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.

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.  

Transformers come in many different flavors of course. One popular type found in and out of industry is the pad mount transformer. Most people have seen these without knowing what they were seeing. If you look outside an apartment complex, grocery store, or small industrial facility you might see a green colored rectangular shaped enclosure sitting on a concrete pad at ground level. That more than likely would be a pad mount transformer. What makes these unique in the family of transformers is their placement outside of a secure restricted access area such as that of a typical electrical substation. These units are designed to be placed in public access areas eliminating the need for a fenced secure area. These units are fully enclosed and have locking cabinet doors preventing public access to high voltage cable connections.

Although these transformers are fully enclosed and locked, it would be misleading to say they were safe since they are still energized. Concrete posts or steel guard rails are often placed around pad mount transforms to protect them from vehicle traffic. Pad mount transformers are reliable and aesthetically pleasing units with typically underground cable feeds. Although they can be fed from the side or from overhead. These transformers are for outdoor use and as such are usually mineral oil filled.

If you have questions about old pad mount transformers, call MIDWEST. They sell, rent, repair and buy them. They would be more than happy to speak to you.

We like it when we run into something unusual.  Recently we had a request for a replacement transformer.  That wasn’t unusual until they told us they wanted a pad mount dry type transformer. That was a surprise. He was surprised we had one.  The company may have been trying to go green and was under a mandate to no longer use fluid filled transformers. This is becoming more common.  We see quite a few companies that require all replacement transformers to be dry type because of their “Green” policy.  Fortunately most of these companies are commercial facilities and do not use higher voltages and do not have huge loads. In addition, the transformers for commercial facilities are often indoors where dry type transformers are appropriate. Therefore non fluid filled replacement transformers are available and “Going Green” is a good option.  But an outdoor dry type pad mount transformer is uncommon.  Most pad mount transformers are oil filled.  And most pad mount transformers are green, painted green.  Pad mount transformers were developed decades ago for use in subdivision developments.  They were replacements for overhead distribution transformers.  The under ground distribution systems and low profile green box shaped transformers were unobtrusive in the beautiful new subdivisions.  So suburbia went green decades before it became environmentally important, but they did it for cosmetic reasons.

But that is not true for industrial facilities that frequently have higher voltages and larger loads, for example over 25 kv and over 3750 kva.  MIDWEST did have one occasion where a manufacturing plant wanted to eliminate the fire hazard of fluid filled transformers. But it was only because their insurance carrier determined their transformers were so close to the outside building walls that they created a fire hazard.  They were not trying to go green, they were trying to avoid going black.  Dry type replacement transformers would not reasonably work because of the size of their transformers.

One common concern with pad mount transformers is in the area of oil sampling. Companies with large pad mount transformers may sample and test the oil in the transformers annually to determine the reliability of the transformer. The condition of the oil reflects the current state of the transformer. Now here’s where it gets tricky. Large pad mount transformers are designed with two cabinet doors. One door exposes the high voltage cables and bushings and the other door exposes the secondary, low voltage, side. The sample valve can be located in either compartment but most of the time the valve is located in the secondary cable side. Years ago it was not uncommon for an intrepid technician with nerves of steel to pull an oil sample from an energized pad mount transformer, the valve being located at the bottom of the transformer while hot cable lugs were located only a couple feet above the oil valve. With safety becoming a paramount concern in industry today, it is no longer prudent nor standard practice to pull an oil sample on an energized pad mount transformer.  The hot lugs being within short reach of the technician create a shock hazard.  And the secondary side of a transformer is one of the most dangerous arc flash hazards there is. Oil sampling is now performed only during scheduled shutdown of the transformer.

However, there still remains somewhat of a challenge when it comes to pulling an oil sample from a pad mount transformer.  The secondary compartment of pad mount transformers can be a pretty fully house. There may be as many as six cables attached to each secondary bushing, making it very difficult to access the sampling valve through that jungle of vines. One way around this, literally, and we’re finding it used more and more in industry, is extending the sample value system to a small secure box on the exterior of the transformer enclosure. The valve would then be enclosed in a newly created box with a locked access door mounted on the exterior wall of the cabinet. In this way the transformer can be sampled at any time, while energized, without exposing the technician to shock or arc blast hazards.