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Fault Levels & Breakers - Isolated Grid

Fault Levels & Breakers - Isolated Grid

Fault Levels & Breakers - Isolated Grid

(OP)
Recently there has been some discussion here regarding fault levels and equipment withstand ratings, as shown in threads such as  http://www.eng-tips.com/viewthread.cfm?qid=251289.

As part of a review of drawings at an isolated power station that we run, I've come across a particular station that has me a little bit concerned about the fault levels versus the withstand ratings of the equipment. Previously in another thread I'd asked the question about contactor ratings, given that the fault level in that case was low, and that the instantaneous fault level didn't stay very high for long. The thread in question is http://www.eng-tips.com/viewthread.cfm?qid=250053&page=1.

The station I'm now looking at is a lot larger (Three 1.5MW units, and a 0.7MW unit) so issues of fault current disappearing aren't quite the same. The station is generating at 415V, 50Hz using diesel powered generators.

However, the same rating philosophy as the previous thread has been used, this being with the assumption that by the time the breaker has opened, the fault level has reduced to a figure within the ratings of the breaker. I tend to think (as per the first thread above) that this isn't appropriate and the equipment should be rated for the maximum amount.

Clearly the situation is a little different when a parallel connection to a stiff grid is involved, as there shouldn't be any reduction over time in the grid's contribution, but I still believe that the equipment should be sized the same, that is, to be sized to withstand the maximum fault level.

In this case the board has been rated differently, whereby the breakers are rated to withstand a figure close to the fault level (fault level ~83kA, breaker rating Icu 100kA) but the board has been rated to withstand 50kA 1sec. The fault level is quite likely to drop below 50kA within that 1 sec, but I would have thought the board should have been rated higher.

Your comments, as always, are appreciated.

 

RE: Fault Levels & Breakers - Isolated Grid

83 kA  - is this rms or peak?  Is this symmetrical or asymmetrical?  You have to make sure you are comparing similar values.  

When you say "the board", are you referring to the bus bracing?  I'd not be surprised to learn the bus was underrated.  It happens.

But your 83 kA sounds pretty high for the amount of generation on site.  Are you confident in this value?   

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