Variable frequency drives typically have employed dc voltage or current links for power distribution between the input and output converters and as a means to temporarily store energy. The dc link based power conversion systems have several inherent limitations. One of the important limitations is the high switching loss and high device stress which occur during switching intervals. This severely reduces the practical switching frequencies. Additionally, while the cost, size, and weight of the basic voltage sourced PWM drive is attractive, difficulties with input harmonics, output dV/dt and over-voltage, EMI/RFI, tripping with voltage sags, and other problems significantly diminish the economic competiveness of these drives. Add-ons are available to mitigate these problems, but may result in doubling or tripling the total costs and losses, with accompanying large increases in volume and weight. This research investigates the design, control, operation and efficiency calculation of a new power converter topology for medium and high power ac-ac, ac-dc and dc-ac applications. An ac-link formed by an inductor-capacitor pair replaces the conventional dc-link. Each leg of the converter is formed by two bidirectional switches. Power transfer from input to output is accomplished via a link inductor which is first charged from the input phases, then discharged to the output phases with a precisely controllable current PWM technique. Capacitance in parallel with the link inductor produces low turn-off losses. Turn-on is always at zero voltage as each switch swings from reverse to forward bias. Reverse recovery is with low dI/dt and also is buffered due to the link capacitance.

In the last twenty years, industrial and academic research has produced over one hundred new converter topologies for drives applications. Regrettably, most of the published work has been directed towards a single topology, giving an overall impression of a large number of unconnected, competing techniques. To provide insight into this wide ranging subject area, an overview of converter topologies is presented. Each topology is classified according to its mode of operation and a family tree is derived encompassing all converter types. Selected converters in each class are analysed, simulated and key operational characteristics identified. Issues associated with the practical implementation of analysed topologies are discussed in detail. Of all AC-AC conversion techniques, it is concluded that softswitching converter topologies offer the most attractive alternative to the standard hard switched converter in the power range up to 100kW because of their high performance to cost ratio. Of the softswitching converters, resonant dc-link topologies are shown to produce the poorest output performance although they offer the cheapest solution. Auxiliary pole commutated inverters, on the other hand, can achieve levels of performance approaching those of the hard switched topology while retaining the benefits of softswitching. It is concluded that the auxiliary commutated resonant pole inverter (ACPI) topology offers the greatest potential for exploitation in spite of its relatively high capital cost. Experimental results are presented for a 20kW hard switched inverter and an equivalent 20kW ACPI. In each case the converter controller is implanted using a digital signal processor. For the ACPI, a new control scheme, which eliminates the need for switch current and voltage sensors, is implemented. Results show that the ACPI produces lower overall losses when compared to its hardswitching counterpart. In addition, device voltage stress, output dv/dt and levels of high frequency output harmonics are all reduced. Finally, it is concluded that modularisation of the active devices, optimisation of semiconductor design and a reduction in the number of additional sensors through the use of novel control methods, such as those presented, will all play a part in the realisation of an economically viable system.

No description available.

Recently, the formation of charge density wave in 3D structure, Lu5Ir4Si10, had been observed in the X-ray diffraction experiment. At the same time, the transition in Lu5Ir4Si10 was thought to be first-order due to the spike-shaped anomaly in specific heat. The first-order transition usually accompanies with thermal hysteresis. In order to clarify this problem, we measure and analyze the specific heat result of Lu5Ir4Si10. As a matter of fact, we don¡¦t observe the thermal hysteresis behavior within the resolution of our apparatus, and we think the formation of CDW in Lu5Ir4Si10 is strong interchain coupling. In addition, we also perform the resistivity, magnetic susceptibility and specific heat measurements under zero and external field on the isostructure component Lu5Rh4Si10 for comparison, which has also been thought to undergo a CDW transition. We indeed observe the thermal hysteresis behavior no matter on resistivity, susceptibility or on specific heat results, and this phenomenon doesn¡¦t have magnetic effects. The thermal hysteresis features in Lu5Rh4Si10 are attributed to the presence of metastable states due to the pinning of the CDW phase to impurities, and we also discuss some possibilities about it. The specific heat measurement in our research is performed with an ac calorimetry, using chopped light as a heat source. The details of this technique are also discussed.

M. Tech. Electrical Engineering. / The main focus of this dissertation is to present the modelling and simulation of the Centurion suburb of the Tshwane Municipality electric power system using MATLAB/SIMULINK package. The basic system is tested under large and small disturbances to study the dynamic behaviour of the system and the stability margins associated with the different configurations of the system.

