60- Surge Arresters

7/28/2019 60- Surge Arresters

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Surge Arresters


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Surge Arresters

Gaps and gapless

Silicon Carbide and Metal Oxide

Class

Tests and Ratings Installation

Field testing and Failures

IEEE C62.11-1999


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I=kVa

No Gap:


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Series Gap:


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Shunt Gap:


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I=kVa

Silicon Carbide Arresters:

Silicon carbide blocks

Duty Cycle Rating

Protective level

ma Ln Current ka

A

rrestervoltage

Metal Oxide Blocks

Gradingcircuitr

y

Series Gaps

Operating voltage


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Silicon Carbide Arrester

Valve Block

Gap

Elements

Gap

Elements


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Silicon Carbide Arresters

Blocks cannot conduct continuously Series gaps

Fast transients cause the series gaps to short over and insertthe silicon carbide blocks

Gaps must reseal after the arrester operates (gradingcircuitry)

Duty cycle rating is the maximum 60hz voltage where thegaps can still re-seal against power follow current

Subject to external contamination

Doble study shows that 50% of silicon carbide arresterstested cannot meet original protection characteristics

Problem with moisture contamination and gaps changingcharacteristics

Oldest SiC arresters do not have a pressure relief rating


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Arrester

Class:

IEEE C62.11-1999


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Arrester Class:

The key test for determining class is the Pressure Relief Test: Arrester must vent at or below the rated current for both the high and

low current values

Parts of the arrester must not fall outside a circle with the radius equalto the height of the arrester (it can fall down!)


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Metal Oxide arresters

Station Class Metal Oxide arresters were firstintroduced around 1980 for transmissionapplications

Originally three varieties: GaplessWestinghouse 4 discs

Shunt gapGeneral Electric 3 discs

Series gapOhio Brass 3 discs

Today all station class arresters are gapless Intermediate and distribution ratings introduced in

mid 80s (gapless)

Polymer housings introduced in 90s


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MOVDesign

Tests

Arrester class


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Tests and Ratings

Protective Characteristics

Discharge current

Lightning impulse

Switching impulse Arrester Survival

MCOV

Temporary over-voltage (TOV)

Duty Cycle (accelerated aging) Transmission line discharge

Pressure Relief tests (arrester class)

Porcelain vs polymer


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Discharge Current:The surge current that flows through an arrester.

In a gapless arrester the peak voltage that appears across the

arrester at the discharge current is the protective level.A series of 8/20 current waves are used with the peak

amplitudes listed below:

1500a

3000a 5000a

10000a

15000a (500kv only)

20000a (distribution & subtransmission

- unshielded)

40000a

These points are used to compare to the equipment BIL.

The manufacturers published information shall state for each

arrester rating the maximum discharge voltage for each

discharge current listed. IEEE C62.11-1999

IV


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A typical discharge current called the classifying

current is used to determine the lightning and

switching surge protective levels. These currentsvary depending on the nominal system voltage:


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Lightning Protective Levels:

LPL - An 8x20 lightning impulse discharge current ispassed through the arrester to determine the dischargevoltages.

The current magnitude is the classifying current for the

appropriate system voltage. It simulates the currentmagnitude and shape that the arrester would have to shuntto ground due to a lightning stroke coming in on the 138kvline.

Example: an arrester applied on a 138kv system should usea 10ka 8x20 u-sec classifying current. This produces164.9kv at the arrester. This is the protective level (LPL).

The voltage protective level coordinates with theequipment BIL withstand value


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Front-of-Wave Protective

Levels:

FOWThree current impulses (1 u-sec, 2u-sec and 8 u-sec rise) are passed throughthe arrester and the three crest voltages are

plotted against time. Again the current magnitude is the

appropriate impulse classifying current.

The Front-of -Wave protective level is thepoint on the curve at .5 u-seconds

This protective level coordinates with theequipment chopped wave withstand


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Switching Surge Protective

Levels:

A discharge current of 45-60 u-secs rise

time is passed through the arrester todetermine the discharge voltage

The magnitude is the switching surge

classifying current for the appropriatesystem voltage

This protective level coordinates with theequipment switching surge withstand


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Tests and Ratings

Protective Characteristics

Discharge current

Lightning impulse

Switching impulse Arrester Survival

MCOV

Temporary over-voltage (TOV)

Duty Cycle (accelerated aging) Transmission line discharge

Pressure Relief tests (arrester class)

Porcelain vs polymer

l


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Be sure MCOV value is correct

Class must also be

Correct

MOV Nameplate


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MCOVMaximum Continuous Operating Voltage

rating is the maximum designated root-mean-squared(rms) value of power frequency voltage that may be

applied continuously between the terminals of the

arrester.

(Note this is phase to ground rms volts!)

