A new one-cycle directional overcurrent relay

This paper describes a new one-cycle directional overcurrent relay which is not limited in its application by lack of contact co-ordination. That contact co-ordination is important is indicated by the many incorrect relay operations which have been traced to the common practice of adding instantaneous attachment to directional overcurrent relays. For years it has been known that directional control is not sufficient for instantaneous directional over-current relays. This paper describes a new application of “memory action” which for the first time complements directional control to give always correct operation. The new idea is to use “memory action” to obtain a time delay in the operation of the overcurrent unit in the order of one-half cycle. Of course “memory action” is also used in the directional element to retain a high torque level even if the fault causes drastic voltage reduction. Both elements are of the rugged induction cylinder.