Power Transformers Blog – Expert Safety and Usage Info

Power Transformers Defect at Top of Winding

MIDWEST frequently rejects equipment even thought it passes the electrical tests. Here’s an obvious example. MIDWEST’s Switchgear Shop rejected a 225 kva 480 volt delta to 480/277 volt wye dry type electrical transformer. We only had the core and coils, not the enclosure. The insulation resistance, winding resistance, and turns ration test results were all good. The transformer was energized at full primary voltage and had proper secondary voltage for the tap position. This all sounds good. So, why was the transformer classified scrap junk?  An inspection revealed the top two turns of the primary windings of one barrel had been crushed down. Something had been dropped onto these winding turns and pushed them down and separated them. The damage wasn’t physically dramatic. It was almost inconspicuous. But it was fatal to the transformer. In addition, this damage didn’t appear to have caused the fault that blew the fuses that resulted in the owner sending us the transformer to check it out. But we also found one of the ‘unused’ tap tabs was melted off to the point the bolt hole was completely gone. The end of the tab looked very clean. The transformer was connected in a different tap. On top of all this, the side of the winding seemed blackened by an apparent arc blast. Apparently the bolted connection at this tap became loose and the arcing completely destroyed the tap connection. The taps of the transformer were changed in an effort to restore power until a replacement transformer was installed. Sometimes the physical damage to electrical power transformers is very obvious. In this case, the damage to the electrical transformer was not as obvious as one might think.  At least until after it was found. Test results alone can not be used to evaluate the condition of electrical dry type power transformers. The transformer must also be thoroughly inspected. What MIDWEST calls a ‘hard focus’ inspection.

MIDWEST was called to check out an old 3750 kva oil filled transformer. The owner was trying to decide whether to reclaim the oil in the old power transformer or to go ahead and replace it with a new 5000 Kva transformer. They actually needed a 5 Mva replacement power transformer. They had the history of the oil test results. It was apparent the transformer had slowly absorbed moisture over the years and there was serious paper degredation. They wanted MIDWEST to do an under the cover inspection to see if there were visual signs of sludge and tank rust. Pretty straight forward stuff. Their electricians asked if they could look inside the transformer while we had the access cover open. They had to follow our safety precautions.  They were absolutely amazed to see all the wood used inside the transformer. Almost the entire top of the core and coils was hidden under wood.  The wood was used for blocking, for support of the tap changer, and for making the high voltage connects going to the line side of the windings. The electricians were a little skeptical when we explained how much wood was actually used in transformer construction. How common it was. And that 15 Kv high voltage terminations were made on the wood. They were also surprised how clean the inside of the transformer looked, even though the oil was dark and there was visible sludge on flat surfaces, especially on the wood. For the electricians, seeing was believing. High voltage oil filled power transformers are made with wood.