Example 145kv to ground = 83.7kv so the minimum

MCOV for our 138 kv system is 84kv

This is the most important criteria for correctapplication

IEEE C62.11-1999


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Temporary Over-voltage Curves: MOVs can

tolerate voltages over MCOV for short periodsCheck the actual manufactures curves for each arrester.

Note curves for prior duty and no prior duty. The prior dutycurve is for previous transmission switching duty.


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Duty Cycle RatingArrester is raised to an

elevated 60hz voltage (duty cycle rating) andoperated 20 times at the impulse classifying current.

If it doesnt go into thermal runaway it passes the

test. Basically this is equivalent to the old duty cycle

rating for silicon carbide arresters.

Example: an 84kv MCOV translates to a 108kv duty

cycle rating. A 98kv MCOV is a 120kv duty cycle.

This test coupled with the high current discharge testsimulates accelerated aging of the blocks.


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Metal Oxide

Arrester Ratings:


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Switching

Surges:

Z0= L/C

Voltage doubles when closing in on an

open line = 2 P.U. at open line terminal

Assume that High Speed Re-closing traps a negative 1 P.U. charge on the line.

Then when the breaker re-closes the maximum voltage at the open end can

approach a maximum of 3.5 - 4.0 P.U. for multiple reflections depending on

damping (R):

Trapped charge = -1.0 P.U.

3.5 P.U.


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Transmission Line Discharges:


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When an arrester discharges a switching surge the

blocks heat up. Switching surges last much longer

than lightning surges and so the arresters must

dissipate more energy. Repetitive discharges can cause the arrester to fail

if there isnt enough time between to allow for

cooling

The transmission discharge test assures the

arrester will tolerate a standard amount of energy


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Surge impedance Line Length


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The arrester is subjected to 20 surges:

Six consecutive-one minute to cool-six more-

one minute-six more-one minute-two more

The arrester passes if:

discharge test is successful

Power loss is within specs (leakage current)

Transient Network Analysis studies use a value of7 kilojoules/kv of MCOV rating for transmission

arresters


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Pressure Relief Tests:

If an arrester fails internally the arc creates rapidly expanding

gasses that can cause the housing to explode violently unless

the pressure is vented. Arresters are rated on the fault current

magnitude that can pass through the housing. They must vent

successfully at or below the rated current:


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Arc

Chutes:


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Arc

Chutes:

Seal plates


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Failed 396kv arrester at Black Oak substation

Failed from prolonged 60hz over-voltage:


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This arrester actually failed according to the

standard. The pieces didnt scatter very far!

rrester ase e t on structure


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rrester ase e t on structureafter failure.

T i l 46 kV MOV A


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Typical 46 kV MOV Arrester

Polymer Housing


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Failed Polymer Arrester


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Arrester Installation

Groundingcontinuous conductor

Better ground improves arrester

performance

Shortest ground lead length Can monitor leakage current if the lead is

insulated

Lead length & ground lead

Corona rings/Clearances Arc chutes

SiC change-outs


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Arc

Chutes:

Arc chutes should face

away from other

equipment or bushings


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Installed 138 kV Arrester


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Infrastructure

Silicon Carbide ArresterReplacement Program

Change out old SiC arresters on xfmrs starting in2006 during 5 year gauge inspections (xfmrs138kv and above)

Replace old arresters as part of xfmr/breakerchange-outs and pin/cap insulator replacements

If a SiC arrester fails, change out all 6

Dont return SiC arresters to stock

Arrester


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Arrester

Maintenance, Field

Tests & Failures

Cant check protectivelevels in the field!

Moisture intrusion

Leakage current

Power factor

Megger Thermovision

Visual inspections

Failed arresters

L k C t


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watts

vars

Leakage Current:Measure leakage current with the arrester energized

Increasing resistive component of

the leakage current indicates blocks arefailing (losses are proportional to i2)

ma


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Inspection While Out of service:

Weather Tight Housing

Check for cracks in the porcelain or tears or bulges in the

polymer.

Clean all external surfaces of the arresterCoat all external weather tight housing surfaces with

silicon grease to aid in water shedding if environment

is harsh.

Check and clean the ground connections

Inspection and Prep for Testing:


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Effects of Contamination:

Contamination causes an unequal voltage

distribution across the outside surface of

the arrester.

In arresters with internal gaps and grading

circuitry this can also cause an imbalance

of voltage across the gaps and results in

improper operation and prematurefailure of the arrester.

P F t


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watts

vars

Power Factor:Apply 10kv to terminal and measure

leakage current

Resistive component of leakage currentindicates internal moisture contamination

10 kv

P F T


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Electrical Testing

Power Factor Testing

Should perform upon installation to establish benchmark.

This test is generally more effective on Silicon Carbide arresters than

MOV arresters in detecting internal contamination or breakdown of

spark gaps or valve blocks.

Make the measurement with the highest voltage available on the test

set without exceeding the line to ground voltage of the arrester undertest.

The values that are measured are the leakage current (less than 3

milli-amperes) and watts loss (less than 150 mW). These tests can

only be read utilizing a 10kV power factor test set.

