The Shape of a Zone: Distance Relaying Technology from Electromechanical to Digital

The concept of comparing two quantities for protective relaying, as in distance protection, was introduced in the 1920's to address the inadequacies of single-quantity protective schemes such as overcurrent. In electromechanical and solid-state relays this comparison was achieved by analog methods, which are based on indirect or analogous computations. Microprocessor relays fundamentally changed the computation method to digital, while employing some of the major principles that were developed for the analog distance protection. This paper provides a historical, yet technically detailed, review of the evolution of the distance relaying technology, and examines the implementation methods of this protection technique.