The production of residual currents due to harmonic loading

The connection of nonlinear loads such as thyristor drive systems to the power system creates waveform distortion due to the harmonic currents which are injected. The propagation of the harmonic currents throughout the power system is governed by a number of factors including resistance, inductance and capacitance of circuit elements and the system load. The harmonic current at the nonlinear load may be substantially balanced, but due to asymmetry in overhead line and transformer configurations, in particular, residual harmonic currents are produced, which circulate through line capacitance-to-ground and transformer grounded star points. These currents, being of zero sequence order, may cause noise interference in neighbouring telephone circuits. Since the harmonic currents in the power system may become amplified as they progress to the power frequency source, the interference could be considerably increased. The paper addresses the basic theory involved and studies particular systems with a view to using simplified manual calculations. From a knowledge of the residual current magnitude produced by a nonlinear load, noise interference could be determined.