Energy efficiency of the building sector is a key strategy to achieve national climate goals in Sweden and other European countries. In this thesis, several renovation scenarios for a case study office building in Sweden are analysed and compared based on their energy performance, environmental impacts, and economic costs from a life cycle perspective. As a baseline, the case study building was simulated in IDA ICE and compared with the simulated renovation scenarios. For the Life Cycle Analysis (LCA) and the Life Cycle Costs (LCC), the commercially available software OneClickLCA was used. The renovation scenarios were carried out over three rounds: (i) material type scenarios where five insulation materials (glass wool, rock wool, hemp fiber, Expanded Polystyrene (EPS), and Extruded Polystyrene (XPS)) and two frame materials (wood and steel) are compared; (ii) insulation thickness optimization from economic and environmental performance perspectives (iii) comparison of combination with other typical renovation measures such as changing of windows, improving specific fan power, heat exchanger efficiencies, and lightings. The results show that glass wool gives the most economical and environmental performance, followed by rock wool and EPS. When considering other environmental indicators, hemp fiber presents the best environmental option. However, it is not competitive with traditional insulation materials from an economic perspective. The insulation thickness scenarios show different optimal economic and environmental performance points, giving total energy savings of 5 % and 9,5 %, respectively. When considering other typical energy efficiency measures, the highest impact on the energy performance was found when improving the specific fan power (SFP) and switching to LED lights with total electricity reductions (including user-based electricity consumption) of 4 % and 14 %, respectively. Conclusively, the case study showed how the electricity and heating demand of the studied office building could be reduced, and the environmental and economic consequences of the different energy-efficiency measures.

This thesis evaluates the environmental, economic, and social impacts of implementing secondary wood from recycling centers into wood crafts in Stockholm schools. The thesis is conducted in a case study setting as a part of the "Climate Neutral Stockholm 2030" sub-project 3.2, which focuses on promoting reuse and waste reduction through circularity. A comparative consequential life cycle assessment (LCA) and life cycle cost (LCC) within a cradle-to-gate system boundary was conducted in a scenario setting, which compared and quantified the use of virgin wood and secondary wood both environmentally and economically. The environmental profile was quantified for four impact categories: global warming potential (GWP), water use, land use, and cumulative energy demand (CED). Additionally, the social implications of incorporating secondary wood were examined through a survey and interviews with the wood craft teacher and students. Furthermore, the LCA and LCC were conducted with a functional unit (FU) of 0,00106 m3 of wood demand per student. The impacts were aggregated to understand the combined effect of all schools in Stockholm, from grades 3 to 9. The study's results verify that secondary wood is more environmentally and economically beneficial while also being positively perceived by the teacher and students. The environmental findings indicate a total yearly reduction in GWP by 78%, water use by 82%, land use by 99%, and CED by 90% for all schools in Stockholm. From an economic perspective, implementing secondary wood in craft classes leads to a significant cost saving of 81%. From a social aspect, results revealed an overall positive perception from the teacher and students. However, the quality and quantity of wood must be sufficient without excessive additional time or labor requirements. Furthermore, a sensitivity analysis was performed for both LCA and LCC results. It was observed that a percentage change in transportation distance does not significantly change the GWP results. In addition, secondary wooden boxes can become more expensive than virgin wooden boxes if transportation costs are increased by more than 54%. To support the "Climate Neutral Stockholm 2030" project, the study contributes to the city's pursuit of sustainability by showing decision makers the magnitude of impacts that can be reduced through circularity and waste reduction, therefore facilitating and motivating the implementation of a sustainable system in Stockholm. / Detta examensarbete syftar till att utvärdera de miljömässiga, ekonomiska och sociala effekterna av att implementera sekundärt trä, från återvinningscentraler, till träslöjden i Stockholms skolor. Examensarbetet genomförs som en fallstudie inom ramen för delprojektet 3.2 i "Klimatneutrala Stockholm 2030", som fokuserar på främjande av återanvändning och avfallshantering genom cirkularitet. En jämförande konsekvensbaserad livscykelanalys (LCA) och analys av livscykelkostnader (LCC) inom en “vagga-till-port" systemavgränsning genomfördes. Där användningen av nytt trä och sekundärträ jämfördes och kvantifierades