  The COVID-19 pandemic led to partial or total lockdowns in several countries
during the first half of 2020, which in turn caused a depressed electricity
demand. In Great Britain (GB), this low demand combined with large renewable
output at times, created conditions that were not expected until renewable
capacity increases to meet emissions targets in coming years. The GB system
experienced periods of very high instantaneous penetration of non-synchronous
renewables, compromising system stability due to the lack of inertia in the
grid. In this paper, a detailed analysis of the consequences of the lockdown on
the GB electricity system is provided, focusing on the ancillary services
procured to guarantee stability. Ancillary-services costs increased by
{\pounds}200m in the months of May to July 2020 compared to the same period in
2019 (a threefold increase), highlighting the importance of ancillary services
in low-carbon systems. Furthermore, a frequency-secured scheduling model is
used in the present paper to showcase the future trends that GB is expected to
experience, as penetration of renewables increases on the road to net-zero
emissions by 2050. Several sensitivities are considered, demonstrating that the
share of total operating costs represented by ancillary services could reach
35%.
