Kokeellinen taajuusmuunnin DVB-T -lähettimeen

Kiinnostus digitaalisen televisiolähetteen vastaanottamiseen matkaviestimillä on lisääntynyt viime vuosina. Euroopalaista DVB-T -digitaalitelevisiostandardia viimeistellessä perustettiin ad-hoc ryhmä DVB-X tutkimaan, miten standardia voitaisiin muuttaa, jotta liikkuva vastaanotto olisi mahdollista. Samaan aikaan langattomien lähiverkkojen käyttö on lisääntynyt huomattavasti, joten viestintäviranomaiset ovat varanneet uusia taajuuskaistoja mikroaaltotaajuuksilta näiden käyttöön. Koska uuden sukupolven langattomien lähiverkkojen standardi HiperLAN/2 ja digitaalitelevisiostandardi DVB-T käyttävät samankaltaista OFDM-lähetystekniikkaa, yhdistämällä lähiverkkojen kaupalliset RF osat ja DVB-T lähete on mahdollista toteuttaa mikroaaltotaajuuksilla toimiva digitaalitelevisiolähetin. Diplomityössä esitetään ratkaisu, jossa UHF-taajuuksinen digitaalitelevisiolähete nostetaan taajuusmuuntimella 5,8 GHz ISM-kaistalle. Jotta muuntimen kriittisten osien sekä taajuusmuunnoksen ongelmat voitaisiin ratkoa, työssä esitetään lyhyt kertaus DVB-T -tekniikan tärkeimpiin alueisiin. Läpikäymällä viimeaikaisia radioaaltojen etenemismallien mittauskampanjoita alan kirjallisuudesta sekä vastaanottoalueen kartoistus, todetaan linkkibudjettilaskelmalla, että tarvittava lähetysteho on noin 10 W EIRP. Koska vastaanottoalue on kapea, lähetinantenni vahvistaa signaalia 20 dB. Tällöin DVB-T -lähettimen vaatimuksiin kuuluva lineaarinen vahvistin toteutetaan käyttämällä ylimitoitettua 1 W tehovahvistinta 10 dB alle sen maksimitehon. Työssä käsitellään myös muita tapoja linearisoida tehovahvistin, jotta backoff:n käyttö voidaan perustella. Jotta rakennetun taajuusmuuntimen suorituskykyä voitaisiin arvioida, diplomityön loppupuolella esitellään eri mittausmenetelmiä sekä VTT:n Otadigi-lähettimen signaalin voimakkuuden mittauksen tulokset. Mittaustuloksien avulla voidaan myös arvioida, millainen taajuusmuunnin on järkevä toteuttaa

