A New Stability Enhancement Method Using KF Estimation for the PWM-SMC-Based Grid-Tied Inverter under Weak Grid Condition

A sliding-mode control (SMC) technology is widely used in inductor-capacitor-inductor (LCL)-filtered grid-tied inverters (GTIs) due to its strong robustness against parameter drift. However, under weak grid conditions, the steady performance of GTI using conventional pulsewidth-modulation-based SMC (PWM-SMC) degrades, resulting in increased harmonics in grid-injected currents. One reason is that the voltage feedforward term at the point of common coupling (PCC) in the SMC loop causes the phase margin of the inverter output impedance to decrease. Another reason is that the gain of conventional PWM-SMC-based systems is not high enough, especially under weak grid conditions. This article proposes a PCC feedforward voltage reconstruction method (named KF-PWM-SMC) for a PWM-SMC-based controller using Kalman filter (KF) estimation. As a result, the phase and magnitude of the inverter output impedance at the desired frequency can be significantly improved. Moreover, the proposed PCC feedforward voltage extraction method can save the PCC voltage control sensor, thereby improving the reliability of the whole system. A 3-kW experimental GTI based on the dSPACE DS1202 platform has been developed to verify the feasibility of the proposed control strategy.