1. Surge Arresters

2. Surge Arresters Gaps and gapless Silicon Carbide and Metal Oxide Class
IEEE C
Gaps and gapless
Silicon Carbide and Metal Oxide
Class
Tests and Ratings
Installation
Field testing and Failures

3. No Gap:
I=kVa

4. Series Gap:

5. Shunt Gap:

6. Silicon Carbide Arresters:
Protective level
Silicon carbide blocks
Grading circuitry
Series Gaps
Metal Oxide Blocks
Duty Cycle Rating
Operating voltage
I=kVa
Arrester voltage
ma Ln Current ka

7. Silicon Carbide Arrester
Valve Block
Gap Elements
Gap Elements

8. Silicon Carbide Arresters
Blocks cannot conduct continuously
Series gaps
Fast transients cause the series gaps to short over and insert the silicon carbide blocks
Gaps must reseal after the arrester operates (grading circuitry)
Duty cycle rating is the maximum 60hz voltage where the gaps can still re-seal against power follow current
Subject to external contamination
Doble study shows that 50% of silicon carbide arresters tested cannot meet original protection characteristics
Problem with moisture contamination and gaps changing characteristics
Oldest SiC arresters do not have a pressure relief rating

9. Brown is down!!
GE Thyrite
Westinghouse LVS

10. IEEE C
Arrester
Class:

11. 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 equal to the height of the arrester (it can fall down!)

12. Metal Oxide arresters
Station Class Metal Oxide arresters were first introduced around 1980 for transmission applications
Originally three varieties:
Gapless – Westinghouse 4” discs
Shunt gap – General Electric 3” discs
Series gap – Ohio Brass 3” discs
Today all station class arresters are gapless
Intermediate and distribution ratings introduced in mid 80s (gapless)
Polymer housings introduced in 90s

13. Arrester class
MOV Design Tests

14. Tests and Ratings Protective Characteristics Arrester Survival
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

15. 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. I V
IEEE C

16. A typical discharge current called the “classifying current” is used to determine the lightning and switching surge protective levels. These currents vary depending on the nominal system voltage:

17. Lightning Protective Levels:
LPL - An 8x20 lightning impulse discharge current is passed through the arrester to determine the discharge voltages.
The current magnitude is the classifying current for the appropriate system voltage. It simulates the current magnitude and shape that the arrester would have to shunt to ground due to a lightning stroke coming in on the 138kv line.
Example: an arrester applied on a 138kv system should use a 10ka 8x20 u-sec classifying current. This produces 164.9kv at the arrester. This is the protective level (LPL).
The voltage protective level coordinates with the equipment “BIL” withstand value

18. Front-of-Wave Protective Levels:
FOW – Three current impulses (1 u-sec, 2 u-sec and 8 u-sec rise) are passed through the 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 the point on the curve at .5 u-seconds
This protective level coordinates with the equipment “chopped wave” withstand

19. Switching Surge Protective Levels:
A discharge current of u-secs rise time is passed through the arrester to determine the discharge voltage
The magnitude is the switching surge classifying current for the appropriate system voltage
This protective level coordinates with the equipment switching surge withstand

21. Tests and Ratings Protective Characteristics Arrester Survival
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

22. MOV Nameplate
Be sure MCOV value is correct
Class must also be Correct

23. MCOV – Maximum 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 correct application
IEEE C

24. Temporary Over-voltage Curves: MOVs can
tolerate voltages over MCOV for short periods
Check the actual manufactures curves for each arrester.
Note curves for “prior duty” and “no prior duty”. The “prior duty
curve is for previous transmission switching duty.

25. Duty Cycle Rating – Arrester is raised to an elevated 60hz voltage (duty cycle rating) and operated 20 times at the impulse classifying current. If it doesn’t 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 test simulates accelerated aging of the blocks.

26. Metal Oxide Arrester Ratings:

27. Switching Surges: Z0= L/C
Voltage doubles when closing in on an open line = 2 P.U. at open line terminal
Z0= L/C
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 P.U. for multiple reflections depending on damping (R):
3.5 P.U.
Trapped charge = -1.0 P.U.

28. Transmission Line Discharges:

29. 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 isn’t enough time between to allow for cooling
The transmission discharge test assures the arrester will tolerate a standard amount of energy

30. Surge impedance
Line Length

31. 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 of 7 kilojoules/kv of MCOV rating for transmission arresters

32. 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:

33. Arc Chutes:

34. Seal plates
Arc Chutes:

35. Failed 396kv arrester at Black Oak substation
Failed from prolonged 60hz over-voltage:

36. This arrester actually failed according to the standard
This arrester actually failed according to the standard. The pieces didn’t scatter very far!

