Analysis of Static Var Generator Considering the Impacts of Internal Dynamics and Frequency Coupling
Li Longcan1, Xiong Xiaoling1, Luo Bochen1, Zhao Chengyong1, Xu Ying2, Lei Tianxiang2
1. School of Electrical and Electronic Engineering North China Electric Power University Beijing 102206 China; 2. State Grid Economic and Technological Research Institute Co. Ltd Beijing 102209 China
Abstract：Due to its compensation capabilities and fast dynamic response times, the cascaded H-bridge static var generator (SVG) has become widely adopted for enhancing power quality and providing reactive power in modern power systems. However, as a power electronic device predominantly governed by control-driven dynamics, the SVG faces the risk of oscillations. In addition, the mechanisms of wideband oscillations in SVG systems remain insufficiently understood, and effective stability enhancement techniques are lacking. This paper analyzes the cascaded H-bridge SVG's nonlinear and strongly coupled internal dynamic characteristics. Focusing on the low-frequency range below the switching frequency, a harmonic state-space (HSS) complex vector impedance model is developed, incorporating the influence of multiple control loops. A detailed time-domain model is constructed to simulate the system's behavior, and frequency scanning is conducted using Matlab/Simulink. Furthermore, considering the system's frequency coupling relationships, the dynamic interactions of internal currents, output voltages, and three-phase capacitor voltages and their coupling grid impedance are examined. Accordingly, an equivalent single-input single-output impedance model is constructed. A stability analysis of the SVG is performed to evaluate the impact of various control loops and their associated parameters on system stability. The phase-locked loop (PLL) is critical in determining system stability. Consequently, a quantitative analysis of the PLL parameters is conducted for parameter design and optimization. In contrast, the inter-phase voltage balancing loop has a minor influence on system stability. This paper proposes a voltage feedforward impedance reshaping strategy to mitigate the stability challenges associated with SVG grid integration. This approach targets the oscillations introduced by the dominant control loop (the PLL), which can offset its impact. However, since this method involves high-order integration, it is prone to causing system instability. Therefore, an optimized control strategy is proposed, effectively reducing the negative resistance region caused by the PLL, thereby enhancing the overall stability of the grid-connected system. A hardware-in-the-loop experiment is implemented using the RT-LAB platform. The experimental results verify the effectiveness of the proposed methods in improving system stability. The key conclusions of this study are as follows. (1) The PLL and its control parameters significantly influence the system's oscillation risk based on the developed HSS impedance model for multi-frequency coupling in the cascaded H-bridge SVG. In contrast, the inter-phase voltage balancing loop and its control parameters exhibit a negligible impact. (2) The design of the PLL bandwidth needs to consider two critical factors: a considerable PLL bandwidth that reduces stability margins and a PLL bandwidth that aligns with the global voltage balancing loop bandwidth. As a result, additional oscillations from coupling are introduced. (3) A voltage feedforward-based impedance-shaping control strategy is proposed. This strategy mitigates the negative resistance region caused by the PLL, significantly improving the stability of the SVG system. This paper provides measures for the design and optimization of SVG systems.
李龙灿, 熊小玲, 罗博晨, 赵成勇, 徐莹, 雷添翔. 考虑内部动态和频率耦合影响的静止无功发生器振荡风险分析[J]. 电工技术学报, 2025, 40(24): 8108-8122.
Li Longcan, Xiong Xiaoling, Luo Bochen, Zhao Chengyong, Xu Ying, Lei Tianxiang. Analysis of Static Var Generator Considering the Impacts of Internal Dynamics and Frequency Coupling. Transactions of China Electrotechnical Society, 2025, 40(24): 8108-8122.
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