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A measurement method of small-capacitance using transient waveforms is proposed in this thesis. A pi-circuit is used to express the stray capacitors between terminals and those from each terminal to ground. Two measuring modes, differential and common modes, are required to obtain the parameters of the circuit. The parameters are determined by transient current waveforms of the modes with an applied voltage, i.e., the open circuited voltage at the end of the current injection cable. The parameters of the pi-type circuit are obtained from a slope of the transient current waveforms or a waveform fitting by a nonlinear method. These methods enable the derivation without a voltage measurement by a probe connecting across the small capacitance, since the parasitic capacitance of the voltage probe obscures the small capacitance. / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University

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Multiphase flows are simultaneous flows of two or more immiscible fluids in a pipe or vessel. Multiphase flows occur in a wide variety of industrial applications, such as chemical reactors, power generation, oil and gas production or transportation, etc. In most of these applications, efficiency and process reliability depend not insignificantly on the composition and flow morphology of these multiphase flows. Therefore, accurate determination of parameters such as phase fractions and their spatial distribution, as well as measurement of volumetric or mass flow rates, is essential to optimize and ensure correct operation of the equipment. For a better prediction of flow characteristics of multiphase systems, the development and validation of analytical models and CFD codes for simulations of multiphase flows has been promoted for some time in thermofluid dynamics research. For this purpose, the in-depth analysis of multiphase flows with high spatial and temporal resolution is essential. However, to date, there is no universal sensor that can directly measure all the required flow parameters over the full range of all flow conditions. Therefore, several strategies have been developed to solve this problem. For pure measurement of fluid composition and mixture volume flow, for example, the fluid mixture is often conditioned before measurement by separation into individual phases or by homogenization. However, this does not allow any more information about the flow morphology. In situations where the fluid cannot be preconditioned, for example when investigating bubble size distributions or predicting plug flows, imaging techniques such as wire-mesh sensors therefore play an important role because they provide cross-sectional images of the flow in rapid succession. This information can be used to determine phase distributions and identify flow regimes, which in turn can serve as input to other sensors to find optimal operating points. In addition, such information is important for validating models and numerical simulations. Although wire-mesh sensors are very attractive and now widely used due to their high spatial and temporal resolution, the measurement signals obtained from the sensor can be corrupted by energy losses and channel crosstalk under certain conditions. Therefore, a better understanding of the real physical conditions when using wire-mesh sensors is essential to improve the measurement accuracy and to extend the range of applications, e.g., for the measurement of media with very high conductivities or for an accurate quantification of individual phases in three-phase flows. In the present work, the current limitations of existing wire-mesh sensor systems are investigated in detail, thus providing a basis for technical improvements and the development of new methods for better interpretation of the measured values of wire-mesh sensors. For this purpose, the electronic measurement principle and the real sensor geometries are first investigated with respect to inherent energy losses and channel crosstalk. Based on mixing models, a method for visualization and quantification of three-phase gas-oil-water flows even in the presence of dispersions is presented. In addition, nonlinearities of wire-mesh sensors are predicted for the first time by a hybrid model based on the finite element method, which also incorporates the real parameters of the electronic components of signal generation and measurement. This model is subsequently used to generate synthetic data and to test new correction methods. Finally, two methods are proposed to compensate for unavoidable energy losses. The first method allows inherent determination of energy losses that cannot be suppressed by further circuit optimization. The second method allows determination of the voltage drop caused by the impedance of the electrodes when measured in highly conductive liquids. Numerical and experimental analyses show an improvement in the measurement accuracy of wire-mesh sensors with respect to the average and local phase fractions. The deviations of the average phase fraction were reduced from more than 15% to less than 2% and the deviations in local measurements from more than 30% to less than 5%.:Abstract 3 Zusammenfassung 5 Statement of authorship 9 Acronyms 13 Symbols 15 1. Introduction 17 2. State of the science and technology 21 3. Wire-mesh sensor and experimental test facilities 43 4. Three-phase flow measurement based on dual-modality wire-mesh sensor 53 5. Wire-mesh sensor model based on finite-element method and circuit simulation 67 6. Analysis of non-linear effects in measurements of wire-mesh sensor 79 7. Methods for improving the measurement accuracy of wire-mesh sensors 87 8. Conclusions and outlook 97 Bibliography 101 Appendices 111 A. List of scientific publications 113

Over the last two decades there has been an increased use of three-phase AC power systems that may not be connected to the main power grid, such as the power systems on more-electric airplane and all-electric ships. Power-electronic converters are usually a significant part of these systems, which provide excellent performance. But their negative incremental impedance nature increases the possibility of system instability. A small-signal analysis that uses interface impedances defined in the synchronous frame is developed by Belkhayat at Purdue in the mid-90s to access the system stability. The system impedance varies with the operating point. Thus the impedance has to be obtained online at the desired operating point, on even in situ. Literature investigates its use with system models, but the lack of equipment to measure such impedance prevents its use in practical systems. Measurement of impedances of each component enables the prediction of system stability before building the real system. The impedance data can also be used to investigate the instability in the system after it is built. The capability of impedance measurement can save the cost and time of system integrators. After reviewing the state-of-the-art development of impedance measurement systems, the dissertation analyzes several systematical error sources in the system, which includes the signal processing and sampling circuits, the phase estimation for coordinate transformation and the injection device connection, and proposes the solution to reduce their influence. Improved algorithm and system architecture are proposed to increase the measurement speed and accuracy. Chirp signal is used as an excitation signal to extract impedances at a group of frequencies at one time. The use of both shunt current injection and series voltage injection improves the SNR of measured signal. Oversampling, cross-correlation and frequency domain averaging technique are used to further reduce the influence of noise. An instrument is built based on the proposed solution. A voltage source inverter is used to generate the perturbation. A PXI computer is used for real-time signal processing. A PC is used for data post processing and measurement process control. Software is developed to fully automate the measurement. The designed unit is tested with various linear and nonlinear load. The test result shows the validity of the proposed solution. / Ph. D.

