| In recent years,with the rapid development of distributed generation system,more and more distributed generation systems are integrated into the power grid,which has a certain impact on the quality of the power grid.In turn,the stability of the power grid has a certain impact on the normal operation of the distributed generation system.National grid system for distributed generation(dg)standard requirements are very strict,for distributed generation system has the strong ability to cope with power grid failure,failure occurs when the power grid requires the distributed generation system is not to take off the net,at the same time can also through the output active power and reactive power to maintain the stability of power grid operation.Due to the randomness and non-emptiness of power grid faults,it is difficult to reproduce various power grid faults in practice.In order to meet the requirements of power grid adaptability testing for distributed generation system,it is necessary to study special power grid simulators to reproduce various power grid conditions.Based on the previous research on the power grid simulator,a multilevel power grid simulator with energy feedback function is developed.The power grid simulator adopts the energy transformation form of "ac-dc-ac",and studies the topological structure and control strategy of the front and rear rectifying sides of the power grid simulator respectively.Specific research contents are as follows:1.Analyzed the status quo of distributed generation and the challenges brought by grid connection of distributed generation,and summarized the research status quo of power grid simulators.2.Classify the fault types of power grid,and divide them into two categories: static fault of power grid and dynamic fault of power grid.Static faults include unbalance of three-phase voltage and distortion of voltage waveform.Dynamic faults include abrupt change of voltage amplitude and frequency.3.Study the topology structure of rectifying side of power grid simulator,compare the advantages and disadvantages of three-phase uncontrolled rectifying circuit,three-phase semi-controlled rectifying circuit and three-phase fully controlled rectifying circuit,and establish the mathematical model of three-phase voltage-source PWM rectifier.On the basis of the principle of power grid voltage directional control,a power grid voltage directional control strategy with double closed loop structure of dc voltage outer loop and current inner loop is proposed and used as the control method of three-phase PWM rectifier on the front rectifying side of power grid simulator.The simulation model of three-phase voltage source PWM rectifier was built by MATLAB/Simulink simulation software to verify the feasibility of rectifying side control strategy of power grid simulator.4.Conduct specific research on the working principle of multilevel inverter,compare several traditional multilevel inverter topologies,analyze their characteristics,and select h-bridge inverter as the topological structure of the inverter side of the multilevel grid simulator studied.The traditional control methods of multilevel inverter are summarized and classified,the principle of multilevel carrier phase shift PWM control is emphatically studied,and the control method is applied to the control of inverter side of multilevel grid simulator studied in this paper.Through MATLAB/Simulink simulation software,the variation of inverter output voltage harmonic content and its influence on output voltage amplitude were studied when the number of cascade h-bridge elements and modulation ratio(modulation amplitude/carrier amplitude)changed.5.The topology structure of the multi-level power grid simulator with energy feedback function is given,and the simulation model of the five-level power grid simulator is built.The simulation results show the feasibility and effectiveness of the proposed power grid simulator.The experiment platform of multi-level power grid simulator was built to verify some functions of the power grid simulator. |
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