Technical description
2 Protection functions
2.14 Intermittent transient earth fault
protection I0T> (67NT)
VM50.EN004
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75
transient stage I
0T
> in an outgoing feeder can be used to block
the U
0
> backup protection.
Co-ordination with the normal directional earth fault
protection based on fundamental frequency signals
The intermittent transient earth fault protection stage I
0T
>
should always be used together with the normal directional
earth fault protection stages I
0
>, I
0
>>. The transient stage
I
0T
> may in worst case detect the start of a steady earth fault in
wrong direction, but will not trip because the peak value of a
steady state sine wave I
0
signal must also exceed the
corresponding base frequency component's peak value in order
to make the I
0T
> to trip.
The operation time and U
0
setting of the transient stage I
0T
>
should be higher than the settings of any I
0
> stage to avoid
any unnecessary and possible incorrect start signals from the
I
0T
> stage.
Auto reclosing
The start signal of any I
0
> stage initiating auto reclosing (AR)
can be used to block the I
0T
> stage to avoid the I
0T
> stage with
a long intermittent setting to interfere with the AR cycle in the
middle of discrimination time.
Usually the I
0T
> stage itself is not used to initiate any AR. For
transient faults the AR will not help, because the fault
phenomena itself already includes repeating self extinguishing.
Intermittent time
Single transient faults make the protection to pick up, but will
not cause trip if the stage has time to release between to
successive faults. When starting happens often enough, such
intermittent faults can be cleared using the intermittent time
setting.
When a new fault happens within the set intermittent time, the
operation delay counter is not cleared between adjacent faults
and finally the stage will trip. A single transient fault is
enough to start the stage and increase the delay counter by 20
ms. For example if the operating time is 140 ms, and the time
between two peaks does not exceed the intermittent time
setting, then the seventh peak will cause a trip (Figure 2.14-3).
Operation time setting and the actual operation time
When the algorithm detects the direction of the fault outwards
from the bus, the stage picks up and the operation delay
counter is incremented with 20 ms and a start signal is issued.
If the time between successive faults is less than 40 ms, a trip
signal is issued when the operation time is full.