188.8.131.52 Synchronizing Relays. The synchronizing relay monitors two separate systems
that are to be paralleled. It automatically initiates switching when the phase-angle displacement,
frequency difference, voltage deviation, and the operating time of the switching equipment (to
accomplish interconnection) are acceptable.
4.3.8 Pilot-Wire Relays. The relaying of tie lines, either between the industrial system and
the utility system or between major load centers within the industrial system, often present a
special problem. Such lines must be capable of carrying maximum emergency load currents for
any length of time and they must be easily and quickly removed from service when a fault
occurs. A type of differential relaying called pilot-wire relaying responds very quickly to faults
in the protected line. It clears the fault promptly and minimizes line damage and disturbance to
the system, yet is normally unresponsive to load currents and to currents flowing to faults in other
lines and equipment. The various types of pilot-wire relaying schemes all operate on the
principle of comparing the conditions at the terminals of the protected line. The relays are
connected to operate if the comparison indicates a fault in the line. The information necessary
for this comparison is transmitted between terminals over a pilot-wire circuit.
4.3.9 Voltage Relays. Voltage relays actuate at predetermined values of voltage, which may
be.overvoltage, undervoltage, a combination of both, voltage unbalance (comparing two sources
of voltage), reverse phase voltage, and excess negative-sequence voltage (single phasing of a
three-phase system). Adjustments for pickup or dropout voltage and operation timing are usually
provided in these relays. Time-delay is often required to preclude nuisance relay operation by
transient voltage disturbances.
4.3.10 Distance Relays. Distance relays comprise a family of relays that respond to voltage
and current in terms of impedance. This impedance represents an electrical measure of the
distance along a transmission line from the relay location to a fault. The impedance can also
represent the equivalent impedance of a generator or large synchronous motor when a
distance relay is used for loss-of-field protection. Three main types of distance relay and their
usual applications are as follows:
184.108.40.206 Impedance-Type. Impedance-type relays provide phase-fault relaying for
220.127.116.11 Mho-Type. Mho-type relays provide phase-fault relaying for long lines and
generator or large synchronous motor loss-of-field relaying.
18.104.22.168 Reactance-Type. Reactance-type relays provide ground-fault relaying and
phase-fault relaying on very short lines.
4.3.11 Phase-Sequence or Reverse-Phase Relays. Reversal of the phase rotation of a motor
may result in costly damage to machines, periods of lengthy shutdown, and production loss.