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| Transient Synchronous Stability Control for Grid-Connected Direct-Drive Wind Turbine System Driven by Improved Energy Function |
| Li Kaixin1,2, He Pengfei1, Liu Qihui3, Cai Xipeng3, Zhu Yihua3 |
1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources North China Electric Power University Beijing 102206 China;
2. Foshan Power Supply Bureau of Guangdong Power Grid Co. Ltd Foshan 528010 China;
3. State Key Laboratory of HVDC Electric Power Research Institute of China Southern Power Grid Company Limited Guangzhou 510663 China |
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Abstract Transient synchronous stability during low-voltage ride-through (LVRT) presents a critical challenge for grid-connected direct-drive wind turbine systems. Conventional stability assessment methods based on energy functions often yield excessively conservative results, a limitation that can be traced to their neglect of damping effects. This paper proposes a novel transient synchronous stability control method based on an improved energy function. The central contribution is the construction of an improved energy function that incorporates the cumulative effect of system damping through an effective kinetic energy correction term. This formulation directly reduces the conservatism in stability region estimation. Building on this analytical foundation, a dedicated phase-locked loop (PLL) control strategy, which does not require precise real-time estimation of grid parameters, is subsequently developed.
The research was conducted in three stages. First, the control structure and assumptions of the grid-connected direct-drive wind turbine system were analyzed to establish a mathematical model for transient synchronous stability. The stability region was derived using the conventional energy function method, and its limitations were analyzed. To address the core flaw of this method, specifically its omission of damping, an effective kinetic energy correction term was introduced. This enabled the formulation of an improved energy function accounting for cumulative damping, from which a revised stability region was theoretically derived, effectively mitigating the conservatism of the traditional approach. Second, based on the boundary characteristics of this improved stability region, the key factors constraining the stability boundary are analyzed in depth. It is determined that the nonlinear characteristics of damping and unbalanced power constitute a primary factor constraining the transient synchronous stability boundary of the wind-integrated system, and the underlying constraining mechanism is elucidated. Consequently, a transient synchronous stability control strategy based on an improved PLL is proposed, along with a clear methodology for parameter setting. Finally, the effectiveness and adaptability of the proposed control strategy under multiple fault scenarios are comprehensively validated through Matlab/Simulink simulation, hardware-in-the-loop (HIL) testing, and physical experiments.
Simulation and experimental results confirm the efficacy of the proposed method. In fault scenarios where the grid voltage sagged to 65 V (with a stable equilibrium point, SEP) and 45 V (without an SEP), systems employing a conventional PLL lost synchronism, whereas systems with the proposed control maintained stable operation. The proposed strategy sustained synchronous stability for voltages above 40 V, extending the practical stability boundary by approximately 13.4% compared to the static stability boundary of 46.17 V. Furthermore, in compound fault scenarios involving simultaneous voltage sag and increased grid inductance, the proposed strategy maintained stability where conventional control failed. Consistent results from HIL testing and physical experiments verified the strategy's robustness across different experimental platforms.
This study leads to three main conclusions: (1) The proposed improved energy function provides a less conservative and more accurate estimation of the transient stability region compared to traditional methods. (2) The nonlinear characteristics of damping and unbalanced power are the principal factors constraining the transient synchronous stability boundary. (3) The proposed control strategy effectively maintains system synchronization across various fault scenarios without relying on precise grid parameter estimation, thereby enhancing the system’s LVRT capability.
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Received: 18 August 2025
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2026, Vol. 41 
