PWM AC controller-based phase-shifters

This paper discusses the use of a practical force-commutated AC controller in PWM phase-shifters. The operating principles are presented for a basic structure, which is then modified to allow implementation with the currently available GTOs. This high power version of the PWM phase-shifter employs a multi-module AC controller to attain the high voltage levels required for operation at transmission system levels. Moreover, the individual modules are controlled with phase shifted carriers in order to achieve harmonic cancellation. Low distortion waveforms are thus obtained with GTOs switching at low frequencies, resulting in reduced switching losses. A design procedure to calculate the required number of PWM AC controller modules so as to meet given harmonic constraints without filters is presented. The proposed concept is verified with EMTDC-based simulation results