Hy martin, most of the times if you just want to study the effect over the network you can simulate the serial or shunt connected devices as voltage sources. There are many papers that deal with this kind of simple modelling, here you have some:
C. Lehmkoster, Security constrained optimal power flow for an econom- ical operation of facts-devices in liberalized energy markets, Power De- livery, IEEE Transactions on 17 (2) (2002) 603–608.
B. Fardanesh, Optimal utilization, sizing, and steady-state performance comparison of multiconverter vsc-based facts controllers, Power Delivery, IEEE Transactions on 19 (3) (2004) 1321–1327.
A. Lashkar Ara, A. Kazemi, S. Niaki, Multiobjective optimal location of facts shunt-series controllers for power system operation planning, Power Delivery, IEEE Transactions on 27 (2) (2012) 481–490.
If you do not need to develop the whole model by yourself, you can browse the MATLAB/Simulink Help. Those devices have already been modelled and some application examples are available. Depending on your needs, you can either use the detailed models (very accurate models resembling the real device that allow you to measure a good number of parameters at the expense of a higher simulation time); or the phasor type models, which are simpler (mainly based on controlled current sources) and more time-efficient.
Provided that your study focuses on the network flows and voltage control, I would rather use the phasor type models that you can find in the MATLAB/Simulink Help.