ABB RELION REX640 Technical Manual page 810

Section 4
Protection functions
804
The calculated U/f ratio is scaled to a value based on the nominal U
the highest allowed continuous voltage (in % U
parameter Voltage Max Cont to change the basis of the voltage. The measured voltage
is compared to the new base value to obtain the excitation level.
The excitation level (M) can be calculated:
E
f
m
M
=
U
Volt Max continuous
n
⋅
f 100
n
GUID-2ACF26AA-430C-436F-8AEB-F626E84368D2 V1 EN
M excitation level (U/f ratio or volts/hertz) in pu
E internal induced voltage (emf)
f measured frequency
m
U nominal phase-to-phase voltage
n
f nominal frequency
n
If the input frequency (f
calculation of the excitation level is disabled and forced to zero value. This means that
the function is blocked from starting and operating during a low-frequency condition.
The calculated excitation level (U/f ratio or volts/hertz) VOLTPERHZ is available in
the Monitored data view.
Level detector
Level detector compares the calculated excitation level to the Start value setting. If the
excitation level exceeds the set limit, the module sends an enabling signal to start
Timer.
Timer
Once activated, Timer activates the START output. Depending on the value of the
Operating curve type setting, the time characteristics are according to DT or IDMT.
When the operation timer has reached the value set by Operate delay time in the DT
mode or the value defined by the inverse time curve, the OPERATE output is activated.
In a drop-off situation, that is, when the excitation level drops below Start value before
the function operates, the reset timer is activated and the START output resets after the
time delay of Reset delay time for the DT characteristics. For the IDMT curves, the
reset operation is as described in the
For the IDMT curves, it is possible to define the maximum and minimum operating
times via the Minimum operate time and Maximum operate time settings. The
Maximum operate time setting is used to prevent infinite start situations at low degrees
) is less than 20 percent of the nominal frequency (f
m
Timer characteristics
1MRS759142 C
/f ratio. However,
n n
) can be defined by setting the
n
chapter.
Technical Manual
(Equation 201)
), the
n
REX640