Insulation system is an inseparable part of electrical equipment. In this study, one of the most important aging factors in insulation systems known as partial discharge (PD) is targeted. PD phenomenon has been studied for more than a century and yet new technologies still demand the investigation of PD impact. Nowadays, electrification is penetrating into various fossil-fuel-based industries such as transportation system that demands the reliability of electrical equipment under various harsh environmental conditions. Due to the lack of knowledge on the behavior of insulation systems, research in this area is intensively needed. The current study probes into the partial discharge phenomenon from two aspects and the groundwork for both aspects are provided by experimentation of multiple PD types. In the first goal, a finite-element analysis (FEA) approach is developed based on measurement data to estimate electric field distribution. The FEA model is coupled with a programming scheme to evaluate PD conditions, calculate PD metrics, and perform statistical analysis of the results. For the second target, it is aimed to use deep neural networks to identify and discriminate different sources of PD. The measurement data are used to generate thousands of phase-resolved PD (PRPD) images that will be used for training deep learning models. To meet the characteristics of the dataset, a deep residual neural network is designed and optimized to discriminate PD sources in an accurate, stable, and time-efficient way. The outcome of this research enhances the reliability of electrical apparatus through a better understanding of the PD behavior and lays a foundation for automatic monitoring of PD sources. / Doctor of Philosophy / Electrical equipment functions properly when its conductive elements are electrically insulated. The science of dealing with insulation systems has become more prominent in recent years due to the novel challenges and circumstances introduced by the rapid electrification trend. As an instance, the electrification trend in transportation systems can impose a multitude of environmental, thermal, and mechanical constraints which were not traditionally considered. These new challenges have led to an accelerated deterioration rate of insulation materials. To address this concern, this study targets the experimentation and modeling of the main aging mechanism in electrical equipment known as partial discharge (PD). A numerical model based on finite-element analysis (FEA) is developed that agrees with the test results and can accurately predict the aging of insulating materials due to the PD phenomenon. Moreover, the growing interest toward electrification of the aviation industry (as a response to the climate change crisis) requires the study of insulating materials under low-pressure (high-altitude) conditions. Theoretical and experimental data confirm the more frequent occurrence of PDs and their higher intensity under low-pressure conditions. Safety of operation is the highest priority in airborne transportation, yet no study has addressed the condition monitoring system as a necessary asset of the electric aircraft. To address this research gap, this work develops a dielectric online condition monitoring system (DOCMS) that actively monitors the deterioration level of insulation using deep learning methods. Based on standardized measurements under low-pressure conditions, the data are preprocessed to train the deep neural network with the pattern of PD activities. The proposed scheme can achieve >82% with short-term signals emitted measured from the system.

High voltage power transformer is one of the most important and expensive components in today's power transmission and distribution systems. Any overlooked critical fault generated inside a power transformer may lead to a transformer catastrophic failure which could not only cause a disruption to the power system but also significant equipment damage. Accurate and prompt information on the health state of a transformer is thus the critical prerequisite for an asset manager to make a vital decision on a transformer with suspicious conditions. Partial discharge (PD) is not only a precursor of insulation degradation, but also a primary factor to accelerate the deterioration of the insulation system in a transformer. Monitoring of PD activities and the concentration of PD generated combustible gases dissolved in the transformer oil has been proven to be an effective procedure for transformer health state estimation. However current commercially available sensors can only be installed outside of transformers and offer indirect or delayed information. This research is aimed to investigate and develop several sensor techniques for transformer health monitoring. The first work is an optical fiber extrinsic Fabry-Perot interferometric sensor for PD detection. By filling SF6 into the sensor air cavity of the extrinsic Fabry-Perot interferometer sensor, the last potential obstacle that prevents this kind of sensors from being installed inside transformers has been removed. The proposed acoustic sensor multiplexing system is stable and more economical than the other sensor multiplexing methods that usually require the use of a tunable laser or filters. Two dissolved gas analysis (DGA) methods for dissolved hydrogen or acetylene measurement are also proposed and demonstrated. The dissolved hydrogen detection is based on hydrogen induced fiber loss and the dissolved acetylene detection is by direct oil transmission measurement. / Ph. D.

