Change in Transformer Inrush

[mgtrp]

We are having issues with one of our step-up transformers tripping on inrush almost every time we energise it.

The transformer is 3/5 MVA and is a star-connected autotransformer with delta tertiary. It is a step-up between a four generators and the grid, and has been in-service and trouble-free for almost 10 years.

We are able to energise it from the grid without issue, but when we try to energise from the generators, it almost always trips immediately due to "switch-on-to-fault" instantaneous overcurrent. I am confident that the relay setting is not a problem because it has not changed, and we are also seeing the high currents on the generator protection relays (pick up of overcurrent elements on all four units, and one has seen sufficiently high current to trip).

I presume that an increase in transformer inrush would be caused by the core/windings shifting, or alternately by the transformer becoming magnetised. We will look to have magnetising current measured at some stage, although I'm not sure that we'd have past test results to compare against on this unit.

Any other suggestions of possible causes, and methods to diagnose/repair?

Thanks!

 

[mgtrp]

To close the loop:
The transformer is no longer tripping when energised. A technician travelled out to site, and checked the transformer. The transformer was tested and came back with virtually identical results to when it was commissioned, so no concerns there. He also reported that the transformer noise was nothing out of the ordinary.

So, with the transformer working correctly and no further testing, we are calling this case closed, albeit with reservations.

 

[electricpete]

Thanks for the feedback?

Does it no longer trip when you energize from generator?

Have you formed an opinion as to the cause?

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(2B)+(2B)' ?

 

[mgtrp]

Nope, no more tripping, everything is working normally.

All I can think of is that we had some remenance, and that now it is gone.

 

[Skogsgurra]

Very good to have that feedback! Has the grid voltage been changed? A few volts may suffice to make a difference.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[mgtrp]

No, nothing else has been changed.