Fuse and Cable selection for PFC bank

[necr]

Hi there.

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 (procedure) the rating of the fuses protecting correction capacitors.

 

[RRaghunath]

Necr,

You need to look at the impact of harmonics on the overloading of cables / capacitors as well.

 

[jghrist]

The capacitor current at rated voltage and with no harmonics can be determined by:

(Mvar x 1000)/kV/sqrt(3)

(2.8 x 1000)/3.3/sqrt(3) = 489 amps

This assumes a three-phase capacitor bank and that the voltage is 3.3 kV phase-to-phase.

If the voltage is higher than the capacitor voltage rating, actual Mvar will be higher than rated (proportional to square of the voltage). You may have a harmonic resonance with the source which causes high harmonic voltages and currents in the capacitor.