Power Transformers Blog – Expert Safety and Usage Info

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.

We received feedback from a new Technician who was scrapping out an old 5000 kva oil filled power transformer. It was over 40 years old and hadn’t been in service for 20 years. The primary voltage was 13,800 and the secondary ranged from 120 volts to 600 volts. These voltages made it an unusual transformer, but it was worth more for scrap copper and steel than for reliable reuse. In addition we had limited knowledge of the history of this 5 Mva transformer.  The test results were barely acceptable, not good.

The young technician had never seen the inside of an old oil filled power transformer, especially such an unusual power transformer out of a foundry. The things he found interesting, a veteran oil transformer repair technician would take for granted. He was surprised how tightly the core and coils fit inside the tank and how the high and low voltage bushings connected to the transformer. He was amazed by the over 3” diameter round solid copper secondary bushings. He saw how the liquid level gauge and temperature gauge for a 5 Mva oil filled transformer worked. The level gauge inside the power transformer had a simple small rod with a cork at the end. The temperature gauge was just a well extending through the wall of the 5 Mva transformer with a temperature gauge installed in the well. The transformer had a basic tubing and pressure gauge system to add nitrogen or dry air.  All very basic stuff, but once he saw all this stuff and the interior of the transformer, these basic things made a lot more sense. Good experience. If a picture is worth a 1000 words, then this simple experience was worth 10,000. Whether a 5000 Kva transformer, 10 Mva, 20 Mva or 50 Mva oil transformer, some of these transformer basics are the same.