I. Introduction
In the early days of electricity generation and transmission, system protection consisted of men in bowler hats watching ammeters. If the current went too high, the operator pulled a large knife switch and fanned the arc out with his hat. Automatic relays using current coils and magnetic disks followed. Voltage was added as an operating quantity increasing the options for protection. Combining voltage and current gave us distance relays of all forms. Frequency was recognized as a critical quantity following the 1965 blackout in the northeastern US and eastern Canada.