Abridgment of high-speed protective relays

During the past year or so, studies of stability have been made to determine methods of preventing loss of synchronism upon the occurrence of faults. Of the several methods found, the most obvious and effective is the high-speed isolation of the faulty section of the line; and this of course means high-speed relays and high-speed breakers. Investigation so far has indicated that the time required depends upon the type of fault. Since a three-phase short circuit prevents the flow of synchronizing power, it is the most serious type and must be cleared in from six to ten cycles. This demands the use of relays which will operate “instantaneously.” Recent suggestions have been numerous, and these are discussed, this discussion covering relays operating at normal frequency and those which have been operated or suggested for higher superimposed frequencies. While there is a number of difficulties attendant on the design of high-speed relays — these troubles depending on the type and construction of the relay — still high-speed relays may be made to operate on any of the present well-known principles, such as impedance principle, current balance principle, etc. They may employ either a mechanical structure or may make use of thermionic or gas-filled tubes. Attention is given to a mechanical relay of the impedance type operating with a speed of one cycle or less. Some discussion is also given of the reactance type relay with mention of its limitations, particularly that of the extra time required for its initialing element to operate. The effect of resistance at the point of fault (arc resistance) is discussed and the conclusion drawn that, for extremely high-speed operation, it does not interfere with satisfactory relay performance. This is because of the time required for the arc resistance to increase to an appreciable value.