Parallel generators with different winding pitch

[PrebenJakobsen]

Hi all,
I have an offshore installation with seven 50Hz generators (3 to 7MW) that can run in parallel on a 6kV distribution system. They are all grounded by NER. (8,2 to 17,3 ohm)
The original installed generators has a winding pitch that produces 3rd harmonic and the new installed generator produces almost none which results in a high 3rd harmonic current that heats it's NER. We have made some measurements that shows variation depending on numbers of generators in parallel, load power factor etc. and the heat is substantial.

We are now trying to find the best way to solve the problem, and i can see IEEE C.62.92 gives some guidance on the issue. Which one should i buy? Does anyone have better references?

Best Regards
Preben Jakobsen

 

[dpc]

Louie Powell published an IEEE paper years ago regarding similar issues. You might do s eearch for that. If you use a common grounding resistor for all the generators, you will eliminate the zero sequence current path you have now that is allowing the 3rd harmonic current to flow. I think that would eliminate your resistor heating issue.

What is the time rating of the resistors? If only 10 seconds, you will need to make some guesses regarding the continuous current rating. But it may be that the resistor heating is tolerable if not too extreme.

 

[bentov]

Don't know if they have these for medium voltage but worth a look:

http://www.mirusinternational.com/d...ating-Current-when-Paralleling-Generators.pdf

 

[waross]

A common neutral connection will still leave circulating currents between the generators. With different pitches, the wave forms will be different. At any point where the wave forms do not coincide, there will be a difference of potential which may will circulate between the generators or may result in the generators passing part of the the reactive current back and forth between them several times a cycle.
You may be able to reduce the circulating current by tweaking the voltage settings a little.

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

 

[7anoter4]

First of all I found this sentence in New Zeeland Neutral Earthing
Resistors/Reactors (NER’s) (including resonant reactances) Application Guide [it is said there is an extract from IEC 60071-2 but I did not find it in this standard):
ch. 5.2.5 Resistance-Earthed System:
“To avoid any serious transient over-voltage problems, the value of neutral resistance should lie within the boundaries:
2X0< R0 <= Xc0
where Xc0 is the zero sequence capacitive charging reactance of the network (Xc0 can be calculated).”
Second, I think the NER of the last generator has to be substituted with another which should withstand the [measured] current-as dpc said- and to increase the resistance value in order to mitigate the current.