A Robust Control Method for Reliability Improvement of Cascade H-Bridge STATCOM Under DC-Link Capacitance Uncertainty

This paper proposes a robust DC-link voltage controller designed for a multilevel-based static synchronous compensator (STATCOM), addressing both DC-link capacitance degradation and load variations. The uncertainty in DC-link capacitance is modeled as an external perturbation, leading to the development of a second-order sliding mode controller (SOSMC) based on a twisting algorithm. This controller effectively manages these uncertainties, providing high stability and robustness against parameter variations and external disturbances. Furthermore, it reduces unwanted chattering and enhances overall system performance. The impact of DC-link capacitance uncertainty on the reliability of multilevel converters is analyzed, comparing the proposed SOSMC with traditional proportional-integral (PI) controllers in the Simulink MATLAB environment. The results demonstrate that the SOSMC method outperforms the PI controller under 33% uncertainty in DC-link capacitance over 5 years. The proposed control scheme not only meets reactive power demands but also effectively manages uncertainties in DC-link capacitors. Additionally, the twisting algorithm maintains an acceptable total harmonic distortion (THD) index on the AC side, thereby improving overall reliability while reducing maintenance costs.