11kv Generator Grounding

[genhead]

A 2mva 11kv generator is to be added as stand-by to an existing 11kv substation. The generator will operate in island mode when the utility supply fails. It will operate in parallel with the utility supply for peak shaving. The incoming transformer is grounded via a neutral-grounding resistor. The generator will need its own ngr when it runs in island mode (utility supply transformer is isolated). What about when paralled with the utility supply? Would the generator ngr have to be disconnected, or can both ngr remain connected?

 

[slavag]

Hi.
we have so much threads on the topic.
You can search in the Forum.
Actually, good practic it's only one grounding point in the
substation for avoid circulation current and heating of generator stator by 3hd harmonics.
From other hand, you can work with both NGR are connected.
Please search in the Forum also term hybrid generator grounding, for my pinion it will be must for the industrial generators, but of course not for 2MVA.
Best Regards.
Slava

 

[Kiribanda]

genhead,

It is difficult to give a solid answer without checking your single line diagram.

In these co-gen systems there is one main requirement to satisfy which is always a Utility requirement. That is, the HV side of your stepup transformer is always "EFFECTIVELY" grounded with or without the generator.To satisfy that you have to have solid grounded WYE on the HV side. Then to avoid most of other problems you have to have DELTA on the 11 kV LV side. So the step up transformer is "mostly" to be YNd.

Now your generator is always WYE. So you have two choices.

1) Ground the generator star point by a suitably sized distribution transformer so that your 11 kV generator is high resistance grounded

2) Ground the generator neutral using a suitably sized low resister so that your 11 kV generator is low resistance grounded

The chioce depends on your local codes, country and the existing grounding system details.

Also do a system study with the generator and ensure the Utility side is effectively grounded.

By doing above you are ensuring a helathy ground fault detecting/ tripping system even the generator is islanded.

Hope this helps.

Kiri

 

[dpc]

As Slava said, ideally, using only one NGR for the entire system is best, but it can be complicated when there are multiple sources. Switching of the neutral can be done, but introduces another failure point and invites possibility of someone making a mistake.

Since the transformer will (I hope) be de-energized when the generator is operating in island mode, its NGR will not be functioning. So I think you will have to provide a separate NGR for the generator. You may want to get information on the expected harmonic voltages produced by the generator and look at the circulating current magnitude if both NGR are in service. It may not be a big issue. You could also put a switch or contactor in the generator neutral such that the generator NGR is only used in island mode. I don't like putting switching devices in neutrals, but it is done, especially outside the US.

If the generator will be running islanded only infrequently, you could consider using a high-resistance grounding system for the generator neutral. That would greatly reduce concerns regarding circulating currents when operating in parallel with the grid.

The downside of the HRG is lack of selectivity for ground faults in the island mode.

The hybrid grounding scheme Slava mentioned is a good option if you're feeling ambitious. It was recently developed by an IEEE Working Group and combines high-resistance and low-resistance grounding techniques.