Fully Digital Control of 27-Level Cascade Inverter with Variable DC Voltage Sources

Cascade multilevel inverters have been widely applied for the applications of high voltage and high power systems. The input power sources for a cascade multilevel inverter requires separate dc voltage sources, available dc sources include batteries, photovoltaic arrays and fuel cells...etc. If variations or fluctuations occur at the input voltage, the harmonic distortion of the inverter output voltage will increase. The conducting angles of switches in the conventional cascade multilevel inverters are obtained by the use of Fourier series to minimize the harmonic distortion of output voltage. The traditional method is complex and difficult to implement on a single chip, especially when the input voltage sources can not remain constant and stable. A switching control algorithm is proposed for cascade multilevel inverter with variable dc voltage sources. The proposed control combines the output voltage feedback and the feedforward voltage method to achieve dynamic control and low distortion of the inverter output voltage. Analysis, simulations and experimental results are presented to demonstrate the superiority of the proposed system.