Cable Testing: On-site Test

Cable Testing:

On-site Test

Dr. Asawin Rajakrom 11 July 2019 1

Presentation Outline Defects in cable and cable accessories Detection of defects

Cable testing concept Tests with different voltages Standards for on-site cable testing

Defects in Cable & Accessories 1. 2.

3. 4. 5. 6. 7.

8. 9.

Protrusions on the semi conductive layers Stripping void between the insulation and semi conductive layers, conductor and insulation shield Voids Moisture Impurities (Metal, Amber) Water tree from conductor shield Water tree form insulation shield Electrical tree caused by partial discharge Bow-tie tree

Defects in Cable & Accessories

Defects in Cable & Accessories

Water Tree ➢

Water trees are formed only in the presence of a liquid.

➢

The applied voltage must be greater than zero.

➢

The site at which propagation begins can be inclusions, micro-voids, irregularities, or ionic contaminants which have seeped from the extruded shields into the insulation.

➢

As partial discharges are not necessary for the inception and propagation of water trees.

➢

Propagation time through the insulation wall is long (years).

Water Tree

Water Tree

Four distinct phases of the treeing process: 1. Moisture permeation

2. Initiation of water trees 3. Propagation of water trees 4. Conversion to electrical trees leading to failure

Water Tree in Polymeric Insulation

Bow- tie- tree growing from an included particle inside the insulation of a 30 kV- PE- cable

Water Tree in Polymeric Insulation

Puncture Punctureout outofofa aventedvented-tree tree

Water Tree and Electrical Tree electrical field water disturbance field time outer semiconducting layer

conditions for development of "water trees" "bow-tie trees"

insulation

inner semiconducting layer conductor

"vented trees"

Electrical Tree Electrical trees are formed only under the influence of partial discharges in the insulation wall or at the shielding layers. These discharges may occur at voids, inclusions or asperities. The applied voltage must be above the corona inception level for electrical trees to form.

Water Tree and Electrical Tree

Stress enhancement areas, sites of possible electrical trees (ET), created by a water tree (WT).

Water Tree and Electrical Tree

Vented water tree (WT) and electrical tree (ET) growing into each other from opposite screens

Water Tree and Electrical Tree

Electrical trees (ET) are linking two contaminants and another ET is emanating from the conductor screen.

Water Tree and Electrical Tree Comparison of Water Treeing - Electrical Treeing

Water Treeing (WT)

Electrical Treeing (ET)

Already seen at small field strength (e.g. < 1kV/mm)

At high local field stress

Extremely slow tree growth (e.g. over 6-10 years)

Very fast tree growth in PE or XLPE insulation

No partial discharge recognizable

Accompanied by partial discharge

No visible channels

Long channel structures (visible trees)

Not visible without special coloring procedure

Clear indication of an electrical breakdown

How to Avoid Water Trees • Insulation material should be clean. • Shield material should be clean. • The insulation should be extruded tightly over the conductor shield. • Shield/insulation interfaces should be smooth and clean. • The curing process should minimize formation of microvoids. • There should be no water in the strands.

XLPE Process to Avoid Water Trees Dry cured Triple extrusion Super clean XLPE compound Super smooth semi conductive compound • Contamination free raw material handling system • Computerized process control • Inline dimension control • • • •

Space Charge

DC hipot output negatively charge up void. These trapped space charges remain after test. When AC is reapplied, there’s a high difference of potential across very little insulation. Leads to electrical trees – cable fails.

Partial Discharge in Cables • A localized electrical breakdown in the electrical insulation system under voltage stress that only partially bridged the insulation between conductors. • A consequence of local breakdown either as a result of – An electric field enhancement within or on the surface of the insulation, or – A region of low breakdown field

• Occurred at: – – – –

Voids or cavities within the insulation or at interfaces Interfacial cavities in cable and accessory interfaces High-resistance insulation shield or broken neutral Electrical trees initiated from protrusions, voids, or water trees

Partial Discharge in Cables

Several kinds of cable defect 1. 2. 3.

interface gap formation cavities cavities eventually with conductive sharp points

Partial Discharge in Cables

Partial Discharge in Cables Acoustic

Heat

PD

Chemical reaction

EMW

Light

Partial Discharge in Cables Voids may be formed in insulation systems due to manufacturing or installation defects, ageing, water tree growth. Continued stress and overvoltages can initiates PD in voids.

