Question

How to size a CT that accompanies a protective relay? use feeder with Load max= 389A.

please more explanations? thank you

Answer 1

For the sizing of CT for protective relay we have to calculate ;

E_{max-calculated} = ALF x (R_CT x I_sec + VA Burden Selected / Isec)

where ALF = accuracy limiting factor

R_{CT = resistance of secondary CT}

E_max-required = K_TD x (I_FSC/CT RATIO) x (R_CT + R_L + nR_R)

Where K_ALF = Accuracy Limit Factor

K_TDF = Transient Dimension Factor

E_{max-calculated} is always greater than the E_{max- required} for the safe CT sizing

Required Accuracy Limit Factor = K_TDF x (I_F/I_P)

Calculated Accuracy Limit Factor = K_ALF x (Selected burden + I_SEC X R_CT).(Required burden + I_SEC X R_CT)

For correct CT sizing Calculated ALF should be greater than Required ALF.