| dc.description.abstract | ELECTRICITY will most probably be the dominant form of energy in the upcoming
future. Triggered by the decline of fossil fuels along with their elevated
and volatile prices, the threat that they represent to national energy security, and
growing environmental concerns as consequence of their negative effect on climate
change, Europe has decided to undertake an ambitious and multidisciplinary project
called the SuperGrid. Such an initiative will serve as a transcontinental highway
for renewable energy, [1], allowing for geographical smoothing effects to help minimize
the disadvantages inherent to the intermittent renewable sources. In more
simple words, this so-called SuperGrid will interconnect the offshore wind and wave
power from the North sea, with the photovoltaic (PV) and concentrated solar power
(CHP) from the south, as well as geothermal and biomass generation, and indeed,
large hydro-power dams (located in Norway and Switzerland among other countries)
that will serve as Europe’s energy storage. The main challenge is of course the
continental-size distances of the interconnections, which were not possible using the
traditional AC technology. Nowadays, such ambitious projects have become feasible
as high voltage direct current (HVDC) technology enters the scene, thanks to
semi-conductor breakthroughs and advances in power electronics; more precisely,
voltage source converters (VSC)-based high voltage direct current (HVDC) links.
It seems to be getting clear that the Modular Multilevel Converter (MMC) proposed
by Professor Marquart in 2003 [2] has emerged as the the most suitable power
converter for such application, since it has several advantages with respect to its
predecessors, such as its high modularity, scalability and lower losses.
When this research project was undertaken in 2011, there was not yet a clear
control strategy defined that could be used to operate the MMC in an optimal and
stable manner since the topology was still very new. Moreover, this strategy needed
to take into account the final HVDC application and withstand grid fault scenarios.
Thus, the present Thesis was carried out with the aim of overcoming such crucial issues,
by proposing a control philosophy in the MMC natural phase coordinates that
was obtained from the application of mathematical optimization using Lagrange
multipliers to the energy-based model of the MMC for balanced and unbalanced
grid conditions. Furthermore, global asymptotic stability issues involving theMMC
were addressed, and a simple and local approach for ensuring the global stability of
the complex MMC-multi-terminal HVDC system was implemented. | nb_NO |
