Research On Reactive Power Control For Improving Voltage Stability Of Multi-terminal HVDC Transmission System

With the development of high-voltage direct current transmission,multi-terminal direct current transmission systems developed on the two-terminal DC transmission system has received more and more attention,which is more economical and flexible than traditional HVDC.However,the operation control of the MTDC system is more complicated,and the stability of the AC bus voltage of the converter station is an important problem that plagues the normal operation of the system.Especially when the receiving end is a weak AC system,the system is very sensitive to the change of the power of the converter station.The too much change of the reactive power absorbed by the converter station will directly affect the stability of the AC bus voltage.Therefore,reactive power controller is used to improve the voltage stability of the MTDC receiving end AC system.First of all,The research status of MTDC technology,system voltage stability and reactive power control at home and abroad are summarized in this paper.It is pointed out that the reactive power control can make full use of the reactive power adjustment capability of the DC system to avoid the instability of the system voltage caused by the reactive power compensation device.Then summarize the principle of MTDC technology from the aspects of wiring,operation mode and basic control method.Comparing the serial and parallel wiring methods,it is concluded that the parallel wiring has more advantages in engineering applications.Then,a parallel three-terminal MTDC system model is established in PSCAD/EMTDC simulation software,and the main circuit and control system of the model are introduced in detail.Then analyze the effects of two control strategies,the constant extinction angle(CEA)and the constant ignition angle(CIA)on system voltage stability.Through simulation comparison,it is concluded that the MTDC system based on CIA type control is more stable and faster than CEA type control.Finally,The basic principle of reactive power control and the reactive power characteristics of the inverter are analyzed in this paper,A control method is proposed,which adds a reactive power controller to the inverter side constant power control and added reactive power feedback.By adjusting the DC voltage to change the reactive power consumption of the inverter,the reactive power exchange between the AC and DC systems is controlled at a set level to achieve the goal of improving the system voltage stability.Using the three-terminal MTDC model for simulation,the simulation results show that the reactive power additional controller can improve the system'sfault recovery capability when the inverter has a three-phase grounding short circuit fault.Thereby improving the voltage stability of the receiving AC system.At the same time,A parameter optimization method based on genetic algorithm is proposed.The objective function based on the control signal deviation is constructed to optimize the PI parameters of the reactive power controller.The results show that the controller parameters obtained based on the optimization method make the effect of the reactive power additional controller better.