Analysis of low frequency circular current characteristic of paralleling converter

With increasing capacity of converter system, technology of paralleling modular has become a research hot spot in power electronics field. According to the frequency and amplitude of output AC voltage, converter constituted of paralleling modular can be divided into constant voltage constant frequency (CVCF), variable voltage variable frequency (VVVF), variable voltage constant frequency (VVCF). CVCF is often used in Uninterruptible Power Supply (UPS), power system of aerospace. Other structures are often used in the field of motor speed control, large capacity wind power grid and reactive compensation. The few research on converter constituted of paralleling modular based on neutral point clamped three level topology (NPC). The paper researched the following aspects: derivate Mathematical model of converter constituted of paralleling NPC modular; analysis effect on zero sequence circulation by different DC bus connected structures; discuss effect on the amplitude and frequency of circulating current by DC bus and the midpoint voltage fluctuation; last, experiment results verify the theoretical Correctness.