ABB Relion 670 Series Technical Reference Manual page 558

Section 9
Frequency protection
9.1.2.1
9.1.2.2
552
voltage is lower. If the frequency remains below 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, that is, if the
voltage is lower than the set blocking voltage IntBlockLevel the function is blocked and
no PICKUP or TRIP signal is issued.
Measurement principle
The fundamental frequency of the measured input voltage is measured continuously,
and compared with the set value, PUFrequency. The frequency function is dependent
on the voltage magnitude. If the voltage magnitude decreases the setting IntBlockLevel,
SAPTUF (81) gets blocked, and the output BLKDMAGN is issued. All voltage
settings are made in percent of the setting 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 underfrequency protection SAPTUF (81) can be either a settable
definite time delay or a voltage magnitude dependent time delay, where the time delay
depends on the voltage level; a high voltage level gives a longer time delay and a low
voltage level causes a short time delay. For the definite time delay, the setting
TimeDlyOperate sets the time delay.
For the voltage dependent time delay the measured voltage level and the settings VNom,
VMin, Exponent, t_MaxTripDelay and t_MinTripDelay set the time delay according to
figure 281 and equation 147. The setting TimerOperation is used to decide what type
of time delay to apply.
Trip signal issuing requires that the underfrequency condition continues for at least the
user set time delay TimeDlyOperate. If the PICKUP condition, with respect to the
measured frequency ceases during this user set delay time, and is not fulfilled again
within a user defined reset time, TimeDlyReset, the PICKUP output is reset, after that
the defined reset time has elapsed. Here it should be noted that after leaving the
hysteresis area, the PICKUP condition must be fulfilled again and it is not sufficient
for the signal to only return back to the hysteresis area.
On the output of SAPTUF (81) a 100ms pulse is issued, after a time delay
corresponding to the setting of TimeDlyRestore, when the measured frequency returns
to the level corresponding to the setting RestoreFreq.
1MRK505222-UUS C
Technical reference manual