Thesis (M.Eng.)(Control)--University of Pretoria, 2004. / Summaries in English and Afrikaans. Includes bibliographical references.

Thesis (Ph. D.)--Drexel University, 2005. / Includes abstract and vita. Includes bibliographical references (leaves 120-128).

Thesis (M.S.)--Ohio University, June, 2005. / Title from PDF t.p. Includes bibliographical references (p. 90-92)

Thesis (M.S.)--Ohio University, November, 2006. / Title from PDF t.p. Includes bibliographical references.

Thesis (Ph. D.)--University of Glasgow, 1994. / Includes bibliographical references. Print version also available.

The generation, transmission and distribution of electricity underpins modern technology and constitutes a necessary element for our development and economic functionality. In the recent years, as a result of environmental concerns and technological advances, private and public investment have been steadily turning towards renewable sources of energy, resulting in a growing penetration of those in the power network. This poses additional challenges in the control of power networks, since renewable generation is in general intermittent, and a large penetration may cause frequent deviations between generation and demand, which can harm power quality and even cause blackouts. Load side participation in the power grid is considered by many a means to counterbalance intermittent generation, due to its ability to provide fast response at urgencies. Industrial loads as well as household appliances, may respond to frequency deviations by adjusting their demand in order to support the network. This is backed by the development of relevant sensing and computation technologies. The increasing numbers of local renewable sources of generation along the introduction of controllable loads dramatically increases the number of active elements in the power network, making traditionally implemented, centralised control dicult and costly. This demonstrates the need for the employment of highly distributed schemes in the control of generation and demand. Such schemes need to ensure the smooth and stable operation of the network. Furthermore, an issue of fairness among controllable loads needs to be considered, such that it is ensured that all loads share the burden to support the network evenly and with minimum disruption. We study the dynamic behaviour of power networks within the primary and secondary frequency control timeframes. Using tools from non-linear control and optimisation, we present methods to design distributed control schemes for generation and demand that guarantee stability and fairness in power allocation. Our analysis provides relaxed stability conditions in comparison with current literature and allows the inclusion of practically relevant classes of generation and demand dynamics that have not been considered within this setting, such as of higher order dynamics. Furthermore, fairness in the power allocation between loads is guaranteed by ensuring that the equilibria of the system are solutions to appropriately constructed optimisation problems. It is evident that a synchronising variable is required for optimality to be achieved and frequency is used as such in primary control schemes whereas for secondary frequency control a dierent synchronising variable is adopted. For the latter case, the requirements of the synchronising feedback scheme have been relaxed with the use of an appropriate observer, showing that stability and optimality guarantees are retained. The problem of secondary frequency regulation where ancillary services are provided from switching loads is also considered. Such loads switch on and off when some prescribed frequency threshold is reached in order to support the power network at urgencies. We show that the presence of switching loads does not compromise the stability of the power network and reduces the frequency overshoot, potentially saving the network from collapsing. Furthermore, we explain that when the on and o switching frequencies are equivalent, then arbitrarily fast switching phenomena might occur, something undesirable in practical implementations. As a solution to this problem, hysteresis schemes where the switch on and off frequencies differ are proposed and stability guarantees are provided within this setting.

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In practical multibody systems, various factors such as friction, joint clearances, and external events play a significant role and can greatly influence the optimal design of the system and its controller. This research focuses on the use of gradient-based optimization methods for multibody dynamic systems with the incorporation of joint friction. The dynamic formulation has been derived in using two distinct techniques: Index-1 DAE and the tangent-space formulation in minimal coordinates. It employs a two different approaches for gradient computation: direct sensitivity approach and the adjoint sensitivity approach. After a comprehensive review of different friction models developed over time, the Brown McPhee model is selected as the most suitable due to its accuracy in dynamic simulations and its compatibility with sensitivity analysis. The proposed methodology supports the simultaneous optimization of both the system and its controller. Moreover, the sensitivities obtained using these formulations have been thoroughly validated for numerical accuracy and benchmarked against other friction models that are based on dynamic events for stiction to friction transition. The approach presented is particularly valuable in applications like robotics and servo-mechanical systems where the design and actuation are closely interconnected. To obtain numerical results, a new implementation of the MBSVT (Multi-Body Systems at Virginia Tech) software package, known as MBSVT 2.0, is reprogrammed in Julia and MATLAB to ensure ease of implementation while maintaining high computational efficiency. The research includes multiple case studies that illustrate the advantages of the concurrent optimization of design and control for specific applications. Efficient techniques for control signal parameterization are presented using linear basis functions. A special focus has been made on the computational efficiency of the formulation and various techniques like sparse-matrix algebra and Jacobian-free products have been employed in the implementation. The dissertation concludes with a summary of key results and contributions and the future scope for this research. / Doctor of Philosophy / In simpler terms, this research focuses on improving the design and control of complex mechanical systems, like robots and automotive systems, by considering factors such as friction in the joints. Friction in a system can greatly affect how it performs for the desired task. The research uses a method called gradient-based optimization, which essentially means finding the most optimal parameters of the system and its controller such that they achieve a desired goal in the most optimal way. Before a model for such a system can be developed, various techniques need to be researched for incorporation of friction mathematically. A model known as Brown McPhee friction is one such model suitable for such an analysis. When optimizing any system on a computer, an iterative process needs to be performed which may prove to be very expensive in terms of computational resources required and the time taken to achieve a solution. Hence, proper mathematical and computational techniques need to be employed to ensure that the resources of a computer are utilized in the most efficient way to get the solution is the quickest way possible. Among the various novelties of this research, it is worth noting that this method that allows for simultaneous design and control optimization, which is particularly useful for applications such as robotics and servo-mechanical systems. Considering the design and control together, leads to more efficient and effective systems. The approach is tested using a software package called MBSVT 2.0, which was specifically developed as part of this research. The software is available in 3 languages: Julia, MATLAB and Fortran for universal access to people from various communities. The results from various case studies are presented that demonstrate this simultaneous design and control approach and highlights its effectiveness making the systems more robust and better performing.

