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[pt] O escoamento trifásico na indústria do petróleo é caracterizado pela presença das fases gás, óleo e água. A presença da terceira fase (água) traz complexidade a esse processo, visto que pode provocar a formação de diversos novos padrões de escoamento, além dos já conhecidos para escoamento bifásico. Adicionalmente, a presença de uma fase líquida dispersa na outra pode formar uma emulsão, alterando significativamente a viscosidade e, assim, influenciando diretamente na perda de carga. O foco do presente trabalho é na previsão do padrão de golfadas com dispersões de água e óleo utilizando um modelo transiente unidimensional de Dois Fluidos. A presença da água é modelada através da solução da equação de conservação de massa para a fase água. Visando prever com precisão a queda de pressão, assim como a distribuição das frações volumétricas de cada fase ao longo do domínio, desenvolveu-se um modelo de fechamento algébrico para avaliar o escorregamento entre as fases líquidas. Com o modelo proposto, os resultados obtidos para a velocidade de escorregamento no escoamento água/óleo foram comparados com dados experimentais e de outros modelos, apresentando um excelente desempenho. O modelo foi então utilizado para analisar o escoamento trifásico no padrão de golfadas. As previsões para a queda de pressão e características das golfadas (comprimento, frequência e velocidade de translação) foram comparadas com dados experimentais da literatura e os resultados são promissores. / [en] The three-phase flow in the oil industry is characterized by the presence of the gas, oil and water phases. The presence of the third phase (water) adds complexity to this process, since it can cause the formation of several new flow patterns in addition to those already known for two-phase flow. Additionally, the presence of a dispersed phase into another can form an emulsion, altering significantly the viscosity and consequently influencing directly the pressure drop. The focus of this job is in the prediction of the slug flow with dispersions of water and oil using a one-dimensional transient Two Fluid model. The presence of water in the flow is modelled with the solution of an equation of conservation of mass. Intending to predict with precision the pressure drop, as the volumetric phase distribution of each phase throw the domain, an algebraic closure model was inserted to assess the slip between the liquid phases. With the proposed model, the results obtained for the slip velocity of the water/oil flow were compared with experimental data and other models, showing excellent performance. The model was then used to analyze the three-phase flow in the slug pattern. The predictions for pressure drop and characteristics of the slugs (length, frequency and translation velocity) have been compared with experimental data from the literature and the results are promising.

Yes / This paper presents a fault detection method in three-phase induction motors using Wavelet Packet Transform (WPT). The proposed algorithm takes a frame of samples from the three-phase supply current of an induction motor. The three phase current samples are then combined to generate a single current signal by computing the Root Mean Square (RMS) value of the three phase current samples at each time stamp. The resulting current samples are then divided into windows of 64 samples. Each resulting window of samples is then processed separately. The proposed algorithm uses two methods to create window samples, which are called non-overlapping window samples and moving/overlapping window samples. Non-overlapping window samples are created by simply dividing the current samples into windows of 64 sam-ples, while the moving window samples are generated by taking the first 64 current samples, and then the consequent moving window samples are generated by moving the window across the current samples by one sample each time. The new window of samples consists of the last 63 samples of the previous window and one new sample. The overlapping method reduces the fault detection time to a single sample accuracy. However, it is computationally more expensive than the non-overlapping method and requires more computer memory. The resulting window sam-ples are separately processed as follows: The proposed algorithm performs two level WPT on each resulting window samples, dividing its coefficients into its four wavelet subbands. Infor-mation in wavelet high frequency subbands is then used for fault detection and activating the trip signal to disconnect the motor from the power supply. The proposed algorithm was first implemented in the MATLAB platform, and the Entropy power Energy (EE) of the high frequen-cy WPT subbands’ coefficients was used to determine the condition of the motor. If the induction motor is faulty, the algorithm proceeds to identify the type of the fault. An empirical setup of the proposed system was then implemented, and the proposed algorithm condition was tested under real, where different faults were practically induced to the induction motor. Experimental results confirmed the effectiveness of the proposed technique. To generalize the proposed meth-od, the experiment was repeated on different types of induction motors with different working ages and with different power ratings. Experimental results show that the capability of the pro-posed method is independent of the types of motors used and their ages.

