Stability Enhancement Control Strategy for Grid-Forming Transformerless Energy Storage System under Low Grid Impedance Conditions
Zhu Guannan1,2, Chen Min1,2,3, Wang Pengcheng1,2, Liang Zhaopei1,2, Zhang Yaoyu1,2
1. College of Electrical Engineering Zhejiang University Hangzhou 310027 China; 2. ZJU-Hangzhou Global Scientific and Technological Innovation Center Zhejiang University Hangzhou 311200 China; 3. Research Institute of Zhejiang University-Taizhou Zhejiang University Taizhou 318000 China
Abstract：Grid-forming energy storage technology serves as a critical solution for enhancing power system stability. Transformerless energy storage systems, characterized by high efficiency, modularity, and direct medium/high-voltage grid integration, have emerged as the preferred choice for large-scale grid-connected energy storage. However, the reduced electrical distance between transformerless systems and the grid results in significantly lower grid impedance, posing severe challenges to the stability of grid-forming control. The underlying mechanism lies in the voltage-source operation of grid-forming converters: under low grid impedance conditions, minor voltage deviations between the converter and grid can trigger substantial current surges, ultimately leading to instability. To address these challenges, this study establishes a full order small signal model to analyze the impact of low grid impedance on stability and proposes impedance enhancement strategies. The research begins by developing a dynamic model that integrates virtual synchronous generator (VSG) control, voltage-loop regulation, and grid interactions. Pole trajectory analysis reveals two critical instability mechanisms: 1) Excessively low grid inductance shifts system poles to the right-half plane, inducing instability; 2) Insufficient grid resistance reduces damping ratios, exacerbating oscillatory behavior. These combined effects diminish system stability margins and may provoke subsynchronous oscillations. To mitigate these issues, a dual-layer impedance enhancement strategy is proposed: (1) Physical impedance reconstruction: The equivalent internal voltage control strategy repurposes filter inductance as coupling impedance by relocating the controlled voltage from the point of common coupling (PCC) to the converter side. This hardware-free modification enhances physical coupling impedance without requiring additional components. (2) Adaptive virtual impedance: A composite virtual impedance module combines static impedance for damping optimization and a dynamic current-limiting component. The static virtual impedance elevates damping ratios near to 0.707, while the current-limiting module dynamically adjusts impedance parameters based on real-time overcurrent thresholds, ensuring fault current suppression. In the analysis of impedance enhancement effect, it is shown that equivalent internal voltage control causes the dominant pole of the system under strong power grid to shift to the left into the stable region, while the introduction of adaptive virtual impedance further enhances damping characteristics and improves dynamic response performance. The proposed impedance enhancement strategy enhances the system stability by introducing filtering impedance at the physical level and superimposing virtual impedance at the control level, thereby increasing the equivalent coupling impedance of the system from a single grid impedance to the combined effect of the three. Experimental validation on a cascaded H-bridge transformerless energy storage platform under zero grid impedance conditions confirms the strategy's effectiveness. The proposed method eliminates oscillatory instability observed in conventional approaches, achieving smooth active power step responses without overshoot. During grid frequency fluctuations (±0.5 Hz), the system provides 0.67(pu) active power support, demonstrating effective grid-forming capabilities. Under symmetrical voltage sags (0.5(pu)), it delivers 0.5(pu) reactive power while constraining currents within 1.2(pu) safety thresholds, validating robust fault ride-through performance. Experimental and theoretical analyses confirm: (1) The proposed impedance enhancement architecture synergizes physical-layer reconstruction with control-layer virtual compensation, demonstrating superior stability improvement over conventional methods through coordinated impedance augmentation. (2) A pole trajectory analysis-based parameter optimization framework achieves concurrent enhancement of stability and dynamic performance, with virtual impedance implementation optimizing damping ratios to eliminate oscillatory instabilities. This work validates the effectiveness of the proposed strategy in extreme low-impedance scenarios, providing technical support for grid-forming transformerless energy storage applications in power grids.
