Research On Control Strategy Of Flexible DC Transmission System

With the development of new energy and the expansion of power grid scale,HVDC technology will play an important role in the construction of smart grid in China.Compared with AC transmission,HVDC has its unique advantages,in particular,the flexible DC transmission technology can control active and reactive power independently,quickly and flexibly.When adopting the traditional control strategy,the flexible DC transmission system is difficult to participate in the dynamic regulation of the frequency and voltage of the grid.In this paper,the virtual synchronous machine control technology is applied to the flexible DC transmission system,which enables the converter station to simulate the synchronous motor to improve its output characteristics,thus providing some help for the stable operation of the power grid.Firstly,the structure and working principle of modular multilevel flexible DC transmission system are analyzed,the mathematical model is set up,and the parameters are designed,then the control strategies at various levels are designed,including the vector decoupling control,valve level control,circulation suppression and modulation based carrier phase shift modulation strategy based on modulation wave reconfiguration,the effectiveness of the controller is verified by simulation.Aiming at converter level control,based on analyzing the structure of synchronous generator and establishing a mathematical model.The feasibility of applying virtual synchronization technology to flexible HVDC transmission system is analyzed,The active frequency controller,reactive power voltage controller and current controller based on proportional resonance are designed for the inverter side and the rectifier side respectively.By setting up the small signal model,the control parameters of the virtual synchronous machine are designed.Finally,the simulation results are based on the PSCAD simulation platform,The simulation results show that the virtual synchronous machine technology can make the converter station of the flexible DC transmission system have the characteristics of synchronous motor,and can provide inertia,damping and frequency support for the system,and improve the stability of the power grid.