Linear couplers for bus protection

THE linear coupler transformer is a constant mutual reactance connecting the primary circuit to the relay. It introduces a new principle into the protective relaying art, a principle^∗ that is fundamentally sound and that eliminates completely, at its source, the most troublesome problem that has been standing in the way of simple high-speed bus protection. That problem is saturation of the current transformers by the d-c transient current that flows for a number of cycles after the occurrence of a fault. Its solution consists of dispensing with the iron, a solution that appears so obvious on the face of it that it is fair to inquire why it was not adopted years ago. This paper might stop right here were it not for the answer to the last question.