This thesis attempts to investigate the performance of various reactive power compensation devices, examine the mechanism of reactive power compensation for HVDC systems, and develop guidelines for the design of reactive power compensation schemes for HVDC systems. The capabilities of various reactive power compensators to enhance power system stability are compared in both steady and transient states. An understanding of the capabilities of these compensators provides a basis for further investigation of their performance in HVDC systems. The reactive power requirements of HVDC converters are studied. The voltage dependencies of the HVDC converters at different control modes are derived, which allow for predictions of how HVDC converters impact AC system voltage stability. The transient performance of reactive power compensation options for HVDC Systems is studied by comparing their behavior during DC fault recovery, Temporary Overvoltage (TOV), and commutation failure. How to quantify the system strength when reactive compensators are connected to the converter bus is investigated. A new series of indices are developed based on the Apparent Short Circuit Ratio Increase (ASCRI). The inertia of a synchronous condenser and its impact on the frequency stability of an AC/DC system are discussed. By modelling the inverter side AC system in greater detail, the frequency stability and rotor angle stability following fault transients is examined based on time domain simulation. Finally, a guideline for designing dynamic reactive power compensation for HVDC systems is proposed.

This thesis attempts to investigate the performance of various reactive power compensation devices, examine the mechanism of reactive power compensation for HVDC systems, and develop guidelines for the design of reactive power compensation schemes for HVDC systems. The capabilities of various reactive power compensators to enhance power system stability are compared in both steady and transient states. An understanding of the capabilities of these compensators provides a basis for further investigation of their performance in HVDC systems. The reactive power requirements of HVDC converters are studied. The voltage dependencies of the HVDC converters at different control modes are derived, which allow for predictions of how HVDC converters impact AC system voltage stability. The transient performance of reactive power compensation options for HVDC Systems is studied by comparing their behavior during DC fault recovery, Temporary Overvoltage (TOV), and commutation failure. How to quantify the system strength when reactive compensators are connected to the converter bus is investigated. A new series of indices are developed based on the Apparent Short Circuit Ratio Increase (ASCRI). The inertia of a synchronous condenser and its impact on the frequency stability of an AC/DC system are discussed. By modelling the inverter side AC system in greater detail, the frequency stability and rotor angle stability following fault transients is examined based on time domain simulation. Finally, a guideline for designing dynamic reactive power compensation for HVDC systems is proposed.

[pt] Este trabalho apresenta um modelo de previsão de curto prazo para cargas reativas horárias. Tal modelo é uma extensão do método desenvolvido por P. C. Gupta no qual utiliza-se um procedimento de previsão que combina idéias e procedimentos de duas abordagens: o método de amortecimento exponencial e o modelo de Box e Jenkins. Até os dias atuais os estudos nesta área foram elaborados para potência ativa da carga. Como praticamente não existem na literatura especializada métodos de previsão para a carga reativa, este trabalho pretende iniciar a modelagem destes dados, o qual é de grande importância para o novo modelo do Setor Elétrico Brasileiro. Desta forma o desenvolvimento desta ferramenta tem a intenção de aprimorar a avaliação do comportamento desta carga, contribuindo para o melhor desempenho operacional do sistema elétrico. Com a evolução das técnicas, este trabalho utiliza também métodos inteligentes para fazer a previsão de carga reativa. Sendo assim foram utilizadas Redes Neurais Artificiais, a título de comparação dos resultados com o modelo proposto. / [en] This dissertation presents a reactive load forecast model for power systems. The model is an extension of P. C. Gupta s method, including ideas and procedures from exponential smoothing and Box and Jenkins models. A wide variety of studies has been developed in the last years on load forecast model for active power only. On the other hand, reactive load forecast works haven t been commonly implemented. The introduction of this new concept intends to fulfill this gap of reactive load forecast studies due to their importance to the Brazilian Power System. An improved load behavior evaluation and a better electric system operation could arise with the adoption and development of this tool. Comparisons with as neural network models were also done.

