CTs problem 1

[PUNGLU]

We have three CTs on a feeder rated 400/1, 36 kV. The CT on one phase failed and we can't get one urgently. We have one rated 200/1, 36 kV. Is it possible to use the 200/1 in conjuction with the others considering that the relays are electromechanical on which the plug settings can be adjusted.

 

[ScottyUK]

At full load, assuming full load to be 400A, you will have 2x nominal current flowing in your relay. Can your plug settings accommodate this? Do you have a residually connected earth fault element? If so then forget it.

You've told us so little about the relaying scheme that it is impossible to give a yes/no answer.


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

 

[PUNGLU]

The scheme is an overcurrent with residual earthfault with the o/c setting at 100% and E/F setting at 0.15%. The plug settings can actually accomodate the required settings from 10% to 200%

 

[jghrist]

Does the CT thermal factor allow using the 200/1 ratio? Do the 400/1 CTs have a 200/1 tap? If so, then you can set all to 200/1 and change the plug setting to suit. You cannot use the residually connected earth fault element if you use unequal ratios.

 

[ScottyUK]

A CT group used with a residually connected E/F relay must be made up of 'identical' CTs. Identical in this case primarily concerns ratio, winding resistance, and kneepoint voltage. Unfortunately there is no way to incorporate unmatched CTs of differing ratios into a residual group for an electromechanical relay.

Sorry.


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