Complementary Grid Forming: Inertia Gathered from Constrained Resources

Power-electronic contributions of inherent inertia require grid-forming (GFM) control schemes such as ‘virtual synchronous machine’. In order to minimize cost and expenditure, the available resources should be fully exploited. Constraints of the resource at/behind the DC link require an AC behaviour slightly or strongly deviating from that of a synchronous machine. For two outstanding aspects of those deviations, this article proposes clear-cut approaches.
The notion ‘complementary grid forming’ (C-GFM) is proposed for contributors of inherent inertia which cannot fully comply with scientific GFM criteria, be it due to only one or several deviations. The definition of C-GFM draws a sharp demarcation towards GFL (grid following) and a soft line towards full GFM. To reflect the particularities of power-electronic inherent inertia, the P-contribution time constant TP is proposed for general use. For controller-agnostic description and reporting of inertia, a minimal set of rules called tailored TP covers the parameterization and operation of power-electronic inertia contributors. Tailored TP allows for the shaping of inertia with a maximum of degrees of freedom. Utilization examples are oriented at constraints of resources and also depict novel possibilities of shaping inertia contributions in order to keep the maximum of network participants operational during extreme events.