Combined AC and Multi-Terminal HVDC Grids – Optimal Power Flow Formulations and Dynamic Control

The trend towards renewable generation and more efficient and flexible load behavior in power systems is generally known. Nevertheless, no actual and future power system works without the connection between production and consumption. Today, and probably even more in the future, climate conditions define the location of generation while load centers remain in the same place. Therefore, the transmission system remains essential. Nowadays, several point-to-point high voltage direct current (HVDC) connections are in operation. There are four convincing reasons why the grid should contain more HVDC parts in the future: lower transmission losses, capability of long cable connections, higher controllability, and the planned refurbishing of the existing transmission infrastructure due to their age, which gives a good opportunity to switch from AC to DC technology. A high cost share of an HVDC connection are the converter stations. Therefore, it is possible that the future HVDC system is constructed as a meshed grid instead of only point-to-point connections. Before a transmission system operator (TSO) will agree to install such an multi-terminal HVDC (MTDC) grid, the following points need to be clarified: what is the influence to the grid in steady state and dynamic operations? How could the new expensive parts be beneficial for the TSO? This thesis provides the tools to analyze and improve the steady state status of a combined grid. Furthermore, a controller will be proposed to use the flexibility of the voltage source converter (VSC) stations to share frequency containment reserves between asynchronous AC control areas. To calculate the steady state behavior of a combined AC and HVDC grid the existing algorithms for AC grids needed to be expanded. The goal is to minimize the cost of operating the power system by changing the power setpoints of the converters and generators wherever possible