Voltage stability is the ability of a power system to maintain acceptable voltages at all buses under normal and abnormal operating conditions. The scarcity of reactive power or reactive power imbalance is the main reason of voltage instabilities. The energy supply of Germany will be dominated by renewable energy sources (RES) within energy transition actions leading to a decrease in conventional power plants especially in the transmission grid. Since the conventional power plants are still main reactive power sources, key challenge in the future will be the provision of system services such as control of reactive power to maintain voltage stability in the transmission networks. A wide range of technology that is able to provide the required dynamic reactive power compensation is already available. However, in order to find the most effective implementation, it is necessary to investigate and compare these different technologies for a voltage stable grid operation. The main challenge regarding comparative studies that incorporate reactive power compensation devices is the development and the implementation of reliable comparison strategies. This thesis analyses the impact of reactive power on the voltage stability phenomena both in long-term and short-term time frames under various grid situations. Voltage stability margins in the long-term time frame are quantified by gradually increasing the reactive power injection into certain buses until the voltage collapses. Voltage stability in the short-term time frame is assessed by applying grid faults and simulating the time-domain grid response. A transient voltage severity ratio (TVSR) is developed in this thesis as the main comparison metric to evaluate and compare the performance of investigated compensation devices. Additionally, TVSR and other developed comparison metrics in this thesis are used to determine the optimal settings of the controller parameters of compensation devices. As a result, the submitted thesis shows a practical and reliable approach to analyze the performance of different compensation devices under different grid situations.

No description available.

<p>Until just a few years ago wind farms where excluded from many of the requirementsthat can be found in grid codes. But as the numbers of wind farms have grown as wellas the sizes of them, the requirements to connect them to the grid have becomemore stringent. In this thesis it has been investigated if it’s possible to design a controlsystem that controls the reactive power from an offshore wind farm, so that the gridcode requirements regarding reactive power are fulfilled. By controlling the reactivepower the dynamic variations in the voltage are decreased. The regulator should alsobe able to help the wind farm to handle the fault conditions that are described in thegrid code.An offshore wind farm outside the coast of Western Europe is now being planned.Data from this wind farm has been used in this thesis. The wind farm has a total of 54wind turbines with more than 300 MW all together. The reactive power is controlledvia the generators and also by disconnecting and connecting four shunt reactors.A model of the wind farm has been built in Simpow, as well as design of the regulator.Simpow is a simulation program developed by ABB that enables simulations of powersystems. Different scenarios have been simulated to see if it is possible to control thewind farm in the desired way. The results show that the wind farm manages to handlefault conditions as described in the grid, and it is also possible to control the reactivepower in a desirable way.</p>

Until just a few years ago wind farms where excluded from many of the requirementsthat can be found in grid codes. But as the numbers of wind farms have grown as wellas the sizes of them, the requirements to connect them to the grid have becomemore stringent. In this thesis it has been investigated if it’s possible to design a controlsystem that controls the reactive power from an offshore wind farm, so that the gridcode requirements regarding reactive power are fulfilled. By controlling the reactivepower the dynamic variations in the voltage are decreased. The regulator should alsobe able to help the wind farm to handle the fault conditions that are described in thegrid code.An offshore wind farm outside the coast of Western Europe is now being planned.Data from this wind farm has been used in this thesis. The wind farm has a total of 54wind turbines with more than 300 MW all together. The reactive power is controlledvia the generators and also by disconnecting and connecting four shunt reactors.A model of the wind farm has been built in Simpow, as well as design of the regulator.Simpow is a simulation program developed by ABB that enables simulations of powersystems. Different scenarios have been simulated to see if it is possible to control thewind farm in the desired way. The results show that the wind farm manages to handlefault conditions as described in the grid, and it is also possible to control the reactivepower in a desirable way.

