Guest Paper: 500kV Bushings Failures and Bushing Oil Sampling Program Monitors

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Mike Lau, B.ASc, Senior Engineer
Generation Technical Services Department, BC Hydro

Abstract

In June 2001, a 500KV bushing ( CGE, type U ) failed and resulted in a catastrophic failure of a generator step-up transformer in one of our major generating facility. Moisture ingress was the primary suspected cause of failure. Since then, an oil sampling program on bushings was launched and a high percentage of bushings were found to have either high moisture and/or high combustible gases. One 230KV ( CGE Type F ) bushing was found to have high combustible gases and subsequent investigation revealed heavy tracking on the innermost Herkolite cylinder. A few other 500KV bushings ( Bushing Company Type 500HA67B ) was found to have moisture ingress through the top filling plugs. Amidst all these bushing testing and change-out activities, another 500KV ( A hybrid unit -- ABB Type O plus C and CGE Type U ) bushing failed while in service on a relatively new ( in service in 1994 ) shunt reactor in November, 2001. Two months later, another 500KV ( similar hybrid unit ) bushing on the same shunt reactor also failed and resulted a huge fire which burnt for hours.

Investigation on the failures of these hybrid bushings continues at the time of writing and no firm conclusion on the cause of failures can be drawn.

This paper provides some details on the bushing failures and oil sample results.

Introduction

Small oil volume apparatus, such as oil-filled current transformers, potential transformers and bushings contain small amount of oil which could be easily contaminated and greatly reduce its insulation integrity. The failures of these apparatus are usually explosive with ensuing fires and detrimental to both equipment and personnel.

Until recently, in BC Hydro, except for Doble testing, no other routine testing are specified for these apparatus. Oil samples had been taken from some makes of CT’s in the past but only for diagnostic purposes and oil sampling was certainly not a routine maintenance task.

However, with recent failures of 500KV bushings and poor test results on oil samples taken from other bushings, the current maintenance practices on bushings needed to be re-examined.

Background

Failure of a 500KV CGE Type U Bushing (Note : Not to be confused with GE Type U)

n June 2001, at the G.M. Shrum Generating Station, one 500KV bushing on one of the unit transformer, T4C, failed violently with all the porcelain ( both above and below the mounting flange ) blew up into pieces the size of a fist. Oil was drained out of the conservator tank through the bushing and fires started both on top and at the bottom of the tank. Fortunately, the sprinkler systems operated ( the heat was so intense that the sprinkler system for the adjacent bank also operated) and cooled down the adjacent equipment allowing the fire crew put out the fire eventually. An internal explosion was evident from the bulging and welding cracks on the tank. There were severe damages internally that the transformer was scraped. (Photos 1,2,3, and 4).

Investigation of the failed bushing confirmed a dielectric failure with the paper insulation punctured through from the center draw rod, at a location about one third of the way down from the top terminal, to the grounded capacitance tap. While the construction of the CGE Type U bushings ( manufactured by Canadian General Electric in Guelph, Ontario )was different from that of a GE type U ( ie no herring-bone inking process ), we were surprised to see the capacitance layers were made up of paper coated with some semi-conductive graphite like material. Smears of this material was evident in many places ( Photo 5 ).

The subject bushing was Doble tested in Oct 1998 and both C1 and C2 insulation power factors were "good".

No definite cause of failure was determined and external moisture ingress was the primary suspected cause.

Photo 1 : GMS Generator Step up Transformer banks	Photo 2: Overview of faulted bushing on GMS T4C
Photo 3 : GMS T4C Overview of faulted bushing	Photo 4 : GMS T4 C – Removal of faulted bushing
Photo 5 : GMS T4C Bushing - Tracking on semi-conductive paper

Initial Oil Sampling Program

Prior to the above bushing failure incident, oil samples were rarely taken from small oil volume apparatus including oil-filled bushings. After the incident, it was decided that additional information on the condition of the bushings is needed and a small scale oil sampling program was launched. Oil samples were taken from all other 500KV bushings in GM Shrum Generating Station. The results was very alarming: Of a total of 41 other bushings (of different manufacturers including CGE, Bushing Company, and Toshiba ), 11 bushings (27%) were found to have either unacceptable DGA results or unacceptable high moisture content.

The results are included in Appendix 1.

Further investigation on those bushings with unacceptable results were carried out and detailed below.

Investigation of Bushing Problesms

Of the 11 bushings that there found bad, 8 were found to have high moisture content ( 23ppm to 72 ppm ); 3 were found to have abnormal amount of combustible gases. All 11 bushings were removed from services for further investigation and dryout .

A. High Moisture Content

A1.The GMS T4A bushing ( CGE Type U, with 62 ppm of H2O ) was removed from service and sent to Powertech Lab for partial discharge ( PD ) test. It failed the PD test. The unit was subsequently drained, dried out extensively with high vacuum and heat , refilled under vacuum. Subsequently, it passed the PD test and returned to service without incident.

