With increasing level of distributed generation¡]DG¡^on radial feeders in electric distribution systems, it could cause over-voltages as well as under-voltages depending on several factors including DG capacity, locations, and the strategy of voltage regulation. This thesis describes the typical and proposed voltage control strategies that could allow the increase of DG interconnection capacity. By using probabilistic load flow technique, voltage regulation performance for cases with different levels of DG outputs, demands and voltage control strategies are presented. They are compared by using a voltage profile improvement index and a risk assessment technique.

This thesis presents a new power circuit configuration to be used in shunt active power filters. A new control algorithm based on the linear voltage control suitable for the proposed circuit is introduced. The system is analysed both in time and frequency domains. The practical implementation of the system proves its suitability for the proposed task. The switching frequency of the proposed circuit is much lower than that in other active filters. The switching losses are then considerably reduced, in addition to the fact that the switching devices can withstand larger values of currents being switched on and off at lower frequencies which is an advantage to this circuit. The component sizes (capacitors and inductors) in the proposed circuit are also much smaller than those in other filter configurations. In addition, the thesis presents a new method for categorising the active filter systems proposed in the surveyed literature. The survey includes a comparison of these techniques showing their respective merits and drawbacks. The thesis also includes an implementation of a reference current generator that is suitable for single-phase applications without the need for excessive computations. The technique involves a modified Fourier analysis, which is suitable for active filtering applications.

Σήμερα στα συστήματα ηλεκτρικής ενέργειας μεγάλο ποσοστό της ισχύος παράγεται από αιολικά πάρκα αλλά και από θερμικούς σταθμούς. Η διείσδυση κατανεμημένης παραγωγής στα συστήματα χρειάζεται ιδιαίτερη μελέτη. Σε αυτή την εργασία ερευνήθηκε η λειτουργία ολόκληρων συστημάτων διανομής που περιείχαν διάφορα αιολικά πάρκα και θερμικούς σταθμούς. Αρχικά εξετάστηκε το θέμα του εντοπισμού των απωλειών ισχύος σε συστήματα διανομής με κατανεμημένη παραγωγή. Παρουσιάστηκαν μια νέα σχετικά προσέγγιση στον εντοπισμό των απωλειών με βάση τις τροφοδοσίες ρευμάτων καθώς και μία μήτρα εμπεδήσεων. Τα παραπάνω μπορούν να χρησιμοποιηθούν για τη στατιστική ανάλυση των απωλειών βάσει της γραμμικής παλινδρόμησης ή την εκτίμηση της συνδιακύμανσης μεταξύ φορτίου και παραγωγής. Στη συνέχεια περιγράφεται το πρόβλημα της βραχυπρόθεσμης ευστάθειας τάσης σε δίκτυα με υψηλή διείσδυση κατανεμημένης παραγωγής. Εστιάζουμε στην αναπαράσταση του δικτύου κατά τη διάρκεια και μετά από την εμφάνιση σφάλματος ως ισοδύναμο κύκλωμα Thevenin τάσης και εμπέδησης. Επίσης διερευνάται η επίδραση γειτονικών σύγχρονων γεννητριών, ανεμογεννητριών τύπου Δανίας, SVC, και STATCOM στις παραμέτρους Thevenin. Τελικά εξετάζεται το πρόβλημα του ελέγχου τάσης και άεργου ισχύος. Ιδιαίτερη προσοχή δίνεται στους περιορισμούς τροφοδοσίας πραγματικής και άεργου ισχύος οι οποίες επιβάλλονται από τα όρια του πλάτους της τάσης. / Nowadays in power systems a large amount of power produced from wind farms and Heat Power Plants (HPP). High penetration of distributed generation in power systems is a subject to study. Τhe focus of this project is on the operation of an entire distribution system with several wind farms and HPP. Firstly, the subject of allocation of power system losses in a distribution system with distributed generation is treated. A new approach to loss allocation based on current injections and an impedance matrix is presented. The formulation can be used for statistical analysis of the losses based on linear regression or estimates of covariances between production and load. Secondly, the problem of short term voltage stability in networks with high penetration of DG is assessed. The focus is on the representation of the network during and after a fault as a Thevenin equivalent voltage and impedance. The influence of adjacent synchronous generators, Danish concept wind turbines, SVCs and STATCOMs on the Thevenin parameters have been investigated. Finally, the problem of voltage and reactive power control is investigated. Special focus is on the constraints for active and reactive power injection which are imposed by the voltage magnitude limits.

