Generalization of oscillation loop and energy flow analysis for investigating various oscillations of renewable energy systems

The renewable energy system can stimulate more dynamics and increase the possibility of the oscillatory instability. The dynamic model becomes significantly complicated due to the large numbers of grid-connected renewable energy systems and time-varying features of their control parameters. Therefore, it is critical to investigate the various oscillations of renewable energy systems preferably without the high-dimensional dynamic modeling. The existing monitoring-based methods cannot provide the mechanism analysis of emerging oscillations or quantitative analysis of the damping feature of oscillations of renewable energy systems. In this paper, firstly, the generalized oscillation loop is designed based on the second-order differential operations of a concerned control loop of a renewable energy system to analyze the mechanism of the emerging oscillation associated with this control loop. On this basis, a generalized energy flow analysis is proposed for the quantitative analysis of the damping feature of oscillations in this control loop. Then, taking the grid-side control loops of the full converter-based wind generation system as an example, the key physical quantities of emerging oscillations are further derived and the damping feature assessment of concerned oscillations is conducted. Finally, the developed methodology is verified to investigate the various oscillations of the full converter-based wind generation system in the case studies. The results demonstrate that the eigenvalues of concerned oscillations can be accurately estimated through the generalized energy flow analysis without the whole grid modeling; additionally, the oscillatory stability of a RES can be enhanced by improving the damping performance of emerging oscillations with the help of the mechanism analysis, which lays a foundation for setting converter control parameters and configuring additional damping devices.