

E •t = constant n ≥ 12...17

10

Withstood values Breakdown values

Routine tests

Type test

Successfully in service

PQ test

1 0.1

1

10

100

1000

10000

100000

Time [h]

Breakdown values and electric ageing for XLPE cables according to [Weissenberg 86], [Peschke 99], [Olshausen 01], [Weissenberg 04-1] and values of practical experiences of Brugg Cables

2. Ageing and Life Expectancy

27

Test values and values of practical experiences of silicone elastomer based cable accessories of Brugg Cables

2.1.3 Thermal ageing

2.2 Volume effect of real polymeric arrangements

Chemical reactions, such as oxidation or creation of radicals, are influenced by temperature. They occur faster at higher temperatures. In the case of insulat-

The dielectric strength of a material also depends

ing materials, these reactions can lead to a change

on the volume at which the electric field is applied.

in the dielectric properties of the materials and can

This relation is called volume effect. The reason for

start after a relatively short time if the temperature

the volume effect is that the breakdown is initiated

continues to increase. An example of this is the fact

and propagates along small irregularities and weak

that the loss factor of an insulating material will in-

points of the material. With increasing volume, the

crease with the creation of polar molecules.

probability of having more weak points in the ma-

To reduce these kinds of reactions in polymers, small amounts of stabilisers and anti-oxidants are

terial also increases. This is a simple statistical effect.

added. However, even with a package of different

If the dielectric strength at a given volume VA is

stabilisers, it is not possible to stop these reactions

measured and the corresponding Weibull distribu-

entirely. Consequently, it is essential to determine

tion with the slope parameter b is known, the die-

the electric lifetime curve of the insulating materi-

lectric strength of a sample with volume VB can be

als as used in high voltage cables and accessories.

determined according to the following equation

Thermal ageing of materials is also used to deter-

[Küchler 96]:

mine the long-term performance of polymer properties. Polymer samples are stored in ovens at a higher temperature than that of the actual application temperature and remain there for several

𝑉! 𝐸#%"$! 𝑉! = 𝐸#%"$! 𝑉! 𝑉!

! !

(Eq. 2-2)

weeks or months. Material properties are then measured on samples that were stored at different

In which:

temperatures. From the results, the ultimate elon-

Ebreak:

electric field at breakdown

gation after several years at application tempera-

V:

Volume

b:

slope parameter in Weibull distribution

ture can be extrapolated.

The relation of the volume effect is of considerable practical relevance for cable systems. It is observed that companies with new products in the market tend to increase the thickness of the insulation in order to be on “the safe side”. But although in-

Failure probability

28

Failures due to ageing

Short-term failures

creased insulation thickness for a given voltage

Occasional failures

level brings a lower electric field, this is only half of the issue. The volume effect causes a higher number of potential weak points in the material, causing in turn a higher probability of breakdown processes and consequently a shorter lifetime.

Time

Bathtub curve representing failure occurrence in technical systems (such as cables and accessories)

Depending on the quality of the material, the advantage of a lower electric field with increased insulation thickness may almost be compensated by the volume effect in the material.

2.3.2 Short-term failures Short-term failures are often listed under a group of failures called “teething problems”. In general, they result from: - Improper design

2.3 Life expectancy 2.3.1 Basic failure behaviour

- Improper materials - Production failures - Installation failures

Life expectancy of the cable system is difficult to estimate. Electric and thermal ageing processes are known and lifetime expectancy can be estimated to a certain extent.

These types of failures usually occur during the first few weeks of the application. To avoid shortterm failures, a great variety of tests are done be-

However, knowledge of electric and thermal ageing

fore the product is delivered to the customer. A

is only one aspect in the process of making lifetime

number of different development tests as well as

predictions of high voltage power cables and their

type and prequalification (PQ) tests should prevent

accessories. In practice many other factors also

(or at least limit) the occurrence of short-term fail-

need to be considered. The sum of the factors in-

ures caused by improper design or improper mate-

fluencing the lifetime of the cable system lead to a

rials. Because of this, type and PQ tests have to be

failure curve that is high at the beginning, low in

repeated once a design or materials have changed

the middle and increases at the end of the lifetime.

significantly.

This behaviour is not only valid for cable systems, it is also typical for most technical systems (including cars, electronic items etc.). Due to its shape, the curve is referred to as “bathtub curve”.

The occurrence of production failures should be prevented by routine tests. To exclude production failures of accessories, pre-tested slip-on bodies are recommended. Pre-testing gives the customer

Factors influencing the lifetime of technical sys-

the proof that the equipment on-site has not suf-

tems, such as a cable system, can be distinguished

fered from production failures.

in: - Short-term failures - Occasional failures and - Failures due to long time ageing

The occurrence of installation failures should be limited by after installation tests. For additional security, additional partial discharge (PD) measurements may be used during after installation tests.

2. Ageing and Life Expectancy More information on tests for cable systems is given in Chapter 3. More information on PD tests for cable systems is given in Chapter 8.

Routine test of a cable drum after production

2.3.3 Occasional failures

29 Possible occasional failures in a cable systems and suggested measures of prevention Type of occasional failure

Suggested measure of prevention

Digging into the cable

Laying the cable in a tunnel or concrete duct or covering the cable system with concrete elements

Terrorist attacks or other human induced violence

In general very difficult; “hiding” the cable in underground concrete ducts, inserting terminations in strong housings

Shooting of insulators

Using grey insulators that are less visible than brown insulators, inserting terminations in strong housings

Bird picking of composite insulators

Using porcelain insulators in areas with a large number of birds that are potentially known for birds picking; using special composites

Earthquakes

Strengthening the fundament of the terminations, appropriate fixing of the cable and accessories to the fundament

Lightning strokes

Insulation coordination with surge voltage limiters for the systems and use of deflectors of solid materials

Once the cable system does not suffer from shortterm failures, the basis for a long lifetime is laid. From then on, only occasional failures can impair the performance of the cable system. Occasional failures mainly result from external influences. They may be: - Induced damage of the cable system by humans, such as digging into a cable - Human induced violence to the system, such as terrorist attacks or shooting on insulators - Systematic naturally induced destruction of the system, such as bird picking of composite insulators - Occasional types of naturally induced destruction, such as earthquakes or wind storms

In general, it is possible to prevent most occasional failures. However, this requires a certain effort and the owner of the system must consider whether such an effort and expenditure is in their interest or not. Possible measures to prevent or reduce the occurrence of occasional failures are listed in the table below.

Concrete ducts to protect the cable

30 than 40 years can be expected to be achieved, with a potential of an even longer lifetime being possible. To achieve a long lifetime of the cable system, the following occurrences must be particularly avoided or kept to a minimum: - Unnecessary high number of transient overvoltages, such as lightning switching impulses - Very high short circuits - Unnecessary high mechanical stress Protected transformer terminations for the power supply of a football stadium; installed in a building for protection (and other reasons)

- Constant high water pressure, especially in joints and terminations - Rodents and termites in the vicinity

Example 2-1:

Æ Cable system  General rules of operation for organic materials, such as XLPE P An increase of the operating temperature by 8 to 10°C reduces the service life by half. P An increase of the operating voltage by 8 to 10% reduces the service life by half.

Terminations with grey porcelain insulators for less visibility

2.3.4 Failures due to ageing Since high voltage cable systems have been in operation for decades, a wealth of experience already exists on the subject of long-term behaviour and ageing of cable systems. Factors that influence the ageing are: - Electric field - Temperature - Mechanical load - Moisture

Once short-term and occasional failures have been excluded, the above-mentioned factors run within the specified limits and an unnecessary overload of the cable system can be excluded, lifetime of more

Chapter 3: Tests and Standards

Chapter 3

Tests and Standards

31

32

3. Tests and Standards

3.1 Tests

33 In general, all materials and designs must undergo extensive development tests before considered for

3.1.1 Basic idea of testing Testing reveals the technical limits of products and systems. Other reasons for testing include quality

use in commercial products. A typical development test is to investigate the mechanical properties of polymeric materials.

control of the material, checking the design, simulating ageing processes or minimising production and installation failures. As a result, tests for cable systems can be typically categorised into different types as listed in the table below.

Overview of typical tests for cable systems Type of test Development test

Type test

Prequalification (PQ) test

Purpose of application To reveal the technical limits of the material and/or the system To stimulate ageing processes To examine material and design of single elements, such as joints, terminations and cables

Determination of the mechanical strength of a polymeric material in the application laboratory at Brugg Cables

To examine material and design of whole cable systems To stimulate first ageing processes

3.1.3 Type tests

Routine test

To detect production failures

Overview

After installation test

To detect installation failures

After a product has been developed, a type test must be carried out. According to IEC 60840, type tests are “tests made before supplying on a general commercial basis a type of cable system or cable or

3.1.2 Development tests

accessory covered by this standard, in order to demonstrate satisfactory performance characteris-

Development tests are carried out to investigate the

tics to meet the intended application. Once suc-

limits of the applied materials and the chosen de-

cessfully completed, these tests need not be re-

sign variants. Development tests are usually de-

peated, unless changes are made in the cable or

structive tests. This means that the material or the

accessory materials, or design or manufacturing

component, such as high voltage polymer cables or

process which might change the performance

high voltage accessories, are tested until break-

characteristics” [IEC 60840].

down occurs.

Put more simply, the standard says that the type

The results of such tests show the security margin

test determines the right dimensioning of the mate-

of the material and the component. Development

rial and design of the components that were devel-

tests must consider all possible factors that may

oped. A type test must be passed if the product is

influence the cable system throughout its lifetime.

to be sold.

These are electric, mechanic and thermal loads, the

IEC standard type tests for high voltage cables and

influence of moisture and all combinations of the

accessories

above-mentioned factors. The results of develop-

IEC 62067.

ment tests are a matter of the company and are subject to strict secrecy policy.

are

described

in

IEC 60840

and

34 IEC 60840 and IEC 62067 not only use different

equipment being tested but also from laboratory

voltage levels, they also employ different proce-

use and installation. To reduce the costs, as many

dures. IEC 60840 allows a separate testing of acces-

devices as possible should be tested at the same

sories, while IEC 62067 considers the testing of the

time.

whole “cable system”, that is, cables and accesso-

The standard design of a type test for a cable sys-

ries in the same sequence.

tem contains the following elements:

Relevant IEC type test standards for high voltage cables and accessories Standard

Voltage range

IEC 60502

1 kV ≤ Um ≤ 36 kV

IEC 60840

36 kV < Um ≤ 170 kV

IEC 62067

170 kV < Um ≤ 550 kV

- Cable with at least one segment of a minimum length of 10 m between the accessories (other segments must have a minimum length of 5 m between the accessories) - Outdoor termination with porcelain insulator - Outdoor termination with composite insulator - Joint - Back-to-back joint (consisting of an SF6 termination and a transformer termination)

Layout of type test The tested equipment is subject to very high stress, making it unable for commercial use afterwards. In addition, type tests are time intensive and expensive. Various costs accumulate, not only from the

Type tests are conducted in high voltage test laboratories, either at the cable or accessory manufacturer or at an independent test laboratory.

Outdoor terminations (1 composite & 1 porcelain insulator)

(Cross-bonding) Joint Cable (> 5m)

Back-to-back joint (SF6 & transformer termination) Cable (> 5m)

High voltage test transformer Cable (> 10m)

High current transformer (for thermal heating) Typical layout of a type test

3. Tests and Standards

35 d)

PD tests at ambient and high temperature

e)

Switching impulse voltage test (required for cable systems with Um ≥ 300 kV)

f)

Lightning impulse voltage test with 10 positive and 10 negative impulses followed by a power frequency voltage test

g)

PD tests, if not previously carried out in d) above

h)

Tests of outer protection for buried joints, thus containing a water immersion and heat cycling test and a separate different

Type test of a 550 kV cable system at an independent test laboratory

voltage test at the joint i)

Examination of cable system with cable and accessories after completion of the

Test sequence

tests

According to IEC 60840, the type test on accessories for voltages 36 kV < Um ≤ 170 kV shall be subjected to the following sequence: a) b)

Partial discharge (PD) test at ambient tem-

Besides these tests on the cable systems a type test according to the standards IEC 60840 and IEC 62067 includes also tests on the material of the cable.

perature

These material tests include the measurement of

Heating cycle voltage test with 20 cycles of

the resistivity of the conductor and insulation

an 8 h heating period and a 16 h cooling

screen and the mechanical properties of the cable

period at a voltage of 2U0

insulation material.

c)

PD tests at ambient and high temperature

d)

Lightning impulse voltage test with 10 pos-

Electric field in the equipment

itive and 10 negative impulses followed by

In both standards, the type test must be carried out

a power frequency voltage test

on the part of the equipment in which the highest

e)

PD tests, if not previously carried out in c)

electric field occurs. The electric field E(x) in the ca-

f)

Tests of outer protection for buried joints,

ble insulation is given by:

thus containing a water immersion and heat cycling test and a separate different voltage test at the joint g)

Examination of the accessories after completion of the tests above.

According to IEC 62067, the type test on cable systems for voltages 170 kV < Um ≤ 550 kV shall be subjected to the following sequence: a)

𝐸 𝑥 =

Bending test on the cable followed by installation of accessories and a partial dis-

𝑉 𝑥 ∙ 𝑙𝑛

𝑅 𝑟

(Eq. 3-1)

Eq. 3-1 shows four important facts for the electric field distribution in cables and accessories. These are: 1.

The highest field in the cable occurs at the inner diameter of the cable insulation.

2.

The highest field in a slip-on element of accessories occurs at the outer diameter of the

charge test at ambient temperature

cable insulation.

b)

Measurement of tan δ

c)

Heating cycle voltage test with 20 cycles of

highest value of the electric field is higher

an 8 h heating period and a 16 h cooling

than that which occurs at a larger conductor

period at a voltage of 2U0

3.

At a smaller conductor cross-section, the

36

4.

cross-section (assuming that the insulation

that for a cable with a large conductor cross-section

thickness is the same).

the thickness of the insulation is smaller than for a

At the outer semiconducting layer, the high-

cable with a smaller conductor cross-section.

est value of the electric field which occurs at

Consequently, type tests for high voltage accesso-

a larger conductor cross-section is higher

ries and the cables are typically carried out at a

than that which occurs at a smaller conduc-

conductor cross-section of 2500 mm2. Usually with

tor cross-section (assuming that the insula-

such a cable the requirements for the type test of the cable and that for the accessories are fulfilled.

tion thickness is the same).

Lower values of the cable cross-sections are covered by the standard, the electric field in the accessories being lower.

R1

Example 3-1:

r1

E

Æ A type test with the following values:

E(r2) E(r1) x

-

Cable of manufacturer A

-

Accessories of manufacturer A

-

U0 = 76 kV

-

Conductor cross-section = 2500 mm2

-

Diameter over inner conductor = 63 mm

-

Insulation thickness = 12.3 mm

 Is the accessories type test valid for a cable with the following details:

R2 r2

Schematic distribution of electric field in a cable with small (orange) and large (grey) conductor crosssection

-

Cable of manufacturer B

-

Accessories of manufacturer A

-

U0 = 76 kV

-

Conductor cross-section = 630 mm2

-

Diameter over inner conductor = 34 mm

-

Insulation thickness = 17.3 mm

P According to Equation 3-1, the electric field at the type tested cable and accessories is:

Taking the different factors of the electric field dis-

-

at inner cond. of the cable = 7.3 kV/mm

tribution and demands of the standards into ac-

-

at outer cond. of the cable = 5.3 kV/mm

count, one could think that cables and accessories need to undergo two different sorts of type test. One test should be made “for the cable” with the lowest possible conductor cross-section (as the electric field is highest at the inner semiconducting layer). Another test should be made “for the accessories” with the highest possible conductor crosssection (as the electric field is highest at the outer semiconducting layer of the cable, which is the same as the inner part of the joint body). However in reality the assumption to have the same thickness of the insulation independent of the cross section of the conductor is not correct. Cable manufacturer usually design the cable in such a way, that for a certain voltage level, the electrical field strength at the conductor is the same, independent of the conductor cross section. This means

According to Equation 3-1, the electric field at the cable, which shall be applied, and the accessories is: -

at inner cond. of the cable = 6.4 kV/mm

-

at outer cond. of the cable = 3.2 kV/mm

The calculated values show that for the accessories of manufacturer A, which shall be applied with a cable of manufacturer B, the electric field is lower at the outer semiconducting layer than in the type test. According to IEC 60840, the cable and accessories can be from different manufacturers.

9 Considering both facts, it can be concluded that the type test (of the accessories) is valid and the accessories of manufacturer A can be applied with that type of cable of manufacturer B.

3. Tests and Standards

37

Duration of type test The longest part of the test is the heating cycle voltage test with a duration of 20 days. The test itself lasts approx. 30 days including all other electric test sequences and the test of the outer protection for the buried joint. The installation of the equipment together with the time required for preparing the laboratory means that the full duration of a type test typically lasts about two months. To ensure that the test parameters meet the standards, the type tests are either carried out at an independent type test laboratory or are witnessed by a representative of an independent test institute.

Type test certificate Once the type test is passed, a type test certificate is given. This certificate lists all equipment tested, the various test procedures and the result of the test. With a type test certificate, the commercial

Example for cable (above) and accessory (bottom) data in a type test report

sale of the product can be made.

3.1.4 Prequalification tests Layout of tests A type test must be successfully completed before a prequalification (PQ) test is started. While a type test demonstrates satisfactory performance characteristics of the cable or the accessories (joints and terminations), the PQ tests examines the compatibility of the cable with the accessories. PQ tests are only required for cables systems if the calculated nominal electrical stresses at the conductor screen will be higher than 8 kV/mm or at the insulation screen higher than 4 kV/mm. According to IEC 62067, PQ tests are “tests made before supplying on a general commercial basis a type of cable system covered by this standard, in order to demonstrate satisfactory long term performance of the complete cable system. The prequalification test needed only be carried out once unless there is a substantial change in the cable system with respect to material, manufacturing process, design and design levels” [IEC 62067]. PQ tests are time intensive and expensive, being carried out for 365 days. Laboratory costs make the tests

particularly

expensive.

Consequently,

as

38 many devices as possible should be tested at the

a)

same time.

Heating cycle voltage test with a test period of 8760 h (1 year) at a voltage of 1.7U0. Parallel to the voltage load, 180 heating

The layout of a PQ test is similar to that of a type

cycles have to be made. These heating cy-

test, it typically contains:

cles must be at least 8 h, whereby the con-

- Full sized cable with a total length of approxi-

ductor temperature shall be maintained

mately 100 metres

within the stated temperature limits for at

- Outdoor termination with porcelain insulator

least 2 h of each heating point. The heating

- Outdoor termination with composite insulator

cycle shall be followed by a cooling period

- Joint

of at least 16 h*

- Back-to-back joint (consisting of an SF6 termi-

b)

Lightning impulse voltage test with 10 positive and 10 negative impulses

nation and a transformer termination) c)

Examination of the cable system after completion of the above tests

* According to the standard, the minimum time for the heating cycles is (24 h x 180 = 180 days). Once the heating cycles are finished, the heating of the cable can be stopped. However, the voltage test of 1.7U0 for one year must still be finished. The heating and cooling cycles usually occur over a period of one year, e.g. 16 h heating and 32 h cooling.

PQ test certificate Similar to the type test, the PQ test is carried out PQ test of a 245 kV cable system at an independent test institute

Test arrangement According to IEC 62067, “the test arrangement shall be representative of the installation design conditions e.g. rigidly fixed, flexible and transition arrangements, underground and in air.” [IEC 62067]. This means that the 100 m test cable length should be typical of the application, which is usually buried directly, in air or in a (concrete) tunnel.

either at an independent test institute or is witnessed by a representative of an independent test institute. Once passed, a PQ test certificate is given. In general, the commercial sale of the cable system is made with the PQ test certificate. Since the PQ test takes a long time, it is not untypical of installations with cables and accessories to be made in the field while the PQ test is still running. Such procedure has to be agreed between the cable manufacturer and the final customer. The PQ test certificate has to be delivered as soon as the test was passed.

It is further stated in IEC 62067 that “Ambient conditions may vary between installations and during the test and are not considered to have any major

3.1.5 Requalification tests

influence” [IEC 62067]. Taking into consideration

Layout of tests

that the test conditions in Mexico City in summer may be significantly different throughout the whole year than in St. Petersburg in winter, the detailed layout of the test should be discussed between the cable manufacturer and the customer in detail.

“Tests for the extension of the prequalification of a cable system”, often called “Requalification tests” is a new test possibility in the latest version of IEC 62067. This type of test has been introduced into the standard to consider changes in the material or production process without the necessity of

Test sequence

carrying out a complete (extensive and expensive)

According to [IEC 62067], the PQ test on the cable

PQ test. Such changes may be a new production

system shall be subjected to the sequence as:

facility (e.g. extrusion line) on a proven cable de-

3. Tests and Standards sign and cable material. The requalification test re-

39 h)

quires a valid prequalification test. Similar to the PQ test, the requalification test exam-

Lightning impulse voltage test followed by a power frequency voltage test

i)

ines the compatibility of the cable with the accesso-

PD tests, if not previously carried out in f) above

ries.

j)

According to [IEC 62067], the standard design of a

k)

Tests of outer protection for buried joints Examination of the cable system with ca-

requalification test is similar to that of a type or PQ

ble and accessories shall be carried out af-

test. It usually contains:

ter completion of the tests above

- Full sized cable with a total length of approximately 20 metres - Outdoor termination with porcelain insulator - Outdoor termination with composite insulator - Joint - Back-to-back joint (consisting of an SF6 termination and a transformer termination)

Test arrangement The test arrangement for a requalification test is similar to that of a type test with the exception that the minimum total cable length should be 20 m [IEC 62067].

l)

The resistivity of semi-conducting screens shall be measured on a separate sample

* According to the standard, the heating shall be applied for at least 8 hours with 2 hours of stated conductor temperature. The cooling period shall be 16 hours. The cycle of heating and cooling shall be carried out 60 times [IEC 62067]. ** According to the standard, the minimum number of heating cycles with voltage of 2.0 U0 shall be 20. The heating cycles can be interrupted.

Requalification test certificate Similar to the type and PQ test, the requalification test is carried out either at an independent test institute or is witnessed by a representative of an independent test institute.

Test sequence According to [IEC 62067], the requalification test on the cable system shall be subjected to the following sequence: a)

Since failures in the production never can be ex-

Bending test without final partial discharge

cluded, routine tests are carried out to detect pro-

test followed by installation of the accesso-

duction failures before the product (e.g. such as a

ries that are part of the tests for the exten-

cable drum) is delivered to the customer.

sion of the prequalification b)

3.1.6 Routine tests

According to IEC 60840, routine tests are “tests

A partial discharge test is applied after the

made by the manufacturer on each manufactured

bending test to check the quality of the in-

component (length of cable or accessory) to check

stalled accessories

that the component meets the specific require-

c)

Heating cycle test without voltage*

ments” [IEC 60840].

d)

Measurements of tan δ

In a routine test, high voltage tests and partial dis-

e)

Heating cycle voltage test**

f)

PD tests at ambient temperature and high temperature. This test shall be carried out after the final cycle of item e) above or, alternatively, after the lightning impulse voltage test in item h) below

g)

Switching impulse test (required for Um ≥ 300 kV)

charge (PD) measurements are carried out. In order to avoid unnecessary electric ageing effects, the test values for routine tests are less than those for type tests. The outline of the routine test, the requirements and the procedures for cables and accessories are dependent on the voltage level and listed in the relevant standards. After each test, a test certificate is given. It shows the equipment that was tested, the voltage range, the PD level, the date of test and the test person.

40

3.1.7 After installation tests Tests according to IEC After installation tests are made to detect failures that have occurred during the installation process, especially during the installation of accessories. According to IEC 60840, electric tests after installation are “tests made to demonstrate the integrity of the cable system as installed”. They are tests on new installations and “are carried out when the installation of the cable and its accessories has been completed” [IEC 60840].

Routine test of a slip-on element for high voltage joints in the fully screened test laboratory of Brugg Cables

Since cables and accessories are tested during the routine test in the factory, failures in the cable system can only occur during the transport. Damage of accessories, particularly the sealed slip-on ele-

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Sample of a routine test protocol (the diagram in the lower part shows the recorded PD level)

After the test, the slip-on elements of the accessories are cleaned. In order to be free of moisture, the slip-on elements are sealed in a plastic bag. To ensure the correctness of the test, a copy of the test certificate is also sealed in the bag. The procedure for cable drums is similar. After the routine test, the cable ends are cleaned and sealed, the cable drum is protected and the drum is delivered to the customer. The advantages of routine tests followed by the sealing of the devices are considerable. Once tested and sealed in the routine test rooms, the devices leave the factory at an extremely high quality level.

AC and DC tests According to the standards, after installation tests can be made with DC and AC. While DC tests for oil-cable systems are often used, DC tests pose a significant danger to polymeric insulations. Due to the character of the polymeric insulation, DC load causes space charges within the insulation. Such space charges can cause additional ageing or early breakdown in the insulation of the polymeric cable and accessories [Riechert 01]. In addition, DC tests do not detect all possible faults and can be dangerous for the accessories of AC polymer cable systems, the so called faults induced by DC testing [CIGRE-21.05 02]. This comes as the electric field control in AC and DC is of a different nature. Consequently, after installation tests are more frequently applied to AC voltage. Although not forbidden by the IEC standards, DC tests for polymer cables should not be applied.

3. Tests and Standards

41

High voltage tests with resonance systems The main reason why DC tests are applied is the effort needed for tests with ac equipment on site. In the last few decades, resonant test systems have been applied very successfully to electric after installation tests. A resonance system consists of the capacity “C” of the cable and the inductivity “L” of a transportable coil. Once energised, the circuit L and C oscillates at a resonant frequency of ω. Due to the capacity “C” of the cable, which is different for each arrangement (it depends on the cable geometry, the cable length, temperature etc.), the resonance frequency varies in each test. The standards take this into consideration and it is

After installation test of a 420 kV cable system of longer length with a heavy mobile resonance system (phase 2 under test)

stated that “The ac test voltage to be applied shall be subjected to agreement between the purchaser

Since resonance systems have been applied very

and the contractor. The waveform shall be substan-

successfully over the last few decades, it is no

tially sinusoidal and the frequency shall be be-

longer necessary, even dangerous, to carry out DC

tween 20 and 300 Hz” [IEC 60840], [IEC 62067].

tests for high voltage polymeric cable systems.

The equipment required for the test also depends significantly on the capacity “C” of the cable and therefore on the cable geometry and cable length. For cables with a high capacity, such as cable lengths of several kilometres, much more powerful equipment is required than for cables with a lower capacity, such as cable lengths of some metres. In general, the longer the cable length, the higher the power for the resonance system. For testing of cables with high capacity (high currents), resonance test systems can be put in parallel. For testing cables of high voltage, resonance systems can be put in series.

24 hour soak test For those applications to which a resonance test cannot be applied, such as with particularly long cable length or with resonance systems that are not available or too expensive, a test at nominal voltage can be carried out. This kind of test is called “soak test” and is made for 24 hours at U0 [IEC 60840], [IEC 62067]. The above-mentioned after installation tests are certified as either “passed” (without breakdown) or “not passed” (breakdown in the main insulation). It is worth mentioning that in case of a failure, soak tests run a higher risk than tests with a resonance system, as the whole energy capacity of the net can discharge at the place of failure. This can cause considerable damage at the place of failure. Therefore, tests with a resonance system should be favoured over 24h soak tests or if being done, the 24h soak test should be conducted out with no load in the system.

PD measurements Partial discharge (PD) measurements are an established method to assess the majority of electrical After installation test of a 420 kV cable system of short length with a light mobile resonance system (phase 2 under test)

equipment in power systems. For many insulation systems, PD measurements are a useful tool to evaluate the quality of the insulation.

42 The correct design of cable systems is checked in

PD tests can be expensive. This is due to the costs

type and prequalification tests, whilst the produc-

incurred by additional equipment and personnel, as

tion quality of cables and accessories is checked in

well as the time consuming processes of preparing,

routine tests according to the relevant standards.

carrying out and finalising the tests.

However, on-site installation poses a potential risk

The application of a PD test must therefore be care-

as far as defects are concerned. Although a final

fully considered. In most cases, it is the decision of

high voltage test after installation is carried out, the

the customer as to whether such a test should be

mentioned deteriorations cannot be fully excluded.

carried out or not. Further information on the types

In response to such difficulties, additional PD

and structure of PD tests is given in Chapter 8.

measurements can be helpful. PD measurements are typically used as measurements during after installation tests.

