Design and testing of an armature-reaction-compensated permanent magnet synchronous generator for island operation

At present, permanent magnet synchronous generators (PMSGs) are of great interest. Since they do not have electrical excitation losses, the highly efficient, lightweight and compact PMSGs equipped with damper windings work perfectly when connected to a network. However, in island operation, the generator (or parallel generators) alone is responsible for the building up of the network and maintaining its voltage and reactive power level. Thus, in island operation, a PMSG faces very tight constraints, which are difficult to meet, because the flux produced by the permanent magnets (PMs) is constant and the voltage of the generator cannot be controlled. Traditional electrically excited synchronous generators (EESGs) can easily meet these constraints, because the field winding current is controllable. The main drawback of the conventional EESG is the relatively high excitation loss. This doctoral thesis presents a study of an alternative solution termed as a hybrid excitation synchronous generator (HESG). HESGs are a special class of electrical machines, where the total rotor current linkage is produced by the simultaneous action of two different excitation sources: the electrical and permanent magnet (PM) excitation. An overview of the existing HESGs is given. Several HESGs are introduced and compared with the conventional EESG from technical and economic points of view. In the study, the armature-reaction-compensated permanent magnet synchronous generator with alternated current linkages (ARC-PMSG with ACL) showed a better performance than the other options. Therefore, this machine type is studied in more detail. An electromagnetic design and a thermal analysis are presented. To verify the operation principle and the electromagnetic design, a down-sized prototype of 69 kVA apparent power was built. The experimental results are demonstrated and compared with the predicted ones. A prerequisite for an ARC-PMSG with ACL is an even number of pole pairs (p = 2, 4, 6, …) in the machine. Naturally, the HESG technology is not limited to even-pole-pair machines. However, the analysis of machines with p = 3, 5, 7, … becomes more complicated, especially if analytical tools are used, and is outside the scope of this thesis. The contribution of this study is to propose a solution where an ARC-PMSG replaces an EESG in electrical power generation while meeting all the requirements set for generators given for instance by ship classification societies, particularly as regards island operation. The maximum power level when applying the technology studied here is mainly limited by the economy of the machine. The larger the machine is, the smaller is the efficiency benefit. However, it seems that machines up to ten megawatts of power could benefit from the technology. However, in low-power applications, for instance in the 500 kW range, the efficiency increase can be significant.

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Kirjallisuusarvostelu

Kunnan talouden tasapaino monitulkintaisena ilmiönä

Lectio praecursoria.

Forest cover and its change in Unguja Island, Zanzibar

Tropical forests are sources of many ecosystem services, but these forests are vanishing rapidly. The situation is severe in Sub-Saharan Africa and especially in Tanzania. The causes of change are multidimensional and strongly interdependent, and only understanding them comprehensively helps to change the ongoing unsustainable trends of forest decline. Ongoing forest changes, their spatiality and connection to humans and environment can be studied with the methods of Land Change Science. The knowledge produced with these methods helps to make arguments about the actors, actions and causes that are behind the forest decline. In this study of Unguja Island in Zanzibar the focus is in the current forest cover and its changes between 1996 and 2009. The cover and changes are measured with often used remote sensing methods of automated land cover classification and post-classification comparison from medium resolution satellite images. Kernel Density Estimation is used to determine the clusters of change, sub-area –analysis provides information about the differences between regions, while distance and regression analyses connect changes to environmental factors. These analyses do not only explain the happened changes, but also allow building quantitative and spatial future scenarios. Similar study has not been made for Unguja and therefore it provides new information, which is beneficial for the whole society. The results show that 572 km2 of Unguja is still forested, but 0,82–1,19% of these forests are disappearing annually. Besides deforestation also vertical degradation and spatial changes are significant problems. Deforestation is most severe in the communal indigenous forests, but also agroforests are decreasing. Spatially deforestation concentrates to the areas close to the coastline, population and Zanzibar Town. Biophysical factors on the other hand do not seem to influence the ongoing deforestation process. If the current trend continues there should be approximately 485 km2 of forests remaining in 2025. Solutions to these deforestation problems should be looked from sustainable land use management, surveying and protection of the forests in risk areas and spatially targeted self-sustainable tree planting schemes.

