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

1MRS 750942-MUM
4.1.3.4.
ABB Automation
Combined Overvoltage and Undervoltage Relay
Technical Reference Manual
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Positive-phase-sequence protection

Instead of the phase-to-phase voltage measurement the high-set undervoltage stage
can be set to be based on the positive-phase-sequence voltage. The relay calculates
the voltage on the basis of the two phase-to-phase voltages U
The function based on the positive-phase-sequence voltage can be applied to
disconnecting a smaller power plant from the outside network. This may be
necessary in case a fault somewhere else in the network causes a condition, critical
for the power plant. An example of this kind of a fault is a short circuit either on the
transmission or distribution network level.
The situation may be critical for the power plant for different reasons. For example,
due to the relay operation caused by a fault situation, the power plant may be left to
feed an isolated network. In this case, there is a risk that the isolated network, in an
asynchronous state compared to the rest of the network, is reconnected to the
network, e.g. as a result of an autoreclosure. Another hazardous situation would be
that the power plant falls into an asynchronous state during the fault condition. Both
of these hazards may be prevented if the power plant is disconnected from the
network quickly enough by opening the connecting circuit breaker.
The benefit of the disconnecting relay assembly based on the positive-phase-
sequence voltage is that the voltage value during the network fault or after it
measures well the criticalness of the fault for a smaller power plant. When the
positive-phase-sequence voltage falls below the critical limit, the power plant has to
be disconnected from the network.
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