2.13 Earth fault protection I0>
(50N/51N)
2 Protection functions
Technical description
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VM50.EN004
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.
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
(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.
Using the directional earth fault stages (chapter 2.12) in
undirectional mode, two more stages with inverse operation
time delay are available for undirectional earth fault
protection.
Inverse operation time (I
0
> stage only)
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 the I
0
> stage.
The inverse delay types are described in chapter 2.23. 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
Setting groups
There are two settings groups available for each stage.
Switching between setting groups can be controlled by digital
inputs, virtual inputs (communication, logic) and manually.
