Impact on Renewable Energy Sources on Harmonic Levels in the Future UK Power Grid: Modelling, Assessment and Mitigating Solutions

The UK is committed to reducing its greenhouse gas emissions by at least 80% by 2050, relative to 1990 levels. Meeting this target will require a significant shift in the way energy is used (for example, through electrification of heat and transport) and generated. To reduce our dependency on fossil fuels, significant levels of Renewable Energy Sources (RESs) will need to be integrated into the power grid. RESs include energy sources that do not rely on fossil fuels and nuclear energy, such as solar, wind, tidal and wave. Some sources of renewable energy have been successfully incorporated (for example, pumped-hydro), and they are generally not classified as 'RESs' as this term refers to devices that are connected to the grid by means of power converters.

While the benefits of RESs are undisputable and their installation should be facilitated, the integration of large amounts of these devices requires the development of new paradigms and methods to maintain a reliable and safe power system. RESs have certain characteristics that differentiate them from traditional sources: they are less controllable than traditional energy sources, they cause unintended power flow patterns, and they impact voltage and current waveforms and the overall power quality of electricity.

This proposal will focus on the study of harmonic propagation in the UK power grid due to RESs. Harmonics are current and voltage components at frequencies multiple of the fundamental, and they are generated by the operation of the power converters. While low harmonic levels are tolerated and do not impact grid operation, increasing harmonic levels have detrimental effects including: increased losses, reduced efficiency, misoperation and reduced lifetime of equipment, and nuisance protection tripping. While the costs of harmonics are not easy to determine because of their long-term effect, it has become accepted by the power grid operators and users that increasing harmonic levels are detrimental for efficient grid operation. More importantly, they may compromise the future integration of RESs.

This proposal aims at assessing the harmonic impact of RESs in the future power grid. Carrying out this assessment requires the development of accurate models of both RESs and of the power system. At the same time, these models requires some form of simplification because of the number of components involved. Previous research has focused on either detailed power converter models, or the use of a large power system model with simplified converter representation. This NIA aims at combining both aspects in a model which is able to represent correctly harmonic generation from RESs, the transfer of harmonics between voltage levels, and the representation of statistical variations of harmonic levels in the system, for varying levels of penetration of RESs.

This research will involve close collaboration with two industrial partners, National Grid and Measurable Ltd, and with the University of Texas at Austin.

The impact of this research will be widespread: the models developed will help the academic community in understanding the best approaches to study harmonic behaviour of large energy systems. This work will assist the system operator in assessing the impact of new RESs on the power system. Furthermore, this research will inform the future developments of power quality standards and of harmonic mitigating solutions.

The ultimate goal of this project is to minimise the occurrence of harmonic problems in the future power grid, allowing for a smooth and clean integration of increasing amounts of RESs, not only in the UK, but on a global scale. Furthermore, it will benefit the UK as a whole in terms of maintaining the operations and efficiency of this ubiquitous infrastructure, whilst helping the government to meet targets for reducing CO2 emissions.

One of the main changes to the UK power grid in recent years has been the influx of Renewable Energy Sources (RESs), mostly in the form of wind and solar energy. In 2013, 14.9% of the UK electrical generation was from renewable sources; in 2017, this figure had risen to 27.9%, and it will continue to increase, as the Paris Agreement on climate change will take effect from 2020 and the UK government will phase out coal-fired electricity generation by 2025.
RESs are connected to the grid by means of voltage source converters (VSCs). One of the side effects of these devices is the increasing level of harmonics injected in the power grid. Harmonics affect the system's ability to perform at optimal levels by increasing power losses, causing malfunctioning in the equipment, and nuisance tripping. These detrimental effects have an impact on the users of electricity and on the system operators, who invest large amount of resources in studying the impact of RESs on the power grid, developing mitigating solutions, and upgrading equipment when necessary.

This project will investigate the transfer of harmonics between different voltage levels caused by RESs, by creating an innovative model of the UK power grid that includes both transmission and distribution system representation. The impact of increasing levels of RESs on harmonic levels and on system impedance will be addressed and suitable mitigating solutions will be identified. The research will be undertaken in partnership with National Grid, Measurable, and the University of Texas at Austin.
The main Impact and Beneficiaries will be:

Impact on UK economy and the power industry: This research will deliver new tools and techniques to improve the knowledge and capability of engineers within the power industry to conduct harmonic studies involving RESs. Specifically, 'National Grid' will benefit through the development of computer models that will assist their management and planning of harmonics associated with RESs. This project will enable their power system engineers to perform more accurate and effective harmonic studies, and therefore to plan more efficiently system upgrades, mitigating solutions, and an overall reduction in power quality problems. 'Measurable' will benefit from an understanding of harmonic impact on energy measurements and to improve their capability to measure energy usage at the residential and commercial levels. Manufactures of energy meters will be also learn best practices to perform power measurements under non-sinusoidal operating conditions. The impact of this research will also extend to other industries: power converter equipment manufacturers (e.g. ABB, Siemens, and GE) will benefit from the models by understanding the harmonic impact of their devices on the grid.

Impact on policy - This research will indicate if the harmonic limits currently included within engineering standards will be sustainable in the future, or if amendments will be required. This research will validate the limits being introduced in the latest version of the UK Engineering Recommendation, in particular with regards to provisions for harmonics between the 50th and the 100th component. Also, given the PI's on-going engagement with the North American standardisation body, the results will also inform the groups involved in revision of standards IEEE 519.

Impact on society: The general public will be engaged by this research as it provides an engineering solution to a problem associated with renewable energy generation that supports the transition to a low-carbon society. The PI will continue her engagement with outreach activities in the local area by promoting the value of STEM subjects to school children. Finally, this research will train and develop a PDRA, a PhD student, and other post and undergraduate students in power quality modelling, providing skills that will be highly valuable to both academic and industrial sectors.