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 transformers, however, have a small error in ratio and phase angle between the primary and
secondary currents, depending upon variations in manufacture, the magnitude of current, and the
connected secondary burden. These errors will cause a differential current to flow even when the
primary currents are balanced. The error current may become proportionately larger during fault
conditions, especially when there is a direct current component present in the fault current. The
differential relays, of course, must not operate for the maximum error current which can flow for
a fault condition external to the protected zone. To provide this feature, the percentage
differential relay has special restraint windings to prevent improper operation due to the error
current on heavy through-fault conditions while providing very sensitive detection of
low-magnitude faults inside the differentials protected zone.
4.3.5 Current Phase-Balance Relays. In some cases phase-balance current relays can provide
an acceptable substitute for differential protection. A negative-sequence current relay is a more
sensitive device that also detects unbalanced phase currents. In applying these relays it is
assumed that under normal conditions the phase currents in the three-phase supply to the
equipment and the corresponding output signals from each phase current transformer are
balanced. Should the fault occur in the motor or generator involving one or two phases or should
an open circuit develop in any of the phases, the currents will become unbalanced and the relay
will operate. In addition to protecting against winding faults, the phase-balance current relay
affords protection against damage to the motor or generator due to single-phase operation.
4.3.6 Ground-Fault Relays. Ground-fault relays may be used to provide improved protection
when the power system is intentionally grounded and ground-fault current can flow through the
conductors. This is often an overcurrent relay connected to sense the resultant current (vector
summation of currents through all conductors of a feeder) or the current flow through the
grounded conductor. The ground relay can be set to pick up at a much lower current value than
the phase relays, because the vector summation of the currents flowing through the conductors of
a feeder is normally zero. Overcurrent relays used for ground-fault protection are generally the
same as those used for phase-fault protection, except that a more sensitive range of minimum
operating current values is possible since they see only fault currents and not load currents.
Relays with inverse-, very inverse-, and extremely inverse-time characteristics, as well as
instantaneous relays, are all applicable as ground-fault relays.
4.3.7 Synchronism-Check and Synchronizing Relays.
4.3.7.1 Synchronism-Check Relays. The synchronism-check relay is used to verify that
two alternating current sources are within the desired limits of frequency, voltage, and phase
angle to operate in parallel. Synchronism-check relays should be employed for switching
applications on systems known to be normally paralleled at some other location. When used
for these applications, synchronous relays ensure the two sources have not become electrically
separated or displaced by an unacceptable phase angle.
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