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Earthing Transformers
Earthing Transformer are used to create a neutral point in a three phase system, which provides possibility for neutral earthing. The earthing can be through an arc-suppression reactor, a neutral earthing reactor or resistor or directly.
Can be produce the following types:
Oil-type: up to 123 kV (BIL: 450 kV class)
Dry-type: up to 36 kV (BIL: 170 kV class)
With auxiliary winding on request.
The design can be a transformer with just one winding, which is zig-zag connected. The zero sequence impedance of such a winding is normally quite low, but it can be increased if the purpose of the is to limit the current through the transformer in case of an earth fault somewhere in the power system.
An alternative connection to zig-zag is star/delta connection where the delta connected winding will compensate the zero sequence magnetic field so it will be confined to a leakage field between the star and the delta winding and make the zero sequence impedance of the transformer relatively small. It is possible to provide the earthing transformer with a secondary winding for continuous auxiliary station supply.
When the earthing transformer is going to be used together with an arc-suppression reactor, the rated current (and its duration) of the earthing transformer will be determined by the data for the arc-surpression reactor.
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