There can be several methods for representing a DGU inverter model as a state space system. However, I'm looking for a particular kind of representation that is used to describe non linear models

usually its like this

x = f(x) + k(x)D+ g(x)u

y= h(x)

has been used by mostly by authros Ali Bidram, Ali davoudi, F.Lewis et all. in their papers.

(i have attached one paper)

whereas the standard form I have used before is

x= Ax +Bu

y= Cx

I am trying to find what the matrices f(x) , k(x), g(x) and h(x) can be. One mapping method is given in "non-linear Systems" by Hassan. K. Khalil. but Im not sure if the results are accurate about what I want.

it says f(x)= Ax, g(x) = B, h(x)= Cx for a special case where the system is considered linear.

The problem is, I cant find any details regarding this, almost anywhere in the associated literature