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Rad definira pojam viših harmonika te objašnjava negativne utjecaje viših harmonika na sve uređaje i čimbenike u elektroenergetskom sustavu. Također, prikazani su i pokazatelji kojima se može kvantificirati određeni harmonik te njegov utjecaj u mreži. Objašnjeno je koji uređaji generiraju više harmonike, prikazane su tehnike modeliranja viših harmonika te je pojašnjeno pri kojim uvjetima vrijede različiti modeli. Nadalje, prikazani su načini modeliranja nelinearnih opterećenja te topologije takvih uređaja. Predstavljene su metode za ublažavanje viših harmonika. Norme koje obuhvaćaju kvalitetu električne energije kojima pripadaju ograničenja pojedinih harmonika također su sadržane u ovom radu. Praktični dio rada sadržava simulaciju u programskom paketu DigSilent PowerFactory s podatcima koji su dobiveni mjerenjem kvalitete električne energije prema normi HRN EN 50160:2012 na dva mjerna mjesta u ruralnom području u blizini Osijeka. Simulacija se sastoji od četiri različita scenarija te su uspoređene fizikalne veličine (generirane programskim paketom) koje nam prikazuju utjecaj viših harmonika na elektroenergetsku mrežu. Dobiveni rezultati simulacija ukazuju da su najbolji rezultati dobiveni kada je aktivni filter snage spojen na kraj izvoda jer su tada najmanje vrijednosti viših harmonika te ukupnog harmonijskog izobličenja. U radu je objašnjeno zašto se ne priključuje više takvih filtera u mrežu ovakve topologije. This thesis describes the concept of harmonics and explains the negative effects of harmonics on all devices and components in the power system, presents indicators that can quantify a particular harmonic and its impact in the network. It is explained which devices generate harmonics, the techniques of modeling harmonics are presented, and the conditions under which different models are valid are explained. Furthermore, the ways of modeling nonlinear loads and the topology of such devices are presented. Methods for mitigating harmonics are presented. Standards that include the quality of elecrtical energy, which includes the limitations of individual harmonics, are also contained in this thesis. The practical part of this thesis contains a simulation in the software package DigSilent PowerFactory with data obtained by measuring the quality of electrical energy according to HRN EN 50160: 2012 at two measuring points in a rural area near the city of Osijek. The simulation consists of four different scenarios and compares the physical quantities (generated by software) that show the impact of harmonics on the power grid. It was concluded that the best results were obtained when the active power filter was connected to the end of the lead because then the values of harmonics and total harmonic distortion were the lowest and it was explained why no more such filters are connected to the network of this topology.

U ovom završnom radu opisana je analiza i proveden proračun magnetskih polja u okolišu trofaznih vodova. Na početku rada koncizno su opisani mogući stupovi dalekovoda te detaljno obrazloženo kako se i od čega izrađuju temelji i stupovi dalekovoda. Zatim su opisani temeljni utjecaji magnetskih veličina na raznoliku živu te neživu prirodu, prikazana je tablica ograničenja magnetskih veličina u Republici Hrvatskoj. Na kraju rada provodi se teorijski pregled analitičkog izračuna magnetskog polja, pojednostavljena shema jednog vodiča protjecanog električnom strujom postupno se proširuje i na kraju dobivamo zadovoljavajuću aproksimaciju dalekovoda u 41 stvarnosti. Primjenom te aproksimacije na zadanim primjerima stvarnog dalekovoda izračunate su magnetske veličine i nacrtana krivulja koja nam grafički predočuje izračunate vrijednosti. The final thesis describes analyze and calculation of magnetic fields in surroundings of three-phase lines. At the beginning of the thesis, the possible columns of the transmission line are described concisely, how and from which material are the foundations and columns of the transmission line made is explained in detail. Then the basic effects of magnetic magnitudes on the diverse living and inanimate nature are described, a table of magnetic magnitude limitations in the Republic of Croatia is presented. At the end of the thesis, a theoretical review of the analytical calculation of the magnetic field is preformed, the simplified scheme of one conductor with electric current is gradually expanding and finally we get a satisfactory approximation of the transmission line in reality. By applying this approximation to the given examples of the real transmission line, magnetic magnitudes are calculated, a curve is drawn that graphically presents the calculated values.

