OFFSHORE wind farms need to develop technologies that fulfill three main objectives:Efficiency, power density and reliability. The purpose of this thesisis to study an HVDC transmission system based on series connection of the turbineswhich theoretically meet these three objectives. A new topology of matrixconverter operated at high frequency is proposed. This converter is studied usingdifferent modulation algorithms. Simulation and experimental results demonstratedthat the converter can be operated as a current source converter with highefficiency. An optimal control based on a linear quadratic regulator is proposedto control the matrix converter as well as the converter placed on shore. Resultsdemonstrated the high performance of this type of control and its simplicity forimplementation. An stationary state study based on non-linear programming andMontecarlo simulation was carried out to determine the performance of the conceptfor long-term operation. Series connection is an efficient technology if and only ifthe differences in the effective wind velocity are small. This aspect limits the numberof wind turbines that can be connected in series, since a numerous number ofturbines will lead to high covariances in the distribution of the wind. A complementarystudy about active filter and reactive power compensation was carried outusing an optimization-based algorithm.

As the costs of large-scale power generation and transmission rise, distributed generation is becoming a prevalent alternative used by a growing number of both residences and businesses. Distributed generation systems typically consist of two main components: a small-scale, often high-efficiency or renewable power source, such as a fuel cell, solar panel, or wind turbine, and a power electronic converter to convert the raw power produced by the source to a usable form. In North America, the majority of power used in residential and light commercial locations is provided in a form known as single-phase three-wire, or split-phase. This consists of two half-phase AC voltages, each of 110 to 120V rms, and one combined AC voltage of 220 to 240V rms. It is therefore necessary for distributed generation systems to supply power in this same form so that it can be used by standard loads such as lighting or appliances, and the excess power can be fed back into the distribution grid. The most common type of converter used to make this conversion is the voltage-sourced inverter (VSI). There are, however, some advantages to using a current-sourced inverter (CSI) instead. These include improved output voltage waveform quality, built-in voltage boost, and built-in overcurrent protection. However, there are also two obstacles that have prevented the adoption of current-sourced inverters to date. The first obstacle to the use of current-sourced inverters is that they require a DC current input to operate. Therefore, a circuit and control algorithm must be developed to produce a DC current from a low DC voltage source. The first part of this thesis deals with the generation of a suitable DC current. The second major obstacle to adopting current-sourced inverters is that no algorithm for producing single-phase three-wire outputs with a CSI presently exists in literature. The second part of this thesis develops such a switching algorithm, using a three-leg current-sourced inverter. The algorithm is demonstrated using simulation and experimental results, which show that the proposed system is able to successfully generate balanced output voltages under unbalanced loading conditions while equalizing switch utilization and minimizing output voltage ripple.

As the costs of large-scale power generation and transmission rise, distributed generation is becoming a prevalent alternative used by a growing number of both residences and businesses. Distributed generation systems typically consist of two main components: a small-scale, often high-efficiency or renewable power source, such as a fuel cell, solar panel, or wind turbine, and a power electronic converter to convert the raw power produced by the source to a usable form. In North America, the majority of power used in residential and light commercial locations is provided in a form known as single-phase three-wire, or split-phase. This consists of two half-phase AC voltages, each of 110 to 120V rms, and one combined AC voltage of 220 to 240V rms. It is therefore necessary for distributed generation systems to supply power in this same form so that it can be used by standard loads such as lighting or appliances, and the excess power can be fed back into the distribution grid. The most common type of converter used to make this conversion is the voltage-sourced inverter (VSI). There are, however, some advantages to using a current-sourced inverter (CSI) instead. These include improved output voltage waveform quality, built-in voltage boost, and built-in overcurrent protection. However, there are also two obstacles that have prevented the adoption of current-sourced inverters to date. The first obstacle to the use of current-sourced inverters is that they require a DC current input to operate. Therefore, a circuit and control algorithm must be developed to produce a DC current from a low DC voltage source. The first part of this thesis deals with the generation of a suitable DC current. The second major obstacle to adopting current-sourced inverters is that no algorithm for producing single-phase three-wire outputs with a CSI presently exists in literature. The second part of this thesis develops such a switching algorithm, using a three-leg current-sourced inverter. The algorithm is demonstrated using simulation and experimental results, which show that the proposed system is able to successfully generate balanced output voltages under unbalanced loading conditions while equalizing switch utilization and minimizing output voltage ripple.

