500KV bushing on the unit transformer
2008-08-25

 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. 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. PCORE capacitor bushings for transformer and oil circuit breaker applications are a proven design based on a capacitor core with aluminum foils and high dielectric paper impregnated with dried, degassed oil. PCORE POC bushings meet all ANSI/IEEE standards for outdoor apparatus bushings, where these standards apply for voltage classes 25 kV through 500 kV. When an application results in overload above ANSI/IEEE standards, a bushing having a higher current rating is recommended. PCORE bushings are designed to carry the short term overloads specifed by ANSI/IEEE standards. PCORE POC bushings for other applications (e.g., oil-to-oil, oil-to-gas, outdoor-to-indoor air, etc.) are available for various voltage classes and current ratings. A mechanical bushing is a cylindrical lining designed to reduce friction and wear inside a hole, or constrict and restrain motion of mechanical parts. One type of bushing is the "threaded insert", a hardened metal insert with a threaded fixing hole which allows one assembly to be fixed to another by means of a screw or threaded bolt. The use of threaded insert avoids the need for a separate nut and washer on the other side of the fixed material. Such inserts are usually fitted into sheet material by a tool which operates on a similar principle to riveting; alternatively, some rivets can themselves incorporate a bushing. A high voltage seismic bushing charged with an insulating material has a procelain tube having an adapter rigidly connected to one end of the tube. The adapter is flexibly connected to a mounting flange such that the porcelain tube is free to move relative to the mounting flange in the event of an earthquake. The means for flexibly connecting the adapter to the mounting flange utilizes a spring mechanism to absorb the energy through friction to attenuate the movement of the porcelain tube. A resilient buffer member is interposed between the adapter and the mounting flange to provide a predetermined spacing therebetween and absorb the impact due to the movement of the adapter relative to the mounting flange. A sealing member seals the interface between the adapter and the mounting flange for containing the insulating material charged therein.