AbstractWith the increasing share of DG and RES, the changing framework in the energy market, the number of traditional synchronous generators decreases, hence primary sources for voltage control are missing. Furthermore, the voltage in the power system is changing much more rapidly, following system load variations and intermittent RES infeed. In unanticipated low-RES infeed/ high load cases, voltage drops appear dramatically, mainly when there are no devices for voltage control. Considering this, TSOs are increasingly looking for additional resources within the distribution system to be deployed in greater coordination. Hence, it is crucial to develop tools/ approaches for the management and planning of voltage/ control of reactive power in future control centers. Bearing in mind that reactive power is always a local challenge and most up-to-date DG and RES units, especially connected at the distribution level, are technologically capable of controlling voltage, these generation units’ capabilities must be considered from the perspective of operational control as further flexibilities. Since reactive power must be transferred to the TSO network, intensive cooperation between TSO and DSO demanding simultaneous coordination is required. Therefore, the optimal planning and operation of DG and RES in the distribution networks to contribute to local and regional power management and reactive power provision to the transmission networks must be enabled.The goal of this Master Thesis is to demonstrate the benefits of a coordination mechanism through high-level Use Case development between DSO and TSO. The coordination scheme depends on an OPF tool attributing Multi-Objective (MO) optimization, which is envisaged as an effective means to resolve the future operational challenges. This contains cooperation between two real-time OPFs running at DSO and TSO control centers. An interoperation chain depending on sequential optimizations and exchange of relevant information and setpoints is defined.The use case is implemented in simulation in order to manage long term voltage variations. The simulation includes models of DGs and their reactive power controllers and models of network components such as OLTCs. The OPF is implemented using the GAMS, and MATLAB is used to orchestrate the data exchange and managing input and output data for simulation. The results demonstrate the benefits of TSO-DSO coordination at the operational level with the fewer variable voltage profiles and closer to the required references. Simultaneously, the results also illustrate the impact of rerouting reactive power on the total loss in the network.