基于单输入单输出等效序阻抗的微网VSG并网稳定性分析及虚拟阻抗设计

doi:10.19595/j.cnki.1000-6753.tces.230077

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摘要

在分布式发电高渗透率的微电网中,虚拟同步发电机（VSG）因可为其提供必要的频率和电压支撑而被广泛关注,其并网稳定性研究已成为热点问题。本文首先在获得VSG输出序阻抗矩阵的基础上,通过建立单输入单输出（SISO）等效模型并结合Nyquist准则对VSG进行了并网稳定性分析;接着,针对正负序之间存在的频率耦合现象,基于所建SISO等效序阻抗模型,对比分析了考虑频率耦合效应前后的VSG输出阻抗特性,揭示了频率耦合效应对VSG并网稳定性的影响机理以及造成VSG并入强电网失稳的根本原因;最后,在上述机理分析的基础上,提出了一种用于提升系统稳定性的虚拟阻抗简化设计方法,该方法从提升系统的幅值裕度的角度出发,直接利用SISO等效输出阻抗与电网阻抗二者的交互关系实现虚拟阻抗的定量计算,简化了虚拟阻抗的设计过程。仿真结果验证了稳定性分析的正确性以及虚拟阻抗设计方法的有效性。

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关键词 ：虚拟同步发电机, SISO等效序阻抗模型, 频率耦合效应, 稳定性分析, 虚拟阻抗设计

Abstract：

In microgrid with high penetration of distributed generation, the generalized Nyquist criterion based on return rate matrix or return difference matrix is often used to analyse the grid-connection stability of virtual synchronous generator (VSG), but the calculation process is complicated. Besides, existing studies have not identified the root cause of the instability of VSG's integration into the strong grid, and the mechanism analysis of the influence of frequency coupling effect on the output impedance characteristics of VSG remains to be deepened. In terms of stability improvement, existing virtual impedance design methods do not take full advantage of the characteristics of VSG output impedance and its interaction with the grid impedance, and there is still room for further simplification. Therefore, this paper conducts a comprehensive and systematic research on the mechanism analysis of microgrid VSG grid-connection stability and the design method of virtual impedance.
Firstly, on the basis of obtaining VSG output sequence impedance matrix, this paper analyses the grid-connection stability of VSG by establishing single input single output (SISO) equivalent model and combining with Nyquist criterion. Secondly, based on the established SISO equivalent sequence impedance model, the influence mechanism of frequency coupling effect on the output impedance characteristics of VSG and the root cause of the instability of VSG's integration into the strong grid are compared and analysed. Finally, based on the above mechanism analysis, this paper proposes a virtual impedance simplified design method to improve system stability, from the perspective of improving the amplitude margin of the system, this method directly uses the interaction relationship between SISO equivalent output impedance and grid impedance to realize the quantitative calculation of virtual impedance, which simplifies the design process of virtual impedance.
The results are as follows: first, the stability analysis method based on SISO equivalent sequence impedance model can easily obtain accurate stability analysis results, the results show that the instability of the system is caused by the sub-synchronous oscillation near the fundamental frequency of 50Hz. Second, the influence of frequency coupling effect on VSG positive sequence output impedance is mainly concentrated around the fundamental frequency 50Hz. Considering the frequency coupling effect before and after, VSG positive sequence impedance changes from capacitive to inductive in the frequency band affected by frequency coupling effect. Lastly, near the critical frequency point where VSG's positive sequence output impedance shows negative resistance capacitance, the influence of grid inductance changes on its impedance characteristics can be ignored. Therefore, the virtual impedance design method based on SISO equivalent sequence impedance proposed in this paper can quickly and accurately obtain the critical threshold of virtual inductance.
The following conclusions can be drawn from the above results: (1) The stability analysis method of VSG grid-connection system based on the SISO equivalent sequence impedance model can simplify the analysis process and lay a foundation for the subsequent quantitative design of virtual impedance. (2) Although the frequency coupling effect only affects the positive sequence impedance characteristics near the fundamental frequency 50Hz, but this is the key to judging the stability of the VSG grid-connection. Through further analysis, the negative resistance capacitance of VSG positive sequence output impedance in a specific frequency band is the root cause of the instability of VSG's integration into the strong grid, the virtual impedance value can be quantitatively designed according to this point to improve the system stability margin. (3) On the basis of the above conclusions, this paper proposes a virtual impedance simplified design method based on the SISO equivalent sequence impedance model. From the perspective of improving the amplitude margin of the system, this method can be used to design virtual inductance directly by calculating or measuring VSG positive sequence output impedance. It avoids the repeated iteration and complex calculation required by traditional methods, and has good applicability to different grid impedance and control parameters.

Key words： VSG SISO equivalent sequence impedance model frequency coupling effect stability analysis virtual impedance design

收稿日期: 2023-01-18

PACS:

TM712

基金资助:

国家自然科学基金资助项目（52207102）

通讯作者: 郭志博男,1998年生,硕士研究生,研究方向为新能源发电系统建模与控制。E-mail：gzb_ncepu@163.com

作者简介: 刘欣男,1980年生,博士,副教授,硕士生导师,研究方向为新能源发电系统建模与控制,电力电子系统电磁兼容和瞬态特性研究。E-mail：liuxinhust@163.com

引用本文:

刘欣, 郭志博, 贾焦心, 王利桐. 基于单输入单输出等效序阻抗的微网VSG并网稳定性分析及虚拟阻抗设计[J]. 电工技术学报, 0, (): 20235427-20235427. Liu Xin, Guo Zhibo, Jia Jiaoxin, Wang Litong. Microgrid VSG Grid-Connection Stability Analysis and Virtual Impedance Design Based on Single-Input Single-Output Equivalent Sequence Impedance. Transactions of China Electrotechnical Society, 0, (): 20235427-20235427.

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