Power Transformers Blog – Expert Safety and Usage Info

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.

We received feedback from a new Technician who was scrapping out an old 5000 kva oil filled power transformer. It was over 40 years old and hadn’t been in service for 20 years. The primary voltage was 13,800 and the secondary ranged from 120 volts to 600 volts. These voltages made it an unusual transformer, but it was worth more for scrap copper and steel than for reliable reuse. In addition we had limited knowledge of the history of this 5 Mva transformer.  The test results were barely acceptable, not good.

The young technician had never seen the inside of an old oil filled power transformer, especially such an unusual power transformer out of a foundry. The things he found interesting, a veteran oil transformer repair technician would take for granted. He was surprised how tightly the core and coils fit inside the tank and how the high and low voltage bushings connected to the transformer. He was amazed by the over 3” diameter round solid copper secondary bushings. He saw how the liquid level gauge and temperature gauge for a 5 Mva oil filled transformer worked. The level gauge inside the power transformer had a simple small rod with a cork at the end. The temperature gauge was just a well extending through the wall of the 5 Mva transformer with a temperature gauge installed in the well. The transformer had a basic tubing and pressure gauge system to add nitrogen or dry air.  All very basic stuff, but once he saw all this stuff and the interior of the transformer, these basic things made a lot more sense. Good experience. If a picture is worth a 1000 words, then this simple experience was worth 10,000. Whether a 5000 Kva transformer, 10 Mva, 20 Mva or 50 Mva oil transformer, some of these transformer basics are the same.  

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.

The internet might be a great tool to find a replacement reconditioned or used 20 MVA transformer, fast. But the internet information could be deceptive and expensive. Nothing wrong or illegal, but it just might not be as good a deal as you think. Consider this example of what has happened to MIDWEST. A customer calls for a 25 MVA Transformer. Say a 138 Kv to 34.5 Kv, oil filled power transformer. And say our sell price is $300,000.00, plus freight. This should be a straight forward transaction.  And often these transactions happen fast, especially when someone has had a transformer fail. But, in this example, the buyer is a transformer ‘lister’ who marks up our price a whopping 30%.  A ‘lister’ is an entity that just lists equipment on their web site. They have no inventory themselves. They have limited technical value. They basically just have the information. This is not an indictment of all listers. Some are very good. But, in this case the ‘lister’ has been contacted by an electrical equipment dealer who also does not stock large equipment, much less large transformers. The dealer marks up the lister’s price a whopping 30% also, because he thinks the buyer just has to have the transformer. The dealer has been contacted by an electrical contractor, who also thinks he can mark it up 30%, because his customer, the actual end purchaser and user of the transformer, just has to have it. The problem is, by the time the shark’s get done marking up the price, the cost has doubled to over $650,000.00 and the end user knows they can buy a brand new unit for much less than $400,000.00.  They feel taken, so they decide to cripple along until they get a brand new transformer. 

This same scenario can happen with high and low voltage switchgear, circuit breakers, and any number of other type electrical equipment.  So how can you tell if you are talking to an intermediary or directly to the source of the equipment? The best bet is to be talking to someone who knows the equipment. Is an expert on the equipment you want to purchase.  Can give you detailed additional information immediately rather than making you wait two hours for answers to each question. Deal with the experts.

What do you do if your manufacturing company suddenly loses its main 5000 kva, 34.5 kv to 480 volt transformer? You have orders pending. And if you can’t fill them, a key account may go elsewhere. You have employees suddenly out of work.  You need to heat the buildings.  And you are told it will take 10 weeks to get a new replacement transformer. This is one of a manufacturer’s worse nightmares. What to do? This happens more frequently than one may realize. MIDWEST actually recommends a “Disaster Action Plan,” on the shelf, ready to go, in case of such a nightmare.

MIDWEST receives these calls all too frequently. You might be fortunate and MIDWEST have a replacement reconditioned or used 5000 kva transformer that can be shipped immediately. No one has new transformers this size in stock. And many companies will not accept anything but a new replacement power transformer. In that case, the best solution is to rent a temporary transformer that will provide power until a new replacement transformer can be specified and manufactured. A new replacement transformer may take 10 weeks.

Renting transformers, and other electrical equipment, is a common solution to this critical problem. The first big question to ask when such an emergency occurs is, “Do you want power back on permanently as quickly as possible or do you just want power back on as quickly as possible?”  There is a huge difference in the time to perform either of these solutions. And way too often there is the presumption that one just must get power back on permanently. This presumption continues until the customer realizes how long it will actually take. Meanwhile many hours, even days, are wasted by chasing the wrong solution.

So the most critical event is to decide how fast one wants power restored.  MIDWEST has transformers, temporary high voltage cables and low voltage cables, and switches in stock.  But, more important, is the knowledge that temporary power installations can be a life saver for a facility that is suddenly down.

This blog concerns a very simple maintenance procedure MIDWEST performs on electrical arc furnace transformers. It has saved our customers tons of money over the years. First a little information on arc furnaces and arc furnace transformers. 

Electrical arc furnaces are basically monstrous arc welders. Instead of the electrodes being pencil thin, they might be 14 inches or more in diameter. And there are three electrodes. The arcing is extremely loud and creates a lot of dust. The dust collectors take care of most of the dust, but there always seems to be dust that gets on to the secondary bus bar of the arc furnace electrical transformers. Whether McGraw Edison, Pennsylvania Transformer, or many of the other arc furnace transformer manufacturers, dust can be a real enemy of the secondary bus bar.

