2.12 Directional earth fault protection
I0
φ
> (67N)
2 Protection functions
Technical description
64
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VM50.EN004
Sector
This mode is used with networks earthed with a small
resistance. In this context "small" means, that a fault
current may be more than the rated phase currents. The
trip area has a shape of a sector as drawn in Figure 2.12-3.
The base angle is usually set to zero degrees or slightly on
the lagging inductive side (i.e. negative angle).
Undir
This mode makes the stage equal to the undirectional stage
I
0
>. The phase angle and U
0
amplitude setting are
discarded. Only the amplitude of the selected I
0
input is
supervised.
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.
Calculated signal I
0Calc
for rigidly and low impedance
earthed networks. I
0Calc
= I
L1
+ I
L2
+ I
L3
= 3I
0
.
Additionally the stage I
0
> has one more input signal
alternative 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 trip. When starting happens often enough, such
intermittent faults can be cleared using the intermittent time
setting. The mode should be Undir. The phase angle detection
of I
0
in directional mode is insecure.
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.
Two independent stages
There are two separately adjustable stages: I
0
> and I
0
>>.
Both the stages can be configured for definite time delay (DT)
or inverse time delay operation time.
