Restricted Earth Fault Relay

A restricted earth fault relay for protecting a three-phase transformer including a Y-connected winding,

• phase currents and a neutral point current of the Y-connected winding being defined to have the same polarity in a direction toward a neutral point,
• the restricted earth fault relay comprising:
• a first differential amount calculation unit that calculates a first differential amount based on a zero-phase current and the neutral point current, the zero-phase current being based on the phase currents;
• a first suppression amount calculation unit that calculates a first suppression amount based on a scalar sum of the phase currents and the neutral point current;
• an operation determination unit that outputs a protection signal for protecting the three-phase transformer, when the first differential amount and the first suppression amount are within an operating region;
• and
• an output control unit that locks output of the protection signal by the operation determination unit,
• the output control unit including:
• a current change detection unit that detects changes in the phase currents;
• a second differential amount calculation unit that calculates an absolute value of an addition current as a second differential amount, the addition current being obtained by adding the phase currents and the neutral point current;
• and
• a second suppression amount calculation unit that calculates a second suppression amount based on the phase currents and the neutral point current,
• the second suppression amount calculation unit
• calculating subtraction currents for the respective phase currents, the subtraction currents being obtained by subtracting the phase currents in a cycle earlier than a current cycle from the phase currents in the current cycle, and
• calculating, as the second suppression amount, a maximum value of absolute values of the subtraction currents in the phases and an absolute value of the neutral point current,
• the output control unit further including a signal output unit that outputs a lock signal for locking the output of the protection signal, when the changes in the phase currents are detected and when the second differential amount and the second suppression amount are not within a second operating region.