Parallel-Operating Three-Phase Voltage Source Inverters – Circulating CurrentModeling, Analysis and Mitigation

The maximum realizable power throughput of power electronic converters may be limited or constrained by technical or economical considerations. One solution to this problemis to connect several power converter units in parallel. The parallel connection can be used to increase the current carrying capacity of the overall system beyond the ratings of individual power converter units. Thus, it is possible to use several lower-power converter units, produced in large quantities, as building blocks to construct high-power converters in a modular manner. High-power converters realized by using parallel connection are needed for example in multimegawatt wind power generation systems. Parallel connection of power converter units is also required in emerging applications such as photovoltaic and fuel cell power conversion. The parallel operation of power converter units is not, however, problem free. This is because parallel-operating units are subject to overcurrent stresses, which are caused by unequal load current sharing or currents that flow between the units. Commonly, the term ’circulatingcurrent’ is used to describe both the unequal load current sharing and the currents flowing between the units. Circulating currents, again, are caused by component tolerances and asynchronous operation of the parallel units. Parallel-operating units are also subject to stresses caused by unequal thermal stress distribution. Both of these problemscan, nevertheless, be handled with a proper circulating current control. To design an effective circulating current control system, we need information about circulating current dynamics. The dynamics of the circulating currents can be investigated by developing appropriate mathematical models. In this dissertation, circulating current models aredeveloped for two different types of parallel two-level three-phase inverter configurations. Themodels, which are developed for an arbitrary number of parallel units, provide a framework for analyzing circulating current generation mechanisms and developing circulating current control systems. In addition to developing circulating current models, modulation of parallel inverters is considered. It is illustrated that depending on the parallel inverter configuration and the modulation method applied, common-mode circulating currents may be excited as a consequence of the differential-mode circulating current control. To prevent the common-mode circulating currents that are caused by the modulation, a dual modulator method is introduced. The dual modulator basically consists of two independently operating modulators, the outputs of which eventually constitute the switching commands of the inverter. The two independently operating modulators are referred to as primary and secondary modulators. In its intended usage, the same voltage vector is fed to the primary modulators of each parallel unit, and the inputs of the secondary modulators are obtained from the circulating current controllers. To ensure that voltage commands obtained from the circulating current controllers are realizable, it must be guaranteed that the inverter is not driven into saturation by the primary modulator. The inverter saturation can be prevented by limiting the inputs of the primary and secondary modulators. Because of this, also a limitation algorithm is proposed. The operation of both the proposed dual modulator and the limitation algorithm is verified experimentally.

Rasitusten älykäs seuranta

Tässä diplomityössä käsitellään monikappalesysteeminä mallinnetun toimilaitteen tai mekaanisen systeemin kappaleissa vaikuttavien rasitusten, siirtymien ja jännitysten laskentamenetelmiä. Työhön sisällytettyjen menetelmien valinta on toteutettu 2000-luvulla virtuaalisuunnittelua käsittelevissä tiedelehdissä julkaistujen artikkelien pohjalta. Työn tarkoituksena on muodostaa kirjallisuuskatsaus uusien laskentamenetelmien ominaisuuksista ja metodiikasta, mitä voidaan tarvittaessa soveltaa virtuaalisuunnittelun tarpeisiin. Kaksi esiteltävistä menetelmistä on optimointimenetelmiä (RBDO ja ESL). Muissa menetelmissä käsitellään muun muassa venymien rekonstruointia ja hankauskitkasta komponentteihin kohdistuvia jännityksiä. Moving frame-menetelmässä sovelletaan kelluvan koordinaatiston periaatetta, yksi menetelmistä perustuu selkeästi osarakennetekniikkaan ja yhdessä kappaleiden joustokäyttäytymistä mallinnetaan muotofunktioiden avulla. Lisäksi on kolme soveltavaa esimerkkiä rasitusten seurannasta teollisuuskoneissa. Laskentamenetelmät ovat luonteeltaan ja sovelluskelpoisuudeltaan erilaisia. Optimointimenetelmät ovat parhaimmillaan rakenteiden jatkokehitystyössä, siinä missä muut menetelmät soveltuvat joko olemassa olevien rakenteiden mallintamiseen tai kokonaan uusien systeemien suunnittelutyökaluiksi. Tätä eroavuutta voidaan pitää hyvänä asiana, jotta voidaan valita parhaiten omiin tarkoituksiin soveltuva menetelmä.

