  With Europe dedicated to limiting climate change and greenhouse gas
emissions, large shares of Renewable Energy Sources (RES) are being integrated
in the national grids, phasing out conventional generation. The new challenges
arising from the energy transition will require a better coordination between
neighboring system operators to maintain system security. To this end, this
paper studies the benefit of exchanging primary frequency reserves between
asynchronous areas using the Supplementary Power Control (SPC) functionality of
High-Voltage Direct-Current (HVDC) lines. First, we focus on the derivation of
frequency metrics for asynchronous AC systems coupled by HVDC interconnectors.
We compare two different control schemes for HVDC converters, which allow for
unilateral or bilateral exchanges of reserves between neighboring systems.
Second, we formulate frequency constraints and include them in a unit
commitment problem to ensure the N-1 security criterion. A data-driven approach
is proposed to better represent the frequency nadir constraint by means of
cutting hyperplanes. Our results suggest that the exchange of primary reserves
through HVDC can reduce up to 10% the cost of reserve procurement while
maintaining the system N-1 secure.
