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Evaluation of the Effectiveness of Dissolved Gas Analysis on Bushings

Colin Clark BC Hydro

Mike Lau BC Transmission Corporation

November 2004

ABSTRACT BC Hydro has been using Dissolved Gas Analysis (DGA) to assist with condition assessment of oil impregnated paper condenser type bushings since 2001. Fourteen bushings have been repaired or replaced in the last three years due to abnormal DGA results indicating potential failures. This paper will present the DGA results together with the corresponding power factor test results and the findings of teardown investigations for suspected bushings. Examples showing the effectiveness of DGA and some of the discrepancies between the DGA and power factor results are included in the paper. INTRODUCTION Prior to 2001 power factor tests were the only routine condition assessment tests specified for bushings at BC Hydro. Over the past three years, bushings have been replaced based on DGA that would not have been identified with power factor tests alone. The potential for bushing failures and the consequential damages associated with bushing failures is increasing as the equipment population ages. The trend of overloading and operating equipment beyond nameplate ratings further increases the risk of catastrophic failures. This risk could be mitigated with condition assessment techniques and DGA of bushings has proven effective in complementing standard power factor tests. BACKGROUND Prior to 2001 oil samples were rarely taken from small volume apparatus at BC Hydro. A 500kV bushing failure at a major generating station prompted oil sampling in an attempt to have a better assessment of bushings. The initial bushing failure destroyed the transformer even though the power factor test of the failed bushing three years earlier was considered acceptable. Since 2001, over 550 oil samples from bushings at major generating stations and key transmission stations have been evaluated. Evaluation of the samples has assisted in identifying the following bushing repairs and replacements:

3 - abnormal combustible gas levels -- scrapped. 3 - abnormal combustible gas levels -- manufacturer refurbished. 2 - combination moisture ingress and combustible gas levels -- scraped. 6 - high moisture -- dry-out repairs followed with Partial Discharge/AC Withstand

tests and eventually returned to service. 2 - High C2 Power Factor without combustible gas levels (for comparison).

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A formal program has been implemented at generating stations for sampling bushings every six years during regular maintenance outages. Key transmission equipment bushings are sampled less formally. INVESTIGATION OF BUSHINGS - HIGH C2 POWER FACTOR Two bushings were replaced in the last few years based on high C2 power factor readings. Neither bushing had abnormally high combustible gas levels to support bushing deterioration.

WSN 5RX4-A: 500kV Micanite 1968, Serial 225659 The first Micanite bushing removed from service had a high C2 power factor (24.8%). Powertech Labs conducted Partial Discharge (PD) and AC Withstand Tests up to 656kV. The bushing failed the tests with a PD of 23pC at 318kV. The bushing power factors were tested before and after the PD tests with a substantial increase in C2 power factor and capacitance. Oil samples were also taken before and after the tests with mixed results. The gasses show a slight increase in H2 and consumption of O2 after the PD test. Without the C2 power factor this bushing would have been returned to service. DGA results alone would not have been sufficient to replace the bushing.

WSN 5RX4 (A) Bushing 500kV 600A Manufacturer Micanite (68) Serial:225659 Field Lab Cap & Tan Delta

Test Nameplate Feb-04 Before PD After PD C1 %PF 0.38 0.47 0.33 0.36 C1 Cap 473 472.6 475.6 476.7 C2 %PF 24.8 12.3 36.5 C2 Cap 1780 1937 2142 2169

DGA

Sample:Field Before PD After PD Gas ppm (v/v) ppm (v/v) ppm (v/v) H2 27 28 38 O2 279 5990 1640 N2 38600 70500 71100 CO 50 67 79 CO2 155 288 264 CH4 11 10 12 C2H2 < < < C2H4 1 2 2 C2H6 17 17 19 H2O 11 3 2

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WSN 5RX5-C: 500kV Micanite 1966, Serial 223304 This bushing had a C2 power factor of 3.15% compared to the previous measured power factor of 0.39%. The bushing was replaced, but upon subsequent external visual inspection, no capacitance tap problem was noted. The C2 power factor was retested and measured 0.74%. The 3% power factor could not be reproduced. DGA showed no abnormal gas levels. The discrepancy between the two C2 power factor results is inconclusive. Since both the latest C2 power factor and the DGA results are normal, this bushing is now being held as an emergency spare.

