| 1. |
- Heuvelmans, Matthijs, 1982-, et al.
(författare)
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Power Semiconductors for Voltage Source Converters in HVDC and STATCOM Applications
- 2015
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- With a steady increase in electricity consumption in both developed and developing regions combined with demand for sustainable energy infrastructure, trends towards more renewable energy sources and more open electricity markets are becoming more prominent. High Voltage DC (HVDC) connections and Static Synchronous Compensators (STATCOM) are two technologies that play a part in fulfilling this increasing demand. HVDC is beneficial in cases where production and consumption are geographically separated or for sub-sea cable transmissions. STATCOM helps to increase the capacity of AC transmission in addition to enhancing ac-grid voltage quality. A key component in both HVDC and STATCOM converters are the semiconductor switching devices. Switching devices have a fundamental impact on performance levels that can be obtained in terms of efficiency, reliability and functionality. This paper serves two purposes. The first is to give a historical overview of switching devices employed in HVDC transmission systems and STATCOMs. This starts with the use of mercury arc valves some 100 years ago and it continues with the semiconductor switching devices that are currently being employed in HVDC and STATCOM applications. A second purpose of this paper is to indicate developments in switching technology that are of interest for HVDC and STATCOM. In order to do this in a structured manner, the technologies are compared in terms of efficiency, reliability and functionality. Developments that are discussed in this paper are the emergence of Silicon Carbide (SiC) devices and the improvement of Insulated Gate Bipolar Transistor (IGBT) and Integrated Gate Commutated Thyristor (IGCT) devices in silicon. Currently, applications are based on silicon based thyristors or IGBT technology. Line commutated converters based on thyristors are cost-effective and efficient; however, due to lack of controlled turn-off capability, functionality is limited. Using voltage source converter technology with semiconductors having turn-off capability, such as the IGBT, increased functionality is obtained. The IGCT, a gate controlled thyristor with turn-off capability, has lower conduction losses compared to an IGBT with the same active area. In case a modular multilevel converter is used, the switching frequency of the individual switches can be reduced for the same performance. This leads to a shift towards the importance of having low conduction losses as opposed to low switching losses. A further shift can be achieved by using soft-switching techniques. Regarding SiC devices, different maturity is reached for unipolar and bipolar devices. Unipolar devices in SiC have been marketed successfully at low voltages (≤ 1700V). An example is SiC Schottky diodes for power factor correctors. For the voltages typically applied in HVDC and STATCOM, the drift region resistance would impose serious limitations on efficiency (if high current densities are used). Bipolar devices such as SiC IGBTs and IGCTs have been demonstrated in laboratory setups and results have been published; however, reliable operation is currently impeded due to the propagation of crystal lattice defects which causes rapid degradation of such devices.
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| 2. |
- Jonsson, Annie, et al.
(författare)
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Landskapets förmåga att hålla biologisk mångfald : – en indikator för biologisk mångfald och ett planeringsverktyg för prioritering av markanvändning
- 2022
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Rapporten beskriver ett nytt verktyg framtaget för att underlätta planering av grön infrastruktur på landskapsnivå och ger via fallstudier exempel på hur verktyget kan användas. Projektet har utförts i ett samarbete med expertis inom teoretisk ekologi, ekologi, biodiversitetsinformatik och tillämpad matematik.Forskningsprojektet har utvecklat en modell för att uppskatta ett landskaps för-måga att hålla biologisk mångfald i dess olika biotoper (Biotope Biodiversity Capacity Indicator, BBCI). Ett teoretiskt ramverk baserat på ekologisk kunskap har tagits fram som grund för modellen.Syftet med BBCI är att modellen ska kunna användas som ett planeringsverktyg för att:stärka biologisk mångfald i ett landskap,förbättra förutsättningarna för arter att använda hela landskapet ochskapa bättre förutsättningar för hänsyn till biologisk mångfald i samband med samhällsutveckling.För att testa och beskriva verktygets användbarhet har fyra fallstudier genomförts med olika fokus:Analys av fragmenteringen i ett barrskogslandskap som sköts med särskild naturhänsyn i Västernorrlands län.Analys av barrskogsvärdekärnors kapacitet för biologisk mångfald inom Västra Götalands län som synliggör vikten av kommunöverskridande samverkan.Analys av potentiella målkonflikter mellan två biotoper, ädellövskog och öppen mark med skyddsvärda träd i Valle.Analys av kapacitet för biologisk mångfald hos äldre ädellövträd i ett landskap mixat med urbana miljöer och landsbygd, Mjölby kommun.Parallellt med utvcklingen av BBCI har en tät dialog och samverkan skett med olika intressenter och slutanvändare. Den breda dialogen har medfört ett effektivt kun-skapsutbyte mellan olika parter.Rapporten avslutas med att beskriva utmaningar och verktygets utvecklings-potential både avseende pedagogik och teknik men även hur modellen kan byggas på och utvecklas med ytterligare funktioner för en breddad tillämpning.
