Optimal Active Power Allocation For VSC-MTDC Transmission System

In recent years,with the upgrade of the concepts of Chinese economic development,the role of renewable energy in the energy structure of China is increasingly important.At the same time,VSC-HVDC transmission system based on voltage source converter has unique technical advantages in solving the problems of wind power connection volatility and randomness,being far away from load center and so on,and has become the main direction of future power grid development,due to its independent and rapid regulation of active and reactive power,noncommutation failure,providing reactive power compensation to AC bus,and being easy to construct multi-terminal DC system.However,as the loss of VSC converter station is relatively large,and with the increase of transmission capacity of VSC-HVDC,the power allocation of VSC converter station has more and more prominent influence on the operation economy of the AC/DC system.How to realize the optimal distribution of active power of VSC converter stations in VSC-MTDC and improve the system operation economy has become the focus of many scholars.Therefore,it is necessary to study the active power optimal allocation problem of VSC-MTDC system.Firstly,a day-ahead optimal dispatch model of VSC-MTDC system was established.The optimization objective was to minimize the total active power loss of the AC/DC system in one day.The constraints on the power regulation range,the number of regulation times and the minimum duration of power state of the VSC converter station were considered.A VSC-MTDC active power optimization method based on genetic algorithm is proposed,and the active power planning curve of VSC-MTDC system is optimized.The results of examples show that the proposed model and method can effectively reduce the total active power loss of AC/DC system with VSC-MTDC,and realize the optimal allocation of active power of the VSC converter station.Then,the day-ahead optimal dispatch problem of VSC-MTDC system with static security constraints is studied.A static safety analysis method combining flow transferring relativity factor method and active power sensitivity method is proposed,which can realize fast and accurate calculation of breaking power flow and correction control.First,flow transferring relativity factor method is used to calculate the new current values on the other branches after the branch is broken,so as to judge whether the branch is overload.Then the active power sensitivity method is used to calculate the sensitivity of each branch power flow in relation to the change of injected power of the AC node where the VSC converter station is located.Based on this,the active power of the VSC converter station is adjusted.The results of examples show that the VSC-MTDC day-ahead optimal dispatch model taking into account the static security constraints and the proposed method,can not only improve the operation economy of AC/DC system with VSC-MTDC,but also ensure the safety of the system.Finally,the stochastic dynamic economic dispatch problem of VSC-MTDC system considering the uncertainty of wind power output is studied.A stochastic dynamic economic dispatch model for VSC-MTDC system active power optimal allocation was established.The optimization objective was to minimize the expected value of the total daily active power loss of the AC/DC system with VSC-MTDC under various wind power scenarios.An optimal dispatch method of VSC-MTDC system considering the randomness of wind power output was proposed based on state space approximate dynamic programming method to solve the optimization model.A three-stage method is proposed to solve the state space,and the similarity function is established to obtain the optimal active power allocation.The results of examples show that the optimized VSC converter station power plan can significantly reduce the total active power loss of AC/DC system and ensure the system safety.