Formula for Knee Point voltage calculation of a132/11kV CT

chandanasushma
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I have been using 800-400-200/1-1-1-1-1A CT with three class PS cores,5P10-20VA and 0.2-20VA.How to find knee point voltage and its burden. My supplier cannot provide the data. Its given by APTRANSCO that transformer differential protection has a formula of 40I(Rct+10). Please explain me the terminology and procedure. So I need your help sir.

Asked by chandanasushma on 9/26/2011 7:39:09 AM

Steven
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CT can be specified according to IEC or BS. The formulae you have looks like the one to convert from IEC to BS. The Siemens Power Engineering Guide (you can search for and download), gives the conversion as:

Uk = In * Kssc * (Rct + Rb) / 1.3

Uk = knee voltage
In = nominal current (1 A in your case)
Kssc = rated symmetrical short-circuit current factor (i.e. 5P10 -> 10)
Rct = secondary winding resistance
Rb = rated resistive burden (20 VA in your case)

Uk = 1 * 10 * (Rct + 20) /1.3

This does not tie up with the formulae you gave, so I’m thinking there is something wrong. Perhaps you should ask APTRANSCO where they got their formulae from.

Answered by Steven on 9/27/2011 7:12:57 AM

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