Development of Magnetic Models to Assess Transformers’ Susceptibility to Geomagnetic Disturbances A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy in the Faculty of Science and Engineering 2018
quarta-feira, 27 de dezembro de 2023
Development of Magnetic Models to Assess Transformers’ Susceptibility to Geomagnetic Disturbances A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy in the Faculty of Science and Engineering
Development of Magnetic Models to Assess Transformers’ Susceptibility to Geomagnetic Disturbances A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy in the Faculty of Science and Engineering 2018
Yufan Ni
School of Electrical and Electronic Engineering
Abstract
During the peak years of solar activity, the magnetic field held by the solar wind has an
impact on the Earth’s magnetic field and induce an electric field on the Earth’s surface. The
Geomagnetically Induced Current (GIC) is generated between two neutral points of
transformers. The GIC can do severe harm to a power system, including to its transformers.
The worst GIC event caused a power system blackout for several hours in Quebec in 1989.
The research aims to build a representative model of core saturation and carry out simulation
studies to understand the performance of transformer cores in the high flux density region.
This in turn helps to identify the design features that need to be taken into account when
assessing the capability of a transformer to withstand over-excitation.
ATP is a kind of user-maintained software so it allows self-developed code to be added into
the software package. The results simulated by the existing ATP models are inaccurate
compared to the measured results. In addition, the existing models cannot provide flux
distribution results, so it is difficult to understand the process of how the core is pushed into
the deep saturation region by DC offset.
A new model is developed to include the equivalent electric and magnetic circuit
representations, taking flux leakage, in particular, into consideration. The flux leakage paths
are composed of the oil gaps and tank in series. This model is validated by the consistency
shown between the measured and simulated HV winding currents of a 5-limb transformer.
The peaks of magnetising currents are identified with the peaks of magnetic flux which
saturate the core.
The model can identify the design features, such as the core structure, dimension of flux
leakage paths and winding impedance that need to be taken into account when assessing the
capability of a transformer to withstand over-excitation. A 3-limb model and a 5-limb core
model are built to assess the susceptibility to GIC for different core types in high flux density
region. The delta winding plays a role in holding the 3rd harmonics and unbalanced current
generated by core saturation, and in delaying the core saturation. Lastly, Transformers are
simulated under realistic GIC waveforms, for situations with and without load.
The new model is expected to be coded into ATP to conduct a GIC study for a power system.
terça-feira, 26 de dezembro de 2023
Contribution to the DC-AC conversion in photovoltaic systems :Module oriented converters Oswaldo Lopez Santos-THESE En vue de l'obtention du DOCTORAT DE L’UNIVERSITÉ DE TOULOUSE
Abstract
These last years, a growing interest in power electronic systems has been motivated by the emergence of distributed renewable energy resources and their interconnection with the grid. In this context, the need of low power topologies fed by a few photovoltaic modules avoiding the use of transformers opens the study of special converters and the associated control strategies ensuring stability, reliability and high efficiency. A resulted generic device known in the commercial and scientific literature as “microinverter” or “module integrated converter” performs a plug and play product together with the PV module called an “AC module”.
This work is devoted to the study of a transformer-less single-phase double-stage grid-connected microinverter. The proposed topology has a non-isolated high-gain boost type DC-DC converter and a non-isolated buck type DC-AC converter connected in cascade through a DC bus. The DC-DC converter permanently extracts the maximum power of the PV module ensuring at the same time a good performance coping with power changes introduced by the change in the environmental conditions. The DC-AC stage injects the power extracted by the DC-DC stage into the grid ensuring a high level of power quality. The research efforts focus on the involved control functions based on the sliding mode control theory, which leads to a simple implementation with a comprehensive theoretical description validated through simulation and experimental results.
After giving the state-of-the-art in the first chapter, the manuscript is divided into four chapters, which are dedicated to the Maximum Power Point Tracking (MPPT), the DC-DC stage and its control, the DC-AC stage and its control and the complete microinverter. A new Extremum Seeking Control (ESC) MPPT algorithm is proposed.
The single-switch quadratic boost converter is studied operating as a Loss-Free-Resistor (LFR) obtaining a high DC output voltage level with a safe operation. The full-bridge converter is controlled as a Power Source Inverter (PSI) using a simple sliding-mode based tracking law, regulating the voltage of the DC bus and then ensuring a high power quality level in the grid connection. Finally, the three building blocks are merged to obtain a sliding mode controlled microinverter constituting the main result and contribution of the work.
VIEW FULL TEXT: https://theses.hal.science/tel-01146805v1
segunda-feira, 25 de dezembro de 2023
How did the USSR miss stealth technology? Part 1 P.Yu. Ufimtsev and the edge wave method-Como a URSS perdeu a tecnologia furtiva? Parte 1 P.Yu. Ufimtsev e o método da onda de borda
The discovery that actually enabled the development of the Lockheed F-117, the first truly stealthy aircraft, was achieved by the Russian physicist Pyotr Ufimtsev, who in 1962 published a paper on a new method (not the first) for estimating edge diffraction, which became known as the Physical Theory of Diffraction. Ufimtsev postulated that Maxwell's equations would make it possible to calculate the behavior of radar waves relayed from a reflective object. The radar return would depend in part on the shape of the object.