Reactive power, denoted as volt-ampere reactive (VARs), is fundamental to ac power systems and is due to the complex impedance of the loads and transmission lines. It has several undesirable consequences which include increased transmission loss, reduction of power transfer capability, and the potential for the onset of system-wide voltage instability, if not properly compensated and controlled. Reactive power compensation is a technique used to manage and control reactive power in the ac network by supplying or consuming VARs from points near the loads or along the transmission lines. Load compensation is aimed at applying power factor correction techniques directly at the loads by locally supplying VARs. Typical loads such as motors and other inductive devices operate with lagging power factor and consume VARs; compensation techniques have traditionally employed capacitor banks to supply the required VARs. However, capacitors are known to have reliability problems with both catastrophic failure modes and wear-out mechanisms. Thus, they require constant monitoring and periodic replacement, which greatly increases the cost of traditional load compensation techniques. This thesis proposes a reactive power load compensator that uses inductors (chokes) instead of capacitors to supply reactive power to support the load. Chokes are regarded as robust and rugged elements; but, they operate with lagging power factor and thus consume VARs instead of generating VARs like capacitors. A matrix converter interfaces the chokes to the ac network. The matrix converter is controlled using the Venturini modulation method which can enable the converter to exhibit a current phase reversal property. So, although the inductors draw lagging currents from the output of the converter, the converter actually draws leading currents from the ac network. Thus, with the proposed compensation technique, lagging power factor loads can be compensated without using capacitor banks. The detailed operation of the matrix converter and the Venturini modulation method are examined in the thesis. The application of the converter to the proposed load compensation technique is analyzed. Simulations of the system in the MATLAB and PSIM environments are presented that support the analysis. A digital implementation of control signals for the converter is developed which demonstrates the practical feasibility of the proposed technique. The simulation and hardware results have shown the proposed compensator to be a promising and effective solution to the reliability issues of capacitor-based load-side VAR compensation techniques.

Η παρούσα διπλωματική πραγματεύεται τη διόρθωση του συντελεστή ισχύος με διάταξη αντιστάθμισης που περιλαμβάνει πυκνωτές οι οποίοι τροφοδοτούνται από ζεύγος αντιπαράλληλων θυρίστορ. Η διάταξη αυτή τοποθετείται παράλληλα με το φορτίο και στόχος είναι να επιτευχθεί πλήρης αντιστάθμιση της άεργου ισχύος παλμοδοτώντας κατάλληλα τα ελεγχόμενα ημιαγωγικά στοιχεία. Τα ημιαγωγικά στοιχεία που χρησιμοποιήθηκαν, δηλαδή τα θυρίστορ, έχουν υπολογιστεί και επιλεγεί με βάση τις προδιαγραφές της διάταξης που έπρεπε να κατασκευαστεί. Η εργασία αυτή εκπονήθηκε στο Εργαστήριο Ηλεκτρομηχανικής Μετατροπής Ενέργειας του Τμήματος Ηλεκτρολόγων Μηχανικών και Τεχνολογίας Υπολογιστών της Πολυτεχνικής Σχολής του Πανεπιστημίου Πατρών. Σκοπός είναι η μελέτη της λειτουργικής συμπεριφοράς διατάξεων που χρησιμοποιούνται για τη μείωση της άεργου ισχύος που παράγεται από τη λειτουργία εξοπλισμού μεσαίων και μεγάλων καταναλωτών. Η άεργος ισχύς προκύπτει είτε από τη διαφορά φάσεως της βασικής αρμονικής του ρεύματος με την τάση του δικτύου, που οφείλεται στη φύση του φορτίου, είτε από την ύπαρξη ανώτερων αρμονικών όταν έχουμε μη ημιτονοειδείς κυματομορφές τάσης και ρεύματος, δηλαδή στην περίπτωση που έχουμε διακοπτικά στοιχεία. Αποτέλεσμα της δημιουργίας άεργου ισχύος είναι η επιβάρυνση του δικτύου, λόγω επιπρόσθετων απωλειών και κατ’ επέκταση η οικονομική επιβάρυνση του καταναλωτή. Αρχικά έγινε διερεύνηση της λειτουργικής συμπεριφοράς του κυκλώματος και αποφασίσθηκε να προσομοιωθεί χρησιμοποιώντας το λογισμικό προσομοίωσης του Matlab, το Simulink. Η προσομοίωση γενικά παρέχει τη δυνατότητα εξοικονόμησης χρόνου, εφόσον είναι συνήθως ευκολότερη η μελέτη της επίδρασης μιας παραμέτρου στη συμπεριφορά του συστήματος σε σύγκριση με την πραγματοποίηση της ίδιας μελέτης στο εργαστήριο. Ως πρώτο βήμα της διαδικασίας προσομοίωσης μελετήθηκε η αξιοπιστία των μετρητικών διατάξεων που περιλαμβάνονται στη βιβλιοθήκη του λογισμικού. Διαπιστώθηκε ότι ενώ για ημιτονοειδή μεγέθη τα αποτελέσματα είναι ακριβή, για μη ημιτονοειδή μεγέθη είναι ανακριβή. Για το λόγο αυτό αναπτύχθηκαν μετρητικές διατάξεις ενεργού, άεργου και φαινόμενης ισχύος στο περιβάλλον του Matlab, Simulink. Στη συνέχεια έγινε προσομοίωση της διάταξης αντιστάθμισης της άεργου ισχύος με πυκνωτές όπου αναδεικνύεται η ανάγκη χρήσης πυκνωτών διαφορετικής τιμής, καθώς το φορτίο μεταβάλλεται. Κατόπιν έγινε διερεύνηση της λειτουργίας με πυκνωτές ελεγχόμενους από διακόπτες, στη συγκεκριμένη περίπτωση θυρίστορ, μέσω του προγράμματος προσομοίωσης ηλεκτρικών κυκλωμάτων Simulink του Matlab. Ερευνήθηκε η λειτουργία της διάταξης με ωμικό και ωμικό επαγωγικό φορτίο, και με διαφορετικές γωνίες έναυσης. Τα τέλος έγινε προσομοίωση της υπό μελέτη διάταξης και ο καθορισμός και η δοκιμή της φιλοσοφίας ελέγχου . Για την επιβεβαίωση των αποτελεσμάτων της προσομοίωσης κρίθηκε απαραίτητη η κατασκευή πειραματικής διάταξης. Για το λόγο αυτό σχεδιάστηκαν και υλοποιήθηκαν: α) το κύκλωμα ισχύος και όλα τα απαραίτητα περιφερειακά για τη λειτουργία του 10 β) το κύκλωμα παλμοδότησης γ) η πλακέτα του μικροεπεξεργαστή Ιδιαίτερη έμφαση δόθηκε στο κύκλωμα ανίχνευσης του μηδενός, καθώς η τάση πάνω στα θυρίστορ είναι μη ημιτονοειδές μέγεθος. Το επόμενο βήμα ήταν η κατασκευή των προγραμμάτων παλμοδότησης για το μικροεπεξεργαστή. Ο μικροεπεξεργαστής που χρησιμοποιήθηκε είναι ο dsPIc30F4011 της Microchip. Αρχικά κατασκευάστηκε ένα απλό πρόγραμμα παλμοδότησης για τον έλεγχο της σωστής λειτουργίας της διάταξης και κατόπιν το πρόγραμμα που χρησιμοποιήσαμε για τη βέλτιστη δυνατή αντιστάθμιση. Τέλος έγιναν οι μετρήσεις στο εργαστήριο έχοντας προς αντιστάθμιση ένα RL φορτίο στο οποίο μεταβάλλαμε την αντίσταση και ως εκ τούτου τις τιμές της παρεχόμενης ισχύος. / -

