Research On AC/DC Hybrid Microgrid Control Based On Virtual Synchronous Generator Technology

The AC/DC hybrid microgrid can realize flexible interconnection of AC subnet and DC subnet through flexible DC transmission,which greatly improves the absorption capacity of new energy and new loads,and AC/DC hybrid microgrid based on flexible DC transmission has become a new development direction in the field of microgrid.According to the load characteristics,based on distributed energy storage,independent operation of AC subnet and DC subnet can be constructed,power electronic device is the core component of distributed power supply,its response is fast and lack of inertia,which is not conducive to the stable operation of the subnet.The existing subnet parallel operation mostly adopts the rigid control technology combining fixed DC voltage and fixed active power,and the subnet needs to bear the power disturbance in the network,and the "source-grid-load-storage" distribution of the AC and DC flexible transmission system is difficult to control,so that the subnets do not have the power disturbance equalization and voltage/frequency support capabilities,and the risk of system instability will be increased when the load fluctuation is large.In view of the above problems,this paper proposes a modular AC/DC hybrid microgrid based on distributed energy storage.The subnet has an independent bus structure,and the flexible HVDC transmission network is connected to form a whole through the grid-connected converter.The battery and grid-connected converter in the AC subnet adopt virtual synchronous generator technology and the battery and grid-connected converter in the DC subnet adopt virtual inertial technology to realize the parallel power supply of the battery and the grid-connected converter in the subnet and the flexible interaction between the power of the subnet and the DC transmission network,and improve the inertia of system operation.Consistency control is adopted between subnets to improve the power mutual assistance between subnets.The specific research content is as follows:(1)Research on open-loop control and frequency characteristics of AC microgrid based on VSG technology.Firstly,how to use VSG technology to realize parallel power supply between AC subnet battery and grid-connected converter is studied,and an improved VSG technology suitable for grid-connected converter is proposed considering the voltage of flexible HVDC transmission network to improve the subnet anti-disturbance ability and flexible HVDC transmission network stability.Secondly,the equilibrium point of system operation is derived in detail,and a small signal analysis model of AC microgrid based on VSG technology is established,and then the characteristic value sensitivity analysis method is used to determine the control parameters that have the greatest impact on the system frequency,and an optimization scheme combining the root trajectory method and particle swarm optimization algorithm is proposed to coordinate and optimize the key control parameters.(2)Research on voltage consistency control of DC microgrid based on virtual inertia.In order to improve the mutual assistance capability of DC subnet,a voltage consistency control strategy of multi-subnet DC microgrid is proposed according to the multi-agent consistency theory,and the overload power of the disturbed subnet is distributed to other subnets through voltage consistency control on the basis of virtual inertial control and improved virtual inertial control to achieve parallel power supply between the DC subnet battery and the grid-connected converter.Secondly,the open-loop control model of DC microgrid and the closed-loop control model using consistency control are established,and the influence of control parameters on system stability is analyzed.Finally,the simulation verifies that the voltage consistency control can further enhance the voltage stability of the DC microgrid.(3)Research on voltage/frequency consistency control of AC/DC hybrid microgrid based on VSG technology and virtual inertia.The operating conditions of AC/DC hybrid microgrids are complex,and it is difficult to coordinate AC frequency and DC voltage stability.This paper combines VSG technology and virtual inertial control technology to realize the AC subnet frequency and DC subnet voltage stability control of the designed modular AC/DC hybrid microgrid.The AC frequency and DC voltage are normalized,and a voltage/frequency consistency control strategy is proposed to achieve voltage/frequency consistency control through the grid-connected converter.Secondly,in view of the voltage/frequency deviation generated by the primary control,on the basis of realizing the voltage/frequency consistency control,the system voltage and frequency deviation is eliminated through the secondary control to improve the power quality of the AC/DC hybrid microgrid.