How to model the three phase pwm rectifier?
You can use the following equations:
$$\begin{bmatrix}V_a \\ V_b \\ V_c\end{bmatrix}=R_f
\begin{bmatrix}I_{ag} \\ I_{bg} \\ I_{cg}\end{bmatrix}+L_f\dfrac{\mathrm{d}}{\mathrm{d}t}
\begin{bmatrix}I_{ag} \\ I_{bg} \\ I_{cg}\end{bmatrix}+
\begin{bmatrix}V_{ax} \\ V_{bx} \\ V_{cx}\end{bmatrix}
$$
where \$V_{[a,b,c]x}\$ are the voltages taken from the points between \$L_f\$ and the sources and referenced to ground. This will take care of the neutral point. However, I don't see anything dynamical in your equations, just the usual form for a switching bridge with series RL.