This thesis proposes a novel consolidated approach for substantial compaction of HVDC lines that includes both new tower geometries as well as novel control concepts. This is based on a thorough discussion on the basic overhead line design parameters and their impact on the right of way width and tower height. Then the electrical aspects of the new approach such as dc overvoltage assessment and lightning performance are investigated. The required horizontal clearances between pole conductors and tower members, as a component of the right of way width, depend on the maximum expected overvoltages. Detailed electromagnetic transient models for the point to point MMC HVDC with different transmission configurations, all including the proposed dc overhead line, are developed for this thesis. The models are used to assess fault contingencies that result in the most significant overvoltage stresses on the HVDC transmission line for finding minimum air clearances and for the design of overvoltage limiting devices, such as surge arresters. New control approaches are proposed that significantly reduce the dc side overvoltage and consequently minimize the required air clearances for maximum compaction of the HVDC overhead lines and also reduce the required surge arrester size for line insulation. Because power transmission lines are the most exposed component within a power system, they are subject to lightning strikes which, in turn, are the main cause of disruption to power flows. This thesis will include an analysis of lightning occurrence on the proposed compact transmission line in order to assess the risk of pole faults. The focus of this analysis is mainly on evaluation of the critical lightning currents that cause fast front overvoltage stresses that may result in insulation failure. / May 2017

Offshore wind power is attracting increasing levels of research and investment. The use of HVDC transmission and the development of DC grids are topics with similar high levels of interest that go hand in hand with the development of large scale, far from shore wind farms. Despite increased capital cost of some components, DC power transmission can have significant advantages over AC transmission, in particular in the offshore environment. These advantages are well established for large scale, long distance point to point transmission. This thesis assesses the suitability of a multi-terminal DC power collection network, with short cables and relatively small amounts of power, addresses a number of the technical challenges in realising such a network and shows methods for overall system cost reduction. Technical and modelling challenges result from the interaction between power electronic DC-DC converters and the cables in a DC transmission network. In particular, the propagation of the ripple current in bipole DC transmission cables constructed with a metallic sheath and armour is examined in detail. The finite element method is used to predict the response of the cable to the ripple current produced by the converters. These results are used along with wave propagation theory to demonstrate that cable design plays a crucial role in the behaviour of the DC system. The frequency dependent cable models are then integrated with time domain DC-DC converter models. The work in the thesis is, broadly, in two parts. First, it is demonstrated that care and accuracy are required in modelling the cables in the DC transmission system and appropriate models are implemented and validated. Second, these models are combined with DC-DC converter models and used to demonstrate the practicality of the DC grid, make design recommendations and assess its suitability when compared with alternative approaches (e.g. AC collection and/or transmission).

During the three decades, the torsional impact on turbine-generator sets due to power system disturbances has been extensively discussed in many research works. However, most of them are focused on the fatigue damage of turbine shafts due to large-signal disturbances. For example, network faults occur. Obviously, the torsional effect subject to small-signal disturbances has not received much attention. In fact, although the small disturbances would not immediately damage the turbine mechanism, the cumulative long-term damaging effects may not be negligible under certain circumstances. Many operating conditions in power systems may lead to small disturbances on blades; for examples, shedding loads, switching transmission line, resetting control system parameters, and harmonics etc. Nevertheless, others only cause short-term or transient non-resonant disturbances occasionally except the power system subharmonics which could results in electro-mechanical resonance. Therefore, two types of subharmonics in power systems are proposed so as to investigate the toque impact and long-term fatigue life expenditure in turbine shafts and blades. Firstly, from the steady-state disturbance viewpoint, the long-term cumulative fatigue estimation based on the three-year project of the GE Co. shows that there are potential damages for both the shafts and the blades of the nearby generators caused by the subharmonic excitations of the HVDC link. The fatigue life sensitivity works are also carried out to provide the recommendations for the safety operation. The optimal damper type and disposition scheme for depressing the resonant torque and prolonging the turbine lifetime is consequently motivated, which is based on participation factor of linear systems with the electromechanical analogy. The effectiveness of this scheme on suppressing vibration torque arising from network faults is also satisfying. In addition, the authors propose the new electromechanical supersynchronous resonance phenomenon for the turbine-generators near the inverter station owing to asymmetric line faults near the rectifier station. Secondly, the dramatic real and reactive power consumption during the melting period of an electrical arc furnace load. The voltage flicker pollution is mainly caused by the reactive power fluctuation while the stochastic subsynchronous oscillation in turbine mechanism is excited by the electromagnetic torque of the subsynchronous frequency which is induced by the real power fluctuation. Such a small stress imposed on the low-pressure long turbine blade combined with its evitable corrosive environment contributing to the corrosion fatigue effect. Although the voltage flicker severity at the point of common coupling is still within the limit, the blade may have been damaged from the long-term corrosion fatigue life expenditure estimation. In other words, the conventional voltage flicker limit established to make human-eye comfortable might not protect the blade from damaging risk. The long-term influence resulted from the electric arc furnace loads cannot always be neglected. It is necessary to take care of the blade material intensity and operating environment. If there is the potential of blade damage, one has to strengthen the output capacity at the power plant or separate the peak load durations among the steel plants to limit the over-fluctuation real power of the generator.