To mitigate high voltages in transmission systems with low demands, traditional solutions often consider the installation of reactive power compensators. The deployment and tuning of numbers of VAr compensators at various locations may not be cost-effective. This thesis presents an alternative method that utilises existing parallel transformers in distribution networks to provide reactive power supports for transmission systems under low demands. The operation of parallel transformers in small different tap positions, i.e. with staggered taps, can provide a means of absorbing reactive power. The aggregated reactive power absorption from many pairs of parallel transformers could be sufficient to provide voltage support to the upstream transmission network. Network capability studies have been carried out to investigate the reactive power absorption capability through the use of tap stagger. The studies are based on a real UK High Voltage distribution network, and the tap staggering technique has been applied to primary substation transformers. The results confirm that the tap staggering method has the potential to increase the reactive power demand drawn from the transmission grid. This thesis also presents an optimal control method for tap stagger to minimise the introduced network loss as well as the number of tap switching operations involved. A genetic algorithm (GA) based procedure has been developed to solve the optimisation problem. The GA method has been compared with two alternative solution approaches, i.e. the rule-based control scheme and the branch-and-bound algorithm. The results indicate that the GA method is superior to the other two approaches. The economic and technical impacts of the tap staggering technique on the transmission system has been studied. In the economic analysis, the associated costs of applying the tap staggering method have been investigated from the perspective of transmission system operator. The IEEE Reliability Test System has been used to carry out the studies, and the results have been compared with the installation of shunt reactors. In the technical studies, the dynamic impacts of tap staggering or reactor switching on transmission system voltages have been analysed. From the results, the tap staggering technique has more economic advantages than reactors and can reduce voltage damping as well as overshoots during the transient states.