On the need for strict second harmonic limits

Even harmonics, especially the second harmonic, cause waveform distortions characterized by asymmetries between the positive and the negative half-cycles. Certain single-phase converter topologies, when supplied with asymmetrically distorted voltage wave-forms, respond by injecting significant levels of direct current, even when the mean value of the voltage is zero. In turn the DC will bias transformers causing a host of undesirable effects. This paper concludes that the existing recommendations for permissible levels of the second harmonic must be stricter and proposes a ban on half-controlled three-phase converters.