A Hybrid Observer for Cell Voltage Estimation of Modular Multilevel Converters Without Internal Voltage Measurements

The modular multilevel converter (MMC) has become a famous topology for high-power applications due to its modularity, scalability, and superior output waveform quality. For the MMC to work, all cell voltages must be known. Therefore, voltage sensors are installed in each cell. This results in a high number of voltage sensors, which complicates the system, increases costs, and reduces reliability. Observers can be used to estimate the required voltages, thereby reducing the number of sensors. This paper proposes a hybrid observer (HO) approach for estimating cell voltages in modular multilevel converters without the use of internal voltage sensors in each cell. Instead, solely the internal cluster currents, the DC-link voltage, and the grid voltages are measured. The design is based on six hybrid observers running in parallel, implemented on a field-programmable gate array (FPGA) to achieve fast execution times of 1$\mu$s. An adaptive observer (AO), implemented on a digital signal processor (DSP), completes the design and estimates the sum capacitor voltages required for the outer energy control loop of the MMC, and additionally provides a common-mode voltage estimate to the hybrid observers. The observer design is theoretically derived, and the proper functionality is validated experimentally under steady-state and transient conditions. Also, parameter mismatches are considered to demonstrate the robustness of the proposed observer approach.