Research On Control Parameter Optimization And Operation Optimization Of VSC-MTDC

Voltage source converter based high voltage direct cur rent(VSC-HVDC)transmission is a new type of direct current transmission technology base on voltage source converter(VSC)and pulse width modulation(PWM).Because of its full controlled power electronic devices,the advantages of VSC-HVDC conclude decoupling active power and reactive power,supplying power to the passive network and avoiding commutation failure.Therefore,it is suitable for connecting offshore wind power station,integrating distributed energy resource,interconnecting different AC system and forming mult i-terminal direct current(VSC-MTDC)system.The VSC-HVDC has received extensive attention of academia and industry both at home and abroad.In this thesis,the control strategy,control parameter optimization and operation optimization of VSC-MTDC are studied.The main contents of this thesis are as follows:In this thesis,the mathematical model of VSC in dq0 rotation coordinate system is established,and then the double closed-loop control strategy of VSC-HVDC is deduced.The inner loop controller adopts the feed-forward compensation structure to achieve the decoupling of active power and reactive power,while the outer loop controller chooses control strategy among constant DC voltage,constant active power,constant reactive power and constant AC voltage according to different application scenarios.The control strategies of VSC-MTDC such as DC voltage master-slave control,DC voltage deviat ion control and DC voltage droop control are studied based on the studies of VSC-HVDC control strategies.The advantages and disadvantages of each control strategy are analyzed,and the control structure of each control strategy is designed.A method to determine the control parameters of the VSC-MTDC system is proposed,which can guarantee the controller to control the system accurately and stably in both steady state and transit ion process.Firstly,the initial value of each controller's control parameters is determined to minimize the steady-state deviation of controller 's outer loop control variables,thus the stable operation of system is guaranteed.By dividing time interval according to the difference of control characteristic region in transition process,the evaluation index of VSC-MTDC control characteristics is constructed and then used as objective function.Finally the optimal control parameters are obtained by using the simplex algorithm to optimize the initial values of the control parameters.Simulation results show that the control parameter tuning method proposed in this thesis can improve the control precision and dynamic response of the control system.A hierarchical optimization control system for VSC-MTDC is proposed,which can optimize the system's network loss while guaranteeing the stable operation of the system.The primary control level uses the DC voltage droop control strategy,which is the foundation of the control system.The secondary control level first collects the active power of each converter station,takes the network loss as objective function and a series of equalit y and inequality as constraints,then calculates the optimal power flow of the system.Finally The secondary control level gives the new reference values to controllers of the primary control level and the minimum network loss of the system is reached.