By 2030, it is expected that 80% of all electric power will flow through power electronics systems. Wide bandgap power modules that can tolerate higher voltages and currents than silicon-based modules are the most promising solution to reducing the size and weight of power electronics systems. These wide-bandgap power modules constitute powerful building blocks for power electronics systems, and wide bandgap-based converter/power electronics building blocks are envisaged to be widely used in power grids in low- and medium-voltage applications and possibly in high-voltage applications for high-voltage direct current and flexible alternating current transmission systems. One of the merits of wide bandgap devices is that their slew rates and switching frequencies are much higher than silicon-based devices. However, from the insulation side, frequency and slew rate are two of the most critical factors of a voltage pulse, influencing the level of degradation of the insulation systems that are exposed to such voltage pulses. The shorter the rise time, the shorter the lifetime. Furthermore, lifetime dramatically decreases with increasing frequency. Thus, although wide bandgap devices are revolutionizing power electronics, electrical insulating systems are not prepared for such a revolution; without addressing insulation issues, the electronic power revolution will fail due to dramatically increased failure rates of electrification components. In this regard, internal partial discharges (PDs) have the most effect on insulation degradation. Internal PDs which occur in air-filled cavities or voids are localized electrical discharges that only partially bridge the insulation between conductors. Voids in solid or gel dielectrics are challenging to eliminate entirely and may result simply during manufacturing process. The objective of this study is to develop a Finite-Element Analysis (FEA) PD model under fast, repetitive voltage pulses, which has been done for the first time. The model is coded and implemented in COMSOL Multiphysics linked with MATLAB, and its simulation results are validated with experimental tests. Using the model, the influence of different parameters including void shape, void size, and void air pressure on PD parameters are studied. / M.S. / To decarbonize and reduce energy consumption for commercial aviation, the development of lightweight and ultra-efficient all-electric powertrain including electric motors, drives, and associated thermal management systems has been targeted. Using wide bandgap (WBG) power modules that can tolerate high voltages and currents can reduce the size and weight of the drive. However, the operation of WBG-based power converter can endanger the reliability of the electrified systems, most importantly, the insulation system. In this study, it is attempted to model the impact of such threats to the insulation system using numerical models.

A finite element analysis (FEA) driven, automated numerical optimization technique is used to design electric field grading structures in a PCB-integrated bus bar for a 10 kV bondwire-less silicon-carbide (SiC) MOSFET power module. Due to the ultra-high-density of the power module, careful design of field-grading structures inside the bus bar is required to mitigate the high electric field strength in the air. Using Bayesian optimization and a new weighted point-of-interest (POI) cost function, the highly non-uniform electric field is efficiently optimized without the use of field integration, or finite-difference derivatives. The proposed optimization technique is used to efficiently characterize the performance of the embedded field grading structure, providing insights into the fundamental limitations of the system. The characterization results are used to streamline the design and optimization of the bus bar and high-density module interface. The high-density interface experimentally demonstrated a partial discharge inception voltage (PDIV) of 11.6 kV rms. When compared to a state-of-the-art descent-based optimization technique, the proposed algorithm converges 3x faster and with 7x smaller error, making both the field grading structure and the design technique widely applicable to other high-density high-voltage design problems. / M.S. / Innovation trends in electrical engineering such as the electrification of consumer and commercial vehicles, renewable energy, and widespread adoption of personal electronics have spurred the development of new semiconductor materials to replace conventional silicon technology. To fully take advantage of the better efficiency and faster speeds of these new materials, innovation is required at the system-level, to reduce the size of power conversion systems, and develop converters with higher levels of integration. As the size of these systems decreases, and operating voltages rise, the design of the insulation systems that protect them becomes more critical. Historically, the design of high-density insulation system requires time-consuming design iteration, where the designer simulates a case, assesses its performance, modifies the design, and repeats, until adequate performance is achieved. The process is computationally expensive, time-consuming, and the results are not easily applied to other insulation design problems. This work proposes an automated design process that allows for the streamlined optimization of high-density insulation systems. The process is applied to a 10 kV power module and experimentally demonstrates a 38\% performance improvement over manual design techniques, while providing an 8 times reduction in design cycle time.

