2.7 Directional earth fault
protection I0φ> (67N)
2 Protection functions
Technical description
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Additionally the stage I
0
> has one more input signal
alternative to measure current peaks to detect short restriking
intermittent earth faults:
I
01Peak
to measure the peak value of input I
01
.
Intermittent earth fault detection
Short earth faults make the protection to start (to pick up), but
will not cause trip. When starting happens often enough, such
intermittent faults can be cleared using the intermittent time
setting. The mode should be Undir. The phase angle detection
of I
0
in directional mode is insecure.
When a new start happens within the set intermittent time,
the operation delay counter is not cleared between adjacent
faults and finally the stage will trip. By using input signal
I
01Peak
a single one-millisecond current peak is enough to start
the stage and increase the delay counter by 20 ms. For example
if the operating time is 120 ms, and the time between two
peaks does not exceed the intermittent time setting, the sixth
peak will cause a trip.
Two independent stages
There are two separately adjustable stages: I
0
> and I
0
>>.
Both the stages can be configured for definite time delay (DT)
or inverse time delay operation time.
Inverse operation time
Inverse delay means that the operation time depends on the
amount the measured current exceeds the pick-up setting. The
bigger the fault current is the faster will be the operation.
Accomplished inverse delays are available for both stages I
0
>
and I
0
>>. The inverse delay types are described in chapter
2.14. The device will show a scaleable graph of the configured
delay on the local panel display.
Inverse time limitation
The maximum measured secondary residual current is 10xI
0N
and maximum measured phase current is 50xI
N
. This limits the
scope of inverse curves with high pick-up settings. See chapter
2.14 for more information.