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Soares Book on Grounding mentions that an ungrounded system is capacitively coupled to ground and the existance of leakage capacitance. I understand what capacitance means, but can anyone explain to me what the actual physical implications there are on the system due to this leakage capacitance. Also, why is this not such of an issue on a grounded system????

Thanks,

Laura J

Trying to keep it short and simple, so ask for more details if you want.

All AC systems have a natural capacitive coupling to ground. In a balanced system the associated leakage current cancels simialr to the way the balanced phase currents cancel. But if one of the phase conductors is grounded then there is no canceling of the charging current.

In a low resistance grounded wye system the overcurrent protective devices should operate when a phase goes to ground preventing further problems. However in high-resistance grounded wye systems the OCPD will not open causing the charging currents form the other two phases to increase by a factor of 3 (this is caused by the change of the phase angles and the increased voltage to ground).

The charging current in a high resistance ground system is minimal so even a 3 times increase is not usually a safety hazard. The primary reason we are concerned at all has to do with the settings and operation of protective realys and other ground monitoring equipment.

 
To effectively damp phase-to-ground overvoltage in an ungrounded system (with complete equipment grounding and bonding in place) the capacitance between phases and ground must be damped by a resistor connected from neutral-to-ground.

“Charging current” for 480V systems is roughly 1 Ampere/1,000kVA of system capacity. [In the case of delta systems, the neutral has to be derived from zig-zag or wye-primary—delta-secondary transformers.]

Here is my favorite story about the very serious problems that can occur with low-voltage ungrounded distribution.




[This message has been edited by Bjarney (edited 09-30-2004).]