Power Transformers Blog – Expert Safety and Usage Info

There are many technical reasons for using an oil filled power transformer or for using a dry type power transformer. But sometimes the reasons might be a bit of self interest. One example that MIDWEST runs into deals with customers that have a great need to monitor the condition of their power distribution equipment. There may be many reasons for this. Some, certainly justified, say for a mission critical facility, and some less important. But for a MIDWEST field service engineering technician, they would much rather test the dielectric fluid, oil, of a transformer than go through the work of testing an equal Kva dry type transformer. Dry type transformers have to be taken out of service and that usually means weekend or holiday work. Weekend or holiday work is not family friendly. It would take a technician maybe 10 minutes to sample the fluid in a 2000 kva oil filled substation power transformer, including a thorough technical and safety inspection of the transformer. The sample is turned over to the lab for a complete battery of tests. The results of these tests are very informative, especially when there is a history of test results. Lots of data and Electrical Power Engineers love lots of data. The tests may include a group of standard dielectric fluid tests, plus combustible gas-in-oil tests, furan tests, and power factor tests. And, of course, the ever important and super sophisticated smell and visual test performed by the experienced service technician on site.  A nice technical package. The site inspection of the transformer by an experienced technician is extremely important.  Whereas the visual inspection and dielectric tests performed on a 2000 kva dry transformer have very limited value in real time or for historical data tracking. So, if the choice is 4 hours during a weekend or over a holiday versus 15 minutes during normal work hours and with much better information, the choice would be 15 minutes and the oil filled transformer.

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.

PCB (polychlorinated biphenyl), that little man made molecule that was so good it just would not go away, that is, without the help of man.  The problem with PCB is also its strength. Unlike many things on the planet, PCB is a highly inert molecule meaning it does not easily combine with other molecules in nature. Drop it off in a field somewhere, come back in 100 years and it will still be there waiting for you in pretty much the same shape you left it. How many things can you say that about? From approximately 1930 through 1979 PCB was used as a dielectric fluid and coolant in old electrical transformers and capacitors. Since 1979 Federal register regulation 40 CFR 761 has regulated the manufacturing, processing, distribution and use of PCB’s in Commerce.  Today the use of PCB in existing electrical devices has all but disappeared. Today MIDWEST is actually surprised when we find a PCB transformer. 

One popular misconception was that old transformers created PCB as a by product of the electro magnetic process. This myth grew from the fact that many mineral oil filled transformers were found to contain low levels of PCB in their oil. The truth of the matter is not that mystical. The oil in these transformers became cross contaminated with PCB from PCB contaminated pumping equipment used to recondition the oil in these units in the days before PCB was regulated.

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.