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Guru

Join Date: Oct 2009
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11kV Generator

04/10/2013 9:26 AM

Can a 11kV, 1000kVA Diesel Generator tolerate a transformer inrush current of 660A for about 650 milli seconds?

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Guru
Engineering Fields - Power Engineering - New Member

Join Date: May 2007
Location: NYC metropolitan area.
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#1

Re: 11kV Generator

04/10/2013 12:35 PM

Is this some type of experiment or emergency condition where it is necessary to charge the 11kv side of an unloaded transformer by randomly closing the generator breaker? As you know the charging current depends upon the state of the transformer, where on the voltage sine wave closure occurs, and decays exponentially from the max; so stating that the current is 660A for 650ms doesn't adequately describe the current wave, it could be a lot more severe under worst case conditions.

"Tolerate" or "Supply"? A lot is going to depend upon how robustly the generator was built and the type of excitation system it has. More than likely it will never be able to supply 12x rated current under the severe over-excitation/saturation conditions that will exist at that current level, making it difficult for the terminal voltage to be maintained anywhere near normal after breaker closure, then everything spirals downhill until the protective relaying/devices operate.

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Guru

Join Date: Dec 2010
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#2

Re: 11kV Generator

04/10/2013 6:28 PM

I guess the impedance of the generator is about the same as that of the transfo. The generator should be able to stand a short circuit for 3 seconds, but the total impedance, including transfo, would be double that - halving the current with winding forces 1/4 (1/22) that for full short. I seem to remember that an on-site repeat of "factory" S/C test is permitted, so long as voltage is 80% of rated.

I think it likely the generator s/c current is less than 660 A. You need to get generator data but I think "toleration" is likely.

Of cause, the voltage is going to be low - can we suppose that is not a problem? Is the transformer unloaded (secondary) when switched-in?

It occurs to me you might use a dodge like energising the (400V?) secondary of the 11 kV transfo through a small 11/0.4 kV transfo to boot up the big transfo before switching 11 kV.

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