Flexible operation of parallel grid-connecting converters under unbalanced grid voltage

According to the European grid code, grid-connecting converters should always be connected to the grid under grid voltage unbalance. However, grid voltage unbalance severely affects the stable operation of the converters and leads to the various power issues, such as active power oscillation, DC-link voltage ripple, and overloading. Moreover, under grid voltage unbalance, the active power delivery ability is decreased due to the converter's current rating limitation. In this paper, a thorough study on the current limitation of the grid-connecting converter under grid voltage unbalance is conducted. In addition, based on the principle that total output active power should be oscillation free, a coordinated control strategy is proposed for the parallel grid-connecting converters. The case study has been conducted to demonstrate the effectiveness of this proposed control strategy.