Converting ungrounded system into High resistance grounded system- Does size of the system matters?

[r1patel]

I got a 500KVA, Delta/Star transformer supplying a 3 phase 3 wire heater load in our chemical process plant which suffered recurring failures. Per observations of failure mode - it seemed to be a result of transient high voltage typical of ungrounded system.

With measured system charging current to only 0.37Amp, required neutral resistance is calculated to 561.75Ohm (=360/SQRT(3)/0.37A). To me, 600Ohm, 150Watt resistance should serve the purpose, at least theoretically ?! Or is this a too small a system to go HRG way.

I have no previous experience of such conversion and seeking second opinion.

 

[davidbeach]

You went the wrong direction in your resistor selection. 550 or probably better 500 ohms would be a better choice. 500 kVA seems on the small side. On the other hand I wouldn't go below 5-10A in the sizing. At that point you have a chance of tracking down a ground fault; I doubt that a 0.37 amp ground fault could be found.

 

[r1patel]

Thanks davidbeach,
You are absolutely correct in pointing out to choose the lower value of resistance to exceed Ic by Ir as a basis for designing HRG.

Next, I was planning to connect voltage relay (#59) rated 110V to sense the ground fault. The system is pretty simple with only one load equipment to look into for locating the fault and therefore no pulsing control etc may be required. Am I missing somthing?

Thanks in advance!!