ABB Relion REQ650, Applications Manual

Page: 337 / 340

UMT User management tool
Underreach A term used to describe how the relay behaves during a fault
condition. For example, a distance relay is underreaching
when the impedance presented to it is greater than the
apparent impedance to the fault applied to the balance point,
that is, the set reach. The relay does not “see” the fault but
perhaps it should have seen it. See also Overreach.
UTC Coordinated Universal Time. A coordinated time scale,
maintained by the Bureau International des Poids et Mesures
(BIPM), which forms the basis of a coordinated
dissemination of standard frequencies and time signals.
UTC is derived from International Atomic Time (TAI) by
the addition of a whole number of "leap seconds" to
synchronize it with Universal Time 1 (UT1), thus allowing
for the eccentricity of the Earth's orbit, the rotational axis tilt
(23.5 degrees), but still showing the Earth's irregular
rotation, on which UT1 is based. The Coordinated Universal
Time is expressed using a 24-hour clock, and uses the
Gregorian calendar. It is used for aeroplane and ship
navigation, where it is also sometimes known by the military
name, "Zulu time." "Zulu" in the phonetic alphabet stands
for "Z", which stands for longitude zero.
UV Undervoltage
WEI Weak end infeed logic
VT Voltage transformer
3I
O
Three times zero-sequence current. Often referred to as the
residual or the earth-fault current
3U
O
Three times the zero sequence voltage. Often referred to as
the residual voltage or the neutral point voltage
1MRK 505 291-UEN A
Section 17
Glossary
331
Application manual