Power Transformers Blog – Expert Safety and Usage Info

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.

MIDWEST received an emergency call from a customer who had lost a 30 Mva 138 Kv to 13.8 Kv oil filled power transformer.  There was no storm or other unusual occurrence and they said it failed “last night.”  We are skeptical of transformers that appear to fail in the middle of the night when the sky is clear and nothing else unusual happened. We prepared to get a replacement reconditioned 30 Mva oil filled power transformer, and to arrange the resources to replace the transformer if necessary. Meanwhile MIDWEST Field Services went to check out the transformer. Differential relay protection took the transformer off line.  But we found the problem immediately in the form of a toasted raccoon and slightly blasted secondary bushing. We cleaned the secondary bushing such that it could be energized, tested the transformer, and figured someone at the Utility would want to use the fried raccoon as a wall ornament. But there were no takers. The transformer was okay. There was what we considered an unusual discussion as to why the raccoon was on the transformer. The consensus of management was that it did not make sense. Something more dramatic had to be in play, because one lousy raccoon could not be the cause of so much trouble and expense. The inquisition focused on the need for a good reason a raccoon would climb on top of the transformer. We explained that this was not an unusual occurrence. It was rather common. We explained that raccoons like the warm transformers and tend to climb over everything near their food supply. There were woods next to the substation. We recommended they put screen mesh along the bottom of the substation fencing and at the gate, to keep raccoons out.  And we suggested they move the park bench, used by employees as an outdoor lunch table, to an area away from the substation.

This is a horror story from an electrical contractor about an event that took place around 1970. But the event reinforces the requirement to follow protocol to the letter when working on a large, say 50 Mva oil filled power transformer.  This was a 30 MVA, 40 MVA, 50 MVA 138000 volts to 13800 volt oil filled power transformer. This is the story of a 10 cent washer making a 50 mva transformer useless.

It was not a MIDWEST project. And MIDWEST’s protocol is that everything is removed from the top of a transformer before the manhole covers are removed. Everyone has removed everything from their person and clothes and they must be wearing special coveralls and other secure clothing and all tools are secured. 

In this case, all the bolts and washers for the manhole covers were left on top of the transformer and placed six feet away from the three manholes. The work on the 50 mva transformer was complete and the crew was lowering a manhole cover in place and “kind of dropped it” the last ½ inch. That caused one of the flat washers to bounce up, roll about 6 feet and plunk into one of the other open manholes on the top of the 138 Kv 50 Mva transformer. The chance of this happening was zero. Well almost zero. The transformer had to be hauled back to the manufacture for detanking to find the washer. A very sad day for that crew and a very expensive washer.

We don’t follow protocol just for what we know can happen. We also follow it for things we can’t image would happen.

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.  

Over the years MIDWEST has had to do some creative and sometimes risky work. These things typically occurred when there were very unusual or strange circumstances.  And they were usually performed against our advice to customers that had no easy or inexpensive alternative. When we say risky, we are not talking about risk to life safety. This following example was a very simple task, unless something went wrong. We just needed to replace the lower main valve of a 2000 kva oil filled power transformer without draining the transformer first.  The owner could de-energized the transformer for a short time, but could not tolerate an outage long enough to drain the oil, change the lower main valve, and replace the oil.  They were willing to take on the risk of changing the value with the transformer full, and de-energized and grounded, of course. Actually, we have changed valves on smaller and larger power transformers, but the circumstances were always different.  We knew in this case we would end up with possibly two gallons of messy waste oil, even if things went well. Our procedure was as follows. Loosen the existing lower valve from the 2000 kva power transformer lower valve stem. Prepare the replacement valve such that it could be immediately installed after the existing valve was removed. Secure the area. A large spill pan was placed under the existing transformer valve and stem. All the needed tools and the new replacement valve were placed inside a sturdy large clear plastic bag, including a pipe cap, just in case. A vacuum was pulled on the transformer. The plastic bag was secured about the pipe stem at the bottom of the old transformer. The bag was big enough to hold and manipulate the valves.  Then, like two lab rats, one technician removed the old valve while a second technician held the new valve adjacent the existing valve. The old valve was removed and slide to the side just as the new valve was slide into place and screwing onto the pipe stem. Works well if you catch the threads on the first try.  Either way, one ends up with a blast of old oil all over the inside of the bag.  If the valve quick exchange fails, the cap is put on the stem. It is amazing what happens inside the clear plastic bag in the seconds it takes, hopefully, to change the valve.  This task may sound like a good idea, but we do not recommend it. And we have a hundred years of experience working on old and new power transformers. We’re good at it and we like it. But we know old oil filled electrical power transformers are good at hiding defects until you try to work on them.  Sometimes the messy work is the fun work on transformers.  

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.   

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 was challenged by a technical skeptic to give him just one example of a large old oil filled transformer that obviously outlasted its useful life expectancy.  Apparently he got into a heated argument with some old timer electricians who claimed they trusted the old transformers a lot more than new stuff they had seen.  It may not be wise to argue with experienced electricians.  They have ways to argue with enthusiasm and sometimes expressions that leave little doubt as to the strength of their convictions.  And they have real world experience, usually a good trump card. 

 MIDWEST gave him the example of two 2000 Kva old substation transformers built in the 1950s that have been running at 107% full load for the last 20 years.  These are not rebuilt or reconditioned transformers.  But they are pretty inefficient and about 3 times as large as a new oil transformer the same kva.  MIDWST is not suggesting old transformers are better than new.  But, be careful when buying new transformers.  Specify quality and reliability, or you may get cheap and disposable.

A salvage company called MIDWEST and complained because they had an old transformer filled with silicone oil and they couldn’t get rid of it. It seemed to them that no one really wanted an old silicone oil transformer. At least no one would pay as much as they wanted for it.  They wanted to know why, because they knew oil filled transformers were used everywhere. Since MIDWEST Switchgear Division deals in used and new electrical power transformers, they came to us for some quick help. The simple answer is that silicone is not oil.  Transformer oil is mineral oil, much like you put into your car, but it is refined differently.  To use the term silicone oil doesn’t make sense.  You could say silicone dielectric fluid, but not oil.  Silicone filled transformers provide fire protection for indoor use. And since silicone is very expensive, it is not needed or used in outdoor fluid filled power transformers.  Regular oil filled transformers must be installed in a secure vault if installed indoors.  But they are used every where outdoors. Silicone is really never used outdoors. Therefore, for many installations, silicone has lost favor as a transformer fluid. It especially does not lend itself to informative routine dielectric testing or combustible gas-in-oil testing, which is commonly used to monitor the condition of old and new electrical power transformers used in industry.  It is a simple case of supply and demand. And there is not a great demand for old or refurbished or retrofilled, so called silicone oil filled transformers. We referred them to Dow Corning 561 Silicone Transformer Liquid if they wanted to know more about silicone. In addition, there are now substitutes for silicone dielectric fluid on the market.