Research On Mutual Fault Characteristics And Backup Protection Of AC/DC Hybrid Grid

With the continuous reduction of fossil fuels and environmental degradation,renewable energy has been widely used in power grids.Connected to renewable energy,traditional AC grid becomes AC/DC hybrid grid.However,renewable energy will bring about enormous change of fault characteristics in AC grid,which will influence the cooperation between protections.Therefore,in order to ensure the safety of AC/DC hybrid grid,it is necessary to research the fault characteristics and relay protections of AC/DC hybrid grid.In this paper,mutual fault characteristics of AC/DC hybrid is studied and based on that,backup protections suitable for AC/DC hybrid grid are put forward.The achievements of the paper are listed as the following:Firstly,mathematical model is established for typical double-terminal VSC-HVDC system and a low voltage ride through(LVRT)control strategy coordinated with DC chopper circuit is analyzed.The fault characteristics of the converter is analyzed in detail.When an LVRT-controlled converter is connected to AC/DC hybrid grid,mutual fault characteristics is studied in detail,which establish the foundation for the subsequent analysis of backup protections.The characteristic of branching factor in AC/DC hybrid grid is studied in detail and modified Zone II of distance protection based on branching factor is proposed.The change law of branching factor is analyzed when the converter is connected,and setting principle of modified Zone II of distance protection is proposed,as a backup protection for Zone I of distance protection.The PSCAD simulation verifies the reliability and sensitivity of proposed protection.In addition,relationship between DC 100 Hz component and AC sequence component are specifically studied when unbalanced AC faults occur.The backup protection based on DC 100 Hz component is put forward and coordination between DC side,Zone III of distance protection and overcurrent protection is also realized.Feasibility and validity of the proposed method are verified by simulations.This paper preliminarily completes the analysis of mutual fault characteristics in AC/DC hybrid grid.Two AC and DC backup protections suitable for AC/DC hybrid grid are also proposed.The protection schemes have certain practical significance to the reliable operation of AC/DC hybrid grid.