Heat and other forms of energy released by PD cause erosion of the internal surface of void.

Continued erosion forms channels that develop into so-called electrical trees in insulation.

Cable Failure Continued PD produced further erosion until the electrical tree bridges the insulation

Partial Discharge in Cables

Electric Stress Distribution With No Void ( 12.7 kV rms on the cable conductor )

Electric Stress Distribution With a Void ( 12.7 kV rms on the cable conductor )

PD in Insulation Cavity

Partial Discharge in Cables PD Patterns

Partial Discharge in Cables PD Patterns

PD Tests What PD parameters are measured? • PD inception voltage (PDIV)

• PD density

• PD extinction voltage (PDEV)

• Phase angle of PD pulse

• PD location

• Phase resolved PD plot

• PD magnitude

• PD magnitude vs. voltage plot

• PD repetition rate

Voltage Sources

Resonance Test System (AC)

Oscillating Wave Test System (DAC)

Very Low Frequency (VLF)

PD Tests PD test methods: • Offline test: – An elevated AC voltage of about 1.5-2.0 U0 is applied to generate PD and then a proprietary digital signal analysis platform is used to detect transient signals that are generated at the discharge site and travel through the cable to the detection equipment – Test voltage source can be selected from VLF, resonance or damped AC voltage tester

• Online test: – Cable circuit is under normal operating voltage and loading condition – Sensors – HFCT or CCV – are used to detect transient signals and digital signal analysis platform is used to capture and record signals

• Tests shall be done in comparative manner, i.e. PD growth

PD Offline Test

Typical test setup for offline PD testing

PD Offline Test Generation of Test Voltage (DAC)

HV Source

HV Solid State Switch

Inductor L

Test Object: power cable Test Object: Power Cable

S

Cc

For short cable an additional CC is required.

Embedded PC Process Unit 200 MHzControl AD Converter Data Storage

HV Divider HV Divider PD Coupling Capacitor PD Coupling Capacitor adaptive PD detector PD detector

PD Analysis Dielectric losses estimation Source: SebaKMT

Dissipation Factor (Tan d)

Dissipation Factor (Tan d)

voltage

current 0

Dissipation Factor tan d

10

time/sec =

true power = reactive power

U² / R U². w C

=

1 w C. R

Dissipation Factor (Tan d) Simplified Model of a Water Tree

wt

R2

C2

C1

R1

Dissipation Factor (Tan d)

Test sutup for Tan d

Dissipation Factor (Tan d) How to test Tan d ➢ Non-destructive and integral procedure, and is hence useful for assessing the

entire tested cable route eg. gross defects ➢ 6 to 10 measurements are performed at 0.5U0, 1.0 U0, 1.5U0 and 2U0 ➢ Tan delta mean value (MTD) of measurements in the individual voltage steps Delivers information on water trees, i.e. damages caused by water in the insulation of plastic-insulated cables. (These water trees can become electrical trees where partial discharges and breakdowns may occur). ➢ Gives information on the thermal or chemical ageing behaviour of the cable route. ➢

➢ Tan delta standard deviation (STD) of measurements in the individual

voltage steps

➢ Indicates of partial discharges (PD) ➢ Detect moist joints ➢ Mean value difference (DTD) in the various voltage steps ➢ Detect water trees ➢ Detect partial discharges ➢ Detect vaporisation effects (e.g. at terminations)

Cable Testing Concept

Field Distribution in Cables: AC vs DC • AC Conditions: – tan δ = ↓↓ – Ic >> Ir – Capacitive field control

• DC conditions (without consideration of space charges): – C becomes irrelevant – Resistive field control – Defects usually have lower resistance and lower field

