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3.3 Cold load pick-up and inrush
current detection
3 Supporting functions
Technical description
116
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Inrush current detection
Inrush current detection is quite similar with the cold load
detection but it does also include a condition for second
harmonic relative content of the currents. When all phase
currents have been less than a given idle value and then at
least one of them exceeds a given pick-up level within 80 ms
and the ratio 2
nd
harmonic ratio to fundamental frequency,
I
f2
/I
f1
, of at least one phase exceeds the given setting, the inrush
detection signal is activated. This signal is available for output
matrix and blocking matrix. Using virtual outputs of the output
matrix setting group control is possible.
By setting the Pickupf2 parameter for I
f2
/I
f1
to zero, the inrush
signal will behave equally with the cold load pick-up signal.
Application for inrush current detection
The inrush current of transformers usually exceeds the pick-up
setting of sensitive overcurrent stages and contains a lot of
even harmonics. Right after closing a circuit breaker the pick-
up and tripping of sensitive overcurrent stages can be avoided
by selecting a more coarse setting group for the appropriate
over-current stage with inrush detect signal. It is also possible
to use the detection signal to block any set of protection stages
for a given time.
Pick-up
Idle
Cold
load
1
2
3
4
Cold load and inrush
No activation because the current has not been under the
set I
dle
current.
Current dropped under the I
dle
current level but now it
stays between the I
dle
current and the pick-up current for
over 80ms.
No activation because the phase two lasted longer than
80ms.
Now we have a cold load activation which lasts as long as
the operation time was set or as long as the current stays
above the pick-up setting.
Figure 3.3-1 Functionality of cold load / inrush current feature.