Sheath test at the cable

Experience shows that most defects caused during

The functioning of the outer sheath of the high

installation occur in the accessories. As a result, particular focus has been given to the technological development of reliable systems for measuring PD in joints and terminations. A PD inside a joint or termination is usually an extremely low-value signal. The cable system itself usually stands in industrial surroundings with a harsh electric environment. A low measurement

voltage cable is relevant for the long-term performance of the cable system. Therefore, tests for the cable sheath must also be carried out. These tests are made after laying the cable and carried out with DC. The applied voltage is typically 10 kV. In contrast to the main insulation, the application of DC for the cable sheath does not pose a significant danger for the cable sheath.

signal combined with a harsh electric environment makes it extremely challenging to carry out reliable

3.1.8 Alternative methods for after

PD measurements. Once the PD signal has been measured and identified as coming from the acces-

installation tests

sories and the cable, the accessories and the cable

Search for alternative test methods

must be dismantled in order to find the PD source.

The costs and effort on site for high voltage test

It is therefore strongly recommended when consid-

systems have led to a search for alternative test se-

ering PD measurements that the right PD equip-

quences. Currently, two main methods are under

ment is used and professional personnel employed

discussion. One is the test with damped AC voltage

to carry out the test.

(DAC), the other is tests with very low frequency (VLF). Tests with damped AC voltage (DAC) The test circuit for DAC tests mainly consists of a DC voltage source, the cable itself and an external inductance. The inductance and the cable capacitance form a resonance circuit. The cable capacitance is charged to a certain voltage, resonates with the inductance and oscillates with a frequency ranging between 20 and 1000 Hz. The losses of the (oscillating circuit) system cause a damping of the voltage. At certain time intervals, the system is recharged with the DC source. This process is done several times during the period of testing [Gulski

PD measurements at after installation test at a 220 kV outdoor termination

07], [CIGRE-D1.33.05 10]. The advantage of such tests is that smaller test equipment is needed. The disadvantage is that only the first few voltage cycles provide a high voltage

3. Tests and Standards load to the cable. If one wants to have a similar test load as that of a resonance system, the test with DAC must be significantly longer.

43

3.2 Standards 3.2.1 Introduction Standards are vital for use of technical systems in international markets. With standards, the final end user receives the promise that all parts will fit the system in his application. The international Electrotechnical Commission (IEC) is a worldwide organisation for standardisation, comprised of many national electrotechnical committees. The object of IEC is to promote international cooperation on all questions concerning standardisation in the electric and electronic fields [IEC 62271]. It is mentioned that standards and regulations of-

Test of a cable system with damped ac voltage

Today, DAC voltage tests are mainly applied as high voltage tests used for on-site insulation diagnostics of high voltage cable systems. Since the DAC test is not fully covered in IEC, few customers carry out this test. However, it has been successfully done on cable systems testing [Gulski 07].

ten differ between countries. Specific countrystandards are not mentioned here.

3.2.2 Main differences between IEC and IEEE standards High voltage cables and accessories adhere to two major international standards. These are set according to IEC and IEEE (Institute of Electrical and Electronics Engineers). While IEEE mainly covers the American region, IEC is a common standard

Tests with very low frequency (VLF)

throughout most other countries.

Tests with very low frequency (VLF) are typically

In the following tables, the main differences be-

carried out at a frequency of 0.1 Hz. In some cases, tests with frequencies down to 0.01 Hz are consid-

tween the two standards for high voltage cables and accessories are shown with particular focus on

ered as VLF [CIGRE-D1.33.05 10]. The advantage of

routine, type and PQ tests [IEC 60840], [IEC 62067],

such tests is that a lower effort for the test equip-

[IEEE 404], [IEEE 48].

ment is required. The disadvantages are the longer test time and that this kind of waveform is not tested sufficiently with high voltage accessories (for polymer cables). The field grading in high voltage accessories for polymer cables is based on the principle of capacitive field grading. The materials and design are designed to work under ac conditions of 50/60 Hz (or 16 2/3 Hz). A test with VLF at 0.1 Hz, or even 0.01 Hz is compared to 50 Hz at a 500 times (or 5000 times) lower frequency. The field grading properties of the deflectors and middle electrode is not sufficiently known at these frequencies. From today’s point of view, it is not clear whether high voltage accessories for polymer cables will work at these (very low) frequencies appropriately.

44 Main differences between IEC and IEEE standards Test type

IEC standard

IEEE standard

Type test

Considers cable, joints and terminations in one standard

One standard for cables, one for terminations and one for joints

PQ test

For cables and accessories in one standard

Does not consider a PQ test

Routine test

Considers cable, joints and terminations in one standard

One standard for cables, one for terminations and one for joints

Difference between IEC and IEEE for type tests

Test on main insulation

IEC 60840 cable and accessories

IEEE Std. 404-2000 for joints

IEEE Std. 48-1996 for terminations

Clause

Clause

Clause

PD tests

12.3.4

5 pC at 1.5 U0

7.6.1

5 pC at 1.5 U0

8.4.1.5

5 pC at 1.5 U0

AC voltage test

-

-

7.7.1

240 kV, 15 min

8.4.1.1

310 kV, 1 min, dry

8.4.1.2

275 kV, 10 s, wet

Requirements (132 kV)

Requirements (138 kV)

Requirements (138 kV)

DC voltage test

-

-

7.7.2

315 kV, 15 min

8.4.1.9

355 kV, 15 min

Heating cycle voltage test

12.3.6

20 cycles at 2 U0

7.9

30 cycles at 2 U0

8.4.2

30 cycles at 2 U0

PD tests

12.3.4

5 pC at 1.5 U0

7.6.1

5 pC at 1.5 U0

8.4.1.5

5 pC at 1.5 U0

Impulse voltage test

12.3.7

At ambient temp. and 95°C, 650 kV, 10+/10-

7.7.3

650 kV, 10+/10-

8.4.1.7

650 kV, 10+/10-

AC voltage test

12.3.7

2,5 U0, 15 min

-

-

-

-

PD tests

-

-

7.6.1

5 pC at 1.5 U0

8.4.1.5

5 pC at 1.5 U0

AC voltage test

-

-

7.10

200 kV, 6 h

8.4.1.3

210 kV, 6 h

Difference between IEC and IEEE for routine tests IEC 60840 cable and

IEEE Std. 48-1996 for

Test on main

accessories

IEEE Std. 404-2000 for joints

terminations

insulation

Clause

Clause

Clause

Requirements (132 kV)

PD tests AC voltage test

Pressure leak test

Requirements (138 kV)

Requirements (138kV)

9.2

5 pC at 1.5 U0

7.6.1

5 pC at 1.5 U0

-

-

9.3

2.5 U0, 30 min

7.7.1

3 U0, 15 min or

8.5.1

310 kV, 1 min, dry

7.7.3

BIL 650 kV, 10+/10-

-

-

8.5.2

Pressure 2.5x the normal rating, 1h

-

-

3. Tests and Standards

3.2.3 Relevant IEC standards The following list gives an overview of the most relevant IEC standards and their scope for high voltage power cable systems.

IEC 60885-3 Electrical test methods for electric cables – Part 3:

45 IEC 62271-203: High-voltage switchgear and controlgear – Part 203: Gas-insulated metal-enclosed switchgear for rated voltages above 52 kV

IEC 60183: Guide to the selection of high-voltage cables

Test methods for partial discharge measurements on lengths of extruded power cable

IEC 60228: Conductors of insulated cables

IEC 60038 (2009-06): IEC standard voltages

IEC 60229: Tests on extruded oversheaths with a special pro-

IEC 60141 (all parts):

tective function

Tests on oil-filled and gas-pressure cables and their accessories

IEC 60287: Electric cables – Calculation of current rating

IEC 60141-1: Tests on oil-filled and gas-pressure cables and their

IEC 60332:

accessories – Part 1: Oil-filled, paper-insulated,

Tests on electric and optical fibre cables under fire

metal-sheathed cables and accessories for alternat-

conditions

ing voltages up to and including 500 kV IEC 60811: IEC 60141-2: Tests on oil-filled and gas-pressure cables and their

Common test methods for insulating and sheathing materials of electric cables and optical cables

accessories – Part 2: Internal gas-pressure cables and accessories for alternating voltages up to 275 kV

IEC 60853: Calculation of the cyclic and emergency current rat-

IEC 60694: Common specifications for high-voltage switchgear and controlgear standards

ing of cables

IEC 61443: Short-circuit temperature limits of electric cables

IEC 60840 (2011): Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV) – Test methods and requirements

IEC 62067 (2011): Power cables with extruded insulation and their accessories for rated voltages above 150 kV (Um = 170 kV) up to 500 kV (Um = 550 kV) – Test methods and requirements

with rated voltages above 30 kV (Um=36 kV)

46

3.2.4 Relevant IEEE, AEIC, ANSI and ICEA standards

IEEE 400.3-2006 is a guide for partial discharge testing of shielded power cable systems in a field environment

In North America, cable systems are often specified according to: - IEEE - AEIC (Association of Edison Illuminating Companies) - ANSI (American National Standards Institute) - ICEA (Insulated Cable Engineers Association)

AEIC CS7-93: Specifications for crosslinked polyethylene insulated shielded power cables rated 69 to 138 kV.

ANSI / ICEA S-108-720-2004: Standard for extruded insulation power cables rated above 46 to 345 kV

The following list gives an overview of the most relevant IEEE, AEIC, ANSI and ICEA standards and their scope for high voltage cable systems.

IEEE standards and guides IEEE Std. 404.2000 is the standard for extruded and laminated dielectric shielded cable joints rated 2.5 kV to 500 kV (currently under revision – draft version 404-2012 available)

IEEE Std. 48-1996 describes the standard test procedures and requirements for alternating-current cable terminations 2.5 kV to 765 kV.

IEEE Std. 817-1993 describes standard test procedure for flame-retardant coatings as applied to insulated cables in cable trays.

IEEE 525-1987 is a guide for the design and installation of cable systems in substations.

IEEE 1299/C62.22.1-1996 is a guide for the connection of surge arresters to protect insulated, shielded electric power cable systems

IEEE 1300-1996 is a guide for cable connections for gas-insulated substations

IEEE 1406-1998 is a guide to the use of gas-in-fluid analysis for electric power cable systems

47

Chapter 4:

High Voltage XLPE Cables

48

4. High Voltage XLPE Cables

49

4.1 Design and types of high voltage XLPE cables

Conductor The conductor of high voltage cables consists of copper or aluminium. It is typically stranded and -

4.1.1 Cable design

in order to reduce the current losses - the conduc-

Introduction

ductor is covered with semiconducting layer to

tor can be additionally segmented. The inner con-

The development of cables with cross-linked poly-

achieve a uniform and smooth surface over the

ethylene (XLPE) insulation goes back to the 1970’s

single conductor wires.

[Peschke 99]. Since then, production and material technology have improved significantly. The main advantages of XLPE cables are that they are maintenance free and easy to install – vital factors if a highly reliable energy supply is to be achieved. At present, numerous high voltage XLPE cable systems with nominal voltages of up to 550 kV and

Different types of conductors; from left to right, sever-

with circuit lengths up to 100 km (voltage levels up

al round conductors, profile conductors and hollow

to 132 kV) are in operation worldwide [Avila 10],

profile conductors. (Hollow profiles are typically used

[Vogelsang 09], [Swingler 07].

for oil filled cables.)

Structure of high voltage XLPE cables XLPE cables consist of the: - Conductor - Inner semiconducting layer Principle types of segmented conductors; left: segmented full conductor and right: segmented hollow conductor for oil filled cables.

- XLPE insulation - Outer semiconducting layer - Outer conductor screen - Metallic sheath

Example 4-1:

- Outer polymeric sheath

Outer Wire screen & radial Polymeric (XLPE) sheath insulation moisture barrier

Æ The specific gravity of copper is 3.3 times higher than that of aluminium and the volume resistivity of copper is 1.56 times lower than that of aluminium. Æ For achieving the same conductivity as a copper conductor an aluminium conductor must have a conductor cross-section, which is 1.56 times larger than that of the corresponding copper conductor. The weight of such an aluminium conductor will be 2.2 times lower than the copper conductor.

Outer semiconducting layer

Inner semiconducting layer

Conductor

General design of a high voltage polymer cable

50 Depending on the customer’s specification, the

At higher voltage levels, the electric field in the ca-

conductor can be equipped with a longitudinal wa-

ble is usually higher. Thus, cables with higher volt-

ter barrier made of hygroscopic tapes or powder

age levels typically make use of material with a

between the individual strands.

higher degree of cleanliness – a factor which does, however, lead to increased costs. The inner semiconducting layer, the XLPE insulation and the outer semiconducting layer are ex-

Inner and outer semiconducting layer The inner and outer semiconducting layers are applied to achieve a homogeneous field within the insulation. Without the semiconducting layer, an elevated electric field would occur at the conductors, putting an unnecessary high degree of electric stress upon the insulation.

truded in one step. This is done to avoid gas-filled interfaces, voids, dust or any other unwanted particles between the different layers. This process produces a high quality cable and is called “triple extrusion”. The cable insulation is usually extruded in three different ways, with: - Horizontal extrusion lines - Vertical extrusion lines - Catenary extrusion lines.

Electric field without (left) and with (right) inner semiconducting layer Principle of triple extrusion

Polymeric (XLPE) insulation The XLPE insulation of the cable insulates the high voltage at the inner semiconductive layer from earth potential at the outer semiconductive layer. Since the electric breakdown in the insulation is mainly caused by structural irregularities and material pollution, the cleanliness of the raw material is

Maximum el.field strength / kV/mm

essential for the functioning of the insulation.

14 12

Outer conductor screen and metallic sheath The outer conductor screen should: - Conduct the earth fault current in case of a short circuit - Return the capacitive charging current - Provide ground potential

MV material

As high voltage polymeric cables are sensitive to

HV material

moisture, the metallic sheath must provide a radial

EHV material

moisture barrier. In some applications, such as a

10

copper corrugated sheath, the outer conductor and

8

the metallic sheath is the same. As a standard pro-

6

cedure, the screen wires are embedded in hygro-

4

scopic tapes to provide a longitudinal water barrier.

2

The outer conductor can additionally be equipped with optical fibres for temperature monitoring

0

110/132 kV

150 kV

220 kV

400 kV

550 kV

Rated voltage

Maximum electrical field strength in cable insulations and types of materials used for power cables

[Brugg 10].

4. High Voltage XLPE Cables

51

The outer polymeric sheath consists of extruded polyethylene (PE) or polyvinylchloride (PVC) and serves as an anti-corrosion layer and mechanical protection for the metallic sheath. For certain types of installations, such as laying in tunnels or buildings, a flame retardant non-corrosive (FRNC) material is additionally applied to provide fire protection to the cable. On request, the outer sheath can be covered with a semiconducting layer for a factory test and after-laying test at the outer sheath.

4.1.2 Types of high voltage XLPE cables

High voltage XLPE cable with 800 mm2 round copper conductor and lead sheath with additional copper wires

Typical conductor types The inner conductor is usually selected by the necessary current rating. Round conductors and segmented conductors are the most typical conductor shapes. For very high current ratings, conductors with large conductor cross sections (e.g. 2500 mm2) and enamelled wires are used. Segmented conductors are typically used for: - Aluminium conductors with cross section larger than 1200 mm2 - Copper conductors with cross sections larger than 1000 mm2

Metallic sheath

High voltage XLPE cable with 1600 mm2 segmented aluminium conductor and laminated sheath with additional copper wires

Apart from the conductor and insulation, the metallic sheath is the main parameter of a high voltage cable. The selection of the metallic sheath depends on the application of the cable and the economic factors involved. The Brugg Cables’ products below are typical examples of sheath types for XLPE cables and their applications [Brugg10].

High voltage XLPE cable with 2500 mm2 segmented copper conductor with enamelled wires and lead sheath with additional copper wires

52 Aluminium laminated sheath with copper wire screen

Copper laminated sheath with copper wire screen

(Brugg type: XDRCU-ALT)

(Brugg type: XDRCU-CUT)

Features

Typical application

Features

Typical application

- Moisture tight

- In tunnels

- Moisture tight

- Low weight

- In trenches

- Low weight

- In trenches

- Low losses

- In ducts

- Low losses

- In ducts

- Low costs

- In tunnels

- Low costs - Single types of metal

Copper corrugated sheath (Brugg type: XDCUW-T)

Features

Typical application

- Impervious to moisture

- All installations in soil and tunnels

- Highly flexible

- Installations with shallow ground water

- Resistant to deformation, pressure, vibration & corrosion - Welded - Can be installed vertically

- Installations in vertical shafts - Installations that may suffer from vibrations (e.g. at bridges)

Lead sheath without copper wire screen (Brugg type XDPB-T)

Aluminium corrugated sheath (Brugg type: XDRAL-T)

Features

- All installations in soil and tunnels

- Highly flexible

- Installations with shallow ground water

- Res. to deformation, pressure & corrosion - Welded - Can be inst. vertically - Lower costs (than copper type)

Typical application

- Impervious to moisture

- All installations in soil and tunnels

- Seamlessly extruded

- Installations with shallow ground water

- Installations in vertical shafts - Installations that may suffer from vibrations (e.g. at bridges)

Lead sheath with copper wire screen (Brugg type XDRCU-PBT)

Features Features

Typical application

- Impervious to moisture

- Impervious to moisture - Seamlessly extruded - Higher short circuit capacity (than without additional copper wires)

Typical application - All installations in soil - Installations with shallow ground water - Installations at higher short-circuit currents to be expected

4. High Voltage XLPE Cables

53

Metallic sheath with integrated optical

Cable data sheet

sensor

Cable type: XDAL-T 1x750 kcmil 120/76 kV

In general, additional optical fibre sensors can be integrated in each sheath type. Such fibres are

Copper conductor, ROUND, stranded

used for temperature monitoring of the cable. More

Diameter: 23.4 mm

details are given in Chapter 8.

Conductor screen, extruded cond. layer Thickness: 1.2 mm

Diameter: 25.7 mm

XLPE insulation, extruded Thickness: 16.0 mm

Diameter: 57.7 mm

Insulation screen, extruded cond. layer Thickness: 1.0 mm

Diameter: 59.7 mm

Conductive swelling tape Thickness: 0.9 mm

Diameter: 61.5 mm

Laminated sheath: Aluminium foil, Copolymer coated Thickness: 0.25 mm

Diameter: 62.0 mm

Polyethylene sheath, black, extruded Thickness: 3.7 mm Diameter: 69.4 mm Semiconducting outer layer

Al laminated sheath and copper wire screen with fibre Symbolic illustration

optic sensors (Brugg type: XDRCU-ALT)

Technical data: Cable weight: Copper weight: Aluminium weight: Screen short circuit:

Summary of metallic sheath types

6.72 kg/m 3.49 kg/m 0.11 kg/m 5kA/1s

Capacitance per length: Minimum bending radius: Maximum pulling force:

Section Drawing High Voltage XLPE Cable

Issued by: amr Date:

The cable data sheet gives a description of the

0.165 uF/km 1.4 m 24.0 kN

07.12.11

Reference: 712-KM-279

characteristics of the different cable types. Custom-

Example of a cable data sheet

ers, be it end users, consulting companies or accessory manufacturers, can design their devices, such as terminations, cable clamps or others, accordingly.

Overview of advantages and disadvantages of different metallic sheath types Sheath type

Moisture protection

Mechanical protection

Corrosion protection

Diameter

Lead sheath

+++

++

+++

+

--

--

Lead sheath & copper wires

+++

++

+++

+

--

--

Cu corrugated sheath

++

+++

++

o

o

-

Al corrugated sheath

++

++

++

o

+

o

Cu wires & laminated sheath

+

+

++

++

o

o

Al wires & laminated sheath

+

+

++

++

+

+

4.2 Cable layout and system design 4.2.1 General The dimensioning of a high voltage cable system is always based on the specifications and demands of the particular project. The following details represent the minimum amount of information required [Brugg 10]:

Weight

Costs

-

Type of cable insulation

-

Nominal and maximum operating voltage

-

Short-circuit capacity (short-circuit current with statement of the effect time)

-

Transmission capacity or nominal current

-

Cable laying configuration

-

Soil properties

54 For a detailed design of the cable systems, the cable manufacturer must make all necessary calculations. These calculations take into account any pos-

Example 4-2:

Æ A cable with the following values:

sible limitations due to manufacturing, transportation, laying and operation of the cable system.

Radius of inner semicond. layer r = 24 mm

-

Outer radius of insulation R = 45 mm

 The operating capacity of the cable P According to Equation 4-1 and by assuming εr for the XLPE material of 2.3, the operating capacity can be calculated as:

4.2.2 Electric field, capacity and charging current Electric field In general, the electric field of a cable can be re-

𝐶=

garded as a homogeneous cylindrical arrangement. As previously shown in Chapter 1, (Equation 1-24), the electric field E(x) at position x in the cable insulations is given by:

𝐸 𝑥 =

-

0.0556 ∙ 2.3 = 0.20 𝜇𝐹/𝑘𝑚 45 𝑙𝑛 24

9 This is a typical value for the cable capacity of high voltage cables of between 0.1 … 0.3 µF/km).

When voltage is applied to the cable, the cable ca-

𝑈

pacity must be charged. For an alternating voltage,

𝑅 𝑥 ∙ 𝑙𝑛 𝑟

the charging current Ic is therefore:

The above equation shows that the highest electric

𝐼! = 𝑈! ∙ 𝜔 ∙ 𝐶 ∙ 10!!

field occurs above the inner semiconducting layer

𝐴 𝑘𝑚

(Eq. 4-2)

and the lowest below the outer semiconducting In which:

layer.

Operating capacity and charging current As mentioned in Chapter 1, every arrangement of a conductor-insulator-conductor

is

able

to

U0 :

Phase-to-ground voltage in kV

ω:

Angular frequency = 2⋅π⋅f in 1/s

C:

Operating capacity in µF/km

store

charges and hence have a certain capacity. A typical arrangement is a cable with its construction of conductor and inner semiconducting layer – insulation – outer semiconducting layer and screen. Consequently, the operating capacity of a cable de-

The charging current flows while the cable is set under voltage. It should be mentioned that in an ac system the charging current flows in accordance with the frequency of the applied voltage.

pends on the type of insulation and its geometry.

As the current flows with a phase angle of approx-

The following equation applies for all radial field

imately 90°, the charging power is almost purely

cables:

reactive. The charging power SC can therefore be calculated as:

𝐶=

2𝜋𝜀! 𝜀! 0.0556 ∙ 𝜀! 𝜇𝐹 = 𝑅 𝑅 𝑘𝑚 ln ln 𝑟 𝑟

(Eq. 4-1)

In which:

ε0 :

absolute permittivity

εr : R: r:

relative permittivity (for XLPE = 2.3) Radius of outer semiconductive layer Radius of inner semiconductive layer

𝑆! = 3 ∙ 𝑈! ∙ 𝐼!

(Eq. 4-3)

4. High Voltage XLPE Cables

55

Example 4-3:

Æ A cable with: -

Rated voltage U0 = 76 kV

-

Net frequency of f = 50 Hz

-

Average charging capacity of C = 0.25 µF/km

a

 The charging current and charging power for the cable

Trefoil (or triangle) laying of a cable system

P According to Equation 4-2 and calculating the angular frequency to ω = 314 1/s, the charging current can be calculated as: -

Ic = 76kV ⋅ 314 1/s ⋅ 0.25µF/km ⋅ 10-3 = 6 A/km

P According to Equation 4-3 and by taking the charging current, the charging power can be calculated as: -

The mean inductance of a flat-laying arrangement can be calculated as:

Sc = 1.73 ⋅ 76 kV ⋅ 6 A/km = 790 kVA/km

9 For a cable length of 5 km, this would mean considerable values of 30 A and 4 MVA for the charging current and the charging power, respectively.

𝐿#"$! = 2 ∙ 10!! ∙ 𝑙𝑛

1.26 ∙ 𝑎 𝑟!

𝐻 𝑘𝑚

(Eq. 4-4)*

* The above formula provides average values as the inductance for the inner and outer phases are not equal.

In which:

4.2.3 Inductive values of the cable Inductance In Chapter 1, it was shown that each current flow

a:

Distance of phase axis in mm

rC :

Radius of conductor in mm

The mean inductance of a trefoil arrangement can be calculated as:

causes a magnetic field. This means that with current flow, the cables have a certain inductance and reactance. In general, the operating inductance depends on the relation between the conductor axis spacing and the external conductor diameter. For practical arrangements, the two cases of flat and trefoil laying of the cables have to be considered [Brugg 10].

𝐿'!&%"$# = 2 ∙ 10!! ∙ 𝑙𝑛

𝑎 𝑟!

𝐻 𝑘𝑚

(Eq. 4-5)**

** Due to the symmetrical arrangements, the inductance is equal for all three phases.

Reactance In both cases, flat-laying and triangle arrangements, the reactance (sometimes referred to as “inductive reactance”) of the whole cable system can be calculated as:

a

𝑋 =𝜔∙𝐿

Ω 𝑘𝑚

(Eq. 4-6)

Flat laying of a cable system

In which: ω:

angular frequency = 2⋅π⋅f in 1/s

L:

Inductance of either the triangle or flat laid cable system in H/km

56

4.2.4 Cable losses

 The voltage depending losses of the cable

General

P According to Equation 4-6 and calculating the angular frequency to ω = 314 1/s, the voltage depending losses can be calculated as:

In a cable system, two basic types of losses are

- Pd1 = (76kV)2 ⋅ 314 1/s ⋅ 0.25µF/km ⋅ 2.5 ⋅ 10-4 = 113 W/km

present: losses that are caused by the high voltage, the so-called voltage dependent losses, and losses

- Pd2 = (220kV)2 ⋅ 314 1/s ⋅ 0.25µF/km ⋅ 2.5 ⋅ 10-4 = 950 W/km

that are caused by the current, the so-called current dependent losses.

9 For a cable length of 5 km, this would mean a considerable value of 565 W / 4750 W for the voltage dependent losses.

Voltage dependent losses Voltage dependent losses occur at any moment in which the cable is connected to an AC voltage

Current dependent losses

source. They reveal a characteristic of the cable’s

Despite ohmic conductor losses, current dependent

property, namely that the cable can be considered

losses only appear when an ac current flows

as a “capacity” or capacitor. An ideal capacitor

through the cable. They consist of the following

consumes purely capacitive power – or capacitive

components:

current. A real capacitor, such as the cable, not only

- Ohmic conductor losses

has capacitive power but also has reactive power –

- Losses due to the skin effect

or reactive current. The angle between the capacitive current and reactive current is considered as

- Losses due to the proximity effect

tan δ (“tangens delta”). The reactive power thereby

- Losses in the metallic sheath

represents dielectric losses in the cable insulation. These losses can be considered as dielectric losses and are therefore described as “dielectric loss power” Pd. The voltage dependent losses Pd can be calculated as:

𝑃! = 𝑈!! ∙ 𝜔 ∙ 𝐶 ∙ tan 𝛿

Ohmic conductor losses Ohmic conductor losses represent the losses caused by the electron flow through the conductor. As introduced in Chapter 1, they are determined by the conductor material and the temperature. The

(Eq. 4-7)

ohmic conductor losses can be calculated by applying Ohm’s law and the power law as:

In which:

𝑃 = 𝐼! ∙ 𝑅

U0 :

Phase-to-ground voltage of the cable

ω:

Angular frequency

C:

Operating capacity

In which:

tan δ:

Dielectric power loss factor (typical

I:

Current through the cable core

values for XLPE between 1.5 … 3.5 ⋅ 10-4

R:

Resistance at a certain temperature

(Eq. 4-8)

and for EPR between 10 … 30 ⋅ 10-4) The standard conductor cross-sections and admisExample 4-4:

Æ Two different cables with: -

Rated voltage U0 = 76 kV

-

Rated voltage U0 = 220 kV

-

Net frequency of f = 50 Hz

-

Average charging capacity of C = 0.25 µF/km

-

Average tan δ for XLPE of 2.5 ⋅ 10-4

sible DC resistances at 20°C are defined in the IEC 60228.

4. High Voltage XLPE Cables

57

Conductor cross-section and admissible DC re-

-

sistances at 20°C according to IEC 60228

R90Cu = 17.6 mΩ/km ⋅ (1 + 3.9 ⋅ 10 1/K ⋅ 70 K) -3

= 22.4 mΩ/km

Conductor crosssection [mm2]

Copper resistance

Aluminium resistance

Ω /km] [mΩ

[mΩ Ω /km]

240

75.4

125

300

60.1

100

400

47.0

77.8

500

36.6

60.5

630

28.3

46.9

800

22.1

36.7

1000

17.6

29.1

1200

15.1

24.7

1600

11.3

18.6

2000

9.0

14.9

2500

7.2

12.7

Due to the power losses, the conductor temperature usually rises above 20 °C, at which typically a maximum permanent temperature of 90 °C can be allowed. The elevated temperature causes the dc resistance to rise accordingly. The effect was described in Chapter 1. According to equation 1-16

P According to Equation 4-8 and taking the values of the resistance at 90°C, the losses at 90°C can be calculated as: -

PlossAl = (560 A)2 ⋅ 37.2 mΩ/km = 11.7 kW/km

-

PlossCu = (560 A)2 ⋅ 22.4 mΩ/km = 7.0 kW/km

9 For a cable length of 5 km, this would mean considerable values of 60 kW / 35 kW for the conductor losses in the aluminium and copper conductor, respectively. This means that at the same conductor cross-section and the same current flow, at 90 °C, the losses in the aluminium conductor are almost twice as much as those of copper.