5RX5 (C) Bushing 500kV 600A Manufacturer Micanite (66) Serial:223304

Test Nameplate 1996 Jul-04 Oct-04 C1 %PF ~0.33 0.33 invalid C1 Cap 460 ~460 461.6 invalid C2 %PF 0.39 3.15 0.74 C2 Cap 1730 1761 1704

DGA Gas ppm (v/v) Gas ppm (v/v) H2 44 CH4 28 O2 212 C2H2 < N2 71000 C2H4 2 CO 231 C2H6 52 CO2 441 H2O 14

INVESTIGATION OF BUSHINGS - HIGH H2O AND C2H2 Six bushings were replaced during the initial oil sampling program at BC Hydro due to excessive moisture content. Four bushings were CGE Type U and two were Bushing Company Bushings. All six bushings were dried out, and passed the Partial Discharge and AC Withstand tests. They were eventually returned to service with no further problems. Two bushings have been replaced since the start of the sampling program due to high combustible gas levels combined with moisture.

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ROS T1-X1: 230kV CGE Type F The first bushing was a 230kV CGE Type F, replaced in 2001. The dismantling of the bushing revealed moisture ingress and tracking, but no power factor tests were available, as this bushing was not equipped with a capacitance tap.

ROS T1-X1 230kV CGE Type F Gas ppm (v/v) Gas ppm (v/v) H2 854 CH4 845 O2 9755 C2H2 2096 N2 84398 C2H4 1583 CO 495 C2H6 293 CO2 4425 H2O 64

WSN 5RX5-A: 500kV Micanite 1966, Serial 223292 49 ppm of Acetylene and 25 ppm of water were detected in the oil sample. The bushing power factors were considered marginally acceptable, with a slight increase in measured C2 power factor (0.8% to 0.99%) over one year. The capacitance tap was in good condition. Water ingress was visually apparent with heavy corrosion around the top seal and oil reservoir (Figure 1). Removal of the porcelain revealed curled paper (Figure 2).

Figure 1 – Corrosion Figure 2 – Paper Curl Treeing was found starting near the outer pressboard tube around the bottom edge of the ground sleeve and fanning toward the bottom of the bushing (Figure 3, 4).

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Figure 3 – Outermost Paper Layer Figure 4 – Treeing (2nd Paper Layer) Counting inwards, the sixth layer of foil was the inception site. Discharge concentrated along the edge of the foil caused the foil to distort (Figure 5). The five outermost foil layers were offset towards the top of the bushing and not involved in the discharge. Foil as deep as the eighth layer had punch through marks (worm-holes), although the discharge was not at the foil edges.

Figure 5 – Inception Foil

No other discharge sites were found. The increasing power factors matched the deterioration that the DGA results clearly indicated.

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WSN 5RX5 (A) 500kV 600A Manufacturer Micanite (66) Serial:223293

Test Nameplate Jul-03 Jul-04 C1 %PF 0.48 0.58 0.54 C1 Cap 487 489.1 493.5 C2 %PF 0.8 0.99 C2 Cap 1845 1932 1919

DGA Gas ppm (v/v) Gas ppm (v/v) H2 20 CH4 23 O2 1520 C2H2 49 N2 7890 C2H4 29 CO 384 C2H6 14 CO2 5520 H2O 25

INVESTIGATION OF BUSHINGS - HIGH C2H2

WSN 5RX4-C: 500kV Micanite 1968, Serial 228488 This bushing was removed from service based on high combustible gas levels with over 3600 ppm of Acetylene. The tests in the field and tests repeated in the shop confirmed acceptable power factor values. A teardown investigation was conducted to explore the discrepancy between the power factor results and the DGA. The initial inspection noted no water ingress or signs of discharge. Working inward, the fifth step of insulation from the bottom of the bushing had a small charred mark (Figure 6). As layers were removed, the burn mark grew to approximately two inches in diameter originating from the innermost layer of foil (Figure 7).

