27 - Undervoltage Relay (1-ph)

Summary

The under-voltage relay (27-UV) 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. The operating quantity of the relay can be the RMS value of phase voltage, or the positive, negative or zero sequence of a three-phase voltage signals. The operating quantity can be either in per unit or actual value.

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 is presented in below figure. For instance, if we use the default values as mentioned above, 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

The below figure shows the control panel of the 27-UV relay

The following gives a brief description about the parameters of 27-UV relay

Parameter tab of 27-UV Relay

Name	Description	Unit	Sample value
Level 1	pu or V	Undervoltage level 1	0.8
Level 2	pu or V	Undervoltage level 2	0.7
Level 3	pu or V	Undervoltage level 3	0.6
Level 4	pu or V	Undervoltage level 4	0.5
Level 5	pu or V	Undervoltage level 5	0.4
Time 1	sec	Time delay associated with level 1	1.0
Time 2	sec	Time delay associated with level 2	0.5
Time 3	sec	Time delay associated with level 3	0.4
Time 4	sec	Time delay associated with level 4	0.3
Time 5	sec	Time delay associated with level 5	0.2

Input and output signals

The module has 1 input and 1 output.

27-UV Relay I/O

I/O Name	Type (Unit)	Description
V	pu or V	A single dimensional signal, it should be the RMS value of phase voltage, or the positive, negative or zero sequence of a three-phase voltage signals.
Trip	Binary	The output of under-voltage relay. By default it is 0, if an under-voltage scenario is detected, it becomes 1