Losses due to the skin effect Losses due to the skin effect are caused by the displacement of the current towards the conductor surface. In Chapter 1, it was shown that each current has the ability to induce a voltage in a conductor. This voltage induction also takes place in the “own” conductor (the conductor in which the current itself is flowing). With the resistance of the conductor, an additional current flows. The direction of this current causes a higher current to flow

the resistance at 90°C can be calculated:

at the outer diameter area of the conductor and a

𝑅!" = 𝑅!" 1 + 𝛼 90°𝐶 − 20°𝐶

uneven current distribution leads to higher losses.

lower current to flow at the conductor centre. The (Eq. 4-9)

The losses rise disproportionally with current as shown in Equation 4-7.

In which: R90

dc resistance at 90°C

R20

dc resistance at 20°C

α

Temperature coefficient of the resistance of

Conductor

the material;

Example 4-5:

Æ Two different cables with: -

Aluminium conductor of 1000 mm2

-

Copper conductor of 1000 mm2

-

Current of 560 A

Current density

(αAl = 4.0 ⋅ 10-3 1/K, αCu = 3.9 ⋅ 10-3 1/K)

DC

0

AC

Radius

r

 The conductor losses of both cable types at 90°C in 1 km

Principle of the skin effect in cable conductors

P According to Equation 4-9 and taking the dc resistance values of IEC 60228, the resistance at 90°C can be calculated as:

quadratic with the power frequency and the con-

-

R90Al = 29.1 mΩ/km ⋅ (1 + 4.0 ⋅ 10-3 1/K ⋅ 70 K) = 37.2 mΩ/km

Losses due to the skin effect are approximately ductor diameter. They can be reduced by a suitable design of the conductor cross-section, such as segmented conductors and/or enamelled wires.

58 60.0

Rel. resistance increase Ra.c./Rd.c. /%

60 50

46.6

40

On high current spikes, especially in the early tran-

36.9

sient phase of a short-circuit, the cables are sub-

30

jected to high dynamic forces. It is therefore of

20

great importance that the cables are properly fixed,

15.0

particularly when they are to be installed in a close

10 0

4.2.5 Dynamic forces

Segments, insulated Segments, blanc

Hollow d=22

Blank

Conductor type

Related electric resistances for different conductors due to the skin effect

trefoil arrangement. The calculation of dynamic forces for cable systems is important for determining the fixing intervals and layout of the fixing devices. The relevant dynamic forces, their calculation and the right selection of cable clamps are de-

It is worth mentioning that the skin effect is zero at

scribed in Chapter 7.

dc currents as there is no induction.

Losses due to the proximity effect Losses due to the proximity effect are caused by the electromagnetic field of the neighbouring conductors. The closer the neighbouring conductors, the higher the losses – hence the term “proximity effect”. In practice, these losses are less important for high voltage cables with single conductors as these provide a sufficient axis space. In comparison to con-

4.3 Laying of high voltage cables 4.3.1 Laying arrangements Overview Laying of high voltage cables very much depends on the laying specifications of the cable system.

ductor losses, losses caused by the proximity effect

Typical layings are:

in high voltage single-conductor cables are 10% or

- Directly buried

lower [Brugg 10].

- In directly buried cable ducts - In pre-fabricated concrete elements

Sheath losses Sheath losses are generated by the magnetic induction of the conductor currents in the metallic screen and sheath of the cable. They are caused by: - Circulating currents in the system - Eddy currents in the cable sheath

- In (concrete) tunnels

Directly buried cables The advantages of directly buried cables are the fact that they incur the lowest installation costs, that no additional construction measures are need-

- Resulting sheath currents caused by induced

ed and that the cable can be laid with cable laying

sheath voltage (in unbalanced earthing sys-

machines in a suitable terrain. The disadvantages

tems)

include their lack of protection from digging work (which can easily destroy the cable), the difficulty

Particularly during high circulating currents, sheath losses may substantially reduce the current load capacity of the cable. Sheath losses can be lowered significantly by means of special earthing methods. More information on such methods are described in Chapter 7.

of accessing the cable after laying, and the fact that, depending on the temperature resistance of the cable-surrounding material, heat dissipation can be poor, thus limiting the maximum current carrying capability of the cable.

4. High Voltage XLPE Cables Cable

laying

machines

are

59 particularly

cost-

effective for the laying process. The procedure depends on the properties of the soil and the mechanical properties of the cable.

Principle of directly buried cables

Direct cable burying with a machine

Laying in directly buried cable ducts The advantages of laying cables in directly buried ducts are the higher degree of protection and the moderate costs. Another advantage is that building of the cable duct and pulling of the cables into the duct are separate tasks and can be organised at different times. A disadvantage is the heat dissipation difficulty from the cable through the surrounding air in the cable ducts to the surrounding soil. The extent of this problem depends on the construction Directly buried cables

of the cable ducts and the dimensions of the cable. A further disadvantage is the reduced accessibility to the cable after the laying process.

Laying principle of direct cable burying with a machine

Principle of cables in cable ducts

60 Laying in cable tunnels The advantages of cable tunnels can be seen in the fact that they offer the highest degree of cable protection, a high degree of cable cooling levels and easy access to the cable itself. The disadvantage is the higher costs generated by the tunnel work.

Cables at the end of a cable duct

Cables in prefabricated concrete elements The advantages of cables in prefabricated concrete elements are the considerable degree of cable protection and the moderate costs of the laying method. Further advantages are the improved heat dissipation due to heat convection in the concrete el-

Principle of a cable arrangement in a tunnel

ements and the improved accessibility to the cable after the laying process. A disadvantage of such laying is the higher costs that arise due to the prefabricated concrete elements.

Cable arrangement in a round tunnel Principle of cables in prefabricated concrete elements

Example of cables in prefabricated concrete elements

Cable arrangement in a rectangular tunnel

4. High Voltage XLPE Cables

61

4.3.2 Current carrying capacity and

The heat generation in the cable and the heat trans-

temperature calculation

fer to the surrounding area depend on the design and laying conditions of the cable itself. Because of

To enable the cable system to achieve high power

this, the optimal design of the cable is best deter-

transmission capability and to avoid damaging of

mined by the use of modern simulation tools. This

the cable, it is of great importance to know the

calculation is usually done by the cable manufac-

maximum current that can be transmitted via the

turer in close cooperation with the owner or con-

cable system. Therefore, the impedance limits of

tractor of the cable system in which possible or

the cable must be taken into account and the cur-

planned laying conditions are carefully considered.

rent that is responsible for the major part of the

The determination of the maximum current and

heat losses has to be calculated accordingly. For

heat dissipation is a result of such calculations. In

continuous load conditions, the maximum temper-

the case of more complex or demanding situations,

ature at the conductor of an XLPE cable is given as

the calculations are done with the help of finite el-

90°C. In order to achieve a certain current flow –

ement methods (FEM).

and therefore transmitted power –, the heat generated in the cable must be transferred to the surrounding area.

Simulation of temperatures and heat distribution in a software tool for different cable arrangements

62

4.3.3 Reduction of magnetic field In accordance with the inductive values calculated in Section 4.2.3 above, there are several ways of reducing the electromagnetic field, such as: - Reducing the distance of the phases - Laying in trefoil arrangement - Increasing the laying depth of the cable

In addition, special shielding means can also be applied to reduce the electromagnetic field strength around high voltage cables. Such systems are

Electromagnetic field of a cable arrangement in trefoil laying with special protection means (Picture: CFW)

available on the market and can be applied accordingly.

4.4 Cable selection process A broad range of products together with a systematic analysis of the technical requirements needed enables the user to find the right solution, no matter what the application. Additional consultations with engineers of cable producing companies are also helpful when it comes to selecting the most appropriate solution for the type and dimension of high voltage cables [Brugg 10]. Electromagnetic field of a cable arrangement in normal trefoil laying (Picture: CFW)

Laying conditions, mechanical cable protection

Type of insulation, load, voltage level, short-circuit current

Cable type & design

Economic aspects, such as price or losses

Conductor material (Al/Cu)

Route length and layout

Earthing method of sheath

Production, transportation, installation & section length

Economic aspects, safety margin, future load expectations

Conductor crosssection

Short-circuit and thermal rating

Indoor or outdoor, cable length & laying conditions for joints

Selection of cable accessories

Leakage path requirements

Losses, economic aspects

Determination of laying conditions

Local boundaries, safety regulations

Cable selection process according to [Brugg 10]

5. High Voltage Accessories for Polymer Cables

Chapter 5

High Voltage Accessories for Polymer Cables

63

64

65

5. High Voltage Accessories for Polymer Cables

5.1 Introduction A brief history At the beginning of the last century, the technology of high voltage cable systems was based on mass impregnated insulations and oil-paper technology [Peschke 99]. These cable systems were so reliable that they remained on the market until the late 1960s. With the upcoming technology of polymer

Basic design of terminations: - Stress cone slip-on element for field grading - Insulator for mechanical and moisture protection - Insulating compound for high voltage insulation - Connection studs for the cable connection to the external device

cable systems, the trend toward inexpensive and simple polymer designs emerged. This was hardly surprising considering the main advantages of polymer cable technology, being almost free from maintenance, easy and fast to install and environ-

Basic design of joints: - Conductor connection to allow current flow through the conductor

mentally friendly. From these reasons, polymer

- Insulation body element for field grading

technologies have successfully established them-

- Cover for mechanical protection

selves for high voltage cable systems. Consequently, this book focuses on the technology of high voltage accessories for polymer cables.

- Cover for moisture protection - Earthing connections

Terminations and joints for high voltage polymer cables Joints and terminations for high voltage cables are called “high voltage accessories”. Terminations (or sometimes referred to as sealing ends) are devices that realise the connection from the cable to an outside device. The connection outside the cable can be an overhead line (outdoor terminations), a transformer (transformer terminations) or a gas insulated switchgear station (GIS) (GIS terminations). Cable joints (often simply called joint or sometimes referred to as splice) are devices that connect two cable segments to each other. In addition to their voltage grading and current carrying capability, high voltage accessories must have the capability of withstanding all environmental influences, such as ultraviolet (UV) radiation, moisture, dirt, dust, salt fog or mechanical load in order to enable the whole cable system to achieve its long lifetime potential. Although the types of high voltage accessories differ, their basic design remains the same and can be described as follows:

Principle layout of a high voltage cable system with terminations and joints

Additional accessories for high voltage cable systems Beside terminations and joints, there are several other devices required for a reliable functioning of a high voltage cable system. These devices are called “additional accessories” and are comprised of certain elements, such as cable clamps, sheath voltage limiters or earthing and cross-bonding boxes. Since these devices are extremely important for a reliable functioning of a high voltage cable system, they are described separately in Chapter 6.

66

5.2 Technologies for slip-on elements 5.2.1 Control of the electric field

In order not to cause a breakdown in the material, the electric field at the end of the semiconducting layer must be controlled down to much lower values. Different types of technology are employed to achieve this kind of field control.

Dielectric breakdown without field control The inner conductor of the cable is at high poten-

Non-linear field control

tial. The outer semiconducting layer carries ground

Non-linear field control is achieved by a non-linear

potential. The polymer insulation of the cable degrades the resulting electric field in between the two layers. To connect a cable to a termination or joint, the outer semiconducting layer must be removed. Without any field grading mechanisms, a very high electric field occurs at the edge of the semiconducting layer. Simulations show that the electric field strength of a standard cable without field control can be in the range of 40 kV/mm and above. The extremely high electric field at the end of the semiconducting layer exceeds the dielectric strength of the material at the point, thus leading to electric discharges and finally to a breakdown.

material, which is applied from the grounded semiconducting outer layer of the cable over the insulation of the cable. These materials consist of a polymer matrix as e.g. polyethylene or silicone with fillers. Typical fillers are: carbon black or doped ZnO particles – the so called microvaristors. These composite materials possess non-linear electrical properties, which means that the electrical properties, as e.g. the volume resistivity, change with changing applied electrical field strength. These filled polymers distribute the electric field over a certain length of the cable termination or joint. This technology is typically used in the medium-voltage sector in conjunction with shrink-on accessories.

Schematic drawing of equipotential lines of the electrical field at the end of a cable without any electrical field control

Schematic drawing of equipotential lines of the electrical field at the end of a cable with a field grading material (green layer) with non-linear electrical properties

Capacitive field control Capacitive field control is mainly used for accessories of oil-filled cables or bushings. For capacitive field control, a considerable number of individual layers are wrapped around the area between the high voltage (conductor) electrode and the grounded semiconducting layer. The individual layers act as electrodes, similar to capacitive voltage dividers, which finally cause a uniform depletion of the electric field along the control element. This technology FEM simulation of high concentration of electric field at the edge of the semiconducting layer at a cable without field control

puts considerable strain on the material and great attention to accuracy during installation must be given.

67

5. High Voltage Accessories for Polymer Cables

Schematic drawing of equipotential lines of the electrical field at the end of a cable with cylindrical metal foils as capacitive layers for electrical field control

Geometric-capacitive field control The most frequently used field control method for polymer cable accessories is based on geometrical

Uniform degradation of the electric field at a cable with geometric-capacitive field control in a termination

shaping of field guiding electrodes, so-called “deflectors”.

A substantial part of the achievable cable termina-

The deflectors are made of semiconducting materi-

tion or joint system strength lies in the skilful selec-

al and are embedded in insulating material. The combination of deflectors and insulating material is called “stress cones”. Stress cones are connected to the semiconducting layer of the high voltage cable. The shape of the deflectors is designed in such a way that the field strength on the outer semicon-

tion of uniform stressing without allowing the tangential components of the field strength at the interface surfaces of the cable to become too high. Between the cable and the insulator the termination is filled with other insulating media such as SF6 gas or silicone oil.

ducting layer can be passed on seamlessly and is

The slip-on elements used for terminations are

continuously reduced. The insulating silicone mate-

called “stress cones”. The slip-on elements used

rial in between has a dielectric strength similar to

for joints are called “slip-on insulation bodies” or

that of the cable and with its snug fit, it provides a

just “insulation bodies”.

cavity-free interface surface for the cable insulation.

The insulation bodies consist of two semiconducting deflectors (one for each side at each side of the insulation body), a semiconducting middle electrode, the insulation material and a conducting layer on the surface of the insulation body. The semiconducting deflectors are used to control the electric field from the ground potential at the outer semiconducting layer towards the high potential of the middle electrode. The middle electrode is used

Schematic drawing of equipotential lines of the electrical field at a cable end with geometric-capacitive field control

Since the electric field is controlled by the geometrical shape of the semiconducting deflectors and the electric field lines cause a capacitive current flow through the insulation material, this method is called “geometric capacitive field control”.

to control the electric field from the conductor clamp – which is at high potential – down to the semiconducting deflectors and the outer conducting layer on the surface of the insulation body, both of which are at ground potential.

68

5.2.2 Semiconducting parts Semiconducting parts must be designed carefully. A somewhat tricky factor in the designing of such contours is their reaction to impulse voltage load. Rapid voltage changes in the high-frequency components of semiconducting parts cause the electric field to penetrate into the semiconducting deflector, especially into those of outdoor terminations. For a few microseconds, this deflector must have the capability of accepting a power of up to 100 kW without injury to the conductive layers. Consequently, solid silicone material, rather than paint-coated material, is strongly recommended. This allows constant optimum performance of the field grading, even at very fast impulse voltage loads. Solid deflectors have a much larger crosssection and therefore lower resistance than that of painted stress cones. This facilitates the process of degrading the power of fast BIL voltages, such as very fast lightning strokes.

Cross-section (top) and field distribution (bottom) in a stress cone for terminations

Cross-section of a middle electrode (left) and a deflector (right) consisting of solid semiconducting silicone Cross-section (top) and field distribution (bottom) in

rubber

an insulation body for joints

Although the principle of geometric-capacitive field control is very similar, stress cones and joint bodies come in various designs and materials. The main ones are: - Cold shrink elements - Three-piece silicone slip-on elements (for joint insulation bodies only) - One-piece EPR/EPDM slip-on elements - One-piece silicone slip-on elements

5.2.3 Comparison of main materials Different main materials Two main insulating materials for slip-on elements (stress cones and joint bodies) are currently in use: - Ethylene Propylene Diene Monomer (EPDM) – also referred to as Ethylene Propylene Rubber (EPR)

69

5. High Voltage Accessories for Polymer Cables - Silicone rubber which can be vulcanised at room temperature (RTV silicones) or at higher temperatures (LSR)

SiR and EPDM The differences between EPDM and SiR are remarkable and listed in the table below.

The following section gives the advantages, disadvantages and application experiences made with these materials [Vogelsang 11]. Properties of EPDM/EPR and silicone rubber

Names

EPDM/EPR

Silicone rubber

Ethylene-Propylene-Diene Rubber (EPDM)

Silicone rubber (SiR)

Ethylene-Propylene-Rubber (EPR) Chemical structure

C-chains as basic

O-Si chains as basic

Binding energy

355 kJ/mol

445 kJ/mol

Range of temperature at which electric and mechanic properties remain stable

- 40 - + 100 °C

- 50 - + 180 °C

Corona and ozone stability

None

High

Elasticity

Limited

High

Flexibility

Poor

Very high

Hydrophobicity

Poor

Good

Mechanical strength

High

Medium

Lifetime factor n

≈ 20

> 40

Breakdown strength

≈ 20 kV/mm

> 23 kV/mm

Costs

Low

Medium

The comparison of SiR and EPDM shows that SiR

ful during the installation of the insulation body

material is particularly advantageous for the appli-

when mechanical tools, such as chain blocks, are

cation of high voltage accessories. This can be seen

being used. On the other hand, increased mechani-

in the prolonged lifetime of SiR material, its lower

cal stiffness results in a poorer fit of the material to

ageing rate, higher electric breakdown strength and

the surface of the polymer cable, thus introducing

the considerably higher temperature range at

the possibility of poor interfaces between the cable

which electrical and mechanical properties remain

and the slip-on element.

stable.

Although in terms of material costs, EPDM offers a

In addition to this, SiR material is extremely elastic,

more economical alternative to that of SiR, if the

facilitating high quality and easy installation of the

lengthy lifetime of the slip-on element is taken into

stress cones and insulation bodies.

consideration (often several decades) as well as the

A disadvantage of the SiR is its low tear resistance.

fact that the termination or joint is only one of sev-

In order not to damage slip on elements, mechani-

eral parts (e.g. the copper tube or the filling com-

cal tools should not be used during the installation process but should rather be performed by hand or with the use of a gas cushion. The increased mechanical stiffness of EPDM, in

pound in a joint or the insulator in a termination is more expensive), the slight increase of price brought about by the use of SiR can be seen as justifiable.

comparison to that of SiR, can be seen as both an

To summarise, SiR provides an excellent material

advantage and disadvantage. The higher mechani-

for the application of slip-on bodies for high volt-

cal strength of this material proves particularly use-

age accessories. This is due to its:

70 - Very

high

breakdown

strength

above

23 kV/mm at 50/60 Hz - Excellent temperature stability of between –50 and +180°C

5.2.4 Cold shrink elements The use of cold shrink element technology for terminations and joints at medium voltage levels has been standard for a number of years.

- Very high lifetime exponent of n larger than 40

This technology has also been recently upgraded to

- Excellent pressure of the slip-on element on

high voltage accessories. Some companies claim to

the polymer cable surface at normal and ele-

have applied this technology up to the level of

vated load conditions due to excellent elasticity

220 kV. However, to date, this technology has rare-

of the SiR

ly been applied in conjunction with high voltage

- Easy installation due to excellent mechanical

cable systems.

properties and elasticity, no high mechanical

The cold shrink method is a slip-on technology in

forces needed

which the elements to be slipped on are already

Because of its numerous advantages, SiR is the most frequently applied material for insulation bodies of high voltage accessories in the range from 72 – 420 kV and the only material used for insulation

pre-expanded

by

the

manufacturer.

The

pre-

expansion can either be performed by mechanical bracings outside the slip-on element or by a tube or plastic spring inside the slip-on element itself.

bodies above 420 kV.

RTV and LSR In the last decade, a new type of SiR has established itself on the market: liquid silicone rubber (LSR) [Vogelsang 11]. The advantage of this new material is the speed of cross-linking reaction in comparison to the RTV silicone material. Another advantage of LSR can be the transparency of the material, which enables inclusions in the material to be detected by optical means. In the case of non-transparent material (and for the time being also for transparent material), such inclusions are usually detected in the final routine test. These tests cost a considerable amount of time and effort.

Different principles for cold-shrink slip-on elements; top: with outside mechanical bracing, middle: with in-

With the aid of visual pre-testing, the time and ef-

side shells as mechanical bracing, bottom: with insert-

fort needed for the tests can be significantly re-

ed plastic spiral for mechanic bracing

duced. When optical detection is allowed by the standards, this could be a significant advantage.

Principle of installing a cold-shrink joint with inside shells and mechanical tools

Transparency of an LSR stress cone

71

5. High Voltage Accessories for Polymer Cables

To summarise, cold shrink elements used at medium voltage levels are undoubtedly advantageous. Used with high voltage accessories, however, the long-term stability of this technology remains to be seen. Principle a cold-shrink element with internal spiral

During installation, the pre-expanded slip-on element is placed over the cable and the preexpansion is released to compress the slip-on ele-

5.2.5 Three-piece silicone rubber slip-on elements

ment onto the cable. Sufficient residual stresses

The distinguishing feature of three-piece silicone

must be present in the material to produce the re-

slip-on elements is the fact that the three main

quired pressure force on the cable surface.

parts are mounted on site. Here, two prefabricated silicone cylinders with integrated field control are pushed from either side onto the conductor connection and fastened in this position. To insulate the voltage-carrying cylinder of the conductor clamp, a third silicone element with conductive field control layers is pushed over.

Example of a cold-shrink element on a cable (in medium voltage joints)

The main advantages of cold shrink technology are the speed at which the slip-on body can be in-

Principle of three-piece slip-on element design

stalled and the fact that no parking position is necessary, making the whole joint smaller in length. In addition, no special tools are required for installation. A major disadvantage is the fact that the slip-on element must guarantee sufficient residual stresses in the material to produce the required pressure force on the cable surface. Any slight gap between the slip-on element and the cable can lead to a breakdown of the joint or termination. Another disadvantage can occur during installation, when the inner spiral must be removed mechanically. Small particles may remain at the interface of the cable to the slip-on element, thus influencing field grading and leading to possible partial discharges and, at worst, an electric breakdown. Another disadvantage is that once the joint body is placed on the cable, it is practically no longer possible to adjust it to the right position. An incorrectly placed slip-on element may lead to a final breakdown of the joint.

The main advantage of the three-piece slip-on element design is that it can be produced costeffectively in the factory and no joint parking position is required. The three pieces need only to be slipped onto each other on site in order to form the full slip-on element of the joint. Another advantage is the reduced number of special tools needed for the installation and that the joint body is of a slightly smaller size than that of a single-piece joint body. Being made out of silicone rubber, the slipon bodies enjoy all the advantages of this material, including a slow ageing rate, a high dielectric strength and extreme temperature stability. The disadvantages of these elements lie in their construction. As the three pieces must be installed on site, additional interfaces within the joint at the area of higher electric field strength are created. Unsuitable preparation conditions – such as during high humidity or in cases where dust or finger-

72 prints are left on the surface of the elements – can

to this, the SiR material has a slow ageing rate, a

result in a higher risk of electrical breakdown of the

high dielectric strength and extreme temperature

joint. Another significant disadvantage is that the

stability, a good basis for a long and reliable life-

whole joint body cannot be pre-tested during pro-

time.

duction in the factory. Any production failures are

A disadvantage of one-piece slip-on elements is the

therefore hard to determine before installation.

necessity of a parking position, which makes the

Similar to cold-shrink element technology, it can be

joint slightly longer. Another disadvantage of SiR

summarised that the three-piece technology is ad-

slip-on elements lies in the slightly higher material

vantageous at medium voltage levels, but used

costs of the silicone. However, compared to the

with high voltage accessories, the long-term stabil-

overall costs of the joint or termination, the slightly

ity of this technology remains to be seen.

higher costs for high-quality slip-on elements are considered to be reasonable.

5.2.6 One-piece silicone rubber slipon elements

General information on assembling of onepiece slip-on elements The assembling of one-piece slip-on elements is

Use of one-piece slip-on element

best illustrated on joints, this process being more

One-piece (sometimes referred to as single-piece)

critical than the assembling of slip-on elements for

silicone rubber (SiR) slip-on elements are most

terminations.

commonly used for high voltage accessories.

More details of general installation processes for

Proven and simple in design, the prefabricated one-

terminations and joints are given in Chapter 7.

piece SiR element accommodates the entire electri-

The assembling of one-piece slip-on elements can

cally stressed area in one piece.

be done in three basic different ways, with: - Cold shrink technology - Mechanical forces - The gas cushioning method

Cold shrink technology The cold shrink method is described in detail in Section 5.2.4. Cross-section of a one-piece slip-on element

Assembling of one-piece slip-on elements by The advantages of one-piece slip-on elements lie in

means of mechanical tools

their design. Being prefabricated, they can be pro-

Another method of assembling is to push the slip-

duced in a clean surrounding in the factory, thus resulting in high quality elements. In addition, the one-piece elements can be pre-tested in the factory. The pre-testing ensures that production failures can already be detected in the factory, thus ensuring that only high quality slip-on elements are delivered on site. A further advantage of one-piece slipon SiR elements is the material itself. As mentioned in Section 5.2.3, SiR elements allow a void-free contact to the cable surface and enable connecting cables with different diameters, tolerances or excentricity, to be attached to each other. In addition

on body to the parking and final position with the aid of mechanical forces. This can be done by hand, particularly for smaller sized bodies, or in the case of larger sized bodies, with the help of additional mechanical tools, such as a pulley.

73

5. High Voltage Accessories for Polymer Cables

Principle for slipping on the insulation body with mechanical tools

Example of introducing gas in the insulation body

The advantage of this method is the reduced

The advantage of this method is that the slip-on

amount of additional equipment needed on site.

body can be mounted without the necessity of high

The disadvantages include the risk of injuring the

mechanical forces. Thus positioned, the joint has

slip-on body whilst in motion due to the application

no

of high mechanical forces, as well as damage to the

stresses inside the silicone element and can be po-

undesired

mechanical

shearing

or

tensile

cable surface during application of the mechanical

sitioned conveniently. This ensures that neither the

tools. In addition to this, slip-on elements of a larg-

joint body nor the cable will be damages during in-

er size require several people or more complex

stallation. Another advantage is, that fewer per-

mechanical equipment to complete the installation,

sonnel are needed for the installation and that the

which again is more time consuming and costly.

slip-on process is very fast, in the region of less than one minute. Consequently, risk is minimised

Assembling of one-piece slip-on elements by means of gas cushion method A more recent method makes use of a gas cushion system. This method was developed to achieve a smooth, easy and stress-free installation of the slipon bodies. With the aid of a pneumatically generated film un-

and time and costs are saved. The disadvantages of this method include the additional time needed for the attachment of the aircushion tools to the slip-on element as well as the fact that more equipment, such as a compressor or nitrogen, is needed on site. However, the time required for these steps is limited, in the region of five to ten minutes, which is very reasonable.

der the slip-on element, the slip-on body can be pushed onto the cable to the parking and back into the end position without the use of high mechanical forces. In order to avoid the introduction of humidity at the interface, dried air (from a compressor) or nitrogen is used, depending on which is most readily available on site.

5.2.7 One-piece EPDM/EPR slip-on elements As mentioned in Section 5.2.3, EPDM/EPR is also used for slip-on elements. The main advantage of EPDM slip-on elements is the lower cost of the material. A major disadvantage is the stiffness of the material that may not always ensure an adequate compression of the slip-on element to the cable surface.

Principle of sliding on an insulation body using com-

Other disadvantages include the lower breakdown

pressed gas

strength, the higher ageing rate, reduced temperature stability and less flexibility compared to SiR.

74

5.2.8 Lapped technology Joints and terminations with lapped technology field grading were used for many decades. Although lapped technology is no longer in use, it is nonetheless worth mentioning, as it is still relevant for a few remaining projects as far as repairs or special solutions are concerned. In lapped technology, tapes of certain conductivity are used. They are wrapped around the area of the conductor clamp to the semiconducting layers of the cable. The tapes are pre-stressed during taping and settle on the cable to a solid polymer material. The challenge of lapping technology is to maintain constant stress during the wrapping of the tapes and to avoid any air or dust inclusions during this process. The main advantage of lapped joints is their ability to adapt cables with different diameters. The disadvantage of lapped joints is their reduced level of reliability due to the more complex installation procedure necessary for the lapping of the tapes. The higher effort and risk during installation, and therefore increased risk in operation, are the reasons why lapped joints are no longer in use for terminations or joints of high voltage cable systems.

