基于无权重变舍入电平控制的MMC-BESS改进定环流SOC均衡策略

doi:10.19595/j.cnki.1000-6753.tces.250689

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Abstract The model predictive control (MPC) method for modular multilevel converter battery energy storage systems (MMC-BESS) faced challenges such as high computational burden, complex weighting factor tuning, and slow State-of-Charge (SOC) balancing. The conventional variable rounding level control (VRLC) method reduces computational complexity. Still, it failed to eliminate the dependency on weighting factors, resulting in a performance trade-off between output current tracking accuracy and circulating current suppression. Therefore, this paper proposes an improved fixed circulating current SOC equalization strategy based on weight-independent variable rounding level control (WI-VRLC). The strategy ensures low computational burden, eliminates weighting factors, enhances power quality, and achieves fast SOC balancing even under non-uniform inter-phase and inter-arm SOC differences.
Two independent cost functions are employed for hierarchical control of output and circulating currents. First, in the output current MPC stage, the reference arm voltage is used to generate three candidate arm combinations through the variable rounding method, with the optimal combination subsequently selected based on evaluation of the output current cost function. In the subsequent circulating current MPC stage, the bridge arm reference voltages for circulating current control are processed through the variable rounding method, yielding two candidate submodule insertion numbers. These are then algebraically combined with the optimal bridge arm combination from the output current MPC stage to produce two new candidate bridge arm combinations. The circulating current cost function is used to evaluate these combinations and determine the optimal number of bridge arm insertions. Finally, the improved constant circulating current SOC balancing strategy calculates the circulating current reference value for SOC balancing, which is incorporated into the circulating current cost function to achieve closed-loop SOC control.
Simulations and hardware-in-the-loop (HIL) experiments demonstrate the method's superiority. In simulation results, the steady-state output current total harmonic distortion (THD) decreases from 2.24% to 1.42% (at modulation index=0.9) and from 1.12% to 0.70% (at modulation index=0.45), while maintaining circulating current performance. Dynamic performance tests reveal that the proposed method exhibits slightly better circulating current response than traditional approaches. Both methods demonstrate excellent output current tracking capability. Regarding SOC balancing performance, the proposed method achieves SOC equalization under both non-uniform SOC distribution and extreme unbalanced conditions, while maintaining stable operation during varying grid-connected power levels and grid voltage sag. Experimental results show that the output current THD is reduced by 17% and 19.6% at modulation indices of 0.9 and 0.45, respectively, without compromising SOC balancing speed or circulating current performance.
Three conclusions are as follows. (1) The hierarchical control architecture eliminates weighting factor requirements, reduces output current THD by ≥17%, and maintains superior circulating current suppression performance. (2) The method's computation remains fixed at five operations per control cycle, regardless of submodule count, thereby ensuring excellent scalability for high-submodule applications. (3) The enhanced fixed circulating current strategy achieves robust SOC balancing under non-uniform SOC distribution conditions.

Key words： Modular multilevel converter battery energy storage system (MMC-BESS) weight-independent variable rounding level control (WI-VRLC) model predictive control (MPC) state of charge (SOC) balancing

Received: 25 April 2025

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TM721

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	Liu Zhan
	Li Quangen
	Zhou Hua
	Fu Jiawei
	Lin Zhifang

Cite this article:

Liu Zhan,Li Quangen,Zhou Hua等. Improved Fixed Circulating Current SOC Equalization Strategy for MMC-BESS Based on Weight-Independent Variable Rounding Level Control[J]. Transactions of China Electrotechnical Society, 2026, 41(8): 2718-2734.

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https://dgjsxb.ces-transaction.com/EN/10.19595/j.cnki.1000-6753.tces.250689 OR https://dgjsxb.ces-transaction.com/EN/Y2026/V41/I8/2718

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