Optimal cell utilisation with state-of-charge balancing control in a grid-scale three-phase battery energy storage system: An experimental validation

Abstract

Cell State-of-Charge (SoC) balancing is essential to completely utilise the available capacity of a Battery Energy Storage System (BESS). Furthermore, redundant cells within a BESS are a key consideration to achieve high reliability. Contrary to conventional converters, the proposed converter is designed using one branch (rather of three) to take advantage of its idle cells, which are one-third of the overall cells. In this article, experimental validation is performed to establish the effectiveness of the proposed converter and the SoC balancing strategy. The experimental results indicate that during the BESS operation where the grid reference voltage (Vref) is equal to 38 V, at least 3 out of 9 Modules (Ms) (36 cells) are idle, with a 15-level three-phase sinusoidal output voltage (Vout(a,b,c)) obtained in proposed/conventional converters. When Vref is increased from 38 V to 50 V, Vout(a,b,c) is not obtained in the conventional converter (as opposed to the proposed converter). Using the proposed converter, Vout(a,b,c) has increased by 33.3% compared to the conventional converter. Moreover, the SoC balancing among 9 Ms, 36 cells, 4 cells in M with the lowest average SoC, and 4 cells in M with the highest average SoC is achieved in 4400 s, 4600 s, 3100 s, and 3550 s, respectively.

Keywords

• Battery Energy Storage System (BESS)
• Cascaded H-bridge multilevel converter
• Cell balancing
• Idle cells
• Redundant cells
• State-of-Charge (SoC)