Research on Transmission Line Control Strategy of AC-DC Hybrid System

The control system is the core of HVDC transmission system. How to improve the operation and control performance of the DC system, how to make full use of the unique advantages of DC control to improve the stability of the power grid, and how to coordinate various control elements in the hybrid system have always been the concerns of the power sector and the control sector. Based on the above background, this paper studies the control strategy of AC/DC hybrid transmission line. Based on the basic structure of hybrid AC/DC transmission, this paper analyzes the principle and mathematical modeling of conventional HVDC and flexible DC transmission technology, and then defines the basic control mode of conventional converter and flexible converter. It is further extended to the topology and control strategy of multi terminal system, and verified by simulation, which lays a theoretical foundation for the follow-up research. Based on the detailed circuit simulation model of HVDC transmission system, considering the relationship between converter transformer ratio and rectifier side trigger delay angle or inverter side turn off angle, the auxiliary controller of tap changer is designed. When the trigger delay angle or turn-off angle is out of limit and the out of limit time exceeds the specified time, the auxiliary control of tap shall be started to make it return to the specified operating range through step-by-step adjustment.