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

1. A method of fault detection in a power transmission line comprising a plurality of sections arranged between a 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 method comprising:

determining at least one first travelling wave voltage or current time derivative 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;

determining at least one second travelling wave voltage or current time derivative 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;

evaluating whether the first travelling wave voltage or current time derivative exceeds a first threshold value;

evaluating whether the second travelling wave voltage or current time derivative exceeds a second threshold value;

evaluating whether the first travelling wave voltage or current time derivative exceeds a third threshold value;

evaluating 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, determining 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, determining 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.

2. The method according to claim 1 , wherein each of the plurality of sections of the power transmission line comprises an overhead line section or a cable section.

3. The method according to claim 1 , further comprising performing, in response to determining that there is a fault occurring in the at least one cable section of the power transmission line, at least one protective action for the power transmission line.

4. The method according to claim 3 , wherein the power transmission line is selectively electrically connectable and disconnectable to a power system at the one end of the power transmission line and at the other end of the power transmission line, respectively, by way of at least one circuit breaker arranged at the one end of the power transmission line and at the other end of the power transmission line, respectively, said circuit breakers being configured to be opened in the event of a fault occurring in the power transmission line and wherein the at least one protective action comprises disabling auto-reclosing for the at least one circuit breaker at the one end of the power transmission line and at the other end of the power transmission line, respectively.

5. The method according to claim 1 , wherein the power transmission line comprises a High Voltage Direct Current transmission line.

6. The method according to claim 5 , wherein the power transmission line is arranged in a bipolar configuration, and wherein the at least one first travelling wave voltage or current time derivative at the one end of the power transmission line comprises at least one differential mode voltage or current time derivative at the one end of the power transmission line, and the at least one second travelling wave voltage or current time derivative at the other end of the power transmission line comprises at least one differential mode voltage or current time derivative at the other end of the power transmission line.

7. The method according to claim 5 , wherein the power transmission line is arranged in a monopolar configuration, and wherein the at least one first travelling wave voltage or current time derivative at the one end of the power transmission line comprises at least one line terminal side voltage or current time derivative at the one end of the power transmission line, and the at least one second travelling wave voltage or current time derivative at the other end of the power transmission line comprises at least one line terminal side voltage or current time derivative at the other end of the power transmission line.

8. The method according to claim 1 , wherein determining at least one first travelling wave voltage or current time derivative at the one end of the power transmission line comprises sensing at the one end of the power transmission line a plurality of travelling wave voltages or currents at different points in time, and

wherein determining at least one second travelling wave voltage or current time derivative at the other end of the power transmission line comprises sensing at the other end of the power transmission line a plurality of travelling wave voltages or currents at different points in time.

9. The method according to claim 8 , wherein the sensing at the one end of the power transmission line of the plurality of travelling wave voltages or currents and the sensing at the other end of the power transmission line of the plurality of travelling wave voltages or current are carried out during substantially the same period of time.

10. 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.

11. A fault detection system 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 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 fault detection system comprising:

at least one voltage and/or current sensing unit configured to:

sense 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; and

sense a plurality of 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; and

the apparatus according to claim 10 .

12. A power system comprising:

at least one transmission line interconnecting at least two different portions of the power system; and

the fault detection system according to claim 11 for detecting a fault in the power transmission line.

13. A non-transitory computer-readable storage medium on which there is stored a computer program product configured to, when executed in the apparatus according to claim 10 , perform a method of fault detection in a power transmission line comprising a plurality of sections arranged between a 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 method comprising:

determining at least one first travelling wave voltage or current time derivative 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;

determining at least one second travelling wave voltage or current time derivative 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;

evaluating whether the first travelling wave voltage or current time derivative exceeds a first threshold value;

evaluating whether the second travelling wave voltage or current time derivative exceeds a second threshold value;

evaluating whether the first travelling wave voltage or current time derivative exceeds a third threshold value;

evaluating 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, determining 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, determining 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.

14. The method according to claim 2 , further comprising performing, in response to determining that there is a fault occurring in the at least one cable section of the power transmission line, at least one protective action for the power transmission line.

15. The method according to claim 2 , wherein the power transmission line comprises a High Voltage Direct Current transmission line.

16. The method according to claim 3 , wherein the power transmission line comprises a High Voltage Direct Current transmission line.

17. The method according to claim 4 , wherein the power transmission line comprises a High Voltage Direct Current transmission line.

18. The method according to claim 2 , wherein determining at least one first travelling wave voltage or current time derivative at the one end of the power transmission line comprises sensing at the one end of the power transmission line a plurality of travelling wave voltages or currents at different points in time, and

wherein determining at least one second travelling wave voltage or current time derivative at the other end of the power transmission line comprises sensing at the other end of the power transmission line a plurality of travelling wave voltages or currents at different points in time.

19. The method according to claim 3 , wherein determining at least one first travelling wave voltage or current time derivative at the one end of the power transmission line comprises sensing at the one end of the power transmission line a plurality of travelling wave voltages or currents at different points in time, and

wherein determining at least one second travelling wave voltage or current time derivative at the other end of the power transmission line comprises sensing at the other end of the power transmission line a plurality of travelling wave voltages or currents at different points in time.

20. The method according to claim 4 , wherein determining at least one first travelling wave voltage or current time derivative at the one end of the power transmission line comprises sensing at the one end of the power transmission line a plurality of travelling wave voltages or currents at different points in time, and

wherein determining at least one second travelling wave voltage or current time derivative at the other end of the power transmission line comprises sensing at the other end of the power transmission line a plurality of travelling wave voltages or currents at different points in time.