Application of Phasor Measurements for Online Monitoring and Adaptive Control of Electromechanical Oscillations

Transmission system operators around the globe are facing challenges related to ensuring the secure and resilient operation of the future power system. These challenges are in large part caused by trends associated with climate change: Fossil fuel power plants are being decommissioned and replaced by renewable energy sources, and both the demand for electricity and imports/exports of electric energy between countries are increasing, which causes faster fluctuations and higher variability in the power flows in the system. This makes it more difficult to design and tune control systems responsible for stabilizing the system. In parallel to this development, heavy investments are being made in measurement and communication infrastructure targeting monitoring, control, and protection of the power system, and the number of controllable devices, in the form of Converter-Interfaced Generation, Flexible
AC Transmission System devices and HVDC links, is increasing. In this thesis, we investigate how the increasing availability of real-time measurements and potential control actuators can be used for the purpose of stabilizing the system. Specifically, periodic small signal rotor angle stability is targeted. If the system is not stable in this sense, standing or growing rotor angle oscillations might appear spontaneously, often referred to as electromechanical oscillations due to the interaction between electrical and mechanical elements. The typical problem related to this instability in today’s power systems is oscillations between weakly connected areas of
generation with high power transfers between them, referred to as interarea oscillations, typically occurring within a frequency range of 0.1 to 0.8 Hz. The system is particularly vulnerable to this instability when it is in an unusual or unscheduled state of operation.
The most important research contributions presented in this thesis target two equally crucial sides of the stability problem: Measuring/monitoring oscillations and damping control for mitigating oscillations. The first contribution is a monitoring method based on statistical learning methods, which provides information about the frequency and observability of oscillatory modes by analyzing phasor measurements. The method is targeted toward online operation, aiming to continuously provide operators with situational awareness in the form of information about the oscillatory modes in the system. Three contributions targeting damping control of oscillations are presented, which represent enhancements of a specific, well known type of oscillation damping controller known as the Phasor Power Oscillation Damper in the literature. Two of the contributions are investigations into how more knowledge about the system (among others, the information provided by the monitoring method) and higher availability of measurements can be exploited for damping control purposes. Another contribution concerns developing this type of controller into a self-tuning, adaptive controller, capable of following changing operating conditions. Simulation results indicate that enhanced performance can be achieved by taking more information into account, and that the adaptive variant of the controller outperforms a comparable variant in the literature under the tested conditions. Finally, a simulation tool was developed throughout the work with the thesis, which is the last contribution. The aim of the tool is to provide an open and transparent platform for prototyping monitoring, control, and protection applications of the future power system, and was developed specifically for testing the presented control methods. With these contributions, it is the aim of this thesis to provide operators with methods and tools to gain increased situational awareness
and new options for remedial action for handling instabilities, with the ultimate goal of facilitating stable, secure, and resilient operation of the future power system.