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 circutit prevents the flow of synchronizing power, it is the most serious type, and must be cleared in from 6 to 10 cycles. This demands the use of relays which will operate instantaneously. Recent suggestions have been numerous and these are discussed. This paper covers relays operating at normal frequency and those which have been operated or suggested for operation at 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), high speed relays may still 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 they 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.