Flexible Power Plant Operation in Electricity Systems with High Shares of Renewables

The massive introduction of variable and only partly predictable renewable energy sources (such as wind and solar power) in modern power systems poses challenges regarding the limited flexibility of power plants, cost-efficient electricity market operation and the system’s reliability. This dissertation presents an operational modeling framework, which is afterwards deployed in case studies for the purpose of gaining new or improved insights on technical and cost-related aspects of electricity generation systems in light of these challenges. At the heart of the developed modeling framework is an operational model of the electricity generation system, called a unit commitment model. This operational model computes the cost-optimal generation schedule for a portfolio of power plants and storage units that respects their technical constraints. Concurrently, the unit commitment model ensures sufficient operating reserves to cope with the uncertainty of intermittent renewable power. By deploying the unit commitment model in combination with a dedicated wind power forecast scenario generation tool, developed for this purpose, the resulting modeling framework is suitable for studying flexible power plant operation, electricity market operations and design changes in systems with high penetrations of intermittent renewable power. This dissertation first addresses the difficulties of managing the variations in renewable power generation and to cope with forecast errors against the backdrop of the limited flexibility of the generation fleet. The impact of the growing share of variable renewables on cycling of thermal power plants is studied. Specific attention is devoted to increased maintenance costs caused by cycling, as they are shown to comprise an important share of the total operational costs, and are significant in comparison to the marginal generation costs. Moreover, the possible trade-off between start-up flexibility and corresponding costs is endogenously included in the scheduling decisions, which leads to cost reductions and reduced cost-optimal operating reserve requirements. Regarding cost-efficient electricity market operation, an important aspect to account for is the significant uncertainty that originates from limitedly predictable intermittent renewables. The case studies in this dissertation demonstrate a considerable cost-saving potential of fixing the final generation schedule closer to the moment of delivery, which is mainly driven by reduced need for operating reserves associated with decreased renewable power uncertainty with shorter forecasting lead times. The cost-benefits are, however, restricted by the flexibility of the generation fleet. Flexibility in the form of operating reserves is required to deal with forecast errors in real time. The benefits of increasing the temporal granularity of operating reserve markets are investigated. The results of a case study show that reserve procurement on a short-term basis (i.e., frequent and dynamic sizing and frequent allocation with high resolution products) leads to considerable total operating cost savings and could mitigate scarcity on operating reserve markets. Furthermore, the studied reserve market design changes facilitate the integration of intermittent renewables. Finally, this dissertation presents a contribution to the field of electricity generation system adequacy assessments. The link between generation adequacy and operational security is studied. The adequacy metrics obtained from operational models that are typically used in today’s adequacy studies are shown to represent a worst-case situation and should be interpreted with care. By incorporating state-of-the-art reserve requirements, the contribution of operating reserves to generation adequacy is accurately captured, leading to more accurate adequacy indices than obtained from commonly used methodologies. These findings should encourage power system practitioners to adopt novel operational power system models for adequacy assessments that accurately estimate the contribution of operating reserves to system adequacy.