Example of the insulation of a lapped joint

5.2.9 Final comparisons and conclusions Each technology can be seen to have its advantages. To conclude it can be said that: - For those whose main concerns relate to costs, cold-shrink, EPR or three-piece elements offer an appropriate solution. - For those looking for quality, reliability and easy installation and therefore for an overall economic solution, one-piece prefabricated slip-on elements of SiR are highly recommendable, preferably by means of the gascushion method of installation.

75

5. High Voltage Accessories for Polymer Cables Overview of advantages and disadvantages of different technologies for slip-on elements 1-piece SiR slip-on bodies

3-piece SiR slipon bodies (joints)

High voltage

Medium voltage

High voltage

Medium voltage

Range of application in diameter over cable insulation

++

+

-

+

Interface behavior

++

-

-

-

Temperature stability

++

++

-

++

Electrical strength and lifetime potential

++

+

+

+

Factory routine test possible

++

-

++

+

Storability

++

++

++

-

Ease of assembly

++

++

+

++

Additional tools required

+

++

+

++

Costs of material

-

-

+

-

++

-

+

--

Originally used

Customer acceptance for HV applications

5.3 Terminations

1-piece EPR/EPDM slip-on bodies

SiR coldshrink bodies

Terminations for polymer cables are comprised of several elements. These are:

5.3.1 Basic design

- A stress cone with deflector

Terminations are items that connect a cable with

- An insulating compound

external devices, such as a transformer (transform-

- An insulator

er terminations), a gas insulated switchgear (GIS terminations) or an overhead line (outdoor terminations).

Terminal stud Corona shield

- A corona shield - A terminal stud.

As explained in Section 5.2, stress cones are premoulded sleeves, which ensure field grading between the outer semiconducting layer and the insulation.

Insulator: – porcelain – polymer – epoxy type

The insulating compound in terminations serves to further degrade the electric stress. With the exception of “dry-type” terminations, silicone oil or SF6 are the type of insulation compounds used, particu-

Insulating compound

larly silicone oil, which is by far the more popular of the two. SiR slip-on stress cone

Which type of insulating compound is employed very much depends on the type of application. Three different types of insulators are available: porcelain or composite insulators for outdoor terminations and epoxy insulators for transformer and

Polymer cable

Principle design of a termination

GIS terminations.

76 The corona shield and terminal stud connect the

Outdoor termination with composite

conductor end of the cable to the external device,

insulators

such as the transformer, the GIS or a high voltage

Composite insulators have been in growing de-

overhead line.

mand over the past decades. The term “composite” refers to the two different materials out of

5.3.2 Outdoor terminations

which they are made. The inside of the composite

Outdoor terminations with porcelain

ables oil and gas tightness and the mechanical

insulators

strength of the termination. The outside is made of

Porcelain insulators have been on the market long-

silicone shields to realise the creepage distance.

insulator consists of a fibre reinforced tube that en-

er than all other insulator types. Their main ad-

The light but strong design of the fibre reinforced

vantages are:

tube, as well as the hydrophobicity of the silicone

- Excellent resistance against ultraviolet (UV) radiation - Resistant against bird-picking - Very high cantilever forces - Excellent track record in numerous countries and climate zones - In service for almost more than 80 years in numerous companies

shields has led to a breakthrough in the technology of composite insulators for high voltage applications in the last few decades. The main advantages of composite insulators are: - Low weight and easy to handle - Good resistance to UV radiation - Excellent hydrophobic behaviour - Shorter creepage distance possible due to the hydrophobicity of the silicone material - Less critical in the event of an explosion or an earthquake - Shorter lead time - Excellent track record in numerous countries and climate zones - In service for more than 30 years in numerous companies

Cross-section of a termination with porcelain insulator

Cross-section of a termination with composite insulator

77

5. High Voltage Accessories for Polymer Cables Insulation filling

with the maximum phase to ground voltage for the

Outdoor terminations are typically filled with sili-

termination.

cone oil. If required, the insulators can also be filled

Overview of different pollution classes according to [IEC 60815-1]

with SF6 gas. It is worth mentioning that when the insulator is filled with SF6, it requires additional

Site pollution severity (SPS) class

Reference unified specific creepage distance (RUSCD)

a

Very light

22.0 mm/kV

Creepage distance

b

Light

27.8 mm/kV

In addition to the issue of voltage level, conductor

c

Medium

34.7 mm/kV

cross-section and the choice of porcelain or com-

d

Heavy

43.3 mm/kV

e

Very heavy

53.7 mm/kV

measures for gas-tightness at the termination and a more costly gas pressure monitoring system.

posite insulator, creepage distance is also one of the most relevant parameters for an outdoor termination. The creepage distance is the distance along the outer side of the termination insulator from

Example 5-1:

high voltage potential to earth potential. In con-

Æ A cable termination for a 138 kV cable system shall be installed in a very heavy polluted environment. The termination supplier offers following creepage distance values for the termination: 3890 mm, 4790 mm and 5580 mm.

trast, the flashover distance is the shortest distance from high voltage to earth potential.

 Which value of creepage distance has to be selected for the termination? P According to the definitions of IEC 60815-1, the RUSCD for a pollution severity class “very heavy” is 53.7 mm/kV. The maximum phase to ground voltage for a 138 kV grid is 145 kV / √3. The minimum creepage distance can be calculated as follows: Minimum creepage distance: 53.7 mm/kV ⋅ 145 kV / √3 = 4483 mm

9 For the application the termination with a creepage distance of 4790 mm should be chosen.

Requirements of different creepage distances can be fulfilled with the same basic geometry, that is, by the simple use of additional intermediate sheds or a longer insulator. Not only does the concept of Creepage distance (red) and flashover distance at a

alternating sheds enable high specific creepage dis-

termination

tance to be achieved, it also provides good wet per-

In order to determine the appropriate creepage dis-

means of self-cleaning. In case of higher creepage

formance as well as an efficient and effective tance for a termination, the pollution severity at the installation site of the termination needs to be known. The standard IEC 60815-1 defines five dif-

distances with larger insulators, more silicone oil for the termination is required. This makes the whole arrangement more expensive and heavier.

ferent pollution classes according to their degree of severity. These are very light, light, medium, heavy and very heavy [IEC 60815-1].

Conductor connection All terminations require a mechanical conductor

In order to determine the appropiate creepage dis-

connection. The main function of such conductor

tance of a termination, the reference unified specif-

connections is to achieve the necessary current car-

ic creepage distance (RUSCD) must be multiplied

rying

capability

and

a

sufficient

mechanical

78 strength. In principle, three main methods are used

tion breakdowns may also result in the destruction

to create the conductor connection, as they can be

of equipment surrounding the termination, leading

welded, pressed or screwed.

to additional costs or outages of the system.

The high mechanical strength of pressed conductor

An explosion of a termination during worst case

connections, together with the fact that they pro-

failures cannot be totally avoided. These are

mote high current carrying capability, makes them

caused by maximum short-circuit currents which

extremely popular especially for outdoor termina-

are too high. However, the impact of such failures

tions.

can be reduced.

Screwed conductor connections also have a suffi-

Explosion resistant terminations are designed to

cient mechanical strength. Their additional ad-

prevent any major part of the termination from fly-

vantage is the ease with which they can be in-

ing out into the surrounding area (few metres)

stalled on site.

when an internal arc at the field grading stress

Although theoretically, welded conductor connec-

cone occurs. The design is constructed so that

tions represent a third option, in reality they are less frequently used, as they run the risk of damaging the cable during the heat treatment that occurs during the welding process.

overpressure occurring during the internal arc exhausts at the overpressure devices at the top and bottom of the termination. In addition, the design and material of the base plate and the insulator termination top is made to withstand any greater forces occurring during a short-circuit current. This ensures maximum protection of the surrounding area.

Example of a pressed conductor connection at a termination

5.3.3 Explosion resistant terminations Although high voltage cable systems are extremely reliable and safe, the risk of breakdowns can never be totally excluded. Failures can be harmful to the surrounding area. At worst, explosions or breakdowns in cable systems, such as those occurring at a termination, can result in harm to life. Termina-

Principle of explosion resistant terminations

79

5. High Voltage Accessories for Polymer Cables

strength in the transformer-side oil chamber and the fact that the transformer termination sometimes requires a corona shield.

Explosion resistant termination with overpressure devices (violet) and enforced mechanical parts (turquoise)

In many cases, the explosion resistant termination is not a standard model. To achieve a greater degree of safety, parts of the termination, such as the base plate, are thicker and of increased strength. Consequently, the termination is heavier. This weight, together with the higher forces occurring during the short-circuit current, means that the mounting of the termination and the steel structure

Classic GIS termination (left) and transformer termination with corona shield (right)

must be able to withstand considerable loads. Because of this, the steel structure and basement of

The connection technology from the GIS to the

the termination need to be reinforced.

termination

is

standardised

and

is

given

in

IEC 62271-209 and EN 50299. The insulator of both, the transformer and SF6 termination, is made of epoxy resin. Similar to outdoor terminations, the inside of the epoxy insulators is normally filled with oil. In exceptional cases, SF6 filling can be used. During installation, the termination is fitted onto the cable up to and including the epoxy insulator and then installed in the GIS or transformer. A main advantage of this classic design is that the termination can easily be applied to a various number of cable diameters and tolerances, making

Installed explosion resistant terminations

it particularly suitable for applications on site where tolerances of cables are sometimes different than previously given.

5.3.4 Classic SF6 and transformer terminations The technology for connecting cables to SF6 filled gas insulated switchgears (GIS) and transformers is very similar. The distinguishing features of the transformer

terminations

are

the

lower

field

80

5.3.5 Dry-type plug-in terminations Design and working principle A big disadvantage of the classic GIS termination is that the termination cannot be tested together with the GIS bay. This means that after the testing of the GIS, the gas compartment has to be opened again. This may not be an issue for the cable termination, but results in a higher effort for the GIS erection process. To meet the needs of GIS manufacturers who do not wish to open their installation after testing, plug-in systems, or so-called “dry-type

Principle design of a dry-type plug-in termination with

plug-in” terminations, have been developed.

the XLPE cable (1), the cable screen connection (2), the

The principle of this application is simple. The ter-

silicone rubber stress cone (3), the connection bolt in-

mination is separated into two main parts, the insulator (often called the “female part”) and the plug-

cluding pluggable current contact (4), the insulator of epoxy resin (5) and the spring assembly (6)

in connection (often called the “male part”). The female part can then be delivered directly to the GIS manufacturer, who can insert it into the GIS in the factory or when erecting the GIS on site. All the work for the GIS erection in terms of termination insertion is then done. The cable connection is usually performed later than the erection of the GIS. When the cable is being connected to the GIS, the male part of the termination can be plugged into the GIS without the necessity of re-opening the GIS, thus the name “plug-in”. However, the term “plug-in” should not lead to the assumption that the cable can be snapped in and out with a twist of the wrist. The work involved at

Example of a dry-type plug-in termination

the installation is considerable, especially for larger conductor cross-sections.

The advantages of such terminations clearly lie on

An epoxy insulator is also used in this system with

the GIS side as well as that of project management,

the appropriate field control elements embedded in

as the gas system of the GIS must not be evacuat-

the insulator. A silicone stress cone performs the

ed again after the GIS test. Consequently, any addi-

field control on the cable side and when slipped on

tional work or travel expenses of the GIS manufac-

in a cavity-free manner to the epoxy insulator.

turer can be avoided. An additional advantage is

Since there is no oil or SF6 used for the insulation

the dry design of the termination. Although classic

(only a larger silicone slip-on body and the epoxy

(oil-filled) terminations are very reliable, there is a

insulator) the termination is called “dry-type”.

strong trend towards dry terminations that do not

To maintain the dielectric strength on the epoxy-

require the handling of oil or SF6. Another ad-

silicon interface, a certain contact pressure must be achieved. Since under thermal load the cable insulation and the silicone material of the termination experience substantial thermal expansion, the pressure is maintained constant by means of a spring system.

vantage of the dry-type plug-in termination is its smaller size. That makes it additionally attractive to GIS manufacturers as they can design a smaller sized system and save material costs. In addition, a reduction in size also requires less time for installation.

81

5. High Voltage Accessories for Polymer Cables

internal life of the termination, such as the shape, size and material of the insulator and stress cone, remains in the hands of the accessory manufacturer. This has led to a variety of designs. It means that although all dry-type plug-in terminations fit into each GIS, they cannot be exchanged between the different accessory manufacturers. Consequently, the male part of manufacturer A has to go with the female part of manufacturer A and so on.

Installation of a dry-type plug-in termination

A disadvantage of this system is that the silicone slip-on body must be made exactly in accordance with the type of cable used. This is because the body must fit exactly into the inside of the epoxy insulator. Because of this, the termination manufacturer must supply a high number of silicone slip-on bodies and the cable manufacturer must guarantee the cable dimensions without a large variation in tolerances. Another disadvantage of the dry-type design is the higher space requirement for installation, as the cables, when being slipped on, have to

GIS termination plus standardised elements (green)

be pushed back somewhat (approx. 1 m).

and individually designed element of the accessory manufacturer (red)

Example 5-2:

Æ A significant difference can be seen in the number of classical slip on bodies needed in comparison to those with a dry-type design. Thus, for a range of 60 – 100 mm of the diameter over cable insulation, only two of the former may be required, whilst approximately 20 of the latter may be needed.

For the end user, the above described variety means that he has to identify the type of accessory supplier when the GIS is tested in the factory or at the latest when it is erected on site. This may be a substantial disadvantage especially when a considerable period of time lapses between the erection of the GIS and the cable system, in particularly when the supplier of the cable system is unknown at an earlier stage.

The normative issue In order to ensure that the terminations of different cables and accessory manufacturers are compati-

Dry-type terminations for transformers

ble with the GIS of various manufacturers, the de-

In most cases, the dry-type plug-in terminations are

sign is standardised in IEC 62271-209.

applied to connect high voltage cable systems to a

However, this standardisation often leads to misinterpretation. The standard determines only the outer dimensions, such as the diameter, the length or the head armature of the termination just enough that the dry-type plug-in termination of each manufacturer fits into the GIS of each manufacturer. The

GIS. However, dry-type plug-in terminations can also be applied to connect cable systems to transformers. The design and working principle is very similar.

82 Further uses for dry-type terminations Dry-type plug-in terminations are standard for the voltage levels of up to 245 kV. Although this type of termination is potentially more problematic for the accessory manufacturer and the installation, the advantages of this product, as far as project management is concerned, leads to a more frequent

5.4.2 Conductor connections Variety of conductor clamps Conductor clamps connect both conductor ends of the cables. They have to fulfil the following main tasks [Peschke 99]: - Carry the rated and maximum permissible

use of dry-type plug-in terminations, especially for

short-circuit current, where the joint must not

GIS. Since they are available up to 550 kV, further

heat up to a greater extent than the conductor

uses for dry-type terminations at higher voltage

in the non-disturbed area of the cable

levels are also expected in the future.

- Provide sufficient mechanical strength to withstand stresses during assembly, stresses of temperature fluctuations and short circuits that occur during operation - Meet the challenges of economic issues, in-

5.4 Joints

cluding cost aspects and ease of installation

5.4.1 Basic design Joints for polymer cables consist of a conductor connection, an insulation body with semiconductive deflectors and middle electrode, a moisture

In principle, three main methods are used to connect the conductor of high voltage cables. These are:

barrier, a mechanical protection and the grounding

- Compression

connections.

- Soldering, welding or brazing - Mechanical or screwed connection

Radial metallic moisture barrier

Conductor clamp

Filling material

Mechanical prtotection

Pressing In power cable connection technology, the pressing of conductors is an established practice. This process is applicable for both aluminium and copper Polymer cable

Deflector

SiR Slip-on insulation body

Middle electrode

Principle design of a joint

conductors. The conductor type spectrum ranges from round to sector-shaped and from multi-wire to solid. For a pressed conductor connection, both conductor ends are connected with a pressing cas-

As described in detail in Section 5.2, the insulation

ing, which is pressed several times at both ends.

body is an element which ensures field grading be-

The advantages are a good conductor connection

tween the outer semiconducting layer, the conduc-

with a low contact resistance, fast fitting and the

tor clamp and the insulation.

ability to connect conductors of different cross-

The insulation body consists of semiconductive de-

sections easily together.

flectors, the semiconductive middle electrode, the

The disadvantages are that the connection points

insulation compound and a conducting layer at the outside of the slip-on body.

are somewhat long and that higher power pressure equipment on site is required, such as pumps or pressing dies.

83

5. High Voltage Accessories for Polymer Cables

The disadvantages arise from the heat that is required for welding. An improper heat shielding may destroy the polymeric insulation of the cable. In addition, a considerable amount of welding equipment is needed on site. A further disadvantage, especially for terminations, is the possibility of corrosion of the connection.

Screw-fastened connections With the increased use of polymer-insulated power cables, Pressing of a copper conductor

Soldering

screw-fastened

connection

technology

(sometimes referred to as mechanical connections or simply screwed connections) has taken on an increasingly important role for high voltage cable connections. Nowadays, the majority of conductor

Soldering is a method of connecting metal materi-

connections are realised with screw-fastened tech-

als with the aid of a melted solder metal. The sol-

nology.

der melting temperature lies below that of the conductor materials to be connected. With copper conductors, the solder connection can only be used if short-circuit temperatures of no higher than 160 °C are assumed. The advantages of soldering are the good and low

The screwed connectors are pushed over the two ends of the cable conductors. After the final arrangement of the cable, the screws in the connector are tightened piece by piece. The convex shape of the screws causes the conductor to expand and presses the single conductor strands onto the inner

resistance of the conductor connection.

surface of the connector. The inner surface of the

The disadvantages arise from the heat that is re-

connector has a rippled structure. This ensures a

quired for soldering, which may destroy the poly-

very low conductor resistance when the conductor

meric insulation of the cable. In addition, a consid-

strands of the cable are pressed onto them.

erable amount of soldering equipment is needed

The application of the required torque is achieved

on site and the soldered connection can only be

with breakaway screws. Their head is designed in

used up to a short circuit temperature of 160 °C,

such a way that it breaks off at a specified torque.

which is usually below that of the cable and the ac-

At the end of the process, only the holes left by the

cessories. Therefore, soldering is not very widely

broken screws need to be filled with a fast-curing

used for the conductor connection of high voltage

epoxy resin.

cable systems.

Two main principles of screwed connectors are used, thus being one-piece and two-piece connect-

Welding

ors. While one-piece connectors are mainly used

Welding is an exothermic bonding process, where-

for cables with smaller conductor cross-sections,

by the two cable conductors are joined together with the help of alloy materials. The result is a permanent connection of the two conductors. The chemical reaction that produces the heat is usually achieved with a mixture of aluminium and copper oxides. The advantages of welding are connections with a low resistance and the fact that the diameter of the connection is only slightly larger than that of the conductor.

two-piece connectors are used for cables with larger conductor cross-sections.

84 Overview of main connection methods The main connection methods, their advantages, disadvantages as well as their typical applications for high voltage cable systems are given in the table below.

Main advantages and disadvantages of different conductor connection methods for high voltage cables Connection type Easy handling of a one-piece screwed conductor con-

Pressed

nector

Main advantages - Low contact resistance - Cables with different crosssections can easily be connected

Soldered, welded, brazed

- Very low contact resistance

Main disadvantages - Greater amount of equipment required on site - Long connection

- A greater amount of additional equipment needed on site - Risk of damaging the cable - Difficult to install

Screwed

- Low contact resistance - Same diameter as cable - Very easy to install

Top: Structure of a two-piece screwed connector; from left to right: empty hole, hole with breakaway screw,

- Must be made separately for cable and conductor size - Higher lead time

- No additional equipment required on site

hole with broken screw, hole with finished epoxy cov-

5.4.3 Moisture and mechanical

ering; Bottom: Finished two-piece screwed conductor

protection of joints

connector

Moisture protection with metal shield

The advantages of the screwed connectors are that they are easily and rapidly installed, they enable the connector to take on the same diameter as the insulation and that no additional equipment is needed for installation on site. The disadvantages are the slight increase in price and that for each different diameter of the cable and

the

conductor

cross-section,

a

different

screwed connector must be made. Another disadvantage is that the connection of cables with different conductor cross-section values is more difficult and expensive.

In all high voltage cables, a metallic shield covers the polymeric insulation (see Chapter 4). This metallic shield protects the insulation against radial moisture and is achieved by a copper or aluminium corrugated sheath, lead sheath or copper and aluminium-laminated sheath. Considering the importance of the reliability of the cable system, as well as the fact that joints are often installed under the influence of moisture or even water, it is surprising that radial moisture barriers have not become a standard in all joints for high voltage cables.

85

5. High Voltage Accessories for Polymer Cables Proper moisture protection of the joint can only be

- Metal sheet with protection box and insulation

realised by a radial moisture barrier made of a metal shield, leading to increased reliability and a prolonged lifetime. The metallic shield can consist of either a metal sheet (usually copper) or a metal tube of stainless steel or copper.

compound - Cu tube with coating of high density polyethylene (HDPE coating) - Cu tube with protection box and insulation compound - Steel housing - Steel housing with protection box and insulation compound

Joints with different designs of moisture protection; top: metal sheet of copper and polyester protection box filled with insulating compound, bottom: copper tube with polymer coating Joints with different designs of mechanical protection;

Variety of designs for mechanical protection

top: heat shrink cover, bottom: stainless steel tube and

Greater reliability and a long lifetime are further

polyester protection box filled with insulating com-

ensured by a first-rate mechanical protection of the

pound

joint. Suited to the diverse applications of the customer’s

Finally,

application, a wide variety of mechanical protection

measures for mechanical and moisture protection

it

should

be

mentioned

that

some

designs are available. Basic protection is provided

fulfil the same effect, such as that of using copper

by a heat shrink cover. For a higher degree of pro-

or stainless steel tubes.

tection, steel or copper (Cu) housing and/or a box of polyester can be chosen. This polyester protection box is filled with an insulating compound, giving excellent sealing and mechanical protection to

5.4.4 Application of joints with different protection degrees

the joint. The different designs of mechanical protection en-

As mentioned above, different types of joint protec-

able the customer to select the best technical and

tions are designed for different applications. The

cost-efficient solution.

recommended application for each joint type

The main protection designs for polymer cable

should be given by the accessory manufacturer. An

joints are: - Metal sheet with heat shrink cover - Protection box with insulation compound

example of such an application table is given below.

86 Applications of joints for polymer cables Type

Radial moisture barrier

Mechanical protection

Advantages

MPAH

Metal sheet

Heat shrink cover

*

*

*

MPAP

Metal sheet

Protection box

*

*

MPCC

Cu-tube

Cu-tube with HDPE coating

* *

*

MPCP

Cu-tube

Cu-tube and protection box

*

*

Application

Extremely compact dimensions Basic sealing against moisture Cost effective Good mechanical protection in different environments Total sealing against moisture Compact dimensions High degree of mechanical protection Total sealing against moisture Highest degree of mechanical protection Total sealing against moisture

*

*

*

*

*

*

*

*

For limited dimensions, such as small manholes In tunnels or concrete manholes without permanent water ingress In all types of laying, such as in tunnels, concrete pits or directly buried installations In buried installations with humid soil In all types of laying, such as in tunnels, concrete pits or directly buried installations In installations with permanent humidity or shallow water In all types of laying, such as in tunnels, concrete pits or directly buried installations In installations with permanent humidity or shallow water

Example for an application table of different joint types

Depending on the type of laying, the different designs of joints should then be applied accordingly. The following pictures show different applications of different joint types.

Joints with protection box and insulation filling in a concrete tunnel

Joints with copper tube and HDPE coating in a cellar

5.4.5 Grounding connections The grounding of joints is essential for the proper working of the cable system. The need and detailed solutions for different groundings of the cable system are given in Chapter 4 and 7, respectively. In this section, the grounding variations of the joint as a product are described. In general, there are three different variations for grounding of joints. These are: - Straight-through connections Joints with protection box and insulation filling in a concrete basement

- Straight-through connections with a direct earthing link - Cross-bonding of joints

87

5. High Voltage Accessories for Polymer Cables

is conducted to either end of the joint, the separation of the two sections is made at the joint itself. Each side of the joint is then connected to a separate outer cable. In most cases, coaxial cables are used for this purpose. More information on this is given in Chapter 7.

Joint with cross-bonding connections Different grounding connections for joints; top: straight-through connection, middle: straight-through connection with grounding link, bottom: crossbonding connection

The straight through-connection can usually be achieved very easily at the joint. The outer sheath of both cable sides is connected to both ends of the metallic protection sections of the joint, which are further connected to each other. The straight-through connection with an earthing link is achieved in the same way as a straightthrough connection with an additional link to the outside of the joint and a grounding link. The cross bonding version must be achieved differently. Since the outer sheath of both cable sides

Joints in a joint bay with cross-bonding cables

89

Chapter 6

Additional Accessories

6. Additional Accessories

6.1

91

Cable clamps

6.1.1 Main requirements Cable clamps are essential for the reliable functioning of a cable system. They protect against mechanical forces during service or in case of failure,

Force of gravity

and ensure that the cable system continues to function reliably during its entire lifetime in all loading conditions. Consequently, their purpose is to: - Reduce mechanical tension on lead plumb of terminations and joints - Compensate expansions due to increasing temperature with load current

Force of gravity FG caused by the weight of the cable at a GIS termination

- Fix the cable when laid in non-horizontal conditions or on ceilings

Example 6-1:

- Guide the cable in horizontal laying - Prevent cable movements at short circuit

Æ A cable system with a cable weight of 30 kg/m and terminations mounted at a height of 5 m  Forces caused by the weight of the cable

6.1.2 Forces in a cable system

P According to Equation 6-1, the force that the cable clamps have to cope with can be calculated as:

General

𝐹! = 30

During operation, different mechanical forces influ-

9 Based on the weight of the cable, the cable clamps have to be designed for at least 1.5 kN.

ence a cable system. These are:

𝑘𝑔 𝑚 ∙ 5 𝑚 ∙ 9.81 ! = 1.5 𝑘𝑁 𝑚 𝑠

- Gravitational force FG Shearing forces during thermal expansion

- Shearing forces FF

Thermal expansion of a cable is caused by a

- Short-circuit forces FS (may occur)

change of its temperature, which results from: - Different ambient temperatures

Gravitational force due to cable weight

- Change of cable load

The gravitational force on the cable FG occurs at

- Short-circuit currents

terminations and at vertical cable laying in an axial direction, as well as horizontal cable laying on ceil-

Although the cable consists of a combination of dif-

ings or walls in a radial direction. Particularly at

ferent materials, the elongation of the conductor is

terminations, all gravitational forces on the lead

the main cause of the forces of the cable. The elon-

plumb or on the termination itself must be avoided.

gation of the conductor can therefore be calculated

The gravitational force is given by the weight of the

as:

cable and can be expressed as:

ΔL = L0 ⋅ α ⋅ Δϑ 𝐹! = 𝑚 ∙ 𝑔

(Eq. 6-2)

(Eq. 6-1) In which:

In which:

ΔL:

Elongation of the cable

m:

Weight of the cable

L0 :

Length of the cable at installation

g:

Acceleration of gravity (g=9.81 m/s2)

α:

Expansion coefficient; αCu = 16.8⋅10-6 1/K, αAl = 23.9 ⋅ 10-6 1/K [Friedrich 93]

Δϑ:

Temperature difference

92 The thermal expansion of the conductor generates

flexible cable laying and requires the cable clamps

shearing forces FF in an axial direction. The shear-

to be positioned at a minimum distance of 2.5 m

ing force can be calculated as:

from one another. The holding forces of the cable clamps have to be strong enough to fix the cable at

FF = α ⋅ Δϑ ⋅ E ⋅ A

(Eq. 6-3)

its position. As a general rule, it can be said that per 1 mm length of the cable clamp, a holding force of 10 N is achieved in an axial direction.

In which:

In flexible cable laying, the cable deflects in either

FF :

Shearing force of the cable

direction with increasing temperature. Such deflex-

A:

Conductor cross-section

ion depends on the distance between two cable

E:

Elasticity module*; ECu= 125 ⋅ 103 N/mm2

clamps, the expansion coefficient and the tempera-

EAl= 72 ⋅ 103 N/mm2 [Friedrich 93]

ture difference. Hence, it can be calculated as:

* Since the metallic conductor is the main influencing factor of these forces, the values for the elasticity

h=

modules are taken from copper or aluminium

2l ⋅ α ⋅ Δϑ π

(Eq. 6-4)

In which: h:

Deflexion of the cable due to increasing temperature

l:

Length of cable between two clamps

Forces during short circuits During short circuits the cable is loaded with both

Shearing force

thermal forces caused by the temperature increase of the short-circuit current flow and mechanical forces caused by the force of the short-circuit current. The force caused by the thermal expansion

Shearing force FF caused by the elongation of the conductor at increasing temperature

can be calculated according to Equation 6-3.

