You are designing a solid state transformer with your project (fllexible electronic transformer) using AC/DC conversion, DC/DC up/down conversion, and DC/AC inversion, is that right?
You did not mention what maximal % of output ripples are permissible.
There's a general tradeoff between the quality of the output signal (voltage/current) and quantity, size of the switching and storage elements (costly).
Smoothness of the output signal, to say minimum voltage or current ripples, depends a lot on your specific design. As mentioned above, a 12 or 24 pulse for the inverter provides you much smoother output ripples, 6 pulse is rougher ripples and needs further filtering if so required.
You can design in a very large expensive cap to even out your DC voltage after conversion, makes the DC voltage rocksolid - but is it even needed? The following
DC/DC converter will chop this DC power into smaller quantities to store in the LC elements. What signal quality do you need at this point? The DC/AC inverter
chops the output signal again. Then, you can use an extra filter stage or two, three or more filters. The lattter is too costly while you only want to achieve accetaple output quality. Many filtering questions can already be accommodated through a clever design of the inverter/converters.
One book I recommend is (not sure whether available outside of NA):
Mohan, Undeland and Robbins, Power Electronics: Converters, Applications, and Design, 3rd Edition, John Wiley & Sons.
Though, there are many other alternative textbooks to give a good understanding of the basic principles of switched mode power electronics.