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FAQs: Modelling Zig-Zag Earthing Transformers in PTW
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 Neutral Earthing Resistor (NER) on the primary side is selected to give the correct fault current.

To create an earthing transformer in PTW, add a two-winding transformer component to your SLD.  Using the component editor, select the transformer type "Grounding Zig-Zag" (see screenshot, below).  PTW will automatically create the dummy bus; users can then enter the NER size (expressed as "Neutral Impedance") and transformer impedance (expressed as "zero-sequence impedance") and rated voltage as required.  PTW also has an option to calculate the required NER size based on the desired full-load current (in this case, the full-load current is actually the earth fault current). 
(Above): The Component Editor view of a two-winding transformer.  The option on the bottom right allows users to select the transformer type "Grounding Zig-Zag".  When this option is selected, the "Bus Connection" will automatically assign a new dummy bus in the "To:" field. 
Transformer and NER impedances can be entered in the "Transformer Impedance" subview; note that the "Rated Voltage" and "Nominal kVA" fields must be filled in.