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

Abstract

Power systems are large and complex systems consisting of thousands of components, like generators, transformers and transmission lines, which function together to ensure that the voltage and frequency are close to the nominal values. Traditionally, the major share of the electricity production was dominated by controllable sources like hydro, coal and nuclear power, where the production can be adjusted at will. To counteract climate change, production based on fossil fuels is phased out and replaced by renewable sources, among others solar and wind power. Since the production from these sources depends on the weather, this introduces larger and faster variations in the flow of electrical power from producer to consumer. Wind turbines and photovoltaics also affect and interact with the rest of the system in a significantly different way compared to conventional generators found in, e.g., hydro power plants. Adding that climate change brings increased frequency and intensity of extreme weather events, which often cause line outages or other disturbances to the grid, it must be expected that system operators are facing significant challenges in the future.

Motivated by this development, Statnett, and corresponding transmission system operators in other countries, are investing in new technology for monitoring of the system: Phasor Measurement Units provide voltage and current measurements with high resolution, allowing detailed dynamic transients to observed and analyzed. The measurement devices are deployed at strategic locations in the system, and continually stream measurements to the operators’ control center. The data stream can be analyzed continuously to extract information about the current state of the system, calculate stability indicators and margins, or propose remedial actions to mitigate incipient instability.

In this thesis, the focus is on investigating how increasing deployment of Phasor Measurement Units can be exploited to contribute to stable, secure and resilient operation of the system, subject to the mentioned expected challenges of the future. Specifically, a particular type of instability referred to as electromechanical oscillations or power oscillations in the literature is targeted. This type of oscillations is typically problematic when transferring large amounts of power between areas of generation via long transmission lines, and can be observed as power oscillations in power plants or grid frequency oscillations. The frequency is typically in the range 0.1 to 2 Hz and the damping is often satisfactory, but in some cases low damped or growing oscillations are experienced. The system is particularly vulnerable to this type of instability when it is in an unusual or unscheduled operating point, e.g., due to maintenance or outage of important components.

The three main contributions to this research field can be summarized as follows:

A method for estimation of frequency and observability of electromechanical oscillations based on phasor measurements. A prototype for online analysis of measurements and live visualization of oscillations is described. Enhancements to a particular type of control algorithm for damping of electromechanical oscillations, referred to as the Phasor Power Oscillation Damper in the literature. Three distinct sub-contributions are described, where the most important is an adaptive variant of the algorithm, capable of adjusting internal parameters to changing operating conditions. A power system simulation tool coded in Python. The tool was developed to support the development of the contributions in the previous point.

With these methods for monitoring and control, it is the aim of this thesis to contribute with 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.