Description

This frequency relay (81) includes three types of frequency elements, namely the over-frequency (81O), the under-frequency (81U) and the rate-of-change-of-frequency (ROCOF) (81R) elements. There are five levels of thresholds available with the 81O and 81U elements, whereas the 81R element features only one level. Each level corresponds to a predefined frequency level and a tripping delay time. When the frequency or ROCOF monitored by the relay exceeds the settings, the relay operates. The model works for 50 Hz or 60 Hz system

Table of Contents

Model Overview

The frequency relay operates whenever the frequency or ROCOF measured from the input voltage signals using a zero-crossing detection logic exceeds the preset settings. A time delay is used to enable the relay to ride through momentary frequency fluctuations and thus prevent nuisance operation. The five-level characteristic of 81O and 81U are presented in figures below:

Characteristics of 81O

Characteristics of 81U

Mask and Parameters

General parameters

Name	Unit	Description
Frequency	Hz	The nominal frequency of the measured voltage signals. Both 50 and 60 Hz are supported in the model.
Freeze Time	s	The freezing time of the 81 relay, when t < freezing time, the 81 relay is not activated.
Sample Time	s	Block sample time

Overfrequency

Name	Unit	Description
Enable Overfrequency	-	Overfrequency element enable/disable
Level 1	Hz	Over-frequency level 1.
Level 2	Hz	Over-frequency level 2.
Level 3	Hz	Over-frequency level 3.
Level 4	Hz	Over-frequency level 4.
Level 5	Hz	Over-frequency level 5.
Time 1	s	Time delay associated with Level 1.
Time 2	s	Time delay associated with Level 2.
Time 3	s	Time delay associated with Level 3.
Time 4	s	Time delay associated with Level 4.
Time 5	s	Time delay associated with Level 5.

Underfrequency

Name	Unit	Description
Enable Underfrequency	-	Underfrequency element enable/disable
Level 1	Hz	Under-frequency level 1.
Level 1	Hz	Under-frequency level 2.
Level 1	Hz	Under-frequency level 3.
Level 1	Hz	Under-frequency level 4.
Level 1	Hz	Under-frequency level 5.
Time 1	s	Time delay associated with Level 1.
Time 1	s	Time delay associated with Level 2.
Time 1	s	Time delay associated with Level 3.
Time 1	s	Time delay associated with Level 4.
Time 1	s	Time delay associated with Level 5.

ROCOF

Name	Unit	Description
Enable ROCOF	-	ROCOF element enable/disable
Level 1	Hz/s	Rate-of-change-of-frequency level.
Time 1	s	Time delay associated with Level 1.

Inputs and Outputs

Inputs

Name	Description
_Vabc	Three-phase voltages
Reset	The reset signal for the relay. The relay will be reset when this signal becomes 1.

Outputs

Name	Description
81O	The output of over-frequency relay. By default it is 0, if an over-frequency scenario is detected, it becomes 1.
81U	The output of under-frequency relay. By default it is 0, if an under-frequency scenario is detected, it becomes 1
81R	The output of rate-of-change-of-frequency relay. By default it is 0, if an under-frequency scenario is detected, it becomes 1.

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