Power Transformers Blog – Expert Safety and Usage Info

Inside of Oil Filled Transformer

Inside of Oil Filled Transformer

There seems to be some mystery around pad mount transformers. And not just the green boxes in residential back yards. Even the larger pad mount power transformers used by manufacturers seem to be a mystery to many folks. During a routine quarterly RCM (Reliability Centered Maintenance) Inspection of a manufacturing plant’s power transformers, MIDWEST detected a small amount of oil weepage on the concrete pad under one transformer. It was a 2000 Kva, 24,940Y/14,400 volt to 480Y/277 volt unit built in 1993 with 740 gallons of oil in it. When we explained to the owners that the main power transformer for their product assembly and finishing area was filled with oil and the oil was weeping out of the transformer, they almost thought we were joking. They had no idea the transformer was oil filled. A little walk, inspection, and explanation and we could tell it was now as clear as mud to them. They needed some pictures of the inside of the transformer. Oil weepage from a power transformer is nothing new, but this 2000 Kva power transformer went from no signs of weepage or leakage to a stained pad within 90 days. The cause was later determined to be a cracked bushing, which is very rare. In our report MIDWEST included pictures of the inside of a similar transformer. The images showed the oil inside the tank. Also showed the bushings extending inside the transformer under the oil and showed the liquid level gauge. Oddly enough, it was the oil level gauge that gave them the epiphany they needed to understand comfortably. These images are always very helpful as a “Seeing is believing” tool.

Here is an example of the success of an Unlimited RCM (Reliability Centered Maintenance) program for large kva and Mva oil filled power transformers. Our Engineering Technicians and Engineers know what the most common problems are for oil filled electrical power transformers, whether 1000 Kva, 5000 kva or 25 Mva. We do the more glamorous transformer tests, but most of these require an outage, which has become unheard of for many large manufacturing facilities and some small municipal utilities. Fluid samples for routines and GCs on dielectric fluid still can be taken safely in some cases. But a hard focused inspection by an experienced, trained, and knowledgeable expert will find many of the probable, but unapparent, deficiencies in oil filled power transformers. Someone following a simple TC Checklist is nearly useless. By TC Checklist, MIDWEST means a “Training Chimpanzee” Checklist. The person performing the service has to think. They have to focus or they will miss subtle things that individually may mean nothing, but collectively may reveal a serious problem or danger. We find things others have looked right at and missed. Some of the things we look for are hard to explain. Others, that seem obvious to the pros, may be invisible to the inexperienced. MIDWEST’s Unlimited RCM Services have saved customers more money than all the transformer testing put together. Here is a simple example of a fluid filled transformer oil leak discovered only by the not so sophisticated “look and feel” test. It wasn’t the ordinary transformer oil weepage or leakage that allowed plenty of time to correct. It was invisible because the transformer was a large pad mount unit and it was leaking into the cable entrance pit under the cable connection compartment. The tank was fire hot, but the cooling tubes and top headers were not unreasonable hot. We could tell the oil level was so low that we were worried the high voltage cables inside the transformer or the core or even the coils might be exposed. Usually a fatal problem. A quick outage was arranged, despite production’s pleas to wait. We found the lowest high voltage bushing was actually cracked and fluid was coming out in a steady little stream. Not just weeping or slowing dripping. A quick temporary repair was made in order to get production back up until a replacement bushing could be purchased. We’ve seen this crisis before. Emergency outage, drop the oil, remove the bushing, make a crazy careful temporary epoxy repair, refill, test, and energize. The only other safe choice was to leave power off until a replacement bushing could be delivered. Terror for production. Later the new bushing was delivered and installed during a scheduled outage. A gas sample was taken and all was well. The transformer was days, if not hours, from failing. If we were not performing Unlimited RCM inspections, they would have had a failure, at great cost to critical production. One additional note. We have a customer who, against our advice, operated a transformer for over 20 years with the temporary repair. Our technical term for that is ‘luck.’ The Unlimited Services by experts works. If you don’t have an electrical power transformer expert, consider starting by using a qualified electrician. A safety note, before you touch an energized transformer, make sure it is properly grounded.

Large Enclosure for General Purpose Dry Type Transformers

For those who have never seen the inside of a general purpose dry type power transformer, you will probably be surprised at how much open space there is inside the enclosure.  The attached image of a dry type general purpose transformer shows just how much space there might be to properly cool off a transformer if it is heavily loaded. The image shows the transformer with panels removed, which allows you to see how much open space there is.  This transformer is special because of the voltages, 3810 volts three phase primary to 240 volts three phase secondary. Power transformers need a lot of space to keep cool.  Even when the transformer is not under load, the iron core still generates a lot of heat.   On the sides of the transformer enclosure, near the bottom and near the top, are vents which allow air to circulate through the enclosure removing the excess heat of the transformer core and coils.  Air is heated by the transformer, rises up and out the top vents while pulling cooler air in through the bottom vents.  Sometimes the vents have filters to keep dust out of the transformer.  If the transformer becomes covered with dust, it will not cool as well.  In serious cases, the transformer may get extremely hot, to the point of eventual failure.  Poor ventilation that results in overheating, shortens the life of the transformer.  Whether a 50 kva, 150 kva, or 500 kva 480 volt to 208 volt, the need to properly cool the transformer is important.  A safety tip, don’t stand near the vents of a general purpose transformer. An electrical fault could cause you serious harm.

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

We received feedback from a new Technician who was scrapping out an old 5000 kva oil filled power transformer. It was over 40 years old and hadn’t been in service for 20 years. The primary voltage was 13,800 and the secondary ranged from 120 volts to 600 volts. These voltages made it an unusual transformer, but it was worth more for scrap copper and steel than for reliable reuse. In addition we had limited knowledge of the history of this 5 Mva transformer.  The test results were barely acceptable, not good.