Example 6-2:

Æ Two cable systems with the following values: -

Cable 1 with copper conductor

-

Cable 2 with aluminium conductor

-

Total length of 300 m for both cables at 20°C

 Length of expansion of the cable system at maximum operating temperature of 90°C. P According to Equation 6-2, the expansion can be calculated as:

Forces during short circuit

ΔLCu = 300 m ⋅ 16.8 ⋅ 10-6 1/K ⋅ 70 K = 0.35 m ΔLAl = 300 m ⋅ 23.9 ⋅ 10-6 1/K ⋅ 70 K = 0.50 m 9 A cable with copper conductors will expand 0.35 m; a cable with aluminium conductors 0.5 m.

Magnetic short-circuit force FS

The mechanical forces during a short-circuit FS are based on magnetic stress that is caused by the short-circuit current inside the conductors.

Shearing forces can be compensated by the flexibility of the cable in a radial direction. This is called

6. Additional Accessories

93

Aside from the distance between the conductors of

𝐹! = 𝛽 ∙

each phase, the maximum real short-circuit current IS provides the basis for the calculation. Potentially, the most damaging scenario is one in which a short circuit occurs far away from the generator. The

In which:

maximum real short-circuit current is given by a

FS :

(Eq. 6-6)

Radial force on the cable per meter due to the short-circuit current in N/m

coefficient depending on the impedance in the power grid and the initial symmetric short-circuit current [ABB 92].

𝐼! = 𝜅 ∙ 2 ∙ 𝐼!"

𝜇! 𝐼!! ∙ 2𝜋 𝑎

µ0 :

Magnetic constant = 4 ⋅ π ⋅ 10-7 N/A2

a:

Distance of the cable axis in m

β:

Geometrical factor representing the kind of

(Eq. 6-5)

cable laying

In which: IS :

Real short-circuit current in kA

For flat cable laying the factor β is about 0.4, for tre-

κ:

Coefficient depending on the impedance in

foil laying this factor is about 0.5.

the power grid at the moment of the shortcircuit (κmax = 1.8) [ABB 92] IK’’:

Initial symmetrical short-circuit current in

6.1.3 Types of cable clamps

kA

To handle all the above-mentioned forces, a combination of several types of cable clamps is neces-

Based on Equation 6-5, the radial force per meter

sary. Typical cable clamps and their associated

on the cable during a short-circuit can be calculated

compensation forces are given in the table below.

as:

Different types of cable clamps and compensation forces Type

Brifix belt

3-core cable clamp

Holding clamp plastic

Holding clamp metal

Fixing clamp short

Fixing clamp long

Example

Compensation of forces in axial direction

No

No

No

Yes, little

Yes, strong

Yes, very strong

Compensation of forces in radial direction

Yes

Yes

Yes

Yes

Yes, strong

Yes, very strong

94

6.1.4 Cable clamps at joints

6.1.5 Cable clamps at terminations

To protect the joint from the above-described forc-

It is vital for the functioning of terminations that the

es, clamps have to be applied on each side of the

cable goes straight into the termination. Otherwise,

joint. Generally, this can be done in two different

the cable becomes bent within the insulator, thus

ways, either by a long fixing clamp or by a short

influencing the electric field grading negatively as a

fixing clamp together with a holding clamp on each

result of which breakdowns may occur.

side of the joint. Long fixing clamps should be placed as close to the joint as possible.

Recommended application of long fixing clamps close to joints in a directly buried installation

Not properly applied cable clamp at a termination (cable may become bent in the termination) Recommended application of normal sized fixing and holding clamps close to joints in a manhole

Properly applied cable clamps at a termination (cable goes straight into the termination)

Fixing clamps at each side of a joint; in this case, the fixing clamps hold the weight of the cable, prevent negative influence of forces caused by thermal expansion and protect the joints from forces caused by a short circuit

Cable clamps positioned next to terminations are used to hold the weight of the cable, guarantee a straight insertion of the cable into the termination, protect the termination from expansion forces caused by temperature increase during current

6. Additional Accessories

95

flow and protect the cable from damages during

metal and polymer to polymer” applies. This

short-circuit currents.

means that for cables with a metal sheath (alumin-

It is generally recommended to apply a fixing clamp approximately 100 – 150 cm below the base

ium or steel), cable clamps without rubber inserts should be applied, whereas for cables with a poly-

plate of the termination and to apply an additional

mer sheath (PVC, PE or similar) cable clamps with

holding clamp approximately 100 cm below the fix-

rubber inserts must be applied.

ing clamp. If adequate space is not available below the termination, a large fixing clamp can also be applied.

Cable clamps next to conduits A fixing cable clamp should be applied at both sides of a conduit. This fixing clamp not only ensures that the deflexion of the cable remains inside the pipe but also compensates the remaining shearing forces. In addition, the fixing clamp guarantees that the cable is not damaged as a result of forces at the edge of the conduit.

Recommended use for fixing and holding clamps at a termination

Properly applied fixing and holding clamps below a termination

Recommended use for fixing clamps next to conduits

Example of fixing clamps as they should be applied next to a conduit

Cable clamps along flat laying of the cable

6.1.6 Cable clamps for cable laying Cable clamps for cables with polymer or metal outer sheath The outer sheath of a cable is usually made of a

Cable clamps positioned along the cable are needed when the cable is not buried. Flat laying is the most popular design for high voltage cable systems, the advantages being that the axial spacing between the conductors reduces the

polymer material (PE or PVC). In rare cases, the

magnetic short-circuit forces, the proximity effect is

outer sheath is reinforced with a metal layer. When

lower and the cooling of the cable is far better.

applying cable clamps, the general rule “metal to

96 for this limitation are costs and short-circuit forces. Consequently, as a general rule, the distance between two cable clamps for flexible laying should be between 2.5 and 3.5 m. However, an exception to that rule is the laying of cables in a trough.

Example of a cable in a flat laying arrangement

Cable clamps of flexible cable installation in flat laying

For flat laying, two different ways of fixing are possible: - Fixed installations - Flexible installations

The advantages of fixed installations include the fact that the cable is protected against undercutting of the minimum bending radius and that the cable remains in the laid position. The disadvantage of fixed laying is that the shearing forces caused by temperature differences must be completely compensated by the holding force of the cable clamps,

Cable clamps of flexible cable installation in flat laying

which requires a considerable number of such

in a cable tunnel (with sneaking of the cables)

clamps. Types of cable clamps for flat laying Brugg Type

Cable clamps of fixed cable installation in flat laying

Example

Max. shortcircuit force

BCT

14 kN

BFB(T)

20 kN

BAF(T)

76 kN

The advantage of flexible installations is the limited number of required cable clamps needed, thus making the laying cheaper. The shearing forces are compensated by the deflection of the cable, which is known as “sneaking”. A disadvantage is that during sneaking, the minimum bending radius of the cable must be considered. Another disadvantage is that the cable does not remain in a fixed position. To profit from the advantages of flexible laying, the distance between the clamps should not be less than 2.5 m. However, the distance between the clamps should also not exceed 3.5 m. The reasons

6. Additional Accessories

97

Cable clamps along trefoil laying of the cable

Types of cable clamps for trefoil laying

The advantages of trefoil laying are the limited amount of space required and the reduced magnetic field of the cables. The disadvantages are higher

Type

Example

short-circuit forces, a higher influence of the proximity effect and poor heat dissipation.

Max. shortcircuit force 9 kN, 1 s

Brifix belt small

6 kN, 3 s 10 kN, 1 s

Brifix belt large

8 kN, 3 s

KH clamp*

25 kN

Similarly, clamps in trefoil laying also require the distance between two clamps to be at a minimum of 2.5 m. *If the magnetic short-circuit forces can’t be compensated by the KH Clamps, forces that occur during short-circuit must be compensated by additional Brifix belts between

Trefoil laying of two cable systems

two cable clamps. These belts must be fixed symmetrically between two KH Clamps.

Application table for cable clamps The following table gives the application for cable clamps with respect to the maximum operating voltage and conductor cross-section: Application table for cable clamps Location of installation

Max. operating voltage [kV]

Conductor cross-section [mm2]

Type BCT

Type BFB(T)

Type BAF(T)

Type BA(T)

Example Both sides of a conduit

60 – 550

120 – 2500

Both sides of a joint

60 – 550

120 – 2500

Yes

1 – 1.5 m below a termination

60 – 170

120 – 2500

Yes

> 170

120 – 2500

2 – 2.5 m below a termination

< 110 110 – 550

Along the cable (distance of 3 m)

< 110 110 – 550

120 – 800

Yes

Yes Yes

Yes

1000 – 2500

Yes

240 – 2500

Yes

120 – 800

Yes

Yes

1000 – 2500

Yes

240 – 2500

Yes

Yes

98

6.2 Surge arresters

Termination

Termination

6.2.1 Fundamentals In high voltage cable systems, the outer (polymer) cable sheath must be protected from overvoltages

SVL

as these may destroy the outer polymeric sheath.

flow incorrectly, causing additional losses and representing a potential danger. Overvoltages in the outer sheath can be caused by

Induced voltage

Once destroyed, the outer metallic screen can suffer from corrosion and the grounding current will

induced voltages resulting from lightning strikes or switching pulses. Surge arresters – in the case of high voltage cable sheath protection, these are

Distance along the cable sheath

Typical application of a sheath voltage limiter at the end of a short cable length

called “sheath voltage limiters (SVL)” – limit the induced overvoltages to an acceptable level. Modern zinc oxide (ZnO) surge arresters consist of Termination

varistor discs enclosed in a polymeric housing. Var-

Termination

istors – a combination of the words variable and resistor – have an extreme non-linear current volt-

Joint

age characteristic. This means that the varistors change within a small voltage range with increas-

Grounding box

ductive material. The high non-linearity is caused by the microscopic structure of the ZnO grains and

Current

the grain boundaries.

Distance along the cable sheath

Typical application of sheath voltage limiters at crossbonding boxes of a cable system

Ik

Uk Voltage

Non-linear U-I characteristic of a ZnO varistor

6.2.2 Application of sheath voltage limiters in cable systems Sheath voltage limiters are installed either at the end of the cable at the termination or at crossbonding boxes.

SVL

Induced voltage

ing voltage from an insulating material to a con-

Sheath voltage limiter at a termination

6. Additional Accessories

99 rester. A safety margin of about 25% is recommended.)

To choose the right SVL for cable sheath protection, it is necessary to calculate any induced voltages that might occur at the point where the arrester is installed. The induced voltage Uind is calculated as:

𝑈"!# = 𝐸 ∙ 𝐿 Sheath voltage limiter in a cross-bonding box (grey item on the top right of the cross-bonding box)

6.2.3 Dimensioning of sheath voltage limiters When it comes to the classification of sheath volt-

(Eq. 6-7)

In which:

𝐸:

Electric field

𝐿:

Length of earthing section

The electric field E can be calculated according to:

age limiters, the following voltage definitions are important: Uc:

Maximum continuous operating voltage

Ur:

Rated voltage

Ures:

Residual voltage

In this context, the rated voltage Ur is the highest permissible voltage at power frequencies of limited

𝐸=

! ! 𝐸"!$# + 𝐸!#$"

(Eq. 6-8)

In which:

Eimag

Imaginary component of the electric field

Ereal

Real component of the electric field

duration and is given by the manufacturer of the SVL (for example: Ur = 1.25 Uc) [Richter 99]. The re-

The values of

sidual voltage Ures defines the protection level of

laying. For a flat laying with an equal distance S be-

the SVL. It is the maximum voltage at the arrester during an 8/20 µs current impulse of the nominal discharge current (e.g. 10 kA).

Eimag

tween the phases

and

Eimag

Ereal

and

Ereal

can be calculated

with the following equations:

When selecting a voltage limiter for cable sheath protection, the following criteria have to be consid-

depend on the cable

𝐸"!$# = −𝜔 ∙ 𝐼 ∙ 10!! ∙ ln

ered:

𝑆 𝐷

(Eq. 6-9)

- The maximum continuous operating voltage UC of the limiter must be higher than the calculated induced voltage (generally for cable

𝐸!#$" = 𝜔 ∙ 𝐼 ∙ 10!! ∙ 3 ∙ ln

sheath protection, the minimum UC of the SVL should be 3 kV, even if the calculated induced voltage is much lower).

4𝑆 𝐷

(Eq. 6-10)

In which:

ω:

Angular frequency = 2⋅π⋅f in 1/s

I:

Short-circuit current in A

(The residual voltages for different current im-

S:

Phase distance in mm

pulses are listed in the data sheets of the ar-

D:

Diameter of cable screen in mm

- The residual voltage of the SVL at the nominal discharge current impulse should be lower than the protection level of the cable sheath.

100 Example 6-3:

Æ To determine a sheath voltage limiter (SVL) placed at a joint, the following data are given: -

Calculated voltage at a cable end of 5.4 kV

-

SVL from a supplier with the rated voltage values of Ur = 3, 6, 10, 15 kV

6.3.2 IP and NEMA protection classes General All earthing and cross-bonding boxes must have a certain degree of protection. This is necessary for

 Which type of SVL should be used?

two reasons. Firstly, to protect the outside from the

P The SVL should have a continuous operating voltage of Uc higher than the calculated voltage at the cable end. Therefore, the SVL with Uc = 6 kV should be chosen.

contacts inside the earthing boxes as these may

9 After selecting the SVL type with Uc = 6 kV, it must additionally be checked whether the residual voltage at the maximum current is below the protection level of the joint. In the data sheet of the arrester, the residual voltage at 10 kA is listed as 21.8 kV, which is below the insulation level of the given joints.

carry dangerous voltage potential. Secondly, to protect the inside contacts of the earthing boxes from outer influences, such as animals, humidity or water, which could influence the performance of the earthing and cable system. Two main systems have been established as a means of providing protection: the “International Protection” (IP) system and the American “National

For more detailed information about the selection of an SVL – as well as for other applications – see the application guidelines for overvoltage protection [Richter 99].

Electrical Manufacturers Association” (NEMA) system. It is worth mentioning that the degrees of protection are valid for all technical systems, not only for earthing devices (IP numbers are therefore also given on household devices, such as hair-dryers).

6.3 Earthing devices for joints and terminations 6.3.1 Fundamentals High voltage cable systems must always be earthed. The earthing, sometimes referred to as grounding, very much depends on cable parameters and on the application of the cable system. This section describes the properties of the products themselves, such as those of cross-bonding boxes for joints. All issues concerning application of the earthing equipment, such as why and how to apply a cross-bonding system, are described in detail in Chapter 7. The following types of devices for the earthing of terminations and joints are available: - Earthing boxes for terminations - Cross-bonding boxes for joints - Earthing clamps for terminations

IP The protection according to IP is based on the standard IEC 60529 [IEC 60529]. In addition to the two letters “IP” the code also contains two numbers. The first number defines the level of protection against penetration of solid objects into the housing. The second number defines the level of protection against penetration of liquids into the housing.

6. Additional Accessories 1st index figure “I”

Example

2

101 Degree of protection

Protection against contact with fingers or solid foreign bod-

2nd index figure “P”

Example

Degree of protection

2

Protection against vertical and diagonal water drops up to 15° angle

3

Protection against vertical and diagonal water drops up to 60° angle

ies with ∅ > 12 mm

3

Protection against tools, wires or similar objects and solid foreign bodies with ∅ > 2.5 mm Same protection as 3,

4

but with ∅ > 1 mm

5

Full protection against contact and protection against interior injurious dust deposits

6

Total protection against contact and protection against penetration of dust

4

Protection against water from a nozzle from all directions 5

6

7

8

NEMA The NEMA code has a successive numbering from NEMA 1 to NEMA 13 and distinguishes between indoor and outdoor applications. The transfer from IP to NEMA enclosure types can only be roughly given. There is no direct comparison between IP and NEMA as the specifications and way of testings are significantly different [Siemon 11], [Moeller 09].

Protection against splashwater from all directions

Protection against ingress of water in case of temporary flooding Protection against ingress of water in case of temporary immersion Protection against ingress of water in case of continuous immersion

102 Analogy transfer of NEMA to IP classes NEMA code

Application

IP analogy

1

Indoor

IP 10 – IP 20

2

Indoor

IP 11 – IP 22

3

Outdoor

IP 54 – IP 55

3R

Outdoor

IP 14 – IP 24

3S

Outdoor

IP 54 – IP 55

4

Indoor / Outdoor

IP 56 – IP 66

4X

Indoor / Outdoor

IP 56 – IP 66

5

Indoor

IP 52 – IP 53

6

Indoor / Outdoor

IP 67

6P

Indoor / Outdoor

IP 67

7

Indoor

8

Indoor / Outdoor

9

Indoor

10

Mining industry

12, 12K

Indoor

IP 52 – IP 54

13

Indoor

IP 54

Earthing box for single-phase earthing with SVL and protection degree of IP 54

6.3.3 Earthing boxes Earthing boxes are used to realise the earthing at terminations or joints. Earthing boxes can be realised for three-phase or for single-phase earthing. Earthing boxes can also be equipped with SVL.

Earthing box for three-phase earthing without SVL and protection degree of IP 54

Since earthing boxes are usually used for the earthing of terminations and are not directly buried, they have a typical protection degree of IP 54.

Earthing box for three-phase earthing with SVL and a protection degree of IP 54

6. Additional Accessories

103

6.3.4 Cross-bonding boxes Cross-bonding boxes are used to realise the interconnection of the outer cable screen of different phases. Similar to earthing boxes, cross-bonding boxes can also be equipped with SVL. Since cross-bonding boxes are used for joints and are usually directly buried, they have usually a protection degree of IP 68.

Disconnecting earthing knife at an outdoor termination

Cross-bonding box without sheath voltage limiter and a protection degree of IP 68

Flexible grounding connections with smaller crosssections at GIS terminations

6.3.5 Earthing clamps for terminations In order to realise the earthing at terminations, solid connections or connections that can be reopened are applied. Amongst the different manufacturers, various types of such connections are available. The appropriate current rating must be ensured (the short-circuit current must also be considered). In order to ensure high current ratings, more than one connection or connections with larger cross-sections are typically applied.

Fixed grounding connections with larger crosssections at GIS terminations

7. Installation and Operation

Chapter 7

Installation and Operation

105

106

7. Installation and Operation

107

7.1 Installation of

cesses to ensure that minimum requirements are met. If trained installers are not available, the com-

accessories

bination of supervisors and personnel with basic qualifications is another option for the installation

7.1.1 Basics

process. However, this should be an exception and it is repeated here that qualified and well-trained

General For a reliably functioning of a high voltage cable system, one of the most important factors is the

installation personnel are a must in order to ensure a high-quality reliable high voltage cable system.

carrying out of a properly conducted and highquality installation process. Failures during installa-

Well-described installation instructions

tion, even tiny inclusions of dust or dirt, can deteri-

As well-described instructions of the installation

orate significantly the electric field and cause dielectric breakdown [Weissenberg 09-1], [Weissenberg 09-2].

steps reduce the risk of failures, installers should be provided with a good description of these. Since some installers may have difficulties with installa-

Since a main cause of cable system failure is instal-

tion instructions, the language and supporting illus-

lation (failures caused by external factors, such as

trations must be presented in a clear and compre-

digging into the cable, are not considered here),

hensible manner.

particular attention must be given to the accessories installation process. For an optimum accessory installation process, the following is essential: - Installation friendly accessories - Qualified installers - Well-described installation instructions - Cleanliness on site

Installation friendly accessories Installation friendly methods for different accessories were described in Chapter 5. There it was shown that single-piece insulation bodies of silicone rubber are the most convenient means for installing high voltage accessories.

Example of clearly described installation instructions

Qualified installers Qualified and well-trained installation personnel are a must. Although due to financial constraints

with illustration and text (Operation 6 for termination installation: Removing cable insulation end)

insufficiently trained and certified installers are sometimes employed, this is not to be recom-

Cleanliness on site

mended for the installation of high voltage acces-

As mentioned above, tiny particles may be very

sories. Considering the costs of the cable system and any failures that might occur, the costs of installer training are negligible. As a means of dealing with the issue of such costs, many cable and accessory

suppliers

offer

on-site

supervision

teams. These teams, consisting of supervisors together with basically qualified installers, provide a cost effective means of conducting installation pro-

harmful for the performance of the cable system. Therefore, cleanliness during installation is essential. Consequently, the installation work should be done under cover in order to protect personnel and equipment from outside disturbances, such as wind, dust, moisture, sun or rain. If the installation is done outside, a container (frequently climatised)

108 or at least a tent should be used. If the installation

ensured. As different countries have different

is done in a tunnel, protection is usually given.

standards of security, local laws must of course be adhered to. Before installation, a cable sheath insulation test and a phase test has to be carried out. This ensures that the cable has not been destroyed during shipment or installation and that the different phases are labelled correctly.

Preparation of the cable In the first step, the cable must be cut. After cutting, the cable has to be straightened and fixed into its final position. In order to avoid any forces at the Tent for installation of terminations in medium temperature and low-dust environment

stress cone, it is essential that the connection inside the termination is straight and, consequently, also the cable end. The straightening of the cable is done with the help of a metal (mostly aluminium) angle.

Climatised container for installation of joints in hot and dusty environment Straightening the cable end in a metal angle

7.1.2 Installation of terminations

Removing cable jacket and sheath

Introduction

After bringing the cable into position, the cable

Although there are several types of terminations,

jacket and metallic sheath has to be removed. How

the general steps for their installation are very simi-

this is actually done depends on the type of metal-

lar. So despite the fact that some outdoor termina-

lic sheath present, which might be of lead, copper

tions can be installed indoors, whilst certain indoor

or aluminium. The copper or aluminium sheath

terminations (GIS or transformer terminations) may

might be corrugated or laminated.

be installed externally, a similar method of installation can be applied.

The polymer outer cable jacket has to be removed whilst taking care not to damage the metallic sheath positioned below. Particular care must be

General preparation before mounting All parts of the termination and the necessary tools should be on site. In addition, the installation location should be clean and personnel safety must be

taken with laminated sheaths (aluminium or copper), as these are much more sensitive than corrugated or lead sheaths.

7. Installation and Operation

109 Due to the limited amount of material, welding of laminated cable screens (especially laminated aluminium screens) is much more difficult and hence requires more sophisticated installation skills. Prior to welding, the metallic screen and the bedding tapes have to be removed up to the tinned area. If present, additional screen wires must be bent back. However, aluminium screen wires should not be bent back too intensively and need to be treated very carefully to avoid breakage.

Removing cable jacket with a hand-driven peeling tool

Insulation cutting The insulation must be removed along the length of the terminal stud. To avoid damage at the conductor, the insulation must be cut up to the inner semiconducting layer of the cable. The final cut should be carefully done manually by a jointer.

Insulation cutting at a termination cable end

Heating of insulation During the extrusion process of the cable, the molecules of the cross-linked polyethylene are preorientated in one direction. Once the cable is heatRemoving cable jacket from laminated sheath; top: heating of the polymer cable jacket, bottom: removal of the cable jacket with the aid of a wire in order not to damage the laminated aluminium sheath below

Metallic screen treatment

ed, such as during current flow, the orientated molecules relax and the insulation shrinks back. To prevent the back-shrinking process of the insulation from affecting the electrical insulation parts of the termination, the shrinking process must be kept to a minimum. This can be achieved by heating and pushing back the polymeric cable insulation. While

To make a proper electric connection of the metal-

the pushing-back is typically done to joints, it is not

lic sheath and cable screen to the termination

needed for terminations, as the cut of the polyeth-

ground, the cable screen must be tinned and weld-

ylene insulation is far away from the stress cone as

ed to the base plate of the termination. Welding

well as from the critical electric regions.

corrugated and lead sheaths is usually not an issue.

110 By heating the insulation for 6 to 12 hours at 80 C, the majority of the insulation shrink-back process can be achieved. For the heating of the insulation, heating tubes or heating mats are typically used. In order not to waste too much time during the installation process, the heating is typically done overnight. After heating, the warm cable should be laid on a frame (or angle) to straighten it. The cable is left in this position until ambient temperature has been reached. It should be mentioned here that for the preparation of the dry-type plug-in termination, the insulation must be heated or pushed back as the cable

Pressing the terminal stud at a cable end

insulation length at the dry-type termination is rather short.

Cable screen connection

Conductor connection to terminal stud

base plate of the termination, thus providing a

The metallic screen of the cable is welded to the

As the conductor connection to the terminal stud is

moisture-tight and strong electric connection.

an electrical contact, it has a certain contact resistance. In order to limit electric losses and avoid unnecessary high temperatures at the terminal stud, a good conductor connection with a low conductor resistance must be made. If present, all tapes between the conductor segments and each single conductor layer inside the conductor must be removed. If swelling powder is present inside a conductor, it should be removed carefully by knocking with a hammer. If present, any varnish on enamelled wires must also be removed. Although this is very time consuming and costly, the varnish has to be removed over the whole length of the conductor connection on each single wire. This can be done by single stranding or sand blasting. Three methods are applied for the conductor connections of terminations: pressing, screwing or welding. If pressing is carried out, the elongation of the conductor must be considered. In addition to this, special crimping tools are needed.

Welded cable screen connection

For earthing purposes, the metallic screen of the cable must be connected outside the termination. The different grounding mechanisms, such as direct earthing or earthing via surge voltage limiters, are realised by equipment at the base plate. More information about the realisation of such earthing is given in Section 7.2.

In the case of a screwed connection with brakeaway screws, the conductor does not suffer from elongation. Also, no additional tools are needed, only a screw wrench. Welding is seldom used to connect the cable conductor to the terminal stud. The risk of damaging the cable with hot welding products as well as the amount of effort and tools needed makes it less suitable.

Removing semiconducting layer and preparation of insulation For preparing the termination, the outer semiconducting layer of the cable has to be removed. This process is done by hand-peeling or machinepeeling, mostly depending on the number of terminations being installed.

7. Installation and Operation

Removing semiconducting layer by hand-peeling

111

Polishing the insulation surface

Mounting of the stress cone After the cable surface has been prepared, the stress cone must be slipped on the cable. In order to reduce friction forces, the cable surface and the stress cone must be greased before the stress cone can be slipped on. It should be mentioned that only special insulation grease which do not affect the material of the stress cone or the semiconducting layer should be used. In general, the stress cone can be slipped on by hand. If this is not possible, such as in the case of Removing semiconducting layer by machine-peeling

very large cable diameters, special stress cone pulling devices can be used. During the slip-on process, damage at the stress cone and cable surface

Since the surface of the cable insulation needs to be very clean and the area where the stress cone is applied extremely smooth, additional steps need to be taken after peeling. One of these is the process of sanding around the area of the stress cone. This is done in several steps, first by machine and then by hand. The sanding is done with sand paper of different grading, moving from rough to fine. In order not to get semiconducting parts into the insulation, the direction of sanding should be towards the semiconducting layer. It should be stressed that no semiconducting parts or particles are allowed in the region of the insulation. After sanding, the cable surface is cleaned using special cleaning agents. To achieve a very smooth insulation surface, the insulation should undergo a final heating in the sanded area. This is done with a hot air gun up to 600 °C. An open flame should on no account be used next to the polyethylene insulation.

must absolutely be avoided. The stress cone must be placed so that the deflectors cover the outer semiconducting layer of the cable. After the stress cone has been mounted, the cable surface and stress cone must be cleaned again. No dust particles are allowed into this area.

112

Mounting of stress cone by hand

Sealing and mounting of insulator To avoid insulation fluid of the termination entering the cable, the areas at the conductor connector and between the insulation body and the metallic cable screen must be sealed. This step is achieved with the aid of different tapes and heat shrinkable sleeves, which are applied below the stress cone

Sealing of the termination at the terminal stud (top) and at the stress cone (bottom)

and at the top of the terminal connection. After sealing, the insulator must be mounted over the cable. The time needed to complete this process depends on the type of insulator. For small and light insulators, such as a composite insulator of 145 kV, the process is simple. The insulator only needs to be put over the cable end. For big and heavy insulators, such as porcelain insulators, lifting equipment must be used. In both cases, the surface of the cable and stress cone must not be damaged whilst the insulator is being mounted. Once the insulator is in place, it must be fixed onto the base plate.

Installation of a composite insulator by hand

7. Installation and Operation

113 Final installation steps and fixing of the termination The final steps of the installation involve the mounting of all remaining additional accessories, such as fixing cable clamps or earthing equipment. To prevent any movements inside the termination as well as forces on the lead plumb, the termination must be fixed with cable clamps. A detailed guideline for the use of cable clamps is given in Section 6.1. To enable the cable system to function appropriate-

Fixing of the insulator at the base plate

After the insulator has been mounted onto the cable, the base plate and gland must be soldered on-

ly, the terminations must be earthed. More detailed information on earthing principles and appropriate devices are given in Section 7.2 and 6.3, respectively.

to the cable sheath. In order not to overheat the cable, the soldering should be divided into at least two steps. For additional cable protection, a tinned copper tube can be mounted in between the gland and the metal cable sheath.

