Abridgment of the application of relays for the protection of power system interconnections

This paper is a compilation of many of the new methods for relay protection required by superpower interconnections. The ideas have been obtained from various sources and represent good present day practise. The general requirements for such interconnections are mentioned with particular reference to protection against phase-to-phase short circuits in those cases where the short-circuit current, under certain system conditions, is less then the full-load current under other conditions. The clearing of accidental grounds is discussed and a new study of residual currents on systems of different types is given. This study indicates the usefulness of certain inverse time limit relays on systems having dead grounded power transformer neutrals at all switching stations. Bus protection and “back-up” methods are described. Some of the new relays which have been developed, or improvements which have been made to older types in order that the new demands may be met, are described; and there is illustrated a typical relay installation similar to that now being placed in service on a 220-kv. interconnection.