Design of state observer for modular multilevel converter

Modular multilevel converter (MMC) is an attractive choice for transformer-less and filter-less high power conversion system due to its ability to upscale to high voltage levels by the use of semiconductors. The capacitor voltages on each converter cell are important variables to be monitored not only for the proper control of MMC but also for the fault detection and protection. At the same time, a series of problems can arise by the use of a large number of sensors to measure the capacitor voltages such as high components counts, complex implementation, and comprised reliability of the MMC system. In order to decrease the number of sensors, lower the cost and improve the reliability, a state observer is proposed in this paper to estimate the capacitor voltages by the measurement of the output currents. First a state space model of the MMC system is developed, and then a closed-loop state observer is established to obtain the capacitor voltages. In this paper the observability of the MMC system is displayed and the calculation of the gain matrix is also described detailedly. No additional sensor is required by this technique and it can easily be implemented in a DSP or microcontroller. The operation and robustness of the proposed state observer are confirmed by the simulation results under different loading conditions, even under typical AC grid faults.