bi-directional dc-dc converter transfer function and controller design

[nuflia]

I’m doing a hybrid power generation project, a part of my work is a bi-directional dc-dc converter that charges and discharges a battery to keep the dc-bus voltage constant 380-420, the battery is 96vdc, I’ve made the Simulink model that works well, the problem is that my bi-directional dc-dc converter does not keep the voltage stable, i know it is due to the modeling of dc-dc converter that I am not able to design the optimum and best PI controllers( voltage and current controllers)
I appreciate any idea or recommendation on how to model the bi-directional dc-dc converter to find the suitable TF and then design the current and voltage controllers that will charge / discharge the battery to keep the DC-bus volatge stable