This paper investigates the impact of power grid strength and phase-locked loop (PLL) parameters on small signal stability of grid-connected doubly fed induction generator (DFIG)-based wind farm. Modal analysis of the grid-connected DFIG wind turbine under different operating conditions and various power grid strengths are investigated at first. Modal analysis results reveal that the DFIG connected to a weak grid may easily lose stability under the heavy-duty operating conditions due to PLL oscillation. The object of this paper is to identify the PLL oscillation mechanism as well as influence factors and propose a damping solution for this oscillation mode. A simplified linear system model of the grid-connected DFIG wind turbine is proposed for analyzing the PLL oscillation. Through the complex torque coefficients method and using this model, the oscillation mechanism and influence factors including the power grid strength and the PLL parameters are identified. To suppress this PLL oscillation, a mixed $H_2/H_{\infty }$ robust damping controller is proposed and designed for the DFIG. Electromagnetic transient simulation results of both single-DFIG system and multiply-DFIG system verify the correctness of the analysis results and effectiveness of the proposed damping controller.

为解决风电变流器控制器“黑/灰箱”化导致的建模难题,提出了一种基于端口测量阻抗的直驱风电机组控制器“黑/灰箱”辨识方法。首先,推导和验证了不同控制方式下机侧变流器直流端口和网侧变流器交流端口的解析阻抗模型;其次,分析了直流端口阻抗和交流端口阻抗的参数灵敏度,论证了控制参数的可辨识性,并通过阻抗的参数灵敏度提取了不同控制方式下直流端口阻抗的特征。在此基础上,提出了基于端口阻抗特征匹配的直驱风电机组“黑箱”控制方式辨识方法;进一步地,提出了一种基于端口测量阻抗的直驱风电机组“灰箱”控制参数辨识方法,即利用阻尼最小二乘算法使端口实测阻抗与理论阻抗在各个频率点处差值的平方和最小,可实现变流器所有控制参数的准确辨识。最后,通过算例验证了所提方法的有效性。

风电并网引发次同步振荡问题对系统稳定运行产生了不良影响，成为制约风电高效利用的瓶颈因素。这类问题引起了工业界和学术界的广泛关注，许多文献提出了不同的分析方法。文章将与风电相关的次同步振荡分析方法归为两类，即时域和频域，全面综述了近十多年来国内外各种方法的研究成果及实践应用，对每种方法进行了比较。考虑到基于阻抗模型的分析方法近期得到广泛关注，文章重点介绍了阻抗分析法，总结了阻抗建模和稳定性判据的最新研究结果。同时，对目前尚未解决的问题及面临的挑战进行总结，并对接下来的研究方向进行展望。

Subsynchronous oscillation occurred in the on-grid wind farms has seriously affected the stability of the whole system and has become a bottleneck to the efficient utilization of wind power. This problem attracts wide attentions from both the industry and the academia. Different study methods have been presented in the literature. In this paper， various analysis methods for subsynchronous oscillations associated with wind power are classified into two categories， namely time domain and frequency domain. A comprehensive review of the methods is given， including their practical application in China and abroad. Considering the fact that the impedance model-based analysis method has received extensive attention recently， the paper focuses on the impedance analysis method and summarizes the latest research results of impedance modeling and stability criteria. Meanwhile， new challenges or open problems are summarized. The direction for future research work is also discussed.

近期,某实际新能源经柔直送出系统出现了40~60 Hz的近工频正序和负序振荡现象,导致新能源降出力运行。新能源经柔直送出系统的近工频振荡较其他频段振荡的机理更为复杂,影响因素更多。针对直驱风电场经柔直送出系统近工频振荡稳定性问题,建立了含正/负序控制的直驱风电机组和柔直送端模块化多电平换流器(modular multilevel converter,MMC)的精细化阻抗模型,从阻抗特性角度揭示了直驱风电场经柔直送出系统产生近工频正/负序振荡的机理;定义了参数相位裕度灵敏度,量化分析了直驱风电场柔直送出系统近工频振荡稳定性的关键影响因素。最后,搭建直驱风电场经柔直送出系统的电磁暂态仿真模型,仿真复现了实际系统的近工频正序和负序振荡现象,并验证了近工频振荡机理分析的正确性。

Positive- and negative-sequence oscillation phenomena close to the fundamental frequency (40-60 Hz) occur in a practical modular multilevel converter-based high-voltage DC (MMC-HVDC) transmission system for new energy integration, which has led to a decrease in the power output of new energy sources. The mechanism of near-fundamental-frequency oscillations is more complex and has more influencing factors than oscillations in other frequency bands. This study focuses on the near-fundamental-frequency oscillation stability of an MMC-HVDC-connected direct-drive wind farm. Refined impedance models of the direct-drive wind turbine and sending-end MMC are established by considering positive and negative sequence controls. Based on the established impedance models, the mechanisms of the near-fundamental-frequency positive- and negative-sequence oscillations between the direct-drive wind farm and the sending-end MMC are revealed. In addition, the parameter phase-margin sensitivity is defined, and the key influencing factors of the near-fundamental-frequency oscillation stability of the interconnected system were analyzed quantitatively. Finally, an electromagnetic transient simulation model of the MMC-HVDC-connected direct-drive wind farm is developed. The near-fundamental-frequency positive- and negative-sequence oscillation phenomena in the actual project are reproduced, and the correctness of the near-fundamental-frequency oscillation mechanism analysis is validated.