Zero Sequence Power Balancing Compensation for Third Harmonic Injection of Multi-Stage Grid-Tied Energy Conversion Systems

DC bus voltage utilization is a key parameter that highly influences the voltage rating of the devices when designing a grid connected inverter. Thus, the maximum voltage rating of devices should be taken into consideration which increases the cost of hardware components. An effective way to improve the DC bus utilization is third harmonic injection. This paper proposes a zero sequence power balancing compensation method for third harmonic injection (THI) through zero sequence reference frame. Compared to the traditional direct injection method in the duty cycle, the new idea adds the third harmonic component to the zero sequence voltage (ZSV) controller. Thus, the injected third harmonic part is controlled through zero sequence controller without introducing extra harmonics to the grid. Also, the zero sequence power balancing method is proposed to compensate for the DC bus oscillation caused by the THI in multi-stage grid connected system. With the zero sequence compensation method, the DC bus controller is further stabilized without generating oscillated harmonics to the current controller for the grid.