High performance hybrid cascaded inverter for renewable energy system

Voltage source inverter is commonly used as the interface to connect the renewable energy source and the power grid. In order to improve the performance of renewable energy system, the topology and control method of the inverter system need to be analyzed. In this paper, a single-phase hybrid cascaded inverter is proposed. It is based on two kinds of power devices - MOSFET and IGBT. The cascaded inverter consists of three H-bridges. The DC voltage of each H-bridge meets the proportional relationship of 1:2:4 and the three modules are connected in series at the AC side. The low voltage bridge is composed of MOSFETs, while the medium and high voltage bridges are composed of IGBTs. This hybrid cascaded inverter can output at most 15 voltage levels at the AC side with rather low switching frequency. At the same time, it can fully exhibit the advantages of different power devices and make the inverter operation flexible. Voltage gradational and PWM carrier modulation methods are adopted in the paper. It is shown that the conversion efficiency reaches up to 98% and the output THD is less than 5% (meets requirement of the IEEE standards). Meanwhile, with different combination of switching states, the distribution of input active power in each H-bridge can be adjusted. As a result, for renewable energy system, larger control freedom is provided and the need of power balance is satisfied. The validity of the inverter system is tested by both simulation and experiment.