Surge Arresters

Surge Arresters Gaps and gapless Silicon Carbide and Metal Oxide Class IEEE C62.11-1999 Gaps and gapless Silicon Carbide and Metal Oxide Class Tests and Ratings Installation Field testing and Failures

No Gap: I=kVa

Series Gap:

Shunt Gap:

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

Silicon Carbide Arrester Valve Block Gap Elements Gap Elements

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

Brown is down!! GE Thyrite Westinghouse LVS

IEEE C62.11-1999 Arrester Class:

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!)

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

Arrester class MOV Design Tests

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

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 C62.11-1999

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:

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

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

Switching Surge Protective Levels: A discharge current of 45-60 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

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

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

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 C62.11-1999

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.

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.

Metal Oxide Arrester Ratings:

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 3.5 - 4.0 P.U. for multiple reflections depending on damping (R): 3.5 P.U. Trapped charge = -1.0 P.U.

Transmission Line Discharges:

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

Surge impedance Line Length

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

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:

Arc Chutes:

Seal plates Arc Chutes:

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

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

Arrester base left on structure after failure.

Typical 46 kV MOV Arrester Polymer Housing

Failed Polymer Arrester

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

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

Installed 138 kV Arrester

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

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

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

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

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.

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

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.

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.

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

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 0.357 0.060 Mid 0.164 0.059 Bot 0.318 0.083

Test Results on the Arrester in Question Z Phase results (4/28/2003) mA Watts Z Top 0.376 0.142 Z Mid 0.309 1.550 Z Bott 0.034 0.211

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 section was still different from a typical reading. The decision was made to replace the arrester assembly.

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

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)

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

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

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

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.

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

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

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.

Infra-red Image of Arresters

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

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.

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.

Thanks for your attention…… Your substation support staff! Luxor substation - 1936