Method and apparatus for detecting fault in a mixed configuration power transmission line

An apparatus for detecting a fault in a power transmission line comprising a plurality of sections arranged between one end of the transmission line and the other end of the transmission line, wherein the plurality of sections of the power transmission line comprises at least two overhead line sections and at least one cable section arranged between the at least two overhead line sections, wherein on a condition that a fault occurs in the power transmission line at least one travelling wave is generated in the power transmission line, the apparatus being configured to:

• determine at least one first travelling wave voltage or current time derivative based on a plurality of travelling wave voltages or currents sensed at different points in time at the one end of the power transmission line, the one end of the power transmission line being included in a first overhead line section;
• determine at least one second travelling wave voltage or current time derivative based on a plurality of sensed travelling wave voltages or currents sensed at different points in time at the other end of the power transmission line, the other end of the power transmission line being included in a second overhead line section;
• evaluate whether the first travelling wave voltage or current time derivative exceeds a first threshold value;
• evaluate whether the second travelling wave voltage or current time derivative exceeds a second threshold value;
• evaluate whether the first travelling wave voltage or current time derivative exceeds a third threshold value;
• evaluate whether the second travelling wave voltage or current time derivative exceeds a fourth threshold value;
• and
• on a condition that a magnitude of the at least one first travelling wave voltage or current time derivative and/or a magnitude of the at least one second travelling wave voltage or current time derivative exceeds said first threshold value or said second threshold value, respectively, determine that there is a fault occurring in at least one cable section of the power transmission line,
• wherein each of the first threshold value and the second threshold value is based on the nominal transmission line voltage or current and an estimated refraction factor of the transmission line indicating the extent of refraction of the at least one travelling wave at a junction between the at least one cable section and an overhead line section adjacent to the at least one cable section;
• or
• on a condition that that the magnitude of the at least one first travelling wave voltage or current time derivative and the magnitude of the at least one second travelling wave voltage or current time derivative exceed said third threshold value and said fourth threshold value, respectively, determine that there is a fault occurring in the at least one cable section of the power transmission line,
• wherein each of the third threshold value and the fourth threshold value is based on the nominal transmission line voltage or current, the estimated refraction factor, and an estimated attenuation of the at least one travelling wave when travelling through the at least one cable section towards the one end of the power transmission line and towards the other end of the power transmission line, respectively.