Το ενεργειακό πρόβλημα αποτελεί ένα από τα πιο πολυσηζητημένα θέματα της ανθρωπότητας που απασχολεί και θα συνεχίσει να απασχολεί τον πλανήτη μας για τις επόμενες δεκαετίες. Μια λύση στο πρόβλημα της ηλεκτρικής ενέργειας είναι η εγκατάσταση και λειτουργία μικροδικτύων είτε σε οικιακή κλίμακα είτε σε επίπεδο εθνικού δικτύου ή ακόμα και σε επίπεδο μιας ολόκληρης ηπείρου. Για την επιτυχή και ευσταθή λειτουργία τέτοιων συστημάτων, κρίνεται απαραίτητος ο έλεγχος των επιμέρους στοιχείων τους ξεχωριστά. Στην παρούσα διπλωματική εργασία μοντελοποιήθηκαν και ελέγθηκαν δύο ηλεκτρονικοί μετατροπείς ισχύος, ο TCSC και ο BOOST, καθώς και η τριφασική ασύγχρονη μηχανή βραχυχκυκλωμένου κλωβού, κυρίαρχα στοιχεία ενός μικροδικτύου. Τα καταστατικά μοντέλα όμως, τόσο των μετατροπέων ισχύος όσο και της μηχανής περιγράφονται από μη γραμμικές διαφορικές εξισώσεις και συνεπώς ο έλεγχος τους δεν είναι καθόλου εύκολη υπόθεση. Συγκεκριμένα, χρησιμοποιήθηκαν μοντέρνες τεχνικές μη γραμμικού και προσαρμοστικού ελέγχου προκειμένου να ελεγχθούν τα συστήματα αυτά και να εξασφαλιστεί κάθε φορά η ευστάθεια του συστήματος στη μόνιμη κατάσταση λειτουργίας. Για τους μετατροπείς TCSC και BOOST σχεδιάστηκαν απευθείας οι μη γραμμικοί - προσαρμοστικοί ελεγκτές ενώ για την τριφασική ασύγχρονη μηχανή εφαρμόστηκε πρώτα έμμεσος διανυσματικός έλεγχος στο στατό πλαίσιο αναφοράς και στη συνέχεια σχεδιάστηκαν οι ελεγκτές και οι εκτιμητές της ροής, της ροπής και της αντίστασης. Τέλος όλα τα συστήματα προσομοιώθηκαν με τη χρήση του εργαλείου Simulink του Matlab και εξήχθησαν τα συμπεράσματα. / The energy problem is one of the most spoken issues of humanity that concerns and will continue to concern our planet for decades to come. One solution to the electrical energy problem is the installation and operation of microgrids either at household level or at the level of a national network or even at the level of an entire continent. For successful and stable operation of such systems, it is necessary to test their components separately. In this Thesis there have been modeled and tested two electronic power converters, the TCSC and BOOST, and the three-phase asynchronous machine of shortcircuited cage, dominant elements of a microgrid. The constitutive models, however, both the power converters and the machine are described by nonlinear differential equations and, therefore, their control is not at all easy. Specifically, modern techniques of nonlinear and adaptive control were used in order to check these systems so that the stability of the system in the steady state be ensured. For the inverters TCSC and BOOST the nonlinear - adaptive controllers were designed directly, while for the three-phase asynchronous machine there was applied firstly the indirect vector control in the stationary frame of reference and then the controllers and the flow, torque and resistance estimators were designed. Finally, all the systems were simulated using the Matlab Simulink tool and conclusions were drawn.