Στην παρούσα διδακτορική διατριβή μελετήθηκε η διασύνδεση ενός υπεράκτιου Αιολικού Πάρκου (ΑΠ) επαγωγικών μηχανών ονομαστικής ισχύος 90MW με ένα απομακρυσμένο ασθενές δίκτυο ΕΡ. Η περίπτωση αυτή παρουσιάζει σημαντικές δυσκολίες λόγω του χαμηλού λόγου ισχύων βραχυκύκλωσης ανάμεσα στις δύο πλευρές και των έντονων διακυμάνσεων της ισχύος που απομαστεύεται από τον άνεμο. Με σκοπό να περιορισθούν οι δυσκολίες αυτές και δεδομένου ότι τα διασυνδεδεμένα άκρα είναι απομακρυσμένα (άρα επιβάλλεται η μεταφορά με καλώδιο ΣΡ), επιλέχθηκε η εφαρμογή της διασύνδεσης του ΑΠ στο δίκτυο ΕΡ μέσω της τεχνολογίας Εναλλασσομένου Ρεύματος/ Συνεχούς Ρεύματος/ Εναλλασσομένου Ρεύματος (ΕΡ/ΣΡ/ΕΡ) με Μετατροπείς Πηγής Τάσης (ΜΠΤ). Η τεχνολογία αυτή επιτρέπει τον ανεξάρτητο έλεγχο της πραγματικής από την άεργο ισχύ και έτσι καθιστά δυνατή τη διασύνδεση δικτύων με χαμηλό λόγο ισχύων βραχυκύκλωσης. Ένα επιπλέον πρόβλημα το οποίο επιλύεται μέσω της χρήσης της τεχνολογίας ΕΡ/ΣΡ/ΕΡ με ΜΠΤ είναι αυτό της δυναμικής αντιστάθμισης αέργου ισχύος των επαγωγικών γεννητριών. Ωστόσο, παρόλα τα οφέλη που αποκομίζονται εφαρμόζοντας την τεχνολογία ΕΡ/ΣΡ/ΕΡ με ΜΠΤ, ο έλεγχος της διασύνδεσης παραμένει μια απαιτητική διαδικασία. Ο σχεδιασμός του συστήματος ελέγχου απαιτεί λεπτομερή γνώση της συμπεριφοράς του συστήματος και ακριβή ρύθμιση ώστε να αποκτηθεί η επιθυμητή έξοδος. Η παρουσία έντονα μεταβαλλόμενων και με θόρυβο εισόδων μπορούν να καταστήσουν πολύ δύσκολη την επιλογή των παραμέτρων ελέγχου οι οποίες παρέχουν σωστή συμπεριφορά σε οποιεσδήποτε συνθήκες λειτουργίας. Σύμφωνα με τα παραπάνω κριτήρια ελέγχου, το σύστημα ελέγχου που επιλέχθηκε για την παραπάνω διασύνδεση σχεδιάστηκε σύμφωνα με την υπολογιστική νοημοσύνη, η οποία βασίζεται στην ποιοτική περιγραφή των ελεγχόμενων διαδικασιών και στη μίμηση του συλλογισμού ενός έμπειρου ανθρώπου-χειριστή της διαδικασίας. Οι κλάδοι της υπολογιστικής νοημοσύνης που εφαρμόσθηκαν είναι τα ασαφή συστήματα και τα υβριδικά νευρο-ασαφή συστήματα. Πιο συγκεκριμένα, για τον έλεγχο της διασύνδεσης του ΑΠ με το ασθενές δίκτυο ΕΡ, σχεδιάστηκαν και μελετήθηκαν τρεις διαφορετικές εκδοχές, οι οποίες περιελάμβαναν: 1. ένα ασαφές σύστημα ελέγχου, 2. ένα νευρο-ασαφές σύστημα ελέγχου, 3. ένα προσαρμοζόμενο ασαφές σύστημα ελέγχου, το οποίο ρυθμίζεται σε πραγματικό χρόνο. Προκειμένου να ελεγχθεί η αποτελεσματικότητα του συστήματος ελέγχου, αλλά και να συγκριθούν οι τρεις εκδοχές του ως προς την αποτελεσματικότητά τους σε διαφορετικές διαταραχές, η αξιολόγηση του συστήματος περιέλαβε τρία μέρη. Στο πρώτο μέρος παρουσιάσθηκε η απόκριση του συστήματος σε μια βηματική αύξηση της μέσης τιμής της ταχύτητας του ανέμου. Η μεταβολή που επιλέχθηκε είναι ιδιαίτερα απότομη με σκοπό να ελεγχθεί η απόκριση του συστήματος υπό ακραίες συνθήκες. Στο δεύτερο μέρος παρουσιάσθηκε η απόκριση του συστήματος σε μια βηματική μείωση της μέσης τιμής της ταχύτητας του ανέμου, εξίσου απότομη με την προηγούμενη, ώστε να σχηματισθεί μια ολοκληρωμένη εικόνα για τη δυναμική συμπεριφορά του συστήματος κατά τη μετάβασή του στις διάφορες περιοχές λειτουργίας. Στο τρίτο μέρος έγινε μια αξιολόγηση της αρμονικής παραμόρφωσης, η οποία προκαλείται από τη διακοπτική λειτουργία του αντιστροφέα, στις κυματομορφές του ρεύματος και της τάσης στο ΣΚΣΔ, σύμφωνα με τα προτεινόμενα όρια του κανονισμού ΙΕΕΕ 519. Μέσω εκτενών αποτελεσμάτων εξομοίωσης αποδείχθηκε ότι το προτεινόμενο σύστημα ελέγχου και στις τρεις περιπτώσεις αντιδρά γρήγορα στις μεταβολές ισχύος του ανέμου και η διασύνδεση ΕΡ/ΣΡ/ΕΡ με ΜΠΤ τροφοδοτεί ομαλά το σύστημα ΕΡ με πραγματική ισχύ υπό σχεδόν σταθερή εναλλασσόμενη τάση. Επιπρόσθετα, λόγω του ελέγχου μεταβλητών στροφών, το ΑΠ οδηγείται σε μέγιστη αεροδυναμική απόδοση, χωρίς