Control of voltage source converters connected to variable impedance grids

The increase in new renewable energy resources is key to achieving carbon reduction targets, however it also introduces new grid integration challenges. The best renewable resource in Scotland is found in remote parts of the country, and as a result new renewable based generation is increasingly subjected to high and variable levels of impedance. Impedances that cause resonances are also increasingly common, given the higher order characteristics of impedance when transformers, filters, subsea cables, compensators and so on are present in the network. For a better understanding of impedance related stability issues, the estimation of the grid impedance using both Thévenin equivalent and wide spectrum techniques is studied in this thesis and integrated into the converter’s control. These estimations inform the controller of the grid conditions, allowing for controller adaptation. In instances where weak grid conditions are severe and the local grid impedance is dominant, a disturbance rejection mechanism called the pre-emptive voltage decoupler (PVD) is proposed. The PVD feeds forward the active current reference and measured voltage, and adapts the reactive current reference as a function of the impedance estimation, to pre-emptively compensate the local voltage for changes in active power transfer. This is justified through small signal analysis using linearised state space models and validated in the laboratory using large inductors and a converter. The control is also made more resilient with an instability detector, proposed to prevent instability when significant grid disturbances occur. Through early detection of sudden power angle changes, stability can be maintained. This is achieved by momentarily reducing the power reference and re-establishing grid parameters. The implementation of the proposed changes improves the steady state stability region from -0.75 – 0.55 pu to -0.85 – 0.75 pu. Further, the nonlinear transient performance is much more resilient, and uninterrupted power flow can be maintained. When the local grid is not dominant, and higher order grid impedances cause undesired resonances, a detection of the resonant frequency allows for an adaptation of the outer loop gains, thus damping the resonances and improving stability. Such grids are also prone to instability, but a reduction of the power reference does not improve stability, on the contrary the reduction of the power reference shifts eigenvalues into the right hand plane. A better preventative measure is to reduce the outer loop gains, and once the frequency of the problematic resonances is identified, final decisions on outer loop tuning can be taken. With this implementation, the stability of the system is maintained and the power output can be recovered within about 1 second.The increase in new renewable energy resources is key to achieving carbon reduction targets, however it also introduces new grid integration challenges. The best renewable resource in Scotland is found in remote parts of the country, and as a result new renewable based generation is increasingly subjected to high and variable levels of impedance. Impedances that cause resonances are also increasingly common, given the higher order characteristics of impedance when transformers, filters, subsea cables, compensators and so on are present in the network. For a better understanding of impedance related stability issues, the estimation of the grid impedance using both Thévenin equivalent and wide spectrum techniques is studied in this thesis and integrated into the converter’s control. These estimations inform the controller of the grid conditions, allowing for controller adaptation. In instances where weak grid conditions are severe and the local grid impedance is dominant, a disturbance rejection mechanism called the pre-emptive voltage decoupler (PVD) is proposed. The PVD feeds forward the active current reference and measured voltage, and adapts the reactive current reference as a function of the impedance estimation, to pre-emptively compensate the local voltage for changes in active power transfer. This is justified through small signal analysis using linearised state space models and validated in the laboratory using large inductors and a converter. The control is also made more resilient with an instability detector, proposed to prevent instability when significant grid disturbances occur. Through early detection of sudden power angle changes, stability can be maintained. This is achieved by momentarily reducing the power reference and re-establishing grid parameters. The implementation of the proposed changes improves the steady state stability region from -0.75 – 0.55 pu to -0.85 – 0.75 pu. Further, the nonlinear transient performance is much more resilient, and uninterrupted power flow can be maintained. When the local grid is not dominant, and higher order grid impedances cause undesired resonances, a detection of the resonant frequency allows for an adaptation of the outer loop gains, thus damping the resonances and improving stability. Such grids are also prone to instability, but a reduction of the power reference does not improve stability, on the contrary the reduction of the power reference shifts eigenvalues into the right hand plane. A better preventative measure is to reduce the outer loop gains, and once the frequency of the problematic resonances is identified, final decisions on outer loop tuning can be taken. With this implementation, the stability of the system is maintained and the power output can be recovered within about 1 second

Modulatory role of adenosine upon GABAergic transmission : consequences for excitability control

