A Large Power, Low-Switching-Frequency Voltage Source Converter for FACTS Applications With Low Effects on the Transmission Line

This paper presents a novel large power voltage source converter for flexible ac transmission systems' applications that employs four three-level, three-phase neutral-point-clamped inverters, combined by means of low volt-ampere rating intermediate magnetic elements. The 12 resulting inverter poles are paralleled up in pairs by using six interphase transformers (IPTs), and their outputs are connected in series by using a simple open-wye/delta transformer, rated to 50% of the power throughput. Due to the inherent harmonic cancelation provided by the transformer and the further harmonic mitigation allowed by the IPT connection, the converter is suited for the use of various low-switching-frequency modulation strategies that obtain a clean output waveform, in some cases comparable to that of a 48-pulse converter. Moreover, the output voltage is doubled by means of the intermediate magnetic elements, which also ensure an equal power sharing between the four inverters. In this paper, the converter components and its operation are explained, and experimental results are provided.