Rule of Thumb for Calculating Capacitive Coupling Charging Current
Rule of Thumb for Calculating Capacitive Coupling Charging Current
(OP)
I have a project that is in design phase that includes paralleled 12.47kV diesel generators feeding a medium voltage distribution system. I am planning to specify a high resistance ground (HRG) for the generators. I understand that the let through current of the HRG must be equal to or greater than the system charging current. How do I estimate the system charging current for the system? Are there any proven rules of thumb for calculating charging current for a 12.47kV distribution system?
The calculated demand load is approximately 15 MVA. The generators are feeding (16) 2500 kVA liquid insulated substation transformers.
The calculated demand load is approximately 15 MVA. The generators are feeding (16) 2500 kVA liquid insulated substation transformers.






RE: Rule of Thumb for Calculating Capacitive Coupling Charging Current
From the industrial power systems data book by GE Dec 21, 1964.
Table one on page 1 .32 has a sable
460 v charging current 3Ico - amps/1000kVA of system capacity to be around .1-2.0. that is without andy contribution from surge protective capacitors for rotating machines.
2400 v is the same.
the charging current must be tested to obtain a perfect number and distances of conductors and rotating machines.
Transformers are usually negligible becuase of the spacing between the transformer core and the windings, and the shielding effect of the xfmr winding layers adjacent to the core prevents the other winding layers from materially increasing the winding to ground.
overhead vs underground is a huge factor.
even if you do your best and get every component - it is still a close approximation at best.
RE: Rule of Thumb for Calculating Capacitive Coupling Charging Current
RE: Rule of Thumb for Calculating Capacitive Coupling Charging Current
The only publicized rule of thumb I could find was from I-Gard (google igard charging current). Here the rule of thumb is 1A/2000kVA for 600V systems and 1A/1000kVA for 4160V systems. Since this system is 12.47kV I'm thinking of estimating 1A/500kVA. With 15MVA of calculated demand load then there would be 30A of charging current. Does this approximation make sense?
RE: Rule of Thumb for Calculating Capacitive Coupling Charging Current
RE: Rule of Thumb for Calculating Capacitive Coupling Charging Current
RE: Rule of Thumb for Calculating Capacitive Coupling Charging Current
Westinghouse Industrial and Commercial Power System Application Series White Paper (PRSC-4A dated January 1978). Title = System Neutral Grounding and Ground Fault Protection.
RE: Rule of Thumb for Calculating Capacitive Coupling Charging Current
The charging current can be dependent on the voltage level and the conducting medium (OHL vs Cable), also take into account the whole network at this voltage level as anything else that is connected to the busbar will contribute to the charging current.
If you are installing a very high impedance ground i suggest you treat it as if you were isolating your star point and consider a neutral displacement relay to pick up a single phase to earth fault.
My attachment is a siemens document outlining same.
Niall Molloy
Electrical Engineer
ESB