Siemens siprotec 7SA522 User Manual

2.2 Distance protection
63
7SA522 Manual
C53000-G1176-C155-3
Negative Sequence
Current 3I
2
On long, heavily loaded lines, the earth current measurement could be overstabilized
by large currents (ref. Figure 2-7). To ensure secure detection of earth faults in this
case, a negative sequence comparison stage is additionally provided. In the event of
a single-phase fault, the negative sequence current I
2
has approximately the same
magnitude as the zero sequence current I
0
. When the ratio zero sequence cur-
rent/negative sequence current exceeds a preset ratio, this stage picks up. It is also
stabilized in the event of large negative sequence currents by a parabolic characteris-
tic. Figure 2-8 illustrates this relationship. A release by means of the negative se-
quence current comparison requires a current of at least 0.2 I
N
for 3I
0
and 3I
2
.
Figure 2-8 Characteristic of the I
0
/I
2
stage
Displacement
Voltage 3U
0
For the neutral displacement voltage recognition the displacement voltage (3·U
0
) is
numerically filtered and the fundamental frequency is monitored to recognize whether
it exceeds the set threshold. The dropout threshold is approximately 95% of the pickup
threshold. In earthed networks (8!7KUHVKROG) it may be applied as additional
earth fault criterion. For earthed systems, the U
0
criterion may be disabled by applying
the ∞ setting.
Logical Combina-
tion for Earthed
Systems
The current and voltage criteria supplement each other, as the displacement voltage
increases when the zero sequence to positive sequence impedance ratio is large,
whereas the earth current increases when the zero sequence to positive sequence im-
pedance ratio is smaller. The current and voltage criteria are logically combined with
an OR-function for earthed systems. It is however also possible to establish an AND
gate of the two criteria (see Figure 2-9). If detection of the displacement voltage has
been disabled by setting 8!7KUHVKROG to infinite, earth fault detection using the
current criterion is still possible in case of CT saturation.
If the relay detects current transformer saturation in any of the phase currents, the
voltage criterion is however an indispensable prerequisite for detection of an earth
fault since unequal current transformer saturation may cause a faulted secondary
zero-sequence current without a primary zero-sequence current flowing in the first
place.

Siemens siprotec 7SA522 Specifications

General

Brand	Siemens
Model	siprotec 7SA522
Category	Protection Device
Language	English