This thesis is dedicated to a comprehensive study of static series synchronous compensator (SSSC) systems utilizing cascaded-multilevel converters (CMCs). Among flexible AC transmission system (FACTS) controllers, the SSSC has shown feasibility in terms of cost-effectiveness in a wide range of problem-solving abilities from transmission to distribution levels. Referring to the literature reviews, the CMC with separated DC capacitors is clearly the most feasible topology for use as a power converter in the SSSC applications. The control for the CMC-Based SSSC is complicated. The design of the complicated control strategy was begun with well-defined system transfer functions. The stability of the system was achieved by trial and error processes, which were time-consuming and ineffective. The goal of this thesis is to achieve a reliable controller design for the CMC-based SSSC. Major contributions are addressed as follows: 1) accurate models of the CMC for reactive power compensations in both ABC and DQ0 coordinates, and 2) an effective decoupling power control technique. To simplify the control system design, well-defined models of the CMC-Based SSSC in both ABC and DQ0 coordinates are proposed. The proposed models are for the CMC-Based SSSC focus on only three voltage levels but can be expanded for any number of voltage levels. The key system transfer functions are derived and used in the controller design process. To achieve independent power control capability, the control technique, called the decoupling power control used in the design for the CMC-Based STATCOM is applied. This control technique allows both the real and reactive power components to be independently controlled. With the combination of the decoupling power control and the cascaded PWM, a CMC with any number of voltage levels can be simply modeled as a three-level cascaded converter, which is the simplest topology to deal with. This thesis focuses on the detailed design process needed for a CMC-Based SSSC. / Master of Science

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The Coal to Liquid facility, Sasol, Secunda operates as a train of processes. Disturbances and capacity restrictions can occur throughout the plant and the throughput fluctuates whenever disturbances occur. When capacity restrictions occur in a subplant and more substances enter the sub-plant than can be processed, the extra substances are flared or dumped and therefore lost. To reduce losses and extra costs and to maximise the throughput of the whole plant, supervisory control is implemented over the whole plant system. Each process in the process train is controlled with regulatory controllers and the overall process is then controlled with a supervisory controller. These two sets of controllers operate in two different layers of control, with the regulatory controllers the faster inner layer. The supervisory control is the outer layer of the two control layers. The supervisory controller takes over the work of the human operator by deciding on the changes in total throughput as well as the set points for each individual process. These set points for each process are then followed with the regulatory controllers. For the regulatory control of the system, different control methods are investigated and compared. The different control methods that are looked at are PI control, Linearised State Feedback control, Fuzzy Logic control and Model Reference Adaptive Control. After an investigation into the various control methods Fuzzy Logic control was chosen for the regulatory as well as the supervisory control levels. Fuzzy Logic control is a rule based control method. Fuzzy variables are everyday terms such as very slow or nearly full. These terms are easy to understand by the operator and multi-variable control is possible with Fuzzy Logic control without an accurate mathematical representation of the system. These facts made Fuzzy Logic control ideal for this implementation. To improve the profit of the Coal to Liquid facility the throughput was maximised. The combination of regulatory and supervisory controllers minimised losses and rejected disturbances. This resulted in a smoother output with maximum profit. / AFRIKAANSE OPSOMMING: Die Steenkool-na-Olie fasiliteit, Sasol, Secunda funksioneer as ’n trein van prosesse. Versteurings en kapasiteit beperkings kan deur die hele aanleg voorkom en die deurset wissel voortdurend wanneer versteurings voorkom. Wanneer kapasiteit beperkings voorkom in ’n aanleg en meer stowwe word in die aanleg ingestuur as wat dit kan verwerk, word die ekstra stowwe gestort en dit gaan verlore. Om verliese en kostes te verminder en om die deurset van die hele aanleg te vergroot, is oorhoofse beheer geïmplementeer oor die hele stelsel. Elke proses in die trein van chemiese prosesse word beheer met regulerende beheerders. Die totale proses word dan beheer met ’n oorhoofse beheerder. Hierdie twee tipes beheerders funksioneer in twee lae van beheer met die regulerende beheerders die vinniger binneste laag. Die oorhoofse beheerder vorm die buitenste laag van die twee beheer lae en neem die werk van die menslike operateur oor deur die veranderinge in die totale deurset, sowel as die stelpunte vir elke afsonderlike proses, te bepaal. Hierdie stelpunte vir elke proses word dan met die regulerende beheerders gevolg. Verskillende beheer metodes is ondersoek vir die regulerende beheer van die stelsel. Die verskillende beheer metodes waarna gekyk word, is PI beheer, Geliniariseerde Toestands Terugvoer beheer, Wasige Logiese beheer en Model Verwysing Aanpassende beheer. Na ’n ondersoek na die verskillende beheer metodes is Wasige Logiese beheer gekies vir die regulerende asook die oorhoofse beheer. Wasige Logiese beheer is ’n reël gebasseerde beheer metode. Wasige Logika veranderlikes is alledaagse terme soos baie stadig of byna vol. Hierdie terme is maklik om te verstaan deur die operateur. Meervoudige-veranderlike beheer is moontlik met Wasige Logiese beheer sonder ’n akkurate wiskundige voorstelling van die stelsel. Hierdie feite maak Wasige Logiese beheer ideaal vir hierdie doel. Om die wins van die Steenkool-na-Olie fasiliteit te verbeter, is die deurset gemaksimeer. Die kombinasie van regulerende- en toesighoudende beheerders beperk verliese en verwerp versteurings. Dit lei tot ’n gladder uitset en ’n maksimum wins.

