Research on Main Parameters Design Method and Circulation Mechanism of Parallel VSC System

Electronic systems and electrical systems usually use parallel VSC to increase drive power or power supply. Parallel VSC systems introduce additional circulation loop apart from 3-phase sequence circuit and circulation current thus appears. Based on modulation model of VSC and analysis of circulation flow path, circulation equivalent circuit of parallel VSC is obtained. The circulating voltage expression based on the BESSEL function is obtained by using VSC's voltage of ac side to the dc pole. Considering the instantaneous power operation interval and circulation inhibition, system parameters design method of parallel VSC is studied. The influence degree of modulation depth, phase difference and carrier frequency deviation on circulation of parallel system are analyzed and main component is zero-sequence circulation. Finally, take switching frequency circulation as example, Simulation model of two-VSC parallel system based on dq decoupling closed-loop is established to analyze the operation transient characteristics. Two situations of same operating conditions and PWM carrier frequency deviation between two VSC are simulated respectively, simulation results show that system parameter design method proposed in this paper is rational. At the same time, an open-loop parallel VSC model under the same electrical parameters is established, and the trend of different parameter deviation on circulation is analyzed.