Multipulse controlled ac-dc converters for harmonic mitigation and reactive power management

Multipulse thyristor converters are being employed in industrial applications to control output current/voltage through adjustment of the thyristors´ triggering angle. For reduced loading conditions, the triggering angle may be raised resulting in low power factor and a high total harmonic distortion (THD) in the source current. In addition, under light loads, overall system efficiency is reduced with rectifiers operated below capacity, and continuous operation of these rectifiers and switching devices reduces their life expectancy. An attractive alternative to meet reduced load conditions while maintaining improved power quality, system efficiency, reliability and device lifespan is proposed. The performance of multipulse thyristor converters is investigated with the proposed control scheme, which involves simultaneous and/or selective control of the thyristor triggering angle and converter pulse number. For pulse control, an appropriate rectifier or pair of rectifiers is disabled or enabled from the high current load depending on the current loading conditions. An 18-pulse parallel thyristor converter is selected to verify the proposed technique. The mathematical model of the 18-pulse thyristor converter is derived and the corresponding simulation model is developed to analyse the performance of the proposed control technique in terms of the above-mentioned criteria.