Research On Control Of Receiving Converter Of VSC-MTDC System Based On Virtual Synchronous Generator

Voltage source converter based multi-terminal direct current transmission system(VSC-MTDC)has the characteristics of multi-source power supply and multi-drop power reception,which has obvious advantages in reliability and flexibility.It is an effective way to build a flexible power grid and promote the consumption of new energy.However,the converter under conventional vector control exhibits zero inertia and low damping characteristics,which cannot provide the necessary frequency support for the power grid.At the same time,with the access of large-scale distributed energy sources that use power electronic devices as grid-connected interfaces,the equivalent inertia and damping in the grid are gradually reduced,which further deteriorates the frequency response characteristics of the system,and it is easy to cause large frequency fluctuations or even collapse.The virtual synchronous generator(VSG)technology can simulates the inertia and frequency modulation characteristics of the synchronous generator,which can alleviate the above problems to some extent.Therefore,this article will introduce the VSG technology into the control of the MTDC system,and study the control of receiving converter of the MTDC system based on VSG control.First of all,this article explains the operating characteristics of the flexible direct power transmission system,studies the MTDC system based on the modular multilevel converter(MMC),and establishes the mathematical model of MMC.On this basis,the design of MMC inner and outer loop controller and modulation strategy is completed.And based on the MATLAB/Simulink simulation platform,a four-terminal MMCMTDC system model is built.In view of the problem that the MMC-MTDC system using conventional vector control cannot provide frequency support for the receiving AC power grid,the VSG control is introduced into the receiving converter.The design of the ontology model,virtual governor and excitation regulator in the VSG control system has been completed.Comparative simulations were carried out using conventional vector control and virtual synchronous machine control respectively,and the superiority of VSG control was verified.Secondly,in order to optimize the transient process of the system,this paper uses the flexible controllability of the VSG parameters to propose an adaptive control strategy suitable for the receiving converter.An adaptive model of virtual moment of inertia is established.The optional range of virtual inertia and damping parameters is determined under the constraints of the performance of the system.Within the range,the moment of inertia and damping can be adjusted in real time.The simulation results prove that the proposed parameter adaptive control can effectively improve the transient response performance of the system.Finally,a VSG coordinated control method with DC voltage control capability suitable for the receiving converter of the MMC-MTDC system is proposed.The VSG coordinated control system is mainly composed of three parts: VSG adaptive control,high-voltage controller and low-voltage controller.Once the DC side system is abnormal,the coordination controller detects the voltage deviation and can quickly adjust the active output of the converter station,realize the rebalance of the DC side power of the system,and stabilize the DC voltage.Simulations were carried out under the conditions of steady-state operation of the system,failure of the main station and power overload of the main station,and the applicability of the coordinated control was verified.