From my calculations I arrived at a non-linear equation :
$$ C\frac{dV_{dc}}{dt} = 3e_q\frac{I_q}{2}V_{dc} - \frac{V_{dc}}{R_{dc}} $$
I have calculated the operating points which are $I_q = 9.58A$, $V_{dc}=400V$ and $R_{dc} = 65.6$ but I got stuck on the next step to find the transfer function of this equation .