Microcracks and Vortices in Superconducting Thin Films

In this work, superconducting YBa₂ Cu₃O_6+x (YBCO) thin films have been studied with the experimental focus on the anisotropy of BaZrO₃ (BZO) doped YBCOthin films and the theoretical focus on modelling flux pinning by numerically solving Ginzburg- Landau equations. Also, the structural properties of undoped YBCO thin films grown on NdGaO₃ (NGO) and MgO substrates were investigated. The thin film samples were made by pulsed laser ablation on single crystal substrates. The structural properties of the thin films were characterized by X-ray diffraction and atomic force microscope measurements. The superconducting properties were investigated with a magnetometer and also with transport measurements in pulsed magnetic field up to 30 T. Flux pinning was modelled by restricting the value of the order parameter inside the columnar pinning sites and then solving the Ginzburg-Landau equations numerically with the restrictions in place. The computations were done with a parallel code on a supercomputer. The YBCO thin films were seen to develop microcracks when grown on NGO or MgO substrates. The microcrack formation was connected to the structure of the YBCO thin films in both cases. Additionally, the microcracks can be avoided by careful optimization of the deposition parameters and the film thickness. The BZO doping of the YBCO thin films was seen to decrease the effective electron mass anisotropy, which was seen by fitting the Blatter scaling to the angle dependence of the upper critical field. The Ginzburg-Landau simulations were able to reproduce the measured magnetic field dependence of the critical current density for BZO doped and undoped YBCO. The simulations showed that in addition to the large density also the large size of the BZO nanorods is a key factor behind the change in the power law behaviour between BZO doped and undoped YBCO. Additionally, the Ginzburg-Landau equations were solved for type I thin films where giant vortices were seen to appear depending on the film thickness. The simulations predicted that singly quantized vortices are stable in type I films up to quite large thicknesses and that the size of the vortices increases with decreasing film thickness, in a way that is similar to the behaviour of the interaction length of Pearl vortices.

Nordic alcohol policy in Europe: the adaptation of Finland’s, Sweden’s and Norway’s alcohol policies to a new policy framework, 1994-2013

The purpose of this thesis is to study how and to which extent Finland, Sweden and Norway have adapted their alcohol policies to the framework imposed to them by the EU and the European Economic Area (EEA) since the mid-1990s. This is done by studying the underlying mechanisms that have influenced the formation of alcohol policy in the Nordic countries in that period. As a part of this analysis main differences in alcohol policies and alcohol consumption between the three countries are assessed and the phenomenon of cross-border trade with alcohol is discussed. The study examines also the development of Finnish, Norwegian and Swedish alcohol policies between 1994 and 2012 and compares the Nordic alcohol policies with other alcohol policies in Europe as the situation was in 2012. The time frame of the study spans from the mid-1990s to the end of 2013 and is divided into three phases. Studying the role of the Europeanisation process on the formation of alcohol policies has a key role in the analysis. Besides alcohol policies, the analyses comprise the development of alcohol consumption and cross-border trade with alcohol. In addition, a quantitative scale constructed to measure the strictness of alcohol policies is utilised in the analyses. The results from the scale are used to substantiate the qualitative analysis and to test whether the stereotypical view of a strict Nordic alcohol policy is still true. The results from the study clearly corroborate earlier findings on the significance of Europeanisation and the Single Market for the development of alcohol policies in the Nordic countries. Free movement of goods and unhindered competition have challenged the principle of disinterest and enabled private profit seeking in alcohol trade. The Single Market has also contributed to the increase in availability of alcohol and made it more difficult for the Nordic EU member states to maintain restrictive alcohol policies. All in all, alcohol policies in the Nordic countries are more liberal in 2013 than they were in 1994. Norway, being outside the EU has, however, managed to maintain a stricter alcohol policy than Finland and Sweden. Norway has also been spared from several EU directives that have affected Finland and Sweden, the most remarkable being the abolishment of the travellers’ import quotas for alcohol within the EU. Due to its position as a non-EU country Norway has been able to maintain high alcohol taxes without being subjected to a ”race to the bottom” regarding alcohol taxes the same way as Finland and Sweden. Finland distinguishes as the country that has liberalised its alcohol policy most during the study period. The changes in alcohol policies were not only induced by Europeanisation and the Single Market, but also by autonomous decision-making and political processes in the individual countries. Furthermore, the study shows that alcohol policy measures are implemented more widely in Europe than before and that there is a slow process of convergence going on regarding alcohol policy in Europe. Despite this, alcohol policies in the Nordic countries are still by far the strictest in all of Europe. From a Europeanisation perspective, the Nordic countries were clearly on the receiving end during the first two study phases (1994–2007), having more to adjust to rules from the EU and the Single Market than having success in uploading and shaping alcohol policy on the European and international field. During the third and final study phase (2008–2013), however, the Nordic countries have increasingly succeeded in contributing to shape the alcohol policy arena in the EU and also more widely through the WHOs global alcohol strategy. The restrictive Nordic policy tradition on which the current alcohol policies in Finland, Sweden and Norway were built on has still quite a solid evidence base. Although the basis of the restrictive alcohol policy has crumbled somewhat during the past twenty years and the policies have become less effective, nothing prevents it from being the base for alcohol policy in the Nordic countries even in the long term. In the future, all that is needed for an effective and successful alcohol policy is a solid evidence base, enough political will and support from the general public.

EVA ja demarit

Vastine tutkija Ville Okkosen artikkeliin: "Elinkeinoelämän yhteiskunnallinen aktivismi politiikan muutoksen taustavoimana 1970-luvulta 1980-luvulle" (HAik 3/2013, s. 293-306).