Research On Virtual Motor Technology Of Common DC Bus System

The common DC bus system is an open DC system,which is connected to the external AC grid by a centralized AC/DC,allowing multiple units such as photovoltaics,fans,energy storage,and DC loads to be flexibly connected through the converter.It is suitable for distributed cell and load plug-and-play.Typical application scenarios include electric vehicle charging stations,regional DC distribution networks,and data centers.Based on the requirement of grid friendliness and stability of DC system,a virtual motor technology applied to common DC bus system is studied.The centralized AC/DC is controlled by a load virtual synchronous motor to simulate the inertial damping characteristics and power characteristics of a conventional motor.The energy storage DC/DC needs to be implemented with load virtual synchronizer,simultaneously,simulate DC motor,flexible charging and discharging according to DC bus voltage to maintain system stability.The main research work is as follows:(1)The topology and overall control scheme of the common DC bus system are analyzed.The topology structure of the common DC bus system is introduced.The main circuit topology of each port converter is determined and the mathematical model is established.The functions and control objectives of each port converter are analyzed.Finally,the overall control scheme of virtual motor for the common DC bus system in this paper is given.(2)The working principle of the load virtual synchronous motor is analyzed in detail,and its power characteristics and inertial damping characteristics are described.Combined with the requirement of AC/DC to build DC bus voltage and improve power quality,the specific implementation method of virtual synchronous motor control strategy of "DC bus voltage control-power loop-voltage and current inner loop" is given,and the principle of DC bus voltage control acting as secondary regulation is analyzed.The effectiveness of load virtual synchronizer to provide frequency and voltage support for external AC network is verified by simulating the fault condition of power grid.Finally,the control stability of virtual synchronous motor is analyzed,and the small signal model of control loop of virtual synchronous motor is established.The influence of virtual inertia,damping and virtual inductance parameters on the stability is analyzed by root locus method,which provides reference for parameter design.(3)Aiming at the problem that virtual synchronous motor affects DC bus voltage stability,based on in-depth analysis of droop control,virtual DC motor control is adopted for energy storage DC/DC.The working principle and control strategy are expounded.The droop control and virtual DC motor control parameters are analyzed by small signal model method respectively.The simulation results show that the virtual DC motor control can increase the inertia damping of DC bus voltage and has better ability to suppress fluctuations.In order to realize the undifferentiated control of power and voltage,the method of quadratic adjustment by translating sag curve is given.Finally,based on the small signal model of virtual synchronous motor,the characteristics and minimum power and capacity allocation of energy storage unit in the primary regulation process are quantitatively analyzed.