Fundamental frequency switching strategy of 120-unit cascaded half-bridge multilevel converter for battery management system

This paper presents a fundamental frequency switching strategy for single phase modular multilevel converter that consists of 120 cascaded battery-based half-bridge units, aiming to generate voltage with low harmonics. The strategy is based on the calculation of the Delay-Time for each unit, after which the half-bridge switches to change the state (connected or disconnected) of the corresponding battery. In this manner, the number of battery cells connected in the circuit is varied and a staircase shape voltage is synthesized. Moreover, the internal resistance of battery can cause the terminal voltage deviation and lead to inaccurate calculation of Delay-Time, which will introduce more harmonics. An improved method is developed to compensate the battery voltage drop. State-space representation is used to describe the circuit and battery model. Simulation is conducted to verify the modulation technique.