Protective Relaying, 4th Edition

Chapter 6

Protection Fundamentals and Basic Design Principles

6.1 Introduction

The best protection technique now and for more than 50 years is that known as differential protection. Here the electrical quantities entering and leaving the protected zone or area are compared by current transformers (CTs). If the net between all the various circuits is zero, it is assumed that no fault or intolerable problem exists. However, if the net is not zero, an internal problem exists and the difference current can operate the associated relays. In general, internal faults provide significant operating current, even for fairly light faults.

Differential protection is universally applicable to all parts of the power system: generators, motors, buses, transformers, ...