Events at Department of Energy Technology

PhD defence by Joachim Steinkohl


07.07.2021 kl. 14.00 - 17.00


Joachim Steinkohl, Department of Energy Technology, will defend the thesis "Reliable Control of Power-electronic-based Power Systems"


Reliable Control of Power-electronic-based Power Systems


Joachim Steinkohl


Professor Frede Blaabjerg


Professor Xiongfei Wang
Associate Professor Pooya Davaari


Professor Huai Wang


Professor Florin Iov, Aalborg University (Chairman)
Professor Monti, Antonello, RWTH Aachen, Germany
Professor Liu, Yilu, University of Kentucky, USA


The transition to a full-renewable based power system is currently under process. The ambitious goals of many countries to limit the usage of carbon-based generation units causes the power systems all over the world to be in the most severe change since their beginning. The increasing integration of power-electronic based power generation units influences the power system's ability to operate in a reliable way. Therefore, transmission system operators have to rethink their decisions on planning and operating the power grid.
Previously dominated by a limited number of central power plants, are power systems now including new generation systems, which are decentralized and operate dependent on the current weather conditions. The arising issues include the lack of system inertia, faster rates of power changes, or harmonic interaction, just to mention a few challenges. However, installing power electronic-based units also allows controlling the system much more flexible than ever before. Controls in the units can be adapted in an infinite number of ways to benefit the power system.
FACTS (Flexible AC Transmission Systems) are known to be the fastest units in the power grid to supply reactive power for voltage control. This allows to more reliably connect renewable power plants, such as offshore wind farms, but also to improve the supply of certain loads, such as remote cities or oil-rigs. The installation of more of these units can highly benefit the system operation. It has to be guaranteed that they all contribute with respect to each other when multiple units are in close proximity. Otherwise, these units can cause interactions between each other, causing undesired voltage fluctuations. Thus, the need for a correct adaption of these units arises when they are being used more and more in modern power grids. 
This Ph.D. project focuses on the interactions between these kinds of units. It is determined how different units have to adapt to the reactive power support of other FACTS devices without communication between them. It is shown that the currently used method in the industry is not sufficient to reliably maintain the dynamic response. A control scheme is proposed, which allows adapting the FACTS units to changes in the power grid during operation and other units nearby. This is required when these units are being used more frequently in modern power grids.
Moreover is the reliable control of the system frequency one of the main challenges when it comes to a full-renewable power system. Micro-grids, where the system is highly limited in space, can already be controlled by power converters. Power systems under transition have to operate in compliance with the existing equipment, such as the remaining generation units and the protection system. Renewable power sources, and especially wind power plants, have the potential to support or even replace conventional generation units for frequency supports. Wind power plants can curtail their output power to keep reserve available, or they can even utilize the rotational energy in the blades to provide short-term frequency support. The transmission system operator now has the challenging task of determining which kind of controls are needed to allow for the reliable control of the system frequency. Therefore it is important to assess the influence of incorporating these new controls into the operational frequency reliability assessment. This also requires the system operators to determine, which controls are optimal for their specific grid conditions.
In order to tackle the issues mentioned above, this Ph.D. project discusses methods, which enable to include wind power plant frequency support into the reliability assessment with all required time-frames. Also, a methodology to find the optimal frequency management is proposed in this project, which allows system operators to compare different strategies easily. System operators are always challenged to find the right balance between gains in frequency quality versus the over-usage of frequency controls and thereby loss of energy production.



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