27 - Undervoltage Relay

Summary

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.

Parameters

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

Name	Unit	Description	Sample value
Frequency	Hz	The frequency of the measured current signals. Both 50 and 60 Hz are supported within the model.	60
Ubase	V	The base voltage (L-L) at the measurement location	500000
Operating Quantity	N/A	Two types of operating quantity are provided in this model: minimum Phase-Neutral Voltage or minimum Phase- Phase Voltage among three phases.	Phase-Neutral Voltage
Level 1	pu	Undervoltage level 1.	0.8
Level 2	pu	Undervoltage level 2.	0.7
Level 3	pu	Undervoltage level 3.	0.6
Level 4	pu	Undervoltage level 4.	0.5
Level 5	pu	Undervoltage level 5.	0.4
Time 1	s	Time delay associated with Level 1.	1.0
Time 2	s	Time delay associated with Level 2.	0.5
Time 3	s	Time delay associated with Level 3.	0.4
Time 4	s	Time delay associated with Level 4.	0.3
Time 5	s	Time 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 name	Type (unit)	Description
Vabc	Input (V)	A 3-dimension voltage input for three-phase voltages, in the order of Va, Vb and Vc.
Reset	Input (binary)	The reset signal for the relay. The relay will be reset when this signal becomes 1.
Q	Output (binary)	The output of under-voltage relay. By default it is 0, if an under-voltage scenario is detected, it becomes 1.
Q_n	Output (binary)	The complementary value of Q.