210 research outputs found

    Functional modelling and prototyping of electronic integrated kinetic energy harvesters

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    The aim of developing infinite-life autonomous wireless electronics, powered by the energy of the surrounding environment, drives the research efforts in the field of Energy Harvesting. Electromagnetic and piezoelectric techniques are deemed to be the most attractive technologies for vibrational devices. In the thesis, both these technologies are investigated taking into account the entire energy conversion chain. In the context of the collaboration with the STMicroelectronics, the project of a self-powered Bluetooth step counter embedded in a training shoe has been carried out. A cylindrical device 27 × 16mm including the transducer, the interface circuit, the step-counter electronics and the protective shell, has been developed. Environmental energy extraction occurs exploiting the vibration of a permanent magnet in response to the impact of the shoe on the ground. A self-powered electrical interface performs maximum power transfer through optimal resistive load emulation and load decoupling. The device provides 360 μJ to the load, the 90% of the maximum recoverable energy. The energy requirement is four time less than the provided and the effectiveness of the proposed device is demonstrated also considering the foot-steps variability and the performance spread due to prototypes manufacturing. In the context of the collaboration with the G2Elab of Grenoble and STMicroelectronics, the project of a piezoelectric energy arvester has been carried out. With the aim of exploiting environmental vibrations, an uni-morph piezoelectric cantilever beam 60×25×0.5mm with a proof mass at the free-end has been designed. Numerical results show that electrical interfaces based on SECE and sSSHI techniques allows increasing performance up to the 125% and the 115% of that in case of STD interface. Due to the better performance in terms of harvested power and in terms of electric load decoupling, a self-powered SECE interface has been prototyped. In response to 2 m/s2 56,2 Hz sinusoidal input, experimental power recovery of 0.56mW is achieved demonstrating that the device is compliant with standard low-power electronics requirements

    Dynamic Stability of a Microgrid With an Active Load

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    Control and Stability of Residential Microgrid with Grid-Forming Prosumers

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    The rise of the prosumers (producers-consumers), residential customers equipped with behind-the-meter distributed energy resources (DER), such as battery storage and rooftop solar PV, offers an opportunity to use prosumer-owned DER innovatively. The thesis rests on the premise that prosumers equipped with grid-forming inverters can not only provide inertia to improve the frequency performance of the bulk grid but also support islanded operation of residential microgrids (low-voltage distribution feeder operated in an islanded mode), which can improve distribution grids’ resilience and reliability without purposely designing low-voltage (LV) distribution feeders as microgrids. Today, grid-following control is predominantly used to control prosumer DER, by which the prosumers behave as controlled current sources. These grid-following prosumers deliver active and reactive power by staying synchronized with the existing grid. However, they cannot operate if disconnected from the main grid due to the lack of voltage reference. This gives rise to the increasing interest in the use of grid-forming power converters, by which the prosumers behave as voltage sources. Grid-forming converters regulate their output voltage according to the reference of their own and exhibit load sharing with other prosumers even in islanded operation. Making use of grid-forming prosumers opens up opportunities to improve distribution grids’ resilience and enhance the genuine inertia of highly renewable-penetrated power systems. Firstly, electricity networks in many regional communities are prone to frequent power outages. Instead of purposely designing the community as a microgrid with dedicated grid-forming equipment, the LV feeder can be turned into a residential microgrid with multiple paralleled grid-forming prosumers. In this case, the LV feeder can operate in both grid-connected and islanded modes. Secondly, gridforming prosumers in the residential microgrid behave as voltage sources that respond naturally to the varying loads in the system. This is much like synchronous machines extracting kinetic energy from rotating masses. “Genuine” system inertia is thus enhanced, which is fundamentally different from the “emulated” inertia by fast frequency response (FFR) from grid-following converters. Against this backdrop, this thesis mainly focuses on two aspects. The first is the small-signal stability of such residential microgrids. In particular, the impact of the increasing number of grid-forming prosumers is studied based on the linearised model. The impact of the various dynamic response of primary sources is also investigated. The second is the control of the grid-forming prosumers aiming to provide sufficient inertia for the system. The control is focused on both the inverters and the DC-stage converters. Specifically, the thesis proposes an advanced controller for the DC-stage converters based on active disturbance rejection control (ADRC), which observes and rejects the “total disturbance” of the system, thereby enhancing the inertial response provided by prosumer DER. In addition, to make better use of the energy from prosumer-owned DER, an adaptive droop controller based on a piecewise power function is proposed, which ensures that residential ESS provide little power in the steady state while supplying sufficient power to cater for the demand variation during the transient state. Proposed strategies are verified by time-domain simulations