A2. Other CGE Type U bushings found to have high moisture content, such as GMS T1B ( 23 ppm ), T12A ( 52 ppm ), and T14A ( 62 ppm ), were all similarly dried out extensively with heat and high vacuum, tested, and returned to service without incident.

A3. GMS T8C ( with 65 ppm of H2O ) and T9C ( with 72 ppm of H2O ) were both Bushing Company bushings, Cat # ART-500HA67B. The location and the likely causes of moisture ingress have been identified: With reference to Figure 1, moisture was found getting inside the bushings through the small oil filling plugs (Item 10) located on the top of the expansion metal can (Item13) and inside the terminal cap (Item 1). The underside surface of the oil filling plugs were found to be rusty and the surfaces inside of the terminal caps were coated with white powdery material indicating signs of moisture ingress.

Further investigation revealed that the moisture somehow managed to penetrate three lay ers of protection; namely ; the outer gasket seal for the terminal caps ( gaskets found to be brittle and cracked ); the inner gasket seal ( gasket found to be in reasonably good order, but the caps were not seating squarely due to the ends of some helical coils improperly protruding outside of their threaded holes ), and finally the sealing washers of the oil filling plugs ( washers found missing completely! ).

Other Bushing Company bushings were checked but so far none was found to have a similar problem.

The two bushings were also dried out, tested, and returned to service without incident.

FIGURE 1: VIEW OF BUSHING COMPANY BUSHING TOP ASSEMBLY

B. Abnormal amount of combustible gases

B1. GMS T10B was found to have high combustible gases ( but only 1 ppm of C2H2 ). This CGE Type U bushing was removed from service and sent to Powertech Labs for testing. Surprisingly, it passed the PD test. However, there is a significant difference in the Dissipation Factor measured before (0.49%) and after ( 0.74%) the AC withstand test. Further investigation is pending, and the bushing will not be returned to service

B2. GMS T7B and GMS T7C – Both have high combustible gases including a fair amount of C2H2 ( 62 and 60 ppm respectively ). These two bushing were Doble tested prior to their removal from the transformer. The Doble test results are enclosed in Appendix 2. It was reported that on the GMS T7C bushing, oil was dripping out of the capacitance tap continuously when the cap was removed. The oil was black and had a burnt smell to it. This bushing failed the PD test miserably.

These two bushings, together with a third one from another station (WSN T4A), which also has high combustible gases ( 3005 ppm of C2H2 ), will be returned to the manufacturer in UK for investigation and possible refurbishment.

B3. A 230 KV bushing, CGE Type F bushing at Rosedale Substation , (ROS T1A Low Voltage bushing), was found to have high combustible gases. It was immediately removed from service. The bushing was subsequently dismantled for examination. Heavy tracking was found on the bottom end of the innermost Herkolite cylinder. See Photo 6. This bushing was scraped.

Photo 6 : CGE Type F bushing : Tracking on the innermost Herkolite cylinder

Expanded Oil Bushing Sampling Program

Encouraged with the success of the oil sampling program, the program has been extended to include other 500 KV bushings in all our generating stations and in some critical transmission stations.

More Failures of 500kV Bushings

As if we don’t have enough bushing problems, in November 2001, another 500 KV bushing failed on a shunt reactor at Kelly Lake Substation ( KLY 5RX5 ). The reactor is a relatively new 3-phase 135 MVAR unit put in service in 1994. Dark oil and burnt paper sprayed out from the joints between sections of the porcelain. No porcelain were broken and no significant damages to the reactor. The bushing was replaced with a new spare. All the bushings as well as the reactor were Doble tested satisfactory and the unit was returned to service in early December 2001.

Two months later, in the mid-night hours of 26 February 2002, another 500 KV bushing on the same shunt reactor failed explosively and resulted a huge fire which burnt for 4-5 hours. It destroyed the adjacent new spare bushing, a surge arrestor and caused significant damages to the adjacent circuit breaker.( see photo 7 ).

These two failed bushings were hybrid units made with ABB condenser core ( with semi-conductive capacitance layers ) and other CGE Type U components. A type problem was suspected and consequently, two other similar 500 KV shunt reactors were removed from service pending further investigation.

Photo 7: KLY 5RX5 Failure of ABB/CGE hybrid bushing.

Conclusion

Failures of small oil volume apparatus, such as bushings, are often explosive with ensuing fires, and are indeed great threats to personnel and equipment. On-line monitoring equipment are readily available to monitor the integrity of bushings by means of unbalanced neutral current, leakage current, power factor etc. However, it would be difficult, if not impossible, to justify economically, on-line monitoring equipment on every bushing.

s demonstrated in this paper, oil testing for DGA and moisture content prove to be a good compliment to other routine testing to provide additional information on the integrity of bushings.

It would also be desirable that a guide or a standard be established for the acceptable gas level limits and interpretation of the DGA results.