The increased penetration of residential-scale photovoltaic (PV) systems in European-style low voltage (LV) networks (i.e., long feeders with high number of connected customers) is leading to technical issues such as voltage rise and thermal overload of the most expensive network assets (i.e., transformer, cables). As these issues significantly limit the ability of LV networks to accommodate higher PV penetrations, Distribution Network Operators (DNOs) are required to proceed with expensive and time-consuming investments in order to reinforce or replace these assets. In contrast to this traditional approach of network reinforcement, which potentially leads to massive capital expenditure, the transition towards active LV networks where controllable elements, existing (i.e., PV systems) and likely to be adopted (i.e., battery energy storage systems, LV on-load tap changer transformers), can be managed in real-time, poses an attractive alternative. Although several active network management schemes have been recently proposed to increase the hosting capacity of PV-rich LV networks, they are mostly based on managing voltage issues only; and, in general, aim to solve technical issues separately. Integrated solutions aiming at managing simultaneously voltage and thermal issues are required, as recent studies demonstrate that both issues can coexist in PV-rich LV networks. More importantly the majority of studies, which commonly neglect the characteristics of real LV networks (e.g., unbalanced, three-phase, radial, multiple feeders with several branches, different types of customers), use complex optimisation techniques that require expensive communication infrastructure and extensive or full network observability (currently not available in LV networks). However, considering the extensiveness of LV networks around the world, practical, cost-effective and scalable solutions that use limited and already available information are more likely to be adopted by the industry. Considering the above gaps in the literature, this Thesis contributes by proposing innovative and scalable active network management schemes that use limited network monitoring and communication infrastructure to actively manage (1) Residential-scale PV systems, (2) Residential-scale Battery Energy Storage (BES) systems and (3) LV on-load tap changer (OLTC)-fitted transformers. The adoption of the proposed active network management schemes, which makes use of already available devices, information and requires limited monitoring (i.e., secondary distribution substation), allows making the transition towards active LV networks more practical and cost-effective. In addition, to tackle the challenges related to this research (i.e., lack of realistic LV network modelling with high resolution time-series analyses), this Thesis, being part of the industrial project 'Active Management of LV Networks' (funded by EDF R&D) and having access to French data, contributes by considering a fully modelled typical real residential French LV network (three-phase four-wire) with different characteristics and number of customers. Moreover, realistic (1-min resolution) daily time-series household (from real smart meter data) and PV generation profiles are considered while a stochastic approach (i.e., Monte Carlo) is adopted to cater for the uncertainties related to household demand as well as PV generation and location.

