| The status of VSC-HVDC has become increasingly prominent in the power grid,with its voltage level continues to increase and transmission capacity to grow,especially when modular multilevel converter technology has matured,the system capacity of VSC-HVDC transmission has already exceeded breakthrough Gigawatts,.Therefore,the impact of VSC-HVDC on regional power grid cannot be ignored any more.Existing MMC-HVDC projects almost use vector control technology without any exception,because vector control has the advantages of fast power regulation and independent control of decoupling.However,with a large scale access to VSC-HVDC systems,the flaws of vector control technology are beginning to emerge.Access of VSC converter station dilutes the installed capacity of the conventional synchronous generator,resulting in the reduction of system equivalent inertia.Furthermore,the external performance of VSC converter station shows zero inertia and damping,which further deteriorates the system operating environment.When the sending-end grid power changes drastically or heavy load changes on the receiving-end grid,the voltage and frequency of AC system can easily cause large fluctuations or even collapse.However,the power of vector control is a fixed value and cannot participate in voltage and frequency regulation of AC system autonomously.Therefore,it is necessary to study a new type of control technology that can involve MMC station in the system's frequency and voltage regulation with inertia and damping support,to realize the effective interaction and cooperation between MMC station and AC system.The above problem can be solved by designing the MMC station as a virtual synchronous generator.Taking into account MMC-HVDC steady state current is very large,the conventional VSG control technology cannot directly be used,and current limiting and voltage support capabilities should be considered simultaneously.Therefore,this paper combines the advantages of vector control and VSG control to study VSG control method of MMC-HVDC transmission system,mainly include:1.The design of bottom control system of VSG control for MMC station is completed.First,each bridge arm is simplified as a voltage controlled voltage source,and taking inverter as an example,the mathematical models of MMC ontology and AC-DC side are carried out in detail.Then the voltage outer loop and the current inner loop controller are designed for the proposed mathematical model.The voltage outer loop adopts the direct power calculation strategy and the current inner loop adopts the PI control strategy.In addition,because the bridge arm uses a simplified model of controlled voltage source,the circulation suppressor is necessary,so a circulating current suppressing controller is designed based on the quasi-proportional resonance PR control method in ?-? coordinate system,which can avoid current coupling in rotating coordinate system and reduce the controller's sensitivity to grid frequency offset.Finally,MMC-HVDC transmission system is built on the Matlab/Simulink simulation platform,and the validity of the proposed mathematical model and the control strategy are verified.2.The design of VSG control for MMC-HVDC is completed.Firstly,the governor and excitation voltage rcegulator in traditional synchronous generator are simplified and extracted in essence.The VSG control mathematical model is established and VSG control system based on vector double closed loop is designed,including rectifier mode VSG and inverter mode VSG.Then,the models of active and reactive power loop transfer functions are established by the small perturbation method,and control parameters of rectifier and inverter station are designed and analyzed.Finally,the main differences between VSG control and conventional vector control of MMC-HVDC are compared on the Matlab/Simulink simulation platform,which shows the superiority of MMC in VSG control,and the correctness of simplified mathematical model and the effectiveness of proposed VSG control system as well.3.The coordinated control strategy based on VSG control for MTDC-HVDC is completed,which can be called DC voltage coordinated control strategy with the function of VSG.DC voltage coordinated controller is divided into three parts,namely high voltage controller,low voltage controller and VSG algorithm module.When the system power flow changes leading to oscillation of the DC voltage,different voltage controller will be triggered,and then the coordinated controller will choose different active current as output autonomously in order to ensure the stable operation of the system.Finally,a four-terminal DC grid simulation model is established on the Matlab/Simulink simulation platform to verify the effectiveness of the coordinated controller. |
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