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| Low-Frequency Stability Analysis of Voltage-Sourced Doubly-Fed Wind Power Grid-Connected System under Small Disturbance |
| Han Yingsheng1, Sun Haishun1, Qin Shiyao2, Zhu Tingmeng1, Wang Dongze1 |
1. State Key Laboratory of Advanced Electromagnetic Engineering and Technology Huazhong University of Science and Technology Wuhan 430074 China;
2. China Electric Power Research Institute Beijing 100192 China |
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Abstract Almost all wind turbines currently used in practical projects use the grid-following control method, which is characterized by the risk of oscillation instability in weak grids, and therefore this control method is inevitably unable to adapt to power system scenarios with increasing penetration of renewable energy sources. To address this issue, some researchers have proposed the grid-forming control method, which can actively support the grid voltage and thus have the ability to operate stably in weak grid conditions. However, the existing work is insufficient in the investigation of oscillation characteristics of grid-forming units, especially for the low-frequency stability and instability mechanism introduced by the virtual synchronous control part. This paper presents an analysis method for this problem. By establishing the generalized Phillips-Heffron model of voltage-sourced DFIG, it can accurately assess the damping characteristics and oscillation risk of the low-frequency mode.
Firstly, a typical grid-forming control strategy applied to DFIG is introduced, including its power outer loop and voltage-current inner loop structure. Secondly, a mathematical model of the grid-forming DFIG-connected power system considering only its slow dynamic part is established, which has the same form as the Phillips-Heffron model used in the low-frequency oscillations analysis of synchronous machines, based on which, the oscillation frequency and electrical damping calculation method of the low-frequency mode dominated by the virtual synchronous control are derived. Thirdly, by applying the aforementioned damping quantification analysis method and electromagnetic transient simulation, the effects of short-circuit ratio, wind power output, and various control parameters on the low-frequency oscillation characteristics of grid-forming DFIG are analyzed. Finally, a stabilization controller attached to the reactive-voltage loop is proposed to improve the low-frequency damping characteristics of grid-forming DFIG and reduce its low-frequency oscillation risk.
The analysis results of the grid-forming DFIG integrated system show that when the grid-side short-circuit ratio increases from 3 to 9, the damping of the low-frequency oscillation mode dominated by the virtual synchronous control will decrease from 2.73s-1 to -0.39s-1, leading to the system oscillation instability. This indicates that the dynamic characteristics of the grid-forming DFIG under weak grid conditions are better than those under strong grid conditions, which is the opposite of the conventional grid-following DFIG. Besides, the analysis of other parameters shows that the damping of the low-frequency mode tends to decrease as the virtual inertia coefficient increases, the virtual damping coefficient decreases, the active power-frequency droop coefficient decreases, and the proportional and integral gains of the voltage loop control decrease, while the reactive power-voltage droop coefficient has almost no effect on the damping of low-frequency mode. The comparison results between scenarios with and without the stabilization controller show that this attached controller can effectively improve the oscillation damping under strong grid conditions, and it works better than just increasing the virtual damping coefficient. The results of the time-domain simulations are all consistent with the damping analysis results, indicating that the proposed method for analyzing the low-frequency oscillation characteristics of grid-forming DFIG is accurate and effective.
The following conclusions can be drawn from the analysis results: (1) When the electrical damping introduced by the coupling of AC system and other control links of the grid-forming DFIG is negative, and its absolute value is greater than the inherent positive damping of the virtual synchronous control, the grid-forming DFIG as a whole will show negative damping characteristics in the low-frequency band and face the risk of low-frequency oscillation instability. (2) The damping of the grid-forming DFIG is smaller under strong grid conditions, contrary to that of the grid-following converter. (3) the proposed method can accurately analyze the low-frequency oscillation characteristics of grid-forming units under small disturbances.
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Received: 13 October 2021
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2023, Vol. 38 
