Research On Energy Stored Quasi-Z Source Cascade Multilevel Photovoltaic Inverters

Photovoltaic (PV) power generation is considered to be one of the most prospective renewable energies due to its pollution-free and environmental-protective characteristic. The power converter is a critical component in the PV power generation system, and its reliability, operating efficiency, and cost are significant for the whole system. Due to the shortcomings of the conventional single-stage or two-stage converters employed in PV systems, more and more researchers focus on the newly proposed Z-Source inverter (ZSI)/Quasi-Z Source inverter (qZSI) and the low harmonic high efficiency cascade multilevel inverter (CMI). According to the research status the dissertation proposes three novel inverter topologies, and applies them in the PV power generation system. Then the basic theory is studied and the control schemes of the proposed inverters when applied in PV system are proposed. The details are as follows:Firstly, considering the characteristic of solar energy and the users’demand, the dissertation proposes an energy-stored qZSI (ES-qZSI) for PV system. The mathematical model of the ES-qZSI is built and the relationship among the system state variables and powers is established. The ES-qZSI can be applied to stand-alone PV system and the grid-tie PV system respectively, and the corresponding control scheme for two systems are investigated. Furthermore, an energy management control scheme is proposed to control the system while taking into account battery safety.Secondly, in view of the advantages of qZSI, ES-qZSI and CMI, the dissertation proposes the qZS cascade multilevel inverter (qZS-CMI) and energy-stored qZS cascade multilevel inverter (ES-qZS-CMI), and then applies them to PV system. The investigation mainly includes the following aspects:For qZS-CMI, qZSI module which is the basic module to make up the cascade multilevel PV system is studied in detail. The dissertation proposes the mathematic model to calculate the2ω ripple component for system state variables accurately. Moreover, the relationship between the2ω ripple and the qZS network parameters is investigated, and the qZS network parameters selection method to limit2ω ripple is proposed. Also, the operating states, the power loss and the operating efficiency of qZSI module are presented in detail. Detailed study on the control strategy of qZSI module based PV system is carried out, and the controller parameters are obtained by using bode plots method. The work will provide important guidance of the parameter design and the device selection for this type cascade multilevel inverters.For qZS cascade multilevel PV power generation system, the control strategy is proposed. With the control strategy, the qZS cascade multilevel PV system can achieve distributed maximum power point tracking (MPPT), balanced DC-link voltage for each module, and the unity power factor operation.For ES-qZS cascade multilevel PV power generation system, the dissertation also proposes the control strategy, with which the system not only can balance the module power difference automatically, but also can satisfy the load/grid demand flexibly, except for the distributed MPPT.Finally, the prototype of ES-qZS PV system, the qZS cascade multilevel PV system, and the ES-qZS cascade multilevel PV system are set up and tested in the lab. Experimental results verify the proposed inverters and their PV systems.