Na7
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MiCOM P741, P742
FIGURE 7: 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.
The detection of breaker opening is made upon one of two scenarios:
(1) The current falls below the instantaneous detection threshold, and does not rise
again before the pulse timer expires; or
(2) A CT current ringdown does not change sign, and remains in one polarity sense up
until the timer expires.
Note The pulse timer length is variable, and adapts according to the
anticipated proportion of DC current offset that may be present in the
measured waveform. The pulse timer initially is fixed at one cycle plus
3ms, as described previously, as upon fault inception the DC offset
could be appreciable. Near the end of the breaker fail time, the pulse
length is shortened to half a power cycle plus 3ms (T/2 + 3ms). The
presumption is that the DC offset in real fault current has decayed,
and that the shorter time is all that is required. The pulse length is
commuted 30ms before expiry of the tBF2 timer (for internally-initiated
CBF) and 30ms before expiry of the tBF4 timer (for externally-initiated
CBF). The reduced pulse length means faster resetting of the current
detector.
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