Reliable Flexibility Provision from Distribution Systems to Enable Higher Grid Utilization by Curative Operation

Operating power grids in the course of the ongoing decarbonization of energy systems is increasingly challenging. More and more decentral renewable energy sources are contributing to the supply of electricity and cause higher transport demands in the power grids. The necessary grid expansion is running behind schedule required to meet the targets of the Paris Climate Accords for reasons of social acceptance and high costs. Therefore, transmission system operators as well as regulatory authorities are investigating new approaches to enable higher network utilization. One way to alleviate the situation, is to shift from preventive n-1 safe grid operation to curative n-1 safe grid operation. To enable such a change, operational degrees of freedom are needed, which can stem from different sources. In this work, the focus is on flexibility provided by distribution grids and how it can be used as an operational degree of freedom for curative grid operation. Flexibility allocated by adapting the schedules of distributed energy sources is provided in such a way, that the resulting power-flow changes relieve congestion in the transmission system. These distributed energy sources can be generators, such as wind-power or photovoltaic power plants, as well as flexible loads, such as power-to-heat plants or heat pumps. Two key questions in the context of this concept are treated in this thesis: Firstly, is there enough flexibility in the German distribution grids to use those grids for curative transmission grid operation? And secondly, how can a distribution system operator calculate the flexibility of his grid that can safely be provided? To answer the first question, an energy system model of Germany in the year 2030 is set up and available flexibility for curative transmission system operation is calculated. Time-steps are selected, in which a loss of transmission capacity leads to critical states in the grid, and the available flexibility in the regions adjacent to the critical corridor is calculated. Results show, that especially power-to-heat can provide valuable flexibility in situations where transmission capacity losses lead to critical states and, together with flexibility from wind-power, in 40 % of these situations the power provided by these two technologies is sufficient to reduce power-line loadings back to safe values. In the remaining 60 % of situations other sources must complement the flexibility necessary for curative operation or the utilization of the grid has to be reduced accordingly. The second question is answered by the development of a fast and robust optimization approach to limit the allowed ranges in power injection changes from individual energy resources in order to guarantee a safe state in the flexibility-providing distribution grid. The derived analytical solution of the boundary conditions that ensure a secure state in the grid can be integrated as a subordinate problem of the optimization, so that a large part of the overall problem can be calculated before optimization. The robustness of the approach is not only shown by the guaranteed compliance with boundary conditions, but also by a stable convergence behavior. This method is successfully tested on both a conceptual test grid and a real distribution grid. This work shows both that flexibility from the distribution grids is available in orders of magnitude relevant for curative grid operation, and that it can be used for such operation. The approach developed in this thesis to calculate and provide flexibility can be applied to real grids and offers the necessary security and speed needed for curative grid operation due to the inherent robustness