Insulation-coordination-fundamentals

Insulation Coordination Fundamentals

Nema 8LA Rev 0 01-16-2019

Webinar Outline 

Initial basics of Insulation Coordination Studies 

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Definitions , Types, Parameters, Purposes

Examples of an Insulation Coordination Study 

Basic Substation , Complex Substation, Transmission Line

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BIL,BSL

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The Backflash

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Traveling Wave Phenomena

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Arrester Fundamentals

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Margin of Protection

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Ground Flash Density

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The Report

Resources for this Webinar 1. Book: “Insulation Coordination of Power Systems” by Andrew (Bob) Hileman, 1999. 2. AR Hileman Software 3. ATP and ATP Draw, XY Plot 4. IEC 60071-1,2,3,4 5. IEEE C62.82.1 and .2 Formerly 1313.1 and .2 (Insulation Coordination Standards) 6. IEEE C62.11 Arrester Test Standards 7. IEEE C62.22 Arrester Application Guide 8. IEEE 1410 and 1243 Improving Lightning Performance of lines

Definition of Insulation Coordination Simple Definition Insulation coordination is the selection of the insulation strength of a system. (Hileman) Better One Insulation coordination is the process where the insulation characteristics of all components of the power system are determined, specified and coordinated to avoid failure due to expected internal and externally occurring surges. (Hileman)

Insulator Arrester

Types of Insulation Coordination Studies  Transformer Protection  Substation Protection Open Air and GIS  Line Protection 

Distribution and Transmission

 Breaker Protection  Generator Protection  Determine clearances  Determine Separation Distances  Determine Arrester Energy and Voltage Ratings.  And on and on and on

Types of Insulation Coordination Studies  Deterministic This is the conventional method where the minimum strength of the insulation is equal or greater than the maximum surge stresses. . Transformer insulation is not statistical in nature. It has one lightning withstand value and one switching withstand value. Therefore a deterministic analysis is all that we can do.

Types of Insulation Coordination Studies  Probabilistic This type of analysis consists of selecting the insulation level and clearances based on specific reliability criterion. Since the insulation strength of air is statistical in nature, we can only determine its probability of Flashover for a given surge. Studies of transmission line performance is based on a flashover rate per year per 100km, and because the flashover parameter is statistical, resulting levels are probabilistic. Studies of substation performance is also probabilistic for the same reason. For this type of study we base the performance on MTBF (Mean Time Between Flashover). More later on this.

Types of Insulation Coordination Studies  Lightning Surge Studies This type of study deals strictly with lightning surges and backflash over surges. Is completed for all system voltage levels.  Switching Surge Studies This type of study is usually for systems above 240kV since it is this type of system that can produce switching surges of relevance. If a lower voltage system has large cap banks, then a switching study is justified.

Parameters of Importance in Studies • Purpose of Study • The Lightning Flash • Ground Flash Density • Shield Failure rate if known • Types of Insulation • BIL and CFO • MTBS and MTBF • Location and Altitude of Study • Cable and Isophase specs

• • • • • • •

Incoming Surge Steepness Backflash Rate (BFR) Calculating BFR Tower Configurations Circuit Physical Dimensions The Transformer Ratings and Capacitance The Arrester • VI Curve • Selecting the Rating

Purpose of Insulation Coordination Studies  Can be to design proper insulation and arrester location from scratch  Can be to validate chosen insulation levels (Very common)  Can be to determine where to locate arresters  Can be to determine cause of failure of equipment (After an incident)  Can be to determine the Width of a ROW (Switching Study)  Can be to provide assurance that equipment is protected properly  Can be to put in the file for future reference  Can be to fulfill a requirement  Can be to …………. and more……

Examples of Lightning Studies  Simple Substation from Chapter 12 of “Insulation Coordination of Power Systems”.  500kV Line-Substation-Generator  69kV Line Study