Rising fuel costs and concerns about air pollution have significantly increased interest in electric vehicles (EVs). EVs are equipped with rechargeable batteries that can be fully recharged by connecting to an external electrical source. However, the wider adoption of EVs is hindered by the need for an on-board charger system that is both lightweight and efficient. EVs utilize two main charging methods: on-board chargers (OBC) for regular charging and off-board (fast) chargers for quick refills of battery pack. Most EVs currently use 400V battery packs paired with 6.6kW or 11kW OBCs, while larger vehicles with over 100 kWh battery packs employ 16.5kW or 19.2kW OBCs, constrained by household voltage and current limits. Some manufacturers are transitioning to 800V battery packs to lower costs and enhance fast charging capabilities, necessitating the development of 800V OBCs with high efficiency and power density. For household use, EVs can charge via OBC in a grid-to-vehicle transfer and can supply energy back to the home or grid (vehicle-to-grid) for emergency use or to support smart grid functionalities, requiring bidirectional OBCs. Advanced power semiconductor devices have been instrumental in advancing power conversion technology. The introduction of power semiconductor devices based on wide bandgap (WBG) materials marks a revolutionary shift, offering potential improvements over silicon-based devices. These WBG devices are capable of achieving higher efficiency, and higher power density in power conversion at higher operation frequency. Elevating the switching frequency diminishes the voltage-second across the transformer, facilitating the utilization of printed-circuit-board (PCB) technology for the windings as opposed to Litz wire implementations. Compared to traditional Litz wire-based transformers, the manufacturing process is significantly streamlined, and the management of parasitic is considerably more straightforward. Furthermore, the integration of resonant inductors with PCB-based transformer results in a reduction in the overall number of magnetic components and improved power density. This dissertation focuses on the DC/DC conversion stage of a bi-directional battery charger. It aims to achieve high power density and high efficiency using a PCB-based integrated transformer, enhancing manufacturing processes. The dissertation details the specific accomplishments in this area: Firstly, a two-stage on-board charger structure for 800 V battery EVs is proposed. The first stage is a four-phase bridgeless totem pole AC/DC converter working at critical conduction mode (CRM) so that soft switching can be achieved for all the fast switches. The second stage is single phase CLLC (1PCLLC) converter which is attractive due to its less component counts of devices and driver circuits. A novel matrix integrated transformer with controllable built-in leakage inductance for bi-directional 1PCLLC converter was proposed. Integrating three UI-core-based (1UI-based) elemental transformers with non-perfectly interleaved winding structures into one 3UI-based integrated transformer can reduce the core loss significantly with a smaller footprint compared with three EI-core-based integrated transformers. The proposed integrated magnetics can be scalable for higher voltage and higher power converters by assembling more 1UI-based elemental transformers. A SiC-based 1PCLLC converter prototype operating at 250-kHz switching frequency for 11-kW OBC applications was built with the proposed integrated transformer, and it can achieve a power density of 250 W/in3 with maximum efficiency of 98.4%. Secondly, the challenge of increased common mode (CM) noise after adopting PCB-based windings in the design was discussed. The inter-winding capacitors between the primary and secondary windings act as a conduction path for high dv/dt CM noise, which can lead to electromagnetic interference (EMI) issues. To address this, a winding cancellation method for an integrated matrix transformer in a 1PCLLC converter was proposed and validated. This approach was tested in an 11-kW 1PCLLC converter. The EMI measurement results align with the analysis, confirming the effectiveness of the proposed method, which achieved a reduction in CM noise by 17dB. Furthermore, the 1PCLLC converter, incorporating the proposed planar matrix integrated transformer and winding cancellation technique, attained a power density of 420 W/in³ and a peak efficiency of 98.5%. Thirdly, to enhance efficiency further, the 1PCLLC converter is substituted with the proposed three-phase CLLC (3PCLLC) resonant converter equipped with three-phase rectifiers. The 3PCLLC converter becomes more promising for high power applications as its lower RMS current stress and automatic current sharing capabilities. It can achieve soft switching under all conditions. In addition, due to the symmetrical resonant tank, it is more suitable for bi-directional operation. Variable DC-link voltage is adopted so that the DC/DC stage can always work at its optimized point, providing best efficiency for the entire battery voltage. An improved core structure for the three-phase integrated transformer was proposed to reduce the core loss and simplify the magnetic components by integrating three primary resonant inductors, three secondary resonant inductors and three transformers into one magnetic component. A systematic method of converter design which includes the design of integrated transformer, converter loss optimization