Virtual DC Machine Control Strategy In DC Distribution Grids

DC distribution grids has great advantages in the access of distributed power,the improvement of new energy penetration,and the enhancement of system controllability,which makes it become the new developing direction of distribution grids in the future.However,many DC terminals of the DC distribution grids are connected to the grid through the power electronic converters,and the connected power electronic equipment with almost no inertia and damping brings challenges to the power quality and the stability of the distribution grids.The instantaneous power fluctuation caused by distributed energy,frequent load switching and large power grid disturbance will affect the quality and stability of DC voltage,so it is necessary to introduce virtual DC machine control to improve the quality of DC voltage.In order to simulate the mechanical inertia characteristics of DC machine and ensure the stability of power grid when providing flexible and controllable dynamic power support at the moment of power imbalance,the virtual DC machine control strategy of DC distribution grids is studied in this paper.The main work of this article is as follows:(1)The topological structure and characteristics of the typical DC distribution network are analyzed,and the mathematical models including energy storage units,connected converter units,load units,and new energy units are established,and their working principle and basic control are analyzed.Based on this,the role that each port can play in the DC voltage regulation is explained,and the voltage quality problem caused by the low inertia of the DC power distribution grids is introduced,which lays a good foundation for the study of the virtual DC machine control strategy of the DC distribution grids.The simulation system built in Matlab/Simulink system also provides a verification platform for subsequent research.(2)Based on the dynamic voltage regulation characteristics of the DC machine connected to the grid,the virtual DC machine control strategy(VDCM)for DC distribution grids is proposed.The speed of the virtual DC machine is compared with the DC bus voltage,to establish the relationship between the DC bus voltage and the inertia of the virtual DC machine,so that the port converter can provide instantaneous power support for the disturbed DC distribution grids,so as to reduce the impact of the disturbance on the DC voltage stability.From the aspects of the control purpose,the controlled object and the response effect,the proposed control strategy is compared with the typical virtual capacitance control strategy in theory and simulation,which shows that the proposed control strategy has its own advantages in suppressing voltage fluctuation and anti-disturbance ability.(3)An adaptive virtual DC machine control(AVDCM)is proposed,which is applied to the dual active full-bridge(DAB)DC-DC converter interface at the energy storage end.In view of the fact that the inertia characteristic simulated by the converter in the virtual DC machine control can not be adjusted adaptively according to the actual working condition of the system,the fuzzy logic controller is added to the control link.By detecting the change rate of the DC bus voltage and the residual capacity of the converter,the response speed and size of the instantaneous power support can be adjusted flexibly.At the same time,the safety of the converter is taken into account and the magnitude of the voltage drop is reduced.(4)The change of the relevant control parameters in the proposed control strategy will affect the stability and dynamic performance of the system.Therefore,based on the established small signal model of the four terminal system,sensitivity calculation and root locus analysis are carried out to reveal the influence of control parameters on the DC voltage stability of the system,which will provide guidance for the selection and design of the parameters in the control strategy.Based on the simulation platform of four-terminal DC distribution grids,the effectiveness of the proposed voltage regulation strategy is verified.