Enhanced power quality control strategy for paralleled inverters in distributed generation

This paper proposes a flexible enhanced power quality strategy for a multi-function distributed generation (DG) in an aggregated system. Being a multi-function DG system, the grid-interfaced inverter can provide very sinusoidal current and harmonic compensating current simultaneously. Different from the classic strategy where the compensating current is directly calculated by the measured harmonic content in the load current, the proposed power quality control strategy presents a more flexible way using a voltage-controlled approach based on the virtual impedance theory. As a result, each individual DG can provide harmonic compensation functionality independently and the power quality of grid current can be enhanced. This paper first develops an aggregated DGs model using continuous timedomain equivalent circuit. The classic and proposed power quality control strategies can then be derived from the developed models. The proposed control strategy is performed by the virtual harmonic impedance method, which can promote the output current of each individual DG itself and compensate harmonics caused by the other DGs. The proposed control strategy can be seamlessly incorporated into the conventional PR control scheme, and maintain both the stead-state and transient performance within the control bandwidth. Moreover, the proposed control strategy is less sensitive to the variable of the grid frequency. Experimental results are provided to validate the correctness of the proposed power quality strategy.