This PhD thesis proposes a bottom-up approach that accurately addresses the operational flexibility embedded in each part of a multi-energy system (MES). Several models which cover the simulations from replicating domestic electrified demands to power system scheduling are proposed. More specifically, a domes-tic multi-energy consumption model is firstly developed to simulate one minute resolution energy profiles of individual dwellings with the installation of prospec-tive technologies (i.e., electric heat pumps (EHPs), electric vehicles (EVs)). After-wards, a fast linear programming (LP) unit commitment (UC) model is devel-oped with the consideration of characteristics of generators and a full set of ancil-lary services (i.e., frequency response and reserves). More importantly, the fre-quency response requirements in low inertia systems are assessed with the con-sideration of three grid frequency regulations (i.e., rate of change of frequency, Nadir and quasi-steady state). Furthermore, the UC model has integrated vari-ous flexibility contributors in MES to provide ancillary and flexibility services, which include pumped hydro storages (PHSs), interconnectors, batteries and demand side resources (i.e., individual EHPs, heat networks, electrolysers). More importantly, the fast frequency response (FFR) provision from nonsynchronous resources is implemented and the demand response application of electrolysers is taken as an example to provide FFR in the UC model. By using the integrated UC model with the consideration of flexibility services provided by resources in the MES, the advantages of multi-energy operation can be clearly identified which can be used to inform system operators and policy makers to design and operate energy systems in a more economic and environment-friendly way.

Variable renewable energy sources are today increasingly integrated in the power system as a step towards the renewable society. The large-scale introduction of converter-based energy sources brings challenges in terms of reduced damping to the power system due to the reduced number of synchronous generators. This can be manifested as high rate-of-change-of-frequency and decreased grid stability. To forestall this reduced performance, it is suggested that the grid-following control of today’s converters are restructured to a grid-forming control, enabling the converter to behave closer to a synchronous machine.   This thesis compares grid-following and grid-forming control and seeks to further describe this grid-forming behavior by applying a grid-forming control method on an energy storage enhanced STATCOM-system. A continuous time model and a linearized model based on state space representations are constructed in order to investigate the grid-forming behavior but also how the converter stability is affected by a restructure from grid-following to grid-forming control.   The results indicate that the investigated grid-forming control method displays a behavior similar to synchronous machines and incorporates the ability to provide frequency response services and so called “synthetic inertia” to the grid.  The results also show that the stability of the converter (the ability to provide a bounded output when the system is perturbed) is ensured when the control method is restructured from grid-following to grid-forming and that the investigated grid-forming method is stable also in weak grid situations.