Flexible Operation of Low-Inertia Power Systems Connected Via High Voltage Direct Current Interconnectors

The replacement of conventional synchronous generators with converter-interfaced generation units calls for increased amounts of flexibility. This paper proposes a novel formulation of the security-constrained unit commitment (SCUC) model applied to a multi-area power system connected via High Voltage Direct Current (HVDC) links. From a system perspective, this paper provides a critical analysis of the synergies and differences between the exploitation of thermostatic loads and HVDC links when providing different layers of flexibility to the system. The former units operate within a local dimension, while the latter enable cross-border exchange of flexibility. Eight different ancillary services are modelled to tackle generation/load outages and uncertainty/variability in renewable energy output. The model is applied to the Great Britain network, which is connected to the Irish network and to the one in Continental Europe. Results suggest a critical review of the operation of future low-carbon HVDC-interconnected systems. Feasibility studies on the benefit for interconnection should no longer neglect considerations on local post-fault frequency dynamics in each area of the system. Then, fundamental changes to the mechanisms that price ancillary services become necessary in order to align these mechanisms with the technical needs of the system.