    A Doppler Lidar system with preview control for wind turbine load mitigation

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    This dissertation focuses on the development of a system for wind turbine in order to mitigate the load from unstable wind speed. The work is divided into 2 main parts: a cost efficient Doppler wind Lidar system is developed based on a short coherence length laser system in combine with multiple length delayline concept; a preview pitch control is developed based on the design of a combination of 2 degree of freedom (2-DOF) feedback / feedforward control with a model predictive control

    Advanced Inverter control for mixed source microgrids

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    This thesis focuses on investigating virtual oscillator control (VOC) and applying it to mixed source microgrids to address several stability issues. A detailed comparison between VOC and droop control in a three-phase system is presented in terms of transient responses of a single inverter under small load disturbances and the synchronization speed in multiple paralleled inverters under various inverter terminal voltage amplitude and frequency regulation settings. In the single-inverter microgrid, it is demonstrated in both simulation and experiment that the two control models produce similar transient responses in the output voltage and current amplitudes. However, VOC has a faster instantaneous frequency transient response whilst still maintaining the terminal voltage amplitude transient response of the droop controller. In microgrids with multiple inverters, the synchronization speed of the VOC is faster than that of the droop control when the terminal voltage’s frequency regulation range is allowed to be wide. The conclusion is verified with different types of loads. A virtual inertia design method for the VOC inverter with a mixed source microgrid is presented to improve the frequency stability issues of the system. The per unit inertia constant of a VOC inverter is derived when coupled with a synchronous generator in an islanded microgrid. The control parameters of the virtual inertia are designed via small-signal analysis. A dual second order generalized integrator - frequency locked loop (DSOGI-FLL) is adopted for digital implementation of proposed virtual inertia based VOC. With the use of virtual inertia block, the frequency nadir is improved by 22% and rate of change of frequency is improved by 29% compared with the unmodified VOC inverter during the transient period induced by load disturbances. Simulation and experimental results verify the enhanced transient response of system frequency. A voltage and current dual-loop control structure is added to the VOC inverter to solve the voltage drop issues at the inverter terminals caused by the inverter dead-time effects, non-ideal semiconductor and LCL filter. A complete small-signal model for a multiple-inverters microgrid with the proposed control structure is presented in order to assess system stability using eigenvalue and participation factor analysis. Analytical results show that the parameter related to the frequency regulation and the integral gain of the voltage controller affect the location of the system’s dominant modes significantly. The stability margin is determined by modifying these control parameters. Experimental results on a laboratory test microgrid verify the predication from the small-signal analysis and time-domain simulations. Finally, a method to limit current in the VOC inverter under large disturbances in a mixed source microgrid is proposed. During a large load change in the islanded microgrid, the inverter based sources may get temporarily overloaded until other generations with sufficient power margin take the remaining load burden. The original VOC inverter lacks the ability to constrain the current within limits during the transient period. The dual-loop structure proposed in this thesis can limit the transient current with the use of virtual impedance. Such virtual impedance is presented by the desired maximum current magnitude and virtual voltage drop. Compared with a recently proposed fault ride through VOC inverter, the proposed virtual impedance based current limitation method can effectively constrain the inverter current within the pre-set value under large disturbances, which augments the range of application of VOC and enhances its robustness

