[Hamid]

What is the influence of capacitor banks existing in short circuit level of power systems?

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[Girl of sky]

According IEEE 141, charged power capacitors can produce extremely high transient short circuit discharge currents, but they are of natural frequency much higher than power frequency and usually of such short duration that the calculated power frequency short circuit duty current is not significantly increased by adding the capacitor discharge. Discharge currents can be calculated as described for RLC circuits and appropriate RLC circuit can be based on power system data. Also according to IEC 60909-0, regarding of the time of short circuit occurrence, the discharge current of the shunt capacitors may be neglected for the calculation of the peak short circuit current. The effect of series capacitors can be neglected in the calculation of short circuit current, if they are equipped with voltage-limiting devices in parallel, acting if a short circuit occurs. In the case of HVDC transmission systems, the capacitor banks and filters need special considerations, when calculating a.c. short circuit currents. As general technical judgment, the reactive power of capacitor bank only effect in sub-transient short circuit currents, I think the influence of parallel charged power capacitor in power frequency regime even may be reduction of short circuit current which contributed by main power source; because the admittance of capacitive circuit can reduce the total impedance of short circuit path.Indeed the capacitor banks can be activated as static reactive power source in power system and they can supply a part of reactive element of fault current.

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