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 was challenged by a technical skeptic to give him just one example of a large old oil filled transformer that obviously outlasted its useful life expectancy.  Apparently he got into a heated argument with some old timer electricians who claimed they trusted the old transformers a lot more than new stuff they had seen.  It may not be wise to argue with experienced electricians.  They have ways to argue with enthusiasm and sometimes expressions that leave little doubt as to the strength of their convictions.  And they have real world experience, usually a good trump card. 

 MIDWEST gave him the example of two 2000 Kva old substation transformers built in the 1950s that have been running at 107% full load for the last 20 years.  These are not rebuilt or reconditioned transformers.  But they are pretty inefficient and about 3 times as large as a new oil transformer the same kva.  MIDWST is not suggesting old transformers are better than new.  But, be careful when buying new transformers.  Specify quality and reliability, or you may get cheap and disposable.

Three answers, NO and NO and NO.  This is a big deal for us. MIDWEST does not sell old, used, or obsolete circuit breakers, bus plugs, transformers or other electrical equipment to an end user, ie the consumer, unless it has been properly serviced, reconditioned, tested, and passed Quality Controls.  We strongly recommend no one buys old electrical equipment “as is.”  That could be very dangerous. There are many “listing” companies and auction type sites that sell “as is.”  For example, eBay does not warranty the used and old electrical equipment sold through their service.  They provide a great service, but they do not provide a warranty on the specific used electrical equipment sold on eBay.  The same may be true with services that just list equipment.  The only people MIDWEST will sell electrical equipment “as it,” are other qualified electrical dealers that have proper Quality Controls.  Old, used, or obsolete circuit breakers, bus plugs, transformers, and switchgear have too many dangerous failure modes to be sold without proper testing.  MIDWEST does not just process orders.  Anybody can do that.  And many do.  We actually have a name for that.  We call it TC, Trained Chimpanzee, work.  You could train a chimpanzee to put old circuit breakers into a box. 

We add value.  Our staff of engineers and technicians service, repair, recondition, and test electrical equipment for a living.  We’ve done it for over 30 years.  That is what we do day after day. And it is what we think others should do if they are going to sell something.  “As Is” is just scary.  When electrical equipment gets unhappy, it can fail with a boom and a flash.  And that can hurt.