The Sottunga-Jurmo shear zone : structure and deformation history of a crustal-scale ductile shear zone in SW Finland

The aim of this study is to gain a better understanding of the structure and the deformation history of a NW-SE trending regional, crustal-scale shear structure in the Åland archipelago, SW Finland, called the Sottunga-Jurmo shear zone (SJSZ). Approaches involving e.g. structural geology, geochronology, geochemistry and metamorphic petrology were utilised in order to reconstruct the overall deformation history of the study area. The study therefore describes several features of the shear zone including structures, kinematics and lithologies within the study area, the ages of the different deformation phases (ductile to brittle) within the shear zone, as well as some geothermobarometric results. The results indicate that the SJSZ outlines a major crustal discontinuity between the extensively migmatized rocks NE of the shear zone and the unmigmatised, amphibolite facies rocks SW of the zone. The main SJSZ shows overall dextral lateral kinematics with a SW-side up vertical component and deformation partitioning into pure shear and simple shear dominated deformation styles that was intensified toward later stages of the deformation history. The deformation partitioning resulted in complex folding and refolding against the SW margin of the SJSZ, including conical and sheath folds, and in a formation of several minor strike-slip shear zones both parallel and conjugate to the main SJSZ in order to accommodate the regional transpressive stresses. Different deformation phases within the study area were dated by SIMS (zircon U-Pb), ID-TIMS (titanite U-Pb) and 40Ar/39Ar (pseudotachylyte wholerock) methods. The first deformation phase within the ca. 1.88 Ga rocks of the study area is dated at ca. 1.85 Ga, and the shear zone was reactivated twice within the ductile regime (at ca. 1.83 Ga and 1.79 Ga), during which the strain was successively increasingly partitioned into the main SJSZ and the minor shear zones. The age determinations suggest that the orogenic processes within the study area did not occur in a temporal continuum; instead, the metamorphic zircon rims and titanites show distinct, 10-20 Ma long breaks in deformation between phases of active deformation. The results of this study further imply slow cooling of the rocks through 600-700ºC so that at 1.79 Ga, 2 the temperature was still at least 600ºC. The highest recorded metamorphic pressures are 6.4-7.1 kbar. At the late stages or soon after the last ductile phase (ca. 1.79 Ga), relatively high-T mylonites and ultramylonites were formed, witnessing extreme deformation partitioning and high strain rates. After the rocks reached lower amphibolite facies to amphibolite-greenschist facies transitional conditions (ca. 500-550ºC), they cooled rapidly, probably due to crustal uplift and exhumation. The shear zone was reactivated at least once within the semi-brittle to brittle regime between ca. 1.79 Ga and 1.58 Ga, as evidenced by cataclasites and pseudotachylytes. In summary, the results of this study suggest that the Sottunga-Jurmo shear zone (and the South Finland shear zone) defines a major crustal discontinuity, and played a central role in accommodating the regional stresses during and after the Svecofennian orogeny.

Att lära för livet hemma och i skolan : elevers uppfattningar av kost och hälsa, konsumtion och privatekonomi samt hushåll och miljö

