A Variable Gain Discrete Time Sliding Mode Observer for MMC Capacitor Voltage Estimation

Modular Multilevel Converter (MMC) is one of the main development directions in the future medium and low voltage DC transmission and distribution field. In engineering, capacitive voltage sensors are usually configured for all submodules to ensure high reliability measurement, but this makes the hardware structure more complex. This paper proposes a control algorithm for online adjustment of observer gain based on discrete-time sliding mode observer (DTSMO). A weighted error judgment energy function is defined and three modes are distinguished. A transition mode is introduced between the approaching mode and sliding mode, and the gain within each mode is adjusted online according to the size of each component error function. This algorithm suppresses the chattering of traditional DTSMO during parameter changes and reduces the voltage error of each observed SM, this observer can eliminate all capacitive voltage sensors within a single bridge arm, and is designed based on the consideration of uncertain parameters outside of MMC. Its stability and convergence have been mathematically proven. Finally, simulation and experimental results of hardware prototypes has verified that the proposed VDTSMO observer has good steady-state and transient performance.