Hybrid cascaded multilevel inverter with improved DC-link voltage control for grid connected systems

A lot of research has been done on balancing of DC capacitor voltage for multilevel converter with little success in converters with higher levels (more than five). Under normal operating conditions, the average DC capacitor can be controlled by slightly shifting sinusoidal modulating waves of phase shifted hybrid PWM technique, however this cannot be easily realized during transient operation or when there is phase to phase imbalance in the output switching pattern. This paper presents investigation of a DC - link voltage balance with the use of balance circuit for upper and lower DC capacitors in a single phase diode clamped inverter composed of two three level legs. Combined with the individual voltage control, a complete voltage controller is developed for a cascaded nine level hybrid model. Mathematical modeling of the whole system is also presented. Robustness of the proposed algorithm under varying operating conditions and modulation indices is verified by simulation results.