Modular Multilevel Converter Control for HVDC Operation: Optimal Shaping of the Circulating Current Signal for Internal Energy Regulation
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- Institutt for elkraftteknikk 
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, , 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  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.