Control strategy for parallel connection systems of three-phase PWM converters in unbalanced grid condition

In unbalanced grid condition, circulating currents, which mainly contain not only the zero-sequence component but also the negative and even the positive component will occur. A novel circulating currents control strategy with the help of the positive and negative synchronous rotating coordinate is proposed in this paper. With the definition of circulating currents, a corresponding mathematical model is derived and analyzed for the paralleled three phase converter system. It can be obtained from the model that main components of the circulating currents, namely the positive, negative and zero-sequence circulating currents can be suppressed by proper control respectively. The corresponding circulating currents controllers are designed. The proposed strategy can be applied to the parallel three-phase PWM converters linked with a common DC bus under unbalanced AC grid conditions and even with different line inductors at AC side. The experimental results verified the validity of the strategy.