CEE Relays Ltd.

87C Whitby Road, Slough, SL1 3DR

Telephone: (01753) 576477        Fax: (01753) 825661

Website: www.ceerelays.co.uk

Modelling Earthing Transformers

Referring to sequence diagrams for a zig-zag earthing transformer, it can be shown that there is no difference, from a fault current point of view, between the model of a zig-zag transformer and a star-delta transformer (as long as the delta side is not loaded).

In PTW, an earthing transformer is therefore modelled as a star-earth/delta transformer with the delta transformer connected to a dummy bus with no loads attached. The NER on the primary side is selected to give the correct fault current.