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.
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.
What do you do if you have a 750 Kva oil filled transformer that has no valves, no gauges, is leaking oil and the owner tells you they can’t possibly do anything for weeks. But they want to know if there is enough oil left in the transformer to make it 45 days, because they can’t stop production. Production verses maintenance, it’s a familiar battle. The reality today is that production wins. One has to find “best fit” solutions. In this case, the transformer was only about 30 years old, so it wasn’t a total dinosaur. Why anyone would buy a naked transformer is the subject of another blog. Both the production manager and maintenance manager were actually worried about their jobs if they lost this transformer, so they were looking for some serious guidance from MIDWEST. They wanted to know if they should get a replacement oil filled transformer. We used a highly technical protocol to determine if the oil level was too low. We first made sure the old power transformer was properly grounded. Then we felt the case of the transformer and then the cooling tubes. Yes, the sophisticate instrument of choice was the human hand. All the cooling tubes were warm and the tank was slightly warmer than the cooling tubes. The heat distribution was as we would expect for a properly cooling old or new oil filled power transformer. We knew from the type of bushings that they extended below the oil level. The oil stain from the leak was fairly large, but it was caused by chronic weepage and not an aggressive leak. So MIDWEST’s opinion was the old obsolete naked power transformer would last until they could schedule repair. Ironically enough, the oil weepage was from the plug that was installed in place of a top valve. So they scheduled the repair per their production schedule. They didn’t need to replace this old oil filled power transformer after all. We repaired the leak, added valves and gauges, sampled the oil, added some new oil, and ran some transformer tests and all was well.
A very important note, MIDWEST made sure the transformer was grounded before touching it. This is an important safety tip. You do not want to become the transformer case ground. Electricity through your body will make you very unhappy.
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.