THD and efficiency improvement in multi-level inverters through an open end winding configuration

A new approach, based on a current controlled Asymmetrical Hybrid Multilevel Inverter (AHMLI), is proposed to reduce power losses and output current distortion on Multi-Level Inverters (MLI) used in AC motor drives, STAT COM devices, Photovoltaic and Wind generators. A key feature of the proposed approach is that the AC machine (motor or transformer) is operated in an open-end winding configuration, being supplied on one end by a main Multi-Level Inverter and, on the other side, by an auxiliary Two-Level Inverter (TLI). The MLI controls the main active power stream; it operates at a low switching frequency and can be equipped with very low on-state voltage drop IGBT devices. The auxiliary TLI is instead operated according to a conventional high frequency two-level PWM technique and acts as an active power filter providing only a null-average power to the AC machine. As the DC bus voltage of the TLI is remarkably lower that that of the main MLI, the auxiliary inverter can be equipped with low switching losses IGBTs, or even Power MOS devices. Simulations and experimental results confirm that using the proposed approach, the phase current harmonic content is remarkably reduced, the efficiency is increased and in motor drive applications, the torque ripple is mitigated.