Robust Higher Order Sliding Mode Control of Grid-Forming Converters with LCL Filter in Weak Grid Scenarios for Fast Frequency Support

This paper presents a nonlinear higher-order sliding mode control (HO-SMC) designed for a droop control-based grid-forming converter (GFM). In weak grid scenarios, where the rate of change of frequency (ROCOF) is notably high, achieving a rapid frequency response becomes imperative. The stable operation of a grid-forming converter using droop control, coupled with classical vector control employing cascaded voltage and current loops (multiloop), faces limitations when higher droop coefficients are applied. This constraint on the application of classical vector control (CVC) in weak grid conditions necessitates alternative solutions. Operating as a grid-forming converter, the grid-connected converter with an LCL filter represents a second-order system. HO-SMC mitigates the switching challenges associated with conventional SMC by integrating robust feedback linearized control. A graphical method is proposed for designing the switching gain using Lyapunov’s direct method to counteract the impact of a matched disturbance. The study demonstrates that the implementation of HO-SMC in the grid-forming converter enhances fast frequency response by increasing the gain margin of the power frequency (P-f) loop. Finally, it is illustrated that the proposed control method also improves the transient response of the converter.