| The global solar Photovoltaic (PV) technology is experiencing a sustained and rapid development with the continuous introduction of supporting policies in PV industry. However, PV power generation is stochastic, instable, and susceptible to natural conditions, such as irradiation and temperature variations. Moreover, partial shading and mismatching of PV panels are also one of the main reasons of power loss. Researches have been dedicated on cascade multilevel inverter (CMI) that can allocate PV panels to several independent H-bridge inverter modules and each module tracks their own maximum power points distributedly, thus to weaken those defects of PV power generation. Nevertheless, traditional H-bridge inverter (HBI) module lacks of boost function so that different PV panel output voltages result in imbalanced dc-link voltages and the inverter KVA rating requirement has to be increased twice with a PV voltage range of1:2. More recently, extra dc-dc boost converter was added into each HBI module to handle PV panel voltage variations. However, such topology is with a two-stage inverter for each module, and many extra dc-dc converters will make the whole system complex, bulky, high cost, and low efficiency.The quasi-Z-source cascade multilevel inverter (qZS-CMI), coupling the quasi-Z-source network into each HBI’s dc link, not only improves advantages of traditional CMI, but also inherits characteristics of quasi-Z-source inverter (qZSI). When applied to PV power generation, this topology presents promising features that: the ability of dc-link voltage balance because each of qZSI module can handle the wide PV panel voltage variation through single-stage power conversion; the power switches in the same bridge leg can conduct together without damage; the modular structure that each H-bridge inverter can be seen as a module with the same circuit topology, modulation, and control; the distributed MPPT that to maximize solar energy; etc. Therefore, the higher reliability, lower cost, and smaller volume can be brough in to PV power systems.The main contribution of this reseach is to investigate control of qZS-CMI based PV power system. Two pulse-width modulation (PWM) methods and the grid-tie control of the entire system are proposed accordingly. The details are as follows.Firstly, a double-line-frequency (2ω) ripple model of qZS network with PV panel terminal capacitance is established for qZS-HBI based PV module, which is used to analyze the influences of impedance parameter to2ω ripples and design impedance parameter to effectively buffer the low-frequency ripples. A small-signal model with stray resistance is established for designing the dc-link voltage control.Secondly, a simple extended, less-resource, and modular multilevel space vector modulation (SVM) for qZS-CMI is proposed on the basis of overview for two-level three-phase qZSI’s SVM. Simulation and experiments verify the proposed method.Moreover, a phase-shifted pulse-width-amplitude modulation (PS-PWAM) is proposed for qZS-CMI, aiming at reducing the power loss of existing PS-SPWM of qZS-CMI. Simulation and experimental results demonstrate its validation, and the qZS-CMI presents lower power loss in the PS-PWAM than PS-SPWM.Finally, a grid-tie control scheme of the qZS-CMI based PV system is proposed, including distributed MPPT, indenpedent dc-link voltage balance control, and grid-tie control with unity power factor. Completed system-level modeling and regulator design process are presented for single-phase qZS-CMI based PV system. The proposed control is then extended to three-phase PV qZS-CMI system.This research aims at investigating the qZS-CMI based PV system in terms of topological level, modulation level, control level, and material level, etc. Simulation and experimental results verify the proposed control, which will promote the application of novel PV inverters and satisfy the high-quality power supply demands. |
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