On some arc furnace transformers, both ends of an individual winding come out of the transformer in such a configuration that there may be only a quarter of an inch between the end winding bus bar. In other words, two massive copper buses, separated by only a ¼”, may have 280 volts potential between them during operation of the electrical furnace transformer.  And the secondary windings are huge masses of bus bar. If these bus bars become coated with foundry dust and the dust is conductive enough, a massive arcing fault may occur at the secondary of the transformer. Unfortunately MIDWEST has responded to such emergencies, and they are a mess. Very expensive repairs.  And so easily preventable.

The unsophisticated solution is to clean the dust and any other contaminates from between the bus bars before they become a threat. One has to do this carefully so as not to damage the seal where the bus comes through the top of the transformer, for example. It may look simple, but only experienced and qualified electrical personnel, who know what they are doing, should do this. In the wrong hands, this maintenance procedure could easily be the cause of a major blowout rather than the prevention.

MIDWEST works a lot with electricians, maintenance mechanics, and other assorted electrical folks that have the every day job of keeping production machines running. When they call for help, for example when they suddenly need a replacement transformer, they want the short answer to their question. Sometimes you can tell from the edge in their voice that they are overwhelmed, in a real jam, and, if we put more burden on them by asking twenty questions about their old transformers, they are just going to stop listening, say thank you, and hang up. They want an immediate short answer to their immediate question.  This isn’t always possible or safe, but, since we have so much field service experience, we usually can get to the point quickly.

Sometimes the simplicity of the request can be amazing. For example, we had an electrician call during his lunch break and ask how he could tell if one of their old dry type transformers, used for production machines, was overheating. They had added a ton of equipment to it over recent years and he was worried the transformer was overloaded and would fail. His boss knew nothing about old, new, replacement electrical transformers and really didn’t want to be bothered with a “maybe problem.” The electrician needed something to get his bosses attention. He couldn’t measure the load, but he just knew it was overloaded.

Our unscientific suggestion was to put his hand on the top of the old power transformer enclosure. If he couldn’t hold it there for a few seconds, the transformer is in danger of being overloaded. The load should be measured and compared to the nameplate rating. If they don’t, they run the risk of the transformer failing when they least expect it, say the day before a holiday. Getting and installing a replacement transformer under emergency conditions can be a lot more expensive than scheduled transformer replacement. You don’t want to get burned.

Safety note. Be sure the transformer is grounded before touching it.

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.  

Oil Filled Transformer

A utility electrician asked MIDWEST how a transformer differential relay worked. He said he wanted a simple answer in English. He had talked to an electrical power transformer engineer and by the time the engineer got done with diagrams and calculations, he said he had no idea what the guy was talking about. He was more confused than ever. This was a practical person who had worked on 5000 kva to 50 Mva oil filled power transformers. He wasn’t an engineer or a test technical.

            So here goes a nice understandable explanation. It’s not very technical and it’s a little figurative. The differential relays for protecting very large Kva and Mva electrical power transformers are basically just measuring the power going into the power transformer and measuring the power coming out of the power transformer. The transformer relays compare what goes in to what comes out and if there is a big enough difference, they tell the protective circuit breaker to “Turn off the transformer!” Because, if there is less coming out than is going in, that means energy went somewhere it wasn’t suppose to. Maybe to ground. And that could make a 20 Mva electrical power transformer very unhappy. The job of the relays is to turn off the power going into an electrical oil filled transformer before a fault destroys the transformer.  The calculations and diagrams are pretty cool, but unnecessary to understand the basic concept. Differential protection is used elsewhere in electrical power systems, especially where there is critical expensive switchgear. You typically would not find differential protection on a 2000, 3000 or 5000 Kva oil filled power transformer. But when the transformers are defined by Mva, say 20, 30, 50 or 100 Mva oil filled power transformers, then differential protection can be very important.

There is an interesting difference in language between industrial electrical engineers and utility electrical power and transmission engineers. One area where this is most evident is their language for electrical power transformers, for MVA versus KVA. For example, their difference in quantitative perspective between a 10 MVA and a 10,000 Kva transformer.

Let’s say we are talking to an electrical utility power transmission engineer and refer to a 10,000 kva oil filled power transformer, say a Siemens or ABB. Their immediate tendency is to think of it as a small transformer, not too important. The tendency is based on language that includes Kva. They automatically think of Kva transformers as small. It has to be small because Kva is a small unit of capacity in their world. And they might assume the voltage must be only 15 kv, 25kv or 35 kv. Now if we were to switch to MVA and spoke of a 10 MVA oil filled power transformer, same size transformer, the utility engineer becomes engaged in the conversation, because now we’re talking their language. 10 Mva, 20 Mva, 100 Mva oil filled transformers, the stuff of real power. This is not a false bias.  It is the language of their world and it makes sense to them.

If we switch our conversation to an industrial or manufacturing electrical power engineer, just the opposite is true. Let’s say we are talking to a plant electrical engineer and refer to a 10 MVA oil filled ABB or Siemens power transformer.  Their immediate thought is that’s a big power transformer. And they would also have a tendency to think the voltage would be 115kv, 240kv. Some high voltage. If we were to change the language to a 10,000 kva Siemens transformer, they might think it’s just a pretty big unit for a large manufacturer. Not a big deal. This is still the language of bias. The way we think. It’s not wrong. It’s just the language we think in. And in each person’s world, it is correct.

In this example, we are talking about the same size transformer and having a little fun with the difference in perspective between the industrial world and the utility world.

In MIDWEST’s world we have to think in Kva and MVA.  Where they may not be the same is the difference in voltages typically found with MVA versus those typically found with kva transformers. This is our bias. MVA is big voltage and Kva is little voltage.  Usually, but of course these biases are not always true.