Schweitzer Engineering Laboratories SEL-701, Instruction Manual

Page: 445 / 468

Date Code 20010719
701 Motor Protection Relay
Glossary
CT–EEPROM
GL.3
CT Abbreviation for current transformer.
Current
Unbalance Element
Protection element that 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 motor rated full load amps or by the average
magnitude, whichever is larger. Unbalance current
causes heating in the rotor of the protected motor. The
unbalance element can trip the motor in the presence of
heavy unbalance to prevent rotor damage due to
overheating. In the SEL-701 Relay, this element works
together with the motor thermal element which also
provides unbalance current protection.
Deassert To deactivate; to remove the logic or electrical
requirements needed to operate a device. To remove a
short-circuit or closed contact from an SEL-701 Relay
input. To clear a logic condition to its false state
(logical 0). To open a normally-open output contact. To
close a normally-closed output contact.
Delta As used in this instruction manual, 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.”
Demand Meter A measuring function that calculates a rolling average
or thermal average of instantaneous measurements
over time.
Dropout Time The time measured from the removal of an input signal
until the output signal deasserts. The time can be user
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.
EEPROM Abbreviation for Electrically Erasable Programmable
Read-Only Memory. Nonvolatile memory where relay
settings, event reports, SER records, and other
nonvolatile data are stored.