Figure 6 – Discharge Mark Figure 7 – Inception Site

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There were six wraps of paper between the conductor and the first foil layer. The foil was bonded to the conductor with a thicker strip that had separated from the foil. A crease in the strip indicated that the conductor had slipped inside the insulation causing the strap to tear (Figure 8). Burning was evident at the joint between the strip and the foil with the soldered section of the strip missing. This bushing would have been returned to service based solely on the power factor test results.

Figure 8 – Torn Bonding Strip

WSN 5RX4 (C) 500kV Manufacturer Micanite (68) Serial:228488

Test Nameplate Feb-04 Jun-04 C1 %PF 0.33 0.36 0.31 C1 Cap 460 457 455 C2 %PF 0.34 0.48 C2 Cap 1755 1750 1728

DGA Gas ppm (v/v) Gas ppm (v/v) H2 9230 CH4 1160 O2 242 C2H2 3670 N2 51300 C2H4 1760 CO 6540 C2H6 399 CO2 271 H2O 3

BUT T1-H2: 230kV CGE Type U, Serial 167351 This 230kV bushing had elevated levels of combustible gasses (625 ppm C2H2), with acceptable power factor results. The bushing was removed from service. Dismantling of the bushing found burn marks on the bottom edge of the ground sleeve near the bushing flange (Figure 9). This bushing would have been returned to service based solely on the power factor test results.

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Figure 9 – Burn Marks Near Flange

BUT T1-H1 230kV CGE Type U Serial:167351

Test Nameplate Field C1 %PF 0.35 0.38 C1 Cap 463 458 C2 %PF 0.39 C2 Cap 7000 7022 DGA Gas ppm (v/v) Gas ppm (v/v) H2 1297 CH4 732 O2 294 C2H2 625 N2 100855 C2H4 793 CO 101 C2H6 147 CO2 254 H2O 3

GMS T7B/T7C and WSN T2A: 500kV Bushing Company, Serials 3BBK13976, 3BBF9972, 3BCK17028

The three Bushing Company bushings were removed from service during the initial oil sampling program. They were sent to the manufacturer, now Trench (UK) Limited, for investigation and refurbishment. The manufacturer generally verified the elevated combustible gas levels although lower levels were measured in 3BBK13976. The field power factor results were verified as acceptable. Bushing 3BBF9972 was not tested in the factory and was deemed unsafe due to potential tap damage. The bushings were dismantled and the condenser for each bushing was considered to be in good condition after 30 years of service. No major fault or trace discharge was found on bushing 3BBK13976. Both remaining bushings were found to have potential tap faults. It was apparent that incorrect potential tap covers were installed in the field and modifications had been attempted to increase the contact between the cover and the tapping stem. Both stems were badly eroded due to arcing (Figure 10) and one was found disconnected from the condenser foil. The potential tap was the only major source of gassing found.

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The bushings were refurbished with new cores and returned to BC Hydro for spare inventory. Figure 11 shows the damaged stem beside the replacement. These bushings would not have been identified for refurbishment without the DGA.

Figure 10 – Eroded Tap Figure 11 – Potential Tap Comparison 500kV Bushing Co. Bushings WSN T2-H1 GMS T7B GMS T7 C

Test Nameplate Result Nameplate Result Nameplate Result C1 %PF 0.475 0.5 0.53 C1 Cap 513 511 507 508 501 515 C2 %PF 0.47 0.48 C2 Cap 9076 9090 9290 9107 9100 8900 Bushing WSN T2-H1 WSN T2-H1 GMS T7B GMS T7B GMS T7C GMS T7C Bushing Co 3BCK17028 Bushing Co 3BBK13976 Bushing Co 3BBF9972 Field Factory Field Factory Field Factory Gas ppm (v/v) ppm (v/v) ppm (v/v) ppm (v/v) ppm (v/v) ppm (v/v) H2 5520 3701 407 32 853 531 O2 7498 15471 6843 29367 383 8484 N2 46993 52077 58684 53845 54601 53160 CO 189 116 932 3 259 165 CO2 433 250 6413 1483 1128 693 CH4 1579 1096 790 70 276 200 C2H2 3005 1948 62 27 60 37 C2H4 2681 1383 1862 463 367 212 C2H6 307 125 302 94 115 58 H2O 3 11 4 11 5 9