både ur miljömässig och ekonomisk synvinkel. Den miljömässiga profilen kvantifierades för fyra effektkategorier: global uppvärmningspotential, vattenanvändning, markanvändning och kumulativ energiförbrukning. Dessutom undersöktes de sociala uppfattningarna av att införa sekundärt trä i skolorna genom en enkät och intervjuer med både eleverna och träslöjdsläraren. En LCA och LCC genomfördes med en funktionell enhet på 0,00106 m3 virkesbehov per elev. Effekterna sammanställdes för att förstå den kombinerade effekten av alla skolor i Stockholm, från årskurs 3 till 9. Resultaten av studien bekräftar att sekundärt trä är mer fördelaktigt både ur miljö- och ekonomisk synvinkel, samtidigt som det uppfattas positivt av läraren och eleverna. De miljömässiga resultaten visar en total årlig minskning av GWP med 78%, vattenanvändning med 82%, markanvändning med 99% och CED med 90% för alla skolor i Stockholm. Ur ett ekonomiskt perspektiv leder implementeringen av sekundärträ i träslöjden till betydande kostnadsbesparingar på 81%. Från en social aspekt visade resultaten en övergripande positiv uppfattning från både läraren och eleverna. Det är dock avgörande att kvaliteten och mängden trä är tillräcklig utan extra tid eller arbetsinsatser. Dessutom utfördes en känslighetsanalys för både LCA- och LCC-resultaten. Det observerades att en procentuell förändring i transportavståndet inte har en betydande effekt på GWP-resultaten. Däremot, kan en trälåda av sekundärt trä bli dyrare än en med nytt trä, om transportkostnaderna ökar med mer än 54%. För att stödja projektet "Klimatneutralt Stockholm 2030" bidrar denna studie till stadens strävan ett uppnå hållbarhet genom att visa beslutsfattare hur stor påverkan som kan uppnås genom cirkularitet och avfallsminskning. Den grunden kan underlätta och motivera implementeringen av ett hållbart system i Stockholm.

AFRY är ett ingenjörsföretag som har efterfrågat en studie som går ut på att ur ett elkonstruktionsperspektiv hitta enklare åtgärder för att bespara industrier pengar ge-nom att utföra energieffektiva investeringar i kontinuerliga elmotordrifter. Detta är extra relevant idag då energikostnaderna ökat anmärkningsvärt de senaste åren. Målsättningen har varit att skapa ett underlag baserat på data från motorer, kablar och frekvensomriktare som talar om vilka investeringar som gör störst nytta ur ett energisparperspektiv vid ett antal olika fall. Arbetet är uppdelat i tre delar där bety-delsen undersöks för systemutförande, dimensionering och IE-klasser och deras på-verkan på energiförbrukningen och livstidskostnaderna. Metoden går ut på att hämta tillverkningsdata från komponenterna som testas, simulera energiåtgången i olika mjukvaruprogram och sedan jämföra utfallen i olika konstellationer mot inköpspri-set. Resultatet är ett underlag där det går att påvisa vilka investeringar ur en elkon-struktionssynpunkt som är mest ekonomiskt gynnsamma. Räkneexempel och olika scenarion finns med eftersom förutsättningarna är beroende av faktorer som elpris, materialkostnader, drifttid, med mera. Initialt var planen att simulera samtliga tre delområden med ett energianalyserings-program utvecklat av AFRY. Flertalet långa fördröjningar och oförmågan att köra programmet på nuvarande operativsystem såg till att planeringen ändrades av stor betydelse under projektets gång. Programmet fick därmed ersättas av andra pro-gram och arbetet har lagt större tyngd på teorin föra att täcka upp samtliga delar.

This thesis investigates the oft-overlooked influence of the classifier's input during the assignment of library classifications and draws upon cultural transfer theory to shed light on the underlying principles that guide the process. Classifiers' personal knowledge, experience, and beliefs, have a critical role in determining the 'aboutness' of a work and its subsequent classification. To explore this theory, this research employs a dual-methods approach, using qualitative analysis of past literature contrasted with a data set formed through a real-world reclassification project at The Swedish Institute in Rome (ISVRoma) using the Library of Congress Classification scheme (LCC), with classifications in the catalogue of the American Academy in Rome (AARome) used for comparison. The resultant data provides a basis for examining the classifier's cultural input and the manifestation of cultural transfer theory in library classification. The findings of this thesis reveal the dynamic nature of classification, which is not a mere static or administrative task, but rather an evolving process deeply embedded in the societal and cultural context. Classifiers are found to act as interpreters, translators, and shapers of knowledge, bringing a profoundly human element to knowledge organisation and retrieval. The classifiers' craft, therefore, extends beyond the implementation of a systemic and controlled vocabulary and involves an active contribution of their knowledge and experience. This contributes to the overall flexibility and adaptability of the classification system. The thesis also underscores the necessity of reassessing our understanding of library classification systems, especially in light of AI's increasing role in this field.

