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The increasing importance of cascaded multilevel converters (CMCs), and the sub-category of modular multilevel converters (MMCs), is illustrated by their wide use in high voltage DC connections and in static compensators. Research is being undertaken into the use of these complex pieces of hardware and software for a variety of grid support services, on top of fundamental frequency power injection, requiring improved control for non-traditional duties. To validate these results, small-scale laboratory hardware prototypes are often required. Such systems have been built by many research teams around the globe and are also increasingly commercially available. Few publications go into detail on the construction options for prototype CMCs, and there is a lack of information on both design considerations and lessons learned from the build process, which will hinder research and the best application of these important units. This paper reviews options, gives key examples from leading research teams, and summarizes knowledge gained in the development of test rigs to clarify design considerations when constructing laboratory-scale CMCs.

Many simplified modular multilevel converter prototypes have been developed over the past few years, typically in order to demonstrate high level control schemes. Many of these prototypes are based on conventional centralised control strategies and are therefore unable to address concerns regarding scalability and the limitations imposed by hardware delays when using distributed control strategies. This paper presents an overview of common control architectures along with the design, construction and initial findings from industrially representative control hardware.

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