Insulation filling One of the last steps involves the filling of the termination with an appropriate insulation. For gas filled terminations, the SF6 gas has to be filled into the termination at an appropriate pressure level. Fluid filled terminations need to be filled with silicone oil. Only special silicone insulation oil, compatible with the insulation body, should be used.

Complete installed (GIS) terminations including cable clamps and earthing equipment

The oil must be clean and free of air bubbles. Terminations without expansion tanks must be

Final installation step for dry-type plug-in

filled with insulation oil only up to a specific level.

terminations

Enough space must be left for the thermal expan-

The installation steps for dry-type plug-in termina-

sion of the oil. Terminations with expansion tanks

tions are very similar to those of standard termina-

will be filled with insulation oil until a certain pres-

tions. However, the final step (filling in the insula-

sure inside the expansion tank has been reached.

tion fluid) is not required.

The filling level and the filling pressure of termina-

For dry-type plug-in terminations, the insulator can

tions have to be calculated for each arrangement.

be installed inside the gas insulated switchgear or

The calculations depend on the oil volume, oil

transformer. The cable with the installed stress

temperature, ambient temperature, maximum and

cone (the “male part”) can now be plugged into the

minimum oil temperature on line and the height of

insulator that is already installed in the GIS (the

the expansion tank in comparison to the termina-

“female part”). During the plug-in process, the

tion. All data are given in the installation instruc-

male part must be plugged in very carefully into

tions of the terminations.

the female part to prevent scratches or damage at the inside of the insulator and to prevent contact grease from polluting the insulator surface.

114 ment or installation and that the right phases have been installed.

Preparation of the cables In a first step, the cables have to be placed, laid down and cut. The cables need to be placed overlapping so that enough length is available for the conductor connection. In general it is recommended to have a little extra overlapping length as this facilitates the installation. In a second step, the cables have to be straightened and then fixed into its final position. To avoid any forces at the insulation body, it is important that the cable is straight inside the joint. In order not to damage the cable surface after it has been peeled off, all parts that are later required on the joint, such as housings, should be positioned on either side of the cable.

Finished male part at a dry-type cable termination

7.1.3 Installation of joints Introduction In contrast to terminations, joints require a greater variety of installation procedures. This is due to the different technologies used for slip-on joint bodies

Cutting of a cable with a saw

and the great variety of product types available. When dealing with high voltage joints, the installation steps described here mainly involve the installation of single-piece slip-on elements.

Removing cable jacket and sheath After bringing the cable into position, the cable jacket and metallic sheath must be removed. The manner in which this is done depends on the type

General preparation before mounting

of metallic sheath present, which might be corru-

All parts of the joint and the necessary tools must

gated, laminated or made of lead.

be on site. In addition, the installation location

For all types, the cable sheath must be removed on

should be clean and personnel safety ensured. As

both sides of the cable along the entire joint length.

different countries have different standards of

A more detailed description of how cable jackets

working security, local laws must be adhered to.

and sheaths should be removed is given in the

Before installation, a cable sheath insulation test

terminations section in Section 7.1.2.

and a phase test has to be carried out. This ensures that the cable has not been destroyed during ship-

7. Installation and Operation

115 Insulation cutting The insulation should be removed with a straight cut along the length of the conductor connection. To avoid damage at the conductor, the insulation should be cut until the inner semiconducting layer of the cable has been reached. The final cut should be carefully done manually by a jointer. As this process is similar to that used for terminations, more pictures are given in Section 7.1.2.

Cutting of a lead sheath

Cable end with properly cut insulation

Heating and insulation pushback During the extrusion process of the cable, the molOverlapping cable length for installation of a joint and starting to remove the cable jacket

Metallic screen treatment

ecules of the cross-linked polyethylene are preorientated in one direction. Once the cable is heated, such as during current flow, the orientated molecules relax and the insulation shrinks back. Back shrinking of the insulation can be very harm-

To ensure that a proper electric connection be-

ful to the joint. If the insulation shrinks back and no

tween the metallic cable screen and the joint is

longer covers the area of the middle electrode of

achieved, the cable screen should be tinned and

the insulation body, the joint will fail. Because of

welded to each side of the cable joint. The welding

this, the shrinking process must be kept to a mini-

of corrugated and lead sheaths is usually not an

mum. This can be achieved by heating the polyeth-

issue. Due to the limited amount of material, weld-

ylene insulation and – in contrast to standard ter-

ing of laminated cable screens (especially laminat-

minations – pushing back the polyethylene insula-

ed aluminium screens) is much more difficult and

tion. The pushback tool needs to be installed before

hence requires more sophisticated installation

mounting the heating system on both cable ends.

skills.

By heating the insulation for up to 6 to 12 hours at

Prior to welding, the metallic screen and the bed-

80°C, the main shrink-back of the insulation can be

ding tapes have to be removed right up to the

achieved. To heat the insulation, heating tubes or

tinned area. If present, additional screen wires

heating mats are typically used. In order not to

must be bent back. However, aluminium screen

waste too much time during the installation pro-

wires should not be bent back several times and

cess, the heating is typically done overnight.

should be treated very carefully to avoid breakages.

After heating, the warm cable should be laid on a frame (or angle) to straighten it. The cable is left in

116 this position until ambient temperature has been

An open flame should on no account be placed

reached. This process ensures that the cable re-

next to the polyethylene insulation.

mains straight.

Heating insulation surface with a hot air gun and view Heating tubes and pushback devices as used at instal-

of the smooth region at the end of the semiconducting

lation of cable joints

layer

Removing semiconducting layer and prepara-

Positioning of insulation body at the parking

tion of insulation

position

To enable insulation properties of the joint to come

Before the conductor is connected, the insulation

into effect, the outer semiconducting layer on both

body must be slipped onto the cable. In the case of

sides of the cable must be removed. Depending on

one-piece slip-on elements, whether silicone rub-

the number of joints being installed, this process

ber or EPDM, the insulation body is placed onto a

can be done by hand or by means of machine-

“parking position” on the cable. For cold-shrink in-

peeling. As this process is similar to that used for

sulation bodies, the insulation bodies are placed on

terminations, more information are given in Sec-

the outer sheath at one side of the cable. Three-

tion 7.1.2.

piece slip-on elements need to be slipped onto

In contrast to terminations, the insulation body co-

both cable sides.

vers the whole length of the peeled cable surface.

To slip the insulation body onto the parking posi-

Because of this, the whole surface of the cable in-

tion, both the cable surface and the inner side of

sulation must be extremely clean and smooth.

the insulation body need to be greased. Only spe-

After the semiconducting layer has been peeled off,

cial insulation grease which do not affect the mate-

sanding must be done. This is done in several

rial of the stress cone or the semiconducting cable

steps, first by machine and then by hand. The sand-

layer should be used.

ing is done with sand paper of different grading,

The slip-on process using mechanical insulation

moving from rough to fine. In order not to get sem-

tools carries two possible risks: the surface of the

iconducting parts into the insulation, the direction

cable insulation may be damaged and, due to the

of sanding should be towards the semiconducting

considerable amount of force needed, the inner

layer. It should be stressed that no semiconducting

surface of the insulation body may get damaged by

parts or particles are allowed in the region of the

friction during the slip-on process.

insulation. After sanding, the cable surface is cleaned using a special cleaning agent. For a very smooth insulation surface, the insulation should undergo a final heating in the sanded area. The heating is done with a hot air gun up to 600 °C.

7. Installation and Operation

117 limits the risk of damaging the cable or insulation body surfaces. This method requires a special adapter for the insulation body. The gas used can be nitrogen or dry air, whichever is most readily available on site.

Conductor connection The conductor connection between the two cables is an electrical contact. Consequently it has a certain contact resistance. In order to limit electric losses and avoid unnecessary high temperatures in Preparing insulation body for installation with gas cushion method

the joint, a good conductor connection with a low conductor resistance has to be made. If present, all tapes between the conductor segments and each single conductor layer inside the conductor must be removed. If swelling powder is inside the conductor, this should be carefully removed by knocking with a hammer. If present, any varnish on enamelled wires must also be removed. Although time consuming and costly, the varnish must be removed along the entire length of the conductor connection at each single wire. This can be done by single stranding or sand blasting. Three methods are applied for the conductor connection of joints: pressing, screwing or welding. If pressing is applied, the elongation of the conduc-

Sliding on insulation body for installation with gas

tor should be considered after crimping. In addition

cushion method

to this, special crimping tools are needed for installation. With specially designed compression connectors, different conductor cross-sections can be connected. The gap between the pressed connection and the surface of the insulation must be covered by supporting tubes. For screwed connections with breakaway screws, the connection does not suffer from an elongation of the conductor. Only a screw wrench is needed for this connection method. Welding is seldom used for cable connections. The risk of damaging the cable with hot welding products and the amount of effort and tools needed

Joint body and mechanical protection of the joint (black coated copper tube in the background) in parking position before finishing the conductor connection

makes it less suitable. More details on the different connections are given in Chapter 5. All conductor connections need a smooth and clean surface without any scratches, edges or small pieces in order not to damage the inner surface of the insula-

The “gas cushion” installation method, in which the insulation body is moved on a gas cushion between the insulation body and the cable insulation,

tion body when slipped over into its final position.

118

Screwed conductor connector (with insulation body in

Applying grease before moving the insulating body

parking position)

into its final position

Clean surface of the shelves at a pressed conductor

Final position of the insulation body

connection after its installation

Cable screen connection and bonding Final positioning of the insulation body

The outer metallic cable screens on each side of the

After connecting the conductor, the insulation body

cable must be connected to the joint. This can be

is put into its final position. For the slip-on process,

done with:

the connector and insulator need to be greased again. To ensure that a proper electric field control is achieved, the middle electrode of the insulation body must be placed exactly over the conductor connection with its deflector ends at the end of the

- Straight-through connection - Straight-through connection and an earthing link - Cross-bonding of the cable screens

semiconducting layer on each side of the cable. In order not to allow the slip-on insulation body to

How the cable screens are connected is determined

settle too much onto the cable surface, thus requir-

by the earthing scheme of the cable system. Fur-

ing higher friction forces when to be moved, the

ther information is given in Section 7.2.

movement of the insulation body to its final position should be done within 1 h after placing it in the parking position.

For joints with straight-through connected cable screens, the semiconducting layer and the cable screens from both sides of the cable have to be connected. The outer semiconductive layer of the insulation body must be connected on both sides with the cable screens.

7. Installation and Operation

119

For joints with direct earthing of the cable screen,

ably. Different types of mechanical and moisture

the method of connection is the same as that of

protections are:

straight-through connections. In addition, the connections are linked to the system earth, which is normally done at an earth-linking box. For joints with cross-bonded cable screens, only the semiconducting layer and the screen on one cable side need to be connected to the insulation

- Heat shrink cover with or without metal sheet - Copper tube with polymer coating - Steel tube with polymer coating - Metal sheet, copper tube or steel tube with additional polyester protection box

body. The cross-bonding link is realised with special cross-bonding cables. In most cases, concentric cables are used, whereby the inner conductor has the same cross-section as that of the outer wires and both are insulated to each other. The crosssection of the cross-bonding cable must not be less than the cross-section of the screen of the high voltage cable. The inner diameter of the crossbonding cable is connected to one side of the cable, the outer wires are connected to the other side. The cross-bonding itself is realised in special crossbonding boxes. More information on possible types of cross-bonding and earthing boxes is given

Common to all types of protection is the fact that if present, the metal cover (metal sheets, copper or steel tubes) must be fixed to the outer screen of the cable. To ensure that the connection is moisturetight, soldering should be used. In many types of joints, a polyester protection box is used, which is additionally filled with an insulating compound. This compound usually consists of two components. Depending on type and manufacturer, the two components must be mixed in a certain ratio within a certain time with special mixing tools. The performance of the compound is deter-

in Section 6.3.

mined by the mixing process and must be done

For cable sheath protection, the use of surge volt-

exactly as described in the mixing-procedure or in-

age limiters is sometimes necessary. Such limiters

stallation instructions. When filling-in the com-

can be used inside the cross-bonding box. More

pound the area of the polyester protection box

information is given in Section 6.2.

around the joint must be tightly sealed, so that no compound can flow out. In addition, it must be ensured that the insulation compound fills the whole volume of the polyester protection box.

Fixing of the joint For directly buried joints, a secure and sturdy underlying base is required. Joints directly buried in earth should be backfilled carefully with sand. To avoid unnecessary mechanical loads, dumping of sand directly onto the joint should be avoided. If the joint is installed inside a manhole, tunnel or cellar, the joint must be fixed on both sides with cable Cross-bonding cables (green cables) at installed joints

Realising moisture and mechanical protection of the joint After having placed the insulation body into its final position, the mechanical and moisture protection of the joint needs to be made. Since many types of mechanical and moisture protection are available on the market, these final steps can differ consider-

clamps. More information on the application of cable clamps is given in Section 6.1.

120 sheath must be sufficiently connected to the earthing system.

7.2.2 Induced voltages at cable screen When laying out a cable system and selecting surge voltage limiters, knowledge of the range of the induced voltage during normal conditions and during failures is essential. Installed and fixed cable joints with copper tube in a

The induced voltage within a cable screen general-

concrete tunnel

ly depends on: - The mutual inductance between core and sheath - The conductor current - The length of the cable

The mutual inductivity LM between core and sheath depends on the: - Mean sheath diameter - Axial spacing between the different phases - Type of laying of cables, these being trefoil or flat laying Installed and fixed cable joints with polyester protection box in a concrete base in soil

- Assumed conditions in the system (normal operating, one-pole short-circuit, three-pole shortcircuit)

The induced voltage and the appropriate earthing

7.2 Earthing 7.2.1 Background of earthing

solution, including additional devices (such as sheath voltage limiters or earthing connections) are usually given by the cable manufacturer. Depending on the calculations of the induced volt-

As described in Chapter 4, high voltage cables have

age, several different types of earthing or bonding

an outer conductor screen. This conducts the earth

systems can be applied.

fault current in case of a short circuit, returning the capacitive charging current, limiting the radial electrostatic field and shielding the electromagnetic field.

7.2.3 Principles of earthing systems

Due to electromagnetic induction, a voltage is in-

Both-end bonding

duced in the outer conductor and metallic screen, thus depending on the operating or short-circuit current. In order to handle all induced voltages and to guarantee a good earth connection during a short circuit, the outer conductor and the metallic

For both-end bonding, both ends of the cable screen are connected to the ground. The advantage of this method is that no standing voltages occur at the cable ends, making it the most secure as far as safety is concerned.

7. Installation and Operation

121

The disadvantage is that circulating currents may

Termination

flow inside the screen as the loop between the two

Termination

earthing points is closed through the ground. As these circulating currents can be as high as the conductor current itself, they can reduce the cable SVL

ampacity significantly. that it is the most disadvantageous earthing system method as far as economic issues are concerned. It is therefore only applied in selected cases and for very short distances and medium voltage systems.

Induced voltage

The losses incurred by both-end bonding means

Distance along the cable sheath

Single-end bonding

Joint bonding Joint bonding is applied to cable systems with one joint in the middle of the system. Here, the cable screen is connected in the joint by means of a straight through version with a grounding connection, which is directly connected to the earth.

Termination

Both-end bonding

Termination

Joint

Single-end bonding Grounding box

For single-end bonding, only one end of the cable

SVL

linearly along the whole cable length and at the “open end” a standing voltage occurs. The open end should be protected with a sheath voltage limiter (SVL). This diminishes the chance of overvolt-

Induced voltage

screen is connected to the earth while the other end is left floating. The induced voltage increases

Distance along the cable sheath

ages occurring inside the cable screen, protects the cable system and ensures that relevant safety re-

Joint bonding

quirements are upheld. More information on the selection of the SVL is given in Section 6.2.

The advantage of joint bonding is that losses

The advantage of single-end bonding is that losses

caused by circulating currents cannot occur. The

caused by circulating currents cannot occur. An-

disadvantage of joint bonding is the voltage that

other advantage is that one end is firmly grounded. The disadvantage of single-end bonding is the voltage which occurs at one end of the termination. This method is typically used for high voltage systems with a length of up to 1 km.

occurs at the terminations. The “open end” should be protected with an SVL. This diminishes the chance of overvoltages occurring inside the cable screen, protects the cable system and ensures that relevant safety requirements are upheld.

122 Joint bonding is used for cable systems with one

long route lengths can consist of several cross-

joint up to a length of approximately 2 km.

bonding systems in a row.

Cross-bonding

Termination

Termination

Cross-bonding is used for long cable segments

Joint

with many joints. The cross-bonding system con-

Grounding box

sheath crossing. At the terminations, the earthing must be solidly bonded to the ground. In an ideal cross-bonding system, the three sections are of

Crossbonding box with SVL

Induced voltage

sists of three sections, each followed by a cyclic

equal length.

Distance along the cable sheath

The advantage of cross-bonding is the absence of

Cross-bonding system for longer distances

residual voltages at the end of the three sections. With no driving voltages, the sheath currents and therefore the losses in the system are zero. In reality, some minor differences between each section and a low current-flow in the sheath do actually

7.2.4 Earthing of terminations As shown above, different layouts of cable systems

cause some losses. However, with a good cross-

require different grounding layouts of the termina-

bonding system, the sheath losses can be kept very

tions. Because of this, terminations are normally

low. Another advantage of regular cross-bonding is

insulated to ground. The different grounding

that at the grounded terminations the voltage is ze-

mechanisms, such as direct grounding, grounding

ro.

via SVL or insulated design, are realised at the termination itself when being installed. Termination

Termination

The insulator of the termination is connected to the base plate. The base plate is mounted on the mechanical support of the termination (that is normal-

Joint

ly on the ground) via small insulators. Between the baseplate of the termination and the support, the

Induced voltage

Crossbonding box with SVL

different earthing schemes can be realised by different devices, such as an SVL or earthing clamps.

Distance along the cable sheath Cross-bonding system

The disadvantages of cross-bonding are the increased amount of additional equipment needed (cross-bonding boxes and cross-bonding cables) and the fact that in reality three sections of equal length cannot always be realised. Cross-bonding is the most typical means of grounding for high voltage cable systems. Very

Direct earthing of GIS terminations with earthing clamps

7. Installation and Operation

123 At cross-bonding joints, the cable screens are separated and connected to the cross-bonding box. Since this is easy to realise and cost effective, coaxial cables are typically used for cross-bonding cables. The cross-bonding of the system is then realised inside a cross-bonding box. More information about the different protection levels and further aspects of cross-bonding and earth-link boxes is given in Section 6.3.

SVL between the termination base plate and the outer sheath of the cable

Cross-bonding box with coaxial cable links and SVL (red-blue-yellow partial discharge sensor in the centre of the picture is not part of the earthing system)

Grounding box and grounding cables for direct earthing at terminations (mounted in the central phase)

7.3 Operation 7.3.1 Terminations in non-vertical

7.2.5 Earthing of joints To realise all the various bonding possibilities as described above, three types of joints must be available:

position Terminations typically operate in a vertical direction. Terminations can however be mounted in other positions. Particularly transformer or GIS

- Straight-through connected joints

terminations are often mounted in a variety of posi-

- Straight-through connected joints with direct

tions, such as horizontally. If required, outdoor

earthing

terminations can also be mounted in non-vertical directions.

- Cross-bonding joints

For non-vertical mounting, both insulation filling At straight-through connected joints, both cable screens are connected inside the joint itself. For

straight-through

connected

joints

with

a

grounding link, both cable screens are connected to each other and to an earthing cable. The earthing of the joint is then realised in an earth-link box.

types, SF6 and silicone oil, can be used. If silicone oil is used, an additional expansion tank is required for each termination at an installation degree of equal or more than 45° (this is a general rule of Brugg Cables, different accessory suppliers may adhere to other rules).

124 However, no matter what the position of the termination, the stability of its location, such as a ceiling for upside-down installation, must be guaranteed.

7.3.2 Terminations on high voltage towers All outdoor terminations can also be installed on high voltage towers. This takes, however, a little more effort for the installation and especially for safety measures of the installation personnel. In addition, the weight of the terminations must be considered. Therefore, outdoor terminations with composite insulators or terminations with insulators without silicone oil (so called non self-carrying or dry-type insulators) are most frequently applied for this purpose.

Expansion tanks in horizontally installed GIS terminations

Installation of terminations on a high voltage tower

Termination in diagonal position

Outdoor terminations with composite insulators on a high voltage tower

Termination in upside-down position

7.3.3 Wind load for terminations In some cases, the issue of wind load on terminations may be of relevance for the cable system.

7. Installation and Operation

125

Any force caused by wind which affects a body is

a cable system. This may be particularly relevant

influenced by the air density, wind speed, the

for regions where the possibility of higher earth-

cross-sectional area (shaded area) of the object, as

quake values occur.

well as the resistance coefficient of the body. In

Since terminations are exposed, the analysis of the

general, this can be calculated as follows:

seismic resistivity mainly focuses on calculations of the termination structure. The calculation of the

𝐹=

1 𝜌 ∙ 𝑐 ∙ 𝐴 ∙ 𝑣! 2

(Eq. 7-1)

In which:

seismic resistance is achieved with the aid of modern Finite Element simulation tools. In the calculation, several cable system related factors are considered, such as the cable weight, the weight of the terminations, the fixing of the cable and termina-

F:

Force on the termination

ρ:

Air density

c:

Air drag coefficient

A:

Cross-sectional area of the body exposed

lation of seismic load gives the distribution of forc-

to the wind load

es on the steel structure, which can then be com-

Wind speed

pared to the steel structure resistance. The compar-

v:

Whereas the cross-sectional area and the air drag

tions, cantilever forces of the termination and more. In addition, external influencing forces are taken into account, such as seismic accelerations. By considering all the influencing forces, the simu-

ison of these values determines the safety factor of the structure.

coefficient are determined by the specific form of the object (the termination), the anticipated wind speed is naturally determined by the location of the assembled system. Wind zone maps are particularly useful for the calculation of the anticipated wind speeds of specific locations. These provide tables of various zones showing the strength of wind speeds together with their corresponding velocity pressure (force per area). When calculating the exerted force on a specific object, the form of the object must be considered. The influence of the form upon the calculation is expressed by the air drag resistance coefficient c. If the dimensionless value is low, then the force of the wind upon the object of a given cross-section is also low (and vice versa). As the exact calculation of the air drag coefficient can only be determined by relatively complex methods of measurements within a wind tunnel, the calculation for terminations can be done by assuming the air drag coefficient of cylinders (c = 0.8).

Simulation result of seismic load for a steel structure of cable terminations (Picture: AK Technology AG)

Once all the cable-related and external data has been put into the simulation tool, additional forces, such as forces during normal operation or during short-circuits, gravity forces or forces during thermal expansion of the cable, can also be calculated

7.3.4 Seismic calculations In some cases, it can be of interest or even necessary to consider the seismic withstanding ability of

easily.

8. Measurements, Monitoring and Diagnostics

Chapter 8

Measurements, Monitoring and Diagnostics

127

128

8. Measurements, Monitoring and Diagnostics

8.1 Introduction and basic definitions 8.1.1 Introduction Measurements of cable systems, together with their associated diagnostics and monitoring, have been in increasing demand over the last years. Three factors were particularly influential in this

129

8.1.2 Basic definitions Discussions on cable systems often vary in their use of key terms. This can be observed in the manner in which “measurements”, “monitoring” and “diagnostics” are used, as well as certain terms within these fields, such as “offline” and “online”. To prevent confusion, the following section clarifies the meaning of these terms as applied here.

development: the increasing popularity of low-cost products, the increasing quality of measurement,

Measurements

monitoring and diagnostics equipment and the age

Measurements determine the state of a (physical)

of the equipment still in service.

system at a certain moment of time under certain

While the focus on quality and reliability of the

conditions. They deliver snapshot information

primary system was a key issue in the past, recent

about the system.

years have seen a shift towards the wish for meas-

In the case of cable systems, an example might be

urements, diagnostics or monitoring of cable sys-

a partial discharge (PD) measurement during an

tems due to a variety of problems occurring around

after installation test. The snapshot covers the pe-

low-cost cables or accessories. Since the meas-

riod in which the after installation test is being car-

urement and monitoring of equipment is, in itself,

ried out. The conditions represent the voltage and

costly and the personnel that operate those sys-

temperature level during the test.

tems too, it is questioned by the authors whether saving by means of low-quality primary equipment and monitoring solutions of any kind is advisable. However, monitoring equipment has become more reliable in recent years and increased knowledge concerning the use of the measured data is now available. This makes the application of monitoring and diagnostic equipment as part of an overall asset management particularly attractive, especially for assets of strategic importance. In addition, the use of monitoring and diagnostics can help to survey the system in order to extend lifetime and operate the system close to its limits –

Monitoring Monitoring is the continual determination of the condition of a (physical) system. The main difference to measurements is that monitoring involves constant surveyance over a prolonged period of time and, thus, provides very different insights into the condition of the system. In the case of cable systems, an example is the distributed temperature sensing (DTS), in which the temperature of the cable is surveyed (monitored) continuously.

solutions that undoubtedly make sense. In the case of cable systems, this might be the diagnostic of oil

Diagnostics

in a termination that has been in operation for dec-

Diagnostics determine the state of the (physical)

ades or temperature monitoring of highly loaded

system with respect to its past. In addition, diag-

cables.

nostics may estimate future behaviour. The mini-

The decision to use measurements, monitoring and

mum requirement for diagnostics is at least one

diagnostic tools is not as clear as engineers would

measurement. An estimation of the future behav-

like it to be. This chapter aims to lift some of the

iour of the system is made with the aid of certain

fog on this topic with the intention of revealing the

information sources, such as the results of different

possibilities and limitations of the various devices

measurements, knowledge gathered on the history

and strategies that are currently available for HV

of the system, applications of existing evaluation

cable systems. It is intended to help the end-user to

schemes and experiences of experts.

make a decision on how best to apply these

In the case of cable systems, this might be a diag-

measures for the benefit of his cable system.

nosis of silicone oil in a termination. The basis

130 measurement could be a gas-in-oil analysis and a determination of the moisture content. Together

8.2 Possible PD phenomena

with the experiences of the experts, the behaviour

in high voltage cables,

of the system could be identified as good or bad

terminations or joints

and the indication could be given whether oil must be replaced or not.

A general look at PD phenomena Partial discharges (PD) are “… localised electrical

Offline When measurements or monitoring actions are done during a period in which the system is out of

discharges that only partially bridge the insulation between conductors and which can or cannot occur adjacent to a conductor.” [IEC 60270].

service conditions, they are called “offline”. Out of

PD usually begins in voids, cracks, or inclusions

service does not necessarily mean that the system

within a solid dielectric, at conductor-dielectric in-

is not under voltage. Offline measurements can be

terfaces, along the boundary between different in-

PD measurements at an elevated voltage during a

sulating materials or in bubbles within liquid die-

service break of the system.

lectrics. Since discharges are limited breakdowns in

Most of the diagnostic measures are done offline.

only a portion of the insulation, they are called partial breakdowns or even partial discharges. In all cases, PD only partially bridges the distance be-

Online

tween electrodes.

When measurements or monitoring actions are

External PD are discharges to the surrounding air,

done during a period in which the system is in service conditions, they are called “online”. Most of the monitoring systems work online. Examples of this are online temperature measurements or monitoring of the cable system.

Typical measurements, monitoring and

the so-called “corona” discharges. Internal PD are discharges in a closed (mostly) solid or fluid insulation system. The reason why PD in an insulating material usually occurs in gas-filled voids within the dielectric is because the dielectric constant of the void is less than the surrounding dielectric. Here, the electric

diagnostics for cable systems

field appearing across the void is significantly

For high voltage polymer cable systems, meas-

higher than across an equivalent distance of the

urements, monitoring and diagnostics are mainly

dielectric. If the voltage stress across the void is in-

used for:

creased above that of the inception voltage for the

- Measurements (or monitoring) of PD for high voltage cables and accessories - Infrared temperature measurements (or monitoring) of terminations - Temperature monitoring of cable systems, mostly made as online monitoring - Moisture monitoring of cables - Leakage and pressure monitoring of oil or SF6 pressure for terminations - Diagnostics of silicone oil in high voltage terminations

gas within the void, then PD activity will start. A calculation example of that effect is given in Example 1-3. Once begun, PD causes progressive deterioration of insulating materials, ultimately leading to electric breakdown.

