scheider electric MiCOM P746, Technical Manual

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Operation P746/EN OP/G31
MiCOM P746 (OP) 5-
9
Internal undercurrent signals are available per phase to confirm that each pole has opened.
To maintain the current criterion active while the signal crosses zero, there is a drop-off timer
associated with the pl(t) signal. The latching duration is variable in order to take all cases into
account:
•
Just after the initiation of the CB fail signal, the waveform can include a DC component,
and the time between two successive zero crossings can thus reach one period.
Therefore, the retrip time shall be set long enough for the DC component to have
disappeared so that the time between two successive zero crossings should be close to
one half-period. Moreover it is important to detect the opening of the circuit breaker
quickly because the end of the back trip timer is near. The drop-off duration is therefore
equal to one half 3ms (13ms at 50Hz, 11.3ms at 60Hz).
0
0
T 2T 3T 4T 5T 6T 7T
tps
tss
Tn1=(T/2) + 3ms (T/2) < t max < T
pl (t)
Tn1 = Switching order
Tp = Trip pole signal
tBF4 - 30ms
P0712ENa
OP
FIGURE 3: CB ELEMENT LOGIC
Principle of the undercurrent function.
Instantaneous current measurements are taken for both the positive and negative half
cycles, ensuring immunity to DC offset waveforms, and CT current ring-down.
The two horizontal dotted lines are instantaneous thresholds, fixed in proportion to the user's
I< setting. The instantaneous threshold is at 70 percent of the I< fundamental RMS setting.
As any current rises above the dotted line instantaneous threshold, this rising measurement
triggers a pulse timer to declare that current is flowing. The duration of the pulse is one full
cycle plus 3ms (T+3ms). It does not matter whether the magnitude of the current stays
above the dotted line further, as the detector is effectively edge-triggered. Current flow has
been declared based on this half cycle, and not until the current falls below the detector
setting is the edge-trigger ready to declare an output again. Whilst current is flowing, on the
rise of current in each half-cycle the pulse timer is retriggered. This sequential retriggering
ensures that current is detected.