2.7 Earth fault protection I0>
(50N/51N)
2 Protection functions
Technical description
64
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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.
Input I
02
for all networks other than rigidly earthed.
Calculated signal I
0Calc
for rigidly and low impedance
earthed networks. I
0Calc
= I
L1
+ I
L2
+ I
L3
.
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
.
I
02Peak
to measure the peak value of input I
02
.
Intermittent earth fault detection
Short earth faults make the protection to start (pick up), but
will not cause trip. 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. By using input signals
I
01Peak
or I
02Peak
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.
Four 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
(IDMT). The other stages have definite time operation
characteristic. By using the definite delay type and setting the
delay to its minimum, an instantaneous (ANSI 50N) operation
is obtained.
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