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 Post subject: AC to DC rectifier with capacitor bank
PostPosted: Wed Nov 10, 2010 3:01 pm 
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How is an AC to DC rectifier with a capacitor bank considered? Looking at a 3MW power converter system with a rectifier feeding DC to multiple DC to AC converters. The rectifier can be treated as being an AC supplied cabinet and the calculation is straightforward. The DC supplied cabinet is the question. If we assume it can't get any more energy than the AC to DC cabinet can supply it is a straightforward answer - same hazard level. If we have to consider the stored energy on the capacitor bank, what calculation do we use? The DC discussions I've seen are centered on systems with a battery bank of some sort. The capacitor bank will give a very high pulse of current but the total energy is limited until the bank is discharged to the level that will be sustained by the input rectifier. The total joules available is much less than even a relatively small battery bank.

Any thoughts?

Thanks,


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PostPosted: Thu Nov 11, 2010 5:27 am 
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this is going from memory from awhile but aren't rectifiers usually limited on the output of current due to the electronics? For instance, the spec sheet on the rectifier may say max fault current output of X amps and shutdown in Y secs at Z amps.
This would give some boundaries on available fault levels.

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PostPosted: Thu Nov 11, 2010 6:47 am 
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For many rectifier systems, the fault current is only about 3 to 5 times full load current.

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PostPosted: Wed Nov 17, 2010 3:54 pm 
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Rectifier capacity

Usually a rectifier circuits fault current at these power levels is limited by the impedance of the AC circuit. The semiconductors may fail shorted long before the fault clears if not properly coordinated with circuit protection on the AC side. If not properly protected the semiconductor may then become the fuse and open up.

If this is the only concern for ARC FLASH calculations then there is no problem, we have it covered. However, there is the capacitor bank with stored energy that has a very fast current rise time and a very high potential peak current but is limited in the total energy available. If we were to add the available energy in the capacitor bank to the calculated available energy from the AC connection would that be a valid/safe/conservative number?


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