Power Transformers Blog – Expert Safety and Usage Info

Power Transformers Defect at Top of Winding

MIDWEST frequently rejects equipment even thought it passes the electrical tests. Here’s an obvious example. MIDWEST’s Switchgear Shop rejected a 225 kva 480 volt delta to 480/277 volt wye dry type electrical transformer. We only had the core and coils, not the enclosure. The insulation resistance, winding resistance, and turns ration test results were all good. The transformer was energized at full primary voltage and had proper secondary voltage for the tap position. This all sounds good. So, why was the transformer classified scrap junk?  An inspection revealed the top two turns of the primary windings of one barrel had been crushed down. Something had been dropped onto these winding turns and pushed them down and separated them. The damage wasn’t physically dramatic. It was almost inconspicuous. But it was fatal to the transformer. In addition, this damage didn’t appear to have caused the fault that blew the fuses that resulted in the owner sending us the transformer to check it out. But we also found one of the ‘unused’ tap tabs was melted off to the point the bolt hole was completely gone. The end of the tab looked very clean. The transformer was connected in a different tap. On top of all this, the side of the winding seemed blackened by an apparent arc blast. Apparently the bolted connection at this tap became loose and the arcing completely destroyed the tap connection. The taps of the transformer were changed in an effort to restore power until a replacement transformer was installed. Sometimes the physical damage to electrical power transformers is very obvious. In this case, the damage to the electrical transformer was not as obvious as one might think.  At least until after it was found. Test results alone can not be used to evaluate the condition of electrical dry type power transformers. The transformer must also be thoroughly inspected. What MIDWEST calls a ‘hard focus’ inspection.

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.