Auxiliary power supply (APS) plays a key role in ensuring the safe operation of the main circuit elements including gate drivers, sensors, controllers, etc. in medium voltage (MV) silicon carbide (SiC)-based converter systems. Such a converter requires APS to have high insulation capability, low common-mode coupling capacitance (Ccm ), and high-power density. Furthermore, considering the lifetime and simplicity of the auxiliary power supply system design in the MV converter, partial discharge (PD) free and multi-load driving ability are the additional two factors that need to be addressed in the design. However, today’s state-of-the-art products have either low power rating or bulky designs, which does not satisfy the demands. To improve the current designs, this thesis presents a 1 MHz isolated APS design using gallium nitride (GaN) devices with MV insulation reinforcement. By adopting LCCL-LC resonant topology, the proposed APS is able to supply multiple loads simultaneously and realize zero voltage switching (ZVS) at any load conditions. Since high reliability under faulty load conditions is also an important feature for APS in MV converter, the secondary side circuit of APS is designed as a regulated stage. To achieve MV insulation (> 20 kV) as well as low Ccm value (< 5 pF), a current-based transformer with a single turn structure using MV insulation wire is designed. Furthermore, by introducing different insulated materials and shielding structures, the APS is capable to achieve different partial discharge inception voltages (PDIV). In this thesis, the transformer design, resonant converter design, and insulation strategies will be detailly explained and verified by experiment results. Overall, this proposed APS is capable to supply multiple loads simultaneously with a maximum power of 120 W for the sending side and 20 W for each receiving side in a compact form factor. ZVS can be realized regardless of load conditions. Based on different insulation materials, two different receiving sides were built. Both of them can achieve a breakdown voltage of over 20 kV. The air-insulated solution can achieve a PDIV of 6 kV with Ccm of 1.2 pF. The silicone-insulated solution can achieve a PDIV of 17 kV with Ccm of 3.9 pF. / M.S. / Recently, 10 kV silicon carbide (SiC) MOSFET receives strong attention for medium voltage applications. Asit can switch at very high speed, e.g. > 50 V/ns, the converter system can operate at higher switching frequency condition with very small switching losses compared to silicon (Si) IGBT [8]. However, the fast dv/dt noise also creates the common mode current via coupling capacitors distributed inside the converter system, thereby introducing lots of electromagnetic interference (EMI) issues. Such issues typically occur within the gate driver power supplies due to the high dv/dt noises across the input and output of the supply. Therefore, the ultra-small coupling capacitor (<5 pF) of a gate driver power supply is strongly desired.[37] To satisfy the APS demands for high power modular converter system, a solution is proposed in this thesis. This work investigates the design of 1 MHz isolated APS using gallium nitride (GaN) devices with medium voltage insulation reinforcement. By increasing switching frequency, the overall converter size could be reduced dramatically. To achieve a low Ccm value and medium voltage insulation of the system, a current-based transformer with a single turn on the sending side is designed. By adopting LCCL-LC resonant topology, a current source is formed as the output of sending side circuity, so it can drive multiple loads importantly with a maximum of 120 W. At the same time, ZVS can use realized with different load conditions. The receiving side is a regulated stage, so the output voltage can be easily adjusted and it can operate in a load fault condition. Different insulation solutions will be introduced and their effect on Ccm will be discussed. To further reduce Ccm, shielding will be introduced. Overall, this proposed APS can achieve a breakdown voltage of over 20 kV and PDIV up to 16.6 kV with Ccm<5 pF. Besides, multi-load driving ability is able to achieve with a maximum of 120 W. ZVS can be realized. In the end, the experiment results will be provided.

