跟网型变换器的小扰动同步稳定机理分析与致稳控制

doi:10.19595/j.cnki.1000-6753.tces.230493

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Abstract

With the increasing integration of renewable energy and power electronic equipment, the power system is characterized by low inertia and weak damping. Consequently, grid-following converters employing a phase-locked loop (PLL) for synchronization face the risk of oscillation instability, especially in complex control coupling. This paper proposes a complex torque analysis model that examines the damping mechanism for small disturbance synchronous instability. To reveal the small disturbance synchronization instability mechanism of grid-following converters, a PLL complex torque analysis mode is built considering the current control (CC) loop and DC-link voltage control (DVC) loop. Accordingly, the accurate damping characteristics and oscillation risk of the low-frequency mode can be assessed. The effects of typical multi-loop control coupling, line impedance, and control parameters on the damping torque of the system are quantified.
Firstly, a typical grid-following topology and control strategy is introduced, including the PLL, current inner current loop, and DC-link voltage outer loop. Secondly, a synchronous dynamic analysis model is established, considering both CC and DVC, resembling in the Phillips-Heffron model used in low-frequency oscillation analysis for synchronous machines. Based on the equivalent block diagram transformation, the complex torque model of the synchronous loop is derived, enabling the examination of equivalent second-order motion driven by unbalanced voltage, as shown in Fig.A1. It indicates that by introducing equivalent voltage components by CC and DVC, control coupling has an uncertain damping effect on the PLL. Thirdly, the intrinsic damping of PLL and additional damping from CC and DVC are extracted using the developed model, facilitating an investigation into the influence of control interaction on the PLL's equivalent motion. Moreover, the study analyzes the impacts of multi-loop control coupling, line impedance, and various control parameters on the low-frequency oscillation characteristics of grid-following systems. Finally, a damping compensator, utilizing d-axis current feedback, is proposed to improve the low-frequency damping characteristics of grid-following and enhance the small perturbation synchronization stability of the converter.

Fig.A1 The complex torque analysis of PLL
The results show that incorrect controller parameters can lead to synchronization instability with oscillation frequencies below 10 Hz, particularly in weak grids. The influence of key parameters on the system's oscillation mode is summarized as follows:
(1) Increasing the proportional coefficient of PLL and the integration parameter of CC can lead to a higher oscillation frequency and exacerbate the negative damping effect.
(2) Amplifying the proportional gain in the CC loop and DVC enhances the system's oscillation frequency but weakens the negative damping effect.
(3) The increase in the grid impedance reduces the oscillation frequency and intensifies the negative damping effect of the system.
Furthermore, leveraging the enhanced PLL, the system exhibits positive damping and remains stable as the grid transitions from strong to weak. As the grid impedance increases from 0.1(pu) to 0.8(pu), the positive damping persists, signifying an improved dynamic characteristic of the grid-following system.

Key words： Grid following converter small signal synchronization stability multi-loop control coupling complex torque analysis damping reshaping

Received: 18 April 2023

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TM464

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	Li Hong
	Liang Junyang
	Wang Zhenmin
	Li Guojin
	Chen Yanming

Cite this article:

Li Hong,Liang Junyang,Wang Zhenmin等. Small Signal Synchronization Stability Analysis and Improved Control Strategy for Grid Following Converter[J]. Transactions of China Electrotechnical Society, 2024, 39(12): 3802-3815.

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https://dgjsxb.ces-transaction.com/EN/10.19595/j.cnki.1000-6753.tces.230493 OR https://dgjsxb.ces-transaction.com/EN/Y2024/V39/I12/3802

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