Study On Control Strategies Of High Voltage Direct Current Transmission System

In order to solve asymmetrical distribution of resources and the power load in our country, and meet the electric power demand in China, government carried out the projects of "nationwide interconnection, delivering power from west to east and exchanging between south and north". Since high voltage direct current transmission technology (HVDC) is suitable for power transmissions of large capacity and with long distance, it gets a rapid development and will be more and more important in our country. The HVDC system is highly controllable. It is of practical significance both at home and abroad how to exploit the high controllability and fast accommodation of HVDC to improve its performance and enhance the operating stability of AC system connected as well. The field has arisen substantial recognition and researches.HVDC system consists of converter station and transmission lines, including converter, smoothing reactor etc., of which converter (rectifier and inverter) is the most important component. Firstly, the paper introduces basic constitution of HVDC and functions of its main elements, and then discusses converter's operating principle and characteristics, which lays a basic foundation for the research into HVDC system control strategies.Secondly, based on a systematic mathematical model, the paper analyzes both the principle and performance of main control and accommodation mode for HVDC system, and then puts forward the control diagram (for basic control and additional control). The mode of basic control is constant current control in rectification side and constant voltage control in inversion side, which ensures HVDC system running normally in constant power. Additional control chooses both sides frequency different signal as input to adjust DC power output and assists system frequency restoration when one side of system is disturbed. Traditional basic control and additional control are both carried out by PI and PID controller.Then, in order to improve operation performance of HVDC system, it emphasizes on fuzzy control's application in HVDC (the important branch in intelligent control). Corresponding to the nonlinear and time-varying characteristic of HVDC, the paper deeply analyzes basic structure and principle of fuzzy control. Combining the fuzzy control with PID control, the paper then designs a new type basic controller and additional controller—fuzzy adaptive PI (PID) controller. They respond timely to systemic changes, and carries out online modification on PI (PID) parameter in accordance with fuzzy guideline to satisfy different requests of parameter at any time, which ensures the system of dynamic and static characteristics.Finally, the paper builds up a 12-pulse wave simulation model of HVDC system in Matlab according to the system control diagram, and simulates the designed basic controller and additional controller with the model above. If only considering basic control, with DC line-to-ground breakdown and AC single-phase earth fault, the HVDC system is simulated respectively in traditional PI control and new control adding fuzzy. For the traditional PI control, considering respectively additional PID control and additional fuzzy adaptive PID control, with large load disturb and distance three-phase short-circuits under the earth fault, the system is simulated. The result proves that the basic controller and additional controller being designed effectively improves transient and static characteristic with good robustness and adaptability.