Advancements in power semiconductor and power converter technology have enabled new low-voltage (LV) and medium-voltage (MV) direct current (DC) distribution systems for a variety of applications. Power electronics converters and DC circuit breakers (DCCBs) are the key components of a DC system and are hence the focus of this work. The combination of growing power density requirements and higher voltages can result in enhanced electric field (E-field) intensities, leaving the system vulnerable to partial discharges (PDs). The manifestation of such PD events gradually degrades the insulation system of the equipment, reducing its lifetime and ultimately leading to total insulation failure. Therefore, inception E-field based insulation design guidelines are developed to help achieve zero-PD operation of power electronics systems with considerations for internal as well as external (surface) E-field distribution. Additionally, surface E-field mitigation methods are experimentally investigated using representative PCB coupons to provide suitable solutions for low air pressure applications. Consequently, E-field management methods consisting of geometry-based techniques are proposed for PCB-based systems to mitigate E-field magnitudes in areas of the system that are prone to peak stresses (e.g. surface interconnections and triple junctions, conductor discontinuities, critical airgaps etc.). Successful design examples are provided including that of a 16 kV rated PCB-based DC bus and a 540 V, 100 kW aircraft generator rectifier unit operating at up to 50,000 ft cruising altitudes. DC circuit breaker (DCCB) technology, though crucial to ensure the safety of DC systems, is still in the early stages of development. As protection devices, their reliable operation is paramount and the selection and sizing of their components are not trivial. In this regard, comprehensive design guidelines are developed for the DC solid-state circuit breaker (SSCB) to ensure that its functional requirements can be met. System analyses and modeling are performed to understand the interactions between the various components, i.e. solid-state device, metal oxide varistor (MOV), and their impact on the breaker operation. A 2.5 kV, 400 A SSCB prototype is designed and verified with experimental results to validate the design approach. Traditional MOV based voltage clamping circuits (VCC) used in solid-state circuit breakers (SSCBs) impose a high interruption voltage on the main solid-state device. The voltage burden arises from the material properties of the MOV which fixes its clamping voltage at a value more than twice its maximum continuous dc voltage rating. A novel and reliable VCC termed as the electronic MOV (eMOV) is proposed to decouple the peak clamping voltage of the MOV from the nominal dc voltage of the system aiming to improve the voltage suppression index (V SI = Vpk/Vdc) of the VCC, thereby reducing the peak system voltage and allowing easier insulation design. By virtue of the proposed circuit, a lower voltage rated device can be used for the main switch enabling higher system efficiency and power density. In all, this work aims to address insulation system design for power electronics converters and systems, ultimately to eliminate PD under specified working voltage conditions for improved electrical safety and insulation lifetime. The implications of high-density integration, unsuitable ambient conditions and higher system voltages are considered to develop a suitable design and assessment methodology for practicing engineers. Techniques to mitigate/ manage E-Field inside and outside (surface) solid dielectric are proposed to attain the above goal. Additionally, design guidelines are formulated for DC SSCBs which are essential to the safety of DC distribution systems and an enhanced VCC is proposed for the same to limit its clamping voltage for easier insulation design. / Doctor of Philosophy / The recent advancements in power conversion technology have promoted the development and use of DC distribution networks for a variety of applications (e.g. electric ships, aircrafts, electric vehicle charging stations etc.). The insulation system of typical power electronics equipment consists of multiple solid insulating media (e.g. PCB dielectric, potting material, conformal coat etc.) separated by air gaps in the assembly. The combination of higher operating voltages, power density targets and unfavorable ambient conditions (e.g. low air pressure) can pose a risk to the insulation system of the equipment, if not addressed. The electric field (E-Field) stresses at certain vulnerable areas can exceed breakdown values of the corresponding media, initiating localized electrical discharge events also called as partial