From the Help in that transformer data box (as trobbins already linked):
"Edit Transformer Properties
"Enter the voltage for the secondary of the transformer. Note that this must be the off-load voltage, which is generally higher than the rated voltage of the transformer.
"For full wave rectifiers, use the voltage for one side only. For example, if the transformer off load voltage is 350-0-350V (700V end to end), the correct value to enter is 350.
"Enter the source impedance of the transformer. If you don't know this, you can click on the [...] buttons to invoke the off load voltage calculator, or source impedance calculator respectively."