The home is an important societal arena for upbringing and learning. A child can experience a feeling of participation in the household he or she belongs to very early in life. In this manner, the home environment constitutes an essential foundation for instruction in the subject of Home Economics. At school, Home Economics pupils should fulfill the intentions that school curriculum has for the subject, that is to say develop the knowledge, skills, and values that allow pupils to be able to take responsibility for their health, finances, comfort, and safety in their close environment. The purpose of this study is twofold. Firstly, the study aims to examine what knowledge and attitudes children and teenagers have acquired from their home environment, close environment, as well as school. Secondly, the study aims to evaluate the effects of instruction in Home Economics, at the 7th grade level, as regards diet and health, consumption and private finances, as well as household and the environment. The study’s methodological foundation focuses on pupils’ understanding of the surrounding world. A phenomenographical approach to the research phenomenon basis itself on the supposition that knowledge is fixed in human beings’ consciousness and experiences. Furthermore, the study stresses individual variations in conjunction with the experienced phenomenon. The empirical portion of the study is based on semistructured interviews of 30 pupils divided into two reference groups. The pupils were interviewed before instruction in the subject of Home Economics started and upon completing instruction. The interview data was analyzed and interpreted in accordance with the “multistage model”. The study results show that upbringing in the home environment is determinative as pertains to understanding of the socio-cultural household environment. Mealtime traditions, for example, are deeply ingrained but nonetheless influenced by lifestyle changes. The study shows that a didactic challenge exists to draw attention to the consequences of poor mealtime habits and stress for everyone raising or educating children and teenagers. Despite good knowledge of what a healthy diet is, the majority of pupils choose fast-food and junk-food when they eat out to save time and money. Studies of pupils’ preparedness for consumption show that a purposeful upbringing in the home in combination with relevant instruction in Home Economics, results in knowledgeable consumers. This study also shows that upbringing in the home environment and instruction in Home Economics requires an intense and conscious focus on the consequences of a household not run in accordance with nature, where the household lifestyle is nonsustainable. Pupils’ understanding is often based on the disregarding of the survival perspective for a comfort perspective. Parents and Home Economics teachers should be able to bring up and teach children and teenagers in a manner that allows children and teenagers to take responsibility for their health, private finances, as well as comfort and safety in the close environment. The method is conscious nurturing and instruction.

The Plastic Capacity of Boiler Supporting Headers

The goal of this thesis was to make a dimensioning tool to determine the plastic capacity of the boiler supporting header. The capacity of the header is traditionally determined by using FE-method during the project phase. By using the dimensioning tool the goal is to ensure the capacity already in the proposal phase. The study began by analyzing the headers of the ongoing projects by using FE-method. For the analytical solution a plain header was analyzed without the effects of branches or lug. The calibration of parameters in the analytical solution was made using these results. In the analytical solution the plastic capacity of the plastic hinges in the header was defined. The stresses caused by the internal pressure as well as the normal and shear forces caused by the external loading reduced the plastic moment. The final capacity was determined by using the principle of virtual work. The weakening effect of the branches was taken into account by using pressure areas. Also the capacity of the punching shear was defined. The results from the FE-analyses and the analytical solution correlate with each other. The results from the analytical solution are conservative but give correct enough results when considering the accuracy of the used method.

Relationships and estimates of longitudinal growth stress in Eucalyptus dunnii at different ages

The objective of this study was to obtain estimates of longitudinal growth stresses in standing trees of the Eucalyptus dunnii Maiden at eight, thirteen, fifteen and nineteen years of age and to determine their relationships with wood characteristics. The longitudinal growth stresses were indirectly measured by the "CIRAD-Forêt" method and estimated from both the dynamic modulus of elasticity and the modulus of elasticity in tension parallel to the grain. The longitudinal residual strain (LRS) and the estimates of the longitudinal growth stresses tended to increase with the age of the material. The LRS correlated positively and significantly with all the growth stresses estimates. The largestes magnitudes were at 13, 15 and 19 years of age. The basic density presented high, positive and significant correlations with the dynamic modulus of elasticity, estimated in the longitudinal direction, for wood saturated and at 12% moisture content, for all the ages assessed. All the growth stresses estimates presented high, positive and significant correlations between themselves.

