State-of-Charge Balancing Control for Optimal Cell Utilisation of a Grid-Scale Three-Phase Battery Energy Storage System Using Hybrid Modular Multilevel Converter Topology Without Redundant Cells

Abstract

Cell state-of-charge (SoC) balancing within each branch of a three-phase battery energy storage system (BESS) and among three branches is crucial to overcome the inability to fully utilise the available capacity of a three-phase BESS. The proposed topology is constructed with one branch instead of three branches to take advantage of its idle cells/modules (Ms) (one-third of the total cells/Ms) and to eliminate the need of SoC balancing among the branches. Contrary to conventional topologies, idle cells/Ms can serve as redundant cells/Ms or can be dropped out of a BESS, thereby leading to a reduction in the cost, control complexity, size, and losses of a BESS. A novel SoC balancing strategy for the proposed topology of a three-phase BESS is introduced in this paper. Moreover, the cell/M activation algorithm is implemented to minimise the duration needed to activate the cells/Ms required to generate voltage for the phases, thereby leading to an improvement of battery operational efficiency. Based on the simulation results, SoC balancing among 3996 cells, 2664 cells, 333/222 Ms, and 12 cells in M with the lowest and the highest average SoC is achieved in 53 min, 48 min, 38 min, 18 min and 53 min, respectively.

Optimal Allocation of BESS to Maximize Efficiency Under Constrained SOC in Parallel Inverter-Based Scaled-Up Slack Bus

“…A. B. Ahmad et al proposed an ESS topology and its related SOC balancing method for grid-scale three-phase BESS [12]. Here, the BESS has been applied as a single-arm to eliminate the need to balance between arms.…”

Design of a bus-based battery equalization with zero-current switching

“…Combined with equation (11), the equivalent resistance R sc can be expressed as the following equation.…”

“…However, the control scheme and circuit structure of this equalization topology are more complex and difficult to implement. [6][7][8][9][10] Ahmad et al, 11 Zhou et al, 12 Lee et al, 13 utilized a C2S topology with an extremely high step-up or step-down conversion ratio when large battery strings are used, resulting in low energy conversion efficiency. The AC2AC topology was chosen, using capacitors as equalizers.…”

An Analysis and Optimization of the Battery Capacity Difference Tolerance of the Modular Multi-Level Half-Bridge Energy Storage Converter

“…The phase-shifted carriers can increase the equivalent switching frequency of the converter, and the magnitude of the equivalent switching frequency is affected by the value of the phase difference θ between carriers of the UA and LA. After amplification and translation, the carrier and modulation waves of the DCCPS-PWM can be transformed into bipolar modulation forms [21].…”

“…Investigating the unbalanced operating principle of the studied topology, reference [20] designs a control strategy to optimize component voltage stress by allocating unbalanced module power, accompanied by a novel fast-balancing charging strategy. In [21], a new three-phase battery ESS with a single branch instead of three branches is proposed, eliminating the need for SOC balancing among branches. Subsequently, a novel SOC balancing strategy is introduced for the proposed topology of the three-phase ESS.…”