A Comprehensive Guide

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Insulation Coordination

Fundamentals

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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)

Arrester

Insulator

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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

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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.

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 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.

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 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.

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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

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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……

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Examples of Lightning Studies

 Simple Substation from Chapter 12 of “Insulation Coordination of Power Systems”.

 500kV Line-Substation-Generator

 69kV Line Study

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

Breaker Disconnect

Switch

CT or CCVT

Station Arresters

Power

Transformer Overhead Shield Wire

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Basic Substation Lightning Study

Incoming Surge

Surge at Trans

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Complex Study

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Complex Insulation Coordination Study

Incoming Line

Switchyard with no transformers

Cross over line to Generator

Station

3 generator step up Transformers

Three generators

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69kV Sub

69kV Sub Transmission Line Study

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69kV Sub Transmission Line Study

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69kV Sub Transmission Line Study

Insulator that flashes over at a specific

voltage

Underbuilt Circuit

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System Fundamentals Relative to Insulation Coordination

1. Insulation

2. Traveling Waves and Reflections, Backflash, and Separation Distance 3. Tower Grounds and

Station Grounds 4. Corona

5. Steepness of Surges 6. Clearances

7. Physical Dimensions 8. Ground Flash Density 9. OHGW

10. Ground Flash Density

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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 Insulation Insulation that completely recovers insulating properties after a disruptive discharge (flashover) caused by the

application of a voltage. This is generally external insulation.

Self restoring Insulator

Terminator with Self-restoring Insulation on outside and non-self-restoring

on inside

Underground Cable with Non-Self Restoring

Insulation

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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.

Self Restoring Insulation Non-Self Restoring

Insulation

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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.

 Statistical BIL is used for insulators means there is a 10% probability of flashover and is used for selfrestoring insulation

 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

Insulator BIL is directly proportional to the strike distance of an insulator

BIL ≈ 15kV x S(inches) And is affected by Altitude

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.

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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.

 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

BSL is proportional to the strike distance of an insulator

BSL= 1080e((0.46 x Strike Distance) + 1)

And is affected by Altitude

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.

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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 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.

Note 1: Insulator withstand voltages are often >2-3 times their operating

voltage.

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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

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CWW

Chopped Wave Withstand

BIL

Basic Impulse Withstand Level

BSL

Basic Switching Impulse Withstand Level

Typical Values 70-1500kVp

Another form of Lightning withstand is CFO

Critical Flashover Voltage

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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.

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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.

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The Backflash

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.

Induced

Induced

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The Backflash

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

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The Backflash

Time = 3

The voltages continue to

increase across all components as the surge crests.

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The Backflash

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…..

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The Backflash

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.

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The Backflash

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.

AC Follow current causing a Line to Ground Fault

Until breaker interrupts

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The Backflash

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.

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

Arresters

CCVTs

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Arresters Note the voltage at the

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

Red = Voltage @ Arrester

Green = Voltage @ Transformer 3 m separation

30 m separation

Separation Distance

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Arresters

the other half of Insulation

Coordination

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Arrester Definition

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• Polymer Housing

• Metal Oxide Varistor (MOV)

• Conductive Spacer

• Strength Member (Fiberglass)

• Spring for Compression

• Rubber Seals

• End Vents and Diaphragms

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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

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Typical Varistor/Arrester V-I Characteristics

|--- Breakdown Region----|

Pre-Breakdown Region

|---|

High Current Region

|---|

Leakage Current Region V1ma or Reference Voltage

Region

TOV Region

Switching Surge Region

Lightning Impulse

Region

Normal Operating Region

20C

200C

Physicists Terminology

Engineering Terminology Vref or Uref

V10kA or U10kA

MCOV or UC (peak) Rated V or Ur peak

SPL LPL

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Arrester Discharge Voltage Curve

Fast Front Voltage

10kA Lightning Protective Level

LPL Switching Surge Protective Level

SPL Faster Front Surges Slower Front Surges

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Insulation Withstand Curve

Arrester Discharge Voltage Curve

Chopped Wave Withstand CWW

Front of Wave Voltage

FOW

BSL BIL

10kA Lightning Protective Level

LPL

Switching Surge Protective Level

SPL

MP1= (CWW/FOW)-1

MP2= (BIL/LPL)-1

MP3= (BSL/SPL)-1

IEEE recommends > .15 or 15%

IEEE recommends >.15 or 15%

IEEE recommends >.20 or 20%

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Clearances and Altitude

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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

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Clearance and Altitude/Elevation

0,600 0,650 0,700 0,750 0,800 0,850 0,900 0,950 1,000

0 2000 4000 6000 8000 10000 12000

Ratio of Altitude to Sea Level

Elevation in Feet

Change in

Withstand voltage

'δ=e-A/26710

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.

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Physical Dimensions

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V V

30 0m NC

V

25 meters

V

2 m AFram

LineA

5 ohms

LCC R(i) R(i)

Sourc

V

2uh 2 meters 5 ohms L_imp ^H

L_Imp

LCC

I ^V

R(i)

I

230kV

200 m

NC

20 m 2 m

Ej 230/13.8

BCT Y

Et

V

Ea

R(i) R(i)

R(i)

I

6.3nF 3m

Eb

V

Chapter 12

Insulation Coordination of Power Systems by Andrew Hileman

Line Entrance

Arrester Transformer

Arrester

Flashover of C-Phase close to substation

6000ft 2000 ft 2000 ft

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.

Backflash 6000 ft out on the line

At Station Entrance

BreakerAt

ArresterAt

Elongated Substation

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Ground Flash Density

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(57)

(58)

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

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The Insulation Coordination Study Report