  Low levels of system inertia in power grids with significant penetration of
non-synchronous Renewable Energy Sources (RES) have increased the risk of
frequency instability. The provision of a certain type of ancillary services
such as inertia and Frequency Response (FR) is needed at all times, to maintain
system frequency within secure limits if the loss of a large power infeed were
to occur. In this paper we propose a frequency-secured optimisation framework
for the procurement of inertia and FR with diverse dynamics, which enables to
apply a marginal-pricing scheme for these services. This pricing scheme,
deduced from a Mixed-Integer Second-Order Cone Program (MISOCP) formulation
that represents frequency-security constraints, allows for the first time to
appropriately value multi-speed FR.
