2.11 Earth fault protection I0>
(50N/51N)
2 Protection functions
Technical description
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Figure 2.11-2. Block diagram of the earth fault stages I
0
>>, I
0
>>> and
I
0
>>>>
Figure 2.11-1 shows a functional block diagram of the I
0
> earth
overcurrent stage with definite time and inverse time operation
time. Figure 2.11-2 shows a functional block diagram of the
I
0
>>, I
0
>>> and I
0
>>>> earth fault stages with definite time
operation delay.
Input signal selection
Each stage can be connected to supervise any of the following
inputs and signals:
Input I
01
for all networks other than rigidly earthed.
From the phase currents calculated signal I
0Calc
for rigidly
and low impedance earthed networks. I
0Calc
= I
L1
+ I
L2
+ I
L3
.
Additionally the stage I
0
> have two more input signal
alternatives to measure current peaks to detect a restriking
intermittent earth fault:
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.
Four or six independent undirectional earth fault overcurrent
stages
There are four separately adjustable earth fault stages: I
0
>,
I
0
>>, I
0
>>>, and I
0
>>>>. The first stage I
0
> can be configured
for definite time (DT) or inverse time operation characteristic