To convert the source to DC you just need an ordinary rectifier. But be sure you design it
for those 2KVA of your UPS. That's the rule for everything on this circuit. It's better to overdo it a little bit instead of destroying all.
Here is a good example (picture) how such a thing work: RECTIFIER
And for the full-bridge, there are thousands of examples out the net but 99% are in inverse direction e.g. drive
a DC motor instead of generating AC, which is just the oposite way to use this device. However I found something
interesting on the net: DSP controlled online UPS
if it's to complicated you can use the technology of a cycloconverter but build it just for one line
and not all three (or more). It works the same way.
Or have a look at brushless-motor driver for RC-helicopters or airplanes. It's also the same thing just
for smaller devices and it has 3 phases (which shouldn't be the problem)
and for the PWM have a look at Wikipedia: PWM wiki
The first picture is exactly what you want to generate.How all that stuff works:
First you get the 60Hz AC and convert it to almost DC with the rectifier. Then it will be filtered
with some capacitors and inductors to eliminate the high-frequency peaks.
After that the current flows into the DC-AC-converter. The bridge-circuit driven by PWM (see wiki)
will convert the DC current to "dirty"
AC of 50Hz and of course 220V.
If you want a proper AC you have to add some filters to the output which smooth the output wave
but I don't think that's really neccessary.