Overhead Shield Wire

Simple Substation Station Arresters

Disconnect Switch Breaker

CT or CCVT Power Transformer

Basic Substation Lightning Study

Incoming Surge Surge at Trans

Complex Study

Three generators

Complex Insulation Coordination Study Incoming Line

Switchyard with no transformers

Cross over line to Generator Station

3 generator step up Transformers

69kV Sub

69kV Sub Transmission Line Study

69kV Sub Transmission Line Study

Insulator that flashes over at a specific voltage

Underbuilt Circuit

69kV Sub Transmission Line Study

System Fundamentals Relative to Insulation Coordination 1. Insulation

7. Physical Dimensions

2. Traveling Waves and Reflections, Backflash, and Separation Distance

8. Ground Flash Density

3. Tower Grounds and Station Grounds 4. Corona 5. Steepness of Surges 6. Clearances

9. OHGW 10. Ground Flash Density

Types of Insulation  External Insulation The distance in open air or across the surfaces of solid insulation in contact with open air that is subjected to dielectric stress and to the effects of the atmosphere. Examples are porcelain or polymer shell of a bushing, support insulators, and disconnecting switches.

Self restoring Insulator

Underground Cable with Non-Self Restoring Insulation

 Self-restoring Insulation Insulation that completely recovers insulating properties after a disruptive discharge (flashover) caused by the application of a voltage. This is Terminator with generally external insulation.

Self-restoring Insulation on outside and non-self-restoring on inside

More On Insulation Non-Self Restoring Insulation Self Restoring Insulation

 Internal Insulation The internal solid, liquid, or gaseous parts of the insulation of equipment that are protected by equipment enclosures from the effects of the atmosphere. Examples are transformer insulation, internal insulation of bushings, internal parts of breakers and internal part of any electrical equipment.  Non-self-restoring Insulation Insulation that loses insulating properties or does not recover completely after a disruptive discharge caused by the application of voltage. Generally internal insulation.

Insulation BIL Basic Lightning Impulse Insulation Level (BIL) The BIL level is the Dry insulation withstand strength of insulation expressed in kV. Is commonly used to describe substations and distribution system voltage withstand characteristics.

Insulator BIL is directly proportional to the strike distance of an insulator BIL ≈ 15kV x S(inches) And is affected by Altitude

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Statistical BIL is used for insulators means there is a 10% probability of flashover and is used for self-restoring insulation

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Conventional BIL is used for Transformers and Cable is the voltage level where there is a 0% probability of Flashover and is applied to non selfrestoring insulation

Note 1: Arresters do not have a BIL rating since their external insulation is self protected by the internal MOV disks. In a sense they have an infinite BIL.

Note 2: Arresters close to an insulator give the insulator infinite BIL.

BSL  Basic Switching Impulse Insulation Level (BSL) The BSL level is the switching surge withstand level of the insulation in terms of kV. BSLs are universally tested under Wet conditions. 

BSL is proportional to the strike distance of an insulator BSL= 1080e((0.46 x Strike Distance) + 1) And is affected by Altitude

Statistical BSL of Insulators

apply to self restoring insulation and represents a 10% probability of flashover. 

Conventional BSL of Transformers and solid dielectrics

apply to non-self-restoring insulation and represents a 0% probability of flashover

Note 1: Arresters do not have a BSL rating since their external insulation is self protected by the internal MOV disks. In a sense they have an infinite BSL.

Note 2: Arresters close to an insulator give the insulator infinite BSL.

Power Frequency Withstand Power Frequency Withstand Voltage This is the highest power frequency voltage an insulator can withstand under wet conditions (low level of contamination). It is affected by creepage distance and strike distance.

Note 1: Insulator withstand voltages are often >2-3 times their operating voltage. Note 2: Arresters will go into conduction if the AC voltage across the unit reaches a 1.25 pu MCOV and above. However they cannot sustain this condition for very long or they will over heat and fail.

Note 3: If the housing is highly contaminated, the housing may flashover at levels below the turn-on voltage of the arrester. Note 4: In highly contaminated areas, extra creepage distance insulators are used to overcome this potentially low flashover voltage. The same policy should be applied to arresters.

CFO and CWW  Critical Flash Over (CFO) Self Restoring insulation only This is the voltage with a 50% probability of flashover of the insulator. It applies to both lightning and switching. It is used to quantify insulation used on transmission and distribution lines. Typically CFO is 4-6% higher than Statistical BIL on an insulator.  Chopped Wave Withstand (CWW) This is a withstand level of equipment. A standard lightning impulse is used but the surge is chopped at 3us, which means the stress is applied for a much shorter time than a standard lightning impulse test and must flashover near the crest of the wave instead of on the tail as it can in BIL tests. The value of this characteristic is about 1.10 times BIL for power transformers and 1.15 times BIL for bushings.

