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.
225 KVA Dry Type Transformer For Sale
Many facilities are declaring their maintenance budget to be revenue neutral. Our recommendations for maintenance of old transformers are both condition based and time based. For example, one may recommend annual infrared scanning and annual dielectric fluid analysis on fluid filled power transformers, especially old or obsolete transformers. Whereas, old indoor dry type transformer should be cleaned and maintained based on their condition, their environment. A foundry would be more condition based than a commercial facility. A problem occurs when the conditioned based maintenance costs are unusually high in a particular year, say this year. Some maintenance supervisors have been told if they add cost in one area, they must remove cost somewhere else. Revenue neutral decision making. This can be roulette in some facilities that have a lot of old or aging equipment that really needs attention. Unfortunately what often happens is time scheduled maintenance is postponed in favor of condition based service. MIDWEST suggests a compromise. Review “Schedule Services” and perform those services on equipment that is most critical and has the poorest service condition. Likewise, service “Condition Based” equipment that is both the most critical and most in need of service. These decisions are only good “if nothing goes wrong.” If something goes wrong, then, of course, it will be attributed to you choosing poorly. You can’t win. You might escape the first year. But as the financial stress on the maintenance dollar continues, what seemed to work last year, now appears more reasonable to “do the same” this year. And next. Until something goes wrong. Then you lose. You chose poorly. We recommend reviewing the maintenance services on obsolete, old, and new equipment each year using the cumulative information of all the prior years. Do not go back just one year. Also, good luck.
Anyone in our business has seen transformers with concrete pads no longer level. MIDWEST frequently sees padmount transformers off kilter because the concrete pad has shifted, usually because the soil in one area has washed out. It occurs far less frequently, but sometimes we see the concrete pad for a large outdoor power transformer has settled on one side, causing the transformer to no longer be level. Slight settling might not pose a problem. But we frequently, using Infrared Scanning, find old oil filled outdoor power transformers that are not cooling properly. This occurs when an old electrical power transformer has long cooling tubes. On some transformers, the oil level may only fill 2/3 of the upper heads for the cooling tubes. If the transformer is slightly out of level, some of the outer cooling tubes on the transformer may tilt high enough that the oil will no longer reach the head and will not circulate. These cooling tubes will actually look cold when viewed with Infrared Thermography.
If you find your transformer is not level or is actually not circulating properly, do not attempt to correct this while the transformer is energized. This is a very bad idea. You are too close if anything goes wrong. And there are things that can go wrong with oil filled transformers that you can’t even imagine. Transformers are very heavy, especially old obsolete transformers. They can get very unhappy and the equipment they are connected to, can get very unhappy and make a big mess, if you try to level them energized. It’s tempting, because it looks so easy. But, if something goes wrong, it’s hard to get out of the way.
MIDWEST was called on an emergency after a contractor tried to level a 3750 kva, 25 kv to 480 volt transformer, that was connect to service bus. The secondary bus faulted at the transformer throat connection and pretty much destroyed everything, transformer and bus. What seemed like a good idea one moment, turned into a catastrophe the next. Again, MIDWEST recommends thinking “consequences, not probability.”
The best test to predict the reliable operating condition of an oil filled transformer is the combustible gas-in-oil analysis. Using a syringe, 40 cc of oil is taken from the transformer and analyzed. When threshold levels of specific gases in the oil are reached, protocol demands appropriate action be taken. This is an extreme oversimplification of a combustible gas-in-oil analysis.
One such case involved a 3000 kva, 25 kv primary, 600 volt secondary, foundry furnace transformer. The oil test results indicated the transformer began generating high levels of acetylene. Additional tests were performed and the transformer was shut down and taken out of service. A temporary transformer was put in its place to continue plant production. The defective transformer was sent out to be rebuilt.
Furnace transformers are special breeds of cats, designed to withstand the stresses of heavy shifting loads. Often times these transformers are pushed to the max in terms of load limits. Based on secondary ampere readings, this transformer was pushed to over 100% of rated load capacity, creating high thresholds of heat in the windings. Transformers don’t much like that too well. Heat puts all kinds of funny stresses on the internals of transformers.
In two months time the rebuilt transformer was put back in service and life continued on.
Two quick qualifiers: One, switchgear manufacturers get it right most of the time. And two, it often takes a pair of fresh eyes to spot the obvious. Now that some perspective has been established we can continue with impunity.
An educational facility had a 13.8 kv primary dry type transformer located in a basement substation. The transformer panels (the skin) had just been pulled per a routine scheduled shutdown testing and maintenance procedure. What our techs found staggers the imagination, the scene echoing with “what were they thinking?” terminology. No, it was not Al Capone’s hidden treasure or Indiana Jones’s arc of the covenant. But the word arc is getting close.
Lying on the concrete floor, like an extension cord, were the standard 15 kv non-shielded switchgear cables. The cables ran from the primary switch compartment to the transformer compartment and had a white chalk like substance running the length of the cable where it made contact with the floor. The chalk like substance was the result of corona (ionization of nitrogen in the air) induced by the intense electrical field between the cables and ground, the concrete floor.
A chemical cocktail soon forms as the corona goes about its business and over time arcing takes place causing carbon tracking (dark carbon deposits) and eventual destruction of the cable or an electrical fault. The cables were replaced and were properly supported in open air with proper clearances. A certain disaster was avoided.
Buy Dry Type Transformers
Some common misconceptions about Transformers:
Transformers can easily be backfed. Yes and No. Transformers cannot always be reversed willy nilly. Due to cost control some manufacturers design transformers which do not allow the primary and secondary sides to be swapped. Contact your favorite electrical engineering office for your case specifics.
Dry type transformer are designs of convenience and can be stuck in a corner or in any confined space with no problem. False. Dry type transformers are designed for indoor use and are air cooled. They require an adequate supply of moving cooler air to operate within their proper temperature range. If dry type transformers are located in indoor substations, the substations should be properly vented to allow good cooler ambient air flow to circulate in and about the transformer.
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.