Insulation of MV, HV and EHV Cables • Examples for Typical Cable Geometry – MV Cable: 24 kV, insulation thickness 5.5 mm, mean operational field strength at voltage peak 3.1 kV/mm – HV Cable: 138 kV, insulation thickness 17.8 mm, mean operational field strength at voltage peak 6.3 kV/mm – EHV Cable: 400 kV, insulation thickness 26 mm, mean operational field strength at voltage peak 12.6 kV/mm

Insulation of MV, HV and EHV Cables Technical Consequences ➢ Field strength increases with cable voltage ➢ Recommended limit for test field strength: 27-30 kV/mm ➢ Allowed ratio between test voltages and operational voltages for MV 4 times and for HV 2 times higher than for EHV XLPE cables ➢ Test methods using higher overvoltages can be applied for MV cables ➢ Using these methods for HV and EHV cables test voltages would have to be reduced, in turn reducing sensitivity of test

Continuous Alternating Voltage Test IEC 60060-3 (2006) High-voltage test techniques Part 3: Definitions and requirements for on-site testing

AC Resonance Testing Series Resonant Circuit

AC Resonance Testing ACTC = conventional test Transformers with Compensating reactors ACRL = Resonant circuits based on tuneable HV reactors (inductance)

ACRF = Resonant circuit based on HV reactors with fixed inductance tuned into resonance by variable Frequency

Circuits for AC test voltage generation 1– test object 2 – voltage divider/coupling capacitor 3 – test transformer 4 – compensating reactor 5 – regulating transformer 6 – switchgear, 7 – control and measuring unit 8 – tuneable reactor 9 – exciter transformer 10 – fixed reactor

AC Resonance Testing

Source: HIGHVOLT

AC Resonance Testing

Source: HIGHVOLT

AC Resonance Testing

Source: HIGHVOLT

1 – Power inverter (400 kVA) 2 – Power inverter (200 kVA) 3 – Fiber optic links 4 – Three identical exciter transformers 5 – Resonant reactors ((110 kV / 194 A) 6 – Voltage divider 7 – Test objects (Cables under test)

AC Resonance Testing

Source: HIGHVOLT

Damped AC System IEC 60060-3 (2006) High-voltage test techniques Part 3: Definitions and requirements for on-site testing

Damped AC System Features ➢ Combination of Resonant and Pulse Technologies ➢ DAC based field test equipment capable of High Test Capacitance @ HV (highest test capacitance 13μF @ 350kV Peak) ➢ DAC wave shape to allow PD testing within power frequency range (20-300Hz) ➢ DAC wave shape simultaneously displays PDIV and PDEV ➢ DAC wave shape visualizes & estimates Dielectric Losses (tan ∂) ➢ DAC represents very low “risk” of cable due to short HV pulse exposure

Summary ➢ Cost effective method to PD test larger, longer & higher voltage cables

Damped AC System Working Principle

Damped AC System Block Diagram DAC Technology

Damped AC System

Damped AC System

Damped AC System PDIV

can be detected simultaneously

PDEV Dielectric Losses = tan∂ can be calculated from attenuation of wave shape

Damped AC System

Damped AC System

VLF System IEC 60060-3 (2006) High-voltage test techniques Part 3: Definitions and requirements for on-site testing

VLF System Features ➢ Makes use of the VLF technology to reduce “reactive” power ➢ Most effective technology for small test capacitance @ fairly modest test voltages ➢ Only suitable wave shape to perform tan∂ measurements ➢ “Electrical Stress” dV/dt approx.500 x less compared to 50Hz ➢ Higher test capacitances are achieved by reducing the test frequency from 0.1Hz to as low as 0.01Hz, ➢ Caution: Results not directly comparable between different frequencies. Stress Level dV/dt very different & # of cycles different

Summary ➢ The only wave shape for VLF tan ∂ measurement, ➢ Cost effective for smaller & lower voltage type cables

