Current differential relay apparatus

A current differential relay apparatus, comprising a first relay and a third relay that perform a relay operation using a first current on a first side of equipment to be protected and a second current on a second side of the equipment to be protected, wherein

• the first relay:
• calculates a first differential current and a first suppression current, using the first current and the second current which have been through a first filtering process for extracting a fundamental wave component;
• and
• performs a ratio differential relay operation based on the first differential current and the first suppression current, and
• the third relay:
• performs an add operation of the first current and the second current which have been through a second filtering process for removing a DC component, and calculates a maximum of absolute values of results of add operations in a reference period as a third differential current;
• performs an add operation of a maximum of absolute values of the first current after the second filtering process in the reference period and a maximum of absolute values of the second current after the second filtering process in the reference period to calculate a result of the add operation as a third suppression current;
• and
• performs a ratio differential relay operation based on the third differential current and the third suppression current,
• the current differential relay apparatus, further comprising:
• a detector that detects a harmonic component superimposed on a current flow through the equipment to be protected, based on the first current and the second current after the second filtering process;
• and
• an output controller that outputs an operation signal for protecting the equipment to be protected, based on a result of operation performed by the first relay, a result of detection by the detector, and a result of operation performed by the third relay.