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On the Electrical Aging of the Insulation in PWM-Fed High-Speed Electric Machines: Analysis, Modelling, and Mitigation
Full text linkThis doctoral thesis represents the completion of a three-year research activity that has been carried out as part of the Ph.D. program in Electrical and Information Engineering, XXXII cycle, of the Polytechnic University of Bari, Italy. The work that is presented in this document addresses some specific aspects of the broader topic of reliability in high-frequency electric drives, which is the early failure of inverter-fed AC machine insulation systems due to the very high dv/dt’s of modern fast switching converters. A selection of the most important research works that have been published in the literature on this subject is reviewed here and can be found in the reference. These works have greatly contributed to the progression of the research and thesis. Hopefully, the content presented in this document will also add a modest contribution to this topic that is becoming more and more relevant in today's electric drive applications.
The background of the thesis, the motivations, and ideas laying behind the research activity carried on the topic of insulation aging are presented. The role of electrification in many application areas and the benefits that the transition to cleaner energy forms can bring to the cause of environment and climate preservation are discussed. In this promising energy transition process, high-speed machines and high-frequency power electronics converters are key technologies to enable enormous energy-saving and to boost the penetration of more electrical systems in both industry and transportation sectors. The growing market diffusion of high-frequency electric drives is driven by the fast developments in the new generation of power switching devices, which are based on silicon carbide and gallium nitride composites, the so-called wide bandgap semiconductor materials. Thanks to these new materials, which present better properties than silicon with respect to breakdown electric field, electron mobility, and thermal stability, modern power switches can operate at higher commutation frequency, with lower losses and higher junction temperature than previous generations. Alongside the numerous advantages of high-frequency devices, new issues arise for their practical implementation because of their fast commutation transients at turn-on and turn-off. In fact, the very short rise and fall time of the applied voltage pulses, in the order of tens of nanoseconds, is source of increased electromagnetic interference that can negatively affect the surrounding low-power electronics, common-mode voltage can appear across the shaft of the machine leading to the circulation of currents along the bearings and subsequent mechanical deterioration, and increased stress on the motor turn insulation. The higher electrical stress on the insulation is the cause of partial discharges within the weak spots of the system, small sparks that delaminate the dielectric material, eventually leading to a turn short circuit and to the fault.
The winding insulation system is a fundamental component of any electric machine, functionally required for the operation of the electromechanical conversion mechanism. The breakdown of the insulation inhibits the operation of the machine, which must be re-wound to be put in service again. Due to the high dv/dt’s in high-frequency power electronic converters, two phenomena occur that can threaten the life duration of motor insulation systems. Voltage waveform reflections along the cable that connects the inverter and the machine produce high overvoltage at the motor terminals when the rise time of the PWM voltage pulses is short. Furthermore, due to the parasitic capacitances of the winding insulation, the distribution of the voltage across the coil is such that, when the pulses are applied to the winding, the first turns are exposed to the higher share of the voltage. Consequently, within the defects of the insulation system, such as air pockets or small clearances, partial discharges are triggered by the electric field exceeding the dielectric strength of the air. Partial discharges are ionic discharge events that modify the physical and chemical properties of the material, making it weaker over time until it cannot sustain the normal field anymore. At this point, the irreversible fault occurs, and the machine must be put out of service. Many studies in the literature have investigated the role of partial discharge on shortening the life of winding insulation systems, and the main effects of PWM voltages on the partial discharge characteristics have been identified.
The main approaches commonly adopted today for the mitigation of the electrical stress on the insulation are the use of inverter output filters, very effective but generally bulky and costly and that often reduce the dynamic performance of the drive, and the combined design of converter and machine to minimize the length of the cables, which has practical limitations in high-speed drives due to the different scaling of the two parts.
Three are three main goals of the research activity presented in this doctoral thesis. The first objective is to achieve a better understanding of the effects of electrical aging on motor insulation, to develop a practical method for the estimation of the aging progression through the measure of electrical quantities such as voltage and current. The second goal is the development of an accurate and easy-to-tune model of the drive components in the high-frequency domain, to be used for both improving the drive design and performing advanced diagnosis features. The third objective is to investigate possible approaches for the mitigation of electrical aging in high-speed electrical machines. Two possibilities are taken into consideration to this purpose: the active regulation of the dv/dt to limit the overvoltage at the motor terminals, and the use of multilevel converters to reduce the voltage stress by increasing the number of voltage levels.
To achieve a deep understanding of the aging phenomenon from a macroscopic point of view, i.e. by mean of easily measurable electrical quantities, a series of electrical aging test has been performed on a group of motors while their characteristics were cyclically measured. The data collected during this aging campaign, together with the experimental setup that has been developed, are presented in the thesis. The data are statistically used to build some indicators of the insulation condition that can be used as an estimation of the aging progression in the insulation system. The first indicator is obtained through the measure of the machine characteristic impedances at different aging steps, while the second and the third are calculated from the high-frequency response of the phase current when a voltage pulse is applied.
