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4.7. Function 81O/U. Overfrequency and underfrecuency protection
There are four protection units of the variation in frequency. Each one of them has an
adjustment which determines whether the unit acts as overfrequency or underfrequency.
The reaction time of the unit is determined by the set operating time.
In case the unit acts like overfrequency, activation of the function occurs at 100%
frequency level set and is reset when the measured frequency is 50 mHz lower than set
start level.
If the unit is set as underfrequency, activation of the function occurs at 100% frequency
level set and is reset when the measured frequency is 50 mHz higher than set start level.
The reset is temporized and the reset time is determined by the reset time setting.
The frequency measurement is done from the voltage of phase B. It takes a minimum of
45 volts at this stage for 81 functions to be operational. If the measured phase voltage is
less than 45 volts, it activates a state bit indicating function blocked. When the frequency
measurement is again valid, function begins in the start state with all bits and counters
reset.
This protection function is adjusted by setting five different parameters:
Function Description Minimum Maximum Step Unit Default
81_1
81_2
81_3
81_4
Overfrequency or Underfrequency
Permission - - Yes/No - No
Type - - (1*) - Overfrequency
Activation level 45.00 65.00 0.01 Hz 50.20
Operating time 0.06 300.00 0.01 s 0.15
Reset time 0,02 300,00 0,01 s 0,20
(1*) Overfrequency or Underfrequency
The frequency measurement is an average value of the frequency measured during 8
cycles. The tripping time will be the adjusted value plus the necessary time to achieve the
measure during 8 cycles