Capacitor Size Reduction of Multilevel Inverters by Utilizing Neutral Shifting

Reduction of size of capacitor is an active area of research as it greatly reduces cost and footprint of the system. A level doubling network (LDN) is a half-bridge with floating capacitor in its dc bus, which doubles the number of ac output voltage levels when connected to a multilevel inverter unit. LDN automatically gets settled at a desired voltage level without closed-loop control. Capacitor of the LDN gets charged for a complete half cycle and discharges for the next half-cycle and therefore results in a higher ripple voltage demanding a higher capacitance. This paper presents a new method to reduce the size of LDN capacitor keeping the voltage ripple within limits. This is achieved by shifting the Neutral point of inverter between positive and negative dc busses of the LDN, which enables the capacitor to charge or discharge more frequently, irrespective of the modulation index and load power factor. In addition, controlling is done in such a way that uniform power distribution ratio among the cells of a single phase leg of the inverter is achieved. Working principle of the proposed method is verified through simulation in MATLAB/Simulink and experimentation on a laboratory prototype.