Synthetic aperture sonar (SAS) is a wide-beam sonar technique commonly used for mapping the seafloor at high resolution. The Acoustics Research Group at the University of Canterbury operates a towed SAS system known as KiwiSAS-IV. This is currently being redesigned with the aim of reducing the weight, size and power requirements of the system. The long term goal is to make it capable of being mounted on an autonomous underwater vehicle (AUV) so that mapping of remote and/or dangerous waters can be accomplished without human interaction. This thesis presents the design of the front-end electronics used to drive the 36 transducers to produce the acoustic beam and receive the returning signals after they have reflected off a target. To achieve sufficient range, the transducers are driven with a 200 Vₚ₋ₚ signal with a maximum frequency of 110 kHz. This design uses class D switching amplifiers to generate these waveforms. The AD9271 integrated circuit, which can handle eight transducers simultaneously, is used to amplify the incoming signals and sample them at up to 50 MHz. This high sampling rate multiplied by all 36 transducers results in an amount of data which is too great for a conventional microprocessor-based system to handle. Instead, an FPGA is used to receive this data, decimate it using multiplier-free cascaded integrator-comb (CIC) filters, and then pass it to the back-end system for further processing and storage. A prototype circuit was created to test the theory developed in this thesis. This showed that the system is capable of generating the necessary waveforms and amplifying, capturing, and decimating the returning signals. However, further refinement is required before it is able to be used in the sonar system.

This thesis introduces a comprehensive methodology to improve electromagnetic transient (EMT) modelling of power cables systems. Several improved modelling and validation techniques are proposed at the parameter estimation, time domain simulation and validation stages of the EMT modelling of transmission lines. A novel approach is developed to model sector-shaped cables in electromagnetic transient type programs. First, the applicability of elemental sub-conductor technique is extended to accurately calculate the frequency dependent impedances of sector-shaped cables. The derived admittance and propagation characteristics of the sector-shaped cable are fitted with rational functions using the method of vector fitting in an EMT-type program. The time domain simulations are validated with the numerical inverse Laplace transform method. A novel frequency domain approach is presented to model cascaded transmission systems. The procedure is based on obtaining four composite propagation functions representing the cascaded system. The performance of the technique does not diminish with increased number of cascaded segments and it preserves the intrinsic details of each line segment. This method is capable of modelling cascaded overhead lines or cables with different characteristic admittances and line lengths. This method can be used to validate EMT models of cascaded transmission systems. An improved generalized transmission line model is introduced which is capable of accommodating time steps greater than the travel time of the line. The time step of the conventional EMT models of transmission lines is constrained by the smallest travel time of the line. When the high frequency transients at the line terminations are not of interest, inaccurate nominal π equivalents are used with large time steps to reduce the computational burden. The proposed model not only is more accurate than the π equivalents, but also degenerates to the conventional frequency dependent EMT model when used with time steps smaller than the travel time. Therefore, the proposed model is highly convenient as it can be used for all types of EMT simulations without resorting to nominal π equivalents when the large simulation time steps must be used to reduce computational burden. / February 2017

