How can I decide the primary current of the Zero-sequence CT?

[Forward]

Dear everyone:

Would you like to give me some hint about how can I decide the primary current for the zero-sequence CT? In my concept, only in the condition of earthing fault or 3 phases is unbalanced, the zero-sequence current created and normarlly it will big especial when the 1 phase directly earthed. but how to calculated the primary current?

you will be highly appreciated.

Thank you.

Forward

 

[dpc]

Are you asking how to determine the turns ratio for the zero-sequence CT?

 

[DanDel]

The primary current through a zero sequence CT will be the vector sum of the currents of all the conductors passed through the CT window, same as any other CT. In normal conditions(no ground fault), this value will be zero. Typically, the CT has a continuous rating equal to the circuit being monitored.

 

[Forward]

Yes, I mean that for the zero-squence CT, we have 100/5,200/5 and so on.How can I decide which sort of ratio I should use, which calculation is based on when I decide the CT ratio?

Thanks a lot.

 

[DanDel]

100/5, 200/5, and so on... the first number is the primary current rating, the second number is the secondary current rating.
The secondary current rating must be determined from the input range of the GF relay being used. The primary rating, as I have said, typically has a continuous rating equal to the circuit being monitored.

 

[dpc]

The selection depends a little on what device you will be connecting to it and how your system is grounded. Ideally, you would like to have sensitivity to low level ground faults but also avoid saturation at high current levels. These goals are somewhat conflicting.

If the system is solidly-grounded, a higher ratio - 200/5 may be in order. For resistance grounded systems, a 50/5 might be more appropriate.

If you are trying to selectively trip overcurrent relays based on this current, then saturation is a concern. If selectivity isn't an issue, then ac saturation isn't as big a problem.

50/5 is very commonly used for this application on feeder circuits.

 

[peebee]

As DanDel has mentioned twice now, though, you should take the size of your monitored circuit into consideration. What is the ampacity of the phase conductors/protection that you're monitoring?