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Class X CT calculations

Class X CT calculations

Class X CT calculations

(OP)
HELP!!!!

I'm trying to spec some class X CT's for a 5.5kV 5MWe generator set into Algeria. The customer has requested that we use Class X CT's for the differential protection. This isn't something we normally use anymore, i think due to the fact that burden's are so low on the electronic relays and also CT's are mass produced these days too.

In the orginal spec i specified 1000/1 Amp 15VA CT's. With this in mind the Generator manufacturer (from whom we purchase the CT's) has come back to me asking for the following information:

1) Minimum knee point
2) Maximum Excitation current
3) Maximum Resistance.

I do have a formulae for the knee point voltage as follows:

Vkp = 2 x Vs   where Vs is the setting voltage

Vs = (If/N) x (Rct + 2RL)

Where If is the fault current in the primary, N is the turns ratio, Rct is the CT secondary winding resistance at 75 deg C, and RL is the maximum lead resistance from the Ct to the relay.

Firstly is this calculation accuate? Secondly how do i calculate both the Max Excitation current and the Maximum Resistance (which i assume is the Rct i need in the formulae above)

Any and all help is greatly appreciated.

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