  Modern electricity grids throughout the world, particularly in islands such
as Great Britain, face a major problem on the road to decarbonisation: the
significantly reduced level of system inertia due to integration of Renewable
Energy Sources (RES). Given that most RES such as wind and solar are decoupled
from the grid through power electronics converters, they do not naturally
contribute to system inertia. However, RES could support grid stability through
appropriately controlling the converters, but currently no market incentives
exist for RES to provide this support. In this paper we develop a methodology
to optimally clear a market of ancillary services for frequency control, while
explicitly considering the participation of grid-forming and grid-following
inverter-based technologies. We propose a mathematical framework that allows to
compute shadow prices for ancillary services offered by a pool of diverse
providers: synchronous and synthetic inertia, enhanced frequency response (e.g.
from curtailed RES) and traditional primary frequency response (e.g. by thermal
generators). Several case studies are run on a simplified Great Britain system,
to illustrate the applicability and benefits of this pricing scheme.
