Well it depends on many different factors and conditions. First it must be identified whether it is designed in order to improve the stability or power quality. Then it depends on how much ripples and THD can be borne, which determines the approximate required capacitance, for a given dc side voltage level. And dc voltage can be designed 1.2-1.5 times the ac side voltage (in rms) as a practical and fast design. However, it is not the best design for sure.

Actually, I have recently authored an article on how to design an optimum point for dc parameters of a specific kind of statcom (APQC), which is already under review. It will soon be accepted I hope! I have obtained a cost function in that article, minimizing which will result in the optimum design of dc link parameters. I, myself, have minimized the cost function in the article using generic algorithm. By the way, it is not as simple as to choose a given value based on the system parameters. I will provide you the article as soon as decision is made.

I hope it had helped you out.

adel