GL.21
Date Code 20090715 SEL-421 Relay
Glossary
Unconditional Tripping—Zero-Sequence Compensation Factor
“best efforts” basis. If no association exists, or if the transport data flow is not
fast enough to support it, events may be lost.
Unconditional Tripping
Protection element tripping that occurs apart from conditions such as those
involving communication, switch-onto-fault logic, etc.
Unstable Power Swing
A change in the electrical angle between power systems for which a control
action cannot return the angular separation between systems to an angle less
than the critical angle.
Untransposed Line
A transmission line with phase conductors that are not regularly transposed.
The result is an imbalance in the mutual impedances between phases.
User ST
Region in GOOSE for user-specified applications.
VA, VB, VC
Measured A-phase-to-neutral, B-phase-to-neutral, and C-phase-to-neutral
voltages.
VAB, VBC, VCA
Measured or calculated phase-to-phase voltages.
VG
Residual voltage calculated from the sum of the three phase-to-neutral
voltages, if connected.
Virtual
Terminal Connection
A mechanism that uses a virtual serial port to provide the equivalent functions
of a dedicated serial port and a terminal.
Volatile Storage
A storage device that cannot retain data following removal of relay power.
VT
Voltage transformer. Also referred to as a potential transformer or PT.
Warm Start
The reset of a running system without removing and restoring power.
Weak Infeed Logic
Logic that permits rapid tripping for internal faults when a line terminal has
insufficient fault current to operate protective elements.
Wye
A phase-to-neutral connection of circuit elements, particularly voltage
transformers or loads. To form a wye connection using transformers, connect
the nonpolarity side of each of three voltage transformer secondaries in
common (the neutral), and take phase to neutral voltages from each of the
remaining three leads. When properly phased, these leads represent the A-
phase-, B-phase-, and C-phase-to-neutral voltages. This connection is
frequently called ‘four-wire wye,’ alluding to the three phase leads plus the
neutral lead.
XML
Extensible Markup Language. This specification developed by the W3C
(World Wide Web Consortium) is a pared-down version of SGML designed
especially for web documents. It allows designers to create their own
customized tags, enabling the definition, transmission, validation, and
interpretation of data among applications and organizations.
Zero-Sequence
A configuration of three-phase currents and voltages with currents and
voltages that occur simultaneously, are always in phase, and have equal
magnitude (3I
0
= I
A
+ I
B
+ I
C
).
Zero-Sequence
Compensation Factor
A factor based on the zero-sequence and positive-sequence impedance of a
line that modifies a ground distance element to have the same reach as a phase
distance element.
Courtesy of NationalSwitchgear.com
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