64REF CT implementation

[niilaryea]

The restrict earth fault on my unit transformer 169MVA 161kv has been operating any time there is fault in the Grid. On one of the disturbance records , it was observed the polarity of the High side CT and neutral CT were the same. Am tempted to reverse the polarity of the Neutral CT. I would appreciate you advice on this.I have have upload the the disturbance and one line.

 

[inpran]

niilaryea,
I suggest you to check the CT connections once before chainging the polarity. The one line diagram is not quite complete in all details.
I assume that the REF is of High Impedence type. If that is the case, are the CT ratio and magnetising characteristics of the HV Side CT and Neutral Side CT identical?
If the Characteristics and ratios and the polarities are all same (and correct as well) I would then look into the relay configuration once. (Have come across few erroneous ones recently)

 

[odlanor]

It seems there are 2 64REF . One for 1 neutral CT and other for 1 neutral and 3 phase CT.

 

[86ed]

I have experienced this same issue in the past. Our GSU neutral bushing CT polarity was reversed. Double check all CT wiring if possible perform all CT polarity testing and finally double checking the CT starpoint direction settings in your relay.

 

[ScottyUK]

Almost certainly a reversed neutral CT.

If you can't get access to the CTs then you can prove the scheme stability by using an LV or MV generator connected to the LV winding of the transformer and using the HV earthing switches to 'steer' current to where you want it to flow, i.e. to force current to flow in the HV neutral. It's a simple test but it's also a fairly high power test which takes a bit of setting up, which is one reason why some commissioning guys on a completion bonus don't bother.


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If we learn from our mistakes I'm getting a great education!

 

[niilaryea]

Thank you all.Comments have been very informing.