The Development Of Off-grid Photovoltaic System Based On Variable Step Size MPPT

Energy and environmental issues are common challenges faced by human society in the 21 st century,and new energy power generation has gradually become the key to resolve the contradiction between them.Solar energy has become the best energy choice available to humans at this stage because of its abundant reserves,clean and pollution-free.Off-grid photovoltaic power generation systems are more practical than grid-connected systems because they are not geographically restricted and do not depend on the public grid.However,it still faces many problems and challenges when it needs to formulate different design schemes according to the actual needs of users.In this paper,the off-grid photovoltaic power generation system is taken as the research object.From the aspect of optimizing the system-related control strategy,the system efficiency is improved,and the reliable operation of the system is guaranteed.Firstly,based on the analysis of typical MPPT control,a segmentation variable step MPPT control method is proposed,which can accurately track the MPP point when the external environment changes,and the time used is the shortest.The power oscillation and overshoot are not obvious.The relative error is the smallest,and the expected effect of fast tracking speed and high steady state precision is achieved.Secondly,in order to stabilize the output power fluctuation of photovoltaic array,keep the input voltage stability of the off-grid PV inverter,ensure the uninterrupted supply of power,this paper studies the charge and discharge control of energy storage battery.By comparing the advantages and disadvantages of several charging and discharging control methods,combined with the actual situation of the system,the constant voltage and current limiting charging and discharging control method is applied to the energy storage device.Then,the topology,modulation mode and output waveform control strategy of off-grid PV inverters are analyzed.Under the premise of satisfying the output power quality,the simulation models of off-grid inverters with linear and nonlinear loads are built respectively.For the non-linear load,the double closed-loop PI voltage control makes the inverter output waveform harmonic content too high.This paper proposes a multi-loop voltage regulation control strategy.The control strategy effectively reduces the harmonic content of the output voltage waveform,and the output voltage and current waveforms are not significantly distorted,which improves the non-linear load capacity of the off-grid photovoltaic inverters.Finally,the overall simulation model of off-grid photovoltaic power generation system is built in Matlab.Considering the user side load and actual capacity,the performance indicators of off-grid photovoltaic power generation system are set.According to the state of charging and discharging of the photovoltaic cell array and the lithium battery,the system is divided into four operating modes.By changing the load and illumination intensity to simulate the application environment of the actual system,the simulation results verify that the simulation results of each unit module in different modes meet the operational requirements and the control strategy is correct.