Reliable Control of Power Electronic based Power Systems

The transition to a full-renewable based power system is currently under
process. The ambitious goals of many countries to limit the usage of carbonbased
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.

Funding: Villum Foundation.