Simplified dynamics and control of Modular Multilevel Converter based on a terminal behavioral model

Abstract

Modular Multilevel Converters (MMCs) are emerging to be an attractive approach for high power applications. Equivalent circuit models and dynamic models for the MMC that provide a faithful representation of system behavior are quite complex given the large number of energy states and control variables. They are not particularly useful in studying the terminal behavior of the converter and for the development of an intuitive control approach to regulate power transfer. This paper reduces the complexity of the MMC analytical model to an equivalent boost-buck converter circuit while providing particularly insightful transparency of the converter's physical operation from a terminal perspective. A control approach which manages throughput power as well as the MMCs internal capacitive states based on terminal quantities is developed eliminating individual feedback of module capacitor voltages to a centralized controller. An extension of the control to mitigate the circulating currents within the MMC caused by bridge voltage harmonics is also developed. The transformed boost-buck converter model and control performance are verified using simulations and experiments using a laboratory scale prototype.