Study of Low-Frequency Interactions between Grid-Forming MMCs located on both sides of a Bipolar HVDC link

Several high-voltage direct current (HVDC) inter-connections in a bipolar configuration are in the design stage and will be operational in the next decade. The HVDC converter stations could be operated in a grid-forming mode to contribute to power system stability. This paper analyses the low-frequency interactions between grid-forming converters placed at the opposite sides and poles of a bipolar HVDC link interconnecting asynchronous AC grids. A small-signal stability analysis carried out using a state-space model determines the interaction eigenvalues and the influence of the grid-forming control parameters on the system stability. The findings reveal the existence of low-frequency interactions between the two grid-forming converters, even though the converters are connected to different poles and asynchronous AC grids. The results are verified through EMT time-domain simulations considering a non-linear model implemented in EMTP software. Furthermore, the influence of the DC side layout on these interactions is studied by replacing the bipolar connection with a parallel monopolar configuration. The results highlight that the observed interactions do not depend on the considered DC connection layouts. Therefore, the outcomes of this work can be extended to parallel monopolar HVDC links interconnecting two asynchronous grids.