ABB REX 521, Technical Reference Manual, Standard Configurations

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1MRS751802-MUM Protection Relay
Technical Reference Manual, Standard Configurations
REX 521
7.1.20. U1<U1U2>
The positive-sequence undervoltage function U1< is designed to be used to protect
small generating plant from asynchronous reclosure with the rest of the network, and
to protect the plant from loss of synchronism.
The function block is also used to protect against unwanted isolation of small
generating plant during one- or two-phase network unbalance. The level of positive
sequence voltage is found to be a good criteria when determining the criticality of
the network fault from the generating plant’s point of view.
Additionally, the function block can be used in unsuccessful reclosure protection in
cases where a small generating plant with a faulted feeder supplies fault current
which is too small to be detected by the current relays, but high enough to maintain
the arc.
The negative-sequence overvoltage function U2> is designed to be used to protect
motors against failure on one or more phases (single phasing), to protect against
excessive imbalance between phases, and to protect machines against reversal of
phase sequence both in forward and reverse rotation direction.
A faulty voltage transformer or sensor, as well as a wrong connection of voltage
measuring devices causes apparent voltage unbalance. The U2> function can also be
used for monitoring the condition of measuring circuits.
The positive-sequence overvoltage function U1> can be used in various applications
instead of an ordinary three-phase overvoltage protection.
Fig. 7.1.20.-1 Function block symbol of U1<U1U2>
Operation accuracies Current measurement:
f/fn = 0.95...1.05: ±1.0%, I = 0.1...10.0 x In
Operate time accuracy ±2% or ±0.5 s
Reset ratio TRIP: (TEMP(%) -0.1)/Trip temperature
START: (TEMP(%) -0.1)/Prior alarm
EN_RESTART: (TEMP(%) -0.1)/Restart inhibit
Three-phase thermal overload protection for devices, 3Ithdev (Continued)
Phase sequence voltage protection
Operation mode Not in use
U1< & U2> & U1>
U1< & U2>
U2> & U1>
U1< & U1>
U2>
U1<
U1>
Start value U2> 0.01...1.00 x Un
BLOCK
GROUP
RES ET
S TART
TRIP
U1<U1U2>