Kalsinaattorin siirtymien ja teräskuoren murtumien tutkiminen

Sementti on yksi eniten maailmassa käytetty rakennusmateriaali ja Suomessa sementin valmistuksesta vastaa Finnsementti Oy kahden tehtaan voimin. Lappeenrannan tehtaalle investoitiin uusi uunilinja vuonna 2007, joka on aikansa kehittyneintä tekniikkaa. Järjes-telmän esilämmitysvaiheen laitteissa, erityisesti kalsinaattorissa, on esiintynyt luvattoman paljon plastisoitumista ja murtumia. Tässä työssä tutkittiin lämpötilan ja sen vaihtelun vaikutusta järjestelmässä syntyneisiin siirtymiin ja murtumiin. Työn alussa luotiin katsaus erilaisiin murtumisilmiöihin ja niiden syntyyn. Vuorauksessa käytettyihin tiilien käyttäytymistä tutkittiin myös, koska ne ovat olennainen osa esilämmi-tysjärjestelmää. Työn kokeellisessa osuudessa aluksi määriteltiin järjestelmässä käytetylle teräkselle lujuusominaisuudet tutkimalla mikrorakennetta sekä suorittamalla veto- että kovuuskokeet materiaalille. Toisessa osiossa mitattiin järjestelmän alasajon aikana tapah-tuneet siirtymät. Laskennallisessa osuudessa tutkittiin väsymismurtuman mahdollisuutta sekä määritettiin kuoressa esiintyvän särön vaikutusta puhkeamis- ja murtumiskuormituk-seen. FEM- analyysissä tarkasteltiin lämpötilan vaikutusta siirtymien suuruuksiin ja pyrittiin löytämään perusteluita, miksi rakenne on todellisuudessa siirtynyt eri suuntiin kuin alun perin oli tarkoitettu. Lisäksi analysoitiin FEM- mallit tiilihyllystä ja kalsinaattorin kuoren tukikehästä, joista tutkittiin lämpökuorman aiheuttamia jännityksiä. Materiaalin osalta selvisi, että kyseessä oli hyvin yleinen rakenneteräs S235 tai sitä vas-taava teräslaatu. Teräs ei ominaisuuksiltaan sovellu kuumiin olosuhteisiin, jonka vuoksi järjestelmässä käytettävän eristevuorauksen kunto on erityisen tärkeä rakenteen käyttöiän kannalta. Vuorauksen kunnosta riippuu, kuinka suuren lämpökuorman teräskuori saa. Väsymismurtuman mahdollisuus jäi minimaalisen pieneksi lämpötilan muutoksen vaiku-tuksesta. Analysoinnin tulosten perusteella voitiin todeta, että lämpötilan hitaasti mutta kohtalaisen suurella vaihteluvälillä sekä tiilien turpoamisen vaikutuksella on erittäin suuri merkitys järjestelmässä esiintyviin murtumiin.

Effect of ethephon on hardening of Pachystroma longifolium seedlings

Immediately after planting, tree seedlings face adverse environmental and biotic stresses that must be overcome to ensure survival and to yield a desirable growth. Hardening practices in the nursery may help improve seedling stress resistance through reduction of aboveground plant tissues and increased root volume and biomass. We conducted an assay to quantify changes in the morphogenesis following application of ethephon on seedlings of Pachystroma longifolium (Ness) I. M. Johnst.during hardening. The results showed no effect of the ethephon treatments on the number of leaves but a reduction of up to 50% in seedling height increment, and an increase in stem diameter increment of up to 44% with the 600 mg L-1 ethephon treatment, which consequently altered seedling Dickson Quality Index. Our results indicate that ethephon may help to promote desired morphological changes that occur during seedling hardening in nurseries.

COMPARISON BETWEEN WOOD DRYING DEFECT SCORES: SPECIMEN TESTING X ANALYSIS OF KILN-DRIED BOARDS

It is important to develop drying technologies for Eucalyptus grandis lumber, which is one of the most planted species of this genus in Brazil and plays an important role as raw material for the wood industry. The general aim of this work was to assess the conventional kiln drying of juvenile wood of three clones of Eucalyptus grandis. The specific aims were to compare the behavior between: i) drying defects indicated by tests with wood specimens and conventional kiln-dried boards; and ii) physical properties and the drying quality. Five 11-year-old trees of each clone were felled, and only flatsawn boards of the first log were used. Basic density and total shrinkage were determined, and the drying test with wood specimens at 100 °C was carried out. Kiln drying of boards was performed, and initial and final moisture content, moisture gradient in thickness, drying stresses and drying defects were assessed. The defect scoring method was used to verify the behavior between the defects detected by specimen testing and the defects detected in kiln-dried boards. As main results, the drying schedule was too severe for the wood, resulting in a high level of boards with defects. The behavior between the defects in the drying test with specimens and the defects of kiln-dried boards was different, there was no correspondence, according to the defect scoring method.

