Cable size of Generator NGR

[Sajid khan]

We have 11kV 2250KW MV Generator and want to Ground it through Neutral Ground Resistor(NGR). The cable size use for this generator is 3X1C-185sq.mm..
The question is what size of neutral cable to be use?? Will it be the same 185sq. mm or it can be reduced??
Regards..


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[iceworm]

What code are you working to, IEC? NEC?
I have no knowledge about IEC.
Under 2017 NEC:
Article 250, part X, Grounding of Systems and Circuits of over 1000V
250.180 (paraphrase) All previous sections (of 250) apply as modified by part X
250.187.B Impedance grounded systems: Must be identified and fully insulated. No mention of size.

So, go to 230.56.B, High Impedance Grounded Neutral Systems, Conductor Insulation and Ampacity: Not less than the maximum current rating of the grounding impedance, in no case smaller than #8 CU.

Most of the MV grounding impedances I have seen are sized to draw ~50A. Which would require a #8 - Same as the minimum.
Depending on your grounding impedance
Note: #8 is 8.3 mm^2

The gen neutral connection to the NGR is not based on the cable size of the generator feeder.

The IEC likely has similar rules, but you will have to check for specifics.

 

[topgone]

We have 11kV 2250KW MV Generator and want to Ground it through Neutral Ground Resistor(NGR). The cable size use for this generator is 3X1C-185sq.mm..
The question is what size of neutral cable to be use?? Will it be the same 185sq. mm or it can be reduced??
Regards..


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Calculate the possible fault that could happen on the neutral of your setup. Choose the appropriate cable to handle the said fault on the neutral cable. Take note that the time rating of the equipment would be just a few seconds--say 10 seconds!

 

[iceworm]

tg -
Excellent point. Most MV NGR I have seen are 50A. For 1kv, that is 320KW under a fault. Most I have seen are set to trip ~ 1 second.

 

[Sajid khan]

tg -
Excellent point. Most MV NGR I have seen are 50A. For 1kv, that is 320KW under a fault. Most I have seen are set to trip ~ 1 second.

Can you elaborate it through equation... Plz ignore my lackness..


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[iceworm]

Excellent point. Most MV NGR I have seen are 50A. For 11kv, that is 320KW under a fault. Most I have seen are set to trip ~ 1 second.

Whoops, that should have been

11 KV​

Can you elaborate it through equation... Plz ignore my lackness..

Maybe: I am not a designer.
NGR are connected gen neutral to ground. Most medium voltage NGR I have seen are sized for 50A.

When a fault occurs, the faulted phase goes to zero Volts to ground - yep it is shorted. This raises the neutral voltage to 11,000/(sqrt(3)) = 6350 Volts above ground..

So sizing the NGR to limit the fault current to 50A would give NGR = 6350/50= 127 ohms.

During a fault, the power developed by the NGR:
P(ngr) = E*I = 6350 X 50 = 318 KW. That is a pretty big heater.

All the ones I have seen are set to trip within 1 second.

As topgone said, the resistors are generally not rated for more than 10 seconds. Which is reasonable, 10 seconds is a really long time for a medium voltage fault. I think the only reason the trip is set as long as 1 second is to keep switching transients from tripping the gen off line

 

[Sajid khan]

Whoops, that should have been

11 KV​

Maybe: I am not a designer.
NGR are connected gen neutral to ground. Most medium voltage NGR I have seen are sized for 50A.

When a fault occurs, the faulted phase goes to zero Volts to ground - yep it is shorted. This raises the neutral voltage to 11,000/(sqrt(3)) = 6350 Volts above ground..

So sizing the NGR to limit the fault current to 50A would give NGR = 6350/50= 127 ohms.

During a fault, the power developed by the NGR:
P(ngr) = E*I = 6350 X 50 = 318 KW. That is a pretty big heater.

All the ones I have seen are set to trip within 1 second.

As topgone said, the resistors are generally not rated for more than 10 seconds. Which is reasonable, 10 seconds is a really long time for a medium voltage fault. I think the only reason the trip is set as long as 1 second is to keep switching transients from tripping the gen off line

Thanks , Now i am clear.. very well explain..


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