[pt] O problema de estabilidade de tensão está intimamente ligado à falta de recursos e questões ambientais que limitam a expansão do sistema de transmissão. Esta realidade em conjunto com o crescimento da carga submete os sistemas elétricos a carregamentos pesados, podendo levar a situações de colapso de tensão. O desenvolvimento de métodos para avaliação do carregamento da rede de transmissão tornou-se necessário e imprescindível para que se possa entender o funcionamento do sistema nestas condições e possibilitar a sua operação de modo correto. Em relatório de força-tarefa internacional está explicitada a necessidade de uma ferramenta computacional capaz de verificar a eficácia das ações de controle de tensão na operação do sistema elétrico em tempo-real. Com esse objetivo, foi desenvolvida uma ferramenta computacional com base no sistema linearizado das equações de fluxo de carga, e de todas as outras equações de controles e limites, julgadas pertinentes, para avaliação do efeito das ações de controle de tensão. Determinase uma matriz de sensibilidade [VCS], voltage control sensitivity matrix, através da qual se pode estabelecer a relação existente entre as tensões controladas e as grandezas controladoras. Os elementos diagonais relacionam a grandeza controladora de cada equipamento com a respectiva tensão controlada. A análise do sinal desses elementos estabelece se uma determinada ação de controle será adequada ou não, isto é, se terá efeito esperado ou oposto. Os elementos fora da diagonal representam a interdependência existente entre os equipamentos controladores de tensão. A matriz de sensibilidade também permite que ações de controle de tensão sejam calculadas com o objetivo de mover um ponto de operação inadequado, ou que esteja suscetível à inadequacidade do controle de tensão, colocando-os em um ponto de operação seguro, onde ações operativas de controle de tensão tenham o efeito esperado. Os resultados da avaliação do método desenvolvido e aplicado a sistemas-teste e reais foram qualitativamente coerentes com aqueles obtidos pela resolução (analítica, iterativa por Newton ou iterativa continuada) das equações de fluxo de carga, validando a proposta. A dimensão da matriz [VCS] é proporcional ao número de barras com tensão controlada em análise. Esta matriz não é esparsa e então, quando utilizada para representar o controle de tensão realizado por um número elevado de equipamentos controladores, sua análise direta pode se tornar complexa. Portanto, uma análise baseada nos autovalores e autovetores associados à matriz [VCS] é aplicada objetivando identificar barras de tensão controlada com efeito reverso, ocorrendo de modo contrário ao esperado, e a existência de controles conflitantes. Buscam-se autovalores negativos, nulos ou próximos de zero. A localização das barras com problemas se dá através do exame dos autovetores e fatores de participação associados a esses autovalores. O cálculo e a análise da matriz de sensibilidade dos controles de tensão são executados de forma extremamente rápida e, portanto, o esforço computacional não impede que a ferramenta desenvolvida possa ser usada durante a operação em tempo-real. / [en] The voltage stability problem has been associated with environment questions and lack of financial resources for transmission system expansion. This reality together with the load growth makes the electrical system to be subject of heavy loadings, what can lead to situations of voltage collapse. The development of methods for evaluation of transmission network loading became necessary and indispensable in order to correct operation of the system under heavy loading conditions. In an international force-task report it is stated the need for analytical tools capable of verifying the voltage control action adequacy in the real-time operation. This work reports the development of a computation tool able to evaluate the effect of voltage control actions on the voltage itself. It is based the linearized load flow equations, including control and limit equations judged pertinent. The tool establishes a sensitive matrix, named [VCS] for voltage control sensitivity matrix, that relates the controlling variables and the controlled voltages. Through the analysis of the sign of each diagonal element, it is possible to know whether a specific control action is adequate or not. Moreover, the off-diagonal elements represent the interdependence among the voltage controller devices of the power system under analysis. The sensitivity matrix also allows the calculation of control actions necessary to move the operating point from an unstable area, or near by, to a secure operating region where all voltage control actions would have the expected effect. The results obtained with the voltage control sensitivity matrix applied on real and test-systems was qualitatively coherent with those calculated by analytical, iteractive by Newton-Raphson and/or continuationiteractive solution of power flow equations, validating the proposed method. The [VCS] dimension is proportional to the number of buses with controlled voltage in the system area under analysis. The matrix is not sparse and when there are a large number of voltage control equipments, its immediate analysis may be complex. Therefore, an analysis based on eigenvalues and eigenvectors associated with [VCS] matrix is used in order to identify voltage control action with opposite effect. In this case, the analysis is focused on negative eigenvalues. The identification of conflicting controls existence of any system area under analysis is a by-product. The voltage control sensitivity matrix computation and analysis are performed extremely fast. Therefore, the computational effort does not constrain its use in real-time operation.

