Earth Fault Protection In Earthed Systems; Method Of Operation - Siemens siprotec 7SA6 User Manual

Functions
6.5

Earth Fault Protection in Earthed Systems

General
6.5.1

Method of Operation

Measured Value
6-102
In earthed systems, where extremely large fault resistance may exist during earth
faults (e.g. overhead lines without earth wire, sandy soil, or high tower footing
resistance) the fault detection of the distance protection will often not pick up because
the resulting earth fault impedance could be outside the fault detection characteristic
of the distance protection.
The distance protection 7SA6 has protection functions for such high resistance earth
faults. These options are available:
• three overcurrent stages with definite time tripping characteristic (definite time),
• one overcurrent time stage with inverse time characteristic (IDMT) or
• one zero sequence voltage stage with inverse time characteristic
These four stages are independent of each other and are freely combinable. If the
inverse time 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 signal transmission (refer also to Section
6.6). If the protection is applied in the proximity of transformers, an inrush stabilization
can be activated. Furthermore, blocking by external criteria is possible via binary
inputs (e.g. for reverse interlocking or external automatic reclosure). During
energization of the protected feeder onto a short circuit, any stage — or several stages
— may be switched to undelayed tripping. If a stage is not required it can be
deactivated.
The earth current is the negative sum of the three phase currents, i.o.w. I
–(I + I + I ). Depending on the version ordered, and the configured application
L1 L2 L3
for the fourth current input I
calculated.
If the input I is connected in the starpoint of the set of current transformers or to a
4
separate 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 matching factor ,,SK &7 (address , refer to Sub-section 6.1.1). As
the linear range of this measuring input is severely limited (maximum 1.6 A), this
current is only evaluated up to an amplitude of approximately 1.6 A. In the case 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
from three current transformers connected in a star arrangement must be available
and connected. This allows the processing of the earth current both when very small
and also larger earth fault currents may occur.
If the fourth current input I
or for the earth current of a parallel line, the device calculates the earth current from
the phase currents. Naturally, in this case also, all three phase currents from three
of the device, the earth current can be measured or
4
is otherwise utilized, e.g. for a transformer starpoint current
4
= –3·I =
E 0
, this current I is used
4 4
7SA6 Manual
C53000-G1176-C133-1