Enhancing Secure Grid Operation with Power Flow Controlling Devices and WAMS

The increasing energy consumption and the rise of renewable energy sources in combination with the unbundling of formerly vertically integrated energy suppliers into independent energy production and transmission companies create additional challenges in power system operation and stability. Despite the effort spent and the results gained in research of smart distribution grids and micro grids, transmission systems will inevitably be a part of the electrical power system in the future. On transmission system level, grid expansion in the sense of new construction or reinforcement of AC transmission lines cannot keep pace with the amount of installed generation capacity of renewables leading to a branch loading near the capacity limit. To maintain a stable network operation under these conditions, optimal use of the flexibilities given by power flow controlling devices, e. g. embedded HVDC links and phase shifting transformers, and real time measurements provided by wide area measurement systems is imperative.
In this thesis, three different aspects of the network operation with power flow controlling devices and available wide area measurement systems are addressed.
The first is the evaluation of the influence of different degrees of coordination of power flow controlling devices on the redispatch costs and volume in a flow-based market setting. It is shown thereby, that the redispatch costs only decrease, if power flow controlling devices are coordinated. However, if the power flow controlling devices are used to reduce the loading of the branches controllable by them without coordination, the redispatch costs might even increase depending on the load situation.
The second aspect is the need for accurate state awareness under strained network conditions. Therefore, an algorithm to detect multi branch outages based on a linear model and optimization techniques is developed. The algorithm is able to correctly detect multi branch outages solely using pre-fault topology information and node voltage angle measurements from synchronous wide area measurement systems as input. The algorithm’s performance is evaluated with simulations of the Nordic-32-Bus test system.
The last part of this thesis is dedicated to emergency control strategies of embedded HVDC links. In the first step, existing control strategies for HVDC links are evaluated in a small radial network and compared to a novel control strategy. The simulation results show that the novel control strategy is superior in relieving overloaded branches and equal in terms of maximum load ability and voltage stability. In the second step, an emergency control strategy combining efficient branch relief and voltage support in a meshed network is proposed. This strategy is based on linearized sensitivities to active and reactive power setpoint changes of the HVDC link converters and synchronous node voltage measurements obtained from wide are measurement systems.