Improving Inverter Output Current Controller Under Unbalanced Conditions by Using Virtual Impedance

Nowadays, three-phase inverters have been commonly used in power systems, especially in renewable energy and motor drives applications. In many applications, the inverter has to generate balanced sinusoidal three-phase currents even though it is connected to an unbalanced system. Methods to suppress unbalanced currents commonly use decomposition of the unbalanced currents to their positive and negative sequence components. Thus, the positive and negative sequence currents can be controlled separately. However, this method is complicated because of the existence of sequence transformation block, a lot of signals to be controlled, and affect the transient response significantly. This paper proposes a novel method with detailed analysis to balance the inverter output currents under unbalanced AC side conditions without sequence transformation and affecting the transient response. The proposed method is based on virtual impedance concept that is inserted in series to the inverter output. The proposed virtual impedance is an additional control algorithm that changes the inverter behaviour into the one that has a real impedance connected to it. As it is virtual, the virtual impedance can be designed in such a way so that just the unbalanced signal is affected. Because there is no sequence transformation, the positive and negative sequence currents are not controlled separately. Simulation and experiment results show the effectiveness of the proposed method to suppress 12.13% current unbalance to only 2.27%. Meanwhile, the result is also compared to the conventional dual-reference method, proving the proposed method does not affect the transient response of the current reference.