Development of a scale model of a Modular Multilevel Converters

Modular Multilevel Converters are now being introduced in the power grid. Control systems for these converters are complex with many degrees of freedom. Simulation models are useful for exploring control algorithms, but there is still a need for acquiring experience from real converters. As extensive experiments on full-scale converters are not always feasible due to the cost and potential consequences, reduced-scale models that correctly reproduce the salient characteristics of the original converters are necessary. This paper describes the development of 60 kVA scaled models of modular multilevel converters with 6 halfbridge, 12 fullbridge and 18 halfbridge cells per arm, respectively. Most parameters scale naturally, but the equivalent series resistance and therefore the per-unit losses due to load current tend to increase, giving more damping of oscillation than in the full-scale reference.