A Large-Power Voltage Source Converter for FACTS Applications Combining Three-Level Neutral-Point-Clamped Power Electronic Building Blocks

This paper presents a novel large-power voltage source converter (VSC) arrangement suitable for flexible alternating current transmission system applications, achieved by combining four three-level three-phase neutral-point-clamped inverters by means of intermediate magnetic elements with lower VA rating and size than those of similar existing installations. The 12 inverter poles forming the VSC share a common dc bus and are implemented using standard power electronic building blocks of well-proven reliability. The output voltage is doubled by using an H-bridge configuration, and only a single-secondary grid coupling transformer is required. In addition, the intermediate magnetic elements ensure an equal power sharing between the four inverters. Since they also allow for the harmonic cancellation between the inverter poles and eliminate any third harmonic component created by the modulation, the output voltage waveform has a high quality, often eliminating the need for additional filtering. In this paper, the converter components and its operation are explained, and experimental results for several modulation strategies are provided.