Earth Fault Overcurrent Protection In Earthed Systems (Optional) - Siemens SIPROTEC 7SA522 Manual

2.7

Earth fault overcurrent protection in earthed systems (optional)

2.7.1 Functional Description
Measured Quanti-
ties
7SA522 Manual
C53000-G1176-C155-3
2.7 Earth fault overcurrent protection in earthed systems (optional)
In earthed systems, where extremely large fault resistances may exist during earth
faults (e.g. overhead lines without earth wire, sandy soil) the fault detection of the dis-
tance protection will often not pick up because the resulting earth fault impedance
could be outside the fault detection characteristic of the distance protection.
The 7SA522 distance 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 also 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 stabilization 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.
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 = I + I + I . Depending on the version ordered, and the configured application
0 L1 L2 L3
for the fourth current input I
sured or calculated.
If the input I is connected in the starpoint of the set of current transformers or to a sep-
4
arate earth current transformer, on the protected feeder, the earth current is directly
available as a measured value.
If the device is fitted with the highly sensitive current input for I
with the factor ,,SK &7(address , refer to Subsection 2.1.3.1). As the linear
range of this measuring input is severely restricted in the high range, this current is
only evaluated up to an amplitude of approx. 1.6 A. 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
or for the earth current of a parallel line, the device calculates the zero-sequence
current from the phase currents. Naturally in this case also all three phase currents
of the device, the zero-sequence current can be mea-
4
is otherwise utilized, e.g. for a transformer starpoint current
4
, this current I is used
4 4
151