    Multiterminal HVDC transmissions systems for offshore wind

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    Offshore wind is emerging as one of the future energy vectors. Offshore wind power plants locations provide more strong and constant wind speed that allows to extract more power compared to onshore locations. In addition, as wind turbine components transportation is less restricted to terrestrial infrastructure, bigger and more powerful wind turbines can be installed offshore. In Europe, 1,567 MW of offshore wind power was installed in 2013. It represents the 14\% of the total wind power installed in Europe. Offshore wind power plants near the shore can be connected to the main grid by means of conventional AC technology. However, if these wind farms are installed further than 80-100 km, the use of AC equipment is economically infeasible due to reactive power issues. In these applications, HVDC system based on static converters can be used. The projects build and commissioned nowadays are based on point-to-point connections, where, each wind farm or wind farm clusters are connected to the terrestrial grid individually. Consequently, these lines might be understood as an extension of the AC system. If different offshore wind farms are interconnected between them and connected at the same time to different AC systems, for example, different countries, the DC grid is created. This scenario creates one of the most important challenges in the electrical power system since its creation, more than 100 years ago. The most relevant challenges to be addressed are the stability and operation of the DC grid and the integration and interaction with the AC grid. This thesis addresses various aspects related to the future Multiterminal-HVDC systems for transmission of offshore wind power. First, the voltage control and the system operations are discussed and verified by means of emulations using an HVDC scaled experimental platform built for this purpose. Voltage stability might be endangered during contingencies due to the different inertia time constant of the AC and the DC system. DC systems only have the inertia of the capacitors compared to synchronous machines rotating masses of the AC systems. Therefore, in faulty conditions the power transmitted through the DC system must be reduced quickly and efficiently. For this reason, in this thesis a coordinated power reduction algorithm taking advantage of Dynamic Braking Resistors (DBR) connected to onshore converter stations and the ability of the power plants to reduce the generated power is presented. From the AC and DC grids integration point of view, the connection point between the offshore grid and the AC grid might be located remotely leading to a connection with a reduced Short Circuit Ratio (SCR). In the literature, several issues regarding the connection of transistor-based power converters to weak AC grid have been reported. In this thesis am advanced control for Voltage Source Converters connected to weak grids is presented and tested by means of simulations. From the AC and DC grids interactions, the voltage stability is not enough to operate a DC grid. Transport System Operators (TSO) operates the power flow through the cables and the power exchanged between by the power converters. In this thesis, a novel hierarchical power flow control method is presented. The aim of the proposed power flow control is to obtain the desired power flows changing the voltage control set-points while the system stability is ensured. Finally, a control procedure for offshore wind farms based on Squirrel Cage Induction Generators connected to a single power converter is introduced.L'energia eòlica marina emergeix com un dels vectors energètics del futur. Les localitzacions eòliques marines proporcionen vens més forts i constants que les terrestres, cosa que permet extreure més potència. A més a més, els aerogeneradors marins poden ser més grans i més potents ja que es redueixen les limitacions de gàlib existent en les infraestructures terrestres. A tall d'exemple, l'any 2013 a Europa es van instal.lar 1.567 MW de potència eòlica marina, cosa que representa un 14\% de la potència eòlica instal.lada a Europa. Els parcs eòlics marins poden ser connectats a la xarxa elèctrica terrestre utilitzant emparamenta convencional de corrent alterna, però quan la distancia amb la costa excedeix els 80-100 km l'ús d'aquesta tecnologia es torna econòmicament inviable degut a l'energia reactiva generada en els conductors. Per solucionar aquest problema, s'emparen els sistemes en corrent continua basats en convertidors estàtics. Els projectes construïts o projectats a dia d'avui es basen en esquemes de connexió punt-a-punt, on, cada parc eòlic o agrupació de parcs eòlics es troba connectat a la xara terrestre individualment. En conseqüència, l'operació d'aquestes línies es pot considerar com una extensió de la xarxa d'alterna. Però, si s'interconnecten diferents parc eòlics amb diferents xarxes terrestres d'alterna (per exemple, diferents països) en corrent continua, s'obtenen xarxes en corrent continua. Aquest nou escenari crea un dels majors reptes des de la creació dels sistema elèctric de potencia, ara fa més de 100 anys. Entre aquests reptes hi ha l'estabilitat i l'operació dels sistemes en corrent contínua i la seva integració i coexistència amb les xarxes en corrent alterna. En la present tesis s'han estudiat diferents aspectes dels futurs sistemes multi terminal en alta tensió en corrent contínua (HVDC, en anglès) per la transmissió de potencia generada mitjançant parcs eòlics marins. Primerament, es descriu el control de tensió i els modes d'operació dels sistemes multi terminal i es verifiquen en una plataforma experimental construïda per aquest propòsit. L'estabilitat de tensió dels sistemes en corrent continua, es pot veure afectada durant una falta a la xarxa d'alterna degut a la reduïda inèrcia dels sistemes multi terminal, només formada pels condensadors dels convertidors i els cables. Així la potència que no pot injectar a la xarxa ha de ser reduïda de forma ràpida i eficient. Per això, en aquesta tesis es presenta un sistema coordinat de reducció de potència que utilitza la resistència de frenat dels convertidors de connexió a la xarxa i els mètodes de reducció de potència dels parcs eòlics. Des del punt de vista de la integració de les xarxes en continua i en alterna, el punt d'interconnexió pot estar localitzat llunys dels grans centres de generació, la qual cosa implica tenir una potència de curtcircuit molt reduïda. En la bibliografia científica s'han descrit diverses problemàtiques a l'hora de connectar un convertidor de commutació forçada a les xarxes dèbils. Per tal de pal.liar aquests inconvenients, en aquesta tesis es presenta un algorisme avançat de connexió de convertidors a xarxes dèbils basat en control vectorial. Des del punt de vista de les interaccions i interoperabilitat dels sistemes en corrent alterna i continua, no n'hi ha suficient en garantir l'estabilitat, ja que el propòsit finals dels operadors de xarxa és fer fluir una potencia a traves de la xarxa per tal de satisfer la demanda. Per aquest propòsit en aquesta tesis es presenta un control jeràrquic de control del flux de potència que fixa el flux de potència a traves d'una xarxa multi terminal canviant les consignes del control primari, tot assegurant l'estabilitat del sistema. Per tancar la tesis, es presenta un nou controlador per parcs eòlics basats en aerogeneradors de gàbia d'esquirol controlats per un sol convertidor

