Modeling of Multi Terminal HVDC Systemsin Power Flow and Optimal Power Flow Formulations

Sammanfattning: Nowadays, due to lack of a strong interconnection between electric power systems within EU, there is a concern about the restricted power exchange. On the other hand, one of the factors which results in necessity of improving the level of power exchange is development of renewable energy sources such as offshore wind farms.Multi-terminal HVDC (MTDC) systems are supposed to be one of the cost-effective ways to aggregate a huge amount of renewable energies on one side and on the other side connect it to the main AC system through a common DC network.Possibility of such connections has led to the proposition of a meshed DC grid which connects several renewable energy sources and large AC systems.In order to fully take advantages of such AC-DC systems in the realsize power systems, extensive research has to be carried out to reveal their steady state and dynamic behavior. This thesis addresses different steady state aspects of such hybrid AC-DC systems.In the first part of thesis, we develop a multi-option power flow approach for hybrid AC-DC grids. The main contribution of this approach is that only one additional state variable is added to the AC and DC variables for each slack converter to handle the slack converter losses. Doing so, all AC, DC and converter equations are solved simultaneously. This makes the power flow algorithm much simpler than the sequential approaches where one external iterative loop is assigned to compute the converter losses. Such a high number of iterative loops in the sequential approaches makes the algorithm not only complicated and time consuming, but also unreliable.In the second part of thesis, given the nonconvex nature of Power Flow Optimisation (OPF) problem, a convex OPF formulation for AC grids with embedded DC networks based on the new Line Flow Based (LFB) variablesand in the form of Second Order Cone Programming (SOCP) is developed. SOCPs are a general form of linear programming accompanied by nonlinear constraints in the form of convex cones which can be efficiently solved through Interior Point methods (IPMs).