Publication: Optimal cell utilisation in three-phase battery energy storage systems using a hybrid modular multilevel converter
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Date
2022-05-01
Authors
Bani Ahmad, Ashraf Mohammad
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Abstract
Battery energy storage systems (BESSs) are a promising technology for power grid applications due to their dynamic behaviours. However, cell state-of-charge (SoC) balancing is crucial to overcome the inability to fully utilise the available capacity of a BESS. In addition, using a high number of switches in a cascaded H-bridge BESS can potentially increase cost and power losses. Moreover, the conventional topology does not have the ability to take advantage of its idle cells, at least one-third of its cells are constantly idling in a typical three-phase BESS. This thesis proposes a novel circuit topology and a SoC balancing control for a three-phase grid-scale BESS using a three cascaded hybrid modular multilevel converter (TCHMMC) without redundant cells. The proposed TCHMMC is constructed with one branch instead of three branches to take advantage of its idle cells/modules and to
eliminate the need of SoC balancing among the branches. The reduction in component count is achieved by integrating each individual cell into L-bridge compared with H-bridge. The simulation results indicate that at least 140 out of 333 modules (3996 cells) are in an idle state during the BESS operation. Using 333 modules, the three-phase output power has increased by 58.8 % in the proposed TCHMMC (306 kW) compared with the conventional topology (180 kW). In addition, SoC balancing among 3996 cells and 2664 cells is achieved within 53 min and 48 min respectively. An experimental validation has been performed to demonstrate the effectiveness of the proposed TCHMMC and the SoC balancing control. The experimental results indicate that the three-phase output voltage of the proposed TCHMMC is increased by 33.3 % compared to the conventional topology. Moreover, the SoC balancing among 9 modules is achieved in 73 min.