The transfer function of a system is the ratio of Laplace transform of the output and input functions. You have to define the input and the output functions.
Transfer function cannot be defined if the input or the output function is a complex (nonlinear function) !
For starters, read the chapter on Power System Loads from the book Power System Stability and Control (McGraw Hill) by P. Kundur. There are cross references to that chapter that you can follow up thereafter.