A sequence relay for network protectors

Protectors on many of today's low-voltage distribution networks operate needlessly several thousands of times each year due to current reversals caused by regenerative loads and difference in loading at various points in the network. Such reversals can be permitted without tripping the protectors and yet maintain a desirable selective tripping characteristic by use of a phase sequence scheme of relaying devised by Messrs. H.R. Searing and R.E. Powers ("Application of Phase Sequence Principles"). It employs the fact that practically all power reversals, other than fault conditions, are inherently balanced and that operation of a directional relay can be easily initiated by simple switching operations which draw negative phase sequence currents from the network. The sequence relay which has been designed in accordance with their theory, and described in this paper, performs the dual function of phasing and tripping. Its operating characteristics are suited to application on low-voltage distribution systems in general.