Positive-Phase-Sequence Protection - ABB REU 523 Technical Reference Manual

REU 523
4.1.4.4.
16
Combined Overvoltage and Undervoltage Relay
Technical Reference Manual
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Positive-phase-sequence protection

The high-set undervoltage stage can be set to be based on the calculated
positive-phase-sequence voltage in addition to on phase-to-phase voltage
measurement. The relay will then calculate the voltage based on the two
phase-to-phase voltages, U
The positive-phase-sequence protection function can be applied to disconnecting a
smaller power plant from the outside network, as when there is a fault in the network
which may prove critical for the power plant, e.g. a short circuit either at the
transmission or distribution network level.
A situation of this kind may be critical for different reasons. The power plant may
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, in an asynchronous state compared to the rest of
the network, will be reconnected to the network, e.g. as a result of an autoreclosure.
In addition, the power plant may also fall into an asynchronous state in a fault
situation. These critical situations may 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 falls below the critical limit, the power
plant will have to be disconnected from the network.
The REU 523 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 current measurement.
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1MRS 750942-MUM
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