Design of control system for solid state variable capacitor with minimum DC capacitor

A variable AC capacitor (with capacitance variable from 0 to C_ac) can be implemented by an H-bridge and an additional phase leg connected to an AC capacitor with fixed capacitance, C_ac. This paper proposed a complete control system for the solid state variable capacitor (SSVC) to absorb double line frequency ripple power and other even order frequency ripple power caused by the harmonics of grid voltage. Based on the relationship between the ripple power and the energy of the AC capacitor, the voltage and current reference of C_ac are obtained to absorb different frequency ripple simultaneously. To enhance the steady-state performance, closed-loop compensation for the capacitor voltage reference is adopted to fully eliminate the ripple power to dc capacitor. The AC capacitor control consists of an inner proportional-resonant (PR) current loop and an outer voltage loop. By adopting the proposed control system, SSVC can reduce the dc capacitance to the minimum just for absorbing switching ripples. The total size of capacitor is reduced by 13 times compared to the conventional H-bridge system. Experimental results are shown to verify the effectiveness of the proposed control system.