Integral sliding-mode current regulation for sequence-impedance shaping and oscillation suppression in virtual synchronous generators

Cascade-controlled virtual synchronous generators (VSGs), widely adopted in grid-forming battery energy storage systems (BESS), may experience low-frequency instability when connected to stiff grids characterized by relatively high short-circuit ratios (SCRs) and small equivalent grid impedances. A frequency-coupled sequence-impedance model was developed to investigate the instability mechanism. The conventional proportional-integral (PI) current regulator was replaced with an integral sliding-mode controller incorporating an integral sliding surface and a boundary-layer reaching law, while retaining the outer power and voltage control loops. Small-signal impedance analysis, MATLAB/Simulink simulations, and RT-LAB real-time simulation tests were conducted under different SCR conditions. The baseline model identified a capacitive region in the positive-sequence impedance that resulted in insufficient phase margins at converter-grid impedance crossovers. The proposed controller narrowed the capacitive region and yielded positive phase margins in the analyzed cases. Simulations and real-time tests confirmed the suppression of sustained grid-current oscillations. Compared with a fixed virtual-inductance benchmark, the proposed method shortened recovery time and reduced grid-current total harmonic distortion from 3.38% to 0.54% at SCR = 30 (Lg = 1.5 mH). The results indicate that the proposed integral sliding-mode current regulation improves the operational reliability of grid-forming BESS under the tested grid conditions, thereby supporting power system resilience. These conclusions are limited to the examined balanced three-phase operating conditions and do not establish a universal stability boundary.