Circulating-current pilot-wire differential protection system

A circulating-current a.c. pilot-wire differential protection scheme is described in which compensation is provided for the phase shift of the fundamental-frequency signals transmitted over the pilot-wire circuit. Alternative relay characteristics are considered, with particular reference to the desirability of maintaining simultaneous tripping of circuit breakers at both ends of the protected section under all internal-fault conditions. Factors influencing the choice of preferred relay-tripping characteristics are examined, together with a means of modifying a practical pilot circuit to enable the preferred tripping characteristic to be obtained.