Byggindustrin har i dagsläget en negativ klimatpåverkan och infrastrukturen likaså. Många länder har därför försökt undersöka möjligheten att hitta ett långsiktigt och hållbart alternativ till det konventionella reparationsmaterialet. Olika material undersöks, olika optimerade betongrecept testas för att förstå hur miljöpåverkan har minimeras för att förlänga livslängden hos betongkonstruktioner. Vid reparation av en bro är det viktigt att ta hänsyn till både kostnader och miljöpåverkan under hela dess livscykel. Kostnader som uppstår är investeringskostnader samt drift- och underhållskostnader. Miljöpåverkan från betongkonstruktioner i produkt skedet består av materialframställning, byggtransporter och produktion på byggarbetsplatserna. totala växthusgasutsläppet summeras och beräknas i kg CO2-ekv. Syftet med detta examensarbete är att studera den långsiktiga hållbarheten hos UHPC med hjälp av beräkningsmodeller såsom livscykelanalys och livscykelkostnadsanalys med avsikt att applicera reparationstekniken. Flera UHPC recept ställs mot det konventionella reparationsmaterialet detta för att kunna bedöma miljöpåverkan och kostnadseffektiviteten hos materialen. Dvs om det går det att minska klimatutsläppet och kostnaderna. De jämförda recepten är olika UHPC-recept samt traditionell betong. Recepten appliceras slutligen på en befintlig bropelare för att undersöka de olika receptens tillämpbarhet som reparationsmaterial ur ett hållbarhetsperspektiv. Det saknas tillräckligt med kunskap om UHPC:s långtidseffekter, speciellt om reparationsintervall. Med åtanke på materialets höga draghållfasthet och beständighet tillsammans med UHPC:s strukturella egenskaper har antaganden gjorts att materialet är reparationsfri under konstruktionens livslängd. Det vill säga att bropelaren som undersökts med UHPC i studien inte behövt repareras under sin livslängd. Resultatet från livscykelkostnadsanalysen visar att UHPC är dyrare i både kubikmeter (m3) och kvadratmeter (m2) med tanke på täckskiktets tjocklek än traditionell betong i materialpriset. Men med tanke på att UHPC är underhållsfritt har den en mindre livscykelkostnad. Resultatet från livscykelanalysen visar att UHPC blandningarna har större miljöpåverkan per kubikmeter. Då de olika täckskiktetstjocklek relateras till pelarens längd erhålls resultat där UHPC medför slankare konstruktioner och besparingar upp emot 50% mindre betongvolym (för den 6 m långa pelaren i fallstudien). Med UHPC som reparationsmaterial medför det till att bron inte behöver repareras under dess livslängd. Bropelaren som repareras med UHPC kommer därför ha en mindre miljöpåverkan än den traditionella betongen. Långsiktig hållbarhet och mindre totala växthusgasutsläpp (som är i riktlinje med EU:s och regeringens klimatkrav) erhålls för anläggningskonstruktioner med UHPC. / The construction industry has a negative climate impact and so does the infrastructure. Which is due to frequent repairs that are not sustainable. Many countries have therefore tried to explore the possibility of finding a long-term and sustainable alternative to conventional repair materials. Different materials are examined, different optimized concrete recipes are tested to understand how the environmental impact can be minimized and the service life of concrete structures extended. When repairing a bridge, it is important to take into account both costs and environmental impact throughout its life cycle. Costs that arise are investment costs as well as operating and maintenance costs. The environmental impact from concrete structures in the product phase consists of material production, construction transports and production at construction sites. The total greenhouse gas emissions are summed up and calculated in kg CO2 eq. The purpose of this thesis is to study the long-term sustainability of UHPC using calculation models such as life cycle analysis and life cycle cost analysis with the intention of applying the repair technique. Several UHPC prescriptions are set against the conventional repair material in order to be able to assess the environmental impact and cost-effectiveness of the materials. That is, if it is possible to reduce climate emissions and costs. The compared recipes are different UHPC recipes and traditional concrete. The recipes are finally applied to an existing bridge pillar to investigate the applicability of the various recipes as repair materials from a sustainability perspective. There is a lack of knowledge about the long-term effects of UHPC, especially about repair intervals. Given the high tensile strength and durability of the material together with the structural properties of the UHPC, it has been assumed that the material is repair-free for the life of the structure. That is, the bridge pillar examined with UHPC in the study did not need to be repaired during its lifetime. The results from the life cycle cost analysis show that UHPC is more expensive in both cubicmeters (m3) and square meters (m2) given the thickness of the cover layer than traditional concrete in the material price. However, given that UHPC is maintenance free, it has a lower lifecycle cost. The results from the life cycle analysis show that the UHPC mixtures have a greater environmental impact per cubic meter when the cover layer varies. As the thickness of the different cover layers is related to the length of the pillar, results are obtained where UHPC leads to slimmer constructions and savings of up to 50% less concrete volume (for the 6 m long pillar in the case study). With UHPC as repair material, this means that the bridge does not need to be repaired during its service life. The bridge pillar that is repaired with UHPC will therefore have a smaller environmental impact than the traditional concrete. Long-term sustainability and smaller total greenhouse gas emissions (which are in line with EU and government climate requirements) are obtained for plant constructions with UHPC.

