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| Mechanism and Characteristics of Subsynchronous Oscillation of Grid-Forming Direct-Drive Wind Farm Based on Matching Control |
| Gao Benfeng1, Deng Pengcheng1, Sun Dawei2, Wu Linlin2, Wang Xiao2, Deng Xiaoyang2 |
1. Hebei Key Laboratory of Distributed Energy Storage and Micro-Grid North China Electric Power University Baoding 071003 China;
2. State Grid Wind-Photovoltaic-Energy Storage Hybrid Power Generation Technology Laboratory North China Electric Power Research Institute Co. Ltd Beijing 100045 China |
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Abstract When new energy, mainly wind power and photovoltaic, is connected to the AC grid through power electronic converters on a large scale, the power system is prone to form a weak AC grid environment, which brings severe challenges to the stable operation of traditional grid-following new energy units. Therefore, grid-forming new energy units with better adaptability to weak AC grid have become the forefront and hot spot of research at home and abroad in recent years, and a variety of grid-forming control strategies have been proposed, such as sagging control, virtual synchronous generator control and matching control. Compared with other grid-forming control strategies, matching control only needs to measure the DC voltage to realize the autonomous synchronization function and has a faster response speed, which has been widely used in new energy units, especially direct-driven wind turbines. However, at present, relevant studies of grid-forming new energy units based on matching control mainly focus on the supporting capacity and inertia response characteristics of AC grid, and its subsynchronous oscillation (SSO) stability under different AC grid strength is not clear.
In order to solve the above problems, this paper firstly establishes the small signal model of grid-forming direct drive wind farm (DDWF) integrated to AC grid system based on the modular modeling method. Secondly, the eigenvalue analysis method is used to analyze the main oscillation modes and participating factors of the grid-forming DDWF, and to analyze its stability in the weak AC grid compared with the traditional grid-following DDWF. Then, the damping torque method in synchronous generator is used to analyze the oscillation mechanism of the grid-forming DDWF based on matching control. Finally, the influence of AC grid strength, the number of direct drive fans and reactive power controller PI parameter on SSO mode damping characteristics of the grid-forming DDWF integrated to AC grid system is studied by eigenvalue root locus.
Based on the eigenvalue analysis, it is concluded that the SSO stability of the grid-forming DDWF integrated to AC grid system is mainly affected by the oscillation mode dominated by matching control. Under strong AC grid condition, the oscillation mode dominated by matching control presents negative damping characteristic, and the system has SSO risk. However, compared with the DC capacitance and phase-locked loop (PLL) oscillation modes in the grid-following DDWF, the oscillation mode dominated by matching control in the grid-forming DDWF have better adaptability to the weak AC grid. In addition, when the DC capacitance and the matching control act together, the oscillation mode dominated by matching control has a dynamic characteristic similar to the rotor motion equation of the synchronous machine, which makes the grid-forming DDWF may be unstable due to insufficient damping.
The following conclusions can be drawn from the simulation and experiment analysis: (1) The oscillation mode dominated by matching control in the grid-forming DDWF presents negative damping characteristics under the strong AC grid, and the risk of SSO exists in the grid-forming DDWF integrated to AC grid system. However, the oscillation mode dominated by matching control has better adaptability to weak AC grid, and the grid-forming DDWF can run stably under the weak AC grid. (2) The oscillation mode dominated by matching control has a dynamic characteristic similar to the rotor motion equation of synchronous machine, which makes the grid-forming DDWF may have weakly damping oscillation. (3) Reducing the AC grid strength or the integral coefficient of reactive power controller, increasing the number of direct drive fans or the proportional coefficient of reactive power controller, can increase the damping of the oscillation mode dominated by matching control and reduce the SSO risk of the grid-forming DDWF.
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Received: 21 March 2023
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2024, Vol. 39 