was adopted to design an 11kW 3PCLLC resonant converter. A SiC-based 3PCLLC converter prototype operating at 250-kHz switching frequency for 11-kW OBC applications was built with the proposed integrated transformer, and it can achieve a power density of 330 W/in3 with peak efficiency of 98.7%. Fourthly, the power level of OBC continues to increase to make up the large capacitance battery pack inside the EVs to relief the concern of mileage range. To address this challenge of higher power, a scalable matrix integrated transformer for multi-phase CLLC converter was proposed. A universal method of integrating magnetizing inductance with built-in leakage inductance based on multiple perfectly coupled transformers (PCTs). The integration of built-in leakage inductance can be achieved by connecting several PCTs using a standardized core type for cost considerations or can be further integrated into a customized core with interleaved magnetomotive force polarities across transformer legs to achieve better flux distribution and smaller core loss. The proposed concept can be applied to single-input single-output, and multiple-inputs multiple-outputs integrated transformer applications. A 3x3 PCTs-based integrated transformer built with PCB windings was designed for a 3PCLLC resonant converter, which integrates three primary resonant inductors, three secondary resonant inductors, and three transformers into one magnetic core to simplify the complexity of the converter. The effectiveness of the proposed concept was demonstrated through a high-efficiency, high-power density 3PCLLC DC/DC converter for an 800V 16.5kW OBC. The designed converter can achieve a power density of 500 W/in3 and a peak efficiency of 98.8%. / Doctor of Philosophy / Rising fuel costs and concerns about air pollution have significantly increased interest in electric vehicles (EVs). EVs are equipped with rechargeable batteries that can be fully recharged by connecting to an external electrical source. However, the wider adoption of EVs is hindered by the need for an on-board charger system that is both lightweight and efficient. The dissertation presents advances in OBC technology to address these challenges, focusing on the development of efficient, high-power density OBCs suitable for various EV applications. EVs utilize two main charging methods: on-board chargers (OBC) for regular charging and off-board (fast) chargers for quick refills of battery pack. Most EVs currently use 400V battery packs paired with 6.6kW or 11kW OBCs, while larger vehicles with over 100 kWh battery packs employ 16.5kW or 19.2kW OBCs, constrained by household voltage and current limits. Some manufacturers are transitioning to 800V battery packs to lower costs and enhance fast charging capabilities, necessitating the development of 800V OBCs with high efficiency and power density. For household use, EVs can charge via OBC in a grid-to-vehicle transfer and can supply energy back to the home or grid (vehicle-to-grid) for emergency use or to support smart grid functionalities, requiring bidirectional OBCs. Advanced power semiconductor devices have been instrumental in advancing power conversion technology. The introduction of power semiconductor devices based on wide bandgap (WBG) materials marks a revolutionary shift, offering potential improvements over silicon-based devices. These WBG devices are capable of achieving higher efficiency, and higher power density in power conversion at higher operation frequency. Elevating the switching frequency diminishes the voltage-second across the transformer, facilitating the utilization of printed circuit board (PCB) technology for the windings as opposed to Litz wire implementations. Compared to traditional Litz wire-based transformers, the manufacturing process is significantly streamlined, and the management of parasitic is considerably more straightforward. Furthermore, the integration of resonant inductors with PCB-based transformer results in a reduction in the overall number of magnetic components and improved power density. Addressing cost concerns, a novel, cost-effective single-phase converter design was proposed, achieving high efficiency with integrated magnetics. Additionally, the research tackled the challenge of electromagnetic interference (EMI) through a winding cancellation technique, significantly reducing common-mode noise and further improving the converter's performance. The research introduces an improved core structure for a three-phase integrated transformer, significantly reducing core loss and simplifying the design by combining multiple components into a single unit. This approach facilitated the creation of a high-efficiency, SiC-based converter prototype, demonstrating remarkable power density and peak efficiency compared with state-of-the-art solutions. To accommodate the increasing power requirements of OBCs, a scalable, matrix integrated transformer design was developed for multi-phase converters, optimizing cost and performance. This design simplifies the converter architecture, enhancing efficiency and power density, and is adaptable to both single and multiple output applications. These advancements offer promising solutions to the challenges hindering the wider adoption of EVs. The dissertation underscores the potential of advanced power conversion technologies, including the application of WBG devices, integrated magnetics to streamline converter design and enhance both the efficiency and power density of battery chargers.

