Flexible AC Transmission Systems (FACTS) have been conceived to increase the transmissibility of electric power without the necessity of building new transmission iines. An important family of controllers proposed for FACTS are based on high power solidstate switches, such as the gate-tuni-off thyristor (GTO), which is presently rated at 6kV, 6kA. The FACTS controlier operates at the transmission voltage, in the order of 5ûûkV, and it rnust be mted at hundreds of MVA. The research of this thesis contributes in developing the rnethodologies necessary for the integration of the sotid-state switches so that the FACTS controllers have the voltage withstand and the cwent carrying capacity. The rnethodologies followed in the thesis are based on organizing the solid-state switches firstly in the forrns of: (a) 2-level, voltage-source converter modules or (b) multilevel, voltage-source, converter modules. Thereafter, the modules are connectai in series and/or in parallel. Switching strategies such as: (i) the phase-shifted triangle can-ier technique and (5) the fundamental fiequency switching, are show to be proniising. The thesis addresses the methodologies of unithg the modules: (i) in parallel, (ii) in series and (Üi) in @el groups of series-comected modules. Senes connecti011~ require magnetic transformers. Transforrners also enable 6-phase and 12-phase ac systems to be produced from the standard 3-phase ac utility supply. The thesis has initiated research in incorporating the multi-phase ac syst ems to the multi-rnodular , dti-level convert ers. The thesis has also developed the interface methodology for digital (digital signal ,orocessor, DSP) control of the amy of modules in rd-the. The methodologies of the thesis have ken proven in digital simu1ations and experimentally in laboratory models.