Appendix 1: Dissolved Gas in Oil Results in Bushings

STN	UNIT	PH	DATE	C2H2	C2H4	C2H6	CH4	CO	CO2	H2	H2O	N2	O2	VOLUME
GMS	T1	B	20-Jul-01	<	1	2	15	600	1348	81	23	91308	4775	9.81%
GMS	T1	C	20-Jul-01	<	<	2	10	704	1107	123	10	97893	2116	10.20%
GMS	T1	A	11-Oct-01	<	20	45	17	315	1357	382	4	65714	351	6.82%
GMS	T2	A	06-Oct-01	<	<	15	6	39	1019	5	8	78300	4704	8.41%
GMS	T2	B	06-Oct-01	<	1	10	7	45	1126	6	7	81282	3224	8.57%
GMS	T2	C	06-Oct-01	<	<	10	9	80	1443	31	4	85336	270	8.72%
GMS	T3	A	06-Oct-01	<	<	11	7	71	1737	14	46	74533	2730	7.91%
GMS	T3	B	06-Oct-01	<	<	10	6	57	1341	5	6	80362	2624	8.44%
GMS	T3	C	06-Oct-01	<	2	58	27	214	1898	15	25	57074	1935	6.12%
GMS	T4	B	04-Jul-01	<	<	1	9	597	1173	116	3	80684	1729	8.43%
GMS	T4	A	04-Jul-01	<	3	2	9	568	4828	27	62	76905	3537	8.59%
GMS	T5	C	04-Oct-01	<	1	4	8	90	2589	3	9	75005	1617	7.93%
GMS	T5	A	04-Oct-01	<	<	6	8	73	1055	2	7	70142	2615	7.39%
GMS	T5	B	04-Oct-01	<	1	4	10	145	1756	38	8	79320	402	8.17%
GMS	T6	B	18-Sep-01	<	13	6	3	106	1598	25	5	60017	24002	8.58%
GMS	T6	A	18-Sep-01	<	2	56	66	238	2393	3	7	37855	7439	4.81%
GMS	T6	C	18-Sep-01	<	14	7	21	556	2620	145	3	62248	1984	6.76%
GMS	T7	B	05-Sep-01	62	1862	302	790	932	6413	407	4	58684	6843	7.63%
GMS	T7	C	05-Sep-01	60	367	115	276	259	1128	853	5	54601	383	5.80%
GMS	T7	A	05-Sep-01	<	1	15	25	219	757	442	3	66739	6088	7.43%
GMS	T8	C	22-Sep-01	<	2	16	31	337	1530	404	65	58803	2531	6.37%
GMS	T8	B	22-Sep-01	<	1	34	49	94	479	314	4	49937	572	5.15%
GMS	T8	A	22-Sep-01	2	6	47	32	78	854	298	5	48412	1744	5.15%
GMS	T9	A	22-Sep-01	<	4	64	40	162	1118	265	8	79732	706	8.21%
GMS	T9	C	22-Sep-01	<	4	31	30	221	1335	894	72	65444	853	6.88%
GMS	T9	B	22-Sep-01	<	7	49	28	359	3263	35	13	79760	4358	8.79%
GMS	T10	A	15-Aug-01	<	2	2	12	931	1915	12	5	53334	3185	5.94%
GMS	T10	B	15-Aug-01	1	263	216	638	800	5993	1172	6	57785	905	6.78%
GMS	T10	C	15-Aug-01	<	2	5	27	585	1733	44	7	90869	1196	9.45%
GMS	T11	C	26-Aug-01	<	1	1	4	405	632	71	9	64862	2489	6.85%
GMS	T11	A	26-Aug-01	<	1	1	5	588	903	144	10	71113	1257	7.40%
GMS	T11	B	26-Aug-01	<	2	1	3	334	573	70	7	41533	1753	4.43%
GMS	T12	A	30-Aug-01	<	1	1	4	488	2910	94	52	64272	2465	7.02%
GMS	T12	C	30-Aug-01	<	2	2	3	427	1981	127	11	69358	1691	7.36%
GMS	T12	B	30-Aug-01	<	1	1	6	538	638	72	10	57308	1310	5.99%
GMS	T13	A	30-Oct-01	<	<	1	5	448	474	54	2	55436	1300	5.77%
GMS	T13	B	30-Oct-01	<	1	3	6	483	1571	51	3	84235	1610	8.80%
GMS	T13	C	30-Oct-01	<	<	2	5	243	444	24	4	65088	3709	6.95%
GMS	T14	A	14-Nov-01	<	2	3	6	815	2216	68	62	84741	1007	8.89%
GMS	T14	B	14-Nov-01	<	2	2	10	1181	3085	12	4	64593	7670	7.66%
GMS	T14	C	14-Nov-01	<	2	2	6	786	2240	71	3	81460	3632	8.82%
ROS	T1-X	A	22-Oct-01	2096	1583	293	845	495	4425	854	64	84398	9755	10.47%
WSN	T2	A	14-Nov-01	3005	2681	307	1579	189	433	5520	3	46993	7498	6.82%

Appendix 2: A Double Test Result of GMS T7B Bushing