Accurate models of the electric drive system in the high-frequency domain are required to both improve designs from a system point of view and to include advanced predictive diagnosis features in the drive control system. A series of high-frequency motor models are reviewed and analyzed in this thesis based on the findings from the electrical aging campaign performed. A first modeling approach is made using a lumped-parameter model of the machine and an automatic identification procedure. To improve both accuracy and model tuning time, a second model is proposed, which is based on the automatic identification of an analytical rational function with the measured data. The fitting algorithm that has been employed is described in the thesis, and results are shown about the achieved accuracy level.
The experimental data collected during the electrical aging tests and the developed high-frequency models of the drive components are used to investigate some possible approaches for the mitigation of the electrical stress on the winding insulation system of electric machines. The first method that is considered is the idea of using the estimation of the winding insulation aging conditions as a feedback for an advanced control system for the active regulation of the dv/dt of the PWM pulses to the purpose of extending the life of the drive while optimizing efficiency and performance. Such a system requires different parts to properly function, an effective estimator, a controller based on an optimization algorithm, and a set of active gate drivers. In this thesis, the idea is explored by simulation with some simplifications. The second overvoltage mitigation approach is to have a multilevel voltage waveform applied to the motor instead of a train of PWM pulses. In this way, the level of the overvoltage applied to the motor is reduced, and so is the electrical stress on the insulation. To this purpose, in this thesis, a cascaded H-bridge multilevel converter is compared to a conventional 2-level inverter operating on the same load conditions and with the same overvoltage level at the motor terminals. A simulation model is used to compare the losses, the efficiency, and the thermal requirements in both cases
Advancements in Control, Design, and Parameter Identification of Permanent Magnet Synchronous Machines
Full text linkIn this work, advancements in control, design, and parameter identification of permanent magnet synchronous machines (PMSMs) are presented. These advancements take up opportunities and challenges offered by novel applications and technologies. In fact, PMSMs are increasingly adopted thanks to high efficiency and versatility, covering a wide range of applications. In particular, the use of PMSMs in the industry of small-scale wind turbines is becoming dominant. In this application, direct drive topologies coupled with variable speed and fixed pitch wind turbines are a promising solution because of their simplicity, efficiency, and reliability. In fact, this solution avoids the use of gearboxes and mechanical systems for the control of the pitch angle, reducing losses, costs, and failure risks. In this architecture, the aerodynamic power regulation is entirely entrusted to the control of the permanent magnet synchronous generator (PMSG). While several methods have been developed to achieve the maximum power point tracking, poor effort has been made to regulate the power for wind speeds above the rated one. In this work, this challenge
has been addressed by proposing an innovative control scheme in which an aerodynamic torque observer and a wind speed estimator are involved. The aim of the designed solution is to achieve the maximum power point tracking for wind speeds below the rated one and to extend the power generation in the high wind speed region. In this region, the respect of the safety mechanical limits of the system is the crucial issue. The proposed control scheme has been tested on an experimental setup in the laboratory of Electrical Machines and Drives of the Politecnico di Bari. The achieved results show that the reference power regulation characteristic has been tracked with a good accuracy covering the whole wind speed range of the system. In the context of the design of PMSMs, novel modular topologies are gaining increasing interest, especially in applications where large machines are employed. Thanks to this technology, several advantages in the manufacturing, transporting, and assembling over conventional PMSMs can be achieved. A drawback of PMSMs with modular stators is the presence of additional harmonic
components of the cogging torque, with higher amplitudes and lower frequencies than the ones of PMSMs with a one-piece stator. The minimization of these harmonics is essential to increase the control accuracy and reduce undesired noise and vibrations. Despite several methods have been developed for the cogging torque minimization, these mostly deal with conventional PMSMs. In this work, two novel methods to minimize the cogging torque of modular PMSMs are proposed and compared. Both analytical studies and heuristic procedures are adopted to solve the problem. Simulation results with the finite element method have been presented to validate the proposed methods. The results achieved exhibit a reduction of the cogging torque over 90% and show that conventional methods used for one-piece stator PMSMs are not effective against the additional harmonic components produced by modular stators. Also, the spread of the IoTs (Internet-of-Things), edge and cloud computing
technologies offers novel perspectives on the monitoring and maintenance of PMSMs. Although data-driven approaches can be considered dominant in this context thanks to the availability of big data, the potential of model-based approaches has not been considered and exploited in this novel scenario. Modelbased approaches are based on the parameter identification of the system. Many well-known solutions have been developed to identify the parameters of PMSMs, but these are not feasible in large-scale applications because these are not designed for straight-through processing where the human component is negligible and IoTs and edge-cloud computing can give their best. Therefore, in the present work, the problem of the automated parameter identification processing data produced by the PMSM drive without ad-hoc tests or control actions is considered. This problem has been addressed by means of the design of an innovative algorithm based on coupled Adaline Neural Networks. Data
produced by the PMSM during its ordinary operations are used to feed the Adaline Neural Networks. Moreover, an analytical study has been performed for the convergence and estimation errors analysis of the proposed algorithm. Finally, simulation and experimental investigations have been performed for verification purposes. The results achieved show a good accuracy of the parameter identification, with experimental estimation errors lower than 15% without any manual action
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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