[pt] Após a incidência de alguns colapsos de tensão em sistemas de transmissão de energia a nível mundial, a segurança de tensão tornou-se um assunto de muito interesse nos últimos anos devido à importância do seu impacto. O fenômeno de estabilidade de tensão deve-se a fluxos de potência ativa e reativa excessivos na rede de transmissão e está associado às restrições ambientais e econômicas que impedem a expansão da rede. Atualmente, sabese da existência de uma máxima carga que pode ser alimentada pela rede e é a manifestação mais conhecida do fenômeno mas, também, é possível que o problema manifeste-se pela existência de uma máxima injeção de potência ativa e reativa na rede por geradores e compensadores síncronos. E mais, em situações de carregamento elevado da rede, é possível que ações de controle de tensão tenham efeito oposto ao usual. É apresentado um método seqüencial iterativo de avaliação e reforço para as condições de carregamento da rede em barras de tensão controlada, embora na literatura somente as barras de carga são analisadas. A verificação do comportamento do gerador e compensador síncrono como dispositivo de controle torna-se necessária já que, se funcionar de forma inversa, poderá levar o sistema ao colapso por problemas de tensão. Uma vez que a avaliação do carregamento da rede de transmissão detectou uma barra de geração crítica em um determinado ponto de operação, o reforço consiste do cálculo de ações de controle para aumentar a distância ou margem de potência entre a geração daquela barra e o novo máximo permitido. Muitas vezes isso pode ser conseguido através da alteração do perfil de tensão com a conseqüente redução nas perdas. Muitas outras vezes, o redespacho de potência ativa torna-se necessário. As etapas do método são: identificar a barra crítica, identificar a sub-rede utilizada para transmitir potência ativa dessa barra para as cargas, nessa sub-rede determinar o caminho e ramo mais carregados e, desviar o fluxo de potência do ramo mais carregado para outros. A seqüência é repetida até que as novas margens de potência sejam consideradas aceitáveis. Exemplos numéricos ilustrativos reais com o sistema brasileiro são apresentados. É verificado que o método proposto realmente produz os resultados desejados. / [en] After the incidence of some voltage collapses in the energy transmission systems in the world, the voltage security became an issue of great interest in the last years due to the importance of its impact. The phenomenon of voltage stability is due to the excessive active and reactive power flow in the electrical transmission network and has been associated with environment questions and lack of financial resources for transmission system expansion. Nowadays, it is well-known that there is a maximum power that the network can transmit to a load bus and is the best known manifestation of the phenomenon, but, is not familiar to many that there is a maximum power that can be injected by generators and synchronous compensators into the network. Moreover, in heavy loading conditions is possible that voltage control actions would have the inverse effect. It is shown a sequential iterative method for assessment and voltage security reinforcement in voltage-controlled buses, although the literature only the load buses are analyzed. The verification of the behaviour of the generator and synchronous compensator as control device becomes necessary since, if it works in an inverse way, it can take the system to the voltage collapse. Once the assessment is performed and is detected one generation critical bus in some operating point, the objective of the reinforcement function is to calculate adequate control actions in order to increase the distance or power margin between the actual generation and the new maximum power flow. Several times this may be achieved by voltage profile changes and consequent loss reduction. Sometimes that procedure is not enough and active generation rescheduling is recommended. The stages of the method are: identify the critical bus, identify the sub-network used to transmit active power flow from this bus to load buses, in this sub-network the critical transmission path and critical branch are determined and redirect the power flow from the branch more loaded to others. The sequence is repeated until resultant power margins are judged suitable. Illustrative real life numerical examples with the Brazilian system are provided. It is verified that the proposed method really produces the desired results.

