| As a kind of multilevel technology,modular multilevel converter(MMC)has the characteristics of high integration,high working voltage level and flexible control mode.Thus it has been widely used in the fields of high voltage direct current(HVDC)transmission and new energy power generation.However,it contains a large number of power electronic devices,bring greater disturbance to the system when connected to the power grid,and resulting in potential safety hazards in the operation of the power system.It is of great significance to introduce the control idea of virtual synchronous generator(VSG)to solve this problem.In addition,the coordinated control among ports in multi-terminal direct current(MTDC)transmission system is the key problem to ensure the normal operation of the whole system.This thesis focuses on the MMCMTDC control strategies under VSG.Starting from the underlying issues such as voltage modulation,voltage sharing control,circulation suppression,etc.,the key issues such as MMC-MTDC-VSG control and system-level multi-port coordination control are studied.The main contents are as follows.Firstly,the MMC control principle,mathematical model,topology structure and the principle and implementation of MMC carrier phase-shifting modulation strategy are studied.The capacitance voltage equalization control of submodule and the circulation suppression in bridge arm in MMC control process are emphatically analyzed.Sub-module independent pressure equalization control and circulation suppression control method located in the dq coordinate system are used respectively.The functions of the above models are verified by simulation on MATLAB simulation platform.Secondly,the idea of VSG is introduced into the traditional MMC system to improve its frequency modulation ability.The relationship between VSG and traditional synchronous generator is studied.Additionally,its active-frequency characteristics and reactive-voltage characteristics,as well as the influence of core parameters moment of inertia and damping coefficient on control effect are analyzed.The VSG system of MMC is studied,and the effects under different working conditions are verified in MATLAB.Thirdly,the mathematical model of MTDC is studied,and the control system is divided into different levels.Then we study the influence of system-level coordinated control among multi-port converters on the normal operation of MTDC.Based on the advantages and disadvantages of master-slave single-point DC voltage control and multi-point DC voltage margin control,then an improved controller combining VSG with DC margin control is studied.Considering the influence of core parameters in the system on the control effect,a fuzzy adaptive VSG DC margin control is proposed.In MATLAB simulation platform,the above MMC multi-port control system is simulated and verified. |
PDF Full Text Request