Charging Current

[GRP123]

Hi,

I have two questions :
Q# 1) Is it true that charging current is only avaiable in single conductor cables and is zero in case of three conductor cables because the magnetic field gets cancelled ?

Q# 2) For the first time I came across a situation I noticed that motors contribute to single line to ground fault as well. When I analyzed the short circuit report I found that +ve seq current and -Ve seq. Current are there but zero seq. current is zero.
I was under the impression that only Motors only contribute to three phase current. Is it true that single phase current contribution because of the unbalanced voltage due to ground fault.

I would appreciate a comment.

Thanks

 

[GRP123]

Hi All,

Any commmenst please.
Thanks

 

[rcw retired EE]

Q#1- Charging current flows to "charge up" the capacitance to gorund of a circuit when a circuit is first energized. All three cables in a circuit have capacitance to ground and will have a charging current. If voltages are balanced and the impedances of the individual circuits are balanced, the three phase charging currents will sum to zero.

Cancellation of magnetic field is not related to charging current.

 

[ijl]

Q#2- The positive, negative and zero sequence currents are equal at the fault. The zero sequence current is zero at the motor. The positive and negative sequence currents are nonzero at the motor because of the unbalanced voltages, as you wrote. The motor does not directly contribute to the fault current, but the whole network, including the motors determine the current. In my (limited) experience, the motors tend to reduce the earth fault current. I guess that the reason is that the source must feed both the motors and the fault, which tends to reduce the available voltage. It would be interesting to see a picture of your network.