| The power quality of microgrid is a matter of concern.Compared with large grids,microgrids working in island mode are susceptible to the influence of line parameters and different working conditions of loads in the network due to the limitation of their own capacity.In the process of microgrid operation,different load types may be occurred by voltage deviation,three-phase asymmetry and harmonics.In actual operation,additional compensation equipment can be added to manage voltage deviation,three-phase unbalance,harmonics and other power quality problems.However,the cost of microgrid and the complexity of the system will be increased by the addition of additional equipment,and it is good for the power quality problems of specific nodes,but still needs to be improved for the overall power quality management in the network.Based on the control framework of robust control and adaptive virtual impedance,a control strategy for the voltage source inverter(VSI)of isolated microgrid,and improves the control performance by optimizing the control algorithm of VSI in microgrid is propose in this paper,control performance of VSI output voltage can be maintained and then the overall voltage power quality of microgrid can be maintained.The overall voltage quality of the grid is maintained,the specific work in this paper is as follows:Traditional robust control is employed to be able to solve the parameter ingestion and perturbation problems to a certain extent.However,the flexibility of controller design in designing robust control in microgrid voltage inverters needs to be further investigated.In response to the above problems,an H∞ robust controller for microgrid VSI is proposed to explore the method of designing robust control in microgrid VSI and make the controller more flexible in the design process by introducing weighting coefficients,while taking the grid-side current as a perturbation in the system.The H∞ robust controller is designed to improve the robust control performance of the inverter by introducing the weight coefficients to make the controller more flexible in the design process and taking the grid-side current as a disturbance in the system mathematical modeling into account.However,since robust control cannot completely solve the voltage problem under complex working conditions.To address these deficiencies,an adaptive virtual impedance group is further proposed,which is controlled separately by converting the VSI output AC quantities into the form of sequence components.Firstly,the positive sequence sag control is designed by the idea of split-sequence control,where the power generated by the positive sequence current and the positive sequence voltage is constant and in direct flow.It is used to obtain the reference voltage and frequency.The adaptive positive sequence virtual impedance is designed to compensate the voltage deviation caused by the load switching process,the adaptive negative sequence virtual impedance is designed to offset the negative sequence components,and then the adaptive harmonic virtual impedance set is designed to offset the harmonic components caused by harmonic loads.Through the design of virtual impedance group to manage the power quality problems are caused by different working conditions of loads.On this basis,the stability of VSI in microgrid is analyzed,the open-loop impedance model of VSI is established,and the stability of the closed-loop system is analyzed by analyzing the performance of the open-loop model.The stability of the controller is analyzed,and then the controller is applied to the system to determine the stability of the system by judging the closed-loop stability of the system.the dynamic performance and steady-state performance of the system can be improved by the proposed control strategy,and the harmonic distortion rate of the inverter output voltage can be effectively reduced and stabilized around the rated value under different working conditions,which further ensures the good power quality of the microgrid output. |
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