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81 -Frequency Relay


Description

This frequency relay (81) includes three types of frequency elements, namely the overfrequency (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 as shown below:

81O element characteristic

Characteristics of 81O

81U element characteristic

Characteristics of 81U


Mask and Parameters

General Parameters

The following parameters are tunable:

NameDescriptionUnitVariable = {Possible Values}
Frequency

The nominal frequency of the measured voltage signals.
Both 50 and 60 Hz are supported in the model.

HzFreq_param= {50, 60}

Freezing time

The freezing time of the 81 relay, when t < freezing time,
the 81 relay is not activated.

stunf_param = { [0.10, 60.00] }


Settings - Overfrequency

The following parameters are tunable:

NameDescriptionUnitVariable = {Possible Values}
Enable OverfrequencyOverfrequency element enable/disable-Enable_81O_param= {0, 1}
Level 1Over-frequency level 1.HzOLevel1_param={ [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 2Over-frequency level 2.HzOLevel2_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 3Over-frequency level 3.HzOLevel3_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 4Over-frequency level 4.HzOLevel4_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 5Over-frequency level 5.HzOLevel5_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Time 1Time delay associated with Level 1.sOTime1_param= { [0, 600] }
Time 2Time delay associated with Level 2.sOTime2_param= { [0, 600] }
Time 3Time delay associated with Level 3.sOTime3_param= { [0, 600] }
Time 4Time delay associated with Level 4.sOTime4_param= { [0, 600] }
Time 5Time delay associated with Level 5.sOTime5_param= { [0, 600] }


Settings - Underfrequency

The following settings are tunable:

NameDescriptionUnitVariable = {Possible Values}
Enable  UnderfrequencyUnderfrequency element enable/disable-Enable_81U_param= {0, 1}
Level 1Under-frequency level 1.HzULevel1_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 1Under-frequency level 2.HzULevel2_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 1Under-frequency level 3.HzULevel3_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 1Under-frequency level 4.HzULevel4_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 1Under-frequency level 5.HzULevel5_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Time 1Time delay associated with Level 1.sUTime1_param= { [0, 600] }
Time 1Time delay associated with Level 2.sUTime2_param= { [0, 600] }
Time 1Time delay associated with Level 3.sUTime3_param= { [0, 600] }
Time 1Time delay associated with Level 4.sUTime4_param= { [0, 600] }
Time 1Time delay associated with Level 5.sUTime5_param= { [0, 600] }


Settings - ROCOF

The following settings are tunable:

NameDescriptionUnitVariable = {Possible Values}
Enable ROCOFROCOF element enable/disable-Enable_ROCOF_param= {0, 1}
Level 1Rate-of-change-of-frequency level.Hz/sRf_param= { [-10, 10] }
Time 1Time delay associated with Level 1.srocofDelay_param= { [0, 600] }


Inputs, Outputs and Signals Available for Monitoring

Inputs

NameDescription
VabcA 3-dimension voltage input for three-phase voltages, in the order of Va, Vb and Vc
Reset

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

Outputs

NameDescription
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.

Sensors

NameDescriptionUnit
m

The calculated frequency by the relay/the actuating quantity of the relay.

Hz


Typical Values for Parameters

The parameter section shows the possible values for each parameter. These values would change to fit the situation of the 81 relay model so they cannot be taken as sole values for each parameter.

NameDescriptionUnitTypical value
Frequency

The nominal frequency of the measured voltage signals.
Both 50 and 60 Hz are supported in the model.

Hz60

Freezing
time

The freezing time of the 81 relay, when t < freezing time,
the 81 relay is not activated.

s0.2
Enable OverfrequencyOverfrequency element enable/disable-Enabled
Level 1Over-frequency level 1.Hz60.5
Level 2Over-frequency level 2.Hz61.0
Level 3Over-frequency level 3.Hz61.5
Level 4Over-frequency level 4.Hz62.0
Level 5Over-frequency level 5.Hz62.5
Time 1Time delay associated with Level 1.s2.0
Time 2Time delay associated with Level 2.s0.5
Time 3Time delay associated with Level 3.s0.4
Time 4Time delay associated with Level 4.s0.3
Time 5Time delay associated with Level 5.s0.2
Enable  UnderfrequencyUnderfrequency element enable/disable-Enabled
Level 1Under-frequency level 1.Hz59.5
Level 1Under-frequency level 2.Hz59.0
Level 1Under-frequency level 3.Hz58.5
Level 1Under-frequency level 4.Hz58.0
Level 1Under-frequency level 5.Hz57.5
Time 1Time delay associated with Level 1.s2.0
Time 1Time delay associated with Level 2.s0.5
Time 1Time delay associated with Level 3.s0.4
Time 1Time delay associated with Level 4.s0.3
Time 1Time delay associated with Level 5.s0.2
Enable ROCOFROCOF element enable/disable-Enabled
Level 1Rate-of-change-of-frequency level.Hz/s0.3
Time 1Time delay associated with Level 1.s0.0

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