Power Transformers Blog – Expert Safety and Usage Info

MIDWEST frequently gets calls from someone wanting to sell used electrical equipment. We are very cautious of the biography of the equipment when the seller is someone other than the original owner. When there are any concerns, the equipment is shipped to MIDWEST, then thoroughly inspected and tested, before we consider purchasing. There are many concerns relating to the condition of used electrical equipment, so we may request pictures, prior use, nameplate data, and other details. Usually a visual inspection is the first line of rejection. With a little biographical information we can frequently determine that the equipment is scrap. As an example, we received a request to sell us three used 1000 kva 13,200 volt to 480/277 volt air cooled, dry type power transformers. The seller was not the company that had used the transformer and the condition was described as “In great condition.” He said he had the transformers for over two years and just wanted to sell them. We asked for pictures before shipping. We got the pictures and the transformers look great. No indications of structural damage. Paint condition indicated the transformers were probably used in a clean environment, a commercial building for example, rather than a manufacturing plant. But, the pictures showed the transformers stored outdoors. Outdoors in the rain and whatever might blow into the windings. Immediate rejection. The windings would be wet and contaminated. Because of the location, the moisture in the windings would have frozen in the winter. The steel structure would be rusted. And if they stored these transformers outside, they had little knowledge of the proper handling of 15 kv dry type general purpose transformers. The pictures said it all. Junk. 

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.

At MIDWESTwe often get calls or receive inquiries from individuals or company employees seeking an exact transformer replacement for a particular unit.  The reasons are numerous.  Their existing transformer failed suddenly or they have a transformer that is in the process of failing.  Maybe they are trying to expand their facility with limited funds and do not want to pay for the services of an engineering consultant.  They look around and decide to copy what they already have and reason if they just had another one just like it and moved the walls out a bit, their newly purchased used machine ( another blog at another time) which hopefully will mean more income.  Possibly the building has a new owner and they are looking for a spare transformer (a gold star to them for being pro-active).  ‘Finding another one just like it’ is easy if you are shopping for a jar of Jiff ® or Skippy®.  But looking for running shoes just like the kind you purchased last year can be difficult.  Now you can imagine just how difficult it is to locate an identical twin to transformer in a building over thirty years old, assuming the transformer nameplate exists, is still legible and not covered with paint.  If you are fortunate to have the model or type number, a Google search may yield a possible match. Hopefully it is an available unit for sale and not a line item on a specification for a future construction project.

A GOOGLE search with only the transformer brand will help you find a soulmate who is usually looking for product data or a wiring diagram for that brand of transformer. 

Sadly, some transformer companies are no longer in business, their assets were sold to another company, the factory shuttered and you trail runs cold.

Assuming the transformer was not built for an Arc furnace and is a one-of-a kind relic (see future blog) you can usually replace a transformer that was made by company X with a transformer that was manufactured by Company Y.  To do so requires a complete list of specifications with all available information on the transformer whether you think it is relevant or not.  There are a lot of parameters that need to be known: KVA, footprint, height and weight, impedance and most importantly the primary and secondary voltage and also if this is a single phase or three phase transformer.  You also need to know where the transformer will be used (outdoors, indoors).  If it is a fluid filled unit, what type of fluid?  If it is a dry transformer (conventional dry type or encapsulated).  You have to know if this transformer needs to have high side voltage taps and if the primary and secondary are Delta or Wye.  If it is a large power transformer, where are the bushings and how are they arranged. Doing your homework first before you have the transformer shipped to you from another state will help you avoid the embarrassment, grief and additional charges that will rack up if the replacement transformer arrives and it will not fit or is suitable for the application or you discover your cable stretcher is undersized.

