DC Capacitor rating calculation

It depends on the capacitor technology and the ripple current. If you use electrolytic capacitors, you will need at least twice as much capacitance as film type to handle the ripple current.

The ripple current is composed of two parts.

1-The relatively low frequency at 6X the line frequency for a 3 phases supply. This can be evaluated from charts available in power electronics books.

2- The switching ripple produced by the IGBT chopper. This depends on the switching technique used and the circuit configuration. Look into a good power electronics books for details.

This analysis is sometimes difficult to do and some people will simply measure the current ripple into the capacitors and add more capacitance if the ripple exceeds what is specified in the capacitor data sheet.

Note that the more capacitance you add, the bigger the explosion you will get if your IGBT fails. This is not a joke. An exploding IGBT can propel debris at high speed. Make sure that you have some sort of cover above the assembly at all time. High voltage capacitor banks are also dangerous as a simple bad maneuver around the busbars can ignite an arc that will melt metal and sounds like a shotgun. This is dangerous for the eyes and the ears.

Note: If you are trying to rate the DC capacitors in KVA, you need much more training in power electronics before undertaking this work.

Be careful. You might get a really bad "fireball".