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Topic Title: 60kW turbine connection
Topic Summary: 60kW turbine connection in parallel with DNO supply
Created On: 30 September 2012 11:09 AM
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 30 September 2012 11:09 AM
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GMAC

Posts: 64
Joined: 12 September 2006

I am looking at the protection of a cable that runs from a 60kW,
3 phase wind turbine inverter (inverter located at the turbine) to the DNO's point of connection, which is, in this case only a 20m cable run.
To comply with BS7671:2008 Amendment 1, I need to protect the cable for short circuit, overload, and earth fault.
I was considering using a Merlin MCCB fitted with a Vigi earth fault unit at either end of the cable, so one circuit breaker would disconnect the DNO's supply from a fault in the cable and the other breaker would disconnect the fault in the cable from the wind turbine inverter supply.
The turbine shuts down when the DNO supply is lost, but may not shut down in time to satisfy the time stipulated by BS7671.
The reason for including an earth fault unit is that I don't think the turbine can deliver enough fault current to trip the breaker alone.
I probably don't need it on the DNO end but on longer runs of cable I may find again that there is not enough fault current available.

Any comments appreciated as there doesn't seem to be much guidance on this subject.
 03 October 2012 05:38 PM
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JonathanHill

Posts: 225
Joined: 09 September 2002

Suggest you consult the manufacturer for details and recommendations. I think it'll be most likely that any fault contribution from your WTG will be very short-lived, especially once the DNO connection is lost. Indeed, if it's configured as an induction machine connected in delta, then there'll not be any earth fault contribution of any consequence from the WTG.

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Jonno
 07 October 2012 02:41 PM
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GMAC

Posts: 64
Joined: 12 September 2006

Thanks for your comments Jonathan.
I am trying to get more information from the WTG suppliers, especially in relation to fault kVA that can be developed from their systems.
The ones in particular are permanent magnet synchronous generators with ac-dc-ac inverter/converter power units.
A reply from them suggested the fault level on a 53kW generator/inverter/converter set was 5kVA! Seems extremely low, in fact much less than the normal running output. Have asked for an explanation.
Its possible the settings they have in their inverter/converter sets for inherent protection will satisfy our regulations with respect to fault level and time to clear the fault, but I'm not sure if our regulations allow the use of inverter/converter sets as protection equipment rather than using fuses or circuit breakers?
 07 October 2012 06:01 PM
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JonathanHill

Posts: 225
Joined: 09 September 2002

In my experience, such a low fault contribution from an inverter coupled generator is not unusual. The electronics can detect and respond very quickly to overload/ short circuit conditions, and shut the power electronics down on loss of commutation (from the mains which has already tripped).

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Jonno
 08 October 2012 11:56 AM
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GMAC

Posts: 64
Joined: 12 September 2006

How can the fault kVA be lower than the rating of the generator?
Is it because the fault current is cut off on the rise of the first half-cycle, and the fault current for calculating the kVA is the RMS value of the portion of the current wave seen before it is cut off?
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