Switching effects in directly paralleled three-phase AC/DC/AC converters with separate DC links

This paper analyzes the effects of asynchronous switchings in the case of directly paralleled three-phase AC/DC/AC converters. Unlike in the most previous papers, DC links of the converters are kept separate. Because of this, a path to a zero-sequence circulating current is not formed between the inverters or the rectifiers. However, it is shown that the three-phase currents may contain zero-sequence components. This is a consequence of a current circulating through the converter units. It is also shown that this kind of a circulating current may overcharge the DC link capacitors. In addition to the circulating current, a cross current flows between the inverters because of asynchronous switchings. The formation of both the cross current and circulating current paths is illustrated in detail. Simulation results are presented to validate the discussion about cross currents, circulating currents, and the capacitor charging effect.