Protection of pilot-wire circuits

THIS paper reviews the problem of protecting pilot-wire circuits, briefly discusses the sources of disturbing voltages, describes two types of neutralizing transformers, and illustrates how they are used to distribute the disturbing voltage most favorably throughout the pilot-wire circuit. The recent increase in use of high-speed pilot-wire relays for the protection of important power circuits has accentuated the need of better understanding of the pilot-wire protection problem. Therefore, a review of this general subject and a description of a new type of neutralizing transformer with mathematical formulas to determine its effectiveness, is considered timely.