Method for determining location of phase-to earth fault

1. A method for determining a location of a phase-to-earth fault on a three-phase electric line of an electric network, the method comprising:

monitoring current and voltage quantities of the three-phase electric line at a measuring point;

determining an equivalent load distance curve of the electric line representing a voltage drop along the electric line in relation to a distance from the measuring point which voltage drop is scaled by an equivalent load distance of the electric line which indicates a distance of an equivalent load point from the measuring point which equivalent load point equals to a total load of the electric line modelled to be concentrated in a single point of the electric line;

determining, after a fault occurs in the three-phase electric line, a fault distance line which indicates an estimate of the distance of the fault from the measuring point in relation to the equivalent load distance on the basis of values of the monitored current and voltage quantities during the fault and an equation which relates the monitored current and voltage quantities to the fault distance;

determining a distance from the measuring point at which the equivalent load distance curve and the fault distance line intersect when superimposed; and

selecting the determined distance as the distance between the measuring point and the point of fault.

2. The method according to claim 1 , wherein the determination of the equivalent load distance curve of the electric line comprises:

determining a voltage drop curve of the electric line which indicates a voltage drop on the electric line in relation to a distance from the measuring point;

determining the equivalent load distance of the electric line; and

determining the equivalent load distance curve by multiplying the voltage drop curve by the equivalent load distance.

3. The method according to claim 2 , wherein the voltage drop is represented in per unit values.

4. The method according to claim 1 , wherein the determination of the equivalent load distance curve of the electric line comprises:

a) conducting an earth fault test at a known distance from the measuring point;

b) determining an equivalent load distance corresponding to the known fault distance on the basis of values of the monitored current and voltage quantities during the earth fault test and an equation which relates the monitored current and voltage quantities to the equivalent load distance, wherein the pair of fault distance and the determined corresponding equivalent load distance defines a point;

repeating steps a) and b) one or more times such that the distance from the measuring point at which the earth fault test is conducted is varied each time; and

determining the equivalent load distance curve by the set of points obtained in step b).

5. The method according to claim 1 , wherein the determination of the equivalent load distance curve of the electric line comprises:

c) conducting a first earth fault test with a ratio of a fault current and load current having a first value;

d) determining a first test fault distance line which indicates an estimate of a distance of the fault from the measuring point in relation to the equivalent load distance on the basis of values of the monitored current and voltage quantities during the first earth fault test and an equation which relates the monitored current and voltage quantities to the fault distance;

e) conducting a second earth fault test at the same distance from the measuring point as the first earth fault test with a ratio of the fault current and load current having a second value different from the first value;

f) determining a second test fault distance line which indicates an estimate of a distance of the fault from the measuring point in relation to the equivalent load distance on the basis of values of the monitored current and voltage quantities during the second earth fault test and an equation which relates the monitored current and voltage quantities to the fault distance;

g) determining a point at which the first and second test fault distance lines intersect when superimposed;

repeating steps c) to g) one or more times such that the distance from the measuring point at which the first and second earth fault tests are conducted is varied each time; and

determining the equivalent load distance curve by the set of determined intersection points obtained in step g).

6. The method according to claim 1 , wherein the equivalent load distance curve of the electric line and the fault distance line are represented graphically.

7. The method according to claim 6 , wherein the determination of the distance at which the equivalent load distance curve and the fault distance line intersect is performed visually.

8. The method according to claim 1 , wherein the equivalent load distance curve of the electric line and the fault distance line are represented by one or more equations.

9. A computer program product comprising computer program code, wherein the execution of the program code in a computer causes the computer to carry out the steps of the method according to claim 1 .

10. An apparatus for use in localization of a phase-to-earth fault on a three-phase electric line of an electric network, the apparatus comprising:

means for monitoring current and voltage quantities of the three-phase electric line at a measuring point;

means for determining an equivalent load distance curve of the electric line representing a voltage drop along the electric line in relation to a distance from the measuring point, which voltage drop is scaled by an equivalent load distance of the electric line which indicates a distance of an equivalent load point from the measuring point, which equivalent load point equals to a total load of the electric line modelled to be concentrated in a single point of the electric line; and

means for determining, after a fault occurs in the three-phase electric line, a fault distance line which indicates an estimate of the distance of the fault from the measuring point in relation to the equivalent load distance on the basis of values of the monitored current and voltage quantities and an equation which relates the monitored current and voltage quantities to the fault distance.

11. The apparatus as claimed in claim 10 , comprising:

means for determining a fault on the three-phase electric line and a faulted phase of the three-phase electric line.

12. The apparatus as claimed in claim 10 comprising:

means for determining a distance from the measuring point at which the equivalent load distance curve and the fault distance line intersect when superimposed; and

means for selecting the determined distance as the distance between the measuring point- and the point of fault.

13. The apparatus as claimed in claim 10 comprising:

means for outputting the determined equivalent load distance curve of the electric line and the fault distance line.

14. The apparatus as claimed in claim 13 , comprising:

means for outputting the determined equivalent load distance curve of the electric line and the fault distance line in a graphical form.

15. The apparatus as claimed in claim 13 comprising:

means for outputting the determined equivalent load distance curve of the electric line and the fault distance line mutually superimposed.

16. The apparatus as claimed in claim 10 , wherein the means for determining the equivalent load distance curve of the electric line are configured to:

determine a voltage drop curve of the electric line which indicates a voltage drop on the electric line in relation to a distance from the measuring point;

determine the equivalent load distance of the electric line; and

determine the equivalent load distance curve by multiplying the voltage drop curve with the equivalent load distance.

17. The apparatus as claimed in claim 10 , wherein the means for determining the equivalent load distance curve of the electric line are configured to:

determine, in connection with an earth fault test at a known distance from the measuring point, an equivalent load distance corresponding to the known fault distance on the basis of values of the monitored current and voltage quantities during the earth fault test and an equation which relates the monitored current and voltage quantities to the equivalent load distance, wherein the pair of fault distance and the determined corresponding equivalent load distance defines a point;

repeat the previous step one or more times such that the distance from the measuring point at which the earth fault test takes place is different each time; and

determine the equivalent load distance curve by the set of points obtained.

18. The apparatus as claimed in claim 10 , wherein the means for determining the equivalent load distance curve of the electric line are configured to:

1) determine, in connection with a first earth fault test with a ratio of a fault current and load current having a first value, a first test fault distance line which indicates an estimate of a distance of the fault from the measuring point in relation to the equivalent load distance on the basis of values of the monitored current and voltage quantities during the first earth fault test and an equation which relates the monitored current and voltage quantities to the fault distance;

2) determine, in connection with a second earth fault test at the same distance from the measuring point as the first earth fault test with a ratio of the fault current and load current having a second value, which is different from the first value, a second test fault distance line which indicates an estimate of a distance of the fault from the measuring point in relation to the equivalent load distance on the basis of values of the monitored current and voltage quantities during the second earth fault test and an equation which relates the monitored current and voltage quantities to the fault distance;

3) determine a point at which the first and second test fault distance lines intersect when superimposed;

repeat steps 1) to 3) one or more times such that the distance from the measuring point at which the first and second earth fault tests occur is different each time; and

determine the equivalent load distance curve by the set of determined intersection points obtained in step 3).

19. The apparatus as claimed in claim 10 , wherein the apparatus is a protective relay.