Observer-based second-order sliding mode control for grid-connected VSI with LCL-type filter under weak grid

LCL-type grid-connected voltage source inverters (VSI) have been widely used in distributed generation systems. However, due to the inherent LCL resonance, it brings a stability challenge to the control system. To address this issue, a sliding mode observer-based second-order sliding mode control method is proposed to achieve arbitrary poles placement, which can ensure desired resonance damping and dynamic performance even under weak grid operation. With the help of sliding mode observer (SMO), only the grid current and grid voltage are sensed to realize system stable operation and grid synchronization, which reduces the cost of system. To improve the robustness and dynamic response of LCL-filtered grid-connected inverters under weak grid operation, a super-twisting algorithm (STA) second order sliding mode control (SOSMC) is proposed. Due to the existence of grid impedance in weak grid, the sliding surface would drift, which increases the track error of grid current. To solve this problem, an integral term is added to the sliding surface function. The systematic design approach of SOSMC is presented, and the stability of system under the grid impedance uncertainties as well as external disturbances is proved by Lyapunov theory. Simulation results are finally presented to validate the effectiveness of the proposed control strategy.