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81 - Frequency Relay
- Sylvain Ménard
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
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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