Control of Low-Distortion 27-Level Cascade Inverter with Three H-Bridge Inverter Modules

This paper presents a digital control of 27-level cascade inverter for the applications of uninterruptible power supplies with high-power and/or high voltage. A conventional 27-level cascade multilevel inverter requires the combination of 13 H-bridge (single-phase full-bridge) inverter modules. A simple control of 27-level cascade inverter is proposed by using only 3 H-bridge inverter units with different dc voltage sources. The conducting angles of switches in the conventional cascade multilevel inverters are calculated by the use of Fourier series to minimize the harmonic distortion. The traditional method is complex and difficult to implement. With the proposed method, the switching angles are translated into switching reference voltages of inverter switches, control of various modulation values is also easy to implement with the proposed control. Analytical, simulated, and experimental results demonstrate the superiority of the proposed system.