| In the existing smart grid,the photovoltaic grid-connected system which composes of the photovoltaic panel and quasi Z-source cascaded multilevel inverter is gaining more and more attention as a new technology.In order to reducing the costs and improving the reliability,this technology uses the H-bridge module as the core unit to realize the MPPT(maximum power point tracking)of the photovoltaic array,DC-DC boosting,DC-AC conversion and grid connection.Regarding the quasi Z-source cascaded multilevel photovoltaic grid-connected system as the research object,this paper focuses on the four aspects: the mathematical model of the quasi Z-source H-bridge photovoltaic inverter,the MPPT of the photovoltaic array,the power balance control algorithm of the cascade inverter and the grid-connected modulation technology.The relevant research contents and results are as follows:(1)The mathematical model of the quasi Z-source H-bridge photovoltaic inverter is established in detail.Based on the module of the quasi Z-source photovoltaic inverter,this paper builds the state space equation which includes the inductor of the quasi Z-source network and the capacitance internal resistor,and derives the system dynamic transfer function model.(2)A novel control algorithm of the MPPT is presented in this paper.Based on the mathematical model of the photovoltaic cell,the characteristics and shortcomings of several traditional MPPT algorithms are analyzed and compared.Moreover,an exponential approach law sliding mode control algorithm based on the quasi-Z source circuit is studied and its implementation process is presented since it can solve the problems of the large jitter and slow response speed of the general constant velocity sliding mode maximum power control.Furthermore,the results of the simulation and experiment show that this novel algorithm has greater performance on speed and stability than other traditional sliding mode algorithms.(3)A new modulation wave distribution method of the power balance control for the quasi Z-source cascade inverter is put forward in this paper,which can realize the distribution between the active and reactive power in the whole cascaded inverters.The modulation principle is following: the reactive power is provided by one cascaded unit and the active power is evenly distributed on the other cascaded units.Meanwhile,the constraints on the duty cycle which enforces the short between two devices of the quasi Z-source circuit are derived to achieve the power balance control.At last,the relevant results of the simulation and experiment prove that the stability of the DC bus voltage is guaranteed with the new modulation wave distribution method,the maximum power output of the photovoltaic array is realized with the exponential approach law sliding mode control algorithm and the unit power factor grid-connected is achieved with the modulation wave distribution of the carrier-phase-shifted(CPS)PWM when the input power is imbalance among the cascaded units.(4)A three-state hysteresis modulation strategy is proposed for the quasi Z-source cascaded multi-level photovoltaic inverters.According to the operating characteristics of the photovoltaic quasi-Z source inverter and the operating principle of the two-state hysteresis current control,a three-state hysteresis control system is presented for achieving the photovoltaic MPPT.Moreover,the operating principle of the three-state hysteresis controller is analyzed and simulated with 3-unit quasi-Z source cascaded inverter system.The simulation results present the three-state hysteresis controller has the better steady-state and dynamic response performance than that of the carrier stacking method and the carrier phase shifting method.It can effectively improve the quality of grid-connected current. |
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