Phase-comparison carrier-current relaying system

SINCE ITS INTRODUCTION 6 years ago, phase-comparison relaying has enjoyed wide acceptance. As originally conceived, its field of application was to be largely for medium-length, 2-terminal overhead lines. The operating experience has been so good that the demand has grown for extending its field of application to longer lines, to 3-terminal lines, and to certain cables. Also, its inherent immunity to the effects of mutual inductance makes its use attractive for ground-fault protection even though it may not be applicable for phase-fault protection. In such cases, there was a need to combine the virtues of phase- and directional-comparison relaying. Modifications now have been made to phase-comparison relaying equipment that will provide these desired extensions of its field of application. See Figure 1.