Travelling wave protection of a transmission line

1. A method for travelling wave protection of a transmission line based on information of both voltage and current polarities, the method comprising:

acquiring information of both voltage and current polarities from two ends of a transmission line;

comparing the voltage and current polarities of a first end of the two ends to the voltage and current polarities of a second end of the two ends; and

making a trip decision based on the comparison, wherein making the trip decision comprises blocking a trip if the voltage polarities from the two ends differ and the current polarities from the two ends differ, and if any of the voltage polarities are the same as the current polarities.

2. The method according to claim 1 , wherein the trip decision relates to whether or not to cause a circuit breaker to trip.

3. The method according to claim 1 , wherein making the trip decision comprises:

allowing a trip only if the voltage polarities from the two ends are the same and the current polarities from the two ends are the same and if the voltage polarities differ from the current polarities.

4. The method according to claim 1 , wherein the transmission line is a multi-terminal transmission line having more than two ends, and wherein making the trip decision comprises:

allowing a trip only if the voltage polarities from all ends are the same and the current polarities from all ends are the same and if the voltage polarities differ from the current polarities.

5. The method according to claim 1 , wherein the information of both voltage and current polarities from the two ends of the transmission line is exchanged between the two ends of the transmission line.

6. The method according to claim 1 , where the comparison is based on at least one of phase quantity, phase-phase quantity, differential mode, common mode, zero mode.

7. The method according to claim 1 , wherein the voltage and current polarities are measured at both the two ends of the transmission line prior to the information being acquired.

8. The method according to claim 7 , wherein voltage and current polarities are measured at both the ends of the transmission line within a certain time interval.

9. The method according to claim 1 , wherein the transmission line is part of a power distribution system.

10. The method according to claim 1 , wherein the transmission line is an alternating current, AC, transmission line.

11. The method according to claim 1 , wherein the transmission line is a direct current, DC, transmission line.

12. The method according to claim 1 , wherein the information of voltage and current polarities is acquired from protective relays, one protective relay being placed at each end of the transmission line.

13. An arrangement for travelling wave protection of a transmission line based on information of both voltage and current polarities, the arrangement comprising a processing unit, the processing unit being configured to cause the arrangement to:

acquire information of both voltage and current polarities from two ends of a transmission line;

compare the voltage and current polarities of a first end of the two ends to the voltage and current polarities of a second end of the two ends; and

make a trip decision based on the comparison, wherein making the trip decision comprises blocking a trip if the voltage polarities from the two ends differ and the current polarities from the two ends differ, and if any of the voltage polarities are the same as the current polarities.

14. The arrangement according to claim 13 , wherein the arrangement comprises at least one protective relay placed at one of the ends of the transmission line.

15. The arrangement according to claim 13 , wherein the arrangement comprises two protective relays, one protective relay being placed at each end of the transmission line.

16. The arrangement according to claim 14 , wherein the each protective relay is part of an Intelligent Electronic Device, IED.

17. The arrangement according to claim 15 , wherein the each protective relay is part of an Intelligent Electronic Device, IED.