This paper presents a variable reactance type series compensator with pulse-width modulation (PWM) capability. The scheme consists of force-commutated switches (GTOs), in a bi-directional configuration, to bypass or insert a series capacitor into the longitudinal ac power transmission system. The switches are turned on at zero voltage and turned off at a variable time in the ac cycle, which provides a means for continuously controlling the equivalent reactance of the series compensator. No low order harmonics are generated since the force-commutated switches can operate at frequencies higher than that of the ac mains. The basic operating principles and performance characteristics of the PWM controlled series capacitor are described here. The physical background of the basic difference between PWM controlled series capacitor (CSC) and a static synchronous series compensator (SSSC) in terms of power flow control are also presented. The developed theory has been simulated for a typical longitudinal transmission system of an urban metropolis of this country using digital computer.
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