MV generators grounding

[sberbece]

Hello !

I would appreciate any opinions on this topic.

Recently I had an argument with one colleague, regarding the grounding of MV generators in a small power plant.
There will be 3 gas generators, 3 MW 13.8 kV, windings start connected.
We agreed that an acceptable technical and cheap solution is to ground each generator’s neutral point through a low impedance earthing resistor, which shall limit the ground fault current of each generator to less than 100 A.
However, I have disagreed when my colleague wants to install a switching device on each generator’s neutral circuit, to be able to open the neutral connection, when the generators are working in parallel, in this way to make sure that at any give time will be always only one generator grounded in the system.

I agree that this solution will limit the power plant single phase to ground fault to maximum 100 A, but on the other hand adds the cost of switching devices and call for troubles during operation of the plant. I believe this switching system couldn’t be justified in this case, when it will reduce the ground fault from 300 A to 100 A, in case of all 3 generators working in parallel.

I believe that the simple solution of directly connecting each generator’s neutral to its earthing resistor will be more appropriated.

The IEEE standards C37.102 clause 3.2.2 – Low resistance grounding and IEEE 142/2007, clause 1.7. and clause 1.7.3 – Paralleled generators in an isolated system, let me understood that directly connected low resistance grounding system is widely use and do not present risk with regards to odd(3rd) harmonic circulation.

Saying these, I would like to know your opinion regarding direct connected low resistance impedance versus low resistance impedance connected through a switch.
Also, what are the advantages of using the switching system for generators grounding?

Thank you in advance for your advice.

 

[slavag]

Hello Sberbece.
We had a lot of threads here on the topic.
You can search.
See. Isn't possible say what is a better.
Possible with and without switching.
A hungres systems work with direct connection.
You decide limit grounding fault 100A, is good.
Possible also on 50A.
Please add some information and we'll continue:
Are you work in parallel with grid ? via common step up transformer or direct connection?
I would like recommend read about hybrid connection of generator neutral.
Best Regards.
Slava

 

[dpc]

There are a lot of multi-generator installations with each generator grounded via its own resistor. It generally works fine, and this is probably the most common method in the US. Elsewhere, the neutral switching devices are more common.

The third harmonic current can be an issue, especially if there a wye transformer winding involved, but normally this is manageable if you are aware of it.

As slava says, there has been a lot of discussion about this on earlier threads in this forum.

 

[sberbece]

Thank you for answers.

Slavag, the power plant will work as an island power system, with no connection with a Utility (please see the attached sketch).
I have mentioned in IEEE 142/2007, clause 1.7., the following advice regarding overvoltage & insulation stress: “special attention shall be given to limiting voltage to ground, by grounding the generator’s neutral”

Best regards,
Stefan

 

[slavag]

Hi Stefan.
For my pinion, you can use non switching connection of generator neutral.
Maybe..limit grounding fault on 50A, but of course in case if you have newer digital protection on the downstream breakers and generators ( hope, aren't only two 13.8/0.44kV trafos).
From other hand, I will add some hand switching element for the maintanance purpose of generator.
Hope that help.
Best Regards and Good Luck.
Slava