Possible PD occurrences in cables In cables, only internal PD may occur. Since internal PD are related to gas-filled voids, PD in a cable can occur when there are voids in the polymeric insulation material itself or at the interfaces semiconducting layer – polymeric insulation material. Additional PD may occur at inclusions of different material in the XPLE insulation. At such inclusions, elevated electric field strength values can occur, thus causing electrical tree growth together with PD in the tree channels (which are gas-filled voids).

8. Measurements, Monitoring and Diagnostics

131 Loose contact(s) Scratched corona shield

Microvoids in insulation Voids at interface outer semicon. layer Voids at interface inner semicon. layer

Bubbles in Si-oil Voids at interface stress cone - cable Voids in stress cone

Inclusions of different material in insulation

Possible PD sources in a cable

Possible PD occurrences in joints In joints, only internal PD may occur. They may occur in void inclusions in the slip-on insulation body or at the interface insulation body – cable surface. Additionally, PD may occur at a gap between the cable insulation and the conductor connection. This can occur when the back shrinking of the cable

Possible PD sources at a termination

Conventional and unconventional PD measurements

takes place and appropriate measures (during in-

In some of the publications around PD, the terms of

stallation, such as pushing-back of the insulation)

conventional and unconventional PD measure-

have not been taken properly.

ments are used. Conventional PD detection mainly refers to that according to IEC 60270. Any other

Voids at interface Voids in insuinsulation body - cable lation body

Gaps at cable insulation

means of PD detection is mainly considered as an unconventional PD measurement or detection. More detailed information is given in [IEEE 444] or [CIGRE D1.33.05]. This book focuses on what measurements are best suited to cable systems, a

Possible PD sources in a joint

closer distinction between conventional and unconventional PD measurements is not made.

Possible PD occurrences in terminations At terminations, both internal and external PD may occur in the following locations: - In void inclusions in the stress cone (internal PD) - In gas-filled inclusions at the interface stress cone – cable surface (internal PD) - In gas bubbles in the silicone oil (internal PD) - As creeping discharges due to loose contacts at the conductor-screening electrode(s) (external or internal PD) - On the outside of the termination at the corona shield or at the insulator itself (external PD)

8.3 Measurements of PD 8.3.1 Introduction PD measurements are an established method to assess the majority of electric equipment in power systems. For many insulation systems, PD measurements are a useful tool for evaluating the quality of the insulation. Consequently, PD measurements have become a mandatory part of routine and type testing according to the relevant technical standards for most high voltage assets [IEC 60840], [IEC 62067]. The correct design of cable systems is checked in type and prequalification tests and the production

132 quality of cables and accessories is checked in rou-

8.3.2 Challenges of on-site PD

tine tests. However, on-site installation poses a po-

measurements

tential risk as far as defects are concerned. Small particles, such as dust, humidity or other tiny traces

Experiences show that the highest numbers of all

of substances, or minor damage to the insulation

defects caused during installation are related to ac-

surfaces may go undetected during the installation.

cessories. As a result, particular focus has been

If occurring in critical locations of the high voltage

given to the technological development of reliable

cable or accessories, they can cause defects, which

and sensitive systems for measuring local PD activ-

in turn can lead to severe insulation failures and a

ity in joints and terminations.

reduction in lifetime of the cable system.

A PD inside a joint or termination is usually an ex-

Despite final high voltage tests after installation,

tremely low-magnitude signal. Cable terminations

the possibility of such deterioration occurring can-

usually stand in an industrial environment, such as

not, although unlikely, be fully excluded. In re-

power plants or substations with a harsh electrical

sponse to such difficulties, additional PD meas-

environment. A low measurement signal combined

urements can be helpful. Although PD measure-

with a harsh electrical environment with numerous

ments are typically used as additional measure-

interfering signals and background noise results in

ments during after installation tests, they can also

a poor signal to noise ratio and makes it extremely

be conducted during operation.

challenging to carry out reliable PD measurements. Any kind of measurement must be able to distinguish relevant (mostly internal) PD from external interferences

[Vogelsang 09],

[Rethmeier

09-1],

[Weissenberg 07] [Weissenberg 04-2], [Lemke 06], [IEEE 444], [CIGRE D1.33.05]. The manufacturers of PD equipment have responded to this by developing new and improved measurement methods.

8.3.3 Measurement methods PD pulse phenomena and relevant variables PD measurements at terminations on site (HV source and voltage divider is in the middle of the picture, PD evaluation equipment is on the truck)

A PD impulse has visual, acoustic and electromagnetic manifestations. The detection of PD can therefore be detected by various visual, acoustic or electromagnetic measurements. A PD pulse is a “… current or voltage pulse that results from a partial discharge occurring within the object under test. The pulse is measured using suitable detector circuits, which have been introduced into the test circuit for the purpose of the

Voltage

test” [IEC 60270].

≈ 10 µs

Time

PD measurements at a joint cross-bonding box on site during service of the system

PD impulse according to IEC 60270

8. Measurements, Monitoring and Diagnostics

133

The “…apparent charge q of a PD pulse is that charge which, if injected within a very short time between the terminals of the test object in a specified test circuit, would give the same reading on the measuring instrument as the PD current pulse itself. The apparent charge is usually expressed as Picocoulomb (pC)” [IEC 60270]. The pulse repetition rate n is the “… ratio between the total number of PD pulses recorded in a selected time interval at the duration of this time interval” [IEC 60270]. The pulse repetition frequency N is the “… number of partial discharge pulses per second, in the case of equidistant pulses” [IEC 60270].

Optical discharges in air; top: at a sphere-plane arrangement; bottom: at an insulator under rain (Pictures: TU Dresden – IEEH)

Optical PD detection The PD is caused by collision of electrons. When the electrons collide with atoms, light is emitted. A low intensity of PD causes emission of light in the ultraviolet (UV) range. As the PD increases, the light turns into the visible range. Both can be detected. The visual detection of PD is mainly used as a UV detection method in test laboratories to locate the source of PD or as a means of conducting measurements of connections at high voltage lines, coil ends at high voltage rotating machines or other equipment where external PD can occur.

Example of PD at overhead lines measured with UV method and graphical enhancement; red spots are the graphic display of the physical PD (Picture: UViRCO Technologies)

Recent improvements in the properties of optical fibres and the according sensor electronics have opened another possibility to detect PD by optical means. Such detection requires transparent insulation material that is applied in absolute darkness. This might be the silicone oil and a transparent silicone rubber of stress cones in terminations or transparent silicone insulation bodies in joints. Although the feasibility of such measurements has been shown in principle, more research and development work must be done to show the practicability depending on the different materials available on the market [Habel 11].

134 Acoustic detection

While IEC 60270 describes PD tests for high voltage

The colliding of electrons that causes a PD also

equipment in general, IEC 60885-3 specialises in PD

produces sound. This sound can also be detected.

tests for cables. It contains similar information to

The acoustic detection of PD is seldom used for

that described above [IEC 60885-3]. It is worth men-

measurements in high voltage cable systems itself;

tioning that IEC 60885-3 is currently under revision.

only in exceptional cases for the determination of noises during different types of tests. Acoustic PD detection is mainly used to locate corona with an ultrasonic directional microphone [IEEE 444].

UHF measurements One solution for measuring a low-value partial discharge signal in an electrically noisy environment is the measurement of PD at ultra high frequency

Standard PD measurements according to

(UHF)

IEC 60270 and IEC 60885-3

[IEEE 444].

[Rethmeier 09-1],

[Weissenberg 07],

A measurement of PD with a coupling capacitor is

PD measurements with UHF has been established

one of the longest standing traditions. In this

for decades. It originates from measurements of

measurement, the capacitive current that flows by

gas insulated switchgears (GIS). A common defini-

recharging the PD source is measured with a cou-

tion of UHF is the measurements at a frequency

pling capacitor. Since the amount of charge q is

bandwidth of 300 MHz – 3 GHz. Measurements are

measured, but not directly at the PD-source, it is

typically done at 0.2 – 1.5 GHz [Rethmeier 09-1],

called “apparent charge” [IEC 60270].

[Rethmeier 09-2], [CGRE D1 33.05].

This measurement method is often used in the la-

As mentioned before, each PD event is an electro-

boratory or in applications with low noises.

magnetic signal. When travelling along the cable, this signal is dampened and loses its high frequency parts. By measuring with ultra high frequency,

Filter

Test transformer

Cable capacitance

only the PD signals close to the place of measureCoupling capacitor

ment are detected. Any other (unwanted) signals, like background noise from the switchgear or the overhead lines, are not registered during the

Coupling quadrupole

measurement. PD system

Shielded room

Principle of measurements according to IEC 60270

Joint Propagation of a PD signal on either side of a cable

Internal PD: Low attenuation

Far PD: Strong attenuation

Cable Internal PD

Remote PD signal

Area of PD signal detection Termination

PD measurement on site with coupling capacitor (yel-

UHF-Sensor

Filter and converter

PD measurement system

low part in the middle of the picture) Principle of UHF measurements at a termination

8. Measurements, Monitoring and Diagnostics

135

UHF PD measurements provide a good solution in terms of costs and reliable results. They are therefore often applied for the measurements of PD in cable systems, especially in terminations. However, a highly skilled measurement personnel is necessary for this kind of measurement as selecting the appropriate frequency range for the PD signals is of great importance.

Measurements with directional coupling sensors The purpose of PD measurements with directional

Application of a sensor for DCS measurements

coupling sensors (DCS) is (again) to realise a measurement of low-magnitude PD in an electrical-

The method of DCS measurements works very

ly noisy environment. DCS measurements take into

well. However, as it requires a high number of sen-

account that PD mainly occurs in joints or termina-

sors and expensive electronic equipment, it has

tions and that PD signals travel in each direction of

rarely been realised on site. Today this method is

the cable and will be dampened along the distance

mostly used in laboratories.

to the source [Vogelsang 09], [Lemke 06]. When placing sensors at either side of the joint and

Inductive measurements

measuring the intensity of the PD signal, the direc-

As each PD impulse is an electromagnetic signal

tion of travel of each PD event can be determined. Signals that travel from only one direction are considered to come from the outside of the joint. Signals that can be determined to come from both directions are considered as coming from inside the joint. By applying suitable electronic means, the direction of travel of the PD signals can be determined and, hence, it can be distinguished whether the PD comes from inside the joint or from outside.

that propagates in all directions, it also travels along either side of the high voltage cable and along either side of the earthing connections. The signal at the earthing connections can be measured with an electromagnetic sensor. If the sensor is sensitive enough, low-magnitude PD signals can also be detected [Rethmeier 05], [Weissenberg 04-2]. Therefore, the ground conductor, which is used for cross-bonding of a joint and earthing of a joint or termination, is used for such

1

2

measurements. Since the measuring principle is

3

based on inductive coupling in a current trans-

Joint A B Directional coupling

C D Directional coupling

Signal at coupling output A

B

C

former, it is called “inductive” measurement.

D

PD joint

-

X

X

Noise left cables

X

-

X

-

Noise right cables

-

X

-

X

-

A B C D

1

t

A B C D

2

A B C D

3

t

t

Principle of signal evaluation during DCS measurements at a joint [Vogelsang 09], [Lemke 06]

Inductive measurements at a cross-bonding box of a cable joint (sensor as blue-red-yellow coloured device)

136

8.3.4 Sensor types established for on-site measurements Recommended sensors for terminations UHF PD measurements have established themselves as the standard for PD measurements for terminations on site [Avila 10], [Rethmeier 09-1], [Weissenberg 07], [Lemke 06]. This is a useful reliable method as measurements with low interfering signal deterioration can be made. Another advantage of measuring with UHF Inductive measurements at earthing connection at a termination (sensor as blue-red-yellow device)

sensors is that they can easily be applied after the cable system has been installed. This provides the customer with considerable flexibility as far as additional UHF PD measurements on the terminations

Since inductive sensors are relatively cost effective

are concerned.

and the measurement of three joints in one crossbonding box is possible, this method has now become increasingly popular for on-site PD measurements of joints.

Capacitive-inductive measurements PD events in a joint insulation cause capacitive recharging currents. As such currents can be detected with a small inductive sensor, this method of measuring is called “capacitive-inductive” measurement.

UHF sensor at a termination

Particularly for joints, capacitive-inductive measurements provide extremely reliable PD detection results. However, since the joint must be additionally equipped with sensors, resulting in an increase in effort and costs, this method is rarely used. PDM System

Mounting of a UHF sensor at a termination

A disadvantage of UHF measurements is that they Principle of capacitive-inductive PD measurements in

require somewhat more equipment than that need-

a joint

ed for standard HF/VHF (high frequency/very high frequency) measurements. In addition, a direct comparison of the measured data of UHF measurements to those of HF/VHF measurements is hardly possible. As an example, for most of the PD measurements, such as for standardised PD measurements according to IEC 60270 / IEC 60885-3, the

8. Measurements, Monitoring and Diagnostics PD magnitude is given in pC. For UHF measurements, the determination of PD in pC is not possible; UHF-PD values are mostly given in mV [IEEE 444]. A second example is that for standard PD measurements, a calibration of the devices is necessary. For measurements with UHF sensors, a calibration is not possible, only a so-called “check of performance” can be carried out [IEEE 444]. For these reasons, additional HF/VHF measurements in some applications are expected to remain in use. Since the trending of PD is of no great importance

137 Recommended sensors for joints In the case of PD measurements for joints, two main methods have been established. One method focuses on achieving an exact measurement for each joint; the other on finding an optimum economic solution for the measurement process. As mentioned before, PD measurements with an integrated sensor provide a good and exact detection of potential PD. Hence, the capacitive-inductive measurement principle is used. The sensor is directly integrated into the joint. Because of this, customers requiring this type of measurement must

for cables and terminations and the main ad-

specify their choice before the joints are manufac-

vantage of the UHF measurements (the extremely

tured.

good signal to noise ratio) is highly relevant for measurements at cable systems, the use of UHF PD measurements is expected to become increasingly popular, especially for terminations of high voltage cable systems.

PD measurements using sensors in the crossbonding box provide the customer with an economic alternative to the method with integrated sensors. This method offers several advantages. The PD can be measured at a sufficiently high resolution and only one sensor per joint-bay is required, thus making the system much less expensive. In addition to this, the sensor can be applied after the installation of the cable system. The disadvantage of one sensor per joint bay is that the PD signal, once present, cannot be related to the phase of origin. This can be avoided by installing three sensors in the cross-bonding box. Practical experiences show that inductive PD measurements in the cross-bonding box are very reliable [Weissenberg 04-2].

Inductive PD measurements at site with one sensor PD pattern of noise at a termination, measured with a UHF sensor at 220 MHz (top), 520 MHz (middle) and 820 MHz (bottom)

138

PD pattern of internal discharges in a prefabricated EPR joint (Picture: Power Diagnostix)

Inductive PD measurements at site with three sensors (Picture: OMICRON)

Recommended sensors for cables Since cables are tested in a final routine test and damage during laying is assessed by a sheath test, PD measurements for cables are rarely required.

PD pattern of several flat cavities in silicone fat due to

However, if requested, the PD in a cable can be

improper mounting (Picture: Power Diagnostix)

measured with inductive sensors applied at the termination and/or joint.

8.3.5 PD pattern recognition The display of the PD events (failures as well as noise in high voltage systems) over the time interval of the line frequency can be viewed as so-called “PD patterns”. The physical underlying principle of such patterns

PD pattern of discharges in a wrapped XLPE joint (Picture: Power Diagnostix)

is the expansion of electrons during a PD event that differs according to each different type of failure or noise. For example, surface discharge will cause the electrons to spread in a wide way, whilst the occurrence of a round bubble in a polymer will produce a different type of electron spreading. The different types of electron discharging cause different PD signals, hence different PD pattern. Decades of experiences and measurements have provided a good base of knowledge for relating dif-

PD pattern of contact problems of the field control of a

ferent PD patterns to different failure types [Pdix

wrapped mass impregnated termination (Picture:

08].

Power Diagnostix)

8. Measurements, Monitoring and Diagnostics

139 Terminations UHF PD sensors

1 x PDM

3 x PDM

X-bonding box with inductive PD sensors

Joints

1 x PDM

1 x PDM

3 x PDM

PD measurement device PDM Measurement Server / System PC

PD pattern of small voids in the insulation at the inter-

PD measurement system with sensors in the joint

face of a semiconductive layer during a laboratory

cross-bonding box

setup (Picture: Power Diagnostix) Terminations UHF PD sensors

3 x PDM

3 x PDM

Integrated PD sensors

Joints

3 x PDM

3 x PDM

3 x PDM

PD measurement device PD Measurement Server / System PC

PD measurement system with sensors in the joint PD pattern of delamination of the outer semiconductive layer of a prefabricated EPR joint (Picture: Power Diagnostix)

As mentioned before, both types of PD measurements have their advantages. The former is particularly beneficial for those requiring a wide variety of detailed results. The latter provides an economic solution as it enables an appropriately detailed PD detection to be achieved, whilst still limiting costs. In both cases, measurements at terminations are mostly done with UHF sensors.

Applications for PD monitoring systems PD monitoring systems are usually applied for PD pattern of a floating potential (Picture: HPS Berlin)

8.3.6 PD measurement and

long-term assessments of systems. This requires durable measuring equipment [Avila 10].

Terminations UHF PD sensors

Integrated PD sensors

Joints

monitoring system design Applications for PD measurement systems PD measurements provide a snapshot assessment of the state of a system and are usually carried out parallel to the after installation test. Customers choose between two recommended PD measurement types. The first measures by placing a sensor within each joint; whilst the second measures by placing a sensor in each joint bay in the crossbonding box.

3 x PDM

3 x PDM

3 x PDM

3 x PDM

PD measurement device PD Monitoring Server / System PC

Client control tool

PD monitoring system with sensors in the joint

3 x PDM

140 Variety of system solutions For the customer, it is often difficult to decide whether PD measurements are necessary or not, especially because of the costs of PD systems, which can be considerable. To provide the customers with an economic solution for measuring PD, modern companies provide innovative solutions in which PD systems, or parts of it, can be bought or rented. Renting can be particularly practical, allowing the customer to choose a part-time solution, which makes the whole system more cost effective.

PD monitoring system at terminations

A practical recommendation for the application of the different sensors and system solutions for PD measurements and monitoring is given in the table below.

Electronics of the PD monitoring system close to the terminations Recommended application of sensors for PD measurement and monitoring Signal to noise ratio

Costs

Construction effort

Can be applied after installation

PD measurements with UHF sensors for terminations

Very good

Medium

Low

Yes

For after installation tests, continuous measurements and monitoring

PD measurements with UHF sensors in cross-bonding box

Very good

Medium

Medium

Yes

For after installation tests, continuous measurements and monitoring

PD measurements with integrated PD sensors in joints

Very good

Medium

High

No

For continuous measurements and monitoring

PD measurements with inductive sensors in cross-bonding box

Good

Low

Low

Yes

For after installation tests, continuous measurements and monitoring

Type and location of PD sensor

Recommended application

Systems for PD measurement and monitoring solutions PD measurements

Costs

Availability time of measurements

Effort to customer

Recommended application by considering measurement accuracy and costs

PD measurements with PD equipment and sensors rented

Low

Short

Low

For after installation tests

PD measurements with PD equipment rented and sensors purchased

Low

Medium

Low

For after installation tests and sporadic measurements (e.g. 0.5x/a)

PD measurements with PD equipment and sensors purchased

Medium

Long

Medium

For after installation tests and sporadic measurements (e.g. 1x/a)

Medium

Short

Medium

For after installation tests and a limited time after (e.g. 2 weeks)

High

Long

High

PD monitoring PD monitoring with equipment rented PD monitoring with equipment purchased

For monitoring over a long period for cable systems of particular importance

8. Measurements, Monitoring and Diagnostics

141

8.4 Temperature measure-

tered light one can determine the location of the

ments and monitoring of

scattering, which means the location, where the temperature has changed.

8.4.1 Basics Raman scattering The temperature in high voltage cables is meas-

Spectral position

cables at He

Solid structure of quartz glass

Light wave

ured with special optical fibres. Such measure-

Laser light

ments are based on the effect of Raman scattering.

Stoke

The optical waveguides (fibres) are made of doped

Antistoke

quartz glass, a SiO2 molecule bond. The incoming laser light (photons) interacts with electrons of the molecules.

Besides

the

elastic

scattering

(no

change of the wavelength of the scattered light) there exists also the inelastic scattering, the socalled “Raman scattering”. The energy of the ine-

Wavelength

Principle of Raman scattering

Measurement setup

lastic scattered light differs from the incoming laser

The measurement setup consists of the fibre optic

light just by the energy of the first excitation level

cable, a detection unit and a signal evaluation unit

of the molecules. One part of the scattered light has

– typically a personal computer.

a wavelength larger than the laser wavelength. This scattered light is called the “Stoke” line or the “Stoke” peak. The other part has a wavelength,

Sending and detection unit Semi permeable mirror

Optical fibre (forward)

which is smaller than the laser wavelength. This light is called “Anti-Stoke” line or “Anti-Stoke”

Laser pulse generator Optical fibre (backwards)

peak. The intensity of the “Anti-Stoke” peak de-

Signal receiver

pendents on the temperature, whilst the “Stoke” peak is closely independent of temperature. The relationship between the intensity of the “AntiStoke” and the “Stoke” peak reflects the temperature. By measuring this relation, the temperature can be determined. Usually the incoming laser light is not continuously but pulsed. From measuring the time between sending the pulse and the detection of the scat-

Fibre splice

Emitted light Reflected light Signal evaluation and storage unit

Principle of temperature measurements with fibre optics

142

8.4.2 Applications Sensor in the cable The application of a temperature measurement or monitoring system is very simple. The fibre optic wire can be easily integrated into each type of high voltage cable. In terms of accessibility it is preferable to install the fibre optic cable on the outer sheath. However, other solutions can also be realised.

Measurements at the cable In the case of cables, temperature can be measured along the cable very easily. The spatial resolution of such temperature measurements is less than 1 m, which is an appropriate value for the temperature monitoring of cable systems [Avila 10]. The result of the measurements is a temperature profile along the cable. The advantage of temperature monitoring is that detections of unexpected hot spots in the cable can be conducted as they may occur when the surrounding undergoes change (e.g. a heating line crosses the cable after the cable has been laid or different back-fill material is used after additional construction work close to the cable line). An additional advantage is the possible load management of the cable (with additional calculation systems). The disadvantages are that the lifetime of the fibre optic cable can only be estimated from today’s perspective. Manufacturers of fibre optics claim that a lifetime of about 15 – 30 years is to be expected.

Cable with integrated optical sensors at a lead sheath

Taking into account that the lifetime of a cable is about 40 – 50 years, 15, even 30 years for the lifetime of the fibre optics is probably too short. Another disadvantage is that the electronics and software for the monitoring or load evaluation system must be updated at particular time intervals.

Temperature profile along a cable length

8. Measurements, Monitoring and Diagnostics

143

To conclude, it can be said that temperature monitoring for cable systems makes sense when the cable is expected to operate at the maximum load or when the surrounding area of the laying place is populated or occupied in some way. In such cases, the higher costs can be seen as being justified, as the system can help to prevent overheating and, therefore, failure of the entire cable system.

IR temperature profile of a high voltage termination

8.5 Other measurement and

with a hot spot

monitoring methods 8.5.1 Infrared temperature measurements for terminations Infrared temperature measurements have been in increasing use over the last decades. Indeed, since the drop in price of measurement devices, they have become standard within the field of power engineering, enabling hotspots in the equipment to be detected from outside. For cable systems, such measurements can be used to determine hotspots at terminations. Although rare, the possibility of the termination heating up cannot be entirely excluded. Too much moisture in the silicone oil or partial discharges in the oil may lead to unwanted heating of the termination. This can be detected with a thermal camera.

8.5.2 Water monitoring for cables The presence of moisture inside a cable inevitably leads to a breakdown. Thus, it is vital to ensure that no moisture or water reaches the semiconducting layer of the cable. This is achieved by the use of metallic shields in the cable, such as lead sheaths, corrugated sheaths or the application of laminated sheaths. Should the outer sheath be damaged and water reaches the inside of the cable, the application of a moisture or water sensor can help to warn of water ingress. The advantage of a cable with such an integrated water sensor is that monitoring of moisture or water ingress in the cable is possible with a prompt detection. The disadvantage of this simple and reliable system is the higher costs involved due to the additional sensor.

Cable with integrated moisture sensor IR temperature profile of a high voltage termination in good condition

144

8.6 Other diagnostic methods 8.6.1 Oil analysis for terminations The method of gas-in-oil analysis has been in use

Caused by the fact, that the dielectric strength of silicone oil depends very sensitive on its humidity, the measurement of the humidity of the oil is another important diagnostic tool for high voltage terminations. Both, the gas-in-oil analysis and the measurement

for many years, its main application being for

of the humidity are important and very cost effec-

transformers and oil-filled cable systems. This type

tive diagnostic tools for high voltage terminations.

of diagnostic method is based on the fact that elec-

They have also the advantage that sampling of the

trical discharges and the degradation of material

silicone oil can be done quite fast, so that there is

due to service load and time generates gases in the

only a short interruption time in the operation of

oil. Typical gases that are generated by electrical

the cable system.

discharges are methane, ethane, ethylene and acetylene. If the insulation consists of paper, as e.g. in oil-filled cables, also the gases carbon dioxide and carbon monoxide are generated in the oil.

8.6.2 Diagnostics based on loss-

From measuring the concentrations of these gases

factor and polarisation-

in the insulating oil, one can determine whether

depolarisation measurements

there were electrical discharges or whether the oil had a too high load. In the case of gas-in-oil analysis in terminations for high voltage polymer cables, this method is relatively young and until today there exists no international standard or guide, which explains how to interpret the different gas concentrations. However, after producing and applying oil-filled terminations for more than 30 years, the manufacturers of the terminations got a good knowledge about the interpretation of the different gas concentrations. The most important points for the gas-in-oil diagnostics for terminations filled with silicone oil are: High concentrations of methane (in the range of 1000 µl/L) are not always a sign of electrical discharges. The gas methane is a by-product of the cross-linking process of the XLPE in the cable. Even in a very good degased cable there is always methane left in the XLPE. This methane may diffuse with time slowly into the silicone oil of the termination. Even very small concentrations (in the range of 1 µl/L) of the gas acetylene are a sign that electrical discharges occurred in the termination. If acetylene is detected in the silicone oil of a termination, one has to investigate the reason for the discharges in the termination, to prevent further damage.

Diagnostics of insulation materials based on lossfactor measurements or measurements of the polarisation-depolarisation current have been in use for many decades. The majority of the measurements are still used for medium voltage cables and for certain types of fluid-filled cables. However, for high voltage polymer cable systems, this diagnostic method has not proved itself as useful in finding answers for the reasons behind ageing effects of such a system. Nevertheless, the possibility of future improvements within this area cannot be excluded, as an application of this method for the diagnostics of cable systems, or parts of them, may in time be realised.

9. Tendencies and Future Developments

Chapter 9

Tendencies and Future Developments

145

146

9. Tendencies and Future Developments

General Some years ago, innovations in the business of cable systems were few. After triple extrusions for polymeric cables had become standard and prefabricated slip-on elements had replaced taped joints, it looked as if there was nothing more to be invented. Those involved in developing cable systems technology were busy concentrating on the development of high temperature super conducting cables as components for high-energy underground transport. The liberalisation of the energy market as well as the opening of new large markets, such as Asia and the Middle East, lead to quite a different turn of events so that cable systems become increasingly in demand with new applications being erected in numerous locations. As a technology, cable systems have a bright future. Consequently, developments and trends in cable systems are on the increase. The most relevant trends and tendencies from the

147 tion increases over the next decades, so will the energy demand and, thus, the need for cables.

Another trend is that more suppliers are entering the market. This can currently be seen in Asia – particularly in China or India – as well as in the Middle East, America and Africa. Many companies producing products in the field of cable systems share their knowledge and expertise, such as those producing extrusion machines, or (XLPE) raw materials, as well as the established cable manufacturers themselves. These activities will lead to an increase of cable manufacturers on the market. And although the new cable manufacturers start with low and medium voltage cables, tendencies show that they will go on to extend their product portfolio to include high voltage cables. This is good news for customers and producers of high voltage accessories as product variety increases and costs decrease.

However,

end-users

require

greater

knowledge in order to evaluate the different cable manufacturers and assess the different cables.

authors’ point of view are named below.

They need to be aware of this and having once de-

The trend for cost savings is not mentioned here as

cided to use low-cost cables, they must be able to

it has been, is, and will always be a constant ten-

deal with the related issues technically.

dency amongst the producers of cable systems (and technical systems).

The customer will increasingly require full contractor solutions. This means turn-key solutions, in which civil works are also included in projects.