Tese de doutoramento, Ciências Biomédicas (Neurociências), Universidade de Lisboa, Faculdade de Medicina, 2015Glutamatergic principal cell excitability in the hippocampus is regulated by local circuit neurons that release the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). These GABAergic interneurons exhibit vast structural, physiological and biochemical diversity, innervating both excitatory principal cells and other inhibitory interneurons. In the hippocampus, two classes of interneurons, the cholecystokinin (CCK)- and parvalbumin (PV)-containing neurons, are the most significant and abundant cell type displaying unique and complementary functions in the control of principal cells output. Hence a tuned modulation of inhibitory circuits is of great importance in the control of network hippocampal function. Adenosine, acting through high affinity A1 receptor (A1R) and A2A receptor (A2AR), is a well-recognized endogenous modulator of glutamatergic principal cells excitability. Actions mediated by A1Rs are long-known to decrease hippocampal excitability with neuroprotective effects while actions through A2ARs are associated with increased neuronal excitability and excitotoxicity. However, the role of adenosine to modulate inhibitory transmission is much less known. This work aimed to evaluate and characterize the involvement of A1Rs (Chapter 5.1, p99) and A2ARs (Chapter 5.2, p143) on inhibitory neuronal communication in CA1 hippocampus and its impact on principal cells excitability and in the control of epileptiform discharges. These main goals were achieved by performing ex vivo electrophysiology recordings (field and patch-clamp recordings) from rat and mice hippocampus. Regarding A1R-actions, it was found that tonic - mediated by GABA receptor type A (GABAAR) localized peri- and extrasynaptically - but not phasic - mediated by GABAARs located at synapses - inhibitory transmission in pyramidal cells and CCKpositive interneurons were diminished after A1R activation. The effect was dependent on a signaling cascade involving both protein kinase A (PKA) and protein kinase C (PKC) and was accompanied by decreased GABAAR δ-subunit expression. On the other hand, it was also found that A2AR-mediated increase in pyramidal cells excitability results from a direct increase of glutamatergic transmission in parallel with disinhibition of principal cells by a mechanism that involves increased GABA release from PV-positive cells to other interneurons. Also, A2AR activation or blockage respectively promotes or reduces synchronous pyramidal cell firing in hyperexcitable conditions induced by elevated extracellular potassium or following high-frequency electrical stimulation. Together the results presented in this thesis show for the first time a direct involvement of adenosine receptors in the control of inhibitory network transmission in the hippocampus. This results open new promising perspectives for the involvement of adenosine in the control of physiological hippocampal operations and maladaptive conditions.A transmissão glutamatérgica no hipocampo é continuamente controlada por neurónios inibitórios, denominados interneurónios, que libertam o neurotransmissor ácido gama-aminobutírico (GABA). Estas células apresentam uma grande diversidade anatómica, fisiológica e bioquímica, estando descritos mais de vinte e um tipos diferentes de interneurónios no hipocampo. Estes são capazes de comunicar quer com células principais excitatórias (denominadas células piramidais), quer com outros interneurónios inibitórios, com resultados diferentes para a excitabilidade do sistema. A inibição de células piramidais leva a uma diminuição direta da sua excitabilidade; ao passo que a inibição de outros interneurónios pode resultar na desinibição das células principais e consequente aumento da excitabilidade. Desta grande variedade de interneurónios, destacam-se duas grandes classes que correspondem às duas populações de interneurónios mais importantes e abundantes no hipocampo – os neurónios que expressam colecistocinina (CCK) e os neurónios que expressam parvalbumina (PV). As funções de cada uma destas populações no hipocampo são únicas e complementares no controlo da atividade das redes neuronais. Desta forma, um controlo rigoroso destes circuitos inibitórios é de extrema importância na regulação das funções do hipocampo. A adenosina é um neuromodulador ubíquo do sistema nervoso central que atua através de dois grandes tipos de recetores de alta afinidade – os recetores A1 (A1R) e os recetores A2A (A2AR). Os primeiros têm ações principalmente inibitórias da excitabilidade neuronal, e portanto estão normalmente associados a funções neuroprotetoras, enquanto os segundos atuam no sentido de aumentar a excitabilidade no hipocampo e induzir excitotoxicidade. Enquanto que a função da adenosina no controlo da transmissão excitatória glutamatérgica tem vindo a ser caracterizada há várias décadas, o papel da adenosina na modulação da transmissão inibitória tem sido muito menos explorada. O trabalho apresentado nesta tese tem como objetivo a caracterização das ações dos A1Rs (Capítulo 5.1, p99) e dos A2ARs (Capítulo 5.2, p143) na comunicação neuronal inibitória no hipocampo bem como tentar perceber quais as consequências que uma possível modulação a este nível tem na excitabilidade das células piramidais e no desenvolvimento de atividade do tipo epiléptica. Para responder a estas questões foi planeado e executado um trabalho experimental que envolveu o registo da atividade elétrica neuronal no hipocampo de ratos e ratinhos através de técnicas eletrofisiológicas ex vivo (nomeadamente registos extracelulares e registos de patch-clamp). Relativamente às ações dos A1Rs, foi demonstrado que apenas um tipo de respostas inibitórias, denominadas por respostas tónicas, são afetadas pela ativação dos A1Rs, levando à sua diminuição. Este tipo de resposta tónica tem caraterísticas lentas e prolongadas no tempo e é mediada principalmente por recetores ionotrópicos do GABA do tipo A (GABAAR) que estão localizados em porções peri- e extrasináticas dos neurónios. Pelo contrário, as respostas habitualmente rápidas e concertadas no tempo, denominadas por respostas fásicas, e que são mediadas por recetores localizados nas sinapses, não parecem ser afetadas pela ativação dos A1Rs. Curiosamente, estas ações ocorrem seletivamente em neurónios excitatórios piramidais e numa subpopulação de interneurónios que expressam o neuropéptido CCK. O efeito dos A1Rs na diminuição das respostas tónicas está associado a uma cascata de sinalização intracelular que envolve as proteínas cinase A (PKA) e C (PKC) e é acompanhado pela diminuição da expressão de GABAARs que contêm a subunidade δ, habitualmente implicada nas respostas tónicas. Neste trabalho foi também demonstrado que a adenosina, através dos A2ARs, também influencia a transmissão inibitória no hipocampo. De facto, os efeitos da ativação dos A2ARs levam a um aumento da excitabilidade das células piramidais, que pode ser explicado pela ação destes recetores em dois locais: (1) a ativação dos A2ARs aumentam diretamente as respostas glutamatérgicas sobre as células piramidais; (2) simultaneamente, os A2ARs vão desinibir as células principais através de um mecanismo que envolve o aumento da libertação de GABA dos terminais sinápticos de neurónios que expressam PV e que contactam com outros neurónios inibitórios. Estas ações moduladoras têm implicações importantes em modelos de hiperexcitabilidade neuronal induzida pelo aumento das concentrações extracelulares de potássio, na medida em que a ativação ou inibição dos A2ARs leva a um exacerbação ou diminuição, respetivamente, desta hiperatividade neuronal sincronizada. No seu conjunto, os resultados apresentados nesta tese revelam, pela primeira vez, o envolvimento dos recetores de adenosina na modulação da transmissão neuronal inibitória no hipocampo. Estes resultados poderão abrir novas e promissoras perspetivas relativamente ao envolvimento da adenosina no controlo das funções do hipocampo em condições fisiológicas e patológicas.Network of European Neuroscience Schools; Medical Research Counci

