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27 - Undervoltage Relay


The under-voltage relay (27) operates when the voltage it monitors falls below a set of threshold values. There are five threshold levels available with this relay. Each level corresponds
to a predefined under-voltage level and a tripping delay time. Two types of operating quantities are available: Phase-Neutral Voltage or Phase-Phase Voltage. The model works for 50 Hz or 60 Hz systems.

Icons and diagrams

Model overview

The under voltage relay picks up whenever the line or phase voltage drops below the selected settings. A timer is normally used to enable it to ride through momentary sags and thus prevent nuisance tripping.
The five-level under-voltage relay characteristics are presented in Figure 3 - 5. For instance, if we use the default values illustrated in Figure 3 - 6, level 1 voltage is 0.8 pu and tripping time is 1 sec. It means that when the input voltage is lower than 0.8 pu and continues to be lower than 0.8 pu for more than 1 second, the under voltage relay will be asserted. If the voltage is lower than 0.8 pu for less than 1 second, its timer will reset and will not restart until the voltage amplitude reaches the level 1 voltage threshold again.
All voltage levels and delay times are tunable parameters.

Under voltage relay characteristics


Figure 3 - 6 shows the control panel of the under-voltage relay model.

27 relay parameter panel

The following parameters are tunable:

Parameter tab of 27 Relay

NameUnitDescriptionSample value

The frequency of the measured current signals. Both 50
and 60 Hz are supported within the model.

UbaseVThe base voltage (L-L) at the measurement location500000



Two types of operating quantity are provided in this
minimum Phase-Neutral Voltage or minimum Phase-
Phase Voltage among three phases.


Level 1puUndervoltage level 1.0.8
Level 2puUndervoltage level 2.0.7
Level 3puUndervoltage level 3.0.6
Level 4puUndervoltage level 4.0.5
Level 5puUndervoltage level 5.0.4
Time 1sTime delay associated with Level 1.1.0
Time 2sTime delay associated with Level 2.0.5
Time 3sTime delay associated with Level 3.0.4
Time 4sTime delay associated with Level 4.0.3
Time 5sTime delay associated with Level 5.0.2

Input and output signals
The module has 2 inputs and 2 outputs:

27 Relay I/O

I/O nameType (unit)Description
VabcInput (V)

A 3-dimension voltage input for three-phase voltages, in the order
of Va, Vb and Vc.

ResetInput (binary)

The reset signal for the relay. The relay will be reset when
this signal becomes 1.

QOutput (binary)

The output of under-voltage relay. By default it is 0, if an
under-voltage scenario is detected, it becomes 1.

Q_nOutput (binary)The complementary value of Q.

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