SHEPWM Inverter Based Zero Sequence Circulating Current Reduction

In this paper, a real-time circulating current reduction method for parallel selective harmonic-elimination pulsewidth modulation (SHEPWM) inverters is proposed. SHEPWM techniques are often used in high-capacity inverters. If the inverters are in parallel operation, the zero-sequence path can be established, and the zero-sequence circulating current will circulate among the inverters. The proposed method installs a cascade null-vector control system behind the three-phase SHEPWM modulator. The proposed null-vector control system can be disabled to save switching losses when the zero-sequence circulating current is small, whereas it can be enabled when the zero-sequence circulating current becomes large. The proposed method does not affect the line-to-line voltage waveforms of SHEPWM inverters, and it can easily enable/disable the null-vector control system to provide bumpless transfer.