Abstract
In recent years, ultralow-frequency oscillation has repeatedly occurred in asynchronously connected regional power systems and brought serious threats to the operation of power grids. This phenomenon is mainly caused by hydropower units because of the water hammer effect of turbines and the inappropriate Proportional-Integral-Derivative (PID) parameters of governors. In practice, hydropower and solar power are often combined to form an integrated photovoltaic (PV)-hydro system to realize complementary renewable power generation. This paper studies ultralow-frequency oscillations in integrated PV-hydro systems and analyzes the impacts of PV generation on ultralow-frequency oscillation modes. Firstly, the negative damping problem of hydro turbines and governors in the ultralow-frequency band was analyzed through the damping torque analysis. Subsequently, in order to analyze the impact of PV generation, a small-signal dynamic model of the integrated PV-hydro system was established, considering a detailed dynamic model of PV generation. Based on the small-signal dynamic model, a two-zone and four-machine system and an actual integrated PV-hydro system were selected to analyze the influence of PV generation on ultralow-frequency oscillation modes under different scenarios of PV output powers and locations. The analysis results showed that PV dynamics do not participate in ultralow-frequency oscillation modes and the changes of PV generation to power flows do not cause obvious changes in ultralow-frequency oscillation mode. Ultra-low frequency oscillations are mainly affected by sources participating in the frequency adjustment of systems.
Highlights
There are differences in mechanism and characteristics between ultralow-frequency oscillation and traditional low-frequency oscillation
Reference [8] used the vector margin method to analyze multimachine systems, and the results showed that thermal power units and hydropower units with small time constants of the water hammer effect can increase the vector margin of the system while hydropower units with large time constants of the water hammer effect can reduce the vector margin
The main conclusions are summarized as follows: (1) The dynamics of synchronizers, governors, and turbines were mainly involved in ultralow-frequency oscillation modes, while the dynamics of PV were hardly involved
Summary
There are differences in mechanism and characteristics between ultralow-frequency oscillation and traditional low-frequency oscillation. Built a small-signal model of a hydropower system and analyzed the change of the damping of the ultralow-frequency oscillation mode when the PID parameters of a governor were changed through a characteristic analysis method. Reference [15] focused on the damping of local-mode power system oscillations and pointed out that, through eigenvalue analysis, the impact of PV power generation on the small-signal stability of power systems can be positive or negative. Reference [18] believed that, PV dynamics do not participate in low-frequency oscillation modes, the access of PV changes the output of the synchronous system of an original system and the power flow distribution of the system, thereby affecting the low-frequency oscillation mode. Motivated by the aforementioned limitations, this paper studied the impact of PV access on the ultralow-frequency oscillation mode of a hydropower system.
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