Short-term forecast techniques for energy management systems in microgrid applications

A Dissertation Submitted in Partial Fulfilment of the Requirements for the Degree of Doctor of Philosophy in Sustainable Energy Science and Engineering of the Nelson Mandela African Institution of Science and TechnologyIn the 2015 Paris Agreement, 195 countries adopted a global climate agreement to limit the global average temperature rise to less than 2°C. Achieving the set targets involves increasing energy efficiency and embracing cleaner energy solutions. Although advances in computing and Internet of Things (IoT) technologies have been made, there is limited scientific research work in this arena that tackles the challenges of implementing low-cost IoT-based Energy Management System (EMS) with energy forecast and user engagement for adoption by a layman both in off-grid or microgrid tied to a weak grid. This study proposes an EMS approach for short-term forecast and monitoring for hybrid microgrids in emerging countries. This is done by addressing typical submodules of EMS namely: load forecast, blackout forecast, and energy monitoring module. A short-term load forecast model framework consisting of a hybrid feature selection and prediction model was developed. Prediction error performance evaluation of the developed model was done by varying input predictors and using the principal subset features to perform supervised training of 20 different conventional prediction models and their hybrid variants. The proposed principal k-features subset union approach registered low error performance values than standard feature selection methods when it was used with the ‘linear Support Vector Machine (SVM)’ prediction model for load forecast. The hybrid regression model formed from a fusion of the best 2 models (‘linearSVM’ and ‘cubicSVM’) showed improved prediction performance than the individual regression models with a reduction in Mean Absolute Error (MAE) by 5.4%. In the case of the EMS blackout prediction aspect, a hybrid Adaptive Similar Day (ASD) and Random Forest (RF) model for short-term power outage prediction was proposed that predicted accurately almost half of the blackouts (49.16%), thereby performing slightly better than the stand-alone RF (32.23%), and ASD (46.57%) models. Additionally, a low-cost EMS smart meter was developed to realize the implemented energy forecast and offer user engagement through monitoring and control of the microgrid towards the goal of increasing energy efficiency

