Coordinated Control of Power Systems with HVDC Links

Today’s power systems are large networks of electrical energy sources and load components, connected over long distances mainly through alternating current (AC) transmission and distribution grids. This basic structure currently undergoes significant changes to ensure the stability, availability and sustainability of the future electrical energy infrastructure. As a result, power system operation and development faces several challenges, including volatile injections from distributed renewable energy sources, variable energy demands and tighter security margins. The development of high voltage direct current (HVDC) links provides an alternative solution for the efficient and flexible transmission of electrical energy, that can support the future power system along multiple dimensions. First, during normal operation, HVDC links provide an increased controllability of the AC power system’s operating point. Power system operators can use HVDC links to optimize the AC power flow in the network in order to avoid congestions and to achieve an economic gain. Secondly, during dynamic situations, HVDC links can be used by a fast automatic grid controller to support the power system’s transient stability. This thesis studies the control of HVDC injections in power systems during dynamic scenarios. Coordinated HVDC control has a large potential for the dynamic performance of power systems, for instance by increasing the damping of power oscillations, but is currently not exploited in a systematic way. The aim is to develop a framework for power system control through HVDC transmission links. Starting with results for classical AC networks, the thesis presents power system models, operation approaches and network planning methods in the context of dynamically controlled HVDC links. The modeling of power systems with HVDC links has to incorporate several physical and operational constraints imposed by the HVDC links and the surrounding AC network. A characterization of the resulting constraints on the HVDC injections is particular important if the HVDC links are to be used for dynamic power system control. Classical capability charts of HVDC links assume a strong AC network connected to a single HVDC link with diminishing impedances in the AC transmission system. This results in simple active and reactive power bounds on the HVDC injections. In this thesis, it is shown that this concept can be generalized to constraint sets that are characterized without these simplifying assumptions. The constraint set of admissible