PROTECTIVE RELAY COORDINATION 313- The large motors have long run-up times e.g. 10 to 20 seconds for high speed centrifugal gas
compressors.
In this section the protective relays and their functions are described in a sequence that pertains
to the protected equipment in a power system. The sequence begins with generators because these
are usually the main source of power in the network. At the end of the sequence are the smaller
power-rated equipment. The sequence is:
- Main generators.
- Emergency diesel generators.
- Feeder transformers.
- Feeder cables.
- Feeder overhead lines.
- Switchboard interconnectors.
- Switchboard busbar section circuit breakers.
- Large motors.
- Small motors.
- Static loads.
12.1.1 Relay notation
There are two generally accepted methods of describing and notating relays and protective devices.
The first and earliest system to be rationalised is that developed by the IEEE in its standard C37.2 in
1970, which has also been revised in 1991. Appendix C gives a comprehensive listing and description
of each function. This method uses a simple numbering system of up to two digits, together with one
or two suffixed letters, to identify the function of the device. Its simplicity is an attractive advantage
and most relay and switchgear manufacturers are very familiar with the numbers.
The second system is based on the IEC60255 and its references. The symbols used are compre-
hensive but tend to suffer from poor clarity when photoreduced, as is often required with engineering
drawings. The first method is regularly used in the oil industry and is preferred herein.
Appendix C herein gives the IEEE device numbers that are most commonly used, together
with their descriptions that are typically used in the oil industry.
12.2 Generator Protection
12.2.1 Main generators
For generators in the range of approximately 2 MW to 50 MW the following protection relays should
be provided:-
- Overcurrent (51 V).
- Differential stator current (87).
- Field failure (40).
- Field winding earth fault (58).