How can find a transfer function of the model in base paper

the title is "Development of a Nonlinear Control Strategy for the DC Power Transferring System"

can anybody solve it?

[Azzi Abdelmalek]

Can you explain what is your problem?

Answer 1

[Joel Van Sickel]

Use frequency response estimation if the system cannot be linearized analytically.

https://www.mathworks.com/help/slcontrol/frequency-response-estimation.html

You can then use system identificaiton to fit this frequency response to a transfer function.