Static instantaneous overcurrent relay with low transient overreach

Abstract

A static instantaneous overcurrent relay with reduced transient overreach is provided for use in protecting an a-c power line from overcurrents. The relay operates on an input signal representative of the magnitude of the current in the a-c power line and develops a relay output signal when the magnitude of the current reaches a predetermined magnitude. In a preferred embodiment, the relay includes a level detector circuit for receiving the input signal which is in the form of a pulsating waveform having the same frequency as the a-c power line. The level detector circuit develops a discrete level detector output signal in response to each pulse of the waveform which attains a magnitude representative of the predetermined magnitude. Output circuit means is coupled to receive the discrete level detector output signals and includes temporary inhibit means for inhibiting the development of the relay output signal until the output circuit has received two of the discrete level detector signals. In one form of the invention, the temporary inhibit means includes delay circuit means for receiving the discrete level detector output signals and developing a delayed signal. AND circuit means is then provided to receive the discrete level detector output signals and the delayed signal, resulting in a relay output signal after two level detector output signals are present. In one form of the present invention, the relay has a transient overreach percentage of less than 8% for power line angles up to 75.degree.. The relay of the present invention may be employed in connection with a single phase input signal as well as with a three phase input signal.