ABSTRACT
Phase-comparison systems are applicable to three-terminal line protection, but care must be exercised, particularly with an on–off carrier, in choosing the settings properly. All of the directional comparison relaying systems utilize overreaching distance units. The reach settings must take into consideration the infeed effect. Various types of power system connections exist, some of which may either alleviate or make more difficult the relaying problem. Some of the obvious things that are influential are the following: the transformer connection, the transformer grounding, generation at the tap, load at the tap and outfeed. If the transformer is treated as part of the protected zone for the line relays, and it is connected delta–wye, no ground carrier start relay is required at that location. Of all the systems discussed, only current differential and direct underreaching transfer–trip may be used when an outfeed condition may exist for an internal fault.
