ABB Relion 670 Series Technical Reference Manual page 564

Section 9
Frequency protection
9.2.1
9.2.2
9.2.2.1
9.2.2.2
558
Introduction
Overfrequency protection function SAPTOF (81) is applicable in all situations, where
reliable detection of high fundamental power system frequency is needed.
Overfrequency occurs because of sudden load drops or shunt faults in the power
network. Close to the generating plant, generator governor problems can also cause
over frequency.
SAPTOF (81) is used mainly for generation shedding and remedial action schemes. It
is also used as a frequency stage initiating load restoring.
SAPTOF (81) is provided with an undervoltage blocking.
The operation is based on positive sequence voltage measurement and requires two phase-
phase or three phase-neutral voltages to be connected. For information about how to
connect analog inputs, refer to Application manual/IED application/Analog inputs/
Setting guidelines
Principle of operation
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 corresponding to the chosen time delay, the
corresponding 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.
Measurement principle
The fundamental frequency of the positive sequence voltage is measured continuously,
and compared with the set value, PUFrequency. Overfrequency protection SAPTOF
(81) is dependent on the voltage magnitude. If the voltage magnitude decreases below
the setting IntBlockLevel, SAPTOF (81) is blocked, and the output BLKDMAGN is
issued. All voltage settings are made in percent of the VBase, which should be set as a
phase-phase voltage in kV. To avoid oscillations of the output PICKUP signal, a
hysteresis has been included.
Time delay
The time delay for Overfrequency protection SAPTOF (81) is a settable definite time
delay, specified by the setting TimeDlyOperate.
1MRK505222-UUS C
Technical reference manual