Power Transformers Blog – Expert Safety and Usage Info

MIDWEST Engineering got a call from a plant supervisor that bought a 225 kva general purpose dry type power transformer, 480 volts to 208 volts. They had installed the transformer and energized it. But they swore they got no voltage at the secondary, 208 volt, side. The transformer was tested by MIDWEST before it was shipped, including being energized at full voltage. In addition, an inductive load was put on the transformer as part of our final testing procedure. But they were certain their power transformer had failed because they had just a couple volts at the output when energized at 480 volts on the primary. And they said they didn’t do anything to the defective transformer. So we ended up shipping them a second unit and asked them to send the suspect transformer back. When the suspect transformer arrived, the switchgear shop checked it out and found no visual indications of failure. If an electrical power transformer fails such that there is no voltage on any secondary phases, we would expect to find the failure during a visual inspection. In a new or reconditioned electrical power transformer, during a three phase fault, a lot of energy must go somewhere and it is usually pretty obvious. In this case there was absolutely nothing visual to indicate a failure. Our first transformer test, turns ratio, helped us find the problem immediately. The buyer had changed the transformer taps, but they did not remove the varnish on the unused tap tabs sticking out the side of the transformer windings. Sometimes these tabs are totally insulated with varnish and it has to be cleaned off thoroughly before connecting the tap jumper. This case was a little bizarre because the bolts holding the jumpers did not make contact on the inside of any of the holes in the insulated tap tabs. Pretty strange to not have any contact at any of the three jumpers.  We have seen this problem before, but usually one or two of the tap jumpers makes some contact and a third is open.  When changing taps, one needs to clean the jumpers and the taps so as to make sure there is a good current carrying connection. A poor heating connection can do some real damage and may not be noticeable until it’s too late. So, in a way, they got lucky.

MIDWEST had a customer who wanted to ask a question about a transformer but he said he was afraid we would laugh at him or think he was stupid. We assured him the only stupid questions are those not asked. He wanted to know if there was such a thing as transformer mounting feet. He had checked the internet and didn’t find anything helpful. He was told by an engineer to make sure they removed the mounting feet from a new transformer their company had bought to replace a failed transformer. He was a purchasing agent and thought the engineer was trying to embarrass him so he didn’t ask any questions. He had rented a 1500 kva electrical power transformer from MIDWEST in the past and thought we could help him. And we did. Dry type electrical power transformers have vibration dampening pads between the transformer mounting feet and the frame of the enclosure. These pads dampen the mechanical vibrations caused by the transformer. The mechanical vibrations cause sound vibrations, noise, which can be a problem. Excessive mechanical vibrations can cause a lot of trouble over time. And noise can be a huge negative human factor if people are working in the area. New transformers are shipped with mounting bolts between the transformer mounting feet and the frame of the transformer. Typically these mounting bolts go right through the vibration dampening pads. There are instructions for loosening or removing the mounting bolts such that the mounting bolts do not transfer the transformer vibrations to the enclosure. So, yes 100 kva, 1000 kva and even 5000 kva dry type air cooled electrical power transformers do have mounting feet. And yes the mounting bolts should be loosened or removed per the manufacturer’s instructions.

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.