Influence of stiffness in bolted connections in wooden plane structure of truss type

Trusses are structural systems commonly used in projects, being employed mainly in roof structures, present in most rural buildings. The design of trusses, as well as other structural systems, requires the determination of displacements, strains and stresses. However, the project is developed from an ideal model of calculation, considering free rotation between the elements of a connection. This paper presents a computer program for the analysis of bidimensional wooden trusses with connections formed with two screws per node. The formulation is based on the flexibility method, taking into account the influence of the effect of semi-rigid connections formed by two screws. An example of a structure is presented and analyzed by the program developed here, highlighting the importance of behavior analysis on semi-rigid connections.

Kantavan modulaarisen kehikkorakenteen suunnittelu optimoiduksi tuotteeksi

Diplomityössä tavoitteena oli suunnitella kantava modulaarinen kehikkorakenne, joka soveltuu käytettäväksi erilaisissa tasoratkaisuissa. Suunnittelumetodina käytettiin järjestelmällistä tuotesuunnittelua. Rakenteen tuli olla modulaarinen, valmistettavissa laserleikkaamalla ja toteutettu ruuviliitoksilla. Lisäksi kehikkorakenteen suunnittelussa pyrittiin optimaalisimpaan ratkaisuun niin teknisen kuin taloudellisenkin toteutuksen kannalta. Premekon Oy investoi vuonna 2009 laserleikkuukeskukseen, jossa pystytään leikkaamaan levyjä ja palkkeja. Yhdessä oikeanlaisen tuotesuunnittelun kanssa laserleikkuukeskuksella pystytään valmistamaan tuotteita suurella tuotantotehokkuudella. Kantavan modulaarisen kehikkorakenteen kehittäminen mahdollisti laserleikkuun aiempaa tehokkaamman hyödyntämisen. Työssä tarkasteltiin yksityiskohtaisemmin erilaisten profiilien soveltuvuutta laserleikattavaksi, osien muotoilua, liitosten yhteensopivuutta ja lujuutta.

