There are various possibilities to ground the star point of transformers. Generally, the grounding is defined on the Basic Data page => Grounding / Neutral Conductor. The following options are available here:

• ground the transformer directly in the element. Here you can define the R and X of the grounding / star point impedance directly in the transformer element
• ground the transformer via the substation grounding. Here you connect the transformer to the substation neutral which is grounded e.g. by an impedance
• ground the transformer via a separate terminal. Here you connect the transformer star point to a separate single phase terminal. The single phase terminal can then be grounded by a corresponding element

The attached PowerFactory project “Transformer grounding.pfd” provides examples of the different grounding options in PowerFactory.