Earthing of multiple generators

[veritas]

Hi

I have a situation where two diesel generators operated in parallel (1MW each, 11kV) only have one of them earthed at any one time. I was told this was to prevent circulating currents. Can anyone tell me where these currents come from and how one can calculate them?

Thanks in advance.

 

[trosepe]

What is the winding pitch of the generator and do both generators have the same pitch.
/

 

[veritas]

Hi trosepe

I don't know what the winding pitch is but I do know the generators are identical so I would assume same pitch. I am curious to know why you ask?

Thanks

 

[glendale]

We have eight idendical 3MW sets running in parallel. Each generator neutral is earthed through 25 ohm resistors. We have never experienced problems with circulating currents, even when the generators are not equally loaded. This configuration is only possible as the design/manufacture of the generators is identical, problems generally only occur when machines of differing size/winding configurations are paralleled.

 

[trosepe]

Veritas,

Use this link:
bmcoi.com/CatLit/Power/.../GEN. WINDINGPITCH - LEKX3115.PDF

It will explain how winding pitch realtes to circulating neutral harmonics.

 

[dpc]

Generators normally produce some amount of third harmonic voltage. The resulting third harmonic currents act like zero sequence currents and flow through the neutral and ground. With multiple grounds there can be circulating 3rd harmonic currents. This may or may not cause an actual problem.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[veritas]

Hi trosepe

Is the link correct or complete as I get an error if I paste it into my browser as it is? Really looking forward to what this document has to say.

dpc - yes, this is what I have heard as well. Thanks for this. I am curious to know where the 3rd harmonic currents come from though and how to quantify them. Trosepe's winding pitch appears to be the answer.

Kind regards.

 

[dpc]

Yes it's a function of the pitch. Some pitch configuration have more third harmonic and some less. I seem to recall a pretty good discussion of this in the Standard Handbook for EE (Fink and Beaty) If all generators are identical, the third harmonic voltages are equal so there is little net voltage to drive the current. If the generators connect to a grounded wye winding of a transformer however, it's a different story.





"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[veritas]

Hi dpc

The generators I have are two 11kV ones connected directly to an 11kV network (no trfr in between). They are also identical sets. Thus from what you say it should be okay to earth both of them directly?

Also, would the introduction of neutral earthing resistors not help as they would suppress the 3rd harmonic currents even further? I know there are other issues that arise if you do this like is the EF sensitivity on the protection devices sufficient for an impedance earthed system, etc.

 

[dpc]

Solidly grounding any medium-voltage generator is not ever a good idea, if it can be avoided.

You can provide each generator with its own grounding resistor and ground each one. This is commonly done in the US,maybe less commonly elsewhere. This does reduce any third harmonic circulating current.

You do need to consider the continuous third harmonic current that could flow in this situation when rating the grounding resistors. A grounding resistor with an 10-second rating does not have an official continuous current rating, at least in IEEE/ANSI land. You would need to make sure the resistor could handle whatever continuous current may flow.

Generator manufacturer should be able to provide expected third harmonic voltage. If generators are identical, resulting circulating currents should be pretty small.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[slavag]

Hi
Im second with Dave.
It is not good idea solidly grounding of industriaul MV generators. Low or medium, is OK.

Veritas, are those generators will operated in co-generation with grid?

 

[trosepe]

veritas,
Try this one:

http://www.bmcoi.com/CatLit/Power/TECHNICAL PAPERS/GEN. WINDING PITCH - LEKX3115.PDF