Caused by insulator flashover just past crest. Can cause winding to winding stress in some transformers

Insulation Withstand Characteristics in Graphic Form Typical Values 70-1500kVp

CWW Chopped Wave Withstand

BIL Basic Impulse Withstand Level Another form of Lightning withstand is CFO Critical Flashover Voltage

BSL Basic Switching Impulse Withstand Level

The Backflash When the OHGW on a transmission line is hit by lightning, a rapid series of events takes place. If the system is grounded well than the surge is transferred to earth and there is no effect on the phase conductors. But occasionally a backflash will occur, this series of slides will show you a close up view of the sequence of events.

The Backflash Time = 0 The first event is the strike. Of course there was already a great deal of activity just to connect this line to the cloud, but that is for another sequence. When the strike pins to the wire, it sets up a voltage surge that travels in both directions down the line. (1-50 million volts) This is all happening at nearly the speed of light and until the surge actually finds ground, there is little current flow.

The Backflash CFO Induced

Induced

Time = 1 In a few Nano-seconds, the voltage front meets the down ground and travels toward earth at the tower bottom. While at the same time it is inducing a voltage on to the phase conductors When it reaches earth, the current begins to flow. The voltage along the tower increases rapidly due to ground potential rise. This potential rise is caused by the resistance of the ground rod of the tower. This tower voltage rises as the current begins to flow.

The Backflash CFO

Time = 2 The voltage at the base of the base of the insulators and on the phase conductors increases as the surge increases in amplitude If the voltage at the base of the insulator increases at a faster rate than the induced voltage on phases, it can reach the CFO of the insulator

The Backflash CFO

Time = 3 The voltages continue to increase across all components as the surge crests.

The Backflash CFO

Time = 4 (.5-2 µsec) If the voltage across the insulator exceeds the CFO, it can flashover from the pole down ground to the phase. This is the backflash…… It flashes from the base to the conductor which is intuitively backward since the down ground spends its entire life except for these few microseconds at ground potential. This is the part of the event that we are interested in with insulation coordination studies. What effect this surge will have the substation. But its not over yet…..

The Backflash

Ionized Gas

Time = 5 (20-50 µsec) The lightning stroke is over and the voltages on the lines revert back to their pre-strike levels. But the air around the insulator is seeping with ions and still highly conductive. When the AC voltage reaches a high enough level, it now flashes forward from the phase conductor to the down ground.

The Backflash

AC Follow current causing a Line to Ground Fault Until breaker interrupts

Time = 6 (50 µsec to 200ms) When the insulator flashes over for a second time, power frequency current flows to ground and a fault is now underway on the circuit and will remain there until a breaker interrupts the event. At that point the event is over assuming no damage occurred on the insulator.

The Backflash CFO

The surge that is transferred onto the phase conductor has entered the station within a few µsec, even before the fault was initiated. This is the impulse that becomes the concern of insulation coordination in substations.

The Rest of the Story on Lightning Initiated Traveling Waves

Initial Strike to OHGW

As shown earlier, every tower has a lightning current level that will cause a backflash across the phase insulation. This backflash now sets up another traveling wave on the phase conductor also heads in both directions.

This fast rising surge with a long tail travels on the phase conductor at nearly the speed of light, toward the transformer. The amplitude is approximately equal to the CFO of the line insulator.

As it reaches the arresters, its tail may still be as far back aa the original tower. Remember the surge has a long tail 50-100 µs due to the lightning. The front of the surge is perhaps .5 µs. It took less than a few microseconds to travel from the backflash to the transformer.

The surge is clamped by the arresters at the transformer and a reflection begins to occur as the wave front meets the transformer winding. The winding is like an open circuit to a fast rising surge.

Note the voltage at the transformer is clamped by the arresters.

CCVTs Arresters

The transformer is protected by the arresters, but a surge is reflected back into the system

In C62.22 there is a separation distance calculator that can be used to determine voltages at other locations in a substation. In this case the bushing of the CCVT has flashed over due to a reflection.

Also if the arresters are mounted away from the transformer, voltages at the transformer can be higher than at the arrester due to reflections 30 m separation 3 m separation

Note the voltage at the transformer is higher than at the arresters. This is due to traveling wave reflection

Separation Distance

Arresters

Red = Voltage @ Arrester Green = Voltage @ Transformer

Arresters the other half of Insulation Coordination

Arrester Definition A device that is connected between phase and earth that will clamp a surge to levels below the damage levels of nearby insulation.