The generic concept of quasi-single-stage (QSS) power conversion topology for ac-dc rectification and dc-ac inversion is proposed. The topology is reached by direct cascading and synchronized switching of two variety of buck or two variety of boost switching networks. The family of QSS power converters feature single-stage power processing without a dc-link low-pass filter, a unidirectional pulsating dc-link voltage, soft-switching capability with minimal extra commutation circuitry, simple PWM control, and high efficiency and reliability. A new soft-switched single-phase QSS bi-directional inverter/rectifier (charger) topology is derived based on the QSS power conversion concept. A simple active voltage clamp branch is used to clamp the otherwise high transient voltage on the current-fed ac side, and at the same time, to achieve zero-voltage-switching (ZVS) for the switches in the output side bridge. Seamless four-quadrant operation in the inverter mode, and rectifier operation with unity power factor in the charger (rectifier) mode are realized with the proposed uni-polar center-aligned PWM scheme. Single-stage power conversion, standard half-bridge connection of devices, soft-switching for all the power devices, low conduction loss, simple center-aligned PWM control, and high reliability and efficiency are among its salient features. Experimental results on a 3 kVA bi-directional inverter/rectifier prototype validate the reliable operation of the circuit. Other single-phase and three-phase QSS bi-directional inverters/rectifiers can be easily derived as topological extensions of the basic QSS bi-directional inverter/rectifier. A new QSS isolated three-phase zero-voltage/zero-current-switching (ZVZCS) buck PWM rectifier for high-power off-line applications is also proposed. It consists of a three-phase buck bridge switching under zero current and a phase-shift-controlled full-bridge with ZVZCS, while no intermediate dc-link is involved. Input power and displacement factor control, input current shaping, tight output voltage regulation, high-frequency transformer isolation, and soft-switching for all the power devices are realized in a unified single stage. Because of ZVZCS and single-stage power conversion, it can operate at high switching frequency while maintaining reliable operation and achieving higher efficiency than standard two-stage approaches. A family of isolated ZVZCS buck rectifiers are obtained by incorporating various ZVZCS schemes for full-bridge dc-dc converters into the basic QSS isolated buck rectifier topology. Experimental and simulation results substantiate the reliable operation and high efficiency of selected topologies. The concept of charge control (or instantaneous average current control) of three-phase buck PWM rectifiers is introduced. It controls precisely the average input phase currents to track the input phase voltages by sensing and integrating only the dc rail current, realizes six-step PWM, and features simple implementation, fast dynamic response, excellent noise immunity, and is easy to realize with analog circuitry or to integrate. One particular merit of the scheme is its capability to correct any duty-cycle distortion incurred on only one of the two active duty-cycles which often happens in the soft-switched buck rectifiers, another merit is the smooth transition of the input currents between the 60o sectors. Simulation and preliminary experimental results show that smooth operations and high quality sinusoidal input currents in the full line cycle are achieved with the control scheme. / Ph. D.

This dissertation presents control, analysis, and design of silicon carbide (SiC)-based critical conduction mode (CRM) high-frequency soft-switching three-phase ac-dc converters (inverter and rectifier). The soft-switching technique with SiC devices grounded in CRM makes the operation of the ac-dc converter at hundreds of kHz possible while maintaining high efficiency with high power density. This is beneficial for rapidly growing fields such as electric vehicle charging, photovoltaic (PV) systems, and uninterruptable power supplies, etc. However, for the soft-switching technique to be practically adopted to real products in the markets, there are a lot of challenges to overcome. In this dissertation, four types of the challenges are carefully studied and discussed to address them. First, the grid-tied inverters used for distributed energy resources, such as PV systems, must continue operating to deliver power to the grid, when it faces flawed grid conditions such as voltage drop and voltage rise. During abnormal grid conditions, delivering constant active power from the inverter to the grid is essential to avoid large voltage ripples on the dc side because it could trigger over-voltage protection or harm the circuitries, eventually shutting down the inverter. Hence, in such cases, unbalanced ac currents need to be injected into the grid. When the grid voltages and the ac currents are not balanced, there is a chance for the CRM soft-switching inverter to lose its soft-switching capability. Continuous conduction mode operation emerges, causing hard-switching where discontinuous conduction mode (DCM) operation is expected. This leads to huge turn-on loss and high dv/dt noise at the active switch's turn-on moment. To eradicate the hard-switching problem, two improved modulation schemes are developed; one with off-time extension in the CRM phase, the other by skipping switching pulses in the DCM phase. The DCM pulse skipping is applied for a variety of grid imbalance cases, and it is proven that it can be a generalized solution for any kinds of unbalanced grid conditions. Second, the CRM soft-switching scheme with 2-channel interleaving achieves high efficiency at heavy load. Nevertheless, the efficiency plunges as the output load is reduced. This