Aqueous mixtures of amphiphiles display a rich variety of molecular self-assembly, from defined objects such as micelles in micro-emulsions to micro-heterogeneity in molecular liquids. 2, 2, 2- trifluoroethanol (TFE), a flourinated mono-ol, is an example of a such an amphiphile. Its aqueous mixture is relevant in the biological context, as TFE-water mixtures are known for promoting or inducing the regular secondary structuring in peptides and proteins [1] and are often used in CD and liquid NMR experiments. On the molecular scale, aqueous TFE mixtures are micro- heterogenous, as witnessed by scattering experiments [2]. This particular type of microheterogenous organization may also contribute in promoting the regular structuring in the solvation process of a peptide. For example, at around 40v/v concentration of TFE - where the microheterogeneity shows the strongest signals - the transition from unfolded to folded protein states is also observed [3]. In this work, we aim at characterizing the structuring in TFE-water mixtures using computer simulations. Since recent studies have shown the issues in achieving correct mixing of the TFE and water [4], a lot of consideration was put into the choice of the force field models, the system sizes and simulation times. By calculating static site-site properties, we track the way the mixture evolves with the change in the molar fraction of the co-solvents. The self-association, induced by self H-bonding, and cross-species associations due to the effect of solvent are particularly distinguished. Oftentimes in the literature, the term "clusters" is used for both cases, which hides the fact that these structures are due to different mechanisms of associations. We also present thermodynamic properties, such as enthalpies and entropies of mixing and concentration fluctuations which again highlight the complex nature of the TFE-water mixture. [1] Goodman M, Listowsky I (1962) Conformational Aspects of Synthetic Polypeptides. VI. Hypochromic Spectral Studies of Oligo-γ - Methyl-L-Glutamate Peptides. J. Am. Chem. Soc. 84:3770 [2] Hong DP, Hoshino M, Kuboi R, Goto Y (1999) Clustering of Fluorine-Substituted Alcohols as a Factor Responsible for Their Marked Effects on Proteins and Peptides. J. Am. Chem. Soc. 121:8427 [3] Gast K, Zirwer D, Müller-Frohne M, Damaschun G (1999) Trifluoroethanol-induced conformational transitions of proteins: Insights gained from the differences between ribonuclease A. Protein Science, Cold Spring Harbor Laboratory Press 8:625 [4] Gerig J (2014) Toward a Molecular Dynamics Force Field for Simulation of 40% Trifluoroethanol-Water. J. Phys. Chem. B 118:1471

We study different ethanol mixtures using classical molecular dynamics computer simulations. The theory of fluctuations, developed by Kirkwood and Buff in the 50's [1], allows us to connect integrals of radial distribution functions, obtained through simulations, with a variety of thermodynamic quantites, such as partial molar volumes, compressibility and most importantly, the thermodynamic factor. The value of thermodynamic factor is twofold. Firstly, it is an indicator of the stability of a mixture, with respect to mixing or demixing, since it can be written as a second derivative of the Gibbs free energy with respect to concentration. Secondly, it is particularly useful in comparing experimental and simulation results for diffusion coefficients. Here, we look for the most precise and accurate ways of calculating the thermodynamic factor from finite size systems and see what we can learn about hydrogen bonding associative mixtures and their deviation from ideality. Understanding the structural characteristics on molecular level is important for the understanding of more complex biological structures and processes. [1] JG Kirkwood, F Buff (1951) J. Chem. Phys. 19, 774

Proučavali smo sustav čistog metanola u NpT ansamblu koristeći jednostavni statističko- mehanički Mercedes Benz model. Toplinski kapacitet, koeficijent termalne ekspanzije i izotermna kompresibilnost pokazuju slaganje u temperaturnoj ovisnosti sa rezultatima soft- core modela. Toplinski kapacitet pokazuje i izraziti maksimum na sličnoj temperaturi u oba modela, koji može biti povezan sa pucanjem vodikovih veza pri povećanju temperature, te prevladavanjem van der Waals interakcija nad vodikovim vezama. Procjena greške na temelju usrednjavanja rezultata više simulacija pokazala je manje vrijednosti grešaka nego pojedinačne simulacije. Uvođenjem kontrole omjera prihvaćenosti pomaka kroz korekcije maksimalnih pomaka, dobili smo bolju preciznost i za rezultate procjenjene na temelju jedne simulacije. Ispitali smo preciznost za dvije vrijednosti omjera prihvaćenosti, 0.5 i 0.3. Pritom je omjer prihvaćenosti od 0.5 pokazao nešto bolje rezultate.