να απαιτείται μέτρηση της ταχύτητας του ανέμου ή του ρότορα των μηχανών. / In this thesis is studied the connection of an offshore Wind Farm (WF) with induction generators to a weak ac grid. This case presents several difficulties, as a result of the low short circuit ratio between the two interconnected ends and the fluctuating and unstable nature of the wind power. In order to constrain these difficulties and taking into account the long distance between the two interconnected ends (which necessitates the use of a dc cable), it was chosen to use the technology of the High Voltage Direct Current (HVDC) link based on Voltage Sourced Converters(VSCs) for the connection of the WF to the weak ac grid. This technology presents the advantages of dc transmission and due to the high switching capability of the VSCs’ valves, it can instantly regulate the reactive power and consequently the ac voltage, independently of the real power flow. So, the short circuit ratio between the two ends of the link does not have to be high. Furthermore, through this link, the dynamic reactive power compensation of the induction generations can be achieved. However, despite the benefits which are obtained through the technology of the HVDC link based on VSCs, the control of the link is still a demanding procedure. The design of the control system demands a comprehensive knowledge of the system behavior and accurate tuning in order to achieve the desirable output. The presence of highly fluctuating and noisy input signals can make the selection of the control parameters which ensure proper behavior in any operating conditions very difficult. Taking into account the above requirements, the control theory that was implemented to the control system of the above mentioned link is that of computational intelligence. Computational intelligence-based controllers do not require precise mathematical modeling of the system nor complex computations. They rely on the human ability to understand the system behavior and are based on qualitative control rules. In addition, they have inherent abilities to deal with imprecise or noisy data. The categories of computational intelligence that were used in the control system design are fuzzy systems and hybrid neuro-fuzzy systems. More precisely, three alternative versions of the control system of the link between the WF and the weak ac grid were designed: 1. a simple, manually tuned fuzzy control system , 2. a hybrid neuro-fuzzy control System 3. an adaptive, on-line tuned fuzzy control system. In order to test the performance of the control system and compare its three versions, the test of the system includes three parts. In the fist part was studied the performance of the system under a step increase of the wind speed. The step increase was chosen to be very steep, in order to test the system under extreme conditions. In the second part was studied the response of the system under a respective step decrease of the wind speed, in order to test the system performance under any operating condition. In the third part was examined the harmonic content of waveforms of the current and voltage at the point of common coupling, according to the limits of the international standard IEEE 519. Through extended simulation results it was shown that the proposed control system quickly reacts to the step changes of the wind power and the HVDC link based on VSCs manages to feed the weak ac grid with the power from the wind under almost stable ac voltage. In addition, due to the ability of the control system to adjust the stator frequency of the induction generators in relation to the wind velocity, maximum power absorption of the WF is achieved, without monitoring the wind speed or the rotor speed.

