Discrimination between main and backup protection is provided by the use of relays

Discrimination between main and backup protection is provided by the use of relays

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Protection Zones

The general philosophy for the use of relays is to divide the system into separate zones, which can be individually protected and disconnected on the occurrence of a fault, in order to permit the rest of the system to continue in service wherever possible.

In general, a power system can be divided into protection zones – generators, transformers, groups of generator transformers, motors, busbars, and lines.  It should be noted that the zones overlap at some points indicating that, if a fault occurs in these overlap areas, more than one set of protection relays should operate. The overlap is obtained by connecting the protection relays to the appropriate current transformers as illustrated in Figure.

Primary protection

Primary protection should operate every time an element detects a fault on the power system. The protection element covers one or more components of the power system, such as electrical machines, lines, and busbars. It is possible for a power system component to have various primary protection devices. However, this does not imply that they all have to operate for the same fault, and it should be noted that the primary protection for one item of system equipment might not necessarily be installed at the same location as the system equipment; in some cases, it can be sited in an adjacent substation.

Backup protection

Back-up protection is installed to operate when, for whatever reason, the primary protection does not work. To achieve this, the backup protection relay has a sensing element that may or may not be similar to the primary protection, but which also includes a time-delay facility to slow down the operation of the relay so as to allow time for the primary protection to operate first. One relay can provide backup protection simultaneously to different pieces of system equipment. Equally the same equipment can have a number of different backup protection relays and it is quite common for a relay to act as the primary protection for one piece of equipment and as backup for another.

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