Simulacijama molekulske dinamike smo proučavali binarne tekuće mješavine koje pokazuju specifično strukturno uređenje. Postoji razlika između kompleksnog nereda (complex disorder) / mikroheterogenosti, definiranog prostornim asocijacijama molekula, te jednostavnog nereda (simple disorder), koji karakterizira posve homogene mješavine. Ova strukturna svojstva su povezana s ponašanjem Kirkwood-Buff integrala (KBI) [1]. KBI su definirani kao integrali radijalnih distribucijskih funkcija i kao takvi se mogu izračunati iz simulacija. Također, mogu se mjeriti preko termodinamičkih svojstava. Najzanimljivija opservabla je termodinamički faktor, koji je indikator stabilnosti mješavine. U ovom radu ispitujemo pouzdanostdva pristupa: računanja termodinamičkog faktora iz KBI-ja te suviška Gibbsove slobodne energije. Počinjemo s jednostavnim sustavom, dvokomponentnom Lennard-Jones mješavinom, a potom prelazimo na realističnije modele, poput mješavine alkohol+benzen. Rezultat za obje metode pokazuju kvalitativno slaganje. [1] JG Kirkwood, F Buff (1951) J. Chem. Phys. 19, 774

Sve brži napredak tehnologije je doprinio da se računalne simulacije prometnu u bitan aspekt znanstvenog rada, usporedo teorijskim i eksperimentalnim metodama. Simulacije molekularne dinamike daju uvid u mikroskopsko ponašanje sustava koje u eksperimentu opažamo preko mjerivih makroskopskih veličina. U ovom radu metodom termodinamičke integracije računala se energija solvatacije alkana [1], s ciljem razumijevanja otapanja nepolarnih molekula u polarnom i amfifilnom mediju. Korišten j eprogramski paket za molekularnu dinamiku Gromacs[2]. Simulirani su alkani: metan, etan, propan i butan u vodi i u metanolu. TIP4P/2005 [3] model za vodu se pokazao najboljim u reproduciranju kvalitativnog ponašanja ovisnosti slobodne energije solvatacije o veličini alkana, što je u skladu s podacima iz literature [4]. Vrlo su zanimiljivi trendovi ponašanja otapanja alkana u vodi, budući da ne pokazuju monotonost. Krivulja ovisnosti energije solvatacije o temperaturi alkana u metanolu pokazuje maksimum, što ukazuje na kompeticijske doprinose. Maksimum je najuočljiviji za metan te s produljenjem lanca alkana postaje sve manje izražen i pomaknut prema višim temperaturama. Ovi rezultati također pokazuju da se veći alkani lakše otapaju u metanolu što se može objasniti formiranjem lančastih struktura metanola unutar samog sustava. Simulacije i eksperimentalni rezultati pokazuju kvalitativno slaganje. [1] Frenkel, D. & Smit, B. Understanding Molecular Simulation. Understanding Molecular Simulation 638 (2002). doi:10.1016/B978-012267351-1/50006-7 [2] Gromacs, About Gromacs, July 2017. , http://www.gromacs.org/About_Gromacs [3] Abascal, J. L. & Vega, C. A general purpose model for the condensed phases of water: TIP4P/2005. The Journal of chemical physics 123, 234505 (2005). [4] Vega, C. & Abascal, J. L. F. Simulating water with rigid non-polarizable models: a general perspective. Physical Chemistry Chemical Physics 13, 19663 (2011)

Molecular dynamics simulations were used to study binary mixtures which exhibit a specific structural organization. We distinguish complex disorder, defined by the presence of microheterogeneous spatial associations of molecules and simple disorder, which characterizes totally homogeneous mixtures. These structural properties are related to the behavior of the Kirkwood-Buff integrals (KBI). The KBI are defined as integrals of radial distribution functions and, as such can be obtained from computer simulations. Equally, they can be measured using thermodynamic properties, such as partial molar volumes, compressibility and partial derivative of the chemical potential with respect to concentration. The latter is called thermodynamic factor and is a quantity of a particular interest, since it is an indicator of the stability of a mixture with respect to mixing or demixing. Here we verify the reliability of the two different approaches: the calculation of thermodynamic factor from KBI and from the excess Gibbs free energy. We start with a simple system, two component Lennard-Jones mixture and advance to more realistic models, such as alcohol + benzene mixtures. The results show that the two methods qualitatively agree.

