ABSTRACT
The purpose of protection equipment is to minimize the effects of faults on electrical power systems, which unfortunately can never be entirely avoided. The protection device can take the form of a single protection relay performing only a single measurement or - in the case of an especially valuable item of plant such as a large generator - it can be a protection system comprising several separate functions. A protection device that has detected a fault on the item of primary plant it is protecting issues a trip command to the circuit-breaker or breakers, which then isolate it from the rest of the system. A very important link in the protection chain from the point of view of reliability is the auxiliary supply AS. This is generally the station battery which also supplies the circuit-breaker tripping coils CC and the display and recording devices D&R.
