Evaluation of Emerging Modular Multilevel Converters for BESS Applications

The power conversion system for a battery-energy storage system typically employs a conventional voltage-source converter with battery strings directly connected to the dc bus. This system configuration presents several issues, such as limited efficiency of two-level converter systems and the limited reliability associated with the use of long battery strings. This paper examines three viable multilevel converter solutions for integrating battery energy storage that offer the potential for enhanced efficiency and reliability. These solutions are the modular multilevel converter (MMLC) with battery energy storage distributed into its submodules, the cascaded converter, and the MMLC with battery energy storage centralized on its dc link. The three systems are compared in terms of efficiency, reliability, and module redundancy. It is determined that the MMLC with distributed battery energy storage must operate differently from conventional MMLC systems. Its operation is therefore studied in detail and validated through simulation to demonstrate its suitability for distributed energy-storage integration. The analysis shows that the MMLC with distributed battery energy storage requires the largest number of semiconductor devices for a given power level; however, it also provides the most efficient, reliable, and versatile solution of energy-storage integration.