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Detecting if the Neutral Ground Resistor (NGR) is shorted is something that must be checked at the Ground Fault Protecting Relay. The typical way to do this is to inject a DC component and to measure the voltage drop in the NGR. If no voltage drop is seen, it is assumed that the NGR is shorted and the relay trips. I would like to have Wye-connected PTs, used to monitor the quality of line to ground voltage and NGR monitoring with NGR short circuit condition detection at the same time on a 4160 V transformer secondary.

Having Wye-connected PTs may confuse the ground fault detection relay as for the DC component the PTs provide a low impedance parallel path which can be confused with a shorted NGR and this must be avoided.

Is adding a Potential transformers Ground resistor (PTGR) the only way to achieve both things? Wouldn't this affect the reading of the system monitoring the quality of line to ground voltage?

Update: There are also PTs that are build with a capacitor voltage divisor on their primary but I haven't found any operating at 5 kV

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  • \$\begingroup\$ What is the sensitivity of your ground-fault-detection-relay and what is the maximum current through your potential transformers? In my experience, a (sensitive) ground fault detection relay is set to about 5 amps primary, and the load current of the potential transformer would be about 0.1A. \$\endgroup\$ – Li-aung Yip Apr 7 at 8:44
  • \$\begingroup\$ I have added more details to my question. The load current of the potential transformer for DC is very high. I would like to find what the typical impedance and load current of the potential transformer is for AC. Where can I find values like 0.1 A? Thank you very much \$\endgroup\$ – VMMF Apr 7 at 16:23
  • \$\begingroup\$ A typical specification for a 11kV electromagnetic voltage transformer (no capacitor divider) has a rated output of 700 VA thermal on the secondary. Working backwards to 11kV amps, 700 VA / 11kV * sqrt(3) = 0.11 A. \$\endgroup\$ – Li-aung Yip Jun 12 at 14:11

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