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 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.

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.

MIDWEST replaced the old main electrical power transformers for a manufacturing facility.  When we were done, the outdoor substation looked barren. It looked like the power capacity went way down.  We removed one 1000 kva oil filled three phase Westinghouse transformer, one 1000 kva oil filled three phase GE General Electric Transformer, and three 167 kva single phase oil filled transformers. We replaced all these transformers with one new oil filled 3750 Kva transformer. We replaced the concrete pad, of one of the old 1000 kva transformers, with a larger pad. Installed a new grounding system and replaced the fencing. But the new installation made the substation look bare.  Where there were five old oil filled transformers, there was now one new oil filled power transformer. The new 3750 kva transformer wasn’t much larger than one of the old scrap 1000 kva transformers. But, when you looked at the substation, you saw four abandoned concrete pedestals for the removed transformers. And that made the place look like it was going out of business, rather than growing.

The size of many very old GE General Electric, Westinghouse, and especially Allis Chalmers oil filled electrical power transformers was a third to even 100% larger than a new same Kva power transformer. Pad mount transformers are especially smaller because of their low profile.  The low profile green box pad mount transformers got extensive early use in subdivision developments, where the new owners of dream homes didn’t want ugly electrical overhead lines and transformers outside their windows. The little green boxes were small and everywhere, but there were barely noticeable because of their size and color.

So, for electrical power transformers, new is less, less space that is.  

MIDWEST is frequently asked about some of our unusual experiences over the years.  Things that happened that had no text book solution. Here is one of those experiences, having to do with an old outdoor askarel (PCB) filled transformer. The transformer was a transplant from indoor to outdoor. A little crazy, even in those days. This happened decades ago. Back when old askarel transformers were still sampled.  The fluid dielectric strength test results were horrible, 14 Kv. During their next plant shutdown, we inspected the transformer and found about 4” of water layered and floating on top of the askarel fluid. Askarel fluid weighed about 12.7 lbs. per gallon, so the lighter water, about 8 lbs per gallon, just mostly floated on the top of the dielectric fluid, askarel. The exposed glass rupture disk on top of the old power transformer had cracked when rained turned to ice during the winter. The internal high voltage (13,800 volts) leads, from the high voltage bushings to the transformer windings, actually passed through the layer of water. These leads were insulated, but not insulated against water. The secondary bushing leads were below the free water level.  The plant would be in crisis without this transformer. They had no spare replacement transformer. It would take too long to get a reconditioned, rebuilt or new transformer. So the plant engineer said, “It was working when we turned it off, so, when you’re done, we’re turning it back on.”  We removed the layer of free water; Added R-Temp Transformer Fluid to the proper level; Turned the transformer back on; and no noise, good. All the PCB contaminated fluid and debris was properly disposed at an EPA authorized facility. 

MIDWEST told the customer the transformer needed to be replaced as soon as possible.  There was an incipient, even imminent, failure.  The next time we heard from the customer was seven months later when the 1000 kva transformer failed.  Our advice was ignored. But we were amazed the transformer lasted that long.

What do you do if you have a 750 Kva oil filled transformer that has no valves, no gauges, is leaking oil and the owner tells you they can’t possibly do anything for weeks.  But they want to know if there is enough oil left in the transformer to make it 45 days, because they can’t stop production.  Production verses maintenance, it’s a familiar battle.  The reality today is that production wins.  One has to find “best fit” solutions. In this case, the transformer was only about 30 years old, so it wasn’t a total dinosaur.  Why anyone would buy a naked transformer is the subject of another blog.  Both the production manager and maintenance manager were actually worried about their jobs if they lost this transformer, so they were looking for some serious guidance from MIDWEST.  They wanted to know if they should get a replacement oil filled transformer. We used a highly technical protocol to determine if the oil level was too low.  We first made sure the old power transformer was properly grounded.  Then we felt the case of the transformer and then the cooling tubes.  Yes, the sophisticate instrument of choice was the human hand.  All the cooling tubes were warm and the tank was slightly warmer than the cooling tubes.  The heat distribution was as we would expect for a properly cooling old or new oil filled power transformer.  We knew from the type of bushings that they extended below the oil level.  The oil stain from the leak was fairly large, but it was caused by chronic weepage and not an aggressive leak.  So MIDWEST’s opinion was the old obsolete naked power transformer would last until they could schedule repair.  Ironically enough, the oil weepage was from the plug that was installed in place of a top valve.  So they scheduled the repair per their production schedule. They didn’t need to replace this old oil filled power transformer after all. We repaired the leak, added valves and gauges, sampled the oil, added some new oil, and ran some transformer tests and all was well.

A very important note, MIDWEST made sure the transformer was grounded before touching it. This is an important safety tip. You do not want to become the transformer case ground. Electricity through your body will make you very unhappy.