TABLE OF CONTENTS Switchgear, cables, transformers, overhead lines and other electrical equipment require protection devices in order to safeguard them during fault conditions. In addition, the rapid clearance of faults prevents touch and step potentials on equipment from reaching levels which could endanger life. The function of protection is not to prevent the fault itself but to take immediate action upon fault recognition. Protection devices detect, locate and initiate the removal of the faulted equipment from the power network in the minimum desirable time. It is necessary for all protection relays, except those directly associated with the fault clearance, to remain inoperative during transient phenomena which may arise during faults, switching surges or other disturbances to the network. Protection schemes are designed on the basis of:
safety,
reliability,
selectivity.
The requirements for CTs and VTs associated with relay protection are described in Chapter 5 and fuse and MCB protection devices in Chapter 11. Standard reference texts are provided in the references section at the end of this chapter. They very adequately cover protection theory and particular relays in UK [1 ], US [2 ] and general [3 ] practice. Graphical symbols for switchgear, control gear and protective devices are given in IEC 60617-7. This chapter therefore concentrates on the principal relay protection schemes and typical applications with practical calculation and computer assisted examples.
While the earliest relays were electromechanical in construction, technological developments led to the introduction of solid state or static...
