Figure 40: The Need For Zero-Sequence Current Filtering; Magnetising Inrush Restraint - GE MiCOM P40 Agile Technical Manual

Chapter 6 - Current Differential Protection
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Figure 40: The need for zero-sequence current filtering

Where a transformer winding can pass zero sequence current to an external earth fault, it is essential that some
form of zero sequence current filtering is used. This would also be applicable where in zone earthing transformers
are used. In this product, zero sequence current filtering is automatically implemented in software when a delta
connection is set for the vector compensation.
10.3

MAGNETISING INRUSH RESTRAINT

Whenever there is an abrupt change of magnetising voltage (e.g. when a transformer is initially connected to a
source of AC voltage), there may be a substantial surge of current through the primary winding called inrush
current.
In an ideal transformer, the magnetizing current would rise to approximately twice its normal peak value as well,
generating the necessary MMF to create this higher-than-normal flux. However, most transformers are not
designed with enough of a margin between normal flux peaks and the saturation limits to avoid saturating in a
condition like this, and so the core will almost certainly saturate during this first half-cycle of voltage. During
saturation, disproportionate amounts of MMF are needed to generate magnetic flux. This means that winding
current, which creates the MMF to cause flux in the core, could rise to a value way in excess of its steady state
peak value. Furthermore, if the transformer happens to have some residual magnetism in its core at the moment
of connection to the source, the problem could be further exacerbated.
The following figure shows the magnetizing inrush phenomenon:
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