A Hybrid Observer Approach for Cell Voltage Estimation in Modular Multilevel Converters

Modular Multilevel Cascade Converters (MMCC) have become a well-established power electronics topology during the past decades since their invention. Among the class of MMCCs, the Modular Multilevel Converter (MMC) is widely adopted nowadays. Due to its modularity, i.e., scalability and flexibility, it is often the preferred choice among other converter topologies. However, the modularity comes with the price of many components (e.g., semiconductors, capacitors, and sensors). The controller must measure and keep every voltage within a specific range to ensure system functionality. This paper proposes a hybrid observer (HO) to estimate all system voltages needed to properly control the MMC. Only current sensors are used, and, in contrast to other methods, even the unmeasured (unknown) DC-link voltage can be estimated. The approach is theoretically derived, and its functionality is validated by high-fidelity simulation under different scenarios. The results prove the excellent estimation performance of the proposed HO.