MIDWEST had a curious question from an engineering student about large oil filled electrical power transformers. He couldn’t understand why a 20 Mva power transformer filled with oil didn’t burn when it had electricity actually flowing through it. It was his gut instinct that electricity and any flammable liquid didn’t mix and would result in an immediate explosion or fire. Just made sense to him. He associated electricity with sparks and arcs. In our world, his question seemed ridiculous because we work around 2000 kva, 10,000 kva and 30 Mva oil filled power transformers all the time and it never occurs to us that electricity and oil don’t mix. Actually a quick load check on oil filled transformers is to put your hand on the side of the tank and see how hot it is. Yup, first we check to be sure the transformer is grounded. But we don’t worry about an explosion or fire if the tank is so hot you can’t keep your hand on it. We do worry about other things when a transformer is that hot, but not fire and explosion. Oil has been used for decades in oil switches and oil circuit breakers along with large oil filled power transformers. It’s the coolant and was used to quench the arc when the oil circuit breaker opened to interrupt the circuit. This student saw a video on the internet of a large high voltage oil filled transformer exploding and couldn’t believe there was actually oil in it. We explained oil flash point and fire point. Discussed the need for a source of ignition and that, under normal conditions, there would be no arcing in a transformer. Talked about oil not getting anywhere near hot enough to vaporize into an ignitable mixture with air, oxygen. But he had fixed in his mind that electricity and oil do not mix. We guessed he needed an opportunity to work on an old outdoor 10,000 Kva oil filled power transformer in the middle of Winter. That would probably cure his misconception.
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.