A Robust Design Method for Ultra-Low-Frequency Oscillation Suppression Control in Hydro-Photovoltaic Complementary Systems  
[PERSON]  
1 State Grid Sichuan Electric Power Research Institute, No.16 Jinhui West 2 nd Street, Chengdu 610041, China; [EMAIL_ADDRESS] (X.Z. ); [EMAIL_ADDRESS] (G.C. ); [EMAIL_ADDRESS] (H.S. ); [EMAIL_ADDRESS] (P.S.) [PERSON]  
1 State Grid Sichuan Electric Power Research Institute, No.16 Jinhui West 2 nd Street, Chengdu 610041, China; [EMAIL_ADDRESS] (X.Z. ); [EMAIL_ADDRESS] (G.C. ); [EMAIL_ADDRESS] (H.S. ); [EMAIL_ADDRESS] (P.S.) [PERSON]  
1 State Grid Sichuan Electric Power Research Institute, No.16 Jinhui West 2 nd Street, Chengdu 610041, China; [EMAIL_ADDRESS] (X.Z. ); [EMAIL_ADDRESS] (G.C. ); [EMAIL_ADDRESS] (H.S. ); [EMAIL_ADDRESS] (P.S.) [PERSON]  
2 School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044, China  
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of PV generators further deteriorates the stability problem. Therefore, it is of great significance to study the ULFO suppression control measures of hydro-PV complementary systems [7; 8]. Existing studies have shown that ULFOs are strongly related to the primary frequency modulation process of hydropower generators, so most of the suppression measures for ULFOs use the method of optimizing the control parameters of turbine governors [9; 10; 11]. In [12; 13], the authors use a PSO optimization algorithm to adjust the governor parameters to increase the damping torque to suppress ULFO. In [14], based on the damping torque method, the authors concluded that the ratio of the proportional parameter to the integral parameter is too small, resulting in a small damping coefficient. The optimal PID parameter tuning method was then proposed to address the internal causes of ULFO. The analysis results in [15] showed that the permanent state difference coefficient and water hammer coefficient are too small and will cause negative damping, and the GA-PSO optimization algorithm is proposed for parameter tuning. In [16], the authors proposed a governor parameter optimization method that maximizes system tracking performance under multi-mode robust stability constraints. In [17], the authors used a state subspace identification algorithm and an improved particle swarm optimization algorithm for an optimized design of governor parameters. The above studies mainly suppress ULFOs by properly adjusting the control parameters of the governor. However, there is a contradiction between ULFO suppression and the requirements of the primary frequency modulation on the governor control parameters. In addition, reference [18] proposed a PR-PSS algorithm to prevent ULFO, whose parameter settings are updated adaptively by a deep reinforcement learning algorithm. Reference [19] put forward a design method of PSS for the governor and verifies its effectiveness. The above methods do not take into account the regulation capacity of PV generators to mitigate ULFOs. In addition, PV generators have flexible control capabilities, which can provide more flexible control resources for oscillation suppression [20]. However, the current research on the use of PV generators to suppress oscillations is mainly focused on low-frequency oscillations, and low-frequency oscillations are suppressed by introducing additional damping control in the control loop of PV inverters. In [21], the authors proposed a PV wide-area damping controller based on the correlation identification method. In [22; 23], the authors used all the active power regulation capacity for damping control or set aside half of the active power to provide sufficient margin for damping control. In [24], the authors proposed active power additional damping control of PV generators based on active disturbance rejection control. In [25], the authors used integrated power modulation to design a PV wide-area additional damping controller.