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GL.18
SEL-421/SEL-421-1 Relay
Date Code 20020501
Glossary
Subsidence Current
—
TXD
Subsidence Current
See CT subsidence current.
Synch Reference
A phasor the relay uses as a polarizing quantity for synchronism check
calculations.
Synchronism Check
Verification by the relay that system components operate within a preset
frequency difference and within a preset phase angle displacement between
voltages.
Synchronized Phasor
A phasor calculated from data samples using an absolute time signal as the
reference for the sampling process. The phasors from remote sites have a
defined common phase relationship. Also known as Synchrophasor.
Telnet
An Internet protocol for exchanging terminal data that connects a computer to
a network server and allows control of that server and communication with
other servers on the network.
Terminal
Emulation Software
Software such as Microsoft
®
HyperTerminal or ProComm
®
Plus that can be
used to send and receive ASCII text messages and files via a computer serial
port.
Thermal Demand
Thermal demand is a continuous exponentially increasing or decreasing
accumulation of metered quantities; used in demand metering.
Thermal
Withstand Capability
The capability of equipment to withstand a predetermined temperature value
for a specified time.
Three-Phase Fault
A fault involving all three phases of a three-phase power system.
Three-Pole Trip
A circuit breaker operation that occurs when the circuit breaker opens all three
poles at the same time.
Time Delay on Pickup
The time interval between initiation of a signal at one point and detection of
the same signal at another point.
Time Dial
A control that governs the time scale of the time-overcurrent characteristic of
a relay. Use the time-dial setting to vary relay operating time.
Time-Delayed Tripping
Tripping that occurs after expiration of a pre-determined time.
Time-Overcurrent
Element
An element that operates according to an inverse relationship between input
current and time, with higher current causing faster relay operation.
Torque Control
A method of using one relay element to supervise the operation of another.
Total Clearing Time
The time interval from the beginning of a fault condition to final interruption
of the circuit.
Tower Footing Resistance
The resistance between true ground and the grounding system of a tower.
Transformer Impedance
The resistive and reactive parameters of a transformer looking in to the
transformer primary or secondary windings. Use industry accepted open-
circuit and short-circuit tests to determine these transformer equivalent circuit
parameters.
Tree Resistance
Resistance resulting from a tree in contact with a power line.
TXD
Transmitted data.
Summary of Contents for SEL-421
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