Research On Virtual Capacitor Control And Stability Of DC Distribution Network

Different from the AC distribution network,the power supply in the DC distribution network has low inertia characteristics.When the distribution network can generate energy output fluctuation or load sudden change,the DC bus voltage fluctuates greatly,and the bus voltage quality is facing great challenges.In solving the problem of low inertia of DC distribution network,virtual capacitance control can be used to increase the virtual inertia of the system.The larger the configured capacitance is,the higher the power supply quality is.At present,the existing virtual capacitor control mainly analyzes the voltage fluctuation suppression,stability boundary determination,sensitivity analysis,adaptive inertia control and other aspects.However,the concept of virtual capacitor in the existing virtual capacitor control essentially refers to the virtual inertia coefficient,which reflects the virtual inertia provided to the DC distribution network,but it lacks the corresponding relationship with the physical super capacitor.Therefore,this paper designs a virtual capacitor control strategy equivalent to the super capacitor for the master station unit of radial DC distribution network under master-slave control,and carries out impedance modeling and stability analysis.The main work is divided into two parts:(1)the virtual capacitor control strategy is designed for the connecting converter and energy storage DC-DC respectively.Firstly,it is assumed that a super capacitor is connected in parallel next to the supporting capacitor on the DC side of the converter,Through the simplification and equivalent transformation of the control structure,the conversion process from super capacitor to virtual capacitor is strictly demonstrated,and the respective virtual capacitor control strategies of interconnection converter and energy storage DC-DC are derived respectively,so that the virtual capacitor of the same size can replace the super capacitor to achieve approximately the same voltage fluctuation suppression effect.(2)The impedance model of the main units of DC distribution network under the control of virtual capacitance is deduced.The influence of Virtual Capacitance and other parameters on the stability of DC distribution network is analyzed through the impedance stability criterion.The analysis results show that the larger the virtual capacitance is,the lower the system stability is.The stable operation range of the system should be considered when configuring the virtual capacitance;Compared with energy storage DC-DC,when the interconnection converter is used as the main station,the stability margin of DC distribution network is higher and can provide greater virtual inertia.In order to verify the equivalence between virtual capacitor proposed in the paper and super capacitor and the analysis conclusion of DC distribution network impedance stability under virtual capacitor control,the DC distribution network system is built in Matlab/Simulink for simulation verification.The simulation results show that under the same size of virtual capacitor and super capacitor,the standard unit value difference of the minimum value of DC bus voltage after sudden load increase is less than 1%,and the stability simulation is carried out under different conditions,the stability boundary obtained is consistent with the stability analysis conclusion,so the simulation results verify the correctness of the theory in this paper.