Tendencies towards a preference for cable systems Due to the ongoing urbanisation and the fact that during the past few decades the technology of high voltage cables has proved itself to be of a solid and reliable nature, the trend towards cables (instead of high voltage overhead lines) is bound to continue. This trend is characterised by a greater number of cable manufacturers, which in

This trend can lead into two basic directions. One being that classic civil works companies become customers of cable system producers; the other that cable system producers include civil works in their projects and civil work providers become customers of cable producers. Which direction will actually be taken is, currently, difficult to predict. A solution in which providers, cable manufacturers and civil work providers set up consortia may also occur.

turn leads to lower prices, an increased variety of types of cables and a greater availability of the ca-

The general interest in a reduction of the elec-

bles themselves. In addition, the increase of tech-

tromagnetic field is likely to increase in years to

nical possibilities, such as higher load-carrying ca-

come. Although cable systems are, for the most

pability or higher reliability, will encourage cus-

part, far from being a cause of problems for hu-

tomers to decide in favour of cables. Finally, in the

mans, the sensitivity of the public will lead to a lim-

case of certain applications, such as in urban areas,

itation of acceptable electromagnetic fields. Certain

the public will rarely accept overhead lines, leaving

maximum permissible values for the electromag-

cables as the only remaining solution. As urbanisa-

netic fields are under continuous discussion, the

148 issue being that these values vary from country to country. Whether stringent or less stringent regulations will be put in place has yet to be seen. At the moment, the International Commission on NonIonizing Radiation Protection recommends a maximum value of 100 µT for electromagnetic fields at an AC system of 50 Hz for those areas that have public access. However, clever cable laying will reduce the electromagnetic field values at the ground surface.

Trends towards more accessory suppliers Similar to the trend toward a preference for cable systems, accessories are in increasing demand. Thus, more accessory suppliers are entering the market. In Asia, particularly in China, more companies are starting to produce (high voltage) accessories. These new accessory manufacturers will start with low and medium voltage accessories and go on to extend their product portfolio to include high

Since cable systems require intensive investment

voltage accessories. The future field of accessories

and the utility (or user of the cable system) only

will see the development and production of a

earns money when current is flowing, the load for

greater variety of technologies as well as designs.

cable systems will be increased in future cable

In general, this trend bodes well for the end-user,

systems. The previous state of affairs in which ca-

but from the buyer’s perspective, it will be difficult

bles were only loaded with about 20 – 50 % of their

to anticipate whether the different products will ful-

maximum load capacity will mainly become a thing

fil their expected lifetimes (approximately 30 – 40

of the past. As current load increases, so will the

years) or not. This issue will require a considerable

thermal load of the cables. To prevent overloads of

degree of knowledge and experience on the part of

the cable systems, the application of temperature

the end-user or final customer. Because of this,

measurement or monitoring systems or load calcu-

consulting and training efforts, both from inde-

lations may become increasingly popular. In addi-

pendent technical consulting companies as well as

tion, new designs of cable systems may be devel-

from manufacturers themselves, are expected to

oped or used more frequently. These might be ca-

increase also.

2

bles with 3000 mm conductor cross-sections, cables with enamelled wires, cables with cooled conductors or cable systems with special concrete or backfill material with better thermal conductivity.

As far as accessories are concerned, the trend towards dry-type plug-in terminations will continue. Despite this solution being somewhat more complicated for accessory producers than previous

Although the technology of transmitting electrical

conventional solutions, the advantages for the

power via high voltage direct current (HVDC)

overall projects are unmistakable. This may well

connections has been available on the market for

lead to improved applications of this type, possibly

many decades, it is recently undergoing a renais-

used far more frequently and with voltages up to

sance. In particular, the possibility of transmitting

the highest levels.

power over considerable distances (of more than 100 km) with HVDC underground cable systems is a huge advantage. Since polymer cables for HVDC systems is becoming state of the art up to voltage levels of 500 kV and more suppliers are entering the market (thus making the technology more accepted and causing prices to fall), an increased demand of HVDC cable systems is expected. Aided by the plans for a restructured supply grid (at least in the European market), the market share for HVDC systems is expected to increase in years to come.

Similar to the wish to reduced electromagnetic fields when it comes to operating devices both the public and customers desire a greater degree of safety. Although highly reliable, a breakdown of a termination cannot fully be excluded. To limit the effects upon the surrounding people and assets, an increased use of explosion resistant terminations is to be expected. These terminations limit the effects of an internal breakdown in the termination and provide a higher degree of protection to the surrounding area than do standard terminations. Although more costly, when it comes to public and

9. Tendencies and Future Developments

149

critical applications, such explosion resistant ter-

ular is questionable. The effort required for such a

minations are bound to be used more frequently.

monitoring system is great and the reliability of cable systems from well-established suppliers is ex-

In the case of high voltage accessories, another

tremely good.

trend can be observed in which companies producing medium voltage accessories are pushing their products (and technologies) towards the application in the high voltage range. This trend

Consulting trends

has been underway for several years but at the

Many of the above-mentioned trends, such as

moment it cannot be said how successful it will be.

those demanding a greater variety of products and

However, considerable efforts have already been

technologies in high voltage cables and accessories

made in this area, a fact that can be seen in the par-

or an increase of monitoring and diagnostic tools,

tial implementation of cold shrink technology for

will result in the need for information sources with

high voltage accessories.

the function of explaining the pros and cons of all these. Therefore, it is likely that an improvement in

The variety of applications for accessories is on the increase. As customers become more sensitive to the costs, the variety of applications requiring different designs will cause a larger variety of product designs. Joints will be particularly affected by these changes, as an increase in the cable system length requires the application of more products. It can be anticipated that joints with simple protection degrees will be used for areas with solid basins (such as concrete tunnels); whilst those with high protection will be merely used directly buried in soil.

Upcoming measurements, monitoring and diagnostic applications High voltage cable systems are not the only field affected by new developments. Measurement and monitoring systems are also undergoing significant developments. From blood pressure surveys with the aid of simple watches to full-blown surveys of the entire values of a power station, measurements, monitoring and diagnostic tools and systems are everywhere. How this will affect cable systems is not, as yet, obvious. It can be estimated that partial discharge measurements for after installation tests will become a standard, at least as far as voltages equal to or higher than 245 kV are concerned. Whether or not the continuous monitoring of whole cable systems will become more pop-

the business of cable system consultancy will continue. Such consulting might occur as in-house features available from the cable and accessory manufacturer, or as free consulting businesses within independent companies or engineering offices.

150

10. References

Chapter 10

References

151

152

10. References

153

The references are given in alphabetical order irrespective of the order in which they appear in the book.

[ABB 92]

ABB, Taschenbuch Schaltanlagen, 9. Auflage, ISBN: 3-464-48233-2, Mannheim, Germany, 1992

[Avila 10]

A. Avila, R. Vogelsang, Experiences in manufacturing, testing, installing and operating of 500 kV cable systems including temperature sensing and PD monitoring, CIGRE paper B1_103, CIGRE session, Paris, France, 2010

[BfS 10]

Bundesamt für Strahlenschutz (German Federal office for radiation protection), http://www.bfs.de /de/elektro October 2014

[BIPM 06]

BIPM, Bureau International des Poids et Measures, The International System of Units, 8th Edition, 2006

[BImSchV 97]

26. Verordnung zum BundesImmissionsschutzgesetz von 1997 (26th German regulation for electromagnetic pollution, 1997) http://www.bfs.de/de/elektro/netza usbau/information/Abstract_Keller.p df October 2014

[Brugg 10]

Brugg Cables, Technical User Guide, Product Brochure, 2nd Edition, Brugg, Switzerland, 2010

[CIGRE D1.33.05]

CIGRE, HV On-Site Testing with Partial Discharge Measurements, Brochure of the CIGRE Working Group D 1.33.05, ISBN: 978-285873-XXX-X

[CIGRE-21.05 02]

B. Fainaru, Experiences with AC tests after installation on the main insulation of polymeric (E)HV cable system, CIGRE report of the Task force 21.05, Electra No. 205, 2002

[DIN/VDE 0848] DIN EN 6226, VDE 0848-226, Sicherheit in elektrischen oder magnetischen Feldern im niedrigen und mittleren Frequenzbereich, 2005 [Friedrich 93]

W. Friedrich, Tabellenbuch Elektrotechnik Elektronik, Dümmler Verlag, ISBN 3-427-53024-8, Bonn, Germany, 1993

[Habel 11]

M. Habel, K. Vaterrodt, G. Heidmann, W. Habel, R. Vogelsang, W. Weissenberg, O. Sekula, D. Pepper, H. Emanuel, R. Plath, Optical PD detection in stress cones of HV cable accessories, 8th International Conference on Insulated Power Cables, Jicable’11, Versailles, France, 2011

[Henningsen 09] C.-G. Henningsen, Technische Umsetzung der Verkabelung von Hochspannungsleitungen, ETPKonferenz Kabelanlagen in Mittelund Hochspannungsnetzen, Düsseldorf, Germany, 2009 [ICNIRP]

ICNIRP, Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz), Health Physics 74 (4): 494-522, 1998

[ICNIRP 10]

International Commission on NonIonizing radiation Protection, http://www.isnirp.org, October 2014

[IEC 60183]

IEC 60183, Guide to the selection of high-voltage cables, International Standard, 1984

[IEC 60228]

IEC 60228, Conductors of insulated cables, Third edition, 2004-11

[IEC 60270]

IEC 60270, High-voltage test techniques – Partial discharge measurements, International Standard, Third edition, 2000-12

[IEC 60502]

IEC 60502, Power cables with extruded insulation and their accessories for rated voltages from 1 kV (Um=1,2kV) up to 30 kV (Um=36 kV), International standard, 1997

[IEC 60529]

IEC 60529, Degrees of protection provided by enclosures (IP code), International standard, 2001

[IEC 60840]

IEC 60840, Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV) – Test methods and requirements, International Standard, 2011

[IEC 60885-3]

IEC 60885-3, Electrical test methods for electric cables – Part 3: Test methods for partial discharge measurements on lengths of extruded power cable, First edition, 1988 (is currently under revision)

154 [IEC 62067]

IEC 602067, Power cables with extruded insulation and their accessories for rated voltages above 150 kV (Um = 170 kV) up to 500 kV (Um = 550 kV) – Test methods and requirements, International Standard, 2011

[IEC 62226]

IEC 62226, Exposure to electric or magnetic fields in the low and intermediate frequency range, 2004

[IEC 62271]

IEC 62271-209, High-voltage switchgear and controlgear – Part 209: Cable connections for gasinsulated metal-enclosed switchgear for rated voltages above 52 kV – Fluid filled and extruded insulation cables – Fluid filled and drytype cable-terminations, International standard, Edition 1.0, 2007-8

[IEEE 48]

[IEEE 404]

[IEEE 444]

[Kirchner 09]

IEEE 48-1996, Standard Test Procedures and Requirements for Alternating-Current Cable Terminations 2.5 kV through 765 kV, IEEE, 1996 IEEE 204-2000, Standard for extruded and laminated dielectric shielded cable joints rated 2500 V to 500 000 V, 2000 IEEE, Guidelines for Unconventional Partial Discharge Measurements, IEEE Working Group D 1.33, 2010 M. Kirchner, Aktuelle Trends und Innovationen in der Kabelentwicklung, ETP-Konferenz Kabelanlagen in Mittel- und Hochspannungsnetzen, Düsseldorf, Germany, 2009

[Küchler 96]

A. Küchler, Hochspannungstechnik: Grundlagen – Technologie – Anwendungen, VDI Verlag, ISBN: 3-18-401530-0, Düsseldorf, 1996

[Lemke 06]

E. Lemke, T. Strehl, W. Weissenberg, J. Herron, “Practical experiences in on-site PD diagnosis test of HV power cable accessories in service”, IEEE International Symposium on Electrical Insulation, Ontario, Canada, 2006

[Lide 03]

D. R. Lide, Handbook of chemistry and physics, CRC press LLC, ISBN: 0-8493-0484-9, 2003

[Lindner 93]

H. Lindner, Taschenbuch der Elektrotechnik und Elektronik, Fachbuchverlag Leipzig-Köln, Springer 5. Neubearbeitete Auflage, ISBN: 3-343-00847-8, Leipzig, Germany, 1993

[Moeller 09]

Moeller Schaltungsbuch, http://www.schaltungsbuch.de/ norm036.html 2014

[Pdix 08]

Information from the internet page http://www.pdix.com 2014

[Peschke 99]

E. Peschke, R. v. Olshausen, Cable Systems for High and Extra-High Voltage, Publicis-MCD-Verl., Erlangen, Munich, Germany, ISBN: 389578-118-5, 1999

[Oesterheld 96] J. Oesterheld, Dielektrisches Verhalten von SilikonelastomerIsolierungen bei hohen elektrischen Feldstärken, PhD thesis, TU Dresden, Germany, VDI Verlag, ISBN 3-18-319621-2, 1996 [Olshausen 01] R. v. Olshausen, W. Weissenberg, The electrical long-term performance of cross-linked polyethylene, 30th WIRE conference, 2001 [Reckenpferd 09] Information from the internet page

http://www.reckenpferd.de/tools/vo lumenr.html, October 2014 [Rethmeier 05] K. Rethmeier, W. Kalkner, R. Plath, On-site PD decoupling and localization at cross bonded HV cable systems, 14th International Symposium on High Voltage Engineering, ISH, Beijing, China, 2005 [Rethmeier 09-1]K. Rethmeier, W. Weissenberg, R. Vogelsang, R. Plath, A. Kraetge, M. Krüger, “Benefits of synchronous UHF IEC-compliant PD measurements for effective noise suppression”, 16th ISH, Cape Town, South Africa, 2009 [Rethmeier 09-2]K. Rethmeier, S. Hoek, M. Krüger, A. Kraetge, R. Plath, W. Weissenberg, R. Vogelsang, “IEC-konforme Bewertung von Teilentladungen im UHF-Bereich durch synchrone Impulserfassung an mehreren TESensoren”, ETG Kongress 2009, Diagnostik elektrischer Betriebsmittel, Düsseldorf, Germany, 2009

10. References [Riechert 01]

[Richter 99]

[Siemon 11]

155 U. Riechert, R. Vogelsang, J. Kindersberger, “Temperature effect on dc breakdown of PE cables”, 12th ISH, Bangalore, India, 2001

[Weissenberg 04-2] W. Weissenberg, F. Darid, R. Plath,

B. Richter, Application guidelines for overvoltage protection – dimensioning, testing and application of metal oxide surge arresters in medium voltage networks, ABB High Voltage Technologies, Wettingen, Switzerland, 1999

[Weissenberg 07] W. Weissenberg, T. Wunderlin, O.

Siemon Network cabling solutions, http://www.siemon.com/us/standar ds/nema_comparison.asp, 2014

[Swingler 07]

S. Swingler et. al., Statistics of AC underground cable in power networks, CIGRE Technical Brochure 338, SC-WG B1.07, Electra No. 235, 2007

[TU-Graz 10]

Institut für Heath Care Engineering of Technical University Graz, Austria http://portal.tugraz.at/portal/page/p ortal/TU_Graz/Einrichtungen/Institu te/Homepages/i4470/Veroeffentlich ungen/Oeffentlichkeitsarbeit 2014

[Vogelsang 09] R. Vogelsang, O. Sekula, H. Nyffenegger, W. Weissenberg, Long-term experiences with XLPE cable systems up to 550 kV, 9th CIGRE CIRED conference, CIGRE SC B1, Kranjska Gora, Slovenia, 2009 [Vogelsang 11] R. Vogelsang, H.J. Winter, H. Gramespacher, M. Grunwald, Weissenberg, Silicone technology for reliable performance of joints and terminations for high voltage polymer cables, International Conference on Insulated Power Cables, Jicable’11, Versailles, France, 2011 [Weissenberg 86] W. Weissenberg, Einfluss makros-

kopischer Fehlstellen auf die elektrische Alterung von Polyethylenkabeln bei Wechselspannungsbelastung, PhD Thesis at TU Dresden, Germany, 1986 [Weissenberg 04-1] W. Weissenberg, U. Rengel, R.

Scherer, “EHV XLPE Cable Systems up to 400 kV – More than 10 Years Field Experience”, CIGRE Session, B1-102, Paris, France, 2004

K. Rethmeier, W. Kalkner, “On-site PD detection at cross-bonding links of HV cables”, CIGRE Session, B1-106, Paris, France, 2004 Sekula, T. Strehl, H. Elze, S. Markalous, UHF-PD-Monitoring and On-site-commissioning-test of 400 kV XLPE-insulated cable circuits at Jebel Ali / Dubai, 7th International Conference on Insulated Power Cables, Jicable 07, Versailles, France, 2007 [Weissenberg 09-1] W. Weissenberg, H. Müller, R. Vo-

gelsang, Anforderungen an die Silikonelastomere für HS-KabelGarnituren, 2tes Burghauser Isolierstoff Kolloquium, Burghausen, Germany, 2009 [Weissenberg 09-2] W.

Weissenberg, R. Vogelsang, Langzeiterfahrungen mit Hochspannungskabeln und Garnituren, RCC Fachtagung Werkstoffe, Berlin, Germany, 2009

[Wikipedia 09-1] Information from the internet page

http://www.wikipedia.de to the topic „Angloamerikanisches Masssystem“ („Angloamerican measuring system“), October 2014 [Wikipedia 09-2] Information from the internet page

http://www.wikipedia.com to the topic „Electric power transmission“), October 2014 [Wikipedia 10-1] Information from the internet page

http://www.wikipedia.de to the topic „Elektrostatisches Feld der Erde“, October 2014 [Wikipedia 10-2] Information from the internet page

http://www.wikipedia.de to the topic „Erdmagnetfeld“, October 2014

11. Symbols and Abbrevations

Chapter 11

Symbols and Abbreviations

157

158

11. Symbols and Abbrevations

Symbols

159 Symbol

Description

Symbol

Description

ρ

Resistivity, air density

A a B b

Cross-section Axial phase distance Magnetic flux density Thickness (common); slope parameter in Weibull distribution Capacity Air drag coefficient

ω

Angular frequency

Abbreviations

C c cos ϕ D d E

S s T t

Power factor Diameter Distance / insulation thickness Electric field, electric field strength, elasticity module Force Frequency Magnetic field strength Deflexion of the cable Current length Inductivity, length Number of turns in a coil, pulse repetition frequency Lifetime coefficient / exponent, pulse repetition rate Reference point(s) for the electric charge Power, effective power Electric charge, reactive power Electric charge Resistance, radius of outer semiconducting layer Common for radius, radius of inner semiconducting layer Apparent power Distance Temperature Time

tan δ U V v W X x

Power loss factor Voltage Voltage, volume Wind speed Energy Reactance Position

α

Temperature coefficient, expansion coefficient

β

Geometrical factor representing the type of cable laying

F f H h I l L N n P0, 1 P Q q R r

ε

Permittivity

Φ

Magnetic flux,

κ

Conductivity, coefficient depending on the impedance in the power grid

µ

Permeability

Abbreviation

Description

a.c., ac, AC: BD BIL BImSch

Alternating current Breakdown Basic Impulse Level “Bundesimmissionsschutzgesetz” (German regulation for electromagnetic pollution) Damped ac voltage Direct current “Deutsches Institut für Normung” (German standards organisation) Distributed temperature sensing Extra high voltage (> 220 kV) Electromagnetic force

DAC d.c., dc, DC: DIN DTS EHV EMF EPDM EPR FEM GIS HDPE HF HV IEEE IEC IP IR IPH LSR MV NEMA OPGW PD PE PQ PUR PVC RTV SF6 SI SiR SVL UHF UV VHF VLF WHO XLPE

Ethylene propylene diene monomer

Ethylene propylene rubber Finite element method Gas insulated switchgear High density polyethylene High frequency High voltage (≤ 220 kV) Institute of Electrical and Electronics Engineers International Electrotechnical Commission International degree of protection Infrared German testing institute Liquid silicone rubber Medium voltage (< 72.5 kV) National Electrical Manufacturers Association Optical ground wire Partial discharge(s) Polyethylene Prequalification (test) Polyurethane Polyvinylchloride Room temperature vulcanisation Sulphur hexafluoride System international Silicone rubber Sheath voltage limiters Ultra high frequency Ultraviolet Very high frequency Very low frequency World health organisation Cross-linked polyethylene

12. Appendix

Chapter 12

Appendix

161

162

12. Appendix

163

12.1 SI units and SI prefixes SI units All measures in this book are based on SI units. SI

Most relevant SI coherent derived units with names and symbols according to [BIPM 06] Name

Symbol

Unit

Frequency

f

Hz (1/s)

Force

F

N (m⋅kg/s2)

metric numbers [BIPM 06].

Pressure, stress

p

Pa (N/m2)

Today, it is the world’s most widely used system of

Energy, work, amount of heat

E

J (Nm)

measurement, both in everyday commerce and in

Power

units are derived from the French “le Système international d'unités”, a system for units based on

science and technology. An overview of the seven basic SI units is given in the table below.

The seven basic SI units according to [BIPM 06] Name Length

Symbol

Unit

P

W

Electric charge

q, Q

C (As)

Electric potential difference (voltage)

V (U)

V

(Electric) capacitance

C

F (As/V)

Electric resistance

R

Ω (V/A)

Electric conductance

S (A/V)

l

m

Magnetic flux

Φ

Wb (Vs)

Mass

m

kg

Magnetic flux density

B

T (Vs/m2)

Time

t

s

Inductance

L

H (Vs/A)

Electric current

i, I

A

Celsius temperature

ϑ

°C

Temperature

Τ

K

Illuminance

lx

lx (lm/m2)

Amount of substance

n

mol

Luminous intensity

Iv

cd

SI prefixes In order to address smaller and larger amounts of

Numerous other units are derived from these seven

the given SI units, prefixes are defined. Based on

basic SI units, many of which are used in our daily

[BIPM 06], the most relevant SI prefixes are given

technical life. According to [BIPM 06], the most im-

in the table below.

portant coherent derived units are given in the table below.

Most relevant SI prefixes according to [BIPM 06] Symbol

Unit

Ato

a

10-18

Femto

f

10-15

Pico

p

10-12

Nano

n

10-9

Micro

µ

10-6

Milli

m

10-3

Centi

c

10-2

Deci

d

10-1

Deca

da

10

Hecto

h

102

Some coherent derived SI units have special names

Kilo

k

103

and symbols. The main expressions are given in

Mega

M

106

the table below.

Giga

G

109

Tera

T

1012

Peta

P

1015

Exa

E

1018

Most relevant SI coherent derived units according to [BIPM 06] Name

Symbol

Unit

Area

A

m2

Volume

V

m3

Speed, velocity

v

m/s

Mass density

ρ

kg/m3

Current density

j

A/m2

Magnetic field strength

H

A/m

Name

164

12.2 Conversion table to the metric system In this document, technical data are given in metric values. However, certain different units are used internationally. The most relevant of these have been converted to metric SI units. The focus is therefore to give a relation to the main values as used in this book.

Length values:

Weight values The weight in a metric system is typically given in

Length values in the metric system are typically

kg, g or t (tons). In other countries, such as Anglo-

given in mm, cm, m or km. In other countries, such

American countries, the weight is also given in “lb”

as Anglo-American countries, length values are given in “ft” (feet) or “in” (inch). Often the short

or “lbs” (pound or pounds). The conversion of g,

sign (‘) for feet and (’’) for inch is used. The conver-

kg or t to lbs is defined by [Wikipedia 09-1]:

sion of mm, cm and m to ft and in is defined by

1 kg = 1000 g = 0.001 t = 2.2 lbs

[Wikipedia 09-1]:

1 lbs = 454 g = 0.454 kg

1 foot (‘) = 30.48 cm = 304.8 mm = 0.305 m = 12 zoll 1 inch (‘‘) = 1 zoll = 2.54 cm = 25.4 mm 10 mm = 1 cm = 0.394 inch (’’) = 0.033 feet

0.8

8

inch feet

0.7

6

0.6

5

0.5

4

0.4

3

0.3

2

0.2

1

0.1

0

0

20

40

60

80

100

mm

120

140

160

180

200

0

feet

inch

7

Conversion between mm, inch and feet

12. Appendix

165 2

Cross-section values

The conversion of mm to kcmil or sqin is defined

For cross-section values, the SI unit is square mm,

by [Wikipedia 09-1]:

often written “mm2“, “sqmm”. This unit is most

1 kcmil = 0.507 mm2

frequently used throughout the world. In the US,

1 sqin = 645 mm2

the term kcmil is given, whereas in British coun-

1000 mm2 = 1974 kcmil = 1.55 sqin

tries, the term sqin is used.

5500

4.5

5000

kcmil sqin

4500

4 3.5

4000 3

3000

2.5

2500

2

2000

sqin

kcmil

3500

1.5

1500 1

1000

0.5

500 0

0

400

800

1200

sqmm

1600

2000

0 2800

2400

Conversion between sqmm, sqin and kcmil

Temperature values:

𝑇! = 𝑇! − 32 ∙

The temperature in most countries is typically given in °C (degree Celsius). In the US, the tempera-

5 9

or:

ture is given in °F (degree Fahrenheit). The conver-

𝑇! = 𝑇! ∙

sion of temperature TC in °C to TF in °F is defined by [Wikipedia 09-1]:

9 + 32 5

400 360 320

Temperature / °F

280 240 200 160 120 80 40 0 -40 -80 -120

-80

-60

-40

-20

0

20

40

60

80

Temperature / °C

100

120

140

160

180

200

Conversion between °C and °F

166 Pressure values:

The conversion of Pa, bar and psi is defined by

The pressure in a metric system is given in Pa (Pas-

[Wikipedia 09-1]:

cal) or bar. In other countries, such as Anglo-

1 Pa = 10-5 bar = 0.01 mbar = 0.147 ⋅ 10-3 psi

American countries, the pressure is given in “psi”

10 psi = 68948 Pa = 0.6895 bar

(pound per square inch).

1 psi = 6.895 kPa

100

10000

Pa-psi Pa-bar

1000 100

10 1 0.1

1 0.1

0.01

0.01

bar

psi

10

0.001

0.001 0.0001

0.0001 0.00001

1

10

100

1000

10000

100000

1000000

0.00001 10000000

Pa

Conversion between Pa, bar and psi

Fluid volume values

This conversion takes the US values of fl oz and

The fluid volume in the metric system is given in

gallons. The conversion of l, fl oz and gallons is de-

litre. In some other countries the fluid volume is

fined by [Reckenpferd 09]:

given in fl oz (fluid ounces) and gal (liquid gallons).

1 l = 0.264 US gal = 33.86 US fl oz

In addition to this, fl oz and gal differ between the

1 US gal = 128 US fl oz = 3.78 l

British and the US systems [Reckenpferd 09].

1 US fl oz = 1/128 US gal

30

3500

fl oz gal

3000

25 20

2000 15 1500 10

1000

5

500 0

gal

fl oz

2500

0

10

20

30

40

50

litre

60

70

80

90

100

0

Conversion between litre, fl oz and gal

Brugg Cables Brugg Group In 1896, Gottlieb Suhner founded a cable factory in Brugg, Switzerland, which was the origin for a group of industrial companies belonging today to the Brugg Group. The Brugg Group consists of 5 divisions with 60 companies in 20 countries. Brugg is represented in all the important industrial countries. Approximately 2000 employees are responsible that the following products are manufactured conforming to the best technical know-how and quality standards: •

Power- and telecommunication cables and accessories

•

Special cables for industry and security

•

Process control systems for water treatment and energy management

•

Pipe sytems for the efficient and safe transport and distribution of liquids

•

Ropes for aerial ropeways, elevators and cranes

•

Rope protection systems against natural hazards

Brugg Cables (Brugg Kabel AG) After its founding in 1896, Brugg Kabel AG, which operates under the name Brugg Cables, developed into the largest company within the Brugg Group and is one of the leading cable manufacturers in Switzerland. Brugg Cables has an international presence. With a product line that ranges from high-voltage cables rated up to 500 kV and all the associated connection technology to medium and low-voltage cables and fiber optic systems. In doing so, Brugg Cables is one of the few cable manufacturers in the world that is successful in manufacturing cables and accessories up to the highest voltage level of 500 kV. Its internal R&D department is continuously working on new and innovative products. In close collaboration with its customers, Brugg Cables also develops custom-tailored system solutions. Brugg Cables develops, manufacturers, tests and installs its products according the highest standards of quality and using state of the art production methods.

Brugg Cables Industry AG is a subsidiary of Brugg Kabel AG. The company specializes in the manufacturing of customer-specific cable systems for industrial applications. From the initial concept through to the finished solution, specialists from Brugg Cables Industry AG work hand in hand with customers.

Switzerland Head office Brugg Kabel AG Klosterzelgstrasse 28 CH-5201 Brugg Tel. +41 56 460 33 33 [email protected] Brugg Cables Academy Klosterzelgstrasse 28 CH-5201 Brugg Tel. +41 56 460 33 33 [email protected]

Subject to change 04.2015

Please find more details on the courses currently offered in the online documentation www.bruggcables.com/academy.

Other sales partners in your region can be found under www.bruggcables.com. A member of the Brugg group.