Research On Three-phase Quasi-z-source Cascaded Multilevel Photovoltaic Inverter

With global energy shortage and environmental pollution problems become increasingly prominent,solar photovoltaic(PV)generation has been an emerging industry of world common concern and major development.Currently,the main utility patterns for PV generation systems are grid-connected PV power plants and distributed PV systems.However,the business model of distributed PV systems has not been completely gone through,centralized PV power plants play a major role of PV generation systems.Traditional PV power plant inverters are connected to high-voltage grid by means of line-frequency transformers,resulting in large system size,low efficiency and high cost.In addition,power generation of PV arrays is susceptible to light,temperature,etc.,especially partial shade and mismatch problems,which will seriously reduce the efficiency of the entire power system.To solve these problems,cascaded multilevel photovoltaic grid-connected inverter is a promising solution,which can access to high-voltage grid without transformer,thus to reduce system size and cost.Meanwhile each H-bridge module tracks their own maximum power point distributedly,hence to weaken those defects of PV systems and improve the efficiency.Nevertheless,traditional cascaded multilevel inverter lacks of boost function so that different maximum power point voltages result in imbalanced DC-link voltages,and the capacity of PV inverter is often designed too large.In recent years,some scholars have proposed quasi-Z source cascaded multilevel inverter(QZCMI),which embeds the quasi-Z-source impedance network into the traditional cascaded multilevel inverter.When applied in PV systems,QZCMI shares many merits of two topologies,e.g.single-stage power conversion of boost and inverter,independent control of DC-link voltages;allow shoot through state;distributed MPPT;and so on.These all help to reduce the cost of PV generation systems to improve efficiency.However,research on QZCMI for grid-connected PV systems is still in its infancy,and lacks of in-depth study and analysis.This paper mainly focuses on the mathematical modeling,control method and design of hardware and software platform.The details are as follows:Firstly,steady-state model and dynamic model of quasi-Z source H-bridge inverter are established,providing a theoretical basis for parametric design of experimental platform.According to zero-pole map of transfer function model,influence of quasi-Z source inductance,capacitance and shoot-through duty ratio to dynamic performance is analyzed.Double-line-frequency(2co)ripple model is established to analyze the influence of impedance parameters to 2? ripples,and to design impedance parameters to suppress the low-frequency ripples.Secondly,control strategy of three-phase quasi-Z source cascaded multilevel photovoltaic inverter is proposed,including shoot-through duty ratio control,total DC-link voltage control,grid current control and power balance control.Power balance control is emphatically analyzed,including per-module power balance control and per-phase power balance control,among which,per-module control methods are classified and power imbalanced constraints formula are deduced.In addition,carrier phase shift SPWM based on constant shoot-through duty ratio is designed to boost each power module DC-link voltage and synthesis multilevel output.Simulation results verify the validity of the entire control strategy,which achieves the desired control objectives.Moreover,system 'software and hardware platforms are designed and implemented.Hardware platform part analyses design principles of QZCMI main circuit parameters,develops DSP and FPGA-based master-slave digital control system,and uses touch screen as real-time PC monitor.Software platform part presents flow charts of DSP program,state-machine timing diagrams of FPGA program and control interface of touch screen.Finally,based on built three-phase quasi-Z source cascaded multilevel photovoltaic inverter prototype,some related experiments are completed,including circuit board debugging experiment,single module boost-voltage experiment,prototype current-loop experiment.Experimental results verify the validity of control algorithms and system platform design.