Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The use of power electronic converters provides a more efficient, accurate and dynamic solution to reactive compensation. In this thesis the application of power electronic converters to shunt reactive compensation will be discussed. In particular voltage dips and voltage unbalance are considered as both can be mitigated by means of shunt reactive compensation. A pre-existing uninterruptible power supply is adapted to operate as a shunt reactive compensator. The uninterruptible power supply consists of a 250 kVA three phase voltage source inverter. The modifications are limited to software and control algorithms that do not alter the normal operation of the uninterruptible power supply. Control algorithms are designed and discussed in detail. A typical double loop control strategy is implemented on the power electronic converter. The inner loop consists of a dead-beat current controller. The outer loop consists of three proportional and integral controllers controlling the DC-bus voltage, AC voltage and voltage unbalance respectively. Voltage dips and unbalance are compensated for using only reactive power. Focus is placed on producing a result can be used easily in practice. / AFRIKAANSE OPSOMMING: Drywings elektroniese omsetters wat gebruik word vir newe reaktiewe kompensasie lewer meer effektiewe, akkurate en dinamiese resultate. In hierdie tesis word die toepassing van drywings elektroniese omsetters vir newe reaktiewe kompensasie bespreek. Daar word meer spesifiek na spannings duike en spannings wanbalans gekyk aangesien albei met newe reaktiewe kompensasie verminder kan word. ’n Bestaande nood kragbron is aangepas om as n newe reaktiewe kompenseerder te funksioneer. Die nood kragbron bestaan hoofsaaklik uit ’n 250 kVA drie fase omsetter spanningsbron. Die aanpassings is beperk tot sagteware en beheer algoritmes wat nie die oorspronklike funksionaliteit van die nood krag bron beinvloed nie. Beheer algoritmes word ontwerp en deeglik bespreek. ’n Tipiese dubbel lus beheer strategie word op die drywings elektroniese omsetter toegepas. Die binnelus bestaan uit ’n voorspellende stroom beheerder. Die buite-lus bestaan uit drie proportioneel en integraal beheerders wat onderskeidelik die GS-bus spanning, WS spanning en spanning wanbalans reguleer. Spannings duike en wanbalans is verminder deur slegs reaktiewe drywing te gebruik. Die doel was ook om ’n prakties bruikbare resultaat te lewer.

L’identification de systèmes est un sujet très utilisé à la fois dans le monde académique et industriel. Des nombreuses méthodes d’identification de systèmes invariants dans le temps existent dans la littérature et beaucoup d’algorithmes sont utilisés dans la modélisation pratique des systèmes. Ces outils offrent des résultats satisfaisants, mais ils ne sont pas capables de reproduire le caractère non linéaire présent dans les comportements des systèmes physiques. Ce besoin a conduit à l’apparition de la classe des systèmes linéaires à paramètres variants (LPV), capable de modéliser les aspects non linéaires des systèmes. Dans le cadre de cette thèse, différentes méthodes d’identification « classiques » ont été étudiées et modifiées pour prendre en compte le cas des modèles LPV.Dans un premier temps une étude sur les représentations et discrétisations des systèmes LPV a été réalisée. Ensuite, les méthodes à erreur de prédiction ont été étudiées et appliquées en vue d’identifier le comportement latéral d’un véhicule, en considérant la vitesse du véhicule comme paramètre variant. Les méthodes à erreur de prédiction ont été également appliquées afin de modéliser un convertisseur de puissance Buck, dont le comportement est sensible au changement de la résistance de charge, considérée comme paramètre variant. L’étude a été poursuivie avec la conception d’une loi de commande H_∞ de type LPV, appliquée au cas du convertisseur.Finalement, les méthodes des sous-espaces classiques ont été abordées et modifiées pour identifier les modèles LPV et appliquées au cas du comportement latéral d’un véhicule. / The identification system is a topic widely used both in the academic world and industry. Several methods of identification of time invariant systems exist in literature and many algorithms are used in practice for modeling real systems. These tools offer satisfactory results, but they are not able to reproduce the non-linearity occurring in the behavior of physical systems. The necessity of more has led to the occurrence of the class of linear systems parameter varying (LPV), able to model the nonlinear system aspects. In this thesis, different classical identification methods have been studied and their structures were modified, in order to take into account the LPV models. First, a study of representations and discretization of LPV models was performed. In the sequel, the prediction error methods have been studied and modified in order to take into account LPV models. This method was used to identify the lateral behavior of a vehicle, considering the speed of the vehicle as varying parameter. The prediction error method has also been applied to model a Buck converter, the behavior of which is sensitive to the changes of load resistance, the considered varying parameter. The study was continued with the design of a H_∞ LPV control law, applied to the converter. Finally, subspace methods were studied, modified for LPV models and applied to identify the lateral behavior of a vehicle.

Firstly, this thesis investigates the electrical power harmonics in an attempt to utilize harmonic current and its distortion power in a novel idea to reconvert the distortion power into useful power. This is done, in order to feed different DC or AC loads in single and three-phase power system by using passive or active filters and accordingly, develop a new topology of hybrid active power filter (HAPF). In addition, this circuit can be considered as a power factor corrector (PFC) because it reduces the total harmonic distortion (THD) and improves the power factor (PF). Secondly, this thesis works on a new design of active power factor correction (APFC) circuit presenting two circuits with the same design principle: the first design consists of two active switches without an external complex control circuit, while the second design contains a single active switch with an additional control circuit. The main contribution of this circuit is 98% reduction of the inductor's value used in the newly proposed PFC circuit in comparison with the conventional boost converter which may lead to a huge reduction in size, weight and the cost of the new PFC circuit. Also, the active switches depend on a carefully designed switching pattern that results in an elimination of the third order harmonic from the input source current which decreases the value of total current harmonic distortion (THDI) to (14%) and improves the input PF to (0.99). Consequently, the simplicity of the design without requiring a complex control circuit and without a snubber circuit plus the minimum size of inductor, gives the newly proposed circuit the superiority on other PFC circuits. Thirdly, this research aims to describe the distortion power through submitting two novel power terms called effective active power (Pef ) & reactive power (Qef ) terms with a new power diagram called Right-Angled Power Triangle (RAPT) Diagram. In addition, a novel de nition of total apparent power (St) has been submitted in order to illustrate the physical meaning of (St) in non-sinusoidal systems. The new RAPT Diagram is based on the orthogonality law and depends on geometrical summation to describe the relationship between different aspects (apparent-active-reactive) of power, and different components (total-fundamental distortion), drawing a bridge to connect the time domain with the frequency domain in a two-dimensional diagram.

The degradation predicted by classical DC reliability methods, such as bias temperature instability (BTI) and hot carrier injection (HCI), might not translate sufficiently to the AC conditions, which are relevant on the circuit level. The direct analysis of circuit level reliability is therefore an essential task for hardware qualification in the near future. Ring oscillators (RO) offer a good model system, where both BTI and HCI contribute to the degradation. In this work, it is qualitatively shown that the additional off-state stress plays a crucial role at very high stress voltages, beyond upper usage boundaries. To yield an accurate RO lifetime prediction a frequency measurement setup with high resolution is used, which can resolve small changes in frequency during stress near operation conditions. An ACDC conversion model is developed predicting the resulting frequency change based on DC input data. From the extrapolation to 10 years of circuit lifetime the model predicts a very low frequency degradation below 0.2% under nominal operation conditions, where the off-state has a minor influence.