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INVESTIGATION OF BUSHINGS – LOW POWER FACTOR AND C2H2 KLY 5RX5: 500kV Hybrid ABB/CGE Type O+C There were two bushing failures on a 500kV shunt reactor in the span of three months. Both bushings were Hybrid ABB/CGE Type O+C. No test data exists for the first bushing failure (H1) other than the bushing was 1994 vintage and the suspected failure mode was an ungrounded capacitance tap (Figure 12). After the first failure, the second bushing (H2) was power factor tested and oil sampled for DGA. Both results were considered acceptable and the unit was energized. The ensuing failure and fire occurred two months later (Figure 13). The subsequent investigation found arcing on the outer capacitance tap pin with the inner portion of the pin missing (Figure 14). Later scrutiny found that the bushings in question were manufactured in Guelph, but assembled in Alamo with slightly different components, i.e. different length capacitance tap inserts. Similar hybrid bushings were sent to the factory in Alamo for refurbishment and replacement of the tap inserts. The power factor tests and the DGA did not show conclusive evidence of bushing deterioration.

Figure 12 – First 5RX5 Bushing Figure 13 – Second 5RX5 Bushing

Figure 14 – Missing Capacitance Tap Insert

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KLY 5RX5 H2 500kV Manufacturer ABB/CGE O+C Serial:3013030293

Test Nameplate Oct-94 Dec-01 C1 %PF 0.30 0.36 0.38 C1 Cap 470 460.2 454 C2 %PF .27 0.31 C2 Cap 8207 8210 8225

Bushing KLY 5RX5 H2 ABB O+C Nov-01 Dec-01 Gas ppm (v/v) ppm (v/v) H2 37 41 O2 5594 1386 N2 62355 56611 CO 1090 1034 CO2 3045 3512 CH4 10 10 C2H2 < < C2H4 < 1 C2H6 1 2 H2O 2 3

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EFFECTIVENESS OF DGA BC Hydro has had three years of experience with bushing DGA and our conclusion is that elevated levels of combustible gas are indicative of bushing deterioration. As more bushings are oil sampled the results continue to support investigative findings. This does not imply that DGA is 100% effective in predicting bushing failures or that oil sampling is not without precautions. However, our experience suggests that DGA is an important test for bushing condition assessment. Concerns over sampling procedures are valid and a properly written sampling standard is required to ensure complications are minimized. Sealing of the sampling port is a known problem. The port must be completely sealed to prevent moisture ingress. Loss of oil from sampling bushings is not a major concern if sampling intervals are reasonable. Regardless, topping up bushing oil levels is a standard maintenance procedure that can be done after sampling if required. The amount of oil required for a syringe sample is 50 mL, which is insignificant in high voltage oil filled bushings. Scheduling oil samples during regular maintenance of the associated equipment can minimize costs associated with obtaining samples. The understanding of bushings gained from oil sampling outweighs the difficulties. The possibility of trending gas content in bushings over the bushing life cycle would increase the effectiveness compared to one-time sampling. The bushings in this paper illustrate that DGA aids in assessing bushing condition with supporting power factor results and occasionally may provide insight that is otherwise missed with power factor tests. SUMMARY With present practices of loading equipment above nameplate ratings, testing measures are needed to establish bushing condition. The consequences of bushing failures dictate the importance that should be placed on accurate condition assessment. DGA can be used to supplement standard power factor tests and prevent possible catastrophic failures of bushings. Not only is DGA effective at screening bushings for moisture content, it is also effective at determining discharge in areas of the bushing where capacitance or power factor measurements may or may not be affected. DGA should be one of the methods used to extend the life of our aging infrastructure while limiting our exposure to risk.

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REFERENCES

1. Lau, Mike. “500kV Bushings Failures and Bushing Oil Sampling Program.” March 2002.

2. Graham, John and Tom Sterling, ed. “Investigation of 500kV Bushings Returned

from BC Hydro.” 21 June 2002.

3. Wang, May and John A. Vandermaar, ed. “500kV Bushing Partial Discharge and AC Withstand Tests.” 6 July 2004.

4. Hassanali, Salim. Personal Correspondence. 2004

5. Fancis, Dave. “Kelly Lake Sub Reactor 5RX5 - Poor Health.” 2002