Linear couplers, field tests and experience at York and Middletown, Pa.

THE system of bus protection using linear couplers, or air-core mutual reactors, in place of current transformers was described before the Institute in January 1942.¹ By special design the mutual reactors were made astatic or insensitive to outside fields. Through the use of air core the troublesome problem of saturation of iron by the d-c transient component of short-circuit current was eliminated. At the same time adequate energy was derived for decisive relay operation. The coupler secondaries were connected in a series loop including the relay to form a bus protective system.