Protection Functions
PRS-7367 105
The activation of the BLOCK input resets the timer and deactivates the OPERATE and START
outputs.
Note:
The reset value is the startup setting -0.02Hz
3.18.3 81O Application Scope
81O is applicable in all situations where high levels of the fundamental frequency of power system
voltage must be reliably detected. A high fundamental frequency in a power system indicates that
there is an unbalance between production and consumption. In this case, the available generation
is too large compared to the power demanded by the load connected to the power grid. If the
situation continues and escalates, the power system loses its stability.
The over frequency function detects such situations and provides an output signal suitable for
example for generator shedding. The function can also be used as a sub-nominal frequency stage
initiating load restoring. The over frequency function is very sensitive and accurate and can also
be used to alert operators that frequency has slightly deviated from the set-point and that manual
actions can suffice.
3.18.4 81O Settings
In the following table, i is the protection stage number, it can be set according to the requirements.
Table 3.18-3 81O settings
3.19 Under frequency protection 81URS
3.19.1 81URS Overview
The feeder relay provides a five-stage under frequency protection with independent definite time
delay characteristics, and each stage has the same protection logics.
The calculation of the frequency is based on the voltage sampled values. Four cycles of the
voltage sampled values are fixedly adopted for the frequency calculation.
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