Integration of a storage device to the DC bus of a grid-forming controlled HVDC interconnection

This paper assesses the performance of a high-voltage direct-current (HVDC) interconnection for providing virtual inertia to the power system. The impact of inertia provision on the DC bus dynamics and system frequency when one or both converter terminals are controlled with a virtual synchronous machine (VSM) based gridforming strategy is evaluated. It has been demonstrated that an identical inertial support cannot be provided to both terminals due to a fundamental conflict between the DC voltage control and inertia emulation tasks. Therefore, it has been proposed to integrate a storage device (e.g. flywheel energy storage system (FESS)) with an additional converter to the DC link in order to control the DC voltage. Doing so, the DC voltage control is not couple with the inertia emulation at each substation and the obtained results have shown that a good performance in the DC voltage control as well as a proper inertia support provision are achieved. Finally, a discussion on a simplified approach to size the storage system is presented.