VLF System Working Principle

VLF System The Growth Rate of Electrical trees depends on the Frequency

All test voltage values in RMS

VLF System Dissipation Factor tan ∂ of New and Serviced aged XLPE Cables

VLF System

VLF System

Comparison of Different Tests

Comparison of Different Tests Norms

AC Resonance

DAC

VLF

DC

IEC 60060-3 (2006)

√

√

√

√

IEC 60502-2(2014)

√

√

√

IEC 60840 (2011)

√

IEC 62067 (2011)

√ √

IEC 60229 (2007) IEEE 400 - 2012

√

√

√

√

IEEE400.1 - 2007

√

IEEE 400.2 - 2013

√

IEEE 400.4 - 2015 PD/TD possibility

√

√

Portability Power consumption

√

√

√ √

√

√

√

IEC Standards ❑

IEC 60060-3 (2006) ➢ High-voltage test techniques ➢ Part 3: Definitions and requirements for on-site testing

❑ IEC 60502-2(2014) ➢ Power cables with extruded insulation and their accessories for rated voltages from 1kV (Um =1.2kV) up to 30kV (Um =36kV) ➢ Part 2: Cables for rated voltages from 6kV (Um =7.2kV) up to 30kV (Um =36kV) ❑ 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 voltage above 150 kV up to 500 kV ➢ Test methods and requirements ❑ IEC 60229 (2007) ➢ Electric cables – Tests on extruded oversheaths with a special protective function

IEC 60060-3 (2006) ❑

Due to a variety of external factors not present in factory and laboratory tests such as external electric and magnetic fields, weather conditions, etc.

❑

Apply for electrical equipment with a highest voltage Um greater than 1 kV

❑

On-site high-voltage tests are required: ➢

➢

➢

As withstand tests as part of a commissioning procedure on equipment to demonstrate that transport from manufacturer to site, and the erection on-site complies with manufacturer’s specification; As withstand tests after on-site repair, to demonstrate that the equipment has been successfully repaired, and is in a suitable condition to return to service; For diagnostic purposes, e.g. PD measurement, to demonstrate if the insulation is still free from dangerous defects, and as an indication of life expectation

IEC 60060-3 (2006) ❑

Provide information about: ➢ ➢ ➢ ➢ ➢ ➢

❑

General Definitions Test voltage Measurement of the test voltage Tests and checks on measuring systems Test procedure

Types of voltage to be performed on site: ➢ ➢ ➢ ➢ ➢ ➢

Direct voltage; Alternating voltage; Lightning impulse voltage of aperiodic or oscillating shape; Switching impulse voltage of aperiodic or oscillating shape. Very low frequency voltage; Damped alternating voltage.

IEC 60502-2(2014)

IEC 60502-2(2014)

IEC 60840 (2011)

IEC 60840 (2011)

IEC 62067 (2011)

IEC 62067 (2011)

IEC 60229 (2007)

IEEE Standards ❑

IEEE 400 - 2012 (Omnibus) ➢

❑

IEEE400.1 - 2007 ➢

❑

IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) (Less Than 1 Hz).

IEEE 400.3 - 2006 ➢

❑

IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage.

IEEE 400.2 - 2013 ➢

❑

IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems Rated 5 kV and Above.

IEEE Guide for PARTIAL Discharge Testing of Shielded Power Cable Systems in a Field Environment.

IEEE 400.4 - 2015 ➢

IEEE Guide for Field Testing of Shielded Power Cable Systems Rated 5 kV and Above with Damped Alternating Current (DAC) Voltage.

IEEE 400 - 2012 ❑

Suggested steps for field testing and evaluation of shielded power cable systems a)

Identify testing objectives.

b)

Identify cable systems to be tested.

c)

Review specifications and operating conditions of cable and cable system components to be tested.

d)

Select and apply suitable field tests.

e)

Record information or documentation for analysis.

f)

Perform recommended corrective actions on cable system.