No description available.

Na busca por eficiência energética em serviços e produtos de iluminação pública, a maioria das iniciativas concentra-se em procedimentos voltados estritamente à substituição de tecnologias convencionais por outras mais energeticamente eficientes, considerando somente a fase de operação, onde há mais consumo de energia, e o investimento inicial. No entanto, verifica-se que há uma carência de estudos dedicados à análise da sustentabilidade urbana, em particular com o foco na ecoeficiência das iniciativas, em que pese o crescente interesse das empresas por produtos economicamente e ambientalmente sustentáveis. Portanto, a qualidade do serviço de iluminação pública quanto à sua ecoeficiência (componente importante da sustentabilidade urbana) pode ser desenvolvida utilizando-se o pensamento de ciclo de vida de forma simplificada para avaliar tanto economicamente quanto ambientalmente o empreendimento, de modo a viabilizar a comparação de tecnologias, desenvolvimento de projetos e a tomada de decisão para gestores. O objetivo do presente trabalho é propor um modelo analítico que permita, tanto os desenvolvedores de produtos e projetos como a gestores de serviços de Iluminação Pública IP, gerenciar seus recursos para garantir maior ecoeficiência na transição para tecnologias mais avançadas (como o uso de LEDs em produtos), que mitigam os impactos ambientais e o custo desta transição. Como método, utilizou-se uma abordagem qualitativa, se aplicou a técnica de Avaliação de Ciclo de Vida (ACV) para avaliação ambiental e econômica de produtos e instalações para IP baseados na tecnologia convencional e produtos com tecnologia LED. Com isso obteve-se uma medida da ecoeficiência associada a diferentes soluções. Na sequência, as informações foram analisadas, comparadas e um conjunto de parâmetros foi definido para constituir um modelo para tomada de decisão. Em particular, foi dada ênfase à qualidade da iluminação na definição da Unidade Funcional. Todas as fases do ciclo de vida foram analisadas (extração de matérias primas, fabricação de componentes, integração e fabricação de luminárias, manutenção e descarte). Como resultados, foram obtidos critérios mínimos de comparação para instalações de Iluminação Pública a LED; identificação dos impactos de novas tecnologias (como uso da tecnologia LED) em termos de emissões de gases de efeito estufa (CO2 equivalentes) do setor de IP; identificação dos impactos de novas tecnologias (como uso da tecnologia LED) no Custo do Ciclo de Vida de instalações para IP e foi desenvolvido um Modelo de Análise para integrar o desempenho econômico ao ambiental (ecoeficiência) de produtos e instalações de IP. / Introduction: in the search for energy efficiency services and street lighting products, most of the initiatives focuses on procedures aimed strictly to the replacement of conventional technologies by more energy efficient ones considering only their use phase. However, it turns out that there is a lack of studies dedicated to analyzing this field with a focus on eco-efficiency initiatives, despite the growing interest of companies by products economically and environmentally sustainable. Therefore, the quality of the street lighting service for their eco-efficiency can be developed using the Life Cycle Assessment (LCA) technique in a systematic and objective manner, in order to facilitate the comparison of technologies, project development and decision making for managers. Objective: To generate an analytical model that allows both product and project developers and managers of street lighting services to manage its resources to ensure greater eco-efficiency in the transition to more advanced technologies (such as the use of LEDs in products) that mitigate the environmental impacts and the cost of this transition. Method: it was applied the LCA for environmental and economic evaluation of products and street lighting facilities based on conventional technology and products with LED technology. Further, the information was analyzed, compared and a set of parameters has been set to provide a model for decision-making. In particular, it was given emphasis to the quality of lighting in the definition of a functional unit. Every stage of the life cycle was analyzed (extraction of raw materials, component manufacturing, integration and manufacturing lighting fixtures, maintenance and disposal). Results: we obtained minimum criteria of comparison for Street Lighting LED installations, identifying the impact of new technologies (such as use of LED technology) in the carbon footprint of the street lighting industry. Besides, we identified the impact of new technologies (such as use of LED technology) in the cost of life cycle of street lighting facilities. Lastly, it was developed an Analysis Model to integrate environmental economic performance (eco-efficiency) products and street lighting facilities.

