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 4.4 APPLIED PROTECTIVE RELAYING. This section provides a general discussion of
applied protective relaying and describes the functional zones of protection of a typical power
system.
4.4.1 Zones of Protection. The general scheme of protective relaying divides the power
system into functional zones that can be protected from damage and isolated when faulted to
minimize any service interruption. The power system is divided into protective zones for:
(a)
Generators.
(b)
(c)
Buses.
(d)
Transmission and Distribution Circuits.
(e)
Motors.
A typical power system and its zones of protection are shown in Figure 4-4. Each zone is
associated with a relay or group of protective relays that sense operating conditions within the
zone. When conditions (voltage, current, frequency, temperature, etc.) deviate from the
protective set points, indicating a fault or potential damage to the equipment in the zone, the
relays actuate circuit breakers to isolate only the faulted or endangered zone (most loads continue
to receive electrical power). Protection in adjacent zones may be overlapped to avoid the
possibility of unprotected areas.
4.4.2 Generator Protection. Abnormal conditions that may occur with rotating equipment
include the following:
(a) Faults in the Windings.
(b) Overload.
(c) Overspeed.
(d) Loss of Excitation.
(e) Motoring.
4.4.2.1 Faults in the Windings. Internal faults generally develop as a ground fault in one
of the phase windings and may occasionally involve more than one phase. Differential protection
is the most effective scheme against multiple-phase faults. The currents in each phase, on each
side of a generator phase winding, are compared in a differential circuit. Any difference in
current indicates an unintentional current path to ground and is used to operate a relay.
4.4.2.2 Overload. Most large generators are equipped with resistance temperature
detectors that are connected to thermal relays. The relays operate when the temperature of the
machine exceeds an established value. Generator feeder overcurrent relays may serve as backup
protection for machine overloads.
4-17
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