Transmission System Relay Protection-III

A paper was presented in 1919, followed by Part II in 1921, giving a resumé of the relay protective systems up to those dates. The various important developments since 1921 warrant a third paper on this subject. There has been a general realization of the importance of shortening the time for removing a fault, and this has led to the production of high-speed circuit breakers combined with very fast relays, the total time of clearing trouble being reduced to from two to eight cycles. There is a tendency towards types of relays and schemes which are inherently selective in operation, such as the ``distance´´ relays, balanced or differential schemes, pilot wire relays, etc. Due to the increased reliability of service and to the relative ease of obtaining selectivity and high speed of the relay protective system the growth of the use of parallel lines (usually in pairs) has been rapid. Back-up protection against through short circuits or failures of relays or breakers at the succeeding stations is growing in use and the necessary relays are being added unless the scheme adopted inherently has this covering protection. Due to its fundamental advantages of the detection of a fault in its incipient stage thereby reducing the voltage disturbance to the system, minimizing the damage at the fault, and reducing the stress on the circuit breakers ground protection is approaching the importance of becoming the first line of defense.