Development of a Modular High-Power Converter System for Battery Energy Storage Systems

The integration of storage systems into the grid is becoming increasingly important due to the growing amount of volatile power sources. This paper shows how to design a modular battery energy storage system (BESS) for medium voltage grids. Typically, this system is scalable in power rated from 5 MW up to 100 MW with a storage capacity of several hours. Using power electronic building blocks (PEBBs) a converter for dc grids and ac grids can be built. In this paper, the chosen topology for the ac solution is the cascaded cell converter. The focus is to determine the optimum number of levels, the modulation technique to avoid microcycles of the batteries and to present the efficiency. A formula to calculate the parasitic capacitance of lead-acid batteries is shown and verified by measurements, which is important for the design of such a converter system. Moreover, a new charging strategy for LiFePO4 - the chosen battery technology for the proposed storage system - is introduced, which prolongs the battery's life, reduces the charging time and decreases the life cycle cost. The proposed charging strategy and battery technology compared to lead-acid batteries is also economically evaluated.