Positive-Phase-Sequence Protection - ABB REU 513 User Manual

Combined overvoltaqge and undervoltage relay

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ABB Substation Automation

Products and Systems

2.1.3.4.

1MRS750886-MUM

User's Guide

U/Un

U<<

0.2 Un

U<<

t<<

Fig. 2.1.3.3.-1 Operation of the undervoltage unit when the function of the high-

set and low-set undervoltage stages is internally blocked in case

the voltage falls below 0.2 x U

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.

t<<

REU 513

and U

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