The purpose of this thesis is to design hardware and a control strategy for a fuel cell/battery hybrid power supply. Modern fuel cell/battery hybrid power supplies can have 2 DC/DC converters: one converter for the battery and one for the fuel cell. The hardware for the power supply proposed in this thesis consists of a single DC/DC buck converter at the output terminals of the fuel cell. The battery does not have a DC/DC converter, and it is therefore passive in the system. The use of one single converter is attractive, because it reduces the cost of this power supply. This thesis proposes a method of controlling the fuel cell's DC/DC buck converter to act as a current source instead of a voltage source. This thesis will explain why using the fuel cell's buck converter to act as a current source is most appropriate. The proposed design techniques for the buck converter are also based on stiff systems theory. Combining a fuel cell and a battery in one power supply allows exploitation of the advantages of both devices and undermines their disadvantages. The fuel cell has a slow dynamic response time, and the battery has a fast dynamic response time to fluctuations in a load. A fuel cell has high energy density, and a battery has high power density. And the performance of the hybrid power supply exploits these advantages of the fuel cell and the battery. The controller designed in this thesis allows the fuel cell to operate in its most efficient region: even under dynamic load conditions. The passive battery inherits all load dynamic behavior, and it is therefore used for peaking power delivery, while the fuel cell delivers base or average power. Simulations will be provided using MATLAB/Simulink based models. And the results conclude that one can successfully control a hybrid fuel cell/battery power supply that decouples fluctuations in a load from the fuel cell with extremely limited hardware. The results also show that one can successfully control the fuel cell to operate in its most efficient region.

The Finite-Difference Time Domain (FDTD) method is a very useful numerical simulation technique for solving problems related to electromagnetism. However, as the traditional FDTD method is based on an explicit finite-difference algorithm, the Courant-Friedrich-Levy(CFL) stability condition must be satisfied when this method is used. Therefore, a maximum time-step size is limited by minimum cell size in a computational domain, which means that if an object of analysis has fine scale dimensions, a small time-step size creates a significant increase in calculation time. Alternating-Direction Implicit (ADI) method is based on an implicit finite-difference algorithm. Since this method is unconditionally stable, it can improve calculation time by choosing time-step arbitrarily. The ADI-FDTD is based on an Alternating direction implicit technique and the traditional FDTD algorithm. The new method can circumvent the stability constraint. In this thesis, we incorporate Lumped Element and Equivalent Current Source method into the ADI-FDTD. By using them to simulate active or passive device, the application of method will be more widely.

The primary design criterion for RF transmit coils for MRI is uniform transverse magnetic (B1) field. Currently, most high frequency transmit coils are designed as periodic, symmetric structures that are resonant at the imaging frequency, as determined by the static magnetic (B0) field strength. These coils are excited by one or more voltage sources. The distribution of currents on the coil elements or rungs is determined by the symmetry of the coil structure. At field strengths of 3T and above, electric properties such as the dielectric constant and conductivity of the load lead to B1 field inhomogeneity due to wavelength effects and perturbation of the coil current distribution from the ideal. The B1 field homogeneity under such conditions may be optimized by adjusting the amplitudes and phases of the currents on the rungs. However, such adjustments require independent control of current amplitudes and phases on each rung of the resonant coil. Due to both the strong coupling among the rungs of a resonant coil and the sensitivity to loading, such independent control would not be possible and B1 homogeneity optimization would involve a time consuming and impractical iterative procedure in the absence of exact knowledge of interactions among coil elements and between the coil and load. This dissertation is based on the work done towards the design and development of a RF current source that drives high amplitude RF current through an integrated array element. The arrangement is referred to as a current element. Independent control of current amplitude and phase on the current elements is demonstrated. A non-resonant coil structure consisting of current elements is implemented and B1 field pattern control is demonstrated. It is therefore demonstrated that this technology would enable effective B1 field optimization in the presence of lossy dielectric loads at high field strengths.

