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How does the second harmonic (H2) restraint filter differentiate between the short circuit current and inrush current based only on the second harmonic content?

The waveform from a typical fault in an AC System as defined by the IEC60909 waveform does have some second harmonic (H2) content; however, this should not cause unwanted activation of H2 Restraint for the following reasons:

 a) With the IEC60909 waveform, there is still a strong fundamental A.C. component to the fault current.  Even with "worst case" levels of the initial D.C. component, that would not be enough to activate the H2 Restraint.

 b) The Sepam H2 Restraint function calculates the percentage of H2 with respect to the H1 magnitude.  If we go back to the transformer inrush waveform, the waveform is not symmetrical in the sense that it is almost "missing some half-cycles."  It is this characteristic of the waveform that leads to high H2 levels.  The decaying D.C. component mentioned in IEC60909 does not affect the H2 percentage as much, relatively speaking.

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