Technical description
2 Protection functions
2.10 Directional earth fault protection
I0φ> (67N)
VM259.EN007
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87
Additionally the stage I
0
> have two more input signal
alternatives 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 a trip. (Here a short fault means one cycle or
more. For shorter than 1 ms transient type of intermittent
earth faults in compensated networks there is a dedicated stage
Iot> 67NT.)
When starting happens often enough, such intermittent faults
can be cleared using the intermittent time setting. 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.
Two independent stages
There are two separately adjustable stages: I
> and I
>>.
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
2.24. 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
2.24 for more information.
Setting groups
There are two settings groups available for each stage.
Switching between setting groups can be controlled by digital
inputs, virtual inputs (mimic display, communication, logic)
and manually.