2.7 Earth fault overcurrent protection in earthed systems (optional)
159
7SA522 Manual
C53000-G1176-C155-3
bilization factor (= slope) may be changed by means of the parameter ,SK67$%
6ORSH (address ). It applies to all stages.
Figure 2-68 Phase current stabilization
Inrush Stabilization If the device is connected to a transformer feeder, large inrush currents can be expect-
ed 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 contains a relatively large second harmonic component (twice the
nominal frequency) which is nearly absent during a fault 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 value (QG,QUXVK5HVW), the affect-
ed stage is blocked.
Direction Determi-
nation with Zero
S e q u e n c e S y s t e m
The direction determination is carried out with the measured current I
E
(= –3·I
0
), which
is compared to the reference voltage U
P
(= 3·U
0
).
The voltage required for direction determination U
P
may also be calculated from the
starpoint current I
Y
of an earthed transformer (source transformer) (Figure 2-69), pro-
vided that the transformer is available.
It is furthermore possible to polarize with the zero-sequence voltage 3U
0
as well as
with the starpoint current I
Y
of a transformer. The reference value U
P
then is the sum
of the zero-sequence voltage 3U
0
and a value which is proportional to the starpoint
current 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.
The determination of direction requires a minimum current 3I
0
and a minimum dis-
placement voltage which can be set as 3U
0
>. If the displacement voltage is too small,
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