Cascade multilevel inverter with phase-shift SPWM and its application in STATCOM

A cascade multilevel voltage-source inverter with separate dc sources is proposed for static synchronous compensation (STATCOM). It can solve the size-and-weight problems of conventional transformer-based multipulse inverters and the component-counts problems of multilevel diode-clamp and flying-capacitor inverter. Phase-shift sinusoidal pulse width modulation (PS-SPWM) technique is employed to control the switching devices of each voltage-source inverter, as a result, the devices operate at a higher equivalent carrier frequency. In conclusion, the inverter has such advantages as: 1) reducing power loss because of transform-less design; 2) generating almost sinusoidal waveform voltage with high equivalent carrier frequency; 3) eliminating transformers of multipulse inverters used in the conventional STATCOM; 4) dynamic response fast; and 5) direct connection to power distribution system without any transformer. This paper focuses on feasibility and control schemes of the cascade multi-level inverter. Simulation of a three-phase 5-level inverter with reactive power regulation using MATLAB-Simulink power system blockset is carried out. The analytical and simulated results show the feasibility and superiority of the proposed inverter for static synchronous compensation.