Analysis of Path and Damping Characteristics of Subsynchronous Interaction Between Photovoltaic Plant and Weak AC Grid
Gao Benfeng1, Deng Pengcheng1, Liang Jifeng2, Zhao Yuhao2
1. Hebei Key Laboratory of Distributed Energy Storage and Micro-Grid North China Electric Power University Baoding 071003 China; 2. State Grid Hebei Electric Power Co. Ltd Electric Power Research Institute Shijiazhuang 050021 China
Abstract When a large-scale photovoltaic (PV) plant is integrated into the AC grid through long transmission lines, the interaction between the PV plant and weak AC grid may make the integration face the threat of subsynchronous oscillation (SSO). However, the mechanism of the subsynchronous interaction between the PV plant and weak AC grid are unclear, and quantifying the damping characteristics of the subsynchronous interaction is difficult by the existing analysis methods. Therefore, this paper proposes a damping torque method extended to the dynamic equation of phase-locked loop (PLL) to analyze the subsynchronous interaction between the PV plant and weak AC grid. Firstly, the damping torque method is extended to the PLL dynamic equation to analyze the stability of the PV plant integrated into the weak AC grid system in the SSO mode dominated by PLL. Secondly, the linearization model of the PV plant integration and its corresponding closed-loop transfer function block diagram are derived. Based on the closed-loop transfer function diagram, the coupling relationship and interaction path between the PV plant and weak AC grid are revealed from the perspective of internal disturbance transfer, and the dynamic process of the interaction path affecting SSO mode damping is analyzed. Thirdly, through transfer function derivation, the damping path reflecting the subsynchronous interaction is separated, and the equivalent damping coefficient is used to quantify the SSO mode damping of the subsynchronous interaction. Finally, the influences of PLL control parameters, PV capacity, AC grid strength, PV plant intensity, and temperature on the damping characteristics of subsynchronous interaction are analyzed. The mechanism analysis shows that the PV plant-weak AC grid current disturbance and the grid-side voltage disturbance drive each other at the point of common coupling, forming the subsynchronous interaction path. Then, the subsynchronous interaction path will form a closed-loop damping path through the PLL in the SSO mode dominated by PLL, affecting the system SSO mode damping. Therefore, the damping contribution of the PV plant and subsynchronous interaction are quantitatively evaluated through the damping path separation. The results show that the PV plant and the subsynchronous interaction provide positive damping and negative damping to the system, respectively, resulting in the SSO of the PV plant integrated into the weak AC grid system. In addition, the subsynchronous interaction damping can be decreased by increasing the PLL proportion coefficient, AC grid strength, PV plant intensity or temperature, or decreasing the PLL integration coefficient or PV capacity, thereby improving the stability of the PV plant integrated to weak AC grid system. The following conclusions can be drawn from the theoretical analysis: (1) The subsynchronous interaction between the PV plant and weak AC grid will form the damping path through the PLL, affecting the SSO mode damping of the system. There is the SSO risk dominated by PLL in the PV plant integration. (2) The subsynchronous interaction provides negative damping to the system, which is the dominant factor for the SSO of the PV plant integration. (3) Reasonable design of system parameters according to the negative damping characteristics of subsynchronous interaction can effectively increase the SSO mode damping and better maintain the original positive damping characteristics of the PV plant.
Gao Benfeng,Deng Pengcheng,Liang Jifeng等. Analysis of Path and Damping Characteristics of Subsynchronous Interaction Between Photovoltaic Plant and Weak AC Grid[J]. Transactions of China Electrotechnical Society, 2023, 38(24): 6679-6694.
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2023, Vol. 38 
