Development of a large-capacity high-efficiency ac/dc power conversion system complying with harmonic guidelines

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Large-capacity pulse-width-modulated (PWM) rectifiers consisting of the same power circuit topology as PWM inverters have shown promise in meeting the harmonic guidelines enacted on Oct. 3, 1994. The increased cost and switching loss caused by PWM, however, would make a high-power PWM rectifier rated at 1-10 MW economically impractical. The reason is that GTO thyristors of IGBT's used for the PWM rectifier are subjected to high-frequency switching of the full amount of active power.The authors propose a new ac/dc power conversion system providing a solution to the abovementioned problems hidden in PWM rectifiers. A small-rated series active filter for the purpose of achieving harmonic compensation is integrated with a large-rated double-series diode rectifier for the propose of performing ac/dc power conversion. An ac/dc power conversion system consisting of a 20-kVA diode rectifier and 0.2-kVA series active filter has been designed and constructed to verify the practical viability and justification by experiment. Based on experimental results obtained from the laboratory system, along with theoretical results, this research is summarized as follows.・ A delay time as short as 40μs may make the active filter unstable when the ac/dc power conversion system is installed on a "stiff" power system.・ The authors propose and evaluate a new configuration of the harmonic extraction circuit capable of eliminating the effect of delay time on the system stability.・ A switching-ripple filter is designed and tested. The effect of the filter on the system stability is also discussed theoretically and experimentally.