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

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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.
Protection
Term: Protection
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Current Transformers
Term: Current Transformers
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asked 9/26/2011
chandanasushma1
chandanasushma

3 Answers

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The formula for calculating the knee point voltage of a PS Class CT is:

Vkp > K * If (r) (Rct + Rb)

where,

Vkp = The minimum Knee Point Voltage of the CT
K = Safety factor, generally taken as 2
If (r) = Maximum Anticipated fault current, as reflected on the CT Secondary side
= Maximum anticipated primary fault current /CT Ratio
Rct = Secondary Winding Resistance of the CT
Rb = Two way lead burden of the wires from CT Secondary to the relay
answered 1 month ago EE65 17
EE65
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According to the answer from Sir Steven ,

I think there may be a point need to be confirmed :

About the Rb , Sir Steven said  : Rb = rated resistive burden (20 VA in your case)

and what i understand is Burden = Isn^2 *Rb

In this case Isn=1A, so the Burden =1^2*Rb, Rb is equal to the value of rated burden .

when the Isn=5A, the Burden=5^2*Rb , the Rb = burden/ 25

Please have a check .

Thank you very much .

 

answered 7/10/2012 Qian Minchun 2
Qian Minchun
  • Qian,

    You logic is correct in calculating the resistance of a device burden.

    Re-reading the question and my previous answer, maybe the terminology is not fully clear. Relays have a burden which can be expressed in VA (or resistance according to your formulae). Total burden of a relay circuit is that presented by both the relay and the connecting wirings. Total burden connected to the current transformer is the sum of each individual relay circuit burden for all the connected relays.

    The current transformers supply the necessary VA for the total connected burden. For a 20 VA current transformer, it can supply a total connected burden of 20 VA (or total connected resistance of 0.8Ω at 5A).

    Steven - Steven 7/10/2012

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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 9/27/2011 Steven 167
Steven

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