Research on the mechanism and transmission path of the oscillation in DFIG-connected flexible DC transmission system

•After system is disturbed, energy flows between different subsystems will be redistributed.•Detailed variation patterns of interaction energy links in oscillation process are obtained.•Energy flow path between the subsystems in the oscillation process is depicted.•Oscillation energy between PLL and RSC inner loop d axis subsystem is key energy link.•PLL—RSC inner loop q axis—flexible DC outer loop q axis is key energy transmission path. Concerning the oscillations in DFIG (doubly-fed induction generator)-connected flexible DC transmission system caused by large disturbances, an analysis method based on subsystem partitioning and dynamic energy modeling is put forward. Firstly, the complex high-order system is partitioned into several low-order subsystems, and the dynamic energy model of each subsystem is built using the first integration method. Then, by analyzing the directions and variation patterns of the interaction energy flows between different subsystems in the oscillation process, the redistribution of the primary flow paths of the system’s oscillation energy after large disturbances occur is determined. And then, an oscillation tracing method based on the response characteristics of the oscillation energy links of the subsystems is proposed. In the oscillation process, the interaction energy links between different subsystems are categorized into 'oscillation gaining links', 'oscillation maintaining links' and 'oscillation damping links', according to the contribution of each interaction energy link to the system’s oscillation energy. By analyzing the ‘oscillation gaining links’ corresponding to the dominant interaction energy links of different subsystems, the transmission paths by which the dominant oscillation energy links keeps gaining can be obtained, and the energy source that induces oscillation can be located. Finally, hardware-in-loop tests are conducted on the RTLAB semi-physical simulation platform, the results of which verify the effectiveness of the proposed method. This research paper can provide theoretical support for suppressing the oscillation by adding compensation branches between the subsystems (i.e. damping control strategies based on supplementary compensation branches).