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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Transmission Line Discharges:
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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Transmission Line Discharges:
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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Inspection While Out of service:
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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Inspection While Out of service:
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
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