Precise detection and elimination of grid injected DC from single phase inverters

Abstract

Single-phase voltage source inverters are used for connecting small scale renewable energy sources to low voltage distribution networks. They operate to supply the network with sinusoidal current. If output transformers are not used, these inverters must prevent excessive dc current injection, which may cause detrimental effects in the network. In this study, the causes of dc current injection in a common inverter topology are analyzed. This work explains the design and test of a passive filter circuit precisely measuring the dc component in the inverter output current. The filtered dc signal is then used to control the single phase inverter for the objective to keep the dc injection low - below the standard limit. Characteristics of the proposed method are illustrated using simulation and experimental results.