A simplified two dimensional (2D) model for methanol is proposed and its properties studied. Model is analogous to the 2D Mercedes-Benz water model and treats orientationally dependent hydrogen bonding interactions in a similar way. Methanol molecules are described as two tangentially fused Lennard-Jones disks, one of them having two arms to account for hydrogen bonds formation. Monte Carlo simulations were done to determine thermodynamic and structural properties for a wide range of temperatures. Stable infinite chain crystal structure is maintained at lower temperatures, analogous to the structures observed in real methanol. Thermodynamic quantities, such as heat capacity, compressibility and thermal expansion coefficient were calculated from fluctuations of volume and enthalpy in the system. Results were compared with ones from molecular dynamics simulations performed with standard Trappe and OPLS models and experimental results for real methanol.

We aim to classify binary mixtures according to whether they contain hydrogen bonds forming liquids (water, alcohols) or not (benzene, cyclohexane, toluene, carbon tetrachlo- ride), into complex and simple disorder. We define complex disorder as the presence of microheterogeneous spatial associations of molecules with similar interactions, like hydrogen bonding ones, where simple disorder represents the totally homogeneous mixture. For that purpose we compare experimental and simulation results for thermodynamic, structural (ra- dial distribution factor, structural factor and Kirkwood-Buff integral) and dynamical (diffu- sion coefficient) data. This analysis clarifies the role of the concentration fluctuations versus that of micro-heterogeneous structures. This is done mainly analyzing the k=0 values of the structure factors S(k) and those corresponding to the pre-peak. The existance of structure factor prepeaks at small k, arising from periodicity in radial distribution functions should imply the microheterogeneous structural ordering. The structure factor value at k=0 tells us about the concentration fluctuations which are, with the help from statistical physics, related to the experimentally measureable quantities such as isothermal compresibility. Un- derstanding the microheterogeneity and structural behavior on molecular level can be useful in understanding more complex biological structures and processes.

Ethanol is a hydrogen bonding liquid. When mixed in small concentrations with water or alkanes, it forms aggregate structures reminiscent of, respectively, the direct and inverse micellar aggregates found in emulsions, albeit at much smaller sizes. At higher concentrations, micro-heterogeneous mixing with segregated domains is found. We examine how different statistical methods, namely correlation function analysis, structure factor analysis and cluster distribution analysis, can describe efficiently these morphological changes in these mixtures.

We study binary mixtures which shows different types of structural organization. We distinguish between ideal, regular and complex structure. Complex disorder is defined by the presence of micro-heterogeneous spatial associations of molecules with strong interactions, like hydrogen bonding, while simple disorder represents totally homogeneous mixtures. Regular structure is present in the case of mixtures with less pronounced associations such as in acetone-benzene mixture. To classify these different types of order we compare simulation results for structural data, namely radial distribution functions (RDFs) and Kirkwood-Buff integrals (KBI). Classical molecular dynamic simulations were performed with two simulation packages, Gromacs [1] and ms2 [2], that are based on different force fields. We show that the RDF and KBI results are robust enough and do not depend on details of the potentials. REFERENCES: [1]www.gromacs.org [2]www.ms-2.de

Nedavna otkrića varijabilnosti inteziteta međuzvjezdanih spektralnih linija ukazuju na moguće postojanje brzih i vrlo malih struktura materije u međuzvjezdanom mediju, reda veličine od 10 do 100 astronomskih jedinica (AU). U ovom radu pažnja je bila posvećena zvijezdama aktivnog zvjezdorodnog područja η Carinae, u kojem je vjerojatnija prisutnost spomenutih struktura, zbog gustoće okolnog plina i prašine. Spomenuto potvrđuju i rezultati rada, gdje sam uočila varijabilnost međuzvjezdanih apsorpcijskih linija na većem broju spektara, nego što je to ranije bilo primijećeno. U rezultatima su analizirane vrijednosti radijalnih brzina varirajućih komponenti, vremenske epohe unutar kojih su vrijednosti uočene, te njihova manifestacija na profil pojedine linije., Recent discoveries of variability in interstellar spectral line intensity indicate possible resence of fast and very small structures of matter in interstellar media, ranging from 10 to 100 astronomical units (AU). In this paper, attention was given to the stars of active stellar region η Carinae, where the presence of the mentioned structures is more likely due to the density of surrounding gas and dust. This is also confirmed by the results of this thesis, where I have found variability of interstellar absorption lines in a bigger sample of spectra, than it has been previously observed. Radial velocities of the variable components, the time periods within which the variability was observed and its manifestation on the profile of a particular line have been analysed in the results.