Power Transformers Blog – Expert Safety and Usage Info

High Voltage Switch Interphase Barrier being destroyed by high voltage corona and tracking

If you’re a manufacturing plant and your main electrical power transformer is a 10 MVA, 10,000 kva, oil filled station type transformer, you’re probably not interested in failure modes and probability analysis from your service company. You just want to know everything is being done to make sure it doesn’t failure. In the world of sophisticated test procedures and test equipment and lengthy specifications for the application of the procedures to your 5 MVA, 10 MVA, or even 50 MVA high voltage power transformer, there is still need for old fashion maintenance and inspection services. Too often the main high voltage power transformer for a facility can never be turned off because of the interruption of production or data processing. We know that is the real world today and we have to deal with it in the best way possible to maintain reliability. Periodic combustible gas analyses and routine oil analysis are extremely useful.  But there is still room for some old fashion basic maintenance services. MIDWEST likes “Ultrasonic Scanning” of medium voltage and high voltage equipment for indications of destructive corona or tracking. We recommend this on high voltage equipment, especially metal enclosed. And if you have a 20 MVA oil filled power transformer, you have something in front of it to protect it. If that protection fails, your transformer becomes very unhappy. For example, it is not unusual for us to find destructive corona or tracking damage to the inter-phase barriers in the high voltage switch protecting a power transformer. A good example is a lightning arrester lead that rests against the inter-phase barrier and is slowing destroying the barrier. Eventually one of the barriers will fail, usually a phase to phase fault and frequently during a lightning storm when a voltage surge screams down the line into the high voltage switch. Ultrasonic Scanning of the high voltage transformer main switch would catch this before it fails. Not real sophisticated, but an extremely effective preventive procedure. Find it, Fix it before it fails.

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.

How does an air cooled or ventilated electrical power transformer located outside in the rain survive?  Why doesn’t the rain cause an electrical short? These are the occasional questions of someone who wants to replace an existing dry type or air cooled transformer with a larger power transformer, when the existing power transformer is located outside, exposed to the rain, wind and sometimes snow.  It does look a little strange sometimes, but the general purpose air cooled dry type electrical power transformers are fitted with weather shields. They should be. If they are not, you will find out very quickly the first time it rains. The weather shields are metal shrouds, hoods, fastened over the upper vent openings. They prevent rain and snow from entering the top open vents of the transformer. This is true as long as the rain is falling vertical or possibly at a 45 degree angle. The rain or snow hits the weather shield and drips off rather than landing inside the metal enclosure of the air cooled power transformer.  We do see these 50 kva, 75 kva, or even 150 kva air cooled transformers fail. This really does not happen very often. And when it does, it’s usually because of a very unusual storm. Possibly where the wind is so strong, the rain is being driven almost sideways. More often we see failures due to snow filling the inside of the metal enclosure due to swirling and drifting of snow about and eventually inside the enclosure. We have seen this happen with 5 kv and even 15 kv class transformers. We are not fans of medium voltage outdoor air cooled transformers. As long as the enclosure remains intact, there should be no problem. And as long as there isn’t some unusual weather condition, they should be okay. In our business the term “should be” makes us cringe. When folks use the term “should be,” we think “sooner or later.”  It’s healthy to be paranoid around electrical power.  

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.