Power Transformers Blog – Expert Safety and Usage Info

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.

MIDWEST was asked what we did with perfectly good obsolete or old used transformers. Bit of an oxymoron since obsolete might be understood to mean no longer any good. But the intent of the question was obvious. Transformers that are too old to be sold in the used transformer market might be maintained in MIDWEST’s pool of rental, temporary, and emergency transformers or they are just scrapped. Typically a rental transformer is just for temporary use. But we have had many transformers on rental for over a year. Especially larger Mva transformers. We have a few we didn’t scrap out but kept around solely because of the special voltages or size or physical configuration, to be used in those rare occasions when a manufacturing company, for example, is in trouble with delivery of a special replacement transformer. This usually happens when they had a non typical transformer suddenly fail. The difference between these special temporary transformers, which we sometimes call cling-ons, and our rental transformers is that we’d really like to get rid of the cling-ons. But about every time we think we’ll scrap out an obsolete old unit, we get a desperate call and it is the only thing that will work and the only one the customer can find. So it is resuscitated again and lives on. We do have fewer of these than ten years ago. We just scrapped out a 2000 Kva oil filled power transformer, 13,200 volts to a variable secondary of 120 volts to 600 volts. Weighed 18,000 pounds. A big old power transformer, specially built for a transformer power lab about 60 years ago. We finally got rid of it because we never rented it in two decades; And we did not have a good biography on the unit, although all our test results were good. The install cost as a rental, emergency, or temporary transformer would have been potentially huge because of the oversize and weight due to the many voltage taps. A potential customer would only need one functional voltage and would be paying a premium for a monster oil filled power transformer when they probably could find a unit with their specific voltage. Plus MIDWEST can not rent a transformer we no longer have full confidence in, even during an emergency. In the last month we designated over ten transformers to the scrap heap. Mainly because of a lack of full confidence in their reliability, even though many of them had good test results. MIDWEST knows electrical tests, on used oil filled and dry type power transformers, are not a perfect indication of the condition of the transformers. So MIDWEST looks for reasons to get rid of the old and the cling-ons.