Research On Control Strategy Of Photovoltaic Grid-connected System Based On Unified Power Quality Controller

As the proportion of photovoltaic grid-connected power generation in domestic power sources continues to rise,the power quality requirements for photovoltaic grid-connected grids are also increasing.Due to the particularity of the power generation principle,the unstable output characteristics of the photovoltaic grid connection will bring impact and power quality problems to the large power grid.Non-linear,asymmetrical loads in the power grid can cause voltage fluctuations and flicker and current distortion.Therefore,there are many factors that need to be controlled when there is a need for coordinated operation between the PV power plant and the power grid.Considering the output characteristics of photovoltaic grid-connected power generation and the characteristics of various harmonic sources in the power grid,improving the efficiency of photovoltaic power generation and effectively solving the power quality problem has become an important prerequisite for the safe and stable operation of photovoltaic grid-connected systems.In this paper,a detailed study on photovoltaic grid-connected systems based on unified power quality controllers is carried out.Firstly,the research on the generation characteristics of photovoltaic power source and the mathematical model analysis in the grid structure are carried out.A method based on the optimal initial value for step tracking is studied for the multi-peak characteristics of power under local shadow and varying illumination conditions.By using the intersection of the IU curve and the OM(M point as point(Uoc,Isc))as the initial voltage,the conductance increment method or the disturbance observation method is followed to track the global maximum power point.This method greatly saves the tracking time and has good dynamic response characteristics when the illumination changes.Secondly,the mathematical model of PV-UPQC grid-connected structure is analyzed,and the interaction between multiple inverters of photovoltaic power supply is analyzed.It is concluded that the influence between the input and output of the series and parallel units is mainly determined by the transformer capacity.Control conclusions.At the same time,the direction of energy flow of the entire system is discussed.Then,the UPQC compensation method and compensation strategy are expounded,verified and compared.The shortcomings of the commonly used detection methods are summarized.The formula deduced that the input signal and any sine and cosine signals can be used for two products and one time.The equations are added to obtain the fundamental component,and the positive sequence component and the harmonic component of the fundamental current are obtained according to the power calculation method.An improved method for extracting fundamental waves without phase-locked loop is studied.In the PV-UPQC series and parallel inverter control,the non-fundamental current and the load non-fundamental voltage of the grid to be compensated are obtained by the phase-free loop harmonic reactive detection method,and then the inverter output is modulated by modulation.The non-fundamental current or voltage and current in the same direction are detected to offset the non-fundamental components in the grid current and the load voltage.Finally,the PV-UPQC grid-connected structure is applied in the low-voltage traversal process of the grid-connected system.When the voltage drop of the grid-connected point caused by the grid fault is studied,the power proportional control of the UPQC series unit and the parallel unit is adopted to compensate the load voltage.At the same time suppress the purpose of inverter overcurrent.The simulation results are verified by simulink.The results show that the PV-UPQC grid-connected structure can achieve good compensation effect in the case of symmetric faults and asymmetric faults,which can meet the low voltage ride-through requirements of photovoltaic power plants.