The young technician had never seen the inside of an old oil filled power transformer, especially such an unusual power transformer out of a foundry. The things he found interesting, a veteran oil transformer repair technician would take for granted. He was surprised how tightly the core and coils fit inside the tank and how the high and low voltage bushings connected to the transformer. He was amazed by the over 3” diameter round solid copper secondary bushings. He saw how the liquid level gauge and temperature gauge for a 5 Mva oil filled transformer worked. The level gauge inside the power transformer had a simple small rod with a cork at the end. The temperature gauge was just a well extending through the wall of the 5 Mva transformer with a temperature gauge installed in the well. The transformer had a basic tubing and pressure gauge system to add nitrogen or dry air.  All very basic stuff, but once he saw all this stuff and the interior of the transformer, these basic things made a lot more sense. Good experience. If a picture is worth a 1000 words, then this simple experience was worth 10,000. Whether a 5000 Kva transformer, 10 Mva, 20 Mva or 50 Mva oil transformer, some of these transformer basics are the same.  

MIDWEST frequently replaces old oil filled transformers with more efficient new oil filled power transformers. Being a specialty engineering firm, we usually get involved when the project is messy. We had a recent project involving replacement of very old Allis Chalmers 600 kva and 1000 kva oil filled conservator tank transformers.  These were monster units compared to the size the replacement transformers, which were 1000 kva.  As a training exercise, MIDWEST had a shop crew tear one of the old transformers apart. They quickly found out that the bushings for the old transformers extended twice as far inside the tank as they did outside. These may have been obsolete transformers, but they were built like battleships and who knows how long they may have lasted. The crew got a good look at the workings of the conservator tank.  Then they removed the top. The old transformer had a bolted on top and the core and coil assembly was bolted to the top, such that the transformer core and coil came out of the tank when the top was lifted. The crew was surprised at how small the core and coil assembly was. They thought it probably took up only the bottom third of the tank. We know that new replacement transformers are much smaller and lighter than the old and obsolete transformers used for manufacturing plants across the country. When we do a power transformer replacement, we seldom have to worry about space when the transformer being replaced is 60 years old.  Actually we have to provide a raised concrete pedestal when installing a new oil filled power transformer in place of one of these old units. So, as strange as it may be in this case, for a replacement electrical power transformer, new is small and old is tall. And seeing is believing.   

By Larry Dahlgren

So someone told you should obtain a transformer.

Before you open your wallet and buy the first transformer you meet, let’s consider the situation a bit.

Buy Dry Type Transformers

You might have just moved your equipment to a new facility and the new location seems to be electrically different from your former location.  You do not see the familiar types of disconnects and plugs your former location had or your machines used to be hard wired and now, at their new home, you have a collection of machines with pigtail wires and you are unsure where to connect them.  Maybe you just picked up a spare to one of your existing machines or added a machine because there are many available at auction.

You get them to your shop and now find the nameplate voltage is different than your building voltage.

A.        AVAILABLE VOLTAGE

Is the actual voltage at the new location any different from the old location?

It could be the same and it might just be referred to by some other name.

Maybe that machine you just picked up at the fire sale has a voltage tolerance that will mean it can work just fine on your available supply.

Consider this single phase example:

There are those who refer to the voltage that comes from a typical wall outlet as 120 Volts.  A generation ago that same voltage used to be called 115 Volts or 110 Volts.

Today the do-it yourselfer may connect an electric water heater or electric dryer or range to 240 Volts from a breaker fed from both sides of the common household circuit breaker cabinet.  Thirty years ago the water heaters were connected across 230 Volts from both sides of the fuse box if the thinking is that the wall outlets were good for 115 Volts.  Maybe that water heater used to be wired to 220 Volts coming from both sides of the fuse box that powered all of the 110 Volt outlets. 

This can become complicated even more in multi family buildings, served by the utility with 208Y/120 volts, where each unit is served from two hot legs and the neutral.  In that case the wall outlets are still at 120 Volts but there is only 208 Volts between the two hot legs so the dryer and water heater are fed from 208 Volts.  Trouble is, a lot of folks just say the specialty socket for the stove or range has 220 Volts and then you have to investigate further and measure safely so you know the actual voltage that is present.

The same type of confusion can extend into the three-phase industrial setting. 

People refer to one type of nominal, three phase, three wire voltage as 440, 460 or 480 Volts.  Some refer to another type of nominal, three phase, three wire voltage as 220, 230 or 240 Volts.

The two, common, three phase, four wire, nominal voltages are 480Y/277 Volts and 208Y/120 volts.

Measuring (safely) you may discover that the actual voltage, over time, could vary above and below these nominal values.  The voltage at times could reach as high as 504 Volts or drop to only 456 volts.  Other times the 240 Volt nominal voltage could swing as high as 252 Volts or as low as 228 Volts.  Watch out because occasionally 208 volts can swing as high as 218 Volts.  You are then left to wonder if a measurement of 216 Volts could mean a stiff 208 Volt system or a very soggy 240 Volt system.  Determining the actual voltage and the nominal voltage are critical in the selection of a transformer. 

The same holds true for your wall outlets where the acceptable voltage, according to your local power company, can swing from 95% of 120 volts (114 volts) to 105% of 120 Volts (126 volts).  Should the voltage drops to 90% of 120 Volts (108 Volts), call your power company.  

Hey 114 Volts is only a volt shy of 115 Volts.  This means your parents were right again; this time about wall outlets.

Next time we will discuss the equipment you energize with the voltage you have.

See Part 2

Good Luck, be safe and happy transforming.