Though economically favorable when compared to other renewable energy storage technologies, thermal energy storage systems for concentrating solar thermal power (CSP) plants require additional cost reduction measures to help transition CSP plants to the point of grid-parity. Thermocline packed bed storage is regarded as one potential low cost solution due to the single tank requirement and low cost storage media. Thus sensible heat storage (SHS) and latent heat storage (LHS) packed bed systems, which are two thermocline varieties, are frequently investigated. LHS systems can be further classified as single phase change material (PCM) systems or cascaded systems wherein multiple PCMs are employed. This study compared the performance of SHS, single PCM, and cascaded PCM direct storage systems under the conditions that may be encountered in utility-scale molten salt CSP plants operating between 565°C and 288°C. A small-scale prototype SHS packed bed system was constructed and operated for use in validating a numerical model. The drawbacks of the latent heat storage process were discussed, and cascaded systems were investigated for their potential in mitigating the issues associated with adopting a single PCM. Several cascaded PCM configurations were evaluated. The study finds that the volume fraction of each PCM and the arrangement of latent heat in a 2-PCM and a 3-PCM system influences the output of the system, both in terms of quality and quantity of energy. In addition to studying systems of hypothetical PCMs, real salt PCM systems were examined and their selection process was discussed. A preliminary economic assessment was conducted to compare the cost of SHS, single-PCM LHS, cascaded LHS, and state-of-the-art 2-tank systems. To the author's knowledge, this is the first study that compares the cost of all three thermocline packed bed systems with the 2-tank design. The SHS system is significantly lower in cost than the remaining systems, however the LHS system does show some economic benefit over the 2-tank design. If LHS systems are to be viable in the future, low cost storage media and encapsulation techniques are necessary.

<p>The development of digital detectors for X-ray imaging in medical diagnostics receives an increasing amount of attention. The detector under development at the Department of Radiation Sciences at Uppsala University is a hybrid pixel detector, which consists of a semiconductor sensor mounted onto a readout chip. The readout chip is capable of performing photon counting and has an externally adjustable threshold.</p><p>A simulation tool for the detector and a model applying the linear-systems transfer theory to X-ray hybrid pixel detectors have been developed. Also a characterization of the readout chip has been done. In order to estimate the potential of the detector for diagnostic radiology, we investigate the image quality using the spatial frequency dependent detective quantum efficiency (DQE). By means of the detector simulations, the influence of threshold setting, noise sources, level of exposure and charge sharing on the DQE have been studied. By means of the linear-systems theory, a single analytical expression is provided to obtain the DQE of a hybrid pixel detector.</p><p>The method developed in this thesis will make it possible to optimize a detector design according to a particular medical application. It will also permit modifications and new features to be included without having to construct a full detector system.</p>

The development of digital detectors for X-ray imaging in medical diagnostics receives an increasing amount of attention. The detector under development at the Department of Radiation Sciences at Uppsala University is a hybrid pixel detector, which consists of a semiconductor sensor mounted onto a readout chip. The readout chip is capable of performing photon counting and has an externally adjustable threshold. A simulation tool for the detector and a model applying the linear-systems transfer theory to X-ray hybrid pixel detectors have been developed. Also a characterization of the readout chip has been done. In order to estimate the potential of the detector for diagnostic radiology, we investigate the image quality using the spatial frequency dependent detective quantum efficiency (DQE). By means of the detector simulations, the influence of threshold setting, noise sources, level of exposure and charge sharing on the DQE have been studied. By means of the linear-systems theory, a single analytical expression is provided to obtain the DQE of a hybrid pixel detector. The method developed in this thesis will make it possible to optimize a detector design according to a particular medical application. It will also permit modifications and new features to be included without having to construct a full detector system.

