ABB REU 610 Technical Reference Manual page 28

REU 610
28
Voltage Relay
Technical Reference Manual
Stage U< can also be configured to be used for alarm purposes. When a trip signal is
generated for alarm purposes, the Start/Alarm indicator LED is lit and the fault is
indicated as an alarm instead of a trip.
When stage U< is configured to be used for alarm purposes:
Start signal of stage is not generated
*
Trip signal of stage is generated, but it is indicated as an alarm
*
Stage cannot be used for triggering CBFP
*
Number of starts is increased instead of number of trips
*
When 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 ~
50 ms' start time. When the set operate time at definite-time characteristic or the
calculated operate time at IDMT characteristic elapses, the stage generates a trip
signal. The stage is reset in 70 ms after the phase-to-phase voltages have exceeded
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.
Undervoltage protection based on positive phase-sequence
voltage
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.
The benefit of this function is that the voltage value measured during or after a
network fault is a good measure of how critical the fault is for a smaller power plant.
When the positive phase-sequence voltage value falls below the critical limit, the
power plant has to be disconnected from the network.
REU 610 measuring the positive phase-sequence voltage complements other
methods used to disconnect smaller power plants. The application of these methods
is based on frequency and voltage measurement. The undervoltage protection based
on calculated positive phase-sequence voltage requires that the relay is in three-
phase use.
1MRS755769