Method and apparatus for detecting a phase-to-earth fault

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

monitoring a zero sequence current on the three-phase electric line and a zero sequence voltage in the electric network;

detecting an earth fault in the electric network based on a value of the zero sequence voltage;

determining a difference between a first zero sequence current monitored before the earth fault and a second zero sequence current monitored during the earth fault;

determining a difference between a first zero sequence voltage value monitored before the earth fault and a second zero sequence voltage value monitored during the earth fault;

determining a neutral admittance, based on a ratio between the difference between the first and second zero sequence currents and the difference between the first and second zero sequence voltages; and

comparing the determined neutral admittance, to a predetermined operation characteristic to detect a phase-to-earth fault on the three-phase electric line, wherein the predetermined operation characteristic, when presented on an admittance plane, defines a closed area such that the center of the closed area is offset from the origin of the admittance plane towards a negative susceptance direction and/or towards a negative conductance direction.

2. The method of claim 1 , wherein the comparing the determined neutral admittance, to a predetermined operation characteristic to detect a phase-to-earth fault on the three-phase electric line comprises:

determining whether the neutral admittance is inside or outside said closed area defined by the predetermined operation characteristic on the admittance plane; and

detecting a phase-to-earth fault on the three-phase electric line when the determined neutral admittance is determined to be outside said closed area defined by the predetermined operation characteristic on the admittance plane.

3. The method of claim 1 , wherein the closed area is defined by a circle or an ellipse having a center point that is offset from the origin of the admittance plane towards the negative susceptance direction and/or towards the negative conductance direction.

4. The method of claim 1 , wherein the closed area is defined by a circle whose center point is offset from the origin of the admittance plane towards the negative susceptance direction and/or towards the negative conductance direction such that a segment of the circle is excluded from the closed area, which segment is defined by a line defined by a predetermined conductance value.

5. The method of claim 1 , wherein the closed area is a polygon having three or more sides defined by three or more lines respectively.

6. The method of claim 1 , comprising:

updating the predetermined operation characteristic when the switching state of the three-phase electric line changes or at predetermined intervals.

7. A non-transitory computer readable medium 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 .

8. An apparatus for detecting a phase-to-earth fault on a three-phase electric line of an electric network, the apparatus comprising:

means for monitoring a zero sequence current on the three-phase electric line and a zero sequence voltage in the electric network;

means for detecting an earth fault in the electric network based on the zero sequence voltage value;

means for determining a difference between a first zero sequence current before the earth fault and a second zero sequence current during the earth fault;

means for determining a difference between a first zero sequence voltage before the earth fault and a second zero sequence voltage during the earth fault;

means for determining a neutral admittance, based on a ratio of the difference between the first and second zero sequence currents and the difference between the first and second zero sequence voltages; and

means for comparing the determined neutral admittance to a predetermined operation characteristic to detect a phase-to-earth fault on the three-phase electric line, wherein the predetermined operation characteristic, when presented on an admittance plane, defines a closed area such that a center of the closed area is offset from an origin of the admittance plane towards a negative susceptance direction and/or towards a negative conductance direction.

9. The apparatus of claim 8 , wherein the means for comparing the determined neutral admittance, to a predetermined operation characteristic to detect a phase-to-earth fault on the three-phase electric line comprises:

means for determining whether the neutral admittance is inside or outside said closed area defined by the predetermined operation characteristic on the admittance plane; and

means for detecting a phase-to-earth fault on the three-phase electric line when the determined neutral admittance is determined to be outside said closed area defined by the predetermined operation characteristic on the admittance plane.

10. The apparatus of claim 8 , wherein the closed area is defined by a circle or an ellipse having a center point that is offset from the origin of the admittance plane towards the negative susceptance direction and/or towards the negative conductance direction.

11. The apparatus of claim 8 , wherein the closed area is defined by a circle whose center point is offset from the origin of the admittance plane towards the negative susceptance direction and/or towards the negative conductance direction such that a segment of the circle is excluded from the closed area which segment is defined by a line defined by a predetermined conductance value.

12. The apparatus of claim 8 , wherein the closed area is a polygon having three or more sides defined by three or more lines respectively.

13. The apparatus of claim 8 , the apparatus comprising means for updating the predetermined operation characteristic when the switching state of the three-phase electric line changes or at predetermined intervals.

14. The apparatus of claim 8 , the apparatus comprising a protective relay.

15. An apparatus for detecting a phase-to-earth fault on a three-phase electric line of an electric network, the apparatus comprising:

a monitoring unit configured to monitor a zero sequence current on the three-phase electric line and a zero sequence voltage in the electric network;

a detection unit configured to detect an earth fault in the electric network based on the zero sequence voltage value;

a determination unit configured to:

determine a difference between a first zero sequence current before the earth fault and a second zero sequence current during the earth fault,

determine a difference between a first zero sequence voltage before the earth fault and a second zero sequence voltage during the earth fault, and

determine a neutral admittance, based on a ratio of the difference between the first and second zero sequence currents and the difference between the first and second zero sequence voltages; and

a comparison unit configured to compare the determined neutral admittance, to a predetermined operation characteristic to detect a phase-to-earth fault on the three-phase electric line, wherein the predetermined operation characteristic, when presented on an admittance plane, defines a closed area such that a center of the closed area is offset from an origin of the admittance plane towards a negative susceptance direction and/or towards a negative conductance direction.