Siemens siprotec 7SA6, Manual

Page: 316 / 876

Functions
6-126
7SA6 Manual
C53000-G1176-C156-2
Figure 6-73 Phase current stabilization
Inrush Stabilization If the device is applied to a transformer feeder, large inrush currents can be expected
when the transformer is energized; if the transformer star-point is earthed, also in the
zero sequence path. The inrush current may be a multiple of the rated current and flow
for several tens of milliseconds up to several minutes.
Although the fundamental current is evaluated by filtering of the measured current, an
incorrect pick-up during energization of the transformer may result if very short delay
times are set. In the rush current there is a substantial portion of fundamental current
depending on the type and size of the transformer that is being energized.
The inrush stabilization blocks tripping of all those stages for which it has been acti-
vated, for as long as the rush current is recognized.
The inrush current is characterized by a relatively large amount of second harmonic
(twice rated frequency). This second harmonic is almost non-existent in the short-cir-
cuit current. Numerical filters that carry out a Fourier analysis of the current are used
for the frequency analysis. As soon as the harmonic content is greater than the set val-
ue, the affected stage is blocked.
Direction
Determination with
Zero Sequence
System TThe direction determination is carried out with the measured current I
E
(= –3 · I
0
),
which is compared to a polarization voltage U
P
, which results from the measured
voltage U
E
(= 3 · U
0
).
The direction determination U
P
may also be carried out with the earth current I
E
and
the star-point current I
Y
of an earthed transformer (source transformer) as polarization
value I
P
(Figure 6-74) provided that the transformer is available.
It is furthermore possible to polarize with the star-point current of the transformer as
well as the zero sequence voltage U
E
(reference voltage). The reference value then
is the sum of reference voltage U
E
and a value which is proportional to reference
currents
I
Y
. This value is about 20 V for rated current.
The directional polarization using the transformer star-point current is independent of
voltage transformers and therefore also functions reliably during a fault in the voltage
transformer secondary circuit. It is however a requirement that not all but at least a
substantial amount of the earth fault current flows via the transformer, the star-point
current of which is measured.
3
I
0
I
N
I
Phmax
I
N
blocking of the
pick up
3I0>
Slope:
Iph-STAB. Slope