is not suitable for PV inverters, which take account of light load efficiency in terms of "weighted efficiency". Small inductor currents at light load cause the switching frequency to soar because of its CRM-based operation characteristic, causing large switching loss. To increase the inductor current dealt with by the first channel, a phase shedding control is proposed. Gate signals for the second channel are not excited, increasing the first channel's inductor current, thus cutting down the first channel's switching frequency. To prevent the unwanted circulating current formed by shared zero-sequence voltage in the paralleled structure, only two phases in the second channel working in high frequency are shed. The proposed phase shedding control achieves a 0.5 to 3.9 % efficiency improvement with light loads. Third, due to the usage of SiC devices, high dv/dt generated at switching nodes over the system parasitic capacitance causes substantial common mode (CM) noise compared to that with Si devices. In this case, a balance technique with PCB winding inductors can effectively reduce the CM noise. First, winding interleaving structure is selected to minimize the eddy current loss in the windings. But the interwinding capacitance caused by the winding interleaving structure aggravates the CM noise. Impact of the interwinding capacitance on the CM noise is analyzed with a new inductor model containing the interwinding capacitance. Then, finally, a novel inductor structure is proposed to remove the interwinding capacitance and to improve the CM noise reduction performance. The soft-switching ac-dc converter built with the final PCB magnetics features almost similar efficiency compared to that with litz-wire inductor and 14 to 18 dB CM noise reduction up to 15 MHz. Lastly, the soft-switching technique is extended to inverters in standalone mode. To meet tight ac voltage total harmonic distortion requirements, a current control in dq-frame is introduced. As for the ac voltage regulation at no-load, on top of the improved phase shedding control, a frequency limiting with fixed frequency DCM method is applied to prevent excessive increase in the switching frequency. Then, how to deal with short-circuit at the output load is investigated. Since the soft-switching modulation violates inductor voltage-second balance during the short-circuit, the modulation method is switched to a conventional sinusoidal PWM at fixed frequency. It is concluded that all the additional requirements for the standalone inverters can be satisfied by the introduced control strategies. / Doctor of Philosophy / The world is facing an unprecedented weather crisis. Global warming is getting more severe because of excessive amount of carbon emission. In an effort to overcome this crisis, paradigm of energy and lifestyle of people have changed. Penetration of distributed energy resources (DERs) such as wind turbines, and photovoltaic systems has been dramatically increased. Instead of internal combustion engine vehicles (EVs), electric vehicles hit the mainstream. In these changes, power electronics plays a critical role as the key element of the systems. Especially, three-phase inverter/rectifiers are essential parts in such applications. Most important aspects of the three-phase inverter/rectifier are efficiency and power density. In the past decades, Silicon (Si) power devices were mostly used for the systems and the technology based on Si has almost reached to its physical limits. The switching frequency of Si-based inverter/rectifier is limited below 20 – 30 kHz to reduce switching loss. This impedes high power density due to bulky passive components such as inductors and capacitors. Nowadays, the advent of wideband gap such as Silicon Carbide (SiC) and Gallium Nitride (GaN) power devices gives us a great opportunity to improve the efficiency and the power density with its high switching speed capability, low switching energy and low on-resistance. The SiC power devices are more suitable for DERs and EVs due to higher voltage rating. Using SiC power devices allows to increase inverter/rectifier' switching frequency about five times to have similar efficiency with those based on Si power devices, making the power density high. However, there is still room to push the switching frequency even higher to hundreds of kHz with soft-switching. In this sense, studies on soft-switching techniques for three-phase inverter/rectifier have been intensively conducted. Particularly, soft-switching techniques based on critical conduction mode (CRM) are regarded as the most promising solutions because it does not have any additional circuits to achieve the soft-switching, keeping the system as straightforward as possible. However, most of the studies for the CRM-based soft-switching three-phase inverter/rectifier mainly focus on limited occasions such as ideal operation conditions. For this technique to be widely used and adopted in industry, more practical cases for the systems need to be studied. In this dissertation, the soft-switching three-phase inverter/rectifier under diverse situations are investigated in depth. First, behavior of the soft-switching inverter/rectifier under unbalanced grid conditions are analyzed and control methods are developed to maintain its soft-switching capability. Second, how to improve light load efficiency is explored. Circulating current issue for the light load efficiency improvement is analyzed and a control method is proposed to eliminate the circulating current. Third, a design methodology and considerations of inductors based on PCB magnetics are discussed to reduce electromagnetic noise and improve system efficiency. Lastly, the soft-switching technique is extended to standalone mode applications dealing with strict voltage regulation, no-load operation, and output short-circuit.

