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T HE POWER SYSTEM planning engineer bases major decisions concerning service continuity on expected system performance during faults and other abnormal conditions. Frequently the protective relay system places additional limitations on system perform ance. (An illustration of this is a system which is unstable with 30-cycle relaying, but may be stable with 1-cycle relaying.) One of the continuing objectives of the relay engineer is to minimize or completely remove these re laying limitations to system performance.Multiterminal lines, in particular, frequently have re laying limitations which limit the system capability. This is becoming a costly restriction as more utilities feel the economic need for superimposing distribution require ments on main-line transmission circuits. Relaying Imi tations can also seriously restrict the system's perform ance when existing facilities must be integrated into new transmission requirements.An analysis of these limitations frequently reveals that they are caused by compromises in the relay system, inas much as the relays at each terminal cannot completely determine the necessity for tripping within the time per mitted by other system requirements. Further analysis will indicate that by comparing the electrical conditions at two or more terminals of the protected line, it will be possible to determine the necessity for tripping.Thus, one can say that the limitation is one of com munications rather than of relaying. Communication fa cilities such as leased-wire and power-line carrier lack complete reliability and security. Leased wire has a fur ther drawback that fundamental responsibilities are out side the jurisdiction of the power company. Power-line carrier relaying functions depend upon the condition of the power system. In addition, the number of channels which can be practically applied to a line or a system are limited.Ultimately microwave communications will have none of these defects. Furthermore, since no high-voltage power system components are used, station design can be sim pler, and a minimum of circuit outages is required for construction and maintenance. Tests covering a 2-year period and reported in another paper (Field Tests of" Microwave Relaying Equipment by E. W. Downer, AIEE paper 57-209) demonstrate that presently available mi crowave equipment adequately meets the requirements of protective relaying communication channels. Thus, microwave for relaying has been adopted as a working tool.Microwave relaying systems are now in service or un der construction on 25 transmission line terminals at five locations on the Cleveland Electric Illuminating Company system. The initial installation is providing transfer trip for a 100-mva 132/66-kv autotransformer which does not have a high-voltage circuit breaker. The associated 132-kv line, as well as other 2-and 3-terminal lines, is protected with microwave relaying using a "zone comparison" method. With this scheme, the response of directional fault-detecting relays, calibrated to over reach the far terminal by onl...