Research on Circulation of Parallel Three-Phase Converters in MW Wind Power System

In this paper, modeling of parallel three-phase converters is first presented. Then definition of the circulating currents of paralleled converters and the circulating-current-generating mechanism are clearly explained. Thus, based on this definition, an averaged model of the circulating current is proposed. It is seen from this model that the circulating current consists of mainly the zero-sequence components. The governing differential equation also shows explicitly the relation between the circulating currents and the affecting factors such as different pulse width-modulation strategies, different circuit parameters etc. With this understanding, a simple coordinate control is then presented to reduce the circulating current. The phenomenon of the intrinsic circulating current is also explained. Furthermore, a parallel converters system used for wind power application is constructed, and the proposed control is implemented. Both simulation and experimental results verify the validity of the proposed theory and control.