Electrical Riddle No.58 – Selection of capacitors

Any nonlinear load in the system such as an arc furnace or converter equipment produces harmonics. Filters are used to control the harmonics. If the tuning of the filters is not sharp enough, then there may be excessive harmonic currents through the capacitor bank. Harmonics cause overheating and failure of the capacitor units. Voltage and frequency increases and total harmonic distortion of the voltage or the current place additional stress on capacitors. Capacitors must be able to carry continuously 1.3 times the current flowing with sinusoidal rated voltage and frequency at an ambient air temperature corresponding to its temperature class. With this loading, the voltage must not be higher than 1.1 Ur, no account being taken of transient over voltages. If the limiting conditions stated above are exceeded, the chosen capacitor must be replaced by one with a higher voltage rating and a rated power If the capacitor is fitted on the low-voltage side of the transformer, in the case of networks having a high harmonics content, it is necessary to check whether a voltage resonance at a harmonic present in the network (usually the 5th and 7th harmonic) can occur between the capacitance of the capacitor and the leakage inductance of the transformer. The maximum capacitor rating can be defined approximately as Where Sr_T is the transformer rated power in kVA, and Qc the capacitor rating in kvar, and ukr the rated impedance voltage (in per cent) of the transformer and the feeding network, and v is the number of the highest critical harmonic. REFERENCE: ABB SWITCHGEAR MANUAL