Power Factor Test:


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Example:

During Class A Maintenance on the No. 2transformer at Doubs Substation, theSubstation Crew decided to perform Power

Factor Testing on the high side arresters. Testing revealed an abnormal test pattern on

the Z phase arrester

The next slides show the test results of aneighboring similar arrester as well as theresults from the arrester in question.


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Arrester Details

Nameplate Data of Arrester

ABB

EXLIM

StyleT396SA318AAP

Serial No. 00M3001

T t R lt


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Test Results

on a Good Arrester

All tests were performed with the Doble Power

Factor test set at 10 kV.

Phase results (comparable to all arresters tested ofthe same make and model)

mA Watts

Top 0.357 0.060Mid 0.164 0.059

Bot 0.318 0.083

T t R lt


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Test Results

on the Arrester in Question

Z Phase results (4/28/2003)

mA WattsZ Top 0.376 0.142

Z Mid 0.309 1.550

Z Bott 0.034 0.211

Retest Results


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Retest Results

on the Arrester in Question

Phase results (4/30/2003) after cleaning surfaces

mA Watts

Top 0.360 0.113

Mid 0.236 0.948

Bott 0.311 0.010

The retests did show a slight improvement of the

readings after the cleaning. The middle sectionwas still different from a typical reading. Thedecision was made to replace the arresterassembly.


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Test Results

on the Arrester in QuestionPhase results (4/30/2003) on the ground after

disassembly

mA WattsMid 0.384 1.471

Bott 0.317 0.082

The tests on the ground showed that the bottom

section was of typical values but the middlesection was still different from the typicalreadings. The arrester was sent to ABB inYoungwood, PA for further investigation.

ABB Investigation


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ABB Investigation

6/3/2002

ABB Received the arrester sections and performeda voltage test on all 3 sections.

The test applies the rated voltage to each section

of the arrester. (118 kV for the top 2 sections and82 kV for the bottom section). The resultingleakage current is then read. The expected leakageis less than 1 mA.

The top section and bottom section passed the test. The middle section failed the test when the applied

voltage was only 44 kV (expected to reach 118kV)

ABB Investigation


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ABB Investigation

6/3/2002

Top of Middle section Arrester with Retaining

plate removed and seal plate exposed.


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ABB Investigation


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ABB Investigation

6/3/2002

A close inspection of the seal plate revealed a

crack in the plate.

ABB Investigation


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ABB Investigation

6/3/2002

Removal of first MOV Disc. Notice the

surface contamination on the disc

ABB Investigation


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ABB Investigation

6/3/2002

Removal of entire stack of

MOV Discs. Notice the

surface contamination on all

the discs

Megger Test:


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Electrical Testing Insulation Resistance

An arrester is to act as an insulator a majority of its in service life. Itwill only allow current to flow to ground during high voltage transientsgenerally caused by lightning.

Make the measurement with the highest voltage available on the testset without exceeding the line to ground voltage of the arrester undertest.

Readings should be comparable to similar arresters. The valueshould be greater than 200 Mega Ohms. This test is generally moreeffective on Silicon Carbide arresters than MOV arresters indetecting internal contamination or breakdown of spark gaps or valveblocks.

MOV arresters may show a high insulation resistance value after anoperation but still be failed as an open circuit.

Megger Test:


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Infra-red Image of Arresters

*>24.3

*


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Inspection While in service :

Infra red Imaging

Infra red imaging of in-service arresters may detect damagedarresters

Arresters while in normal service only conduct a few milli-amps of current, therefore, will not produce heat.

If the arrester images indicate a rise in temperature fromambient temperature of 5 degrees Celsius or more,replacement should be considered.

The blocks are probably starting to fail and are conductingtoo much current

Infrared Imaging

Visual Inspection


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Inspection While in service:

Weather Tight Housing The weather tight housing is the covering of the arrester

Generally produced from porcelain or polymer.

Check for cracks in the porcelain or tears or bulges in the polymer.

Make certain porcelain or polymer appears clean and free ofany debris

Visually inspect the ground connections from the base of thearrester. A missing connection will not allow the arrester tofunction as designed.

Multiple arrester operations with improperly sized wire may result in afailure of the ground wire.

Improperly sized ground wire may also result in fire.

Visual Inspection

Failed Arresters:


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Failure assessment Suspect Arresters

Exercise EXTREME caution while investigating problems and

handling suspect arresters. Sealed Silicon Carbide and MOV

arresters may be under pressure due to a build up of fault

gasses. Visually inspect suspect arresters carefully while looking for

burn deposits near arc chutes on Silicon Carbide arresters.

MOV arresters may show very subtle signs of failure such as

deformation of the polymer covering. Generally, MOV

arresters will fail and split the polymer covering or fracture theporcelain cover.

Failed Arresters:


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Thanks for your attention

Documents

60- Surge Arresters