Novel Multilevel Inverters Based on AC-AC Converter for Photovoltaic Applications

A novel family of multilevel inverters consisting of three-level (3L) half-bridge, 3L full-bridge and cascaded modular multilevel inverters for photovoltaic applications is presented in this article. The proposed inverters are based on a buck type ac-ac converter and feature no high-frequency common-mode voltage. They can be isolated from the grid or output load without requiring an external circuitry. The voltage stress of all the high-frequency switches in the proposed half-bridge 3L inverter is half of the input voltage. The proposed cascaded multilevel inverter is highly modular and can reach a high output voltage level using low voltage rating devices. In addition, it offers a unique feature in that a faulty or unnecessary unit can be bypassed without disturbing the circuit operation of the remaining healthy units. The derivation of the proposed inverters, modulation strategies, working principles and common-mode voltage analysis are discussed. A 550 W experimental prototype of the proposed 3L half-bridge inverter and 1.5 kW experimental prototype of the proposed 5L cascaded modular multilevel inverter are constructed and tested. Test results verify the excellent performance of the proposed inverters.