O setor de açúcar e álcool etílico no Brasil tem experimentado um grande crescimento. Contribuem para isto as demandas crescentes do álcool etílico, um biocombustível, e do açúcar. A área industrial produtora de açúcar, um alimento, ainda convive com uma prática industrial ultrapassada, que vem a ser o uso do aço carbono, um material barato, mas com pequena resistência à corrosão. Os aços inoxidáveis são os materiais mais indicados para a substituição do aço carbono, entretanto, eles são considerados caros. Os evaporadores das usinas de açúcar são equipamentos importantes para a sua operação. Quando são usados tubos de aço carbono para a troca térmica, eles começam a ser substituídos a partir do segundo ano de operação devido à corrosão. Enquanto isso, os tubos em aços inoxidáveis podem durar décadas. O presente trabalho objetivou a avaliação do desempenho ambiental e financeiro de tubos para evaporadores construídos com aço carbono e, comparativamente, com os aços inoxidáveis 304, 444 e 439. Para alcançar estes objetivos foram utilizadas as metodologias de Avaliação do Ciclo de Vida (ACV) e de Custeio do Ciclo de Vida (CCV), respectivamente. Neste estudo de ACV foi adotada como função dos sistemas em estudo a massa de água evaporada. Na definição destes sistemas de produtos foram adotados os subsistemas de fabricação das bobinas de lâminas dos aços e fabricação, instalação e limpeza dos tubos. O consumo de gás natural, eletricidade e o transporte foram usados como subsistemas secundários. O período de avaliação destes sistemas foi de trinta anos. A partir dos resultados desta ACV, concluiu-se que os tubos em aço carbono apresentaram maior impacto ambiental que os tubos em aço inoxidável 304, 444 e 439, pois emitiram mais que 4,2 vezes a quantidade de dióxido de carbono; 2,8 vezes a de óxidos de nitrogênio; 3,1 vezes a de materiais particulados, 13 vezes a de materiais suspensos e 4,5 vezes a de resíduos totais. Os tubos em aço carbono consumiram também mais que 11 vezes o total de recursos naturais utilizados (carvão; lignita; calcita; dolomita; óleo; gás natural e minérios de ferro, cromo, níquel, molibdênio e manganês); 1,8 vezes a de água e 5 vezes a de energia. Com relação à emissão de óxidos de enxofre, os tubos em aço carbono emitiram quantidades semelhantes às emitidas pelos tubos em aços 444 e 439 e a metade que os tubos em aço 304. Na elaboração do CCV foram utilizados os componentes de custo compra, transporte, instalação, substituição e limpeza dos tubos e a venda das sucatas. Na avaliação financeira, todos os custos que aconteceram no período de trinta anos foram trazidos ao valor presente. A partir dos resultados obtidos concluiu-se que os aços inoxidáveis 439 e 444 apresentaram-se como opções de investimento mais interessantes que os tubos fabricados em aço carbono, já que apresentaram custos trazidos ao valor presente menores, na proporção de 0,76 para os tubos em aço 439; 0,79 para os tubos em aço 444 com 1,20 mm de espessura e 0,93 para tubos em aço 444 com 1,50 mm de espessura. Os tubos em aço 304 com 1,20 mm de espessura apresentaram desempenho financeiro semelhante aos tubos em aço carbono; enquanto que os tubos em aço 304 com 1,50 mm apresentaram custos ao valor presente 1,19 vezes maior. Os resultados obtidos indicam também que as metodologias ACV e CCV devem ser usadas em conjunto, pois mostram que produtos mais seguros ambientalmente podem vir a ser opções de investimentos também mais interessantes. / The sugar and ethanol sectors in Brazil have tried a great growth. The increasing demands for ethanol, a biocombustible, and for the sugar contributes for this situation. The producing industrial area of sugar, a foodstuff, still adopts the use of carbon steel pipes, a cheap product, of low corrosion resistance. The stainless steels are the materials more indicated for the substitution of carbon steel, however they are considered expensive. In sugar cane plants, the evaporators, are important equipments to operation. Carbon steel pipes are the main product used for the thermal exchange, and these pipes start to be replaced after the second year of operation. Meanwhile, stainless steel pipes can last for decades. This work evaluates the enviromental and financial performance of evaporators pipes constructed with carbon steel and with 304, 444 and 439 stainless steel. To reach these objectives the methodologies of Life Cycle Assessment (LCA) and the Life Cycle Costing (LCC) were applied, respectively. The water mass evaporated was adopted as function of the systems in LCA study. In the definition of these product systems were adopted the manufacturing of steel and the manufacturing, installation and cleanness of the pipes. Natural gas, electricity and transport had been considered. The evaluation time was thirty years. From the LCA results, there were concluded that the steel carbon pipes presented more enviromental impact