Modification of single-tuned filters to avoid parallel resonance and harmonic amplification problems

This paper investigates the amplification problems of harmonic currents in a plastics plant and a suitable mitigation method. Formerly, 5^th and 7^th harmonic filters were installed to eliminate the harmonic currents from rectifiers. However, due to the variation of power system impedance, one of the parallel resonant frequencies is shifted toward the 4^th harmonic, When a small amount of 4^th harmonic current is generated by rectifiers, it is amplified. This causes the reduction of rectifier loading. It is required to turn off the 7^th harmonic filter in order to maintain the rectifier loading, but the power factor is not high enough. Field measurements and computer simulations are used to verify the harmonic amplification problems. A mitigation method which removes half the capacitors of the 7^th filter to shift the parallel resonant frequencies away from the 4th harmonic is proposed. Field measurements indicate that the proposed method may avoid the amplification problems and maintain the power factor