The Provision of Instantaneous Reserve from MVDC Grids in the Transitioning Power System

Abstract:
The phase-out of fossil fuels is transforming the power system, moving from machine-based power plants to converter-based distributed generation. This change poses stability challenges, i.e. during frequency events in low-inertia regions. To address these, recommendations include increasing the use of grid-forming converters. However, the provision of instantaneous reserve by converters differs fundamentally from conventional systems, particularly in terms of activation times and control processes on the DC side. This paper examines these differences and proposes a general distinction between instantaneous and primary power reserve within converter-based systems and their machine-based equivalents. An MVDC grid case study, where onshore wind farms and storage systems are combined via a medium voltage DC grid and feed directly into the transmission grid via a grid-forming converter, demonstrates the provision of instantaneous reserve at transmission level. Offline simulations and real-time validations show the potential of MVDC grids for rapid frequency response, highlighting the need for a distinction of time constants in hybrid AC and DC systems.