Fractionally-rated DC-DC stages for use in multilevel cascaded converter utility-scale battery energy storage systems

Abstract

Multilevel cascaded converters for use in battery energy storage systems (BESSs) can utilise parallel-connected dc/dc conversion stages. These stages decouple each battery string voltage from the associated H-bridge capacitor voltage. Decoupling prevents second harmonic current ripple flowing in the batteries and compensates for variation in battery voltage due to state of charge. Instead of parallel-connected converters, this paper proposes use of series-connected fractionally-rated dc-dc conversion stages. Series-connected converters are shown to maintain the benefits of voltage decoupling but with significantly lower power ratings than their parallel-connected counterparts. Lower power ratings can facilitate a reduction in semiconductor losses and a lower cost for the BESS.