| As people pay more attention to new energy generation,a large number of distributed generation systems are connected to the distribution network.When the distributed photovoltaic grid-connected power generation system is in operation,it will inject harmonic components into the distribution network,which will increase the hazards and negative impact for the safe operation of the distribution network an d the normal operation of the power-using equipment,causing the power quality problem of the distribution network to become increasingly prominent.At the same time,the continuous improvement of power quality requirements of power users has prompted power and related departments to take active and effective measures to ensure the supply of high-quality electrical energy for distribution networks.Therefore,the analysis of harmonics caused by distributed photovoltaic grid-connected power generation access to distribution networks needs further study.Firstly,This paper establishes the mathematical model of the photovoltaic cell,analyzes the influence of light and temperature on the output characteristics of the photovoltaic cell by establishing the mathematical model of the photovoltaic array,and uses the MPPT tracking method of the disturbance observation method to build the MPPT control system of the distributed photovoltaic power generation system.Combined with the PV array simulation model,analyzes the response speed of the system and the changes in output voltage,output current,and output power under conditions of different temperature and light.At the same time,the typical topological structure of distributed photovoltaic access to low voltag e distribution network system is introduced.Then the influence of distributed photovoltaic access location on current harmonics of distribution network and the mechanism of voltage change at the grid point are analyzed,which provides a theoret ical basis for follow-up work.Secondly,taking the example of distributed photovoltaic access to a typical distribution network,the discussion focuses on analyzing the effects of mul tiple distributed photovoltaics in parallel on the harmonic characteristics of low-voltage buses.First of all,the mathematical modeling of the harmonic domain of a single distributed photovoltaic near the load is carried out.Through the theoretical analysis of the characteristics of a single typical LCL distributed photovoltaic harmo nic output characteristics and harmonic distribution features,the equivalent of multiple distributed photovoltaic is proposed by extending the single distributed photovoltaic equivalent model to multiple distributed photovoltaic grid-connected networks.By establishing multiple equivalent circuits of distributed photovoltaics,the harmonic characteristics of multiple distributed photovoltaic parallel connections are theoretically analyzed.Using MATLAB to build a simulation to verify the harmonic output characteristics of a single distributed photovoltaic and multiple distributed photovoltaics.Thirdly,based on the harmonic output characteristics of the single distributed photovoltaic,an LCLLC-type inverter is proposed for grid connection after analyzing the amplitude-frequency characteristics of the LCL,LLCL and LCLLC grid filters.In order to improve the LCLLC's resonant peak,a no-source-damping technique is used in the system for increasing the impedance suppression resonance peak,and the conclusion of the general design of the three types of filters is described.A unified control strategy using active filtering and grid-connection is proposed,which not only improves the distributed photovoltaic self-harmonic output characteristics,but also decreases the grid current distortion rate caused by non-linear loads when there is margin capacity in the inverter.Further,combining the typical structure of multiple distributed photovoltaic access distribution networks,harmonic control measures are proposed in two cases.Firstly,based on the problem of current harmonics caused by multiple distributed photovoltaic on the low voltage side caused by low-voltage side grid,an active power filter control strategy based on LCLLC filter is proposed.Combined with traditional LC passive filters,broadband frequency domain harmonics on the low-voltage grid-connected buses can be achieved,while current harmonics of near-end nonlinear loads can be solved.Secondly,the current harmonic degradation caused by long-distance transmission lines is studied.After analyzing the characteristics of traditional active filters and passive filters,parallel active power filters and parallel resonant LCs are proposed.Passive filter for comprehensive treatment,the resonance frequency of the parallel resonant LC passive filter adopted is set at the fundamental wave frequency,which results in a large impedance to the fundamental wave current and a low impedance to high-frequency harmonic waves.Harmonic currents on the load side of the grid are more likely to flow into passive filters,and simulations verify the effectiveness of the harmonic suppression on the high side of distribution networks containing distributed photovoltaic access.Finally,based on the dSPACE platform,a distributed photovoltaic grid-connected experimental platform was built to verify the correctness of the proposed unified control strategy for distributed photovoltaic grid-connected and active filtering based on LCLLC inverters. |
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