Abstract Adding a dc circuit to an existing transmission line is one method of significantly increasing the power transfer capability of a transmission corridor. The resulting hybrid system has significant coupling between the ac and dc circuits, not only because of the proximity of the circuits, but also from the fact that they may share the same sending end or receiving end ac systems. The resultant interaction produces overvoltages on the dc system which can be somewhat higher than for a conventional dc scheme. This thesis investigates the overvoltages on a hybrid ac-dc transmission system and suggests some design considerations which could be taken into account to reduce stresses on certain critical components which result from such an arrangement. Blocking filters consisting of a parallel L-C combination in series with the dc converter were included to limit the flow of fundamental frequency current in the dc line. This thesis also investigates the proper blocking filter configuration to be used as an incorrectly chosen blocking filter can cause resonance overvoltages on the dc line at fundamental frequency. A method of eliminating dc components of the currents in the transformer windings of a dc converter is presented. The method uses the technique of firing angle modulation. It is shown that merely eliminating the fundamental frequency component on the dc side may not remove this dc component. The impact of such control action at one converter on the other converters in the dc transmission system is also presented. It is also shown that the undesirable side effects of such a scheme include increased generation of non-characteristic harmonies on both the ac and dc sides. The study is performed using an electromagnetic transients simulation program and theoretical calculations.

Abstract Adding a dc circuit to an existing transmission line is one method of significantly increasing the power transfer capability of a transmission corridor. The resulting hybrid system has significant coupling between the ac and dc circuits, not only because of the proximity of the circuits, but also from the fact that they may share the same sending end or receiving end ac systems. The resultant interaction produces overvoltages on the dc system which can be somewhat higher than for a conventional dc scheme. This thesis investigates the overvoltages on a hybrid ac-dc transmission system and suggests some design considerations which could be taken into account to reduce stresses on certain critical components which result from such an arrangement. Blocking filters consisting of a parallel L-C combination in series with the dc converter were included to limit the flow of fundamental frequency current in the dc line. This thesis also investigates the proper blocking filter configuration to be used as an incorrectly chosen blocking filter can cause resonance overvoltages on the dc line at fundamental frequency. A method of eliminating dc components of the currents in the transformer windings of a dc converter is presented. The method uses the technique of firing angle modulation. It is shown that merely eliminating the fundamental frequency component on the dc side may not remove this dc component. The impact of such control action at one converter on the other converters in the dc transmission system is also presented. It is also shown that the undesirable side effects of such a scheme include increased generation of non-characteristic harmonies on both the ac and dc sides. The study is performed using an electromagnetic transients simulation program and theoretical calculations.