Voltage Support of Transmission Grids Using Resources Located in the Underlying Lower Voltage Networks : Unlocking the Potential of Distributed Generation Reactive Power Capabilities

The path to sustainable, fossil-free future leads to inevitable energy transition towards renewable energy sources. A part of this transition includes replacement of conventional transmission system connected power plants with distributed renewables plants such as wind power. Consequently, transmission system loses its main source of reactive power responsible for voltage control – synchronous generators. This dissertation tries to answer if, how and to what extent can the missing reactive power be replaced from underlying Active Distribution Networks (ADN) hosting renewable distributed Inverter-Based Generation (IBG). The first part of this dissertation analyzes potential of ADNs to support reactive power in long-term time frame where dynamics of interest lasts from couple of minutes to couple of hours. The bounds on how much reactive power can be provided or consumed at the point of common coupling are analyzed. The most important factors affecting these bounds are identified. Some of these factors are subject to uncertainties. To analytically represent ADN reactive power capability bounds in the presence of these uncertainties, new probabilistic ADN capability charts are proposed. These charts describe in the compact form the aforementioned reactive power bounds as random variables given as a function of active power exchange at the point of common coupling. Having defined ADN reactive power capability, a new method to calculate flexibility of DSO/TSO interface is introduced. This flexibility is calculated taking into account both ADN provision capability and transmission system need for flexible reactive power at the point of common coupling. The second part of the dissertation analyzes short-term capability of IBGs hosted in ADNs to support voltage recovery of local ADN buses and remote transmission system buses. Special attention is put on relevance of preserving IBG active fault-currents in the areas with high ratio of dynamic load component. The results of case studies show that ADN hosted IBGs can substantially help in voltage recovery of local ADN buses. However, they have limited effect on the voltage recovery of electrically further transmission system buses. The studies presented in this dissertation point out to the substantial potential of supporting reactive power to transmission systems from the underlying ADNs. However, this potential is not enough to fully replace synchronous generators as providers of reactive power. As a solution, the missing reactive power can be replaced by combination of reactive power support from ADNs and other dynamic reactive power resources hosted locally in transmission systems.QC 20210810