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| 3. |
- Jonsson, Annie, et al.
(författare)
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Utvärdering av Hornborgasjöns restaurering : måluppfyllelse och effekter på biologisk mångfald med fokus på vegetation och fågelfauna
- 2017
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Hornborgasjön räknas idag som en av Sveriges rikaste fågelsjöar och är internationellt utpekad som en av Sveriges värdefullaste våtmarker. Sjön har stor betydelse som både rast- och häckplats för en mängd fågelarter. Området är också av stort intresse för friluftslivet och som ett levande kulturlandskap. Under de senaste 150 åren har Hornborgasjön genomgått stora förändringar. En serie av sjösänkningar under 1800 och 1900-talen ledde till kraftig igenväxning och följdes av ett storskaligt restaureringsprojekt i senare tid. Syftet med Hornborgasjöns restaurering var att säkerställa Hornborgasjöns framtid som fågelsjö. Restaureringen är ett av Sveriges största naturvårdsprojekt. I denna rapport har vi utvärderat hur Hornborgasjöns restaurering påverkat vegetation och fågelfauna. Syftet var att analysera om och till vilken grad de biologiska målen med restaureringen uppnåtts.För att utvärdera måluppfyllelsen har vi i första hand jämfört olika naturtypers utbredning mellan åren 1905 och 2010 samt förändringar i fågelfaunan under flera tidsperioder. Analyser av vegetationskartor visar att vass- och buskområden kraftigt reducerats och att en stor öppen vattenspegel och omgivande mader återskapats. Våtmarksfåglarnas numerär har generellt sett återhämtat sig från igenväxningsperioden och är för vissa arter till och med större än vid förra sekelskiftet. För vissa naturtyper och fågelarter, som till exempel vassområden och häckande vadare, är dagens situation dock inte i linje med målen. Det står ändå klart att det övergripande målet och många av de mer specifika delmålen har uppfyllts så att Hornborgasjön idag är en levande våtmark med stort antal häckande och rastande fågelarter.I analyser av fågeldata från senare tid finns indikationer på negativa trender som man behöver vara observant på för att för framtiden säkra en biologiskt rik Hornborgasjö. En utmaning för denna utvärdering har dock varit bristen på högkvalitativa och jämförbara data att basera analyserna på. Vi belyser därför vikten av att ha ett fungerande övervakningssystem som kontinuerligt följer upp statusen i ekosystemet.
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| 4. |
- Roos, Henrik, et al.
