Capacitive Charging Current

[mrg397cs]

We have a 13.8 kV generator neutral grounded with a 100 amp resistor to limit transient overvoltages. The capacitive charging current is calculated to be 30 amperes @ 13.8 kV. The plant will expand with long 15 kV cables and the capacitive charging current may rise to 60 amperes. Is this excessive charging current? Shouls an isolation transformer be added to isolate the existing system from this new charging current?

 

[DanDel]

A neutral grounding resistor is usually used to limit ground fault current. I'm not sure how it could effect transient overvoltages, though a large value resistor can make it possible to have a voltage between neutral and ground during a fault.

 

[rbulsara]

Sounds very high. While all numbers are relative.

Is 30A a steday state value?

What size system, generator and what length are we talking about here?

 

[alehman]

Normally the grounding resistance is sized so the ground fault current slightly exceeds the charging current to minimize transient overvoltages.

 

[jbartos]

Suggestion: Relay trip setting will need to be adjusted if sensitively set at the present time.

 

[Kiribanda]

Hi mrg397cs,

Yes it seems to be very high. If you refer any cable catalogue even a 33 kV cable will not have a charging current more than 2 Amp per km per phase. So it is difficult to understand how you have got 60 Amp as the charging current on 13.8 kV system? Is it per phase or the total?

If your figure is correct then your grounding resister should have 180 A rating at 13.8 kV line voltage. That means you cannot have a High resistance grounding system on 13.8 kV, because it is normally in the range of 2-5 Amp maximum.

That means you have to adopt a Low resistance grounding system.(You are already having a 100 A LR system) The let through current depends on the allowable earth fault current of the generator or 13.8 kV motors (if any). If you adopt that LR earthing then by doing a short circuit calculation you have to make sure that your earthing system is effectively grounded. For that you have to prove at all fault locations with all the new cable lengths

(X0/X1)>0 and<3, (R0/X1)>0 and <1.

I could not understand what you mean by isolating transformers? But it is interesting to know how you have got such a high charging current?

Regards!
Kiribanda