Estimation Methods and Sensor Reduction in Modular Multilevel Converters: A Review

The circuit configuration of the modular multilevel converter (MMC) requires a large number of current and voltage measurements as well as communications within the converter arms and between the arm controllers and the main control circuit. In order to reduce circuit complexity and hardware requirements with their associated cost, while adding an additional level of reliability to the operation of the MMC, a number of methods have been proposed for the estimation / observation of the various internal electrical variables or parameters of the MMC. These include estimations for sub-module capacitor voltages, dc-link voltages, arm currents or total arm energy. Such methods aim to eliminate or reduce the sensors in the circuit, reduce communication requirements or both. This paper provides an overview and classification of the different estimation/observation methods for MMCs currently proposed in the literature.