by Vince

An Electrical Contractor called MIDWEST with an emergency. A customer lost a 1500 kva dry type power transformer. The voltage was 13,200 to 480.  Two of the fuses in the primary load break switch had blown. They checked things out and couldn’t find anything wrong, but wanted MIDWEST to test the transformer anyway. Meanwhile they checked out the secondary switchgear and a 480 volt to 208 volt dry type transformer in the same room as the switchgear. They found no smell and no smoke, so they figured they might get lucky, although we both knew that was not very probably.  When 15kv, 100 amp fuses blow, a lot of energy went somewhere.  The 1500 kva dry type transformer was a typical indoor unit substation type, with LAs, cooling fans and a fan control package. When we got there, the electrical contractor already had the panels off the switchgear and transformer, and everything properly grounded.  They offered to help haul up the test equipment, which is always nice, but we said to hold off for a minute.  It took 15 seconds to get our flashlight and then it took another 15 seconds to inspect the transformer and tell them the transformer had failed and they needed to get a replacement transformer or a rental transformer. Their skepticism was obvious, and for good reason. But we showed them the debris that had blown out the bottom of the center high voltage winding. Then had them look down through ventilation ports of the blown high voltage winding so they could see the blown out turns within the high voltage winding.  They understood immediately.  We ran a quick turns ratio test to document the failure. A case of seeing is believing.

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.

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.

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.  

This MIDWEST blog is about a low voltage, high current transformer used in a hardening process. The transformer was physically located deep in the process equipment. It was very difficult for the owner to access the transformer for maintenance. They just could not maintain it during normal production schedules. The process operators were having more and more trouble controlling the output voltage and current. It got to the point they just could no longer regulated the output current as necessary to perform the metal hardening process. MIDWEST was contracted to remove and replace the transformer or repair it. Whatever could be done. When the transformer was being removed, the problem was found immediately.  There were massive copper bus bars on the secondary side of the transformer to carry the high currents. One of the output bus bar bolted connections had failed. It was held by only four bolts and the bolted connection had become loose. The copper at the connection was discolored and distorted from overheating. The connection contact surfaces were all destroyed by being overheated.  The damaged copper and damaged connection contact surfaces for this damaged transformer could not be repaired. The copper was damaged all the way into the winding. All the other connections were intact. The repair bill was over $50,000.00. This four bolt connection cost over $12,000.00 per bolt to repair.  The transformer replacement was not caused by a transformer failure, but by a simple bolted connection failure. MIDWEST frequently finds dry type transformer failures or replacement transformer projects are caused by the most probable failure mode because the needed preventive maintenance was not performed, because it would have been relatively expensive compared to routine, specified, maintenance. Everyone understands the value of the maintenance dollar. But it does not make sense to continually spend hard to get maintenance dollars on preventing very low probability failure modes and totally ignore the most probably failure mode all together, when the most probable mode would be catastrophic and the low probability failure modes have little consequence.  This is a nightmare for maintenance supervisors. It can be tough to sell critical maintenance procedures that are not found in standard maintenance specifications.

MIDWEST had a curious question from an engineering student about large oil filled electrical power transformers. He couldn’t understand why a 20 Mva power transformer filled with oil didn’t burn when it had electricity actually flowing through it. It was his gut instinct that electricity and any flammable liquid didn’t mix and would result in an immediate explosion or fire. Just made sense to him. He associated electricity with sparks and arcs.  In our world, his question seemed ridiculous because we work around 2000 kva, 10,000 kva and 30 Mva oil filled power transformers all the time and it never occurs to us that electricity and oil don’t mix. Actually a quick load check on oil filled transformers is to put your hand on the side of the tank and see how hot it is. Yup, first we check to be sure the transformer is grounded. But we don’t worry about an explosion or fire if the tank is so hot you can’t keep your hand on it. We do worry about other things when a transformer is that hot, but not fire and explosion. Oil has been used for decades in oil switches and oil circuit breakers along with large oil filled power transformers. It’s the coolant and was used to quench the arc when the oil circuit breaker opened to interrupt the circuit.  This student saw a video on the internet of a large high voltage oil filled transformer exploding and couldn’t believe there was actually oil in it. We explained oil flash point and fire point. Discussed the need for a source of ignition and that, under normal conditions, there would be no arcing in a transformer. Talked about oil not getting anywhere near hot enough to vaporize into an ignitable mixture with air, oxygen. But he had fixed in his mind that electricity and oil do not mix. We guessed he needed an opportunity to work on an old outdoor 10,000 Kva oil filled power transformer in the middle of Winter. That would probably cure his misconception.