performance than 304, 444 and 439 stainless steel pipes, because they emitted more than 4.2 times of carbon dioxide; 2.8 times nitrogen oxides; 3.1 times of particulated materials; 13 times of suspended materials and 4.5 times of total residues. The carbon steel pipes also consumed more than 11 times the sum of all natural resources used (coal, lignite, iron ground, chromium, nickel, molybdenum, manganese; calcite, dolomite; oil and natural gas); 1.8 times more of water and 5 times more of energy. Regards to sulphur oxides emission, the carbon steel pipes emitted the same amount of sulphur oxides as 444 and 439 stainless steel pipes and they emitted half than of 304 stainless steel pipe. In the life cycle costing (LCC) elaboration were applied the cost components: purchase, transportation, installation, substitution, cleanness and the scraps sale. In the financial evaluation, all the costs that have incurred in the period of thirty years have been brought to the present value. As results concluded that 439 and 444 stainless steels presented the best investment options, due to this they have presented less costs brought to the present value, in the ratio of 0.76 for the 439 steel pipe; 0.79 for 444 the steel pipe with 1.20 mm of thickness and 0.93 for 444 steel pipe with 1.50 mm of thickness. The 304 stainless steel pipes with 1.20 mm of thickness presented similar financial performance compared to carbon steel pipes; whereas the 304 steel pipes 1.50 mm of thickness presented costs to 1.19 times higher of the present value. The results also indicate that LCA and LCC methodologies must be used together; therefore, they show that safer enviromental products can become the more interesting investment options.

The interaction of an HVDC converter with the connected power system is of complex nature. An accurate model of the converter is required to study these interactions. The use of analytical small-signal converter models provides useful insight and understanding of the interaction of the HVDC system and the connected system components. In this thesis analytical models of the HVDC converters are developed in the frequency-domain by calculating different transfer functions for small superimposed oscillations of voltage, current, and control signals. The objective is to study the dynamic proprieties of the combined AC-DC interaction and the interaction between different HVDC converters with small signal analysis. It is well known that the classical Bode/Nyquist/Nichols control theory provides a good tool for this purpose if transfer functions that thoroughly describe the 'plant' or the 'process' are available. Thus, there is a need for such a frequency-domain model. Experience and theoretical calculation have shown that voltage/power stability is a very important issue for an HVDC transmission link based on conventional line-commutated thyristor-controlled converters connected to an AC system with low short circuit capacity. The lower the short circuit capacity of the connected AC system as compared with the power rating of the HVDC converter, the more problems related to voltage/power stability are expected. Low-order harmonic resonance is another issue of concern when line-commutated HVDC converters are connected to a weak AC system. This resonance appears due to the presence of filters and shunt capacitors together with the AC network impedance. With a weak AC system connected to the HVDC converter, the system impedances interact through the converter and create resonances on both the AC- and DC-sides of the converter. In general, these resonance conditions may impose limitations on the design of the HVDC controllers. In order to improve the performance of the HVDC transmission system when it is connected to a weak AC system network, a reactive compensator with a voltage source converter has been closely connected to the inverter bus. In this thesis it is shown that the voltage source converter, with an appropriate control strategy, will behave like a rotating synchronous condenser and can be used in a similar way for the dynamic compensation of power transmission systems, providing voltage support and increasing the transient stability of the converter. / QC 20100708

