PROTECTIVE RELAY COORDINATION 319Figure 12.7 Effect of a large load variation on the response of overcurrent relays at the generators. The
diagram shows the response when a large induction motor is started direct-on-line. Four different standing loads
are shown.
for longer than several seconds, see Reference 2. The relay itself should function in approximately
15 milliseconds, and have a sensitivity of about 2 to 5% of the stator rated current. The nominal
setting ranges are between 10 to 40% for 1amp relays and 20 to 80% for 5 amp relays. The low %
settings are usually chosen initially and increased if subsequently found to be too sensitive. This type
of protection is generally applied to high voltage machines above about 2 MW.
Electromagnetic differential current relays are fitted with restraining or biasing, coils in each
secondary circuit of their current transformer. These restraining coils stabilise the relay when large
‘through’ fault currents are present in the windings i.e. feeding an external fault. Stabilisation is
necessary because of mismatch errors and saturation effects in the current transformers, which would
otherwise be large enough to activate the sensitive operating coil.
12.2.4 Field failure relay
If the field is lost in a generator it will attempt to generate power at a low leading power factor and
with a large rotor angle. In most loading situations the generator rotor angle will increase to a critical
value where unstable power transfer will occur, and the generator will no longer be able to remain in
synchronism with the supply. When synchronism is lost the stator current varies in magnitude over
a wide range and at the slip frequency. If this is allowed to continue then it is possible that damage
will result in the stator and rotor windings, and the disturbance in voltage at the connected power
network will be large enough to cause tripping and overcurrents in loads.