Schwatzernov
Electrical
- Sep 5, 2006
- 47
Dear All,
I have one question regarding to the CT sizing for phase overcurrent relay. The question is that the CTs used for motor feeder normally is dimensioned based on continuous current of motor. For example motor 320kW, 6.6 kV, the 5P20 class,50/5 A CT 15VA is selected while the max. three-phase fault at point just below VCB for its feeder is 30kA.
This means that the fault current is 30,000A/50 A = 600 times of its rated primary current. Does this means that if fault occur, the CTs will be always saturated ? so all current will comes to magnetizing part of CTs which there would be very few current flowing to 50/51 relay so relay will not operate for fault. Anyone could explain me ?
While selecting CTs with 30,000A/20 = 1500/5 A for this size of motor is very unusual, how to select suitable CT ?
Thank you
Schwatzernov
I have one question regarding to the CT sizing for phase overcurrent relay. The question is that the CTs used for motor feeder normally is dimensioned based on continuous current of motor. For example motor 320kW, 6.6 kV, the 5P20 class,50/5 A CT 15VA is selected while the max. three-phase fault at point just below VCB for its feeder is 30kA.
This means that the fault current is 30,000A/50 A = 600 times of its rated primary current. Does this means that if fault occur, the CTs will be always saturated ? so all current will comes to magnetizing part of CTs which there would be very few current flowing to 50/51 relay so relay will not operate for fault. Anyone could explain me ?
While selecting CTs with 30,000A/20 = 1500/5 A for this size of motor is very unusual, how to select suitable CT ?
Thank you
Schwatzernov