Hi,
I can't help with proteus...
But I'm not sure the voltages you show are the voltages you really want.
With a low frequency transformer the signal waveform usually is sine. And sine voltage often is often specified as "voltage RMS".
A low frequency transformer has sine_in and sine_out, thus both sides are specifued in V_RMS.
But it seems you don't want to use it as low frequency transformer.
I rather think you want to use the transformer in a circuit, where the input voltage is DC
Note: this is not the transformer input voltage .... and not given as V_RMS.
But maybe you want the output voltage 230V RMS...
And also mind that if the "12V" DC come from a battery then you need to calculate with a battery voltage range of 10.5V up to 14.4V.
Klaus