Inverters [ For increasing supply frequency 50Hz->100Hz or 150Hz ]. I am intrested in using these for welding transformers having ratings 25KVA to 300KVA.

I may be wrong here (I know power transformers, but not the pecularities of welding transformers), read the plate on the welding transformer, it will probably state 50-60HZ.
If the welding transformer is made for 50-60HZ, this is not a good Idea, a power transformer will be made for the frequency of the supply. It will probably burn under heavy load if you feed it 3* the nominal frequency.

Erik