    Deep Controlled Source Electromagnetics for Mineral Exploration: A Multidimensional Validation Study in Time and Frequency Domain

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    The focus of this thesis is the derivation of an independent multidimensional resistivity model utilising land based controlled source electromagnetics (CSEM) with resolution to conductive structures down to 1 km depth. Data is evaluated in both, time and frequency domain. Since the resistivity distribution is strongly multidimensional, besides conventional 1D inversion methods, 2D inversion techniques are applied to the dataset. The objective of the BMBF funded DESMEX (Deep Electromagnetic Sounding for Mineral Exploration) project is the development of an electromagnetic exploration system which can be used for the detection and assessment of deep mineral resources. In order to obtain a high data coverage as well as a high spatial and depth resolution, airborne and ground based methods are combined in a semi-airborne concept. In the framework of the DESMEX project, the University of Cologne conducted large scale ground based long offset transient ­electromagnetic (LOTEM) measurements along an 8.5 km long transect in a former mining area in eastern Thuringia, Germany. Within the LOTEM validation study, an independent multidimensional resistivity model of the survey area was derived, which serves as a reference model for the semi-airborne concept developed within DESMEX and is eventually integrated into a final mineral deposition model. Utilising in total 6 transmitters in broadside configuration, data at 170 electric field stations were recorded during two large scale LOTEM surveys. In addition, a full component magnetic field dataset was acquired with SQUID sensors using a dense station spacing along the transect. For a preliminary evaluation, conventional 1D techniques are applied to the dataset. The individual switch on transients of the electric field can be explained by a 1D approach, the obtained models however indicate a strong multidimensional subsurface with rather large variations in resistivity. For further interpretation, the LOTEM data is analysed in frequency domain. Obtained 1D and 2D inversion models of the electric field component in frequency domain are in a good agreement with the time domain results. Subsequently, a joint multidimensional inversion of the full dataset in frequency domain was carried out, including electric and magnetic field data. Derived 2D inversion models are discussed in terms of sensitivities and resolution capabilities. Shallow high conductive structures are well comparable to inversion results from other conducted reconnaissance surveys and the semi-airborne CSEM model. The dominant conductivity structures can be linked to the occurrence of Silurian graptolite shales

    Enhancement of power system stability using wide area measurement system based damping controller