Rapid expansion of the information technology (IT) sector, market size and consumer interest for off-line power supply continue to rise, particularly for computers, flat-panel TVs, servers, telecom, and datacenter applications. Normal components of an off-line power supply include an electromagnetic interference (EMI) filter, a power factor correction (PFC) circuit, and an isolated DC-DC converter. For off-line power supply, an isolated DC-DC converter offers isolation and output voltage adjustment. For an off-line power supply, it takes up significantly more room than the rest; thus, an isolated DC-DC converter is essential for enhancing the overall performance and lowering the total cost of an off-line power supply. In contrast, data center server power supplies are the most performance-driven, energy-efficient, and cost-aware of any industrial application power supply. The full extent of data centers' energy consumption is coming into focus. By 2030, it is anticipated that data centers will require around 30,000 TWh, or 7.6% of world power usage. In addition, with the rise of cloud computing and big data, the energy consumption of data centers is anticipated to continue rising rapidly in the near future. In data centers, isolated DC-DC converters are expected to supply even higher power levels without expanding their size and with much greater efficiency than the present standard, which makes their design even more challenging. LLC resonant converters are frequently utilized as DC-DC converters in off-line power supply and data centers because of their high efficiency and hold-up capabilities. LLC converters may reduce electromagnetic interference because the primary switches and secondary synchronous rectifiers (SRs) both feature zero-voltage-switching (ZVS) and zero-current-switching (ZCS) for the SRs. Almost every state-of-the-art off-line power supply uses LLC converters in their DC-DC transformations. However, LLC converters face three important challenges. First, the excessive core loss caused by the uneven flux distribution in planar magnetics, owing to the huge size and high-frequency operation of the core. These factors led to the observation of dimensional resonance within the core and an excessive amount of eddy current circulating within the core, which resulted in the generation of high eddy loss within the ferrite material. This was normally assumed to be negligible for small core sizes and lower frequencies. This dissertation proposes methods to help redistribute the flux in the core, particularly in the plates where the majority of core losses are concentrated, and to provide more paths for the flux to flow so that the plates' thickness can effectively be reduced by half and core losses, particularly eddy loss, are reduced significantly. Second, the majority of power supplies in the IT sector are needed to deliver high-current output, but the transformer is cumbersome and difficult to build because of its high conduction losses. In addition, establishing a modular solution that can be scaled up to greater power levels while attaining a superior performance relative to best practices is quite difficult. By increasing the switching frequency to several hundred kilohertz using wide-band-gap (WBG) transistors, printed circuit board (PCB) windings may include magnetics. This dissertation offers a modular and scalable matrix transformer structure and its design technique, allowing any number of elemental transformers to be integrated into a single magnetic core with significantly reduced winding loss and core loss. It has been shown that the ideal power limitations per transformer for PCB-based magnetics beat the typical litz wire design in all design areas, in addition to the unique advantages of PCB-magnetics, such as their low profile, high density, simplicity, and automated construction. Alternatively, shielding layers may be automatically put into the PCB windings between the main and secondary windings during the production process to reduce CM noise. A method of shielding is presented to reduce CM noise. The suggested transformer design and shielding method are used in the construction of a 3 kW 400V/48 V LLC converter, with a maximum efficiency of 99.06% and power density of 530W/in3. Thirdly, LLC converters with a matrix transformer encounter a hurdle for extending greater power, including the number of transformers needed and the magnetic size. In addition to the necessity of resonant inductors, which increase the complexity and size of the magnetic structure, there is a need for a resonant inductor. By interconnecting the three-phases in a certain manner, three-phase interleaved LLC converters may lower the circulating energy, but they have large and numerous magnetic components. In this dissertation, a new topology for three-phase LLC resonant converters is proposed. Three-phase systems have the advantage of flux cancellation, which may be used to further simplify the magnetic structure and decrease core loss. In addition, a study of the various three-phase topologies is offered, and a criterion for selecting the best suitable topology is shown. Compared to the single-phase LLC, the suggested topology has less winding loss and core loss. In addition, three-phase transformers have a lower volt-second rating, and smaller core sizes may be used to mitigate the impact of eddy loss in the ferrite material. In contrast, three-phase systems offer superior EMI performance, which is shown in the loss and size of the EMI filter, and much less output voltage ripple, which is reflected in the size of the output filter. Finally, several methods of integrating resonant inductors into transformer magnetics are presented in order to accomplish a simple, compact, and cost-effective magnetic architecture. By