A Cascaded Control Concept for Modular Multilevel Converters with Capacitor Voltage Estimation using a Kalman Filter

An approach to estimate the submodule capacitor voltages in a Modular Multilevel Converter (MMC) is presented and realized in a hardware test setup. One disadvantage of the MMC topology is the high communication and measurement effort. As the submodule voltages are being estimated, the measurement effort as well as the communication effort between the submodules and the main controller hardware is significantly reduced. Thereby, the reliability of the system also increases. In the hardware test setup, it is verified that using a notch filter in the feedback path of the energy control loop significantly decreases unwanted AC components of the circulating currents within the MMC. Subsequently the current stress as well as the conduction and switching losses in the semiconductors can be minimized. Furthermore, the dynamic performance of the capacitor voltage estimation and control scheme was analyzed via hardware tests.