| CPC H02H 7/045 (2013.01) | 11 Claims |
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1. 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 current change detection unit that detects changes in the phase currents;
a first suppression amount calculation unit that calculates a first suppression amount based on a result of detection by the current change detection unit, the phase currents and the neutral point current,
the first 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
when the changes in the phase currents are detected, calculating a first maximum value or a first addition value as the first suppression amount, the first maximum value being a maximum value of effective values of the subtraction currents in the phases and an effective value of the neutral point current, the first addition value being a value obtained by adding the effective value of the neutral point current to a maximum value of the effective values of the subtraction currents in the phases; and
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.
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