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    Contemporary power networks are gradually expanding incorporating new sources of electrical energy and power electronic based devices. The major stability issue in large interconnected power systems is the lightly damped interarea oscillations. In the light of growth of their incidents there are increased concerns about the effectiveness of current control devices and control systems in maintaining power system stability. This thesis presents a Wide Area Measurement System (WAMS) based control scheme to enhance power system stability. The control scheme has a hierarchical (two-level) structure comprising a Supplementary Wide-Area Controller (SWAC) built on top of existing Power System Stabilisers (PSSs). The SWAC's focus is on stabilising the critical interarea oscillations in the system while leaving local modes to be controlled entirely by local PSSs. Both control systems in the two levels work together to maintain system stability. The scheme relies on synchronised measurements supplied by Phasor Measurement Units (PMUs) through the WAMS and the only cost requirement is for the communication infrastructure which is already available, or it will be in the near future. A novel linear quadratic Gaussian (LQG) control design approach which targets the interarea modes directly is introduced in this thesis. Its features are demonstrated through a comparison with the conventional method commonly used in power system damping applications. The modal LQG approach offers simplicity and flexibility when targeting multiple interarea modes without affecting local modes and local controllers, thus making it highly suitable to hierarchical WAMS based control schemes. Applicability of the approach to large power systems is demonstrated using different scenarios of model order reduction. The design approach incorporates time delays experienced in the transmission of the SWAC's input/output signals. Issues regarding values of time delays and required level of detail in modelling time delays are thoroughly discussed. Three methods for selection of input/output signals for WAMS based damping controllers are presented and reviewed. The first method uses modal observability/controllability factors. The second method is based on the Sequential Orthogonalisation (SO) algorithm, a tool for the optimal placement of measurement devices. Its application is extended and generalised in this thesis to handle the problem of input/output signal selection. The third method combines clustering techniques and modal factor analysis. The clustering method uses advanced Principal Component Analysis (PCA) where its draw backs and limitations, in the context of power system dynamics' applications, are overcome. The methods for signal selection are compared using both small signal and transient stability analysis to determine the best optimal set of signals. Enhancement of power system stability is demonstrated by applying the proposed WAMS based control scheme on the New England test system. The multi-input multi-output (MIMO) WAMS based damping controller uses a reduced set of input/output signals and is designed using the modal LQG approach. Effectiveness of the control scheme is comprehensively assessed using both small signal and transient stability analysis for different case studies including small and large disturbances, changes in network topology and operating condition, variations in time delays, and failure of communication links.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

    Enhancement of power system stability using wide area measurement system based damping controller

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    Contemporary power networks are gradually expanding incorporating new sources of electrical energy and power electronic based devices. The major stability issue in large interconnected power systems is the lightly damped interarea oscillations. In the light of growth of their incidents there are increased concerns about the effectiveness of current control devices and control systems in maintaining power system stability. This thesis presents a Wide Area Measurement System (WAMS) based control scheme to enhance power system stability. The control scheme has a hierarchical (two-level) structure comprising a Supplementary Wide-Area Controller (SWAC) built on top of existing Power System Stabilisers (PSSs). The SWAC's focus is on stabilising the critical interarea oscillations in the system while leaving local modes to be controlled entirely by local PSSs. Both control systems in the two levels work together to maintain system stability. The scheme relies on synchronised measurements supplied by Phasor Measurement Units (PMUs) through the WAMS and the only cost requirement is for the communication infrastructure which is already available, or it will be in the near future. A novel linear quadratic Gaussian (LQG) control design approach which targets the interarea modes directly is introduced in this thesis. Its features are demonstrated through a comparison with the conventional method commonly used in power system damping applications. The modal LQG approach offers simplicity and flexibility when targeting multiple interarea modes without affecting local modes and local controllers, thus making it highly suitable to hierarchical WAMS based control schemes. Applicability of the approach to large power systems is demonstrated using different scenarios of model order reduction. The design approach incorporates time delays experienced in the transmission of the SWAC's input/output signals. Issues regarding values of time delays and required level of detail in modelling time delays are thoroughly discussed. Three methods for selection of input/output signals for WAMS based damping controllers are presented and reviewed. The first method uses modal observability/controllability factors. The second method is based on the Sequential Orthogonalisation (SO) algorithm, a tool for the optimal placement of measurement devices. Its application is extended and generalised in this thesis to handle the problem of input/output signal selection. The third method combines clustering techniques and modal factor analysis. The clustering method uses advanced Principal Component Analysis (PCA) where its draw backs and limitations, in the context of power system dynamics' applications, are overcome. The methods for signal selection are compared using both small signal and transient stability analysis to determine the best optimal set of signals. Enhancement of power system stability is demonstrated by applying the proposed WAMS based control scheme on the New England test system. The multi-input multi-output (MIMO) WAMS based damping controller uses a reduced set of input/output signals and is designed using the modal LQG approach. Effectiveness of the control scheme is comprehensively assessed using both small signal and transient stability analysis for different case studies including small and large disturbances, changes in network topology and operating condition, variations in time delays, and failure of communication links.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
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