Stability Improvement of STATCOM System Using Fractional-Order Proportional Integral (FOPI) Based Control Strategies

Abstract

Proportional (P) controllers are widely adopted in STATCOM systems for their simplicity and fast dynamic response. However, under transient conditions, P controllers exhibit limitations in accurately tracking voltage and current variations, often leading to significant ripples and suboptimal performance. To address this, Fractional Order Proportional-Integral (FOPI) controllers are proposed, leveraging their ability to introduce a fractional-order integrator that provides enhanced tuning flexibility and robustness. The added degree of freedom in FOPI controllers allows for improved handling of non-linear STATCOM dynamics, particularly in mitigating outer-loop capacitor voltage fluctuations and inner-loop current variations. This paper presents a FOPI-based control approach implemented with decoupled current control for enhanced STATCOM performance during transients. The current control is developed assuming steady-state operating conditions to counteract variations induced ripples. While conventional PI controllers must operate at low bandwidths to preserve system stability, this method effectively improves for feedback limitations through a positive gains, and non-integer integral gain leading to improved transient responses. Simulation results demonstrate a substantial reduction in ripple amplitude and improved settling time. The proposed method achieves approximately 70% reduction in load current ripple (from 0.15 A to 0.015 A) and an 80% faster settling time (from 1.2 s to 0.2 s) compared to the conventional P controllers. Additionally, the stability margin improves significantly, with phase margin increasing from -180° to +0.167°. These findings highlight the efficacy of the FOPI-based control scheme in enhancing STATCOM system performance, particularly in mitigating ripple and improving transient response speed.