Power Transformers Blog – Expert Safety and Usage Info

When Infrared Scanning electrical power distribution systems, including general purpose dry type transformers, MIDWEST frequently finds interesting, strange, and even dangerous situations that have nothing to do with Infrared Thermography.  Visualize a make shift rest area in a manufacturing plant. This area was next to the tool room.  There were two panel boards, some wire way with switches above it, and two small general purpose dry type transformers next to the panel boards. One transformer looked like an old 75 kva 480 volt to 208 volt transformer and the other looked like a new, maybe 25 kva general purpose transformer.  Both of the dry type transformers were very warm. On top of the large dry type transformer was a small microwave and a coffee maker.  On top of the small dry type transformer was a cushion.  This looked like a pretty comfortable kitchenette set up.

Anyone electrical should recognize the danger of coffee, liquid, around an open air, air cooled, general purpose dry type transformer. A liquid spill would seem inevitable.  Just a matter of time. Even if the transformer had weather shields, no one should ever be around it with liquid.

The smaller electrical power transformer was just a nice warm seat.  We have seen this a million times, especially in warehouse and other unheated or poorly heated manufacturing and industrial areas.  Usually the folks using these transformers as seat warmers are not electrical and have no idea of the danger below. 

A 75 kva 480 volt air cooled transformer can let out a horrific blast if the primary conductors short out. The transformer would become an instant hot seat and the arc blast from the open top vents could cause horrible burns to anyone nearby.  Add to this the shock hazard. An arcing fault could go to ground and some one touching the transformer could receive a deadly shock. Or something could easily be stuck into the vents and contact live conductors. We see these general purpose air cooled dry type transformers everywhere, and in sizes ranging from 5 kva to 500 kva.  When we see this danger, we politely let the person know the risk of shock or burn.  In a second, that nice warm seat could turn into a deadly hot seat. And that coffee pot stand could be the source of a deadly shock or horrible arcing burn, in a second.

A MIDWEST Thermographer found an interesting problem with a 208 volt, 3 phase, 75 kva dry type general purpose transformer.  The Infrared Scan showed the enclosure of a Square D transformer was much warmer on one side compared to the middle and other side. It was an older transformer and had been in service for many years. An enclosure must re-emit enough heat from the air cooled transformer and the difference in heat from one area of the transformer to another must be substantial enough for the Thermographer to see the difference in heat pattern.  But when you’ve scanned 1000s of these dry type transformers over 15 years, you have a pretty good idea of what’s normal or not.  A load check revealed one phase was at 220 amps, another at about 115 amps, and the third phase 90 amps.  They definitely had a large load imbalance on their old transformer. The maintenance man said they had been having nuisance tripping on the output breaker, but when he checked the load, there was less than 100 amps, 50% load. Unfortunately he only checked the load on one phase and got unlucky enough to pick the lightest loaded phase.  But it was also very easy for our Thermographer to see the load imbalance on the circuit breakers on the load and line side of the transformer.  It is amazing how tough these old air cooled transformers are.  10 kva to 500 kva, general purpose dry type transformers are found everywhere.  Some are real dinosaurs, beyond old, but not obsolete because they are still running.  Some over 70 years. A couple other things about these really old dry type transformers. They were so overbuilt that they can be very forgiving of overloading and they can be really loud.  

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 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.