Patent ID: 7196884
Filing Date: 2007-03-27
Classification: H02H

Abstract:
1. A protective relay configured to prevent generation of a trip signal by a power system element of a three-phase power system when a current transformer connection is lost, the current transformer connection provided by one of a plurality of current transformers coupling the protective relay to the power system element and providing a corresponding plurality of secondary current waveforms of the three-phase power system to the protective relay, the protective relay comprising: a signal logic circuit configured to provide a first binary signal in response to receipt of a plurality of like-phase digitized current sample streams derived from the plurality of secondary current waveforms, a first value of the first binary signal indicating an occurrence of a short circuit in the power system element; a lost current transformer (CT) detection logic circuit configured to provide a second binary signal in response to at least one comparison of at least one calculated current value of the plurality of like-phase digitized current sample streams to at least one threshold value, a first value of the second binary signal indicating the current transformer connection loss and a second value of the second binary signal indicating lack of impairment of the current transformer connection; and a trip logic circuit coupled to the signal logic circuit and the lost CT detection logic circuit, the trip logic circuit configured to generate the trip signal when the first binary signal has the first value and the second binary signal has a the second value, and to prevent generation of the trip signal when the second binary signal has the first value.