Fixed-frequency sliding mode control for power quality improvement of a grid-connected inverter

In this paper, a cascaded sliding mode control strategy is proposed for a grid-connected inverter system to simultaneously improve the power quality of the local load voltage and the injected grid current. This control scheme also enables a novel seamless transfer operation between grid-connected and stand-alone mode. The controller comprises of an inner voltage loop with proportional controller and an outer current loop with proportional-integral controller. This results that both the injected grid current and the inverter local load voltage have a very low total harmonic distortion (THD). The overall controller is designed on the basis of sliding mode control technique and constant-frequency current controller with a hysteresis band. The controller retains the benefit of hysteretic current control having fast dynamic responses and mitigates the drawback of the variable switching frequency. The proposed method is then applied to 6-kVA, 220V, 50Hz grid-connected 1-φ voltage source inverter (VSI) system. The results show that the steady-state response and the dynamic response are quite attractive. Moreover, the THD of load voltage of the inverter and the injected grid current are 1.96% and 2.14% respectively at standard nonlinear load IEC62040-3.