Reliability Evaluation of Voltage Source Converters for MVDC Applications

The emerging Medium Voltage Direct Current (MVDC) distribution networks are becoming more attractive due to their flexible control of power flow and lower losses compared to traditional AC networks. This will significantly increase the wide uptake of renewable energy sources. Voltage source converters (VSCs) are the main building blocks of modern DC systems. Achieving a high reliability is an important aspect of selecting the converters for a specific project. However, it is still unclear which type of VSC is the most suitable solution for MVDC systems. This paper addresses this question from the reliability point of view. Three commonly used VSCs are considered for the analysis namely, two-level (2L), three-level NPC (3L-NPC), modular multi-level (MMC) converters and the cascaded three-level VSC which is currently being built for the first MVDC link in the UK. The results suggest that during the early stage of operation MMC shows a higher reliability. However, in the long run the other VSCs have shown higher reliabilities.