I have a PV Solar system with 1.2MW Capacity connected with Grid & Genset at a same time. Total running load is 2.5MW. Solar provide 700~900KW, 900KW is on Diesel Genset & remaining load is provided from National Grid. All the sources are running in parallel. Issue is that when ever the Grid supply fail the solar goes to 0KW although Diesel Genset is remains in system. How can i avoid solar to go to zero load. although if I restart the solar with Diesel Genset running only it provide maximum available load.

I'm I reading this correctly 1.2MW??? 🤯 How many panels do you have?

Check your inverter manual and settings.  Inverter(s) most likely requires batteries in order to run in Off-Grid mode

Even with the gird and generator running in parallel, if the grid fails there will definitely be some transient that may cause the Inverters to disconnect.

Why they do not reconnect to the generator though I do not know. Maybe some more info such as the make and model of the inverters and any accompanying hardware.

For example do you have something that prevents grid feed-in? If so, it may be possible that when the grid fails, comms to the grid feed-in protection fails (or just reads 0 load) causing the inverters to stop production. Again why it would work if started on the generator alone, I don't know without more info.