Classification and dynamic assessment of droop-based grid-forming control schemes: Application in HVDC systems

Abstract

This paper introduces a novel classification for droop-based control strategies towards emerging interest of utilizing grid-forming control schemes in high voltage direct current (HVDC) systems. Classical droop control for grid-forming operation does not conventionally depend on a phase-locked loop (PLL). It is demonstrated that by including a PLL in droop control, it is possible to define two other droop-based control variants with different capabilities and functionalities. To highlight the differences in performance of each control scheme, the active power response of the power converter is evaluated in case of variation in grid short circuit ratio (SCR) as well as the PLL response time by considering an ideal DC bus. Accuracy in active power regulation, frequency support capability and virtual inertia emulation are the functionalities that have been addressed for the proposed three different control schemes. In a next step, the introduced droop-based grid-forming schemes are assessed by considering the dynamics of both AC and DC side of the power converter at one terminal of an HVDC link. Finally, based on the simulation results, a summary of features for all introduced grid-forming control schemes is presented11This work is supported by the project “HVDC Inertia Provision” (HVDC Pro), financed by the ENERGIX program of the Research Council of Norway (RCN) with project number 268053/E2, and the industry partners; Statnett, Statoil, RTE and ELIA..