Enhanced control of grid-connected inverters with non-linear inductor in LCL filter

In grid-connected inverters, utilisation of an appropriate output filter is vital to improve power quality. However, the filter, which comprises of capacitor and inductor(s), should be cost effective. In this study, an LCL filter with non-linear inverter-side inductor is used to reduce the filter volume and cost. In order to harness the undesirable non-linear behaviour of the LCL filter's inductor, a new control scheme based on proportional-resonant controller is proposed. The control strategy is based on combinational feedback of inverter-side and grid-side currents, with online adaptive estimation of non-linear inductor. With this scheme, the harmonic compensators’ gains can be reduced, while stability margins and dynamic response are improved. Therefore, while a cost-effective filter is used at the inverter output, acceptable total harmonic distortion is achieved. Experimental tests based on a 7 kW digital signal processor controlled prototype inverter are provided to verify the proposed control strategy for grid-connected inverters with LCL output filter benefiting from low-cost non-linear inductor.