Consideration from Viewpoint of Output Impedance Concerning Control Method for Parallel Operation of CVCF Inverters

This paper describes the study results of the inverters running in parallel. In the consideration of the operation algorithm, it is important to reveal output impedance characteristics of the inverters. First, simulation results demonstrate that the output impedance depends on the control scheme of the output voltage and characteristics of the circulating current that flows through the inverter differs. Next, as examples, two control schemes and their experimental results are demonstrated. The two schemes are: “PQ droop method” based control and “common current distribution method” based control. In the both case, since the output impedance is resistive, the phase of the output voltage is adjusted by using the reactive component of the circulating current. The amplitude of the output voltage is not manipulated. In the latter control system, quantity of the output-current feedback changes with the output impedance. This characteristic clarifies that the output impedance among the inverters becomes higher and the output impedance of the loads gets smaller. Moreover, suppression of the circulating current and excellent output voltage characteristics can be maintained.