Multiple Generator Grounding & System Capacitive Charging Current

[sykimk]

We have total six (6) GTGs (all 15 MVA, 11kV generators) and all generators will be connected to a common 11kV bus. i.e. all generators in parallel and individually grounded. We have initially selected 100A NGR for each generator without neutral switching device.

In addition, we also have two 11kV/6.6kV distribution transformers with a vector group of YNyn0+D11. Common 11kV generator bus will also feed these transformers. HV side of these transformers is also grounded with 100A NGR (No neutral switching device). FYI, these transformers are used for feeding our Essential 6.6kV bus and all other distribution transformers have a vector group of Dyn11.

I understood that many references recommend neutral switching device for individually grounded generators and only one generator grounded in the system. But my client does not want to have neutral switching device since there is possibility of ungrounded system operation. It is their biggest concerns and there is no way to change my client’s mind.

Now we had 3rd harmonic circulating current concerns and generator manufacturer recommended 40A NGR for generator neutral grounding to reduce 3rd harmonic circulating current not to damage generators.

But we have lots of 11kV distribution cable feeders in the system, and capacitance charging current of cables (all feeders) will be about 30.1A. For the worst case consideration, I assumed only one generator running and all feeders are closed for ready to energize secondary substations. In this case, the maximum charging current will be about 33.4A. Therefore, I think 40A NGR would be OK.

Now, we are about to make final decision to select 40A NGR (NO neutral switching device) for generator neutral grounding (i.e. individually grounded).

If we have GTG load sharing modules to control active & reactive power and put generators in voltage control mode, does it help us to reduce 3rd harmonic circulating current? As far as I know, generator 3rd harmonic voltage is dependent upon generator loading. If all generators have been equally loaded, I assume there will be no 3rd harmonic circulating current between generators but circulating current will flow between generator and NGR (100A) of transformer primary winding. Therefore, I also need to change transformer primary NGR size from 100A to 40A as well in order to keep low 3rd harmonic circulating current.

Did I understand correctly??

If I remove NGR from transformer primary winding (YNyn0+D11), i.e. ungrounded primary side, are there any adverse outcomes? I assume it would be the best for my case. i.e., individually grounded (40A each) generators in parallel and no additional grounding on the primary winding of YNyn0+D11 transformers (floated wye).

Could you please advise if I missed some points in this situation? Do you have any further recommendations for my case?

Thanks in advance for your advices and recommendations.

 

[waross]

Have you considered a 100 Amp NGR on the 11kV bus and a 10 or 20 Amp NGR on each generator? That will assure a minimum ground current of 100 Amps with a maximum ground current of from 160 Amps to 220 Amps.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sykimk]

Waross// thanks for your advice.

I had also proposed zig-zag transformer application on the 11kV bus with HRG on each generator as well as hybrid grounding scheme for generator but my client does not want to have all those applications in the plant.

Basically, he does not allow us to install any switching device on the neutral grounding resistor.

 

[waross]

Permanently connect an NGR sized for about 100 Amps on the bus. This will ensure that the system always has a ground reference. Permanently connect an NGR sized for about 10 Amps to each generator neutral to provide protection to each generator when the generator breaker is open.
Yes this is a compromise, but if the customer wants protection without switching he may have to accept a compromise.
I would prefer this system to a switched system.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Another option for the generators would be to connect all generator neutrals solidly to a neutral bus and then use a single NGR, of what ever rating, to ground that bus. Then the number of running generators does not affect the current during a ground fault and no switching is required.

 

[slavag]

I think a common ground bus with one switched ( automaticly or manualy )NGR is prefered.
From other hand is possible use one zig-zag on the bus (also switched).

 

[waross]

I like that solution, David.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[odlanor]

Perhaps, for david-solution you have to install ground-sensor CT at generator line side for a seletivity protection(67N) ofground fault in stator winding generator.