increasing the switching frequency to 500 kHz, all six transformers and six inductors may be achieved using four-layer PCB winding. To decrease CM noise, additional 2-layer shielding may be implemented. A 500 kHz, 6-8 kW, 400V/48V, three-phase LLC converter with the suggested magnetic structure achieves 99.1% maximum efficiency and a power density of 1000 W/in3. This dissertation addresses the issues of analysis, magnetic design, expansion to higher power levels, and electromagnetic interference (EMI) in high-frequency DC/DC converters used in off-line power supply and data centers. WBG devices may be effectively used to enable high-frequency DC/DC converters with a hundred kilohertz switching frequency to achieve high efficiency, high power density, simple yet high-performance, and automated manufacture. Costs will be minimized, and performance will be considerably enhanced. / Doctor of Philosophy / The IT industry, market size, and customer interest in off-line power supply continue to grow quickly, especially for computers, flat-panel TVs, servers, telecom, and datacenter applications. Off-line power supplies usually have a DC-DC converter, an EMI filter, and a PFC circuit. A DC-DC converter is needed for an off-line power supply. An isolated DC-DC converter makes an off-line power supply work better and cost less, even though it takes up more space than the rest. But power supplies for data center servers are the most performance-driven, energy-efficient, and cost-conscious industrial applications. It's becoming clear how much energy data centers use. By 2030, data centers will use 7.6% of the world's power, or 30,000 TWh. With the rise of cloud computing and big data, energy use in data centers is likely to go up by a lot. In data centers, isolated DC-DC converters are expected to have much more power without getting bigger and to be much more efficient than the current standard. This makes their design even harder. LLC resonant converters are often used as DC-DC converters in data centers and off-line power supplies because they are very efficient and easy to control. LLC converters may have less electromagnetic interference because both the primary switches and the secondary synchronous rectifiers (SRs) have zero-voltage-switching (ZVS) and zero-current-switching (ZCS). Almost every modern off-line power supply uses LLC converters for DC-DC stage. LLC converters have to deal with three big problems. Due to the large size of the core and the high frequency of operation, the uneven distribution of flux in planar magnetics causes too much core loss. This dissertation suggests ways to redistribute flux in the core, especially in the plates where most core losses are concentrated and provide more flux paths to reduce plate thickness by half and core losses, especially eddy loss. Second, most IT power supplies need to put out a lot of current, but transformers are bulky and hard to build because they lose a lot of current. It is hard to make a modular solution that can scale up to higher levels of power and perform better than best practices. With wide-band-gap (WBG) transistors, the switching frequency can be raised to several hundred kilohertz so that magnetics can be added to PCB windings. This dissertation describes a modular and scalable matrix transformer structure and design method that lets any number of elemental transformers be put into a single magnetic core with much less winding loss and core loss. PCB-based magnetics have a low profile, a high density, are easy to build, and can be built automatically. Their ideal power limits per transformer beat the typical litz wire design in every way. Shielding layers can be added automatically between the main and secondary PCB windings to cut down on CM noise. CM noise is lessened by shielding. The suggested transformer design and shielding method are used to build a 3 kW 400V/48 V LLC converter with a maximum efficiency of 99.06% and a power density of 530W/in3. Third, LLC converters with matrix transformers can't get more power without more transformers and a bigger magnetic size. Resonant inductors, which add to the size and complexity of a magnetic structure, are also needed. By connecting the three phases, three-phase interleaved LLC converters use less energy, but they have a lot of magnetic parts. In this paper, a three-phase LLC resonant converter topology is proposed. In three-phase systems, flux cancellation makes magnetic structures easier to understand and reduces core loss. There is also a study of three-phase topologies and a set of criteria for choosing one. Compared to the single-phase LLC, the topology cuts down on winding and core loss. Three-phase transformers have a lower volt-second rating, and ferrite material eddy loss can be reduced by making the core smaller. The size and loss of the EMI filter show that three-phase systems have less output voltage ripple and better EMI performance. Finally, several ways of putting resonant inductors into the magnetics of a transformer are shown to make a magnetic architecture that is simple, small, and cheap. At 500 kHz, all six transformers and all six inductors can be wound on a four-layer PCB. CM noise can be cut down with 2-layer shielding. With the suggested magnetic structure, a 500 kHz, 6-8 kW, 400V/48V, three-phase LLC converter can reach 99.1% maximum efficiency and 1000 W/in3. This dissertation presents analysis, magnetic design, expanding to higher power levels, and electromagnetic interference (EMI) in high-frequency DC/DC converters used in off-line power supplies and data centers. WBG devices can be used to make high-frequency DC/DC converters with a switching frequency of a few hundred kilohertz that are powerful, easy to use, and can be automated. Both cost and performance will get better.

