ABB REU 610 Technical Reference Manual page 26

REU 610
26
Voltage Relay
Technical Reference Manual
Consequently, the set resetting time ensures that when the stage starts because
of voltage drops, it is not immediately reset. If stage U< has already tripped,
the stage is reset in 7 0 ms after the phase- to- phase voltages have fallen below
0.15 × U . H owever, if stage U< has already tripped and the phase- to- phase
n
voltages have ex ceeded the set start value of the stage, the stage is reset when
the set resetting time has ex pired.
S tage U< tripping can be set to be blocked by the start of stage U<< or U
S tage U< can also be configured to be used for alarm purposes. W hen a trip
signal is generated for alarm purposes, the S tart/ Alarm indicator L E D
and the fault is indicated as an alarm instead of a trip.
W hen stage U< is configured to be used for alarm purposes:
S tart signal of stage is not generated
*
Trip signal of stage is generated, but it is indicated as an alarm
*
S tage cannot be used for triggering CBF P
*
N umber of starts is increased instead of number of trips
*
W hen the conventional protection mode is selected and the voltages fall
below the set start value of the high- set stage, U<<, the stage generates a start
signal after a ~ 5 0 ms' start time. W hen the set operate time at definite- time
characteristic or the calculated operate time at ID M T characteristic elapses,
the stage generates a trip signal. The stage is reset in 7 0 ms after the phase- to-
phase voltages have ex ceeded the set start value of the stage.
The undervoltage stages have a settable drop- off/ pick- up ratio, which is
adjustable between 1.01...1.05 , the default being 1.03 .
Un d er v oltage p r otec ti on b as ed on p os i ti v e p h as e- s eq u en c e
v oltage
The undervoltage protection based on calculated positive phase- sequence
voltage can be applied to disconnecting a smaller power plant from the
outside network, for instance in situations where there is a fault in the
network which can be critical for the power plant, such as a short circuit
either at the transmission or distribution network level.
A situation of this kind can be critical for different reasons. The power plant
can be left to feed an isolated network due to a trip caused by a fault. In this
case, there is a risk that the isolated network, which is in asynchronous state
compared to the rest of the network, is reconnected to the network as a result
of an autoreclosure, for instance. In addition, the power plant can also fall into
an asynchronous state in a fault situation. These critical situations can be
prevented by disconnecting the power plant from the network fast enough by
tripping the connecting circuit breaker.
1MRS755769
<.
1
is lit