Flexible Virtual Inertia Control And Its Stability Of The VSC In DC Distribution Network

DC distribution network has become a choice of the future distribution network with its remarkable advantages in terms of distributed power access,controllability and utilization of renewable energy.The control strategy of DC distribution network plays a key role in optimizing the coordination of power,improving power quality and power supply reliability.However,due to the low-inertia of the DC distribution network,the fluctuation of the system will affect the quality and stability of the DC voltage.Therefore,it is necessary to introduce virtual inertia to improve the DC voltage quality.In order to make the virtual inertia of the system flexible and controllable for the sake of adapting to different types of fluctuations and different stages of disturbance,which can result that the inertia is enhanced when the voltage fluctuation is faster and the inertia is reduced while the system is recovering to shorten the response time,the flexible virtual inertial control strategy and its stability of DC distribution network are studied in this paper.The main research results are as follows:(1)Firstly,the topology of a typical DC distribution network with multiple types of distributed generators(DG)is analyzed in this paper.The voltage source converter(VSC)plays an important role for AC/DC conversion,and the basic principle and mathematical model of VSC are introduced.Secondly,the hierarchical voltage regulation control system of DC distribution network is researched including primary voltage regulation,secondary voltage regulation and tertiary voltage regulation.Finally,the voltage droop control that is widely adopted as primary voltage regulation method is analyzed,which will result in poor quality of the DC voltage due to its voltage deviation.Thus,it is significant for improving the voltage quality to introduce the virtual inertia to DC distribution network.(2)Aiming at the problem that the inertia cannot be adaptively adjusted according to the actual situation in the traditional virtual inertia control method,the inertia of the AC system based on the virtual synchronous generator(VSG)control and the inertia of the DC system are compared and analyzed,and a flexible virtual inertial(FVI)control strategy for the DC distribution network is proposed to provide flexible and adjustable inertia for the disturbed DC distribution network.The serious impact of the system disturbances on the stability of the DC voltage is thereby weakened to further improve the DC voltage quality.The six-terminal DC distribution network simulation platform is established on Matlab/Simulink and the effectiveness of the proposed control strategy is verified by simulation results.(3)When the FVI control is applied to DC distribution network,the change of relevant control parameters will affect the virtual inertia of the system and the stability ofthe DC bus voltage.Therefore,the small-signal model of the six-terminal DC distribution network based on the FVI control method is constructed for small-signal stability analysis of the proposed control strategy,which lays a foundation for exploring the influence of the main control parameters in the proposed control strategy on the DC voltage stability of the system.(4)According to the small-signal model and state space matrix of the system,the influence law of the key control parameters in the proposed control strategy on the DC voltage stability of the system is revealed by the sensitivity calculation and root locus analysis,which provides the theoretical basis for the selection of the control parameters.Finally,the correctness of the theoretical analysis is verified by simulation results.