2.8 Earth Fault Protection in Earthed Systems (optional)
207
7SD5 Manual
C53000-G1176-C169-1
2.8
Earth Fault Protection in Earthed Systems (optional)
The 7SD5 line protection features protection functions for high-resistance earth faults
in earthed power systems. These options are available — depending on the ordered
model:
– Three overcurrent stages with definite time tripping characteristic (definite time),
– One overcurrent stage with inverse time characteristic (IDMT) or
– One zero sequence voltage stage with inverse time characteristic
– One zero sequence power stage with inverse time characteristic
The elements may be configured independently from each other and combined ac-
cording to the user's requirements. If the fourth current-, voltage or power-dependent
stage is not required, it may be employed as a fourth definite time stage.
Each stage may be set to be non-directional or directional — forward or reverse. A
signal transmission may be combined with these four stages. For each stage it may
be determined if it should coordinate with the teleprotection function. If the protection
is applied in the proximity of transformers, an inrush restraint can be activated. Fur-
thermore, blocking by external criteria is possible via binary inputs (e.g. for reverse in-
terlocking or external automatic reclosure). During energization of the protected
feeder onto a dead fault it is also possible to release any stage, or also several, for
non-delayed tripping. Stages that are not required, are set inactive.
In the line protection 7SD5, the distance protection function (order option) can be sup-
plemented by the earth fault protection function. In the case of short-circuits with high
fault resistances, the fault detection of the distance protection often does not pick up
because the measured impedance is outside the fault detection characteristic of the
distance protection. High fault resistances can be found, for instance, in overhead
lines without earth wire or in sandy soil.
2.8.1
Method of Operation
Measured
Quantities
The zero-sequence current is used as measured variable. According to its definition
equation it is obtained from the sum of the three phase currents, i.e.
3·
I
0
=
I
L1
+
I
L2
+
I
L3
. Depending on the version ordered, and the configured application
for the fourth current input
I
4
of the device, the zero-sequence current can be mea-
sured or calculated.
If the input
I
4
is connected in the starpoint of the set of current transformers or to a
separate earth current transformer, on the protected feeder, the earth current is direct-
ly available as a measured value.
If the device is fitted with the highly sensitive current input for
I
4
, this current
I
4
is used
with the factor
I4/Iph CT
(address
221
, refer to Section 2.1.2.1). As the linear range
of this measuring input is severely restricted in the high range, this current is only eval-
uated up to an amplitude of approx. 1.6A. In the event of larger currents, the device
automatically switches over to the evaluation of the zero sequence current derived
from the phase currents. Naturally, all three phase currents obtained from a set of
three star-connected current transformers must be available and connected to the
device. The processing of the earth current is then also possible if very small as well
as large earth fault currents may occur.
If the fourth current input
I
4
is otherwise utilized, e.g. for a transformer starpoint current
or for the earth current of a parallel line, the device calculates the zero-sequence
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