Yliopiston reunalla. Tutkimus suomalaisen avoimen yliopiston muotoutumisesta

Avoin yliopisto on koulutusmuoto, johon kuka tahansa voi osallistua ja jossa voi suorittaa yliopistotasoisia opintoja. Avoimen yliopiston ideassa keskeistä on koulutuksellisen tasa-arvon edistämisen tavoite. Tämän tutkimuksen tehtävänä on kuvata ja analysoida suomalaisen avoimen yliopiston muotoutumista. Tutkimuksella etsitään vastauksia kolmeen kysymykseen: 1. Millaisten vaiheiden kautta avoin yliopisto on muotoutunut? Mitä avoimen yliopiston historiassa on tapahtunut ja millaista keskustelua näistä tapahtumista on käyty? 2. Millaisia diskursseja avointa yliopistoa koskeneessa keskustelussa voidaan tunnistaa ja miten eri toimijatahot ovat näihin puhetapoihin kiinnittyneet? 3. Millaisena koulutuksellisen tasa-arvon toteuttajana avoin yliopisto näyttäytyy tutkimusaineiston valossa? Tutkimuksen aineisto koostuu erilaisista julkisista teksteistä. Aineistossa on mukana useita erilaisia tekstityyppejä: komiteanmietintöjä ja työryhmäraportteja, korkeakoululaitoksen kehittämissuunnitelmia, muita suunnitteluasiakirjoja, tutkimuksia, selvityksiä, puheenvuoroja ja esitelmiä sekä lehtikirjoituksia. Täydentävänä aineistona on lisäksi käytetty tilastoja. Tutkimusaineistoa on analysoitu diskurssianalyysillä. Keskeisenä lähtökohtana analyysissa on, että tekstien avulla tuotetaan avointa yliopistoa koskevia käsityksiä ja merkityksiä. Nämä merkitykset myös muuttuvat ajassa. Aineiston analyysin tuloksena avoimen yliopiston historiassa voidaan erottaa erilaisia vaiheita ja näiden vaiheiden välisiä taitekohtia. Ensimmäiseksi murrokseksi avoimen yliopiston historiassa voidaan paikantaa avoimen yliopiston synty, joka ajoittui 1970-luvun alkupuoliskolle. Avoin yliopisto sai vakiintuneet puitteensa vasta 1980-luvun puolivälissä, jolloin se organisoitiin osaksi yliopistojen täydennyskoulutusta. Tämä voidaan nähdä avoimen yliopiston historian toisena murroksena. Kolmas murros ajoittui 1990-luvulle, jolloin avoimen yliopiston resursointi muuttui ja nuoret tulivat sen näkyväksi opiskelijaryhmäksi. Tämä murros problematisoi avoimen yliopiston ja tutkintokoulutuksen suhteen aiemmasta poikkeavalla tavalla ja avoimen yliopiston tutkintoväylä nousi keskeiseksi keskusteluteemaksi. Tämä jännite purkautui tultaessa 2000-luvulle, ja tutkinnonuudistuksen yhteydessä avoimen yliopiston väylä sai paikkansa suhteessa kahden syklin tutkintoihin. Nyt elämme tutkinnonuudistuksen jälkeistä aikaa, jolloin avoimen yliopiston väylä vertautuu paljolti maisterikoulutuksiin. Kysymys aikuisten asemasta suhteessa tutkintokoulutukseen on kuitenkin edelleen ajankohtainen. Esillä ovat etenkin kysymykset aiemmin opitun tunnustamisesta, aikuisten ohjauksesta sekä avoimen yliopiston suhteesta työ- ja elinkeinoelämään. Keskustelussa avoimesta yliopistosta on paikannettavissa erilaisia positioita, jotka määrittävät avoimen yliopiston merkitystä ja tehtävää. Näitä positioita voidaan nimittää diskursseiksi, jotka konstituoivat avoimen yliopiston paikkaa yliopistokoulutuksen kentällä. Aineistosta on paikannettu neljä eri diskurssia: (1) akateemisia arvoja painottava yliopistollinen diskurssi, (2) osallistumisen tasa-arvoa korostava sivistyksellisen demokratian diskurssi, (3) yksilöllisiä mahdollisuuksia ja innovatiivisuutta korostava joustavuuden diskurssi sekä (4) työelämää, taloudellisuutta ja statusta korostava tehokkuuden diskurssi. Nämä diskurssit käyvät aineiston teksteissä keskinäisiä neuvotteluja ja kantavat merkityksiä suhteessa toisiinsa. Diskurssien välisiä suhteita voidaan kuvata kahden eri dimension kautta. Yhtäältä vastakkaisiksi arvoiksi asettuvat akateeminen eksklusiivisuus ja koulutuksellinen tasa-arvo. Toisena ulottuvuutena on koulutuksen arvottaminen sivistyksen versus hyödyn näkökulmasta. Avoimen yliopiston tehtävä tasa-arvon edistäjänä on eri aikoina mielletty eri tavoin. Avoimen yliopiston historiassa sen merkitystä ja tehtävää on kehystetty erilaisin puhetavoin, ja eri diskurssipositioiden vuoropuhelun kautta myös avoimen yliopiston tasa-arvotehtävästä on eri aikoina keskusteltu eri tavoin. Avoimen yliopiston alkuvaiheessa sen tehtävänä näyttäytyi sivistyksellisen demokratian turvaajana toimiminen. Kun avoimen yliopiston kurssit käynnistyivät, määrittyi toiminta selkeästi aikuisten koulutukseksi. Avoin yliopisto määrittyikin nyt aikuisten toiseksi mahdollisuudeksi hankkia koulutusta, jota vaille he olivat nuorempina jääneet. Avoimen yliopiston puitteiden lukkoonlyömisen jälkeen keskustelussa nousi vahvasti esiin toiminnan ja opetusmuotojen kehittäminen. Avoin yliopisto määrittyikin nyt aikuisten monipuoliseksi ja joustavaksi koulutusmahdollisuudeksi. Tasa-arvoisten mahdollisuuksien luominen näyttäytyi innovatiivisena, dynaamisena ja eteenpäinpyrkivänä toimintana, jossa otettiin huomioon aikuisten erilaiset tarpeet. Relander-ohjelman myötä avoimen yliopiston julkilausuttu tehtävä nimenomaan aikuisten kouluttajana kuitenkin muuttui. Avoin yliopisto näyttäytyi nyt yksilöllisiä tarpeita palvelevana mahdollisuuksien talona, jossa oli sijaa kaikille. Tärkeäksi määrittyi myös opiskelun tavoitteiden ja motiivien moninaisuus. Tutkinnon suorittamisen avoimen yliopiston opintojen kautta tuli olla realistisesti mahdollista. Viimeisimmän murroksen jälkeen avoin yliopisto määrittyy yhä selvemmin työelämän sekä alueellisten tarpeiden kautta. Avoin yliopisto näyttäytyy joustavana ja erilaisia tarpeita palvelevana opiskelufoorumina. Avoin yliopisto palvelee paitsi yksilöiden, myös työelämän ja yritysten tarpeita sekä on osaltaan turvaamassa alueiden kilpailukykyä ja elinvoimaisuutta. Tässä tutkimuksessa tarkastellaan avointa yliopistoa, sen historiaa ja siitä käytyä keskustelua erityisesti tasa-arvon näkökulmasta. Yhtenä keskeisenä tuloksena on, että avoimen yliopiston paikka on ollut aina jollakin tavalla marginaalissa. Tätä ilmentää mm. yliopistollisuuden ja tasa-arvon välinen jännite, jonka ympärille avointa yliopistoa koskeva keskustelu paljolti on järjestynyt. Ylipäätään aikuisten asemaa yliopistossa määrittää tietty epämukavuus ja täyden legitimiteetin puute erityisesti suhteessa tutkintokoulutukseen. Aikuisten koulutuksesta puhutaan yliopiston yhtenä perustehtävänä ja retorisesti voidaan todeta tämän tehtävän tärkeys. Aikuinen opiskelija aiempine osaamisineen ja osaamistarpeineen positioituu kuitenkin yliopistokoulutuksen ja työelämän väliselle rajapinnalle. Aikuisen paikka määrittyykin yliopiston ytimeen nähden selvästi marginaaliin.

