Power factor correction?! What kind of power is this outputting, must be quite a large inductive load to require PFC.
So considering the 400V is coming from rectified mains I'm not really sure what w2 is doing. I can only think that it's there to provide additional regulation of the 400V, perhaps the rectified 400Vdc can dip slightly due to the comparatively low frequency of the mains and w2 can help mitigate this.
Either way w3 is your offset voltage and w1 is your primary winding.
What exactly are you wanting to know about your transformer? You say you would like to know how its constructed, is that just turns ratio or are you interested in the magnetic core ( inductance, material, geometry, winding layers)
Can you post a photo of the transformer?