Power Transformers Blog – Expert Safety and Usage Info

Sometimes simple oil filled power transformer repairs can be made very complicated by seemingly small hidden details. Sometimes the circumstances that create the complication evolved over time. Or, in some cases, it was just poor engineering. In this case the new transformer, switchgear, and substation structure were pieced out to different engineers and contractors, 20 years past. Not a good idea. Here’s an example of a job made hard by the lack of foresight and design coordination   An electric utility had a 20 Mva power transformer with side mount secondary bushings, 15 Kv. The bushings connected to a 15 Kv enclosed busway that ran from the transformer to the 15 kv switchgear inside the building. The bushings were side mounted in a throat for connection to the busway.  One of the bushings had a small leak from the crown gasket. But the oil was pooling and dripping slowly from the throat enclosure. This would normally not be a very complicated repair, even though the leaking bushing would have to be removed to replace the gasket. Access to the bushing connections inside the transformer were very easy. Access for proper oil handling equipment for work on a 115 kv high voltage transformer was easy.  But the bushings were not top mounted. They were side mounted and the bushings had to be removed 24 inches in order to get them out of the transformer side throat.  Usually you just remove the immediate flexible connections to the bushings and one section of busway and have at it. In this case, because of the configuration and supports for the busway, three sections of busway, including a 90, would have to be disassembled. And this was huge bus for a 20 Mva transformer. It was as if the busway was erected first and then the transformer slid in place to connect to the bus. Then overhead structure installed and more infrastructure install adjacent to the transformer such that you couldn’t move or pick the transformer, even if you wanted to, in order to save time.  No thought was given to access or service of the secondary bushings.  So the Utility cleaned up the oil and redefined the leak as a weep, to be monitored.  They couldn’t handle a 48 hour outage to do the repair. Whether this was a 10 Mva, 20 Mva, or 30 Mva oil filled power transformer, the simple leak repair would be a monstrous job.  But sooner or later the old power transformer will have to be repaired.

When Infrared Scanning electrical power distribution systems, including general purpose dry type transformers, MIDWEST frequently finds interesting, strange, and even dangerous situations that have nothing to do with Infrared Thermography.  Visualize a make shift rest area in a manufacturing plant. This area was next to the tool room.  There were two panel boards, some wire way with switches above it, and two small general purpose dry type transformers next to the panel boards. One transformer looked like an old 75 kva 480 volt to 208 volt transformer and the other looked like a new, maybe 25 kva general purpose transformer.  Both of the dry type transformers were very warm. On top of the large dry type transformer was a small microwave and a coffee maker.  On top of the small dry type transformer was a cushion.  This looked like a pretty comfortable kitchenette set up.

Anyone electrical should recognize the danger of coffee, liquid, around an open air, air cooled, general purpose dry type transformer. A liquid spill would seem inevitable.  Just a matter of time. Even if the transformer had weather shields, no one should ever be around it with liquid.

The smaller electrical power transformer was just a nice warm seat.  We have seen this a million times, especially in warehouse and other unheated or poorly heated manufacturing and industrial areas.  Usually the folks using these transformers as seat warmers are not electrical and have no idea of the danger below. 

A 75 kva 480 volt air cooled transformer can let out a horrific blast if the primary conductors short out. The transformer would become an instant hot seat and the arc blast from the open top vents could cause horrible burns to anyone nearby.  Add to this the shock hazard. An arcing fault could go to ground and some one touching the transformer could receive a deadly shock. Or something could easily be stuck into the vents and contact live conductors. We see these general purpose air cooled dry type transformers everywhere, and in sizes ranging from 5 kva to 500 kva.  When we see this danger, we politely let the person know the risk of shock or burn.  In a second, that nice warm seat could turn into a deadly hot seat. And that coffee pot stand could be the source of a deadly shock or horrible arcing burn, in a second.