Method for converting boundary element by extra-high voltage direct current transmission line mode voltage S

The invention discloses a method for converting a boundary element by an extra-high voltage direct current transmission line mode voltage S, comprising the following steps: when the direct current line is in fault, after a protective element is started, the line mode voltage is obtained by utilizing a phase mode transformational theory according to the two-pole direct current voltage detected on a protection installation part; a dispersion line mode voltage signal is selected to perform S conversion, wherein the sample sequence length is 200 points, and the conversion result is 101*200 time frequency complex matrix; mode calculation is performed on each element in the complex matrix; the high-frequency energy and the low-frequency energy of the line mode voltage are extracted according tothe obtained mode matrix; the ratio of the high-frequency energy and the low-frequency energy of the line mode voltage is calculated; the faults in the cell and out of the cell are distinguished according to the ratio of the high-frequency energy and the low-frequency energy. A large amount of stimulated results prove that the method of the invention has better effect.