Differential protective relay for electrical buses with improved immunity to saturation of current transformers

A microprocessor-based protective relay, comprising:

• a first functional block configured to receive a plurality of input current signals, the first functional block performing a ratio matching of the plurality of input current signals;
• a second functional block configured to receive the plurality of ratio-matched input current signals and to apply a Fourier algorithm to generate a corresponding plurality of phasor currents;
• a summer configured to receive the corresponding plurality of phasor currents and to generate a differential phasor current;
• a third functional block configured to receive the corresponding plurality of phasor currents and to determine a restraining current from a maximum one of the phasor currents;
• one or more combinatorial blocks configured to receive the restraining current and the differential phasor current and to determine whether the combination falls in a first range of combination values or a second range of combination values;
• a directional element configured to receive the corresponding plurality of phasor currents, the differential phasor current, and the restraining current, the directional element determining a directional value indicative of the occurrence of a fault on the power system;
• a saturation detector configured to receive the plurality of ratio-matched input current signals, the differential phasor current, and the restraining current, the saturation detector determining a saturation condition of one or more current transformers associated with the power system;
• logic configured to receive the outputs of the combinatorial blocks, the directional element, and the saturation detector, the logic generating a trip control signal:
• if the combination falls in the first range, the combination indicates the occurrence of a fault, and the directional value indicates the occurrence of a fault;
• if the combination falls in the second range, no positive saturation condition is determined, and the combination indicates the occurrence of a fault;
• or
• if the combination falls in the second range, a positive saturation condition is determined, the combination indicates the occurrence of a fault, and the directional value indicates the occurrence of a fault.

A storage medium encoded with machine-readable instructions, comprising:

• one or more instructions for receiving a plurality of input current signals from a plurality of current transformers associated with the power system;
• one or more instructions for performing a ratio matching of the input current signals;
• one or more instructions for generating a corresponding plurality of phasor currents from the plurality of ratio-matched current signals;
• one or more instructions for generating a differential phasor current from the corresponding plurality of phasor currents;
• one or more instructions for generating a restraining current from a maximum one of the corresponding plurality of phasor currents;
• one or more instructions for determining, for the combination of the differential phasor current and the restraining current, whether the combination falls in a first range of combination values or a second range of combination values;
• one or more instructions for determining a directional value based on the corresponding plurality of phasor currents, the differential phasor current, and the restraining current;
• one or more instructions for determining a saturation condition of one or more of the plurality of current transformers based on the ratio-matched input currents, the differential phasor current, and the restraining current;
• one or more instructions for initiating a trip command if the combination falls in the first range, the combination indicates the occurrence of a fault, and the directional value indicates the occurrence of a fault;
• one or more instructions for initiating a trip command if the combination falls in the second range, no saturation is determined, and the combination indicates the occurrence of a fault;
• and
• one or more instructions for initiating a trip command if the combination falls in the second range, saturation is determined to have occurred, the combination indicates the occurrence of a fault, and the directional value indicates the occurrence of a fault.