System grounding

[Mila2015]

I'm feeding a 480v MCC system by (2) MV 4.16KV feeds, through (2) 500KVA Transformers. We're adding a 550KW emergency generator to one of the MCCs. The question came up, whether we should add low impedance resistors to EGEN and XFMRs to limit grounding current. I've seen multiple applications where no resistors where used. Any thoughts on requirements on this, and advantages Vs disadvantages?

 

[electricpete]

I'm not a power system designer. My thoughts fwiw:
Neutral grounding resistors can limit fault current, which may mean less switchgear expense and less damage possible during faults.
It affects protective relaying strategies in many ways from detection to coordination. I tend to think ground fault detection tends to be less sensitive when a neutral grounding resistor is present than it would be in a solidly grounded system.

FWIW, at our power plant, the 4.16kv and 13.8kv systems are resistance grounded and the 480vac systems are completely ungrounded (our only solidly grounded systems are lower voltage levels). It's an oddball design, used in some power plants. We are not subject to the same code requirements as other industrial facilities (NESC vs NEC).

I'm sure others can provide a lot more.



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(2B)+(2B)' ?

 

[Mila2015]

Thanks electricpete! Yes, I agree, I see NGRs more in the MV systems, rarely in 480v systems. Electrical model for that particular plant does show them on 480v systems though. I understand this would affect relay settings since ground faults are now limited to 50-100A depending on NGR size.

 

[RRaghunath]

Earth faults involve earth loop impedance and rarely yield large currents when it comes to LV systems. Hence, it is not common to have resistance grounding in LV systems.
LV systems supplying critical processes are an exception and are high resistance grounded to ensure community of power supplies even with one earth fault.

 

[waross]

If the neutral of a 480 Volt system is not solidly grounded, the code prohibits ever using line to neutral loads.
I a large plant, this prevents the economy of using 277 volt lighting.
If the transformers are now solidly grounded, beware of the law of unintended consequences if you add NGRs.
Also, if you add NGRs to both the transformer, you will need a four pole transfer switch, with the added failure modes.
One of the added failure modes is the possibility of an open neutral and possible subsequent damage.
The safest switching arrangement is a three pole transfer switch, a solid neutral and one NGR on the neutral serving both the gen-set and the transformer. This is also the safest switching arrangement for a solidly grounded neutral.
The solid neutral may not always be feasible due to local conditions.
If it appears that there may be an issue with a three pole transfer switch, my first answer is;
"Please try harder." That being said, I realize that there may be the odd exception to the general rule.

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Ohm's law
Not just a good idea;
It's the LAW!

 

[dpc]

Earth faults involve earth loop impedance and rarely yield large currents when it comes to LV systems.

Line-to-ground faults at 277 V can produce quite significant fault currents.

High-resistance grounding at 480 V is increasingly common in industrial facilities in the US. It allows continuing operation in the event of a single ground fault, similar to ungrounded systems. However, as Bill says, this cannot be done if there are 277 V loads. Low-resistance grounding is never done at 480 V - it's not a safe practice.

Solidly-grounded 480 V systems are far more common.