A new concept of current sensor reduction technique called multi-branch current sensing technique (MCST) is proposed in this dissertation. In the proposed current sensing method, one more branch currents are simultaneously measured several times in a single switching cycle by using a single current sensor. After that, the current reconstruction algorithm is applied to obtain all phase currents information. Compared to traditional single current sensor techniques (SCSTs), the proposed method samples the output of the current sensor regularly, and the current sensing dead-zone is dramatically reduced. Since the current sampling is performed periodically, its implementation using a digital controller is extremely simple. Moreover, the periodical dead-zone and the dead-zone near the origin of the voltage vector space which have been a big problem in the existing methods can be completely eliminated. Accordingly, there is no need to have a complicated vector reconfiguration or current estimation algorithm. The proposed MCST also takes the advantages of a SCST such as reduced cost and elimination of the sensor gain discrepancy problem in the multiple current sensor method. The fundamental concept, implementation issues, and limitation of the proposed MCST are described based on three-phase systems first. After that, the proposed MCST is adopted to two-phase inverters and multi-phase dc-dc converters with little modifications. Computer simulations and hardware experiments have been conducted for a three-phase boost converter, a three-phase motor drive system, a two-phase two-leg inverter, a two-phase four-leg inverter with bipolar modulation, a two-phase four-leg inverter with unipolar modulation, and a four-phase dc-dc converter applications. From the simulations and the experimental results, the feasibilities of the proposed method mentioned above are fully verified. / Ph. D.

The modern day power system is witnessing a tremendous change. There has been a rapid rise in the distributed generation, along with this the deregulation has resulted in a more complex system. The power demand is on a rise, the generation and trans-mission infrastructure hasn't yet adapted to this growing demand. The economic and operational constraints have forced the system to be operated close to its design limits, making the system vulnerable to disturbances and possible grid failure. This makes the study of voltage stability of the system important more than ever. Generally, voltage stability studies are carried on a single phase equivalent system assuming that the system is perfectly balanced. However, the three phase power system is not always in balanced state. There are a number of untransposed lines, single phase and double phase lines. This thesis deals with three phase voltage stability analysis, in particular the voltage stability index known as L-Index. The equivalent single phase analysis for voltage stability fails to work in case of any unbalance in the system or in presence of asymmetrical contingency. Moreover, as the system operators are giving importance to synchrophasor measurements, PMUs are being installed throughout the system. Hence, the three phase voltages can be obtained, making three phase analysis easier. To study the effect of unbalanced system on voltage stability a three phase L-Index based on traditional L-Index has been proposed. The proposed index takes into consideration the unbalance resulting due to untransposed transmission lines and unbalanced loads in the system. This index can handle any unbalance in the system and is much more realistic. To obtain bus voltages during unbalanced operation of the system a three phase decoupled Newton Raphson load ow was used. Reactive power distribution in a system can be altered using generators voltage set-ting, transformers OLTC settings and SVC settings. All these settings are usually in balanced mode i.e. all the phases have the same setting. Based on this reactive power optimization using LP technique on an equivalent single phase system is proposed. This method takes into account generator voltage settings, OLTC settings of transformers and SVC settings. The optimal settings so obtained are applied to corresponding three phase system. The effectiveness of the optimal settings during unbalanced scenario is studied. This method ensures better voltage pro les and decrease in power loss. Case studies of the proposed methods are carried on 12 bus and 24 bus EHV systems of southern Indian grid and a modified IEEE 30 bus system. Both balanced and unbalanced systems are studied and the results are compared.