SHE-PWM cascaded multilevel converter with adjustable DC sources control for STATCOM applications

This paper presents a new variation of the selective harmonic elimination pulse width modulation (SHE-PWM) technique applied to cascaded multilevel converter based STATCOM system. The DC voltage levels of the cascaded H-bridge converter are assumed variables, hence increasing the degrees of freedom in the formulation of the problem. This approach allows additional harmonics to be controlled when compared to the existing family of SHE-based multilevel control for the same converter configuration. The proposed modulation method also shows an interesting feature of fixed switching angles across the entire range of the modulation index, which alleviates the problem of online implementation. The proposed method solely relies on the availability of the variable DC voltage levels, which can be obtained by different topologies. For simplicity, a DC/DC buck converter is utilized in this paper to provide a variable DC voltage levels required by the H-bridges of the five-level converter. The STATCOM system is integrated with a current controller along with a voltage control loop for the buck converter to meet the reactive power demand of different loading conditions. The method is further compared with the conventional carrier-based pulse width modulation (CB-PWM) to show its improved characteristics. The effectiveness and the theoretical analysis of the proposed approach and validated through simulation studies.