Design of a Single Branch of Energy Storage Submodules Connected to HVDC Systems to Support AC Grids

The number of HVDC installations is increasing and the decarbonization of power systems makes it necessary to install storage systems. It might become relevant to assess the synergies between both trends by connecting storage systems on the DC side of HVDC systems. The contribution of this paper is to show the feasibility and the design of such a system to provide three different ancillary services: power oscillation damping (POD), fast frequency response (FFR), or wind power oscillation smoothing. The DC storage system consists of a series connection of sub-modules with energy storage devices connected to each sub-module through a DC-DC converter. For the proposed design methodology, a first simple average model of the DC storage converter is developed to run preliminary EMT simulations to obtain power profiles for the energy storage elements according to the system needs. These profiles are used as inputs for designing the storage system. The design methodology is validated by performing new EMT simulations with a more detailed model which includes a model of the storage elements. Results show that the designed system can provide the expected services and sizing results confirm the technical feasibility of the solution.