Power Transformers Blog – Expert Safety and Usage Info

MIDWEST performed an Infrared Scan of the electrical system for a new customer.  They had two old outdoor oil filled transformers with load tap changers that had not been used in some time.  So when we scanned these old transformers, we made certain we scanned the load tap changer compartments.  Each tap changer was dangerously over heating. The electrician with us wondered why, since they hadn’t operated the tap changers in 20 years.  But tap changer contacts can cause coking, especially if they are not operated.  Later, during a scheduled maintenance outage, MIDWEST found both load tap changer compartments completely coked, full of black sludge.  The sludge had to be removed by hand, like scooping out sticky black mud.  Since they would never use the transformer tap changers again, MIDWEST removed the entire interior operating mechanisms and bussed the terminals. The old transformers were very lucky the tap changers didn’t fault. If they had, it would not have been cost effective to repair them.  It was just a matter of time before a catastrophic fault in a tap changer would have cost them a transformer.  We have seen this problem before in old transformers.  In this case, certain failure was avoided, and they reused the transformer after the repair. Infrared Scanning is very important, even on old, obsolete transformers and switchgear. 

MIDWEST was asked if we ever rejected a transformer even though it passed all our electrical tests.  The answer was “yes.”  We scrapped an old 1000 kva dry type transformer used in a fairly clean room.  We had purchased the old transformer after replacing it with a larger unit in a paper mill.  The transformer was trucked to MIDWEST.  But an outer high voltage winding had physical damage that was found during the “hard focus” visual inspection. The riggers had damaged the outside of one winding with the tie down chains.  All the standard test results were very good.  And we energized the transformer at full voltage.  Everything tested out great.  But our inspection told us the transformer could not be used.  It was junk. Not worth repairing. It had what we call an “Incipient Failure.”  It was damaged and was going to fail.  When, who knows.  A day, a week, two years?  We didn’t know.  But we did know it was not reliable.  The high voltage windings were going to fail, short out.  It was just a matter of time before the used transformer became scrap.  We just hurried it up and declared it junk.  Here is another great example of pure technical knowledge just not being enough.  Experience and Knowledge were needed. 

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.

MIDWEST is frequently asked if it is safe to take an oil sample from a sample valve under the secondary cables of a pad mount transformer while the transformer is still energized?  No, it is not safe. Do not do it.  We think it is extremely dangerous.  This is the location of possibly one of the most dangerous arc flash hazards in a facility.  You can’t wear enough PPE to protect yourself.  Keep away from the secondary.  Just opening the door to inspect it can be very dangerous.  In addition, you would be exposed to unsafe shock hazard.  Our recommendation is to sample the transformer during a scheduled outage.  We know very well in years past this was common practice for many service companies.  Those days are gone.  As they should be.