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

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.

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.