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Short circuit infeed from wind generator

Short circuit infeed from wind generator

Short circuit infeed from wind generator

(OP)
My current work is conceptual design for step-up substation (20/110 kV) for connection of wind generators to the grid. I have to calculate short circuit current on 20 kV switchgear. It is clear about the infeed from 110 kV grid, but I am not sure how to calculate infeed from generators. If they are normal turbogenerators also no problem, but for wind generators according to me the methods should be different. They are not connected directly, and as far as I know there are inverters installed between the generator and step-up transformer (generator voltage)/20 kV in each tower. I suppose there is some current limiting automation in these inverters.
The project is still in conceptual phase, so the manufacturer of generators is not determined. The only available information is that generators will be 3 MVA each.
Would you advice me on this matter ?

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant
 

RE: Short circuit infeed from wind generator

The answer may depend on the purpose of the short circuit calculations.  If you are trying to determine the needed short circuit rating of breakers, etc, you need to take a worst case - maximum fault current.  So I'd assume a direct connection to the wind generators - no inverter.  For coordination purposes, you might need to consider a minimum condition.  

FWIW, most of the wind generators we see in the US do not have a inverter on the generator output.  They are doubly-fed, with an inverter feeding only the rotor circuit.   

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

RE: Short circuit infeed from wind generator

dpc, I wouldn't go as far as saying most wind generators are doubly-fed.  Only the GE units are that way.  The units with inverters on the output are not particularly uncommon either.

RE: Short circuit infeed from wind generator

Yes,I was trying to say that most wind generators with inverters do not have inverters directly on the generator output.

I think GE holds a US patent on the doubly-fed design, but elsewhere in the world they are supplied by other companies, such as Vestas.   

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

RE: Short circuit infeed from wind generator

At one wind plant I am familiar with, there are about 1/3 Vestas machines and 2/3 Siemens machines.  The Siemens machines, and there are others at other plants in the vicinity, all have an inverter in the output circuit; all output power converted to DC and then inverted back to AC.  So, at least for that sample area it would be safe to say that most DO have inverters directly on the generator output.

RE: Short circuit infeed from wind generator

OK, that's interesting.  I have no experience with the Siemens units.

But if the supplier is unknown and the goal is to calculate the maximum available fault current, it might be better to assume there is no inverter on the output.   

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

RE: Short circuit infeed from wind generator

Determine if the inverter is on the generator output or feeding the generator field.
If the inverter is on the field, short circuit current will be determined (mostly) by the generator characteristics.
If the inverter is supplying the total output, the short circuit current will be determined (mostly) by the inverter characteristics.

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

RE: Short circuit infeed from wind generator

Generally wind turbines contribute to the make current, but not the break current. If they are asynchronous generators the excitation collapses quickly after a fault, and if they are DFIG or full conversion types the crowbar circuit in the inverter acts to limit the current. The attached link may be of interest. See page 51.
In terms of protection design and settings wind turbines are not a reliable source of fault current, so the protection schemes for the turbines look pretty much like normal distribution protection schemes would, as if feeding a load rather than a generator.
http://www.ensg.gov.uk/assets/14_06_2005_dgcg0000200.pdf
Regards
Marmite

RE: Short circuit infeed from wind generator

(OP)
Thank you all for your comments. Especially thank you Marmite, that material is very interesting!

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant
 

RE: Short circuit infeed from wind generator

I think you need to be very careful about fault contribution and protection of wind generators, On a recent 60MW I waS loosely involved with the international supplier, along with his DVAr supplier could not provide a reasonable fault model for the genration plant. Needless to say the protection enginners were concerned escpecially when the connection was into a weak rural grid

RE: Short circuit infeed from wind generator

(OP)
The project is still on conceptual stage, so I have to determine only short circuit current level for correct specification of CB's and 20 kV switchgear. Calculation of relay settings will be on a late stage, we will see ...
The positive point is that the grid is quite strong at that area, and guys from respective department of system operator have very good model and are very competent, so at least from their side I don't expect problems.

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant
 

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