how to calculate knee point voltage of current transformer?
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Answer Posted / arvind negi
CT Knee Point Voltage Calculation Sheet
Primary Secondary Volt
Resistance Resistance Length
In MVA In AMP In kV In ohm In
ohm/km in Meter In %
1 CT ratio of the trasformer REF Protection -
1200 1 - - - - -
2 Transformer Rating, MVA 20 - - -
- - - -
3 % Impedance, %Z - - - -
- - - 13.9
4 Voltage of Line Side, KVLV - - -
11 - - - -
5 CT Secondary Resistance, RCT (ohm) - -
- - 6 - - -
6 Resistance of control cable of 2.5 Sq. mm.
- 8.5 -
7 Loop Length of Control Cable - - -
- - - 120
Calculation:
1) Fault Current at Primary Side of CT (kA) =
MVA/*100(%Z*1.732*KLV) 7.55
2) Fault Current at Secondary Side of CT, If (A)=
6.29
3) Cable Lead Resistance, RL (Ohm)=
1.02
4) Knee Point Voltage Vk (Volt) = 2If(RCT+2RL)
101.2
REMARKS : -
Knee Point Voltage of CT provided by CGL for REF CT
= 115V and for Differental CT = 200V are in order.
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