Measurements and finite element modelling of transformer flux with dc and power frequency current

Geomagnetically induced currents (GIC’s) caused by solar storms or other sources of dc excitation in the presence of ac energization can disturb the normal operation of power transformers. If large enough, they cause half-cycle saturation of a power transformer’s core which could lead to overheating due to excessive stray flux. Finite element matrix (FEM) modelling software is of considerable use in transformer engineering as it is able to solve electromagnetic fields in transformers. For many problems, typically involving only specific parts of a transformer, fairly accurate solutions can be reached quickly. Modelling the effects of GIC or leakage currents from dc systems, however, is more complex because dc components are superimposed on ac in transformers with nonlinear electrical core steel parameters. At the beginning of the investigation, FEM models of different bench-scale laboratory transformers and a 40 MVA three-phase three limb power transformer were investigated, but the results did not sufficiently represent the measurement data due to the application of widely used modelling assumptions regarding the transformer joints. Following the preliminary analyses, practical measurements and FEM simulations were carried out using three industrially made model single-phase four limb transformers (1p4L) without tanks. These test transformers resemble a real power transformer because they have high-quality grain oriented electrical core steel and parallel winding assemblies. Practical laboratory measurements recorded during ac testing were used to calibrate 2D FEM models by adding “equivalent air gaps” at the joints. The implementation of this joint detail helped to overcome the shortcomings of the preliminary FEM simulation. Analyses of the electrical and magnetic responses of the FEM models using simultaneous ac and dc then followed. A refined 3D FEM simulation with more detailed modelling of the core joints of 1p4L model transformers agreed more closely with the practical measurements of ac only no-load conditions. Further, the depiction of stray flux leaving the transformer’s saturated core under simultaneous ac and dc excitation showed an improvement in the approach as measured in the physical model. Saturation inductance (Lsat) is an important parameter for input into mid- to low-frequency lumped parameter transformer models that are used in electromagnetic transients software such as PSCAD/EMTDC, but it is not easily measured and is seldom provided by manufacturers. Some Lsat measurements on the 1p4L test transformers are presented in this thesis, along with some 3D FEM analyses. The measurements and FEM analyses investigated “air core inductance” which represents a transformer without a core, and “terminal saturation inductance” which represents deep saturation due to dc excitation. An important finding in this thesis is that “terminal saturation inductance” is the more useful of the two for topological transformer models investigating realistic GIC excitation. Further to this, a new composite depiction of half-cycle saturation with a multi-parametric relationships supported by measurement and simulation is presented. The main contribution of this thesis is that it gives more accurately the electrical response and distribution of the leakage flux under conditions such as those caused by GIC or other sources of leakage dc excitation, as well as including of joint details in the FEM models through calibration with physical models. This calibration can aid transformer modelling and design in industry for mitigation of the effects of GICs, contributing to improved transformer survival during significant geomagnetic disturbances

