Technology and Experience of Fully Numerical Differential Protection Communicating over Metallic Pilot-Wire

Pilot-wire differential relays have a long track-record of providing effective and reliable protection for overhead lines and underground cables in many parts of the world. The conventional analogue or electromechanical types are, however, limited in that they cannot provide the high performance and advanced features which have become commonplace in modern digital protection systems. The authors have developed a numerical, phase-segregated, current differential relay that applies digital communication techniques over conventional metallic pilot communication circuits, thus allowing utilities to re-use their existing copper links while benefiting from the advantages of numerical relay technology. This paper describes how the problems of limited bandwidth and interference which are imposed by the use of metallic circuits have been overcome, and how a high degree of immunity to communication failures has been achieved. Also his paper describes the result of a field test at a power utility in Asia