2.11 InRush Restraint
121
7ST6 Manual
E50417-G1176-C251-A3
Figure 2-36
Logic diagram of the inrush restraint
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 harmonics content exceeds the value set in
2nd HARM.BLOCK
, an
inrush current is detected.
With the parameter
Imax InrushRest
a maximum current amplitude for using this
criterion can be specified. Above this current amplitude, the detection of inrush cur-
rents on the basis of the second harmonics is blocked. This blocking prevents that the
inrush detection feature responds in case of transformer saturation since transformer
saturation, also, causes a large proportion of second harmonics in the current signal.
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