Research On Digital Control Technique Of Single-stage Photovoltaic Pumping System

With the development of economy and society,the demand of energy is increasing.Due to the reserve of traditional energy is limited,so attention is paid to solar energy which is environmentally friendly and renewable.Solar PV power generation has been rapidly developing.As a kind of derivative product of PV power generation,PV pumping system has attracted extensive attention for its virtue of high reliability,high automation and no pollution.PV pumping system mainly consists of PV cells,converter,motor pumper and controller.In a PV pumping system,the problem of maximum power point tracking and pump unit control is always the focus of research.According to the requirements of system designing,this thesis adopts the single-stage AC PV pumping system structure.The output DC voltage of PV array is directly inverted to drive asynchronous motor and pumper.In this paper,the main research work is as follows:Firstly,according to the simplified equivalent circuit of photovoltaic cell mathematical model of photovoltaic cell is established.The principle of photovoltaic cell is analyzed and simulated and the simulation results verify the output characteristics of photovoltaic cell,which provides the theoretical basis for discussing the problem of maximum power point tracking.The advantages and disadvantages of SPWM and SVPWM modulation mode are discussed.And according to the analysis of the advantages and disadvantages,SVPWM modulation mode is selected and the generation mechanism of SVPWM is analyzed.In fixed frequency condition,the V-F two square curve of motor with pumper load is optimized determination by disturbing voltage and observing power factor,and the relationship of power factor and V-F curve is found.So combination method of MPPT and self optimizing of V-F curve is found.Secondly,the steady-state impedance characteristic of asynchronous motor with pumper load is analyzed.Thus PV array with power source load mathematical model is obtained.Combination the project mathematical model of PV array with the load mathematical model,a nonlinear differential equation of PV array output voltage and the load power relationship is gotten.And through the simulation in power control conditions,the trajectory of PV array output voltage is gotten under different conditions.The stability is analyzed and the stable operating condition of system is obtained.Finally,according to phase trajectory analysis of PV array output voltage of system,two MPPT methods without DC side current sensor are researched which are suitable for single-stage PV pumping system-frequency step method and voltage perturbation method.And the methods are verified by simulation and experiment.Experimental results show that these two methods can both track the maximum power point exactly.The correctness and validity of these two algorithms are verified.