Electric Vehicle Powertrain Integrated Charging

Batterieelektrische Fahrzeuge benötigen ein im Fahrzeug eingebautes Ladegerät, um die Energie aus dem Wechselstromnetz für die Gleichstrom- Batterie aufzubereiten. Integriertes Laden ist eine Methode der Integration von Ladefunktionalität in die Antriebsstrangkomponenten, welche während des Parkens außer Betrieb sind, mit dem Ziel, Kosten, Gewicht und Volumen des Ladegerät zu sparen. Das Laden ohne die Sicherheitsmaßnahme einer galvanischen Trennung im Ladegerät ist möglich mit zusätzlichen Maßnahmen gegen elektrischen Schlag, z.B. mit einer Fehlerstromerkennung und entsprechenden Trenneinrichtung. Im Stand der Technik wurden 33 integrierte Ladekonzepte gefunden und bezüglich Antriebsstrangnutzung, benötigte Komponenten, Drehmoment der elektrischen Maschine und Wirkungsgrad verglichen. Im Rahmen dieser Arbeit wird ein neues galvanisch getrenntes integriertes Ladekonzept beschrieben, mit dem Ziel, die Effizienz zu verbessern und gleichzeitig auftretendes Drehmoment in der Maschine zu vermeiden. Der Antriebsstrang wird als DC/DC-Wandler mit der elektrischen Maschine als Transformator im Stillstand genutzt. Berechnungen zeigen eine maximale Effizienz von 88%. Ansätze zur Verbesserung des Wirkungsgrads und zur Integration des Energieflusses im Bordnetz werden in dieser Arbeit vorgeschlagen und diskutiert. Allerdings muss der Rotorkäfig geöffnet werden, um ein Drehmoment während des Laden zu vermeiden. Dies stellt einen ähnlichen Aufwand dar wie die Darstellung eines separaten Ladegeräts. Somit ist dieses Konzept aus heutiger Sicht wegen niedriger Effizienz und hoher Kosten gegenüber einem separaten Ladegerät nicht konkurrenzfähig. Zwei Ladekonzepte ohne galvanische Trennung, die eine sechsphasige elektrische Maschine als in Serie geschaltete Hoch- und Tiefsetzsteller nutzen, werden im Rahmen der Arbeit vorgestellt und bezüglich der benötigten Komponenten, der Effizienz und des Drehmoments des Maschine ausgearbeitet. Die Antriebsstrangverluste werden für die Ladebedingungen mit Gleichströmen analysiert, basierend auf neuen Materialcharakterisierungen für die angewendete Belastung. Es wurden Wirkungsgrade bis zu 93% demonstriert und auch in theoretischen Berechnungen mit einer maximalen Abweichung von ±1% zum experimentellen Befund bestätigt. Zum Schutz gegen elektrischen Schlag bei nicht isolierten Ladekonzepten werden drei Konzepte für eine Fehlerstrommessung präsentiert und anhand von Messergebnissen analysiert. Siliziumkarbid-Inverter-Technologien zeigen in Kombination mit diesen Ladekonzepten Wirkungsgrade, die vergleichbar zu herkömmlichen separaten Ladegeräten sind, und weisen dabei deutlich geringere Kosten auf

Improved transistor-controlled and commutated brushless DC motors for electric vehicle propulsion

The development, design, construction, and testing processes of two electronically (transistor) controlled and commutated permanent magnet brushless dc machine systems, for propulsion of electric vehicles are detailed. One machine system was designed and constructed using samarium cobalt for permanent magnets, which supply the rotor (field) excitation. Meanwhile, the other machine system was designed and constructed with strontium ferrite permanent magnets as the source of rotor (field) excitation. These machine systems were designed for continuous rated power output of 15 hp (11.2 kw), and a peak one minute rated power output of 35 hp (26.1 kw). Both power ratings are for a rated voltage of 115 volts dc, assuming a voltage drop in the source (battery) of about 5 volts. That is, an internal source voltage of 120 volts dc. Machine-power conditioner system computer-aided simulations were used extensively in the design process. These simulations relied heavily on the magnetic field analysis in these machines using the method of finite elements, as well as methods of modeling of the machine power conditioner system dynamic interaction. These simulation processes are detailed. Testing revealed that typical machine system efficiencies at 15 hp (11.2 kw) were about 88% and 84% for the samarium cobalt and strontium ferrite based machine systems, respectively. Both systems met the peak one minute rating of 35 hp

The determination of the parameters of non-linear equivalent circuits for synchronous machines and transformers

Imperial Users onl

Microwave Superconductivity

We give a broad overview of the history of microwave superconductivity and explore the technological developments that have followed from the unique electrodynamic properties of superconductors. Their low loss properties enable resonators with high quality factors that can nevertheless handle extremely high current densities. This in turn enables superconducting particle accelerators, high-performance filters and analog electronics, including metamaterials, with extreme performance. The macroscopic quantum properties have enabled new generations of ultra-high-speed digital computing and extraordinarily sensitive detectors. The microscopic quantum properties have enabled large-scale quantum computers, which at their heart are essentially microwave-fueled quantum engines. We celebrate the rich history of microwave superconductivity and look to the promising future of this exciting branch of microwave technology.Comment: 18 page

Topological Photonics

Topological photonics is a rapidly emerging field of research in which geometrical and topological ideas are exploited to design and control the behavior of light. Drawing inspiration from the discovery of the quantum Hall effects and topological insulators in condensed matter, recent advances have shown how to engineer analogous effects also for photons, leading to remarkable phenomena such as the robust unidirectional propagation of light, which hold great promise for applications. Thanks to the flexibility and diversity of photonics systems, this field is also opening up new opportunities to realize exotic topological models and to probe and exploit topological effects in new ways. This article reviews experimental and theoretical developments in topological photonics across a wide range of experimental platforms, including photonic crystals, waveguides, metamaterials, cavities, optomechanics, silicon photonics, and circuit QED. A discussion of how changing the dimensionality and symmetries of photonics systems has allowed for the realization of different topological phases is offered, and progress in understanding the interplay of topology with non-Hermitian effects, such as dissipation, is reviewed. As an exciting perspective, topological photonics can be combined with optical nonlinearities, leading toward new collective phenomena and novel strongly correlated states of light, such as an analog of the fractional quantum Hall effect.Comment: 87 pages, 30 figures, published versio