(författare)
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Identification of patches with high ecological importance of broadleaved forests and open lands with valuable trees in Valle : Using a biodiversity capacity landscape metric, combining area and connectivity, to examining management options
- 2021
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This study is a complement to previous studies of regional action plans of green infrastructure in Valle (County Administrative Board 2016; County Administrative Board 2019). The aim was to identify the relative importance of biotope patches of broadleaved forest and open lands with valuable trees, respectively, in Valle for the maintenance and enhancement of biodiversity. A testversion of the method Biotope Biodiversity Capacity Indicator (BBCI) was applied on the two biotopes of broadleaved forest and open lands with valuable trees in Valle. BBCI was developed within the project “Landscape biodiversity capacity: a tool for measuring, monitoring and managing” financed by the Swedish Environmental Protection Agency, Miljövårdsanslaget (2019-2021).The results of the BBCI-analysis identified hotspots areas for the two studied biotopes in Valle, that is, areas with important patches for the sustainability of BBCI. For broadleaved forest, the northern part of Valle stands out as highly importantand for open lands with valuable trees the south-southwest parts of Valle stands out as the most important area. In this study, effects on BBCI from restauration of broadleaved forests to open land with valuable trees was evaluated. Calculations of BBCI shows encouraging results of an increased capacity for biodiversity of the biotope with open lands with valuable trees but not a significant decrease in biodiversity capacity of broadleaved forest, despite the area losses and fragmentation caused from the restauration. Both biotopes have BBCI values above one, showing that they are sustainable within a 100 year period. The result can be used as support to management plans of nature reserves in a landscape setting and as recommendations or advice to private land owners. Moreover, the study was produced within the work of BIOGOV, where regional action plans for nature, tourisms, culture and small enterprises were under development.
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| 5. |
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| 6. |
- ANGHOLT, JONAS, et al.
(författare)
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A First Security Analysis of a Secure Intermodal Goods Transport System
- 2013
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The goods transport business involves a lot of money and is a big part of the infrastructure of any European country. There are often many different actors involved in each transport and the communication network is rather complex due to the point-topoint communication structure. It is easy to understand why there is a high demand for increased simplicity and effectiveness. With this in mind, the e-Freight project which is based on PEPPOL has moved towards a standardized solution by developing a communication system based on access points (APs). These APs act as the interface to the system and makes it easy to establish communication between any two connected actors.With PEPPOL and e-Freight as a foundation, VOLVO leads the SITS project in close cooperation with Stena Line and DSV. The goal is to develop a harmonized communication framework that promotes increased sharing of information between actors and enable new applications to increase effectiveness and security in the chain of transportation. This leads to simplified accessibility for actors to a set of services by being connected to an AP. At the same time service providers benefit from being able to easily set up cloud services available for all actors. In addition to the back-office communication between APs, external devices such as cellphones, in-vehicle computers and check-in terminals can communicate directly with each other over short distances. This type of communication is only partially specified and a mutual standard is yet to be decided upon.In this report we have analyzed the SITS project from an IT-security perspective. The back-end system derived from e-Freight is looked into and communication links, access points, protocols, certificate handling etc., are examined. Another concern in the SITS project is the short-range communication between trucks and terminals. Since RFID is a highly potential candidate for use in this area, we have studied the technology by categorizing a typical RFID system into three distinct layers and researched important security threats with the classic CIA approach. Based on the security issues found, countermeasures such as encryption, authentication and protection against man-in-themiddle attacks are reviewed.
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| 7. |
- Balachandran, Arvind, 1994-
(författare)
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Battery Integrated Modular Multilevel Converter Topologies for Automotive Applications
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electric vehicles are rapidly developing in response to the need for increasing sustainable energy sources. The range and lifetime of an electric vehicle are limited by the battery pack. A pack comprises modules with several parallel and/or series-connected cells. Differences in leakage currents and cell in-homogeneities cause individual cell voltage and state-of-charge distribution among the cells to be non-homogeneous. As a result, over time, some cells discharge faster than other cells, thus limiting the total energy delivered by the pack. In order to maximize the energy delivered by the pack, individual cell control is desirable. As a solution, battery-integrated modular multi-level converter (BI-MMC) topologies are proposed, presented, and evaluated. BI-MMC topology consists of either one or two arms per phase, and each arm comprises several cascaded stages of DC–AC converters and is commonly referred to as submodules. BI-MMCs provide increased controllability and potential improvement in the lifetime of the battery pack. Furthermore, BI-MMCs have low output total harmonic distortion, further improving the powertrain efficiency.The first contribution is the design and evaluation of 3-phase and 6-phase BI-MMCs; comparisons are made against a conventional 2-level inverter for a 40-ton 400 kW commercial vehicle. The evaluation considers the total number of submodules, energy rating of the DC-link capacitors, battery losses, capacitor losses, and semiconductor losses. The evaluation showed that the BI-MMCs have lower semiconductor losses than the conventional 2-level inverter. However, the BI-MMCs have higher capacitor and battery losses. The second contribution is the investigation of the impact that the number of series connected cells per submodule has on the total losses of the BI-MMC. The study showed that 5- to 6-series connected cells have the lowest losses. The third contribution is the design principles for optimization of the DC-link capacitors and the MOSFET switching frequency; this is supported by experimental validation for the loss distribution within a submodule. The fourth contribution is a methodology for determining the battery impedance using the full-load converter current. In a conventional battery pack, the battery is connected directly to the fast charger’s DC supply. However, in a BI-MMC, the battery and the inverter are integrated, potentially increasing the DC charging capabilities because higher voltages can be achieved during charging than during operation. The fifth contribution is thus the derivation and investigation of the maximum DC charging power of BI-MMCs assuming the same submodule semiconductor losses during traction. The analysis showed that most BI-MMCs have a maximum DC charging power of about 1MW.