The era of 1950-1970 was marked by brilliant achievements in astronautics, nuclear energy and the aviation industry. It’s the last one I’d like to talk about.
Ufimtsev and the edge wave method
All this enormous scientific potential often transformed from quantity to quality. So, in 1962, a young scientist,
P.Ya. Ufimtsev published a monograph “The method of edge waves in the physical theory of diffraction.”
With the help of such calculations, solutions to a number of diffraction problems for a wedge and disk were obtained, and non-uniform parts of the current were found for these bodies. The theory of physical optics existing at that time did not take into account the uneven part of the current, which was concentrated near the kink. To put it more roughly, it turned out to be possible to more accurately describe the further behavior of electromagnetic waves when falling on surfaces with kinks and sharp edges. More accurately predict the structure of scattered waves. It would seem, what does aviation have to do with it?
Development in the USA
Let's move to the USA. At that time, American aircraft could not boast of their power. The first development of a stealth aircraft in the United States began in 1958. However, the attempts were unsuccessful. The planes were much inferior to the Soviet ones, as demonstrated by the Vietnam War. Their basic science lagged behind. They had not heard about the edge wave method until they accidentally (or maybe not) translated Ufimtsev’s monograph in 1971. Having got their hands on the theory, the missing link, they began development. The project received funding of several billion dollars. Lockheed and Northrop, using Ufimtsev’s theory and computer modeling (this was already possible in the USA in 1975), were able to develop in 1983 the first aircraft completely made using Stealth technology (Stealth - cunning) - the F-117 Night Hawk. one of the F-117 developers stated
Many journalists began to write about the "invisible plane." But this is absolutely incorrect. Firstly, it is visible in the optical range, and secondly, it is inconspicuous in a certain sector of angles for certain radars.
I will share my calculation, carried out while still at the institute. If you look at the scattering diagram of the F-117, you can see how much lower its frontal RCS is. Thanks to this shape, the aircraft re-radiates most of the energy of electromagnetic waves into the upper hemisphere. In the direction of the radar, the radiation is extremely small. The ESR of the F-117 aircraft when irradiated from frontal and tail angles is reduced to 0.1-0.01 m2, which is approximately 100-200 times less than that of a conventional aircraft of similar sizes.
Pyotr Yakovlevich Ufimtsev, who at that time was working at a research institute in Moscow, did not know anything about the fact that an airplane had been developed in the USA based on his monograph. Imagine his surprise when, in 1990, he went to a conference in the USA by invitation, after finishing his report, the audience gave him a standing ovation, but he could not understand how he had gained such fame. In the USSR he was known to few people. But his monograph became a reference book for Lokcheed Martin engineers. After so many years, he got what he deserved. True, not in my home country. In the 90s, no one in Russia needed scientific personnel, the economic crisis and meager salaries. Ufimtsev goes to work at the University of California at Los Angeles as a visiting professor and participates in the creation of the B2 strategic bomber. Now Ufimtsev is 88 years old and lives in America.
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domingo, 24 de dezembro de 2023
Design and characterization of a three-phase current source inverter using 1.7kV SiC power devices for photovoltaic applications Luis Gabriel Alves Rodrigues THÈSE Pour obtenir le grade de DOCTEUR DE LA COMMUNAUTE UNIVERSITE GRENOBLE ALPES Spécialité : Génie Electrique
Design and characterization of a three-phase current
source inverter using 1.7kV SiC power devices for
photovoltaic applications
Luis Gabriel Alves Rodrigues
THÈSE
Pour obtenir le grade de
DOCTEUR DE LA COMMUNAUTE
UNIVERSITE GRENOBLE ALPES
Spécialité : Génie Electrique
Abstract
Classically, the energy conversion architecture found in photovoltaic (PV)
power plants includes solar arrays delivering a maximum voltage of 1kV followed
by a step-up chopper connected to a three-phase Voltage Source Inverter. This
multistage conversion system (DC/DC + DC/AC) is then connected to the
medium-voltage grid through a low-voltage/medium-voltage transformer. In order
to simplify the PV systems, this research work focuses on the study and
implementation of a DC/AC topology employing a single power processing stage:
the three-phase Current Source Inverter (CSI). To deal with the inconvenient of
high conduction losses when implementing this topology, wide-bandgap Silicon
Carbide (SiC) semiconductors are used, allowing to efficiently convert energy
while keeping a relatively high switching frequency. Nonetheless, since the
available power semiconductor modules on the market are not compatible with
the CSI, a novel 1.7kV SiC-based voltage bidirectional module is developed in the
context of this thesis. Hence, the dynamic characterization of the new SiC device
is carried out and serves as the basis for the design of a 60kW CSI prototype.
Finally, the inverter efficiency is evaluated at nominal operating conditions,
employing both a calorimetric and electrical methods. The obtained results
confirm the CSI ability to operate efficiently at high switching frequencies
(η>98.5% @60kHz). The originality of this work lies mainly in the design,
characterization and implementation of the new 1.7kV full-SiC power module
adapted to the CSI topology.
Keywords: Power electronics, DC/AC converters, Current Source Inverter (CSI),
Silicon Carbide (SiC), Photovoltaics, Power module.
VIEW FULL TEXT: https://theses.hal.science/tel-02476261v1/document
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