Cascaded Fractional Order and Sliding Mode Control for an Autonomous Voltage Source Inverter

A Voltage Source Inverter (VSI) is a critical component in the control of autonomous microgrid system as its operation depends on the voltage and frequency control capability of the VSI. Conventionally, a cascaded PI structure is employed for the control of VSI owing to its simplicity and ease in implementation. However, the cascaded PI structure suffers from performance degradation with the system parameter variations. This paper presents a fractional order controller designed for the outer voltage loop with a sliding mode based inner current loop to augment the system's robustness against parameter variations. Compared to a PI controller, a fractional PIλ controller offers an additional degree of freedom λ along with the proportional and integral gains. The fractional controller is designed to improve system robustness along with noise attenuation where as sliding mode based inner current loop is designed to provide fast dynamic performance, reduced overshoots along with the capability to reject exogenous disturbances. The performance of the proposed FOPI+SMC controller is evaluated against a well-tuned PI+PI controller under various operating conditions and disturbance in both simulation and experimental domain.