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FAQs: Modelling Source Impedance in PTW
One of the options in PTW's LoadFlow set-up is whether to model the Source Impedance or not. If the option is not selected, then the voltage at any busbar connected directly to a swing bus source is fixed at the rated voltage, no matter what load is supplied. 
(Left): Screenshot showing the Load Flow Study setup screen.  The Source Impedance is selected or deselected using the check-boxes labelled "Include Utility Impedance" and "Include Swing Generator Impedance". One of these options may not be relevant to your project, depending on which energy sources you have modelled. 
On the other hand, if the source impedance is modelled, then PTW calculates an equivalent positive sequence impedance to be "inserted" between the swing bus source and the busbar. This equivalent impedance is based on the 3-phase short-circuit capacity of the source (either the value entered directly into the Utility component, or the xd'' of a swing-bus generator, as entered in the IEC Contribution subview). The source impedance is not calculated for PQ and PV generators. 
The voltage drop at the busbar is then calculated based on this equivalent impedance and the current being supplied by the generator to support the load on the network. 
Click here to read a discussion of the three different types of generator (SB, PV and PQ). Further discussion about Source Impedance is contained in the section on Running TMS Studies. This article should be read in conjunction with both these articles.