Operation Principle - ABB Relion Technical Manual

1MRK 502 048-UUS A
10.3.6
10.3.6.1
10.3.6.2
Technical manual

Operation principle

Rate-of-change frequency protection SAPFRC (81) is used to detect fast power system
frequency changes at an early stage. It (81) has a settable definite time delay.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. If the frequency recovers,
after a frequency decrease, a restore signal is issued.
Measurement principle
The rate-of-change of the fundamental frequency of the selected voltage is measured
continuously, and compared with the set valuePUFreqGrad. If the voltage magnitude
decreases below the setting MinValFreqMeas in the preprocessing function, which is set
as a percentage of a global base voltage parameter, SAPFRC (81) is blocked and the output
BLKDMAGN is issued. The sign of the setting PUFreqGrad, controls if SAPFRC (81)
reacts on a positive or on a negative change in frequency. If SAPFRC (81) is used for
decreasing frequency that is, the setting PUFreqGrad has been given a negative value,
and a trip signal has been issued, a 100 ms pulse is issued on the RESTORE output, when
the frequency recovers to a value higher than the setting RestoreFreq. A positive setting of
PUFreqGrad, sets SAPFRC (81) to PICKUP and TRIP for frequency increases.
To avoid oscillations of the output PICKUP signal, a hysteresis has been included.
Time delay
SAPFRC (81) has a settable definite time delay, tTrip.
Trip signal issuing requires that SAPFRC (81) condition continues for at least the user set
time delay, tTrip. If the PICKUP condition, ceases during the delay time and is not
fulfilled again within a defined reset time, the PICKUP output is reset after the reset time
has elapsed.
After an issue of the TRIP output signal, the RESTORE output of SAPFRC (81) is set after
a time delay (tRestore), when the measured frequency has returned to the level
corresponding to RestoreFreq. If tRestore is set to 0.000 s the restore functionality is
disabled, and no output will be given. The restore functionality is only active for lowering
frequency conditions and the restore sequence is disabled if a new negative frequency
gradient is detected during the restore period.
Section 10
Frequency protection
329