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| 8. |
- Berg, Sofia, 1980-
(författare)
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Community Robustness Analysis : Theoretical Approaches to Identifying Keystone Structures in Ecological Communities
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Most of the world’s ecosystems suffer from stress caused by human activities such as habitat destruction, fragmentation, overexploitation of species and climate change. These factors affect the reproduction and/or survival of individual species as well as interactions between species in ecological communities. Forthcoming effects of this are altered abundances, direct species loss, and indirect cascading extinctions, with yet largely unknown consequences on community structure and functioning. Today, biodiversity loss is of global concern since human society and welfare depend upon resources and services provided by ecosystems. The importance of considering entire ecological communities as a target for conservation and management has been increasingly recognized due to the interdependencie of species. Our ability to make predictions of the response of ecological communities to stress and biodiversity loss is in need of a deeper understanding of how structure and dynamical processes contributes to the functioning and stability of a community. In this thesis I use mathematical theory and dynamical models to study the response of community structure and resilience to a variety of disturbances affecting species and species interactions, ranging from small perturbations (Papers I-II) to large perturbations (species extinctions, Papers IIIIV).In Paper I we develop Community Sensitivity Analysis (CSA) as an analytical tool to study how a small permanent perturbation to the intrinsic growth rate, or mortality rate, of species is expected to affect i) the resilience (return rate) and ii) the structure (distribution of species equilibrium abundances) of an ecological community. Species interactions are described using Lotka-Volterra predator-prey dynamics. We apply CSA on the pelagic food webs of Lake Vättern and the Baltic Sea, respectively, and find that a change in the mortality rate of large-bodied species has a higher impact on community resilience and structure, compared to a perturbation to small-bodied species. However, analyzing the effect of a proportional change to the growth or mortality rate of species (elasticity analysis) shows that smallbodied species have proportionally larger effects on species equilibrium abundances, but not on resilience. CSA can also be used to study the effect of permanent (absolute or proportional) changes to inter- and intraspecific interaction strengths. For the two pelagic systems used in this study, CSA reveal that changes in the effect of a prey on its consumer tend to affect community structure and resilience significantly more than changes in the effect of a predator on its prey.In Paper II we assess the importance of rare species for the structure and resilience of ecological communities. First we show analytically, for a two species predator-prey system, that a change in the intrinsic growth rate of the rare species affect resilience more than a change in the growth rate of the common species. To test the generality of these results we next apply CSA on complex model food webs. In the analysis we distinguish between four trophic groups, each including only species with a similar trophic position, to separate the effect of abundance from the trophic position of species. Using mixed effect models we find support for our analytical predictions. More precisely, we find a strong negative relationship between the importance (sensitivity) of a species and its equilibrium abundance within all consumer groups and a weaker, but significant, relationship for producer species. The results from this study suggest that rare species can act as keystones through their effect on both community resilience and community structure, regardless of its trophic position.In Paper III we evaluate the risk of food web collapse caused by different trait-based extinction scenarios. In previous studies, groups of species, e.g. rare species, large-bodied species and top predators, have been identified to be relatively more prone to extinctions and other studies have found that extinctions of such species have comparably small effects on the remaining community. Using mathematical models of species dynamics we study the response of ecological communities to species removal (i.e. the proportion of species needed to be primarily removed to cause a 50% reduction in species richness, R50) when species are sequentially removed from the food web based on eight different traits. We show, contrary to some previous studies of sequential extinction simulations, that communities can be very vulnerable to realistic species loss. We furthermore find that the response of communities seems to depend on whether the extinction sequence follows a bottom-up or top-down direction, making it difficult to identify one particular extinction sequence as the most important/severe sequence.Finally, in Paper IV we aim to identify traits of species that can be used to identify keystone species, in terms of causing the highest proportion of secondary extinctions following their loss, in food webs with different degree of disassembly. Moreover, we