NGR - Ground Detection NEC 250.187 (2)

[myntz]

I have a 5MVA 26kV to 4.16kV Delta to Wye transformer with ~3 Ohm NGR in series between Neutral and Ground. Looking for help on NEC 250.187 (2) - "Ground Detector are installed on the system". Does this mean I need a current transformer monitoring the NGR current? If so, is there a general rule of thumb for sizing the CT?

 

[don_resqcapt19]

If you bought an actual NGR, that same company very likely can supply the require ground detector equipment.

 

[David Castor]

Yes, there should be a CT between the neutral point and the ground connection. It could be in the transformer. Then you need a ground relay connected to the CT and finally, you need a breaker that the relay can trip. For 3 ohm resistor, the maximum ground fault current will be 2400/3 = 800 A. That's pretty high. I'd verify the 3 ohm value. Also verify there isn't a single phase transformer between neutral and ground - this would create a high resistance ground.

For 800 A of fault current, my rule of thumb would be probably 800/5 for the CT ratio. You could go lower for more sensitivity, but with 800 A of fault current, it's going to be a pretty obvious situation.

 

[myntz]

If you bought an actual NGR, that same company very likely can supply the require ground detector equipment.

I should have mentioned, this is an existing installation with ancient NGR and protective system. I am replacing the transformer because it is failing. I am going to get a transformer with CT internally so I can tie it to some sort of protective system. My goal right now is to size this CT to prevent over saturation but still works as a protective scheme. I don't believe the current system has anything monitoring the NGR current.

 

[myntz]

Yes, there should be a CT between the neutral point and the ground connection. It could be in the transformer. Then you need a ground relay connected to the CT and finally, you need a breaker that the relay can trip. For 3 ohm resistor, the maximum ground fault current will be 2400/3 = 800 A. That's pretty high. I'd verify the 3 ohm value. Also verify there isn't a single phase transformer between neutral and ground - this would create a high resistance ground.

For 800 A of fault current, my rule of thumb would be probably 800/5 for the CT ratio. You could go lower for more sensitivity, but with 800 A of fault current, it's going to be a pretty obvious situation.

This is a transformer replacement. Currently the NGR is 3ohm without any CT monitoring the neutral line that I can see. We have no documentation or anything and a shutdown is not possible. I am replacing this transformer and having it customized with a neutral CT inside. I know the full current is 800A max during a fault, but I am hesitant on the 800A as this may not be sensitive enough to trip. I am hoping to get find some sort of white paper or rule of thumb for NGR CT sizing for protection. Of course I can go lower, but then I ran into risk of over saturation during a hard fault. So this is my dilemma right now.

 

[David Castor]

Call your local Schweitzer (SEL) engineer. 800/5 should be sensitive enough. At 4160 V, there would rarely be low current arcing faults like you see at 480 V. The relay pickup can usually be set at 0.1 A secondary - that would be 16 A at 2400 V.

If it makes you feel better, most CTs have a thermal rating factor of 2.0, meaning at 400/5 could carry 800 A continuously. Even lower ratios would work, assuming the fault will be cleared promptly. If you're buying the transformer, I'd get a CT on the neutral bushing inside the tank. But there are a lot of option. What's the rating of the resistor - 10 seconds?