Bidirectional Multilevel Converter for Grid-Tie Renewable Energy and Storage with Reduced Leakage Current

This thesis discusses a transformerless multilevel converter (MLC) applied to a domestic level renewable energy system consisting of PV panels and EV batteries in their 2nd life applications. MLCs enable the use of conventional switching devices due to reduced voltage stress. Being able to produce a multilevel output voltage waveform, MLCs require less filtering and therefore may produce better quality waveform when compared to a standard 2-level voltage source converter (VSC). In this study, various modulation techniques for MLCs are implemented and the performance of the converter analysed regarding regulations and standards. The system is designed to have two-stage power conversion, including a DC-DC boost converter for adjusting each stage battery voltage, and maximum power point operation of the PV panels in each module. This provides a stable input voltage for the DC-AC converter stage. The cascaded H-bridge converter (CHB) is selected for the DC-AC conversion due to its isolated DC source requirement. This topology enables the separation of the total DC link voltage into different modules, increasing the accessibility of EV batteries in their 2nd life application. The base system is designed to be coupled without a transformer to the single-phase UK utility grid. A systematic approach is adapted for examining the MLC system. The design procedure starts with system parameter definition and component selection. This is then validated using simulation analysis and hardware implementation to demonstrate the practicability of the system for the planned application. The control algorithm is implemented in a National Instruments (NI) CompactRIO FPGA that can transform graphical programming into VHDL code. To accelerate the implementation and optimisation process, a co-simulation environment is used between NI LabVIEW and NI Multisim software. This ensures the optimisation of control code before compilation and enables testing without having analogue circuitry. Converters without galvanic isolation may exhibit ground leakage currents when coupled with grounded PV panels. This thesis analyses the common-mode and differential-mode voltages that CHB modules generate, and their effect on ground leakage current. The mathematical analysis suggests that leakage current may be supressed solely on changing the modulation method in a CHB converter. A novel leakage reduction pulse width modulation (LRPWM) technique is proposed, which successfully diminishes the ground leakage current to within the limit allowed by VDE-0126-1-1 (withdrawn, accessed in 2018) or IEC 62109-2 standard. The experimental results show that LRPWM has superior performance when compared to conventional MLC modulation technique

Battery-sourced switched-inductor multiple-output CMOS power-supply systems

Wireless microsystems add intelligence to larger systems by sensing, processing and transmitting information which can ultimately save energy and resources. Each function has their own power profile and supply level to maximize performance and save energy since they are powered by a small battery. Also, due to its small size, the battery has limited energy and therefore the power-supply system cannot consume much power. Switched-inductor converters are efficient across wide operating conditions but one fundamental challenge is integration because miniaturized dc-dc converters cannot afford to accommodate more than one off-chip power inductor. The objective of this research is to explore, develop, analyze, prototype, test, and evaluate how one switched inductor can derive power from a small battery to supply, regulate, and respond to several independent outputs reliably and accurately. Managing and stabilizing the feedback loops that supply several outputs at different voltages under diverse and dynamic loading conditions with one CMOS chip and one inductor is also challenging. Plus, since a single inductor cannot supply all outputs at once, steady-state ripples and load dumps produce cross-regulation effects that are difficult to manage and suppress. Additionally, as the battery depletes the power-supply system must be able to regulate both buck and boost voltages. The presented system can efficiently generate buck and boost voltages with the fastest response time while having a low silicon area consumption per output in a low-cost technology which can reduce the overall size and cost of the system.Ph.D

