Control of power quality in inverter-based distributed generation

Power quality is an important additional service of inverter-based interfaces for distributed generators. In grid connected applications the power quality depends on the harmonic content of the current injected at the point of common coupling. By careful design of the power converter and its output filter the switching frequency components in the output current spectrum can be reduced to low levels. The effect of the harmonic distortion of the grid voltage on the output current can be minimised by using an appropriate inverter control strategy. Conventional control methods (manipulation of inverter voltage magnitude and phase) offer active and reactive power control, but not the control of the output current quality. This paper describes a new choice of control structure and explains the interaction between the applied control loops. The inverter is used to control the current in the first element of an LCL filter. A further controller is wrapped around this loop to control power export to the grid. The usefulness of this arrangement in providing big power quality is emphasised. Experimental results from a 10 kVA prototype are used to evaluate the distortion rejection properties and the regulation of active and reactive power control. The results show high quality of generated power and excellent transient and steady state-response to both active and reactive power demands.