discharges (PD). Internal discharges generally occur in the vicinity of material defects, conductor discontinuities or sharp geometric features, while surface discharges may occur along exposed conductor metallizations on insulator surfaces (at the interface of multiple media) or critical air gaps in the assembly. PD events, while not posing any imminent threat, can degrade the surrounding area over time to reduce the operating life of the system and in some cases may cause catastrophic failures. Therefore, irrespective of location, such PD events must be eliminated to improve the overall system lifetime and reliability. Therefore, the main focus of this work is to develop insulation design guidelines and methodologies to achieve zero-PD operation of power converters and DC circuit breakers (DCCBs), both of which are key components of DC systems. A generalized design guideline is proposed to help with the insulation design of power electronics systems. Design techniques are developed to reduce E-field magnitude at critical areas to avoid over-designing the insulation system. Successful converter-level design examples are provided to validate the proposed approaches. DCCB technology is still in the early stages of development. As a protection device, its reliable operation is paramount and the selection and sizing of its components are not trivial. Therefore, in addition to the above insulation design methodology, comprehensive design guidelines are developed for the solid-state device and voltage clamping circuit (VCC) of the DC solid-state circuit breaker (SSCB), to ensure that its functional requirements can be met. Additionally, a novel VCC is proposed for the same to limit its fault interruption voltage for easier insulation design. Both SSCB and VCC prototypes are built and successfully demonstrated in a fault current breaking application. Overall, this dissertation provides a reference for the design and assessment of next generation power electronics converters and DC circuit breakers, to address, specifically, the challenges to their insulation systems.

This thesis presents the development of a double-side cooled 3.3 kV, 100 A SiC MOSFET phase-leg power module for heavy-duty traction applications. Parasitic extraction and thermal simulations of the module showed a parasitic inductance of 2.89 nH and junction temperature of 108.3 °C at a heat flux of 156 W/cm² under a typical water-cooling condition. Electric field simulations identified high electric field stress at the module's outer surface edges exposed to air, posing a risk for partial discharge. To mitigate this risk, a solution that involves covering the critical point in an epoxy was proposed, analyzed, and validated through partial discharge inception voltage tests. Steps for fabricating the module are presented. Static electrical characterization of the fabricated module showed an average on-resistance of 31 mΩ and an average leakage current of 356 nA at VDS of 3 kV, which are similar to those of the unpackaged devices. The module with a double-side cooling design achieved an exceptional power density of 116.6 kW/cm³, more than twice that of any single-side cooled 3.3 kV SiC module. This makes it highly suitable for next-generation electric transportation systems that require high power density and efficient thermal management, such as electric trucks, railways, and eVTOL aircraft. / Master of Science / This thesis presents the development of a highly efficient and compact power module designed for electric vehicles and other high power applications. By utilizing advanced silicon carbide technology and double-side cooling structure, the module achieves outstanding performance, making it ideal for heavy-duty uses such as electric trucks, railways, and eVTOL aircraft. The module operates at 3.3 kV and 100 A, with low electrical losses and excellent thermal management. Extensive simulations and testing demonstrated that the module significantly reduced unwanted electrical effects and maintained a stable temperature under high power conditions. An epoxy coating was applied to critical areas to prevent electrical discharge, enhancing the module's reliability. The fabrication process incorporated packaging techniques like silver-sintering for attaching the semiconductor chips and other components, resulting in strong and reliable connections. Static tests confirmed that the electrical performance of the packaged power module maintained consistently high efficiency compared with the bare chips. Overall, this double-side cooled power module offers more than twice the power density of traditional designs, paving the way for the development of future electric vehicle traction systems that require high power density and efficient cooling.

