Nonlinear Modeling and Improved Suppression of Zero-Sequence Circulating Current for Parallel Three-Level Inverters

The parallel operation of three-level inverters (TLIs) can increase the current capacity and power rating of the system. However, the problem of zero-sequence circulating current (ZSCC) inevitably emerges due to the common-mode voltage difference (CMVD) between the parallel TLIs (PTLIs). Especially, the inverter nonlinear characteristics (INCs) will worsen the ZSCC problem and weaken the suppression effect of ZSCC. In this article, a nonlinear equivalent model of ZSCC is established considering the INCs. Then, the simplified graphical analysis method is proposed to implement the frequency analysis of ZSCC. Furthermore, an improved ZSCC suppression strategy for the PTLIs is proposed. The INCs and original CMVD are regarded as the total disturbance and a nonlinear disturbance observer (NDOB) is designed to estimate it. Based on the NDOB, a feedback-feedforward control method is adopted to suppress ZSCC. According to the amplitude-frequency characteristics of the closed-loop control system, the controller parameters design and performance analysis are performed. The control structure is simple, the parameter design is convenient, and the suppression effect of ZSCC is evidently improved. Finally, the simulation and experimental results verify the validity of proposed strategy.