[1] 李建林, 马会萌, 惠东. 储能技术融合分布式可再生能源的现状及发展趋势[J]. 电工技术学报, 2016, 31(14): 1-10, 20. Li Jianlin, Ma Huimeng, Hui Dong.Present development condition and trends of energy storage technology in the integration of distributed renewable energy[J]. Transactions of China Electrotechnical Society, 2016, 31(14): 1-10, 20. [2] 中关村储能产业技术联盟.中国新型储能累计装机突破50GW,中标价首次跌破0.5元/Wh[EB/OL]. [2024-11-14]. http://www.esresearch.com.cn/report/ info/detail/?id=5862. [3] 蔡旭, 李睿, 刘畅, 等. 高压直挂储能功率变换技术与世界首例应用[J]. 中国电机工程学报, 2020, 40(1): 200-211, 387. Cai Xu, Li Rui, Liu Chang, et al.Transformerless high-voltage power conversion system for battery energy storage system and the first demonstration application in world[J]. Proceedings of the CSEE, 2020, 40(1): 200-211, 387. [4] 刘畅, 吴胜兵, 吴西奇, 等. 35 kV高压直挂大容量电池储能系统[J]. 高电压技术, 2024, 50(2): 881-892. Liu Chang, Wu Shengbing, Wu Xiqi, et al.High-voltage large-capacity battery energy storage system connecting to 35 kV grid without transformer[J]. High Voltage Engineering, 2024, 50(2): 881-892. [5] 黄萌, 舒思睿, 李锡林, 等. 面向同步稳定性的电力电子并网变流器分析与控制研究综述[J]. 电工技术学报, 2024, 39(19): 5978-5994. Huang Meng, Shu Sirui, Li Xilin, et al.A review of synchronization-stability-oriented analysis and control of power electronic grid-connected converters[J]. Transactions of China Electrotechnical Society, 2024, 39(19): 5978-5994. [6] 梁军杨, 李红, 宋国杰, 等. 多时间尺度控制下跟网型变换器的同步稳定性分析与改进控制[J]. 电工技术学报, 2024, 39(22): 7182-7196. Liang Junyang, Li Hong, Song Guojie, et al.Synchronization stability analysis and enhanced control of grid-following converters under multi-timescale control[J]. Transactions of China Electrotechnical Society, 2024, 39(22): 7182-7196. [7] 汪春江, 孙建军, 宫金武, 等. 并网逆变器与电网阻抗交互失稳机理及阻尼策略[J]. 电工技术学报, 2020, 35(增刊2): 503-511. Wang Chunjiang, Sun Jianjun, Gong Jinwu, et al.Mechanism and damping strategy of interactive instability between grid-connected inverter and grid impedance[J]. Transactions of China Electrotechnical Society, 2020, 35(S2): 503-511. [8] Yao Wei, Chen Min, Matas J, et al.Design and analysis of the droop control method for parallel inverters considering the impact of the complex impedance on the power sharing[J]. IEEE Transactions on Industrial Electronics, 2011, 58(2): 576-588. [9] Gao Mingzhi, Chen Min, Zhao Bin, et al.Design of control system for smooth mode-transfer of grid-tied mode and islanding mode in microgrid[J]. IEEE Transactions on Power Electronics, 2020, 35(6): 6419-6435. [10] 屈子森. 高比例新能源电力系统电压源型变流器同步稳定性分析与控制技术[D]. 杭州: 浙江大学, 2021. Qu Zisen.Synchronizing stability analysis and control technology of voltage source converters in power system with high-penetration renewables[D]. Hangzhou: Zhejiang University, 2021. [11] 周京华, 李津. 构网型储能变流器国内外技术标准对比与分析[J/OL]. 电力系统自动化, 2024: 1-22 [2024-11-14]. http://kns.cnki.net/KCMS/detail/detail.aspx?filename=DLXT20240926004&dbname=CJFD&dbcode=CJFQ. Zhou Jinghua, Li Jin. Comparison and analysis of domestic and foreign technical standards of reticulated energy storage converter[J/OL]. Automation of Electric Power Systems, 2024: 1-22[2024-11-14]. http://kns. cnki.net/KCMS/detail/detail.aspx?filename=DLXT20240926004&dbname=CJFD&dbcode=CJFQ. [12] Gao Xian, Zhou Dao, Anvari-Moghaddam A, et al.Stability analysis of grid-following and grid-forming converters based on state-space modelling[J]. IEEE Transactions on Industry Applications, 2024, 60(3): 4910-4920. [13] IEEE. Guide for Planning DC Links Terminating at AC Locations Having Low Short-Circuit Capacities: IEEE Std 1204—1997[S]. IEEE, 1997. [14] 于琳, 孙华东, 赵兵, 等. 新能源并网系统短路比指标分析及临界短路比计算方法[J]. 中国电机工程学报, 2022, 42(3): 919-929. Yu Lin, Sun Huadong, Zhao Bing, et al.Short circuit ratio index analysis and critical short circuit ratio calculation of renewable energy grid-connected system[J]. Proceedings of the CSEE, 2022, 42(3): 919-929. [15] 胡光, 庄可好, 高晖胜, 等. 低惯量交流系统并网变流器次/超同步振荡分析[J]. 