Fuse and Cable selection for PFC bank
Posted 07 December 2002 - 07:32 AM
Recently our PFC bank cables got overloaded and eventually failed. The cables emanate from the 3.3Kv bus bars and terminate at the PFC yard via a duct. The specs of the PFC bank are as follows; Voltage=3.3Kv, PFC bank rating=2.8Mvar and it is required to correct the power factor to 0.85. The cables to the PFC yard are two 150mm^2 capable of carrying 720A. So what i want to know is if the cable are adequate for the task. Further more i wolud like to know how to calculate the rating of the fuses protecting correction capacitors.
Posted 17 January 2003 - 07:44 PM
From the information that you have given, I would expect the current to be in the order of 490 amps in which case the cable you have used should be adequate, however that is base on sinusidal voltage. The presence of harmonics rapidly increases the current flowing into the capacitors and this will put extra stress on the capacitors, cables and fuses.
The capacitors will have a maximum continuous current rating, depending on the design of the capaciotrs and that would be typically 1.2N or 1.4N This data will be available from the manufacturer and is an allowance for an increased current due to harmonics etc. For fusing you should select a fuse that is less than the current rating of the cable and I wouls also suggest less than the overlaod rating of the capacitors as well. If you use fast fuses and switch large banks of capacitors regularly, this can lead to premature fuse failure so you may need to use slow acting fuses.
If you are haveing problems with the PFC being overloaded, that would tend to suggest a high level of harmonics present and the best way to reduce the effect of these harmonics on your capacitors is to add detuning reactors in series with the capacitors. This makes the impedance inductive above the tuned frequency of the capacitor and choke (typically around 180 Hz) and reduces the current due to the fifth harmonics and above.
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