Research On The Operational Characteristics Of Hybrid AC/DC System With MMC-MTDC Grids

Voltage source converter-based multi-terminal direct current(VSC-MTDC)system has become an attractive solution to renewable energy integration,bulk power transmission and AC grid interconnection,for its operational flexibility,immunity against commutation failure and so on.Among various topologies of VSC,the modular multilevel converter(MMC)is the most promising one,due to high capacity,high-quality sinusoidal voltage waveforms,and other benefits.All of the MTDC grids that are in operation and under construction are based on MMC.Many MMC-MTDC grids,e.g.,Zhangbei MTDC grid,will be commissioned in the near future.Studying operational characteristics of large-scale hybrid AC/DC grid with inclusion of MMC-MTDC grid deeply is of great significance to improve the safety and stability of power system.This thesis studies the short-circuit current characteristics,static security characteristics and transient stability characteristics of hybrid AC/DC grid with inclusion of MMC-MTDC.Based on this,the methods to improve the safety and stability of hybrid AC/DC grid with MMC-MTDC grid are explored.The main works and contributions of this thesis are as follows.(1)The calculation method and characteristics of short-circuit current in hybrid AC/DC power system with MMC-MTDC are studied.For DC fault,a companion circuit based short-circuit current calculation method is proposed firstly.Then,the characteristics of short-circuit current of the MTDC grid are analyzed.Finally,the suppression strategy of short-circuit current is studied.For AC fault,the mechanism how a converter station provides short-circuit current to the AC grid is analyzed firstly.Then the short-circuit currents of the near-area AC network of Zhangbei MTDC Grid is studied.Finally,one method of suppressing the short-circuit currents provided by converter stations is put forward.(2)The static security characteristics of hybrid AC/DC power system with inclusion of MMC-MTDC grid are studied.Firstly,an efficient decoupled power flow calculation method for hybrid AC/DC power system with MMC-MTDC is proposed.Validity and efficiency of the proposed method are verified by comparing with the sequential method on the modified New England Grid and North China power grid.Then,based on this,the power flow distribution of North China Power Grid with Zhangbei MTDC Grid is analyzed,and its static security is studied.Finally,aiming at the static security problems,the grid emergency control methods,such as power transfer strategy,are also explored.(3)The transient stability characteristics of hybrid AC/DC power system with MTDC grid are studied.Firstly,the electromechanical transient model of Hybrid AC/DC power system with MMC-MTDC is established,and compared with the detailed electromagnetic transient model established in PSCAD,the correctness of the electromechanical transient model is thus verified.Then,the transient stability characteristics of hybrid AC/DC power system with MMC-MTDC grid are analyzed from two aspects of AC fault and DC fault.Finally,the strategy of limproving the transient stability level of hybrid AC/DC power system with MMC-MTDC is studied.The influence of controller parameters on the transient response characteristics is explored.In addition,a coordinated control strategy to improve the frequency stability of hybrid AC/DC power system with MMC-MTDC is studied,which significantly improves the frequency stability of AC power network.In a word,the study on operational characteristics of hybrid AC/DC power system with MMC-MTDC is of great significance to improve the security and stability of power system.