电工技术学报, 2024, 39(8): 2250-2264. Hu Guang, Zhuang Kehao, Gao Huisheng, et al.Sub/super synchronous oscillation analysis of grid-connected converter in low inertia AC system[J]. Transactions of China Electrotechnical Society, 2024, 39(8): 2250-2264. [16] Gao Mingzhi, Chen Min, Wang Chenxi, et al.An accurate power-sharing control method based on circulating-current power phasor model in voltage-source inverter parallel-operation system[J]. IEEE Transactions on Power Electronics, 2018, 33(5): 4458-4476. [17] Li Xin, Hu Yaowei, Shao Yuting, et al.Mechanism analysis and suppression strategies of power oscillation for virtual synchronous generator[C]// IECON 2017-43rd Annual Conference of the IEEE Industrial Electronics Society, Beijing, China, 2017: 4955-4960. [18] 严文博, 黄云辉, 方正, 等. 低阻抗下构网型变流器稳定性机理分析[J]. 高电压技术, 2025, 51(3): 1444-1453. Yan Wenbo, Huang Yunhui, Fang Zheng, et al.Stability mechanism analysis of reticulated converter under low impedance[J]. High Voltage Engineering, 2025, 51(3): 1444-1453. [19] Du Wei, Chen Zhe, Schneider K P, et al.A comparative study of two widely used grid-forming droop controls on microgrid small-signal stability[J]. IEEE Journal of Emerging and Selected Topics in Power Electronics, 2019, 8(2): 963-975. [20] Zhang Lidong, Harnefors L, Nee H P.Power-synchronization control of grid-connected voltage-source converters[J]. IEEE Transactions on Power Systems, 2010, 25(2): 809-820. [21] 温春雪, 黄耀智, 胡长斌, 等. 虚拟同步发电机接口变换器并联运行虚拟阻抗自适应控制[J]. 电工技术学报, 2020, 35(增刊2): 494-502. Wen Chunxue, Huang Yaozhi, Hu Changbin, et al.Virtual impedance adaptive control for parallel operation of virtual synchronous generator interface converter[J]. Transactions of China Electrotechnical Society, 2020, 35(S2): 494-502. [22] 詹长江, 吴恒, 王雄飞, 等. 构网型变流器稳定性研究综述[J]. 中国电机工程学报, 2023, 43(6): 2339-2359. Zhan Changjiang, Wu Heng, Wang Xiongfei, et al.An overview of stability studies of grid-forming voltage source converters[J]. Proceedings of the CSEE, 2023, 43(6): 2339-2359. [23] 范宸珲, 秦晓辉, 齐磊, 等. 构网型下垂控制中虚拟阻抗的作用、改进及研究前景分析[J]. 电网技术, 2024, 48(6): 2237-2250. Fan Chenhui, Qin Xiaohui, Qi Lei, et al.Analysis of the role, improvement, and research prospects of virtual impedance in grid-forming droop control[J]. Power System Technology, 2024, 48(6): 2237-2250. [24] Liu Teng, Wang Xiongfei.Unified voltage control for grid-forming inverters[J]. IEEE Transactions on Industrial Electronics, 2024, 71(3): 2578-2589. [25] 国家质量监督检验检疫总局, 中国国家标准化管理委员会. 电力变压器第5部分:承受短路的能力: GB/T 1094.5—2008[S]. 北京: 中国标准出版社, 2009. [26] Wen Bo, Boroyevich D, Burgos R, et al.Analysis of D-Q small-signal impedance of grid-tied inverters[J]. IEEE Transactions on Power Electronics, 2016, 31(1): 675-687. [27] Zhao Fangzhou, Zhu Tianhua, Li Zejie, et al.Low-frequency resonances in grid-forming converters: causes and damping control[J]. IEEE Transactions on Power Electronics, 2024, 39(11): 14430-14447. [28] Huang Linbin, Yang Chaoran, Song Meiyan, et al.An adaptive inertia control to improve stability of virtual synchronous machines under various power grid strength[C]//2019 IEEE Power & Energy Society General Meeting (PESGM), Atlanta, GA, USA, 2019: 1-5. [29] 杜步阳, 邵德军, 朱建行, 等. 电压源型变流器并网系统多时间尺度间相互作用[J]. 电工技术学报, 2023, 38(20): 5547-5559. Du Buyang, Shao Dejun, Zhu Jianhang, et al.The interaction between multiple timescales of the grid-tied voltage source converter[J]. Transactions of China Electrotechnical Society, 2023, 38(20): 5547-5559. [30] Huang Liang, Wu Chao, Zhou Dao, et al.A simplified SISO small-signal model for analyzing instability mechanism of grid-forming inverter under stronger grid[C]//2021 IEEE 22nd Workshop on Control and Modelling of Power Electronics (COMPEL), Cartagena, Colombia, 2021: 1-6. [31] 曾志杰. 并网逆变器DQ阻抗模型的建立方法研究[D]. 南京: 东南大学, 2021. Zeng Zhijie.Research on DQ impedance modelling method of grid-tied inverters[D]. Nanjing: Southeast University, 2021.
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