Siemens siprotec 7SA6 User Manual

Functions
6-29
7SA6 Manual
C53000-G1176-C156-2
6.2 Distance Protection
Distance protection is the main function of the device. It is characterized by high meas-
uring accuracy and the ability to adapt to the given system conditions. It is supplement-
ed by a number of additional functions.
6.2.1 Earth Fault Recognition
6.2.1.1 Method of Operation
Recognition of an earth fault is an important element in identifying the type of fault, as
the determination of the valid loops for measurement of the fault distance and the
shape of the distance zone characteristics substantially depend on whether the fault
at hand is an earth fault or not. The 7SA6 has a stabilised earth current measurement,
a zero sequence current/negative sequence current comparison as well as a displace-
ment voltage measurement. Furthermore, special measures are taken to avoid a pick-
up for low-current earth faults in an isolated or resonant-earthed system.
Earth Current 3·I
0
For earth current measurement, the fundamental sum of the numerically filtered phase
currents 3·I
0
is monitored to detect if it exceeds the set value (parameter 3I0>
Threshold, address 1203). It is stabilized against over-operation resulting from un-
symmetrical operating currents and error currents in the secondary circuits of the cur-
rent transformers due to different degrees of current transformer saturation during
short-circuits without earth: the actual pick-up threshold automatically increases as the
phase current increases (Figure 6-15). The reset value is approximately 95 % relative
to the pick-up value.
Figure 6-15 Earth current stage: pick-up characteristic — example
10 x 3I0
I
N
3I0
I
N
I
Ph max
slope
0,1
0,95·3I0>
3I0>
block
release

Siemens siprotec 7SA6 Specifications

General

Brand	Siemens
Model	siprotec 7SA6
Category	Relays
Language	English