This research and development work deals with the design methodology for Cascaded Multilevel Converter (CMC) based Transmission STATCOM (TSTATCOM) and development of a &plusmn / 12MVAR, 12kV line-to-line wye-connected, 11-level CMC. This CMC module constitutes the basic building block of TSTATCOM systems. Sizing of the CMC module, number of H-Bridges in each phase of the CMC, AC voltage rating of the CMC, the number of paralleled CMC modules in the T-STATCOM system, optimum value of series filter reactors and determination of busbar in the power grid to which the T-STATCOM system is going to be connected are also discussed in the thesis in view of IEEE Std.519-1992, current status of HV IGBT technology and the required reactive power variation range for the T-STATCOM application. In the field prototype of the CMC module, the AC voltages are approximated to sinusoidal waves by Selective Harmonic Elimination Method (SHEM) and by the use of an optimized series input filter reactor. The use of n number of HBs in each phase provides us n number of freedom in the application of SHEM. One of them is allocated to the fundamental component while n-1 is for the elimination of low order harmonics. Since n is chosen to five in the prototype system, 5th, 7th,11th and 13th harmonic components are successfully eliminated in the AC voltage waveforms of the CMC module. The equalization of DC link capacitor voltages is achieved according to Modified Selective Swapping (MSS) algorithm. MSS is applied every 400&mu / s period if needed to obtain a perfect equalization of DC link capacitor voltages at the expense of higher switching frequency and hence switching losses. In this research work, an L-shaped laminated bus has been designed and the HV IGBT driver circuit has been modified for optimum switching performance of HV IGBT modules in each HB circuit. The performances of the HB circuit and the resulting 11-level CMC module have been obtained not only in the laboratory but also in the field. Design works for HB and the CMC are based on MATLAB and PSCAD simulations. The laboratory and field performance of the HB circuit and CMC module is found to be satisfactory and quite consistent with the theoretical results and design objectives. In addition to these, 154 kV, &plusmn / 50MVAr T-STATCOM prototype has been designed, implemented and installed at Sincan Transformer Substation-Ankara primarily for the purposes of reactive power compensation and terminal voltage regulation. The T-STATCOM prototype is composed of five parallel operated CMC modules developed within the scope of this PhD thesis research work. The T-STATCOM configuration permits the operation of any number of CMC modules in the range from one to five for experimental purposes. The performance of this T-STATCOM system is also presented in this PhD thesis as a sample application.

The automobile industry is moving fast towards Electric Vehicles (EV); however this paradigm shift is currently making its smooth transition through the phase of Hybrid Electric Vehicles. There is an ever-growing need for integration of hybrid energy sources especially for vehicular applications. Different energy sources such as batteries, ultra-capacitors, fuel cells etc. are available. Usage of these varied energy sources alone or together in different combinations in automobiles requires advanced power electronic circuits and control methodologies. An exhaustive literature survey has been carried out to study the power electronic converter, switching modulation strategy to be employed and the particular machine to be used in an EV. Adequate amount of effort has been put into designing the vehicle specifications. Owing to stronger demand for higher performance and torque response in an EV, the Permanent Magnet Synchronous Machine has been favored over the traditional Induction Machine. The aim of this thesis is to demonstrate the use of a multi level inverter fed Brush Less Direct Current (BLDC) motor in a field oriented control fashion in an EV and make it follow a given drive cycle. The switching operation and control of a multi level inverter for specific power level and desired performance characteristics is investigated. The EV has been designed from scratch taking into consideration the various factors such as mass, coefficients of aerodynamic drag and air friction, tire radius etc. The design parameters are meant to meet the requirements of a commercial car. The various advantages of a multi level inverter fed PMSM have been demonstrated and an exhaustive performance evaluation has been done. The investigation is done by testing the designed system on a standard drive cycle, New York urban driving cycle. This highly transient driving cycle is particularly used because it provides rapidly changing acceleration and deceleration curves. Furthermore, the evaluation of the system under fault conditions is also done. It is demonstrated that the system is stable and has a ride-through capability under different fault conditions. The simulations have been carried out in MATLAB and Simulink, while some preliminary studies involving switching losses of the converter were done in PSIM.

<p>Den situerade ansatsen inom artificiell intelligens har i tidigare experiment visat på stora möjligheter vid utvecklingen av enkla beteenden. Ansatsens framgångar är dock inte lika tydliga när det kommer till utvecklingen av mer komplexa beteenden som i högre grad påminner om de experiment som gjorts inom traditionell artificiell intelligens. I studien utvecklas en agent med ett ”Extended Sequential Cascaded Network” som kontrollarkitektur för att lösa en uppgift som kräver ett ”räkneliknande beteende”. Utvecklingen av nätverket grundas på en situerad syn på kognition, däribland att designern i så liten grad som möjligt skall styra utvecklingen. Experimentets resultat visar på en agent som inte ens löser den enklaste versionen av uppgiften. I diskussionen härleds misslyckandet till svårigheterna med en designeroberoende utveckling.</p>