37. Arrester base left on structure after failure.

38. Typical 46 kV MOV Arrester
Polymer Housing

39. Failed Polymer Arrester

41. Arrester Installation
Grounding – continuous 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

42. Arc Chutes: Arc chutes should face away from other
equipment or bushings

43. Installed 138 kV Arrester

44. Infrastructure Silicon Carbide Arrester Replacement Program
Change out old SiC arresters on xfmrs starting in 2006 during 5 year gauge inspections (xfmrs 138kv and above)
Replace old arresters as part of xfmr/breaker change-outs and pin/cap insulator replacements
If a SiC arrester fails, change out all 6
Don’t return SiC arresters to stock

45. Arrester Maintenance, Field Tests & Failures
Can’t check protective levels in the field!
Moisture intrusion
Leakage current
Power factor
Megger
Thermovision
Visual inspections
Failed arresters

46. Leakage Current: Measure leakage current with the arrester energized
Increasing resistive component of
the leakage current indicates blocks are
failing (losses are proportional to i2) ma
watts
vars

47. Inspection and Prep for Testing:
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 arrester
Coat all external weather tight housing surfaces with silicon grease to aid in water shedding if environment is harsh.
Check and clean the ground connections

48. 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 premature
failure of the arrester.

49. Power Factor: Apply 10kv to terminal and measure leakage current
Resistive component of leakage current
indicates internal moisture contamination
watts
vars

50. Power Factor Test: 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 under test.
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.

51. Example:
During Class A Maintenance on the No. 2 transformer at Doubs Substation, the Substation 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 a neighboring similar arrester as well as the results from the arrester in question.

52. Arrester Details Nameplate Data of Arrester ABB EXLIM
Style – T396SA318AAP
Serial No. 00M3001

53. 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 of the same make and model)
mA Watts
Top
Mid
Bot

54. Test Results on the Arrester in Question
Z Phase results (4/28/2003)
mA Watts
Z Top
Z Mid
Z Bott

55. Retest Results on the Arrester in Question
Phase results (4/30/2003) after cleaning surfaces
mA Watts
Top
Mid
Bott
The retests did show a slight improvement of the readings after the cleaning. The middle section was still different from a typical reading. The decision was made to replace the arrester assembly.

56. Test Results on the Arrester in Question
Phase results (4/30/2003) on the ground after disassembly
mA Watts
Mid
Bott
The tests on the ground showed that the bottom section was of typical values but the middle section was still different from the typical readings. The arrester was sent to ABB in Youngwood, PA for further investigation.

57. ABB Investigation 6/3/2002
ABB Received the arrester sections and performed a 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 and 82 kV for the bottom section). The resulting leakage current is then read. The expected leakage is 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 118 kV)

58. ABB Investigation 6/3/2002
Top of Middle section Arrester with Retaining plate removed and seal plate exposed.

59. Notice the corrosion and signs of moisture
ABB Investigation 6/3/2002
Under side of Seal plate after removal
Notice the corrosion and signs of moisture

60. Notice the corrosion and signs of moisture
ABB Investigation 6/3/2002
Inside arrester with Seal plate removed
Notice the corrosion and signs of moisture

61. A close inspection of the seal plate revealed a crack in the plate.
ABB Investigation 6/3/2002
A close inspection of the seal plate revealed a crack in the plate.

62. ABB Investigation 6/3/2002
Removal of first MOV Disc. Notice the surface contamination on the disc

63. ABB Investigation 6/3/2002
Removal of entire stack of MOV Discs. Notice the surface contamination on all the discs

64. Megger Test: Inspection While Out of service: Electrical Testing
Insulation Resistance
An arrester is to act as an insulator a majority of its in service life. It will only allow current to flow to ground during high voltage transients generally caused by lightning.
Make the measurement with the highest voltage available on the test set without exceeding the line to ground voltage of the arrester under test.
Readings should be comparable to similar arresters. The value should be greater than 200 Mega Ohms. 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.
MOV arresters may show a high insulation resistance value after an operation but still be failed as an open circuit.

65. Infra-red Image of Arresters

66. Infrared Imaging Inspection While in service : Infra red Imaging
Infra red imaging of in-service arresters may detect damaged arresters
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 from ambient temperature of 5 degrees Celsius or more, replacement should be considered.
The blocks are probably starting to fail and are conducting too much current

67. Visual Inspection 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 of any debris
Visually inspect the ground connections from the base of the arrester. A missing connection will not allow the arrester to function as designed.
Multiple arrester operations with improperly sized wire may result in a failure of the ground wire.
Improperly sized ground wire may also result in fire.

68. Failed Arresters: 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 the porcelain cover.

69. Thanks for your attention…… Your substation support staff!
Luxor substation