A Novel Circulating Current Suppression Strategy of Parallel Three-level Inverters in Flywheel Energy Storage System with Zero-sequence Voltage Injection

The parallel operation of three-level inverters can increase the power rating for flywheel energy storage system. However, the zero-sequence circulating current inevitably emerges owing to the excitation of the common-mode voltage difference, which can lead to current distortion and system loss. In addition, parallel three-level inverters have nonlinear characteristics, which can affect the zero-sequence circulating current. Therefore, a novel circulating current suppression strategy with zero-sequence voltage injection is proposed in this paper. A nonlinear equivalent model of the common-mode voltage difference is developed. Then, the zero-sequence voltage injection method is used to suppress the zero-sequence circulating current. Compared with the traditional suppression strategy, the novel strategy can achieve better performance. Simulation and experimental results prove the validity of the theoretical analysis and the feasibility of the proposed strategy.