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Sunday, September 4, 2011

Emergency Shutdown of 50 MVA Transformer due to Oil Leak at the Base of Ground-Connection Terminals for Transformer Active Parts (Core Clamps and Core Laminations)

Our power transformer in our substation has been plagued by various oil leak problems. Considering that the transformer is still 15 years old, it is still considered at its peak performance. But some of its parts had failed already on which the insulating oil inside had found its way to leak. This article is about another weak part of the power transformer that may be a probable source of oil leak.

I. BACKGROUND OF THE PROBLEM:

Last May 19, 2011, it was discovered that the base of ground-connection terminals for transformer active parts of our 50 MVA Transformer had a hairline crack with very minimal oil leak. The oil level at that time on the conservator of 50MVA Transformer Main Tank was at 3/4. The leak was assessed by Maintenance and Test group to be insignificant and can be corrected on the next shutdown of the power transformer scheduled on September 20, 2011, while looking for available materials to be used in the replacement of the base. The oil level was monitored constantly and it remained at ¾ level until August 25, 2011.

During the onslaught of typhoon Mina last August 26-29, Substation Engineers-on-duty observed significant drop of values of the transformer operating temperatures. The usual temperature value for transformer oil, 115kV, 69kV and 13.8kV windings is 55 degrees C. However, last August 28, the temperatures dropped to as low as 33 deg. C. They also further noticed that the oil level was at ½ already.

On August 31, 2011, Engineers-on-duty observed that the level further dropped from 1/2 to 2/5. Due to rainy weather, the leak coming from 50MVA Transformer was hard to determine. Like other materials, insulating oils tend to shrink in volume during cold temperature which has also contributed to the further drop of conservator oil level. Hoping that the oil level will increase when the temperature rises, the Engineers-on-duty waited for the operating temperatures to go up for observation. But on September 1, 2011, the operating temperatures had gone up to 46 degrees C already but the oil level was still at 2/5 level. Furthermore, drops of oil leak from the top of power transformer had increased significantly especially during transformer peak loads.


The above images show the ground-connection terminals for the transformer active part before replacement. Epoxy sealant was used before to mitigate the leak.














II. ACTIONS TAKEN
Reported the situation to the Supervisors. Upon careful evaluation, emergency shutdown was recommended to prevent further oil spill and potential damage to power transformer. SS Engineers coordinated with other substations for the materials needed in the correction of oil leak.
Requested emergency shutdown from Control Center and the 50 MVA Power Transformer was isolated at 1208H. It was found out that the hairline crack on the base of the ground-connection for active part terminals became gap of about 1.5 mm apart. The cracked base of the ground-connection for active part terminal was replaced with fabricated acrylic glass base and the power transformer was successfully energized at 1759H.




The pictures show the whole assembly being removed inorder to take out the cracked base of the ground-connection terminal.







III. ROOT CAUSE OF THE PROBLEM

The hairline crack developed into a large gap because of the shrinking of the base of the ground active part terminal due to the sudden low operating temperature of the power transformer.

 




The images show the retrieved base for ground-connection terminal for transformer active part and the fabricated base made of acrylic glass (thermoplastic glass).






RECOMMENDED ACTION PLAN
1. Replacement parts must be readily available to minimize outage duration.
2. Closely monitor the oil conservator level.



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