Battery Storage System as Power Unbalance Redistributor in Distribution Grids Based on Three Legs Four Wire Voltage Source Converter

This article discusses the application of battery energy storage systems (BESSs) as power redistributors in three-phase distribution grids as an add-on functionality to typical BESS applications, such as congestion management and energy arbitrage. Combining those ancillary services into a single power unit is not yet performed in practice but may constitute an emerging business opportunity to increase the BESS revenues. The unbalanced operation of the BESS voltage source converter (VSC) leads to the circulation of low-frequency current harmonics in the dc-link through the capacitors and the battery cells. Therefore, it is particularly interesting whether relatively large 50- and 100-Hz currents can safely circulate within these components. Analytical modeling and design guidelines for the dc-link of a three-leg four-wire two-level VSC operating under unbalanced loads are detailed. Furthermore, a low-power VSC prototype is used to demonstrate the working principle of the BESS, providing power unbalance redistribution and symmetric power exchange. Additionally, the ICR18650-26F Lithium-ion cells are cycled to reach end-of-life with different current profiles and C-ratings. The analysis shows that charging with a 100 Hz ripple superimposed to the dc current leads to a 10% increment in degradation.