FACTS (flexible ac transmission system) apparatuses employing many alternative configurations of power-electronic switching devices are being considered by others for increasing the power transfer capability of ac transmission lines. This paper introduces special modifications to the basic Dommel algorithm to expedite simulation of systems including arbitrary configurations of individual power-electronic switching devices. The described techniques have been implemented in a prototype transients simulation program. Two time-step sizes are used. Individual switching devices are represented according to simple characteristic curves. The modified algorithm includes iteration of a time-step when required t o provide solutions on the curves. A technique is described for removing numerical oscillations of currents in capacitive loops and voltages a t inductive nodes. Simulation results are presented for an example powerelectronic apparatus.