In the last years, the planning of optimised electric power systems, in addition to a more efficient operation of already installed systems, has resulted in an appealing development of controllers which make transmission systems more flexible. The acronym FACTS (Flexible AC Transmission Systems) has become a synonym of electronic-based controllers that perform a quick and safe control of system parameters. A large number of FACTS controllers have been proposed, most of them already in operation. The construction of a comprehensive controller, which would provide simultaneous control of several system parameters at a reduced cost and size, has been the challenge addressed to power engineering designers. This thesis presents the Unified Voltage Controller (UVC), which is an alternative to the control of power flow. The UVC is based on the control of local and remote voltage vectors of the transmission line ends. The concept of the parameters involved in power transfer is discussed. Equations of space vector control for the UVC are defined, and the pulse-width modulation to control the UVC converters is presented. A simplified three-phase system is simulated to show the efficacy of the UVC when operating as a conditioner of power systems. Several simulations have been developed to show the efficiency of the UVC. The control of the local and the remote voltage magnitude, besides the control of transmission angle have been illustrated through computational simulation.

Ethylene propylene rubber (EPR) cable has been extensively used for distribution of power. The insulation of cable should withstand electrical, thermal, mechanical, and chemical stresses during its operation. It is imperative to measure the data providing dielectric strength of EPR cable for these stresses. Significant improvements on the quality of insulation have been progressing for better performance of cable under these stresses. This study deals with ac voltage stress imposed on the cable. The electrical breakdown strength of 5kV EPR cable under ac voltage has been measured by constructing a suitable test set in Mississippi State University High Voltage Lab.

Chemical sensors based on arrays of polymer-coated surface acoustic wave (SAW) devices are required for defense applications that provide a combination of sensitivity, selectivity, portability and response time. The primary challenge in the development of these polymer-based microsensors is the need to reproducibly deposit high quality, defect-free polymer coatings onto microelectrodes. Coating methods such as air brushing and solvent casting have proven unreliable and I have investigated the possibility of depositing polymer films on microelectrodes using electrostatic processing methods. In this work AC electrospraying was used to deposit nanoscale polymer films onto the surface of microelectrodes. The alternating polarity of the electric field eliminates surface charge accumulation and the polymers were deposited uniformly across both electrically insulating and conducting surface regions. In a different work, DC electrospraying was used to deposit patterned organic coatings onto the surface of microelectrodes. The surface of the microelectrode array consisted of an alternating pattern of insulating, grounded-metallic and ungrounded metallic regions, each with a width of 15µm. The charged particles were deposited only onto the grounded-metallic surface regions where there is an electrical path for charge dissipation. No polymer deposition was observed on the insulating or ungrounded-metallic regions due to the effects of surface charge accumulation. Also, I, DC electrodeposited organic molecules within the pores of ceramic film. Due to electrospraying, this film has a strong built-in electric field that induces Stark effect in the organic molecules, providing a unique new technology for bio and chemical sensing. Electrospinning has been used to produce polymer nanofibers with diameters ranging from a few microns to less than 100 nanometers. Due to mechanical oscillations of the electrically charged fibers during electrospinning, they are usually collected in the form of a non-woven mat without any significant fiber orientation. I have developed a new method for making highly aligned arrays of polymer nanofibers by using an AC coupled DC field to drive the electrospinning process. This new "biased AC electrospinning" method can be used to deposit aligned arrays of polymer nanofibers onto virtually any substrate. Potential applications of well-ordered nanofiber materials include tissue engineering, filtration, drug delivery and microelectronics.

In electrical power distribution and power electronic applications, a transformer is an indispensable component which performs many functions. At its operating frequency (60/50 Hz), it is one of the most bulky and expensive components. The concept of the electronic transformer introduced previously has shown considerable reduction in size, weight, and volume by operating at a higher frequency. In this dissertation, the concept of the electronic transformer is further extended to the auto-connected phase-shifting type to reduce harmonics generated by nonlinear loads. It is shown that with the addition of primary side and secondary side AC/AC converters achieves phase-shifting. With the addition of converters, magnetic components are operated at a higher frequency to yield a smaller size and weight. Two types of auto-connected electronic transformer configurations are explored. In the first configuration, the secondary converter is eliminated and the output is suitable for rectifier type loads such as adjustable speed drives. In the second configuration, the secondary converter is added to obtain a sinusoidal phase-shifted AC output voltage. This approach is applicable in general applications. With the proposed approaches, the th and 7th harmonic in utility line currents, generated by two sets of nonlinear loads, are subtracted within the electronic transformer, thereby reducing the total harmonic distortion (THD) of the line current. The analysis and simulation results are presented. In the second part of the dissertation, the electronic transformer concept is applied to a telecommunication power supply (-48 VDC) system. The proposed approach consists of a matrix converter to convert the low frequency three-phase input AC utility to a high frequency AC output without a DC-link. The output of the matrix converter is then processed via a high frequency isolation transformer to produce -48 VDC. Digital control of the system ensures that the output voltage is regulated and the input currents are of high quality, devoid of low frequency harmonics and at near unity input power factor under varying load conditions. Due to the absence of DC-link electrolytic capacitors, the power density of the proposed rectifier is shown to be higher. Analysis, design example and experimental results are presented from a three-phase 208 V, 1.5 kW laboratory prototype converter.

No description available.