| The depletion of fossil energy,the shortage of electricity,global warming and environmental problems have increased the demand for renewable energy.As a special form of micro-grid,DC Microgrid transmits power and energy at the DC bus voltage,which can improve the overall transmission efficiency.Therefore,one of the objectives of the operation of the DC Microgrid system is to output a stable and smooth DC bus voltage.In recent years,this research has been very popular.Energy transmission is carried out through DC/DC converter or DC/AC converter to meet the requirements of DC load and AC load,which improves the reliability,high efficiency and stability of the power system.In order to obtain a stable and smooth DC bus voltage,this article has launched a series of research on related control technologies.The specific research contents are as follows:First,a detailed study on the DC bus voltage control of the DC busbar photovoltaic power generation unit and the composite energy storage unit of the DC Microgrid system is carried out.On this basis,the DC bus voltage control method and the facing problems in the DC Microgrid are understood and analyzed.The difficulty.In response to these difficulties,corresponding solutions are proposed: for the photovoltaic power generation unit and composite energy storage unit in the DC Microgrid system to transmit energy through nonlinear power electronic converters,the DC bus voltage produces harmonic distortion and the transmission efficiency is reduced.The second and third chapters are discussed.The traditional LC filter cannot filter out the harmonics very well,so the third chapter of this article proposes LLC filter,which filters out the DC bus voltage harmonics,detects the harmonic content,and makes the harmonic value within the control range.At the same time,the DC Microgrid system will also have fluctuations during stable operation.In order to ensure the power quality to solve this problem,a feedforward control strategy of nonlinear disturbance observer on the DC/DC converter side is proposed to track in real time.The DC bus voltage fluctuates,and a stable and smooth DC bus voltage is output.Therefore,Chapter 4 of this article builds a control model based on the above research content using MATLAB/SIMULINK,and obtains some research results:In view of the DC bus voltage control strategy of the DC Microgrid system of photovoltaic power generation units,the use of power electronic converters causes low transmission efficiency,and for the future integration of the DC Microgrid into the AC grid to prevent bus voltage fluctuations and instability,improvements have been made to reduce the DC bus voltage Because of the necessity of harmonic distortion,a filter circuit should be designed on the DC/AC AC side of the composite storage unit to suppress AC current fluctuations.And it eliminates the higher harmonics that the power electronic converter will bring to the DC bus and deteriorate the power quality when the AC load is connected in parallel for energy conversion.And proposed to improve the original system LC filter,the LLC filter model was established,and the experimental simulation was carried out to verify the correctness of the theory.It can be seen that the waveform is stable in 1.0-1.2s and the disturbance range is 3.7%-6.0% by using the LC filter simulation waveform alone.The DC bus voltage simulation waveform with LLC filter is stable in 1.05-1.1s,the harmonic distortion rate is 1.80%,and the fluctuation range is 2.6%-5.6%.The simulation waveform of the DC bus voltage with the LLC filter is 0.1-0.15 s less than the simulation waveform of the LC filter alone,and the disturbance range is reduced 0.4%-1.1% less.Theoretical analysis and simulation results show that the method proposed in this paper can effectively reduce the harmonic distortion,reduce the harmonic content of the DC bus voltage,and increase the transmission content;In order to better control the LLC filter,the previously studied single closed-loop control LLC filter is further changed to a double closed-loop feedforward control LLC filter to suppress the harmonics caused by the nonlinear converter and the AC load to the DC microgrid system,Reduce the waveform distortion rate when parallel AC loads,make the DC bus voltage stable output,and ensure the power quality.From the waveform,it can be seen that the simulation waveform of the DC bus voltage with the double closed-loop control of the LLC filter is stable at 1.0-1.02 s,the harmonic distortion rate is 0.91%,and the fluctuation range is 1.06%-3.79%.From the waveforms,it can be seen that the simulation waveform of the DC bus voltage with the double closed-loop control of the LLC filter is stable at0.8-0.9s,the harmonic distortion rate is 0.42%,and the fluctuation range is0.04%-1.3%.With the optimized control strategy,the waveform is more stable and smooth.Compared with the LLC filter double closed loop control simulation waveform,the stability time of the LLC filter double closed loop feedforward control simulation waveform is shortened by 0.12-0.20 s,the harmonic distortion rate is reduced by 0.49%,and the amount of fluctuation is reduced 1.02%-1.49%;The simulation waveform of the double closed-loop feedforward control of the LLC filter is 0.20-0.25 s shorter than the simulation waveform of the LLC filter alone,the harmonic distortion rate is reduced by 1.38%,and the fluctuation is reduced by 2.2%-4.3%.Compared with the LLC filter double closed-loop control simulation waveform,the stability time of the LLC filter simulation waveform is shortened by0.05-0.08 s,the harmonic distortion rate is reduced by 0.89%,and the fluctuation is reduced by 1.54%-1.81%.Regarding the DC bus voltage fluctuation problem of the DC microgrid in stable operation,the non-linear interference observation double closed-loop feedforward control on the DC/DC side is studied.The purpose is to further obtain a more stable bus voltage during the operation of the DC Microgrid,restrain the disturbance,and effectively improve the stability of the power grid during operation.Through the simulation waveform,it can be concluded that the bus voltage disturbance is significantly reduced when the nonlinear disturbance observer tracking system is running.The disturbance multiple is reduced.The disturbance under the original control strategy is2.24-3.3 times that of the improved strategy.The DC bus voltage stabilization time is reduced from 0.8s under the original double closed-loop control strategy to 0.4s under the DC/DC side voltage loop feedforward control.The bus voltage was quickly improved,and the load output disturbance current was reduced from 35 A to 4.2A.Theoretical analysis and simulation results show that the method proposed in this paper can effectively reduce DC bus voltage fluctuations and ensure power quality. |
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