| With the process of reform and opening.due to the increasing scale of power systems and the scale of new energy grids,the security and stability of power systems has received more and more attention.How to make the power system safe and stable in more economical situations has become more and more important,so the stability analysis of the power system has become an important hot spot for people to study now.In an equivalent dual-supply system,if the system oscillates with some perturbation,the angle of the equivalent electromotive force on both sides of the system may change from 0° to 360°,and the system is in an unstable state.When δ is swung to an angle after 90°.achieving a sudden change in power angle by "phase sequence exchange",pull the originally increasing power angle back to a smaller state,and suppress the occurrence of out-of-synchronization.This new method of improving system stability is called phase sequence changing stability control.Since the transmission power of the system tie line is related to the power angle,when the system power angle is abruptly changed 120° by using the phase sequence changing technique,the transmission power of the contact line will change at this moment.Therefore,by studying the influence of DC power modulation on the power system,this paper proposes a circuit that combines the MMC-HVDC w ith the AC phase sequence changing function.That is,the AC/DC conversion is used to realize the phase sequence changing stability control.(1)The basic concepts and principles of AC phase sequence changing stability control and phase sequence changing stability control based on AC-DC conversion are analyzed respectively,which explains what is the phase sequence changing technique,and how the phase sequence changing technique improves the stability of the system.Proposes a circuit that combines the MMC-HVDC with the AC phase sequence changing function.And the various operating states of the circuit topology,the mutual switching between states,and how the circuit topology combines the DC power transmission and the AC phase sequence changing function are also studied.(2)In the process of studying the phase sequence changing stability control based on AC/DC conversion,the overload capacity of DC line.the influence of the initial power angle of steady-state operation,the fault resection angle and the transmission power distribution of AC and DC lines on phase sequence changing stability control are analyzed and by comparing the power modulation of the DC system in the AC-DC parallel system,the modulation of the system power by the designed circuit is explained.Then,the relationship between the transmission power limit and the short-circuit ratio and the relationship between the strength of the AC system and the effective inertia constant are analyzed to improve the power transmission capability of the DC transmission system.(3)The simulation verification is carried out on the Matlab/Simulink platform,by analyzing the transient simulation results of the three-phase short-circuit fault of the same single machine to the infinite system which add phase sequence changing circuits that have different types of DC transmission systems,study on the control effect of phase sequence changing stability control based on AC/DC transmission on system failure and the advantages of the designed circuit topology. |