In this work, we first analyze the behavior of the acoustic wave from the theoretical point of view using a simplified 1-dimensional model. The model was developed based on the conservation of mass, the conservation of momentum, and the state equation; in addition, the fluid medium obeys Stokes assumption and it is homogeneous, adiabatic and isentropic. Experiment and simulation results show consistency to theoretical calculation. The second part of this thesis focuses on the PD signal analysis from an on-site PD measurement of the in-house design fiber optic sensors (by Virginia Tech, Center for Photonics Technology). Several commercial piezoelectric transducers (PZTs) were also used to compare the measurement results. The signal analysis employs the application of wavelet-based denoising technique to remove the noises, which mainly came from vibration, EMI, and light sources, embedded in the PD signal. The denoising technique includes the discrete wavelet transform (DWT) decomposition, thresh-holding of wavelet coefficients, and signal recovery by inverse discrete wavelet transform. Several approaches were compared to determine the optimal mother wavelet. The threshold limits are selected to remove the maximum Gaussian noises for each level of wavelet coefficients. The results indicate that this method could extract the PD spike from the noisy measurement effectively. The frequency of the PD pulse is also analyzed; it is shown that the frequencies lie in the range of 70 kHz to 250 kHz. In addition, with the assumed acoustic wave propagation delay between PD source and sensors, it was found that all PD activities occur in the first and third quadrant in reference to the applied sinusoidal transformer voltage. / Master of Science

In this thesis, power system transients are considered as an opportunity for development of on-line diagnostics of power components and specifically the insulation systems of power transformers and bushings. A new technique for on-line dielectric response measurement of power transformer bushings is proposed which utilizes natural transients in the power system, such as lightning and switching surges, as stimuli. Laboratory investigations are done on implementation of the proposed technique. Measurement considerations, data acquisition and processing involved in achievement of reasonable accuracy in the Dielectric Response (DR) are presented. Capability of the technique in tracking of the degradation signatures such as moisture content in the insulation has been evaluated and it has shown a good level of accuracy by being compared to the Frequency Domain Spectroscopy (FDS).  The proposed technique is tested on the service-aged 150 kV bushings and feasibility of the technique for monitoring of dielectric properties of power transformer bushings has been assessed; the results are promising for the technique to be used in the real application.  Partial Discharges (PD) behavior under transients has been also studied for different materials in this project. PD behavior of different defects, at different insulation condition, responding to the overvoltage transients in form of superimposed impulses on ac voltages was investigated and it was perceived how their distinctive response and the interpretation of  that, can be useful for their identification. Besides the conventional materials, surface ac PD properties of modified paper with silica and zinc oxide nanoparticles under the superimposed impulses have been assessed in this project. Proper type and optimum concentration level of nanoparticles in the paper are the factors that lead to the improvement of PD behavior in the modified paper under overvoltage transients. / <p>QC 20160525</p>

O crescente aumento na demanda de energia elétrica nacional, associada às alterações regulamentares do setor, em que o tempo que um equipamento permanece indisponível para o sistema, aguardando manutenção significa perda de receita para as companhias de energia, motivou a busca por diagnósticos precisos e utilização de técnicas não invasivas que possam ser aplicadas em transformadores em serviço. Assim, o foco desta pesquisa foi o desenvolvimento de uma arquitetura de sistema inteligente baseado em Redes Neurais Artificiais, que a partir de características extraídas de sinais de emissão acústicas provenientes de sensores distribuídos espacialmente no tanque de transformadores de potência, possa identificar internamente o local de ocorrência das descargas parciais e fornecer as distâncias estimadas entre os sensores e o ponto dessa descarga, e com essas distâncias, utilizando técnicas numéricas de triangulação, o sistema fornece também a coordenada espacial da falha auxiliando no diagnostico de defeito do transformador e no processo de tomada de decisões. / The increasing in the demand for national electrical energy coupled with alterations in the regulation of the sector, where the time which a piece of the electrical system equipment stays out of service means loss of income for the electrical companies, has motivated the search for correct diagnostics and usage of non-invasive technics that can be used in tranformers which are in operation. Therefore, the aim of this research was to develop the design of an intelligent system based on Artificial Neural Nets, which through the characteristics extracted from the acoustic emission signals coming from the sensors spatially distributed in the power transformer tank can identify internally the place of the occourrence of the partial discharges and provide the estimate distances between the sensors and the discharges point, so with these distances using triangle technical analysis the system will also provide the spatial coordinate of the flaw for diagnosing the problem with the transformer and help the process of decision making.