同步机和非同步机电源接入强弱网系统小扰动稳定特性比较与统一分析

doi:10.19595/j.cnki.1000-6753.tces.242348

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Abstract

The grid-connected systems of synchronous generators (SG) and grid-following converters may experience stability issues under weak grid conditions, while the grid-forming converter systems may face stability issues under stiff grid conditions. Nevertheless, a unified understanding of the instability characteristics of these three types of power sources under varying grid strengths is still lacking. To address this, this paper conducts a comparative analysis of the instability mechanisms under stiff or weak grid conditions for the SG and converter-interfaced generations. Firstly, the small-signal stability of the second-order synchronization loop model and the fourth-order model considering the outer loop for the three types of grid-connected systems was compared using eigenvalue analysis. It is determined that the instability issues under stiff or weak grid conditions for all three power source grid-connected systems are produced by the outer loop control. Next, the Heffron-Phillips-like models were constructed. Analysis is conducted from a unified perspective where the synchronization loop is the dominant loop and the outer loop control is treated as an additional branch. Based on the feedback branch transfer function decomposition method, the feedback transfer functions G(s) with different structures were decomposed into a unified second-order oscillation component H₁(s) and a left component H₂(s). A unified analysis of the instability mechanisms under stiff or weak grid conditions is conducted. The conclusions of this paper are as follows:
1) A unified Heffron-Philips-like model was developed for three types of power source grid-connected systems. The study reveals that the different instability phenomena in the weak or stiff grids are due to the involvement of outer loop control. Although the SG and grid-forming converter systems have the same synchronization loop, they exhibit significantly large negative damping coefficients under weak and stiff grid conditions, respectively, due to their differences in the outer loop control structure. This ultimately leads to instability under different grid strengths.
2) A feedback branch transfer function decomposition method was proposed for the analysis of instability mechanisms. Based on this method, the structural differences in the transfer functions used for stability analysis in different grid-connected systems can be eliminated, enhancing the uniformity of stability comparisons and analyses.
3) Since the phase of H₁(jω_n) at the natural oscillation frequency f_n is -90°, the differences in the coefficients of H₁(s) among the three types of power sources lead to different trends in the relative distance between the system oscillation frequency f_s and f_n. Consequently, the grid strengths at the instability also differ. It can be deduced that: in weak grids, the f_n of the SG is close to f_s, leading to instability; in strong grids, the f_n of the GFM-VSC is close to f_s, resulting in instability; and in weak grids, the f_n of the GFL-VSC is far from f_s, with the additional influence of the phase from the additional transfer function causing instability.

Key words： Synchronous generator GFM-VSC GFL-VSC heffron-phillips-like model feedback branch transfer function decomposition method

Received: 26 December 2024

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TM712

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	Liu Qi
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Liu Qi,Zhan Meng,Li Wei等. Characteristics Comparison and Unified Analysis of Stiff or Weak Grid-Tied Small Disturbance Stability of Synchronous Generator and Converter-Interfaced Generations[J]. Transactions of China Electrotechnical Society, 0, (): 258103-258103.

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https://dgjsxb.ces-transaction.com/EN/10.19595/j.cnki.1000-6753.tces.242348 OR https://dgjsxb.ces-transaction.com/EN/Y0/V/I/258103

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