High efficiency hybrid cascade inverter for photovoltaic generation

Power converter is the key element of the photovoltaic generation system. In order to improve the energy conversion efficiency and achieve an economic solution, hybrid cascade inverter (HCI) is employed and implemented to realize DC-AC conversion in this paper. HCI has a similar topology with conventional H-bridge cascade inverter, while the voltages of its DC buses are in a geometric sequence. In the inverter system developed in the paper, they are V₀, 2V₀ and 4V₀ respectively. According to the unbalanced characteristics of DC buses, a control method is developed for HCI, which consists of stair-case wave modulation for high voltage H-bridges and carrier PWM for low voltage H-bridges. This control method can be used not only for the constant DC bus voltage, but also for the variable DC bus voltage to adapt PV array output voltages. The output waveforms and efficiency are evaluated through simulations by using PSPICE and MATLAB. Simulation results verify that the HCI is a high performance inverter with high efficiency (>98%) and low output harmonic components. A 4kW prototype is implemented and the experimental results validate the feasibility of the developed HCI system.