In recent years, ultra-low frequency oscillations (ULFOs) have occurred in power systems with a high proportion of hydropower, which seriously threatens the system stability. With the rapid increase of the proportion of photovoltaic (PV) plants in the power grid, it is feasible to use the regulation ability of PV plants to suppress ULFOs. First, the inadaptability of the traditional Phillips–Heffron model in the analysis of ULFOs is pointed out. Second, a frequency response model of hydro-PV integrated systems is established, and a PV supplementary damping control method for suppressing ULFOs is proposed on the basis of this model. Third, the frequency response model is extended to multihydropower and multi-PV systems, and the supplementary damping control parameters of multiple PV plants are designed collaboratively. Finally, time-domain simulation and experimental results validate the effectiveness of the proposed method.

Aiming at the problem of ultra-low-frequency oscillation (ULFO) in power systems, with an oscillation frequency lower than 0.1 Hz, an oscillation suppression control method is proposed. Firstly, the mathematical modeling of the system is carried out, and a cascade correction control method of the governor of hydropower generators is proposed. Secondly, using the ability of photovoltaic (PV) active power to adjust the system frequency, an additional controller for PV active power is proposed. Then, a unified frequency model considering the two controllers is established for multi-hydro and multi-PV systems, and the structural singular value method is used to coordinate the design of the parameters of the hydropower and PV generator controllers to achieve robust performance. Finally, based on a time-domain simulation of single-hydro and single-PV systems and an actual demonstration system located in Sichuan Province, China, the results show that the control method proposed in this paper has a good suppression effect on ultra-low-frequency oscillation under different working conditions, and meets the requirements of robust performance.