Medium Voltage System Ground Protection

[FaradayFF]

Hello,
For medium voltage 3 wire, 3 phase system is ground fault monitoring typically employed using the residual element in the feeder protection relays? The NEC doesn't really cover the Medium voltage protection requirements apart from conductor protection in MV system, so I guess ground fault could fall in that category.

Thanks,
EE

 

[Julius Right]

IEEE 242/2001 8.3.5 Sensing, relaying, and trip devices

The signal for ground-fault protective devices may be derived from the residual of phase CTs, window CTs, or sensors.

 

[paulengr]

How are you planning on meeting insulation coordination? Ungrounded MV transient overload factor will be around 10 or more. May want to seriously consider high resistance grounding which is pretty standard. Then TOF is 1.0 and you can just put a CT on the resistor or use BYZ CTs everywhere else (run all three phases through a single CT minus the grounds). Residual (phaser math or connect grounds of all 3 CTs) suffers accuracy issues since you add errors of all 3 phase CTs.