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 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.

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.

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.

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.