side is converted to vector group 0. Since you are in the 3-phase system, the conversion occurs via matrices
which are stored for the respective vector-group type. The general description is given in the following
formula. In the same calculation step, the absolute value is corrected:
[foscha08-170712-01.tif, 1, en_US]
with
i
phsx
* Adapted instantaneous value per phase (x = A, B, C)
i
phsx
Instantaneous value per phase (x = A, B, C)
k Absolute-value correction per measuring point
M
SG
Correction matrix
I-DIFF Differential Protection Function
A differential current per phase based on the instantaneous value variables is calculated from the processed
currents (see section Measurand Processing). The direction of current flow is defined as to the protected
object as positive. The fundamental component contribution to the differential current (I
diff
) is calculated via
a Fourier filter (filter length = 1 period (20 sampled values)).
The currents of that measuring point whose RMS value is the largest are used as restraint current. If, for
example, the current collapses at a measuring point as a consequence of current-transformer saturation, the
measuring point that has the largest current is automatically switched to at the point in time of the calcula-
tion.
NOTE
Differential protection does not work with sensitive ground-current transformers. Since sensitive trans-
formers can go into saturation too quickly, false differential currents arise.
[fogrds16-170712-01.tif, 1, en_US]
with
x
Variable (1, 2, 3) for the phase currents (A, B, C)
m Variable (1, 2, ..., N) for the measuring points
N Number of measuring points
n Number of samples per period (n = 20)
FIR Fourier filter
MAX Maximum value determination
Pickup Characteristic
Figure 7-51 shows the pickup characteristic of the differential protection. The characteristic curve branch a
represents the sensitivity threshold of the differential protection (parameter Threshold ) and considers
constant fault currents like magnetizing currents.
Capacitor Bank Protection
7.8 Differential Protection for Capacitor Banks
SIPROTEC 5, Overcurrent Protection, Manual 1049
C53000-G5040-C017-8, Edition 07.2017
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