Understanding the role of VSC control strategies in the limits of power electronics integration in AC grids using modal analysis

International audienceIn this article, the interactions between AC grid and existing VSC control modes are analyzed. First, three VSC control modes, namely grid-feeding, grid-supporting and grid-forming, are compared with each other in order to assess VSC/grid coupling level. Then, in order to underscore the growing role of VSCs in transmission systems, an elementary benchmark network is considered. It includes an equivalent SM (Synchronous Machine), a VSC, a load and a transmission line with a varying electrical distance. Thereafter, a modal analysis is conducted on this model to study the interactions between the different VSC controls and the SM dynamics using a Matlab/Simulink environment. The converter penetration rate is represented by a variable power ratio between the converter and the machine, while the electrical distance by an RL dynamic impedance. This approach makes it possible to assess the stability limits on the VSC share imposed by the system strength. Negative interactions are identified and recommendations for novel converter control strategies proposed. Besides, the influence of the AVR with the VSC in grid-forming as well as PLL-related stability issues with the VSC in grid-feeding or grid-supporting are shown to be the key limiting factors for increasing the penetration of PE-based sources