Research On VSC-HVDC Converter Control Based On Virtual Synchronization

In recent years,the penetration rate of power electronic equipment in the power grid is gradual improve,the technology of HVDC has been rapidly developed.The system of VSC-HVDC based on the voltage source converter has a series of technical advantages,such as no commutation failure,no need for a large number of reactive power compensation devices and filtering equipment.So it have a good space for development in the fields of clean energy grid connection,connection to passive networks and weak AC systems,and black start.However,in some cases,there still have some shortcomings in the analysis and control of VSC-HVDC.The power distribution between the converter stations is unbalanced,the output adjustment of converter is not flexible and the external performance of the converter lacks inertia and damping,etc.which makes the power system more susceptible to Power fluctuations and system failure.In response to these problems,further control research on flexible DC transmission technology has become particularly important.This thesis takes the VSC-HVDC system as the research object,in view of the low-inertia and undamped characteristics of the transient process involved in the VSC-HVDC system under the traditional converter control are easily impacting the power grid and the difficulty of effectively participate in the dynamic frequency regulation of AC system as well as the unbalanced power transmission between AC and DC systems,on the basis of studying the research on the operating mechanism characteristics of virtual synchronous generator technology,a VSC-HVDC receiver-side converter control strategy based on VSG technology is proposed.Firstly,based on the VSC-HVDC system topology model,the feasibility of virtual synchronization of the converter station is analyzed;secondly,The VSG technology with active fuzzy PI droop control is introduced into the HVDC transmission system,by adjusting the droop coefficient,the VSC-HVDC system inverter can guarantee the system’s active balance transmission under steady state operation and transient fault,so that make it have the ability to coordinate control the frequency of the AC system and the voltage of the DC system,and further utilize the flexible and controllable characteristics of the VSG parameters,introduce a fuzzy algorithm,optimize the parameters such as droop control,moment of inertia and damping coefficient,so that we can introduce the VSG technology with parameter coordination to VSC-HVDC transmission system,thereby controlling system frequency,power and DC voltage fluctuations to improve system transient performance;Finally,a three-terminal system was built in Matlab/Simulink for simulation verification.The results show that the VSC-HVDC system can improve the inertia level and attenuate frequency variation of the AC grid after adopting the VSG parameter coordinated optimization control strategy with the proposed power fuzzy PI drooping.The system frequency,power and DC voltage fluctuations are reasonably controlled,the voltage regulation control can also provide better support in steady state and transient state,and enhance the stability and reliability of the AC/DC system availability.