GL.3
Date Code 20170927 Instruction Manual SEL-751 Relay
Glossary
CT—Event Summary
number. Used to ensure that the information was received without data
corruption.
CT
Abbreviation for current transformer.
Current
Unbalance
The SEL-751 calculates the magnitudes of the measured phase currents,
calculates the average of those magnitudes, determines the magnitude with the
largest deviation from average. It then calculates the difference between the
magnitude average and magnitude of the phase with the largest deviation from
the average. Finally, the relay calculates the percent unbalance current by
dividing the difference value by the CT nominal current or by the average
magnitude, whichever is larger.
Deassert
To deactivate; to remove the logic or electrical requirements necessary to
operate a device. To remove a short-circuit or closed contact from an SEL-751
input. To clear a logic condition to the false state (logical 0). To open a
normally-open output contact. To close a normally-closed output contact.
Delta
A phase-to-phase connection of voltage transformers for electrical measuring
purposes. Typically, two voltage transformers are used with one primary lead
of the first transformer connected to A-phase and the other lead connected to
B-phase. The second voltage transformer is connected to measure the voltage
from B-phase to C-phase. When two transformers are used, this connection is
frequently called “Open-Delta.”
Dropout Time
The time measured from the removal of an input signal until the output signal
deasserts. The time can be settable, as in the case of a logic variable timer, or
can be a result of the characteristics of an element algorithm, as in the case of
an overcurrent element dropout time.
Directional Supervision
The relay uses directional elements to determine whether protective elements
operate based on the direction of a fault relative to the relay.
DNP (Distributed Network
Protocol)
Manufacturer-developed, hardware-independent communications protocol.
EEPROM
Abbreviation for Electrically Erasable Programmable Read-Only Memory.
Nonvolatile memory where relay settings, event reports, SER records, and
other nonvolatile data are stored.
Ethernet
A network physical and data link layer defined by IEEE 802.2 and
IEEE 802.3.
Event History
A quick look at recent relay activity that includes a standard report header;
event number, date, time, and type; maximum fault phase current; and targets.
Event Report
A text-based collection of data stored by the relay in response to a triggering
condition, such as a fault or command. The data show relay measurements
before and after the trigger, in addition to the states of protection elements,
relay inputs, and relay outputs each processing interval. After an electrical
system fault, use event reports to analyze relay and system performance.
Event Summary
A shortened version of stored event reports. An event summary includes items
such as event date and time, event type, fault voltages, currents, etc. The relay
sends an event report summary (if auto messaging is enabled) to the relay
serial port a few seconds after an event.
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