电工技术学报2025，Vol.40Issue(9)：2697-2711,15.DOI:10.19595/j.cnki.1000-6753.tces.242382
低电网阻抗下构网型直挂式储能系统稳定性提升控制
Stability Enhancement Control Strategy for Grid-Forming Transformerless Energy Storage System under Low Grid Impedance Conditions
摘要
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.关键词
直挂式储能/构网/稳定性/虚拟阻抗/虚拟同步机Key words
Transformerless energy storage/grid-forming/stability/virtual impedance/virtual synchronous generator分类
动力与电气工程引用本文复制引用
朱冠南,陈敏,王鹏程,梁钊培,张耀予..低电网阻抗下构网型直挂式储能系统稳定性提升控制[J].电工技术学报,2025,40(9):2697-2711,15.基金项目
国家电网有限公司总部管理科技项目(5500-202329500A-3-2-ZN)和台州市-浙江大学科技合作项目资助. （5500-202329500A-3-2-ZN）