Synchronous machines (SMs) are a vital part of today's world, and precise modeling is important for studying their stability. In this thesis, a small-signal analysis is done on the d-q frame for the AC SM. Starting from formulating the SM's abc frame equations, a d-q transformation is done based on the SM rotor frame so that the inductance from the abc frame representation will be fixed. Then, the SM's fundamental parameters are obtained from standstill frequency response testing. In the interest of having the most complete model possible, a governor controller and an exciter controller have been designed, and their performance has been evaluated according to the International Standard ISO-8628. A d-q steady-state analysis has been carried out and the resulting small-signal perturbation has been added to the steady-state equations. The model has been analyzed with exciter control only, with governor control only, and with both controllers, and in the end, the small-signal d-q impedance model for the SM has been verified by simulations and experiments in the Center for Electronics Systems (CPES) at Virginia Tech. The impedance measurement unit (IMU) was built by CPES. It is designed for measuring three-phase AC power systems and DC systems. Nevertheless, even though the single-phase system can be connected to the IMU, the impedance result in the end is not correct. Modifications have been made to the IMU so that it is able to calculate the single-phase AC power system impedance. The experimental results demonstrate that implementation has been completed. / Master of Science / Emergency diesel generators are critical in the event of widespread blackouts. Generators are typically synchronous machines (SMs), and stability studies are crucial from the standpoint of the power system. With knowledge gained through stability analysis, if instability occurs, a solution can be quickly formulated based on phenomena that has been observed during the event. In order to study the system stability, an accurate model is essential. The small-signal d-q impedance model will be derived by the Center for Power Electronics Systems (CPES) at Virginia Tech based on an AC generator that is rated at 28 kW, and the impedance results will be verified by both MATLAB/Simulink simulations and experiments. The impedance measurement unit (IMU) will serve as the equipment to do the measurement. However, the IMU can only correctly calculate impedance for a three-phase power system. For the single-phase AC power system, due to the three-phase phase locker loop (PLL) calculating the wrong angle, the IMU cannot properly determine the impedance. However, modifications have been applied to the IMU so that it can automatically calculate the impedance for a single-phase AC power system.

This dissertation presents work conducted at the Center for Power Electronics Systems (CPES) at Virginia Polytechnic Institute and State University. Chapter 1 introduces the concept of impedance measurement, and discusses previous work on this topic. This chapter also addresses issues associated with impedance measurement. Chapter 2 introduces the analyzer architecture and the proposed algorithm. The algorithm involves locking on to the voltage vector at the point of common coupling between the analyzer and the system via a PLL to establish a D-Q frame. A series of sweeps are performed, injecting at least two independent angles in the D-Q plane, acquiring D- and Q-axis voltages and currents for each axis of injection at the point of interest. Chapter 3 discusses the analyzer hardware and the criteria for selection. The hardware built ranges from large-scale power level hardware to communication hardware implementing a universal serial bus. An eight-layer PCB was constructed implementing analog signal conditioning and conversion to and from digital signals with high resolution. The PCB interfaces with the existing Universal Controller hardware. Chapter 4 discusses the analyzer software. Software was written in C++, VHDL, and Matlab to implement the measurement process. This chapter also provides a description of the software architecture and individual components. Chapter 5 discusses the application of the analyzer to various examples. A dynamic model of the analyzer is constructed, considering all components of the measurement system. Congruence with predicted results is demonstrated for three-phase balanced linear impedance networks, which can be directly derived based on stationary impedance measurements. Other impedances measured include a voltage source inverter, Vienna rectifier, six-pulse rectifier and an autotransformer-rectifier unit. / Ph. D.

Bibliography : leaves 237-251. / x, 251 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Mechanical Engineering, 1994

This thesis targets on design of portable device for impedance measurement. Theoretical knowledge related to the construction of a portable impedance analyzer is summarized in a separate chapter. This section also compares some commercially available impedance analyzers. In addition, this thesis contains a description of the principle of operation of the impedance analyzer, which is divided into individual function blocks and supplemented by control circuits. A large part of the work focuses on the design of the analog measurement circuit, where suitable measuring methods are also presented. The functionality of the analog blocks of the device was verified by circuit simulation in the PSpice program and by experimental tests. Selection of some components is mentioned in the relevant chapters, where the essential parameters from datasheets are listed. The output of the work deals with a designed prototype of the device, which was tested. Based on the findings some modifications of the device were proposed for practical usability. The attachment of the thesis contains final version of designed printed circuit board.

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