Estimation of sub-module capacitor voltages in modular multilevel converters

This paper presents a method for estimating the voltages at the individual sub-module capacitors of Modular Multilevel Converters (MMCs). The estimation method is designed according to a predictor-corrector scheme with a model-based prediction stage utilizing the arm current measurements and the internal modulation signals of the control system. The corrector stage is based on measurement of the total output voltage from each arm of the MMC. Thus, only two voltage measurements for each phase of the MMC must be continuously processed by the control system instead of individual voltage measurements for each sub-module. This can significantly reduce the communication requirements for the MMC control system and by that simplify the implementation. The principles of the proposed estimation method are described in detail in the paper, and the operation is demonstrated by simulation results. Estimation accuracy and settling time of the proposed method have been investigated with respect to the number of sub-modules in each arm of the MMC. Quantization effects due to limited resolution of the arm voltage sensors and the influence of parameter deviations have also been considered. The presented results illustrate the response of the proposed algorithm and indicate its potential for limiting the communication requirements and increasing system reliability by reducing the control system dependency on individual sub-module capacitor voltage measurements.