An AC system’s voltage unbalance by a fundamental frequency negative sequence system is usually the main cause for the emission of non-characteristic harmonics by current source converters as used in conventional HVDC systems. This emission takes place on both sides of each 12-pulse converter. On the DC side mainly a 2nd harmonic voltage appears driving a 2nd harmonic current. The magnitude of this harmonic current can exceed the magnitudes of the characteristic harmonics even if no low order resonance exists. Further non-characteristic harmonics generated by the converter under such unbalanced supply voltage conditions have frequencies with a frequency distance to the characteristic harmonics of 2 times the fundamental frequency. The main technical drawbacks are the unintended coupling between both AC systems and the risk of thyristor over-stresses by DC current discontinuities at low power transfer levels. On both AC sides the largest 2 non-characteristic current harmonics generated by a 12-pulse HVDC converter under unbalanced supply voltage conditions are a negative sequence system of the fundamental harmonic and a positive sequence system of the 3rd harmonic. Also on the AC sides further harmonics are emitted by the converter with a order number distance of 2 to the orders of the characteristic harmonics. However, in practical AC system operation special attention has to be paid to the 3rd harmonic distortion level, in particular when low order resonance appears between the system impedance and the impedance of the converter station AC filters. In order to avoid the above mentioned problems, large smoothing reactors and sometimes large blocking filters are installed on the DC side and the voltage distortion on the AC sides is reduced by AC filters. However, these filters require an expensive high component rating if they are tuned to the 2nd or 3rd harmonic respectively. The work shows that a modification of the valve firing can reduce the levels of the 2nd and 3rd harmonic without investment into additional primary equipment. Furthermore, this offers the chance to reduce the minimum power transfer level since also the risk of an intermittent DC current can be reduced. A corresponding algorithm and a control strategy are proposed. However, the calculation of an appropriate firing pattern requires a detailed modelling of the processes within the converters, especially the formation of the harmonics and the harmonic transfer between AC and DC sides. The work proposes a component vector model for the calculation of the harmonics. This model assumes that each harmonic consists of a first component representing the ideal conversion process, a 2nd component representing the impact of different commutation angles and in the case of the modified firing a 3rd component considering the impact of the intended non-equidistant firing. The work shows, that the harmonic component vectors resulting from voltage unbalance and from firing modulation can be treated separately and superimposed linearly. The calculation of the harmonic component vectors is performed applying the method of switching functions. For the consideration of the commutation and firing angle differences the modelling of switching functions based on differential impulses is proposed. However, especially an accurate representation of the above mentioned 2nd component vector requires a correct calculation of the commutation angles and their valve-specific differences. The investigations of this work have revealed that the conventional method of calculating the commutation angles – assuming an ideal smoothed DC current - may not produce results of sufficient accuracy. This is especially true in the case of a high ripple of the DC current, e.g. smoothed with a small smoothing reactor. A small smoothing reactor is typical for HVDC back-to-back applications. Therefore a new analytical method for the calculation of the commutation angles has been developed which in particular considers the typical pulse form of the DC current and additionally the impacts of the voltage unbalance and of the proposed modification of the firing on the ripple shape of the DC current. Moreover, as this analytical method requires the instantaneous values of the DC current at the instants of valve firing, a further analytical method for the calculation of these discrete current values has been developed. The equations are valid under the same conditions as the new ones for calculation of the commutation angles, i.e. resistive-inductive AC system fundamental frequency impedances, any degree of DC current smoothing between ideal smoothing and a ripple at the limit for current discontinuities. Symmetrical conditions, supply voltage unbalances and non-equidistant firing as proposed are applied. It is shown that, using this method, also the discrete values of the DC current at the end of the commutation intervals can be determined. In practice one of these discrete current values indicates the minimum value during one period of the fundamental frequency. This offers the chance for a more exact analytical determination of the limit for the appearance of DC current discontinuities. For typical parameters of a back-to-back installation the new methods and the new analytical equations have been compared with simulation results showing excellent correlation for typical voltage unbalances of not more than 1...2% and firing angle differences of not more than 2.5°. This verification is performed for the harmonics, the commutation angles and the discrete values of the DC current at the firing instants as well.