We have transfer functions for other generation sources like non-reheat steam turbine (1/1+st). I am looking for same kind of transfer function for Renewable sources.
Why do you consider frequency control to be part of a transfer function? It is an operating parameter. A stable system will work to maintain its operating parameters and will have enough reserve power available to do this. If your system is so stressed that it cannot maintain its set frequency then you have many other problems beyond frequency control.