Research on zero-sequence current calculation method for VSC and circulation current of parallel VSC

Renewable energy usually adopts flexible grid connection technology which based on three-phase voltage source converter (VSC) and droop control to connect with the distribution. Zero-sequence current of three-phase VSC has influence on on-state loss current and zero-sequence circulating current. Fourier decomposition of zero-sequence voltage of three-phase VSC at valve side based on bessel function was analyzed. Steady-state zero-sequence equivalent circuit of three-phase VSC based on equivalent model of modulation was derived, and the calculation method of zero-sequence current for VSC was obtained. Simulation model of three-phase VSC using set electrical parameters was built; value of zero-sequence current at the switching frequency was obtained based on simulation model while modulation depth changed. Meanwhile, value of zero-sequence current at the switching frequency was obtained based on calculation method mentioned in this article. Correctness of proposed calculation method was verified by comparison of two methods. In addition, the trend of main components of zero-sequence current of VSC and zero-sequence circulating current increment of parallel VSC were analyzed while modulation depth changed, conclusions were consistent with the theoretical analysis.