8.2.7 Operation principle
Overfrequency protection SAPTOF (81) is used to detect high power system frequency.
SAPTOF (81) has a settable definite time delay. If the frequency remains above the set
value for a time period greater 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.
8.2.7.1 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 discussed in the Basic IED Functions chapter and is set as a percentage
of a global base voltage parameter VBase, SAPTOF (81) is blocked and the output
BLKDMAGN is issued. All voltage settings are made in percent of the global parameter
VBase. To avoid oscillations of the output PICKUP signal, a hysteresis has been included.
PICKUP
PICKUP
TRIP
Pickup
&
Trip
Output
Logic
Definite Time Delay
TimeDlyOperate
BLOCK
ANSI09000033-1-en.vsd
Frequency Comparator
f > PUFrequency
TRIP
BLKDMAGN
BLOCK
OR
freqNotValid
ANSI09000033 V1 EN
Figure 82: Schematic design of overfrequency protection SAPTOF (81)
1MRK 511 287-UUS A Section 8
Frequency protection
197
Technical manual
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