三电平变流器模块化并联系统中点电位与零序环流机理建模与协调控制

doi:10.19595/j.cnki.1000-6753.tces.241311

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Abstract The modular parallel configuration of three-level converters has become a favored solution for high-power electrical energy conversion due to its superior output waveform quality and robust scalability across power levels. However, challenges such as neutral point potential (NPP) fluctuations inherent to the three-level topology and zero-sequence circulating currents (ZSCC) significantly affect the efficiency and reliability of the system. Existing studies primarily focus on either achieving NPP balance or suppressing ZSCC independently. This paper develops integrated models for NPP and ZSCC and proposes a coordinated control strategy to simultaneously balance NPP and suppress ZSCC.
Firstly, the simplified models of NPP and ZSCC in a parallel converter system are derived to explain their excitation mechanisms and transfer functions. Then, this paper establishes a model based on space vector pulse width modulation (SVPWM) to analyze the detailed dynamics of neutral point currents and zero-sequence excitations during each modulation cycle. The results reveal that NPP and ZSCC fluctuate three times the fundamental frequency. Adjusting the operational timing of redundant small vectors through a proportional-integral (PI) control loop can suppress the fluctuation. Additionally, to elaborate on the coupling interactions between NPP and ZSCC, the detailed models are refined to incorporate ZSCC into the neutral point current for NPP fluctuations in the zero-sequence voltage. This analysis demonstrates that their interaction is influenced by the inherent coupling term (arising from physical characteristics) and control coupling term (introduced by minor vector adjustments).
Secondly, it is critical to eliminate control-based coupling to ensure independent regulation of NPP or ZSCC. An adjustment factor K is designed: setting K₁=K₂ enables independent NPP balance, while K₁=-K₂ significantly suppresses ZSCC without amplifying NPP fluctuations. For dual control objectives, a coordinated control strategy is proposed. Four operational conditions are identified and categorized into two types. (1) When V_p>V_n, i_z<0 or V_p<V_n, i_z>0, appropriately adjust the dwell time of P-type small vectors. (2) When V_p>V_n, i_z>0 or V_p<V_n, i_z<0, the adjustment of the short vector dwell time has conflicting effects on NPP and ZSCC, and the priority of a more critical or preferred objective is needed. This strategy can continuously balance both targets with optimized effectiveness.
Finally, the proposed strategy is validated through Matlab/Simulink simulations and tests on a laboratory prototype comprising two parallel hybrid 3L-ANPC converters with identical parameters. Under inverter mode and unit power factor, NPP fluctuations are reduced by 60%, and ZSCC decreases to one-third of its initial value. Experimental results further confirm the robustness of the strategy under both inverter and rectification modes, even under low power factor conditions. Compared to the existing strategies, such as overlapping control methods, the proposed coordinated control demonstrates superior adaptability and dynamic performance.

Key words： Three-level converter parallel system zero sequence circulation current suppression neutral point potential balance coordinated control strategy

Received: 23 July 2024

PACS:

TM46

Fund:国网山西省电力公司科技资助项目（52053023001L）

Corresponding Authors: 向鑫男,1990年生,研究员,博士生导师,研究方向为新能源发电并网,柔性直流输配电与大容量直流变换。E-mail: xiangxin@zju.edu.cn

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	Zhang Min
	Chen Hongzhou
	Fan Rui
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	Li Wuhua

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Zhang Min,Chen Hongzhou,Fan Rui等. Modeling and Coordinated Control of Neutral Point Potential and Zero-Sequence Circulating Current in Modular Parallel Three-Level Converters[J]. Transactions of China Electrotechnical Society, 2025, 40(18): 6014-6028.

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https://dgjsxb.ces-transaction.com/EN/10.19595/j.cnki.1000-6753.tces.241311 OR https://dgjsxb.ces-transaction.com/EN/Y2025/V40/I18/6014

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