Operate solar farm and supply the load through alternative circuit

Case: 20 MW solar farm is connected to grid at 66 KV or 110 KV, suddenly there is grid failure. Is it possible to operate solar farm and supply the load through alternative circuit(Island mode)? if YES how?

You're looking at a large scale science fair project, since the inverters by listing and code require grid time base and voltage to be within limits before they operate - modification of this would be dangerous. You could theoretically construct a means to switch the PV array output off the grid and to provide these standards to "fool" the inverters into switching back on, but at the power level you're talking about it would not be an insignificant expense in engineering and hardware. Grid intertied solar systems are really not meant to be used for standalone power; they were developed to "cheaply" offset site grid energy costs using the local site and/or grid as their "load." To try to convert one into an off-grid source means you will be fighting the entire design concept implemented.