| In recent years,with the gradual depletion of fossil fuels and the impact on the ecological environment,high proportion of renewable energy power generation has become the future development trend,in which photovoltaic system has become a very promising power generation method because of its rich storage and other characteristics.However,with the rapid growth of electric load of electric arc furnace and rolling mill,it brings great challenges to the grid connection of photovoltaic system.In order to solve the problem of grid voltage distortion caused by a large number of distribution network loads(mainly high proportion of arc furnace loads)connected to the distribution network,the dynamic analysis of the distribution network voltage is carried out,and the photovoltaic grid connected converter based on the new phase-locked strategy is very important.Therefore,in order to reduce the harmonic distortion rate and improve the quality of grid connected voltage,the main research contents of this paper are as follows:(1)The voltage characteristic analysis of distribution network considering typical load is carried out.Firstly,based on the working principle and typical parameters of several typical loads,such as conventional load(constant power load,constant impedance load)and unconventional load(AC arc furnace load,rolling mill load),the mathematical models of different loads are established.Then,according to the mathematical model of the load,the simulation model is built by PSCAD/EMTDC software,and the accuracy of the simulation model is corrected by the measured data,and the voltage characteristics of the corrected load model are analyzed.Finally,combined with the actual situation of a substation in Anshan,Liaoning Province,the arc furnace load and rolling mill load are simulated to connect to the distribution network,and the radiation range of their influence on the supply voltage of the distribution network and the influence degree on the total harmonic distortion rate of the distribution network are analyzed.(2)The photovoltaic system model with new phase-locked strategy is established.According to the generation principle and grid connection criteria of the actual photovoltaic power generation system,the photovoltaic array based on maximum power tracking(MPPT)control of incremental conductance method and the photovoltaic grid connected converter model based on cascaded DC/DC and DC/AC photovoltaic grid connected structure are established,and a phase-locked strategy(MAF-SASOGI)of the photovoltaic grid connected converter with self-adjusting double second-order generalized integrator(SOGI)with maximum amplitude filter(MAF).This phase-locked strategy can overcome the disadvantages of the traditional phase-locked strategy,such as the serious distortion of grid connected current caused by inaccurate phase-locked,the secondary pollution of grid voltage,and the influence of photovoltaic grid connected due to the large harmonic component in the separated positive sequence voltage when the distribution network voltage distortion is very serious.(3)The simulation and verification analysis of the phase-locked strategy of photovoltaic grid connected considering the voltage distortion of distribution network are carried out.Combined with the actual operation parameters of a substation in Liaoning Anshan power grid,the power system simulation model is built by using PSCAD/EMTDC simulation software.In this system,the traditional d-q phase-locked loop,the phase-locked loop based on SOGI and the phase-locked loop based on MAF-SASOGI are used to grid the photovoltaic system.Finally,the validity of the self-adjusting dual SOGI(MAF-SASOGI)PLL with ideal low-pass filter is verified by analyzing the phase-locked angle,grid current,grid power,harmonic content of each sequence and the voltage of 10 k V bus in the distribution network under three kinds of phase-locked strategies when the distribution network voltage is distorted. |
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