What’s Inside • Polymer Housing • Metal Oxide Varistor (MOV) • Conductive Spacer

Distribution Arrester

• Strength Member (Fiberglass) • Spring for Compression • Rubber Seals • End Vents and Diaphragms

Station Arrester

Voltage Current (V-I) Characteristics

VI Characteristics of an Arrester or Disk is the essence of the MOV. The resistance of the MOV disk is a function of the voltage stress across the terminals.

Example 50kV MCOV Arrester

Typical Varistor/Arrester V-I Characteristics Pre-Breakdown Region |--------------------------------------|

Physicists Terminology |---------------------- Breakdown Region--------------------------------|

High Current Region |---------------------------------------|

SPL V10kA or U10kA

V1ma or Reference Voltage Region TOV Region

Vref or Uref Rated V or Ur peak MCOV or UC (peak)

Switching Surge Region

20C

Leakage Current Region Engineering Terminology

200C

Normal Operating Region

LPL

Lightning Impulse Region

Arrester Protective Characteristics in Graphic Form

Fast Front Voltage Arrester Discharge Voltage Curve

10kA Lightning Protective Level LPL

Faster Front Surges

Switching Surge Protective Level SPL Slower Front Surges

Calculating Margin of Protection Chopped Wave Withstand CWW

MP1= (CWW/FOW)-1

BIL Insulation Withstand Curve

IEEE recommends > .15 or 15%

Front of Wave Voltage FOW

BSL

MP2= (BIL/LPL)-1 IEEE recommends >.15 or 15%

MP3= (BSL/SPL)-1 10kA Lightning Protective Level LPL

Arrester Discharge Voltage Curve

IEEE recommends >.20 or 20%

Switching Surge Protective Level SPL

Clearances and Altitude

Clearances Phase to phase and phase to ground clearances are often the purpose of a study. They are easily calculated once the maximum voltage on a line is determined. With arresters, the NEC clearances can be reduced near the arrester and along ROW if studies are completed. For example, Lightning Impulse withstand of Air at STP is a linear function at 450kV/m

Clearance and Altitude/Elevation 1.000

Change in Withstand voltage

0.950

All external insulation is affected by altitude. Specifically in this case, the clearance between lines needs to be increased to attain the same withstand voltage at sea level.

Ratio of Altitude to Sea Level

0.900

0.850

0.800

'δ=e-A/26710 0.750

0.700

0.650

0.600 0

2000

4000

6000 Elevation in Feet

8000

10000

12000

Physical Dimensions

Elongated Substation

Backflash 6000 ft out on the line

6000ft

AFram

L_Imp I

L_imp

H

2000 ft

2000 ft

Chapter 12 Insulation Coordination of Power Systems by Andrew Hileman

V

Eb

V

LCC

V

2 m 200 m

Flashover of C-Phase close to subs tation

Ej

NC

At 20 m Arrester

Et

V

V

230/13.8 BCT Y

3m

2m

Sourc V

V

Ea

5 ohms

R (i)

25 meters

R (i)

At Station Entrance

At Breaker

Line Entrance Arrester

I R (i)

230kV

R (i)

30 0m

R (i)

I R (i)

NC

LineA

LCC

V

6.3nF

Transformer Arrester

Surges travel at ~980ft per µs on an overhead line. In this elongated station, It can be seen here that the surge first appears at the metered points at different times based on the distance from the initial surge. 2uh 2 meters 5 ohms

Ground Flash Density

Lightning Strike Rate Worldwide Optical Flash Density = Flashes/year/km 2 Divide by 3 to get GFD

Lightning Incidents and Intensity

Ground Flash Density Is used to calculate the • Backflash rate on a line • The challenge rate to a line • The outage rate of lines • Steepness of a surge on a line • The MTBF of a substation

The Insulation Coordination Study Report

Webinar Overview Subjects covered 1. 2. 3. 4. 5. 6. 7. 8. 9.

Definitions Examples of Studies Insulation Fundamentals Backflash Concept Traveling Wave Concept Arrester Fundamentals Clearances and Physical Dimensions Lighting Ground Flash Densities The Report