Induction practices : experiences of newly qualified teachers

The thesis consists of four studies (articles I–IV) and a comprehensive summary. The aim is to deepen understanding and knowledge of newly qualified teachers’ experiences of their induction practices. The research interest thus reflects the ambition to strengthen the research-based platform for support measures. The aim can be specified in the following four sub-areas: to scrutinise NQTs’ experiences of the profession in the transition from education to work (study I), to describe and analyse NQTs’ experiences of their first encounters with school and classroom (study II), to explore NQTs’ experiences of their relationships within the school community (study III), to view NQTs’ experiences of support through peer-group mentoring as part of the wider aim of collaboration and assessment (study IV). The overall theoretical perspective constitutes teachers’ professional development. Induction forms an essential part of this continuum and can primarily be seen as a socialisation process into the profession and the social working environment of schools, as a unique phase of teachers’ development contributing to certain experiences, and as a formal programme designed to support new teachers. These lines of research are initiated in the separate studies (I–IV) and deepened in the theoretical part of the comprehensive summary. In order to appropriately understand induction as a specific practice the lines of research are in the end united and discussed with help of practice theory. More precisely the theory of practice architectures, including semantic space, physical space-time and social space, are used. The methodological approach to integrating the four studies is above all represented by abduction and meta-synthesis. Data has been collected through a questionnaire survey, with mainly open-ended questions, and altogether ten focus group meetings with newly qualified primary school teachers in 2007–2008. The teachers (n=88 in questionnaire, n=17 in focus groups), had between one and three years of teaching experience. Qualitative content analysis and narrative analysis were used when analysing the data. What is then the collected picture of induction or the first years in the profession if scrutinising the results presented in the articles? Four dimensions seem especially to permeate the studies and emerge when they are put together. The first dimension, the relational ˗ emotional, captures the social nature of induction and teacher’s work and the emotional character intimately intertwined. The second dimension, the tensional ˗ mutable, illustrates the intense pace of induction, together with the diffuse and unclear character of a teacher’s job. The third dimension, the instructive ˗ developmental, depicts induction as a unique and intensive phase of learning, maturity and professional development. Finally, the fourth dimension, the reciprocal ˗ professional, stresses the importance of reciprocity and collaboration in induction, both formally and informally. The outlined four dimensions, or integration of results, describing induction from the experiences of new teachers, constitute part of a new synthesis, induction practice. This synthesis was generated from viewing the integrated results through the theoretical lens of practice architecture and the three spaces, semantic space, physical space-time and social space. In this way, a more comprehensive, refined and partially new architecture of teachers’ induction practices are presented and discussed.

