Analysis and suppression of circulating currents in parallel-operated T-type three-level PWM rectifiers

In high power applications, parallel-operated T-type three-level PWM rectifiers have been employed to overcome the power limitation of single rectifier and achieve a flexible operation with improved efficiency. However, zero sequence circulating current (ZSCC) problem emerges when rectifiers are directly parallel-connected in both ac and dc sides. In this paper, the circulating current problem in parallel-operated T-type three-level rectifier system is analyzed, and a new ZSCC suppression method is proposed. The influences of system structure and dc-link capacitor voltages balance algorithm are discussed, and small positive voltage vector duration is adjusted to both suppress ZSCC and balance neutral-point voltage. Unlike the past suppression methods in three level topology, no extra circuit or high speed communication are needed. The proposed method is verified with both simulations and experiments.