[pt] A falta de recursos e a questão ecológica tem limitado a expansão do sistema de transmissão. Esta realidade, em conjunto com o crescimento da carga, faz com que os sistemas elétricos trabalhem bastante carregados. Esta nova condição pode levar a situações de colapso de tensão. O desenvolvimento de métodos para a avaliação do carregamento da rede de transmissão tornou-se necessário e imprescindível para que se possa entender o funcionamento do sistema nestas condições e possibilitar a sua operação de modo correto. O carregamento da rede de transmissão pode ser avaliado por condições nodais associadas ao máximo fluxo de potência ativa e reativa que pode ser transmitida dos geradores para as cargas. Estas condições nodais são avaliadas por uma ferramenta analítica com base em modelo matemático, simples mas poderoso, de uma direta interpretação física do fenômeno. Índices abrangentes e significativos são obtidos a partir da matriz Jacobiano do sistema. Eles indicam a região de operação na curva V x P, Q a margem em MVA para o máximo carregamento, a importância relativa entre as barras, e uma medida de dificuldade de transmissão. O controle de tensão influi diretamente nas condições de operação e portanto, deve ser incluído na modelagem do problema. Usualmente são: controle de tensão por ajuste de tap de LTCs e controle de tensão (local e remota) por injeção de potência reativa. O controle é restrito pelos limites dos taps de transformadores, limites de injeção de potência reativa e limites de tensão. Neste trabalho é mostrada a forma como se deve incluir os controles e limites relacionados com a tensão, na ferramenta analítica de avaliação do carregamento da rede de transmissão. O índice calculado apropriadamente é nulo no ponto de máxima transferência de potência. É mostrado que os controles e limites de tensão do sistema podem e devem ser representados na matriz Jacobiano do sistema linearizado de equações que modelam o sistema, exceto os da barra em análise, mesmo se esta tiver sua tensão controlada. / [en] The lack of investiments and the ecological matter have limited the expansion of the transmission system. This reality, together with the growth of the load, makes the electric systems to work heavy loaded. This new condition can lead to situations of voltage collapse. The development of methods for evaluation the loading of the transmission network became necessary and indispensable in understanding the performance of the system under these conditions and in making possible the operating in a correct way. The transmission system loading can be evaluated by nodal conditions associated to the maximum active and reactive power flow that can be transmitted from generators to loads. These nodal conditions are assessed by an analytic tool based on a a mathematical model. Simple but powerful, derived directly from a physical interpretation of the phenomenon. Meaningful and comprehensive indexes are obtained from a partition of the jacobian matrix of the system. They indicate the operation area in the V x P, Q curve, the margin in MVA for the maximu loading, the relative importance among the busbars, and a measure of transmission difficulty. The voltage control is of paramount importance on operating consitions and therefore should be included in the problem modeling. Usually they are voltage control by tap adjstment of LTCs and voltage control (local and remote) by injection of reactive power. The control is constrained by limits on the taps of transformers, on reactive power injection and on voltage magnitudes. In this work it is shown how the controls and limits related with the voltage should be include in the analytical tool for the evaluation of the transmission system loading. The index calculated adequately is null in the point of maximum power transfer, positive in the upper half and negative in the lower half of the V x P, Q curve. It is shown that the voltage controls and limits of the system can and should be represented in the Jacobian matrix of the linearised system of equations, except on bus under analysis, even if it is a voltage controlled bus.