Performance and scalability of isolated DC-DC converter topologies in low voltage, high current applications

Fuel cells are a promising alternative for clean and efficient energy production. A fuel cell is probably the most demanding of all distributed generation power sources. It resembles a solar cell in many ways, but sets strict limits to current ripple, common mode voltages and load variations. The typically low output voltage from the fuel cell stack needs to be boosted to a higher voltage level for grid interfacing. Due to the high electrical efficiency of the fuel cell, there is a need for high efficiency power converters, and in the case of low voltage, high current and galvanic isolation, the implementation of such converters is not a trivial task. This thesis presents galvanically isolated DC-DC converter topologies that have favorable characteristics for fuel cell usage and reviews the topologies from the viewpoint of electrical efficiency and cost efficiency. The focus is on evaluating the design issues when considering a single converter module having large current stresses. The dominating loss mechanism in low voltage, high current applications is conduction losses. In the case of MOSFETs, the conduction losses can be efficiently reduced by paralleling, but in the case of diodes, the effectiveness of paralleling depends strongly on the semiconductor material, diode parameters and output configuration. The transformer winding losses can be a major source of losses if the windings are not optimized according to the topology and the operating conditions. Transformer prototyping can be expensive and time consuming, and thus it is preferable to utilize various calculation methods during the design process in order to evaluate the performance of the transformer. This thesis reviews calculation methods for solid wire, litz wire and copper foil winding losses, and in order to evaluate the applicability of the methods, the calculations are compared against measurements and FEM simulations. By selecting a proper calculation method for each winding type, the winding losses can be predicted quite accurately before actually constructing the transformer. The transformer leakage inductance, the amount of which can also be calculated with reasonable accuracy, has a significant impact on the semiconductor switching losses. Therefore, the leakage inductance effects should also be taken into account when considering the overall efficiency of the converter. It is demonstrated in this thesis that although there are some distinctive differences in the loss distributions between the converter topologies, the differences in the overall efficiency can remain within a range of a few percentage points. However, the optimization effort required in order to achieve the high efficiencies is quite different in each topology. In the presence of practical constraints such as manufacturing complexity or cost, the question of topology selection can become crucial.

Power cycling lifetime estimation of IGBT power modules based on chip temperature modeling

In this doctoral thesis, methods to estimate the expected power cycling life of power semiconductor modules based on chip temperature modeling are developed. Frequency converters operate under dynamic loads in most electric drives. The varying loads cause thermal expansion and contraction, which stresses the internal boundaries between the material layers in the power module. Eventually, the stress wears out the semiconductor modules. The wear-out cannot be detected by traditional temperature or current measurements inside the frequency converter. Therefore, it is important to develop a method to predict the end of the converter lifetime. The thesis concentrates on power-cycling-related failures of insulated gate bipolar transistors. Two types of power modules are discussed: a direct bonded copper (DBC) sandwich structure with and without a baseplate. Most common failure mechanisms are reviewed, and methods to improve the power cycling lifetime of the power modules are presented. Power cycling curves are determined for a module with a lead-free solder by accelerated power cycling tests. A lifetime model is selected and the parameters are updated based on the power cycling test results. According to the measurements, the factor of improvement in the power cycling lifetime of modern IGBT power modules is greater than 10 during the last decade. Also, it is noticed that a 10 C increase in the chip temperature cycle amplitude decreases the lifetime by 40%. A thermal model for the chip temperature estimation is developed. The model is based on power loss estimation of the chip from the output current of the frequency converter. The model is verified with a purpose-built test equipment, which allows simultaneous measurement and simulation of the chip temperature with an arbitrary load waveform. The measurement system is shown to be convenient for studying the thermal behavior of the chip. It is found that the thermal model has a 5 C accuracy in the temperature estimation. The temperature cycles that the power semiconductor chip has experienced are counted by the rainflow algorithm. The counted cycles are compared with the experimentally verified power cycling curves to estimate the life consumption based on the mission profile of the drive. The methods are validated by the lifetime estimation of a power module in a direct-driven wind turbine. The estimated lifetime of the IGBT power module in a direct-driven wind turbine is 15 000 years, if the turbine is located in south-eastern Finland.