Instantaneous overcurrent element for heavily saturated current in a power system

摘要

An instantaneous over-current element for use in a microprocessor based protective relay for a power system includes a digital filter circuit 20 and a peak detector circuit 22 which are both responsive to a secondary current waveform from a current transformer that itself is responsive to the current waveform from the power line, the filter determining the magnitude of the current waveform and the peak detector determining the peak magnitude of the current waveform. A circuit 24 determines the distortion of the output from the current transformer, and element 60 compares the distortion against a predetermined standard. In response to the output of the comparison element the switch 29 either connects the output of the filter circuit 20 to a fault determination circuit 82 when the distortion is less than the predetermined standard or connects the output of the peak detector 22 to the fault determination circuit when the distortion exceeds the predetermined standard. This will ensure the reliability of the operation of the over-current element, and in particular that it does not under-reach or even fail completely in the circumstance that the current transformer is saturated during the fault.