M
E
f
U
f
Volt Max continuous
m
n
n
=
⋅
100
(Equation 133)
M excitation level (U/f ratio or volts/hertz) in pu
E internal induced voltage (emf)
f
m
measured frequency
U
n
nominal phase-to-phase voltage
f
n
nominal frequency
If the input frequency (f
m
) is less than 20 percent of the nominal frequency (f
n
), the
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
Chapter 4.5.8.5 Timer
characteristics
.
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 of overexcitation. The
Time multiplier
setting is used for scaling the IDMT
operate times.
The activation of the OPERATE output activates the BLK_RESTART output.
For the DT characteristics, the deactivation of the OPERATE output activates the
cooling timer. The timer is set to the value entered in the
Cooling time
setting. The
BLK_RESTART and COOL_ACTIVE outputs are kept active until the cooling timer is
reset. If the excitation increases above the set value during this period, the OPERATE
output is activated immediately. For IDMT, the deactivation of BLK_RESTART and
COOL_ACTIVE depends on the curve type selected.
1MRS757644 H
Protection functions
620 series
Technical Manual
675
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