IEEE 400 - 2012 ❑ ❑

Tests: For the purpose of this guide, several test categories are considered: From the application point of view, there are three categories of tests: ➢

Installation test: ➢ ➢

➢

➢

Acceptance test: ➢ ➢ ➢

➢

A field test conducted after cable installation but before the application of joints or terminations. Intended to detect shipping, storage, or cable installation damage with the advantage of testing cable sections only. Care should be taken to have a proper interface for tests at cable ends to avoid excessive leakage or a possible flashover. Temporary terminations are generally required. A field test made after cable system installation, including terminations and joints, but before the cable system is placed into normal service. To demonstrate that the transportation, handling and installation have not damaged the cable system components; To identify poor workmanship as well as to demonstrate that the equipment has been successfully repaired after an on-site repair of new components and significant defects in the insulation have been eliminated.

Maintenance test: ➢ ➢ ➢ ➢

A field test made during the operating life of a cable system. To assess the present condition of in-service cable systems Test data serve as reference for future evaluation and be used for trending to enhance diagnostics. Test data on other cables similar in design and service conditions can be used to establish decision criteria

IEEE 400 - 2012 ❑

From the technical point of view, there are five broad sets of tests: ➢

Diagnostic test: A field test made during the operating life of a cable system to assess the condition of the cable system and, in some cases, locate degraded regions that can result in a failure.

➢

Non-monitored or simple withstand test: A diagnostic test in which a voltage of a predetermined magnitude is applied for a predetermined time duration. If the test object survives the test it is deemed to have passed the test.

➢

Monitored withstand test: A diagnostic test in which a voltage of a predetermined magnitude is applied for a certain time period. During the test, other properties of the test object are monitored to help determine its condition and also evaluate if test duration needs to be extended or may be reduced.

➢

Offline testing: The cable system under test is disconnected from the service power source and energized from a separate field test power supply.

➢

Online testing: The cable system under test is energized by its normal service power source, usually at 50 Hz or 60 Hz. This type of test enables temporary or permanent monitoring.

IEEE 400 - 2012 ❑

Field testing methods: a)

Voltage withstand

b)

Dielectric response ❖

❖ ❖ ❖ ❖

c)

Dissipation factor (tan delta) Leakage current Recovery voltage Polarization/Depolarization current Dielectric spectroscopy

Partial discharge ❖ ❖

Electrical measurement Acoustic measurement

d)

Time-domain reflectometry

e)

Thermal infrared imaging

IEEE 400.1 - 2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage ❑

Point standard for testing laminated insulated cables with HVDC

❑

Includes testing procedures

❑

Provides guidelines for test voltages

❑

Methods of evaluation

❑

➢

Current-time relationship

➢

Resistance values

Not recommend for service aged solid dielectric cables

IEEE 400.2 - 2013 IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) (Less Than 1 Hz) Point standard for testing laminated insulated cables with HVDC ❑

Point standard for VLF withstand tests

❑

Presents rational for VLF versus DC

❑

Test parameters for tan delta

❑

Test values in appendix

IEEE 400.3 - 2006 IEEE Guide for Partial Discharge Testing of Shielded Power Cable Systems in a Field Environment Point standard for testing laminated insulated cables with HVDC ❑

Background information on partial discharge detection and location

❑

Interpretive guidance provided

❑

Technology has improved sensitivity of measurements

❑

Very good and very bad cables identified

❑

Remaining life cannot be predicted with great accuracy

IEEE 400.4 - 2015 IEEE Guide for Field Testing of Shielded Power Cable Systems Rated 5 kV and Above with Damped Alternating Current (DAC) Voltage ❑

Provides for use of damped alternating current voltages for field testing

❑

Guidelines for evaluation of test results

❑

DAC applications advanced diagnostic testing

❑

Most common use partial discharge and dissipation factor

MEA Standards for Cable Tests (115 kV)

MEA Standards for Cable Tests (24 kV)

MEA Standards for Cable Tests (24 kV)

MEA Standards for Cable Tests (24 kV)

MEA Standards for Cable Tests (24 kV)

“Thank you”

[email protected]