The increasing penetration of distributed energy resources, in particular Photovoltaic (PV) production units, and the ever-growing use of power electronics-based equipment has led to specific concern about Power Quality (PQ) in the Low-Voltage (LV) grid. These include high- and low-order current harmonics as well as voltage distortion at the point of common coupling. Solutions to overcome these issues, meeting international grid codes, are being proposed in the context of smart energy management schemes.This work proposes a novel three-phase topology for a PV system with enhanced PQ mitigation functionality, tackling the corresponding control challenges.First, a single-stage current-source inverter PV system with active filtering capability is preferred to the more common two-stage voltage-source inverter topology with additional voltage-step-up converter. The system also guarantees a nearly unitary displacement power factor in the connection to the grid and allows for Maximum Power Point Tracking (MPPT) with direct control of the PV array power. The grid-synchronised dq-axis grid current references are generated for the mitigation of nonlinear load low-order current harmonics, without the need for additional measurements. Active damping is used to minimise grid-side filter losses and reduce high-order harmonics resulting from the converter switching.Results on a 500W laboratory prototype confirm that active damping reduces the switching harmonics in the grid currents and active filtering properly mitigates the low-order current harmonics. The MPPT algorithm works effectively for various irradiance variations. Second, a PV system with a novel Indirect Matrix Converter (IMC)-based unified power quality conditioner topology is developed for enhanced current and voltage compensation capability, with compactness and reliability advantages. PQ issues such as current harmonics, and voltage sags, swells, undervoltage and overvoltage are mitigated by the shunt and series converters, respectively.The more common Space Vector Modulation (SVM) method used in IMCs is developed for this specific topology. In particular, a new shunt converter modulation method is proposed to additionally control the PV array current with zero switching vectors, resulting in a specific switching sequence.A direct sliding mode control method is also studied separately for the shunt and series converters, so that the zero-vector modulation method of the shunt converter can be used, with no sensitive synchronisation of the switching signals; this contrasts with the SVM method. A new dc link voltage modulation method with 12 voltage zones, instead of 6, is proposed to help overcome the limitation in the choice of shunt converter switching vectors due to the positive dc link voltage constraint.Results are obtained for the direct method on a 1 kW laboratory prototype with optimised IMC dc link connection and alternative shunt converter switching transitions to guarantee a positive dc link voltage. Current and voltage compensation capabilities are confirmed by tests in various operating conditions. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

[pt] Casos reais de blecaute, caracterizados por afundamento de tensão, indicam que os procedimentos normais para o controle automático de tensão podem agravar o nível de tensão. Isto ocorre porque, em situações especiais, as ações de controle têm o efeito oposto ao esperado da sua lógica de projeto. Essas situações especiais podem ser identificadas através de ferramenta computacional que, baseada no sistema linearizado das equações de fluxo de carga e de todas as outras equações de controle consideradas pertinentes, determina uma matriz de sensibilidade dos controles através da qual se pode estabelecer a relação existente entre as tensões controladas e as grandezas controladoras dos dispositivos de controle de tensão. O controle de tensão em geradores síncronos é realizado por reguladores automáticos de tensão (RAT) que, na prática, não têm lógica de controle para diferenciar relação direta e inversa entre a tensão controlada e a tensão de excitação do gerador. Assim, se a relação é inversa, o resultado da ação do RAT é inadequado (oposto ao desejado) e pode levar o sistema ao colapso por baixa tensão, por exemplo. Visto isso, esse trabalho apresenta um método que, através de mudanças adequadas nas grandezas dos geradores determinadas pela análise da matriz de sensibilidade dos controles, permite alcançar um novo perfil de tensão partindo de pontos de operação com problema de adequação das ações de controle por geradores. / [en] Actual recent blackouts which were charactherized by voltage sags suggest that normal process for the voltage control can aggravate the voltage level. It happens because, in special situations, the automatic voltage control has the opposite effect of its logic of conception. In order to identify situations like those, we developed a computational tool to evaluate the effect of voltage control based on the linearized system of power flow equations and selected control equations. The tool calculates the voltage control sensitivity matrix which relates the controlling variables and the controlled voltages of voltage control equipments. The voltage control on synchronous generators is performed by automatic voltage regulators (AVR) that, in practice, have no control logic to differentiate direct and inverse relation between the controlling variable and the excitation voltage of generator. Therefore, if the relation is inverse, the result of the action of the AVR is inadequate (opposite to desired) and may lead the system to collapse due to low voltage, for example. Hence, this paper presents a process to achieve the new voltage profile, starting from operating points with adequacy problems of voltage control actions, through appropriate changes in variables of generators determined by analysis in voltage control sensitivity matrix.

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