Spacecraft system study: A study to define the impact of laser communication systems on their host spacecraft

The mutual influence of a laser communication system and its host spacecraft and the degree to which the mutual influence limited acquisition, tracking and pointing processes were investigated. A laser klink between a low earth orbiting (LEO) satellite and a geosynchronous earth orbiting (GEO) satellite was used as a baseline. The laser link between satellites was a generic channel transferring 500 Mbps data from the LEO to GEO using the GaAlAs laser as the laser light source. Major aspects of pointing and tracking with a satelliteborne optical system were evaluated including: (1) orbital aspects such as spacecraft relative motions, point ahead, and Sun snd Moon optical noise; (2) burst errors introduced by the electronic and optical noise levels; (3) servo system design and configurations, and the noise sources such as, sensor noise, base motion disturbances, gimbal friction torque noise; (4) an evaluation of the tracking and beacon link and the type of sensors used; (5) the function of the acquisition procedure and an evaluation of the sensors employed; and (6) an estimate of the size, weight and power needed for the satellite system

The Albatross voyage : a study on the effect of the internet on expedition communication

Since the advent of the Internet expeditioners travelling alone or in small groups to remote locations have been using email and websites to communicate with sponsors, supporters, friends and family. This brings new communication opportunities to expeditioners and at the same time may be changing the expedition experience. The author used the experience of participating in an 11-month sailing voyage around the world as a platform for this research by project. The purpose of the voyage was to raise awareness of the plight of the albatross. The author examined how the Internet shaped this expedition, the purpose of expedition communication and how meaning was constructed for a remote audience. Using a constructionist epistemology, a phenomenological approach is used in the development and analysis of a narrative account of the voyage, its preparation and aftermath. The Exegesis contains a detailed account of one Leg of the Voyage, from Cape Town to Melbourne, with a particular focus on the online communications. A full account of the voyage, again with a particular emphasis on communications, is contained in the Durable Record. The potential availability of Internet communication was found to have been intrinsic to the design of the expedition and enabled the Voyage to forgo traditional media sponsorship and yet still attract and engage a worldwide audience through an institutional community website. The lack of use of the Internet’s capability to support online discussion and build communities on this Voyage and other expedition websites sampled, is explored and it is proposed to be an expression of these expeditioners possible disinterest in feedback. The building of online communities around an expedition website is suggested to be an area of opportunity with the potential to engage site visitors, to enable new insights to be gained into the life of expeditioners, and to provide greater exposure for sponsors. A simple classification of expeditions by their leadership, organisation, purpose and membership is proposed and the differing purposes of communication in each considered. The use of email on the voyage was found to facilitate and improve contact with supporters, family and friends. Whilst adding a sense of security it was also found to be time consuming, stressful, power hungry and to build a sense of obligation to communicate. Its general reliability led to exchanges on non-critical matters and to a reduction in the feeling of remoteness that some expeditioners may be seeking. Whilst on this voyage technical and budgetary constraints limited the full use of the Internet’s capability to utilise text, images and sound to construct meaning, the effective and timely use of a short daily narrative, aimed at a known audience is demonstrated. The author justifies drawing learning from his participation in the Voyage through the use of experiential learning and reflective learning theory arguing that the preparation for expeditions, participation in them and mediated reflection following them, are rich learning environments provided that good records are kept, particularly a frequently written personal journal