With fast development of semiconductor industry, the transistors in microprocessors increase dramatically, which follows the Moore’s law. As a result, the operating voltages of the future microprocessors follow the trend of decreasing (sub 1V) while the demanding currents increase (higher than 100A). Furthermore, the high slew rates during the transient will reach 1200 A/us. All these impose a serious challenge on a Voltage Regulator (VR) or Voltage Regulator Module (VRM). In order to meet requirements of the next generation microprocessors, four new ideas are proposed in this thesis. The first contribution is an accurate analytical loss model of a power MOSFET with a Current-Source Driver (CSD). The impact of the parasitic components is investigated. Based on the proposed loss model, a general method to optimize the CSD is presented. With the proposed optimization method, the CSD improves the efficiency from 79.4% using the conventional voltage source driver to 83.6% at 12V input, 1.5V/30A output and 1MHz. The second contribution is a new continuous CSD for a synchronous buck converter. The proposed CSD is able to drive the control and Synchronous Rectifier (SR) MOSFETs independently with different drive currents enabling optimal design. At 12V input, 1.5 V/30A output and 1MHz, the proposed CSD improves the efficiency from 79.4% using a conventional voltage source driver to 83.9%. The third contribution is a new discontinuous CSD. The most important advantage of the new CSD is the small inductance (typically, 20nH at 1MHz switching frequency). A hybrid gate drive scheme for a synchronous buck converter is also proposed. The idea of the hybrid gate driver scheme is to use the CSD to achieve switching loss reduction for the control MOSFET, while use the conventional voltage source driver for the SR. At 12V input, 1.3V/25A output and 1MHz, the proposed CSD improves the efficiency from 80.7% using the voltage source driver to 85.4%. The final contribution is new self-driven zero-voltage-switching (ZVS) non-isolated full-bridge converters for 12V input VRM applications. The proposed converter achieves the duty cycle extension, ZVS operation and SRs gate energy recovery. At 12V input, 1.3V output and 1MHz, the proposed converter improves the efficiency from 80.7% using the buck converter to 83.6% at 50A. / Thesis (Ph.D, Electrical & Computer Engineering) -- Queen's University, 2009-04-23 08:59:12.699

As is predicted by Moore’s law, the transistors in microprocessors increase dramatically. In order to increase the power density of the microprocessors, the switching frequency of the Voltage Regulator (VR) is expected to increase to MHz level. However, the frequency dependent loss will increase proportionally. In order to meet requirements of the next-generation microprocessors, three new ideas are proposed in this thesis. The first contribution is a new bipolar Current Source Driver (CSD) for high frequency power MOSFET. The proposed CSD alleviates the gate current diversion problem of the existing CSDs by clamping the gate voltage to a flexible negative value during turn off transition. Therefore, the proposed driver turns off the MOSFET much faster. For buck converters with 12 V input at 1MHz switching frequency, the proposed driver improves the efficiency from 80.5% using the existing CSD to 82.5% at 1.2V/30A, and at 1.3V/30A output, from 82.5% to 83.9%. The second contribution is an accurate analytical loss model of a power MOSFET with a CSD. The current diversion problem that commonly exists in CSDs is investigated mathematically. The inductor value of the CSD is optimized to achieve minimum loss for the synchronous buck converter. The experimentally measured loss matches the calculated loss very well. The efficiency with the optimal CSD inductor is improved from 86.1% to 87.6% at 12V input, 1.3V/20A output in 1MHz switching frequency and from 82.4% to 84.0% at 1.3V/30A output. The third contribution is a new inductorless bipolar gate driver for control FET of buck converters. The most important advantage of the driver presented in this thesis is that it can turn off the power MOSFETs with a negative voltage, which will significantly reduce the turn off time and thus switching loss. In addition, the proposed bipolar gate driver has no inductor in the driver circuit; therefore it can be fully integrated into a chip. For buck converter with 5V input, 1.3V/25A load, in 2 MHz frequency, the proposed gate driver increases the efficiency from 75.8% to 77.8% and from 72.9% to 76.5% at 5V input, 1.3V/25A load, in 2.5 MHz switching frequency. / Thesis (Master, Electrical & Computer Engineering) -- Queen's University, 2010-07-30 14:06:04.003

No description available.

Line current distortion is an important issue to a high-power current source rectifier(CSR) system. There are two main challenges related to this issue. First, the CSR input LC resonance can be affected by the variation of the source inductance from the power system and the effects of the CSR DC side circuit, which may lead to a line current distortion higher than expected. Another challenge is that the traditional high-power CSR using Selective Harmonic Elimination (SHE) pulse-width modulation (PWM) technique attempts to eliminate certain harmonics in the PWM current, which limits its ability for line current harmonic control. To control the CSR line current harmonics, this thesis focuses on two aspects: 1) the analysis and design of CSR input filter to avoid unexpected input LC resonance, and 2) the development of a new PWM scheme that can compensate the effects of the grid voltage harmonics and DC link current ripples. The thesis work has been validated by simulations and on an experimental CSR prototype. / Power Engineering and Power Electronics