Capacitor required for harmonic filter duty will be required to accept given magnitude of various orders of harmonic current which will increase the nominal line current & hence the thermal duty on capacitor. In addition, voltage due to these harmonic currents will appear across the capacitor & will cause an increase in the R.M.S. voltage. Some degree of derating is, therefore , necessary for capacitors used as harmonic filters.
The normal current overload permitted in typical power capacitor specification is 1.3 multiplied by nominal current , & in determining this factor due attention has been paid to the fact that a capacitor connected to an ac power system may be required to accept an unknown magnitude of unspecified harmonic currents. In the case of a properly designed harmonic filter the magnitudes & orders of the harmonic currents are known & hence, provided that the permitted over - voltages & KVAR loading are not exceeded , the de-rating factor for the capacitors may not be as great as would be expected.
While Designing this Capacitors due care is taken to avoide imbalance in detunning with reactor over long life life operation. |