analyze if the loss of a species that triggers a cascade of many secondary extinctions are the same species being identified as a keystones using Community Sensitivity Analysis. To answer these questions we randomly remove species from a set of 100 model communities. We analyze the relationship between the number of secondary extinctions following the randomly removed species and a range of species traits in communities where i) 75-100% of the initial number of species remain, ii) 50-75% of all species remain, iii) 25-50% of all species remain and iv) only 0-25% of all species remain. We find that the variation in secondary extinctions explained using species traits increases when the degree of food web disassembly and food web connectance are taken into account. The most important trait varies for different degrees of food web disassembly and also depends on whether basal species can go primarily extinct or not. However, due to correlation between most important traits, we conclude that the key status of different traits is rather robust against structural changes in the model food webs. Interestingly, food webs seem to be most sensitive to a random species loss after the loss of more than 25% of all initial species, suggesting that there is a threshold from which secondary extinctions increases. We also conclude that species being identified as keystones, based on the effect of their loss, are to some extent the same species being identified as having the largest effect on community structure and resilience, respectively, following a small perturbation.
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| 9. |
- Berg, Sofia, et al.
(författare)
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Ecological communities are vulnerable to realistic extinction sequences
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Loss of species will directly change the structure of ecological communities, which in turn may cause additional species loss (secondary extinctions) due to indirect effects (e.g. loss of resources or altered population dynamics). The vulnerability of food webs to repeated species loss is expected to be affected by food web topology, species interactions and the order in which species go extinct. Species traits such as body size, abundance and connectivity probably determine the vulnerability to extinction of species and, thus, the order in which species go primarily extinct. However, how different sequences of primary extinctions affect the vulnerability of food webs to secondary extinctions, when species abundances are allowed to respond dynamically, is not well understood. So far, only one study has incorporated species dynamics when assessing the effect of different extinction sequences on community structure, and only a limited number of extinction sequences have been evaluated. Here, using complex model food webs and including population dynamics, we analyze the effect of 33 extinction sequences on community structure using R50 (the proportion of primarily removed species needed to cause a 50% reduction in species richness) as a measure of community robustness to secondary extinctions. As expected, we find community structure to be highly vulnerable to removal of primary producers. More surprisingly, removing species based on traits that are strongly linked to the trophic position of species (such as large-bodied species, rare species, species with a high net effect, species with a high trophic position) are found to be as destructive as removing only primary producers. Such top-down oriented removal of species are often considered to correspond to realistic primary extinctions of species, but earlier studies, based on topological approaches, have not found such realistic extinction sequences to have any drastic effect on the remaining community. Thus, our result suggests that ecological communities could be more vulnerable to realistic extinction sequences than previously believed.
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| 10. |
- Berg, Sofia, et al.
(författare)
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Rare but important: perturbations to uncommon species have disproportionately large impact on ecological communities
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The majority of species in the ecosystems of the world are rare. Because the contributions to community biomass and productivity of many of these species are small it has been suggested that loss of rare species should have relatively small ecological consequences. However, the extent to which rare species affect the structure and stability of ecosystems is largely unknown. Using a theoretical approach, based on analytical methods, we here investigate how perturbations to rare as well as common species affect the structure (distribution of equilibrium abundances of species) and resilience (recovery rate) of complex ecological communities. We show that, contrary to expectation, resilience and structure of ecological communities are generally more sensitive to perturbations to rare than to common species. We find the explanation for this to lie in the cause of rarity: rare species tend to interact strongly, on a per capita basis, with other species. Our results suggest that many rare species are likely to fill important ecological roles in ecosystems.
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