Design and Demonstration of Embedded Inductors for High-Voltage Integrated Voltage Regulators

Increased functionalities and computational capacity of today’s electronic systems have resulted in the need for higher power density. Current multi-stage 48 V to 1 V power delivery networks shows efficiencies of 75% or lower. Substrate-embedded inductors can enable the miniaturization of power modules and Integrated Voltage Regulators (IVRs) making possible single-stage down-conversion of 12 V to 1 V or 48 V to 1 V, improving both the system efficiency and regulation bandwidth. The design rules of inductor for single-stage high-conversion-ratio IVRs are quite different and challenging compared to low voltage converter like 1.7 V to 1 V. With extensive design exploration and experimentation, we have validated a novel inductor and fabrication technology along with a novel design methodology. We have demonstrated over 42 fabricated embedded inductors with 7 different designs and 6 different magnetic materials spanning an inductance range from 10 nH to over 500 nH, DC resistance from 14 mOhm to 40 mOhm, and saturation current from 100 mA to over 5 A. We have proposed and validated a new inductor power loss calculation method that includes the effect of frequency, duty cycle, and large-signal (or hysteresis) losses, and only circuit quantities such as inductance and resistance, current ripple, and power loss need to be measured. This new method evolves in an inductor design framework that allows predicting the performance of complex embedded inductors using a discrete toroidal inductor that takes only one day to fabricate. We have demonstrated an inductor with 60 nH, density of 12 nH/mm2, 23 mOhm of DC resistance, a maximum current of 5A, a current density of 1 A/mm2, and an inductance to DC resistance ratio of 2850 nH/Ohm. However, for 12 V to 1 V single-stage IVRs, more advances need to be made for the magnetic materials. We have determined that the required magnetic material needs a relative permeability of 65, loss tangent less than 0.015, saturation field over 6 kA/m, and large to small signal losses ratio of 4. Finally, a scalable small-signal SPICE model is presented. This model allows obtaining an ultra-wide-bandwidth inductor circuit representation with any amount of inductance (for a given magnetic material) using a single model to measurement fitting. We believe these new technologies will allow obtaining improved designs of inductors, magnetic materials, and IVRs to power the next generation of high-performance computing (HPC) platforms.Ph.D

Experimental study of the quantum phase-slip effect in NbN nanowires

Coherent quantum phase-slip (QPS) in a superconducting nanowire is the dual phenomenon to the well-known Josephson effect. Josephson junctions form the basis of superconducting electronic circuits with a wide range of applications, and each of those circuits has a corresponding dual quantum phase-slip device with a dual purpose. Examples that draw particular attention are a new quantum standard of electric current, and a quantum phase-slip qubit. The aim of this project is to develop methods of design, fabrication, and measurement of quantum phase-slip nanowires, and to demonstrate the potential of these devices for technological application. In our experiments we incorporate NbN nanowires into a superconducting loop and bias the loop with a magnetic flux. The state of the nanowire-embedded loop is then read out by coupling to a high quality coplanar waveguide resonator. In this thesis we present the results of two such experiments. First, we fabricated NbN nanowires using a neon focused-ion-beam, and measured their properties at T=300 mK. Periodic tuning of the resonant frequency of the readout resonator revealed that magnetic flux is transferred to the interior of the loop with flux-quantum-periodicity. Our measurements confirm that the flux-quantum transfer is mediated by incoherent quantum phase-slips occurring in the nanowires, and that these incoherent QPS can be fully controlled with an external bias. In the second experiment, nanowire-embedded NbN loops were fabricated by electron-beam lithography and cooled to T=10 mK. The resonant frequency tuning exhibited avoided crossings, which is evidence of coherent coupling between the resonator and a coherent quantum two-level system. We numerically fit these avoided crossings to the Jaynes-Cummings model to extract the properties of the two-level system, and find a good fit with the design parameters of our nanowire qubit. Finally we discuss whether the observation of coherent dynamics is evidence of coherent QPS in the EBL-fabricated nanowire