than the set time delay the TRIP signal is issued. To avoid an unwanted trip due to
uncertain frequency measurement at low voltage magnitude, a voltage controlled
blocking of the function is available from the preprocessing function, that is, if the voltage
is lower than the set blocking voltage in the preprocessing function, the function is
blocked and no PICKUP or TRIP signal is issued.
8.1.7.1 Measurement principle
The frequency measuring element continuously measures the frequency of the positive
sequence voltage and compares it to the setting PUFrequency. The frequency signal is
filtered to avoid transients due to switchings and faults in the power system. If the voltage
magnitude decreases below the setting MinValFreqMeas in the SMAI preprocessing
function, which is described in the Basic IED Functions chapter and is set as a percentage
of a global base voltage parameter, SAPTUF (81) gets blocked, and the output
BLKDMAGN is issued. All voltage settings are made in percent of the setting of the
global parameter VBase.
To avoid oscillations of the output PICKUP signal, a hysteresis has been included.
Frequency Comparator
f < PUFrequency
PICKUP
PICKUP
TRIP
BLOCK
Comparator
f > RestoreFreq
BLOCK
OR
DefiniteTimeDelay
TimeDlyOperate
TimeDlyRestore
RESTORE
100 ms
Pickup
&
Trip
Output
Logic
ANSI09000034-1-en.vsd
TRIP
BLKDMAGN
freqNotValid
ANSI09000034 V1 EN
Figure 79: Simplified logic diagram for SAPTUF (81)
1MRK 511 287-UUS A Section 8
Frequency protection
193
Technical manual
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