Research On Restoration Of Power Systems With Multi-DC System Hybrid Feed

With the widespread application of conventional high voltage direct current transmission systems(Line Commutated Converter based high voltage direct current,LCC-HVDC)in the field of power transmission and the rapid development of flexible DC power transmission systems(Voltage Source Converter based high voltage direct current,VSC-HVDC),the shape of China’s power grid is gradually developing towards a large-scale AC-DC hybrid power grid.The change in the shape of the power network has improved the convenience of system operation and control,but also increased the complexity of recovery after a power blackout.The change in the shape of the power system network has also provided new ideas and measures for AC system recovery.This article studies the integration of LCC-HVDC and VSC-HVDC into the black-start system recovery process for a variety of DC mixed-feed AC system networks.It is of great significance to make full use of the advantages of each DC system to improve the existing black-start recovery strategy and meet the requirements for the development of new generation power grids.This article first introduces theoretically the operating characteristics and control strategies of VSC-HVDC and LCC-HVDC participating in system recovery,and clarifies the role of the DC system in the black start process.Using PSCAD software to build models for different DC participating system recovery,the feasibility of VSC and LCC participating in system restoration at the same time was verified by simulation.Clarifying the starting control method of each DC system to lay the foundation for the subsequent recovery of conventional AC and flexible DC participating receiving-end AC systems.Critical impedance of starting LCC-HVDC using VSC-HVDC independently is analyzed.For the multi-DC feed receiving-end system without black start,according to the short-circuit capacity and system inertia requirements of the conventional DC system startup,the feasibility of starting a conventional DC system directly by the flexible DC system is analyzed.A connection impedance representing the reactive power supporting capacity of VSC to LCC is defined.This impedance determines the voltage level of the LCC receiving-end converter bus,and clarifies that the connection impedance is a key factor that determines whether VSC-HVDC can start LCC-HVDC or not.The critical connection impedance is solved using the Newton optimal power flow method.Based on the PSCAD software,a simulation model was established to verify the feasibility of starting of conventional DC by flexible DC independently and correctness of the proposed critical connection impedance,which can provide theoretical support for subsequent research on different DCs participating in black start recovery.Research on network reconfiguration of black start restoration of receiving-end AC system with hybrid DC feed.When the receiving-end AC system with multi-DC feed is restarted,VSC is used as the black-start power source to restore the receiving-end network,which can give full play to advantages of VSC and LCC systems and improve the system recovery process.Aiming at the receiving AC system with multi-DC feed,the role and characteristics of VSC and LCC when participating in system recovery are analyzed,and a method for obtaining the short-circuit capacity of the receiving-end system with the VSC drop point is proposed.Based on this,a multi-objective optimization model for the network reconstruction of the receiving-end AC system with hybrid DC feed is established.Because the transmission power of the LCC system is related to the system recovery path and system strength,an improved SPEA2 algorithm is proposed to characterize the particle encoding method by the recovery path,which can optimize non-tree recovery networks.Then,the optimal grid reconstruction scheme that takes into account all power sources is solved.