It is difficult to distinguish the faults of the left and right sides of the T zone when a T-type connection appears in the multi-terminal flexible HVDC project

so the protection scheme of the multi-terminal flexible HVDC line should be studied. This paper proposes a multi-terminal flexible HVDC line protection scheme based on the first-order difference of line mode inverse traveling wave. Firstly

The propagation characteristics of line mode inverse traveling wave are analyzed when the fault occurs in line 1

line 2

and T districts

and the formula of line mode inverse traveling wave for faults at different positions is derived. The variation trend of the line mode inverse traveling wave passing through the protection device at different locations is revealed. Secondly

the first-order difference is used to characterize the change characteristics of line mode inverse traveling wave. The first difference of the line mode inverse traveling wave of the two protective devices within a specified time is calculated

and their positive and negative values are used to determine the left and right faults in the T-zone and the faults in the T-zone. The absolute sum of first-order difference is used to judge the fault inside and outside the rectifier side of line 1 and the fault inside and outside the inverter side of line 2. The positive and negative voltage amplitude ratio is used for pole selection. Finally

the simulation results show that the scheme can use the first-order difference sum of linear mode inverse traveling wave as the direction criterion to determine the fault area

and the first-order difference absolute value of linear mode inverse traveling wave can be used to identify the fault inside and outside the region.