Introduction

The Rogowski coil (RC) current sensors presented here operate on the same principle as coils that were first introduced in 1912 for magnetic field measurements.  At that time, the coils could not be used for relay protection because their output power was not sufficient to drive electro-mechanical relays.  However, with today's microprocessor-based equipment, RC current sensors are suitable for such applications.  Current transformers have traditionally been used for protection and measurement applications, in part because of their ability to produce the high power output needed by electromechanical equipment.  Microprocessor-based equipment makes high power output unnecessary.  This article presents novel solutions for differential protection of power transformers, busbars, generators, and large motors.  The initial projects included differential protection of EAF transformers, which are the first such applications in the U.S. and most likely in the world.