High Current Levels Research Articles

Influence of end-effect on torque-speed characteristics of various switched flux permanent magnet machine topologies

Purpose– The purpose of this paper is to investigate and compare the influence of end-effect on the torque-speed characteristics of three conventional switched flux permanent magnet (SFPM) machines having different stator/rotor pole combinations, i.e. 12/10, 12/13 and 12/14 as well as three novel topologies with less permanent magnets (PMs), i.e. multi-tooth, E-core and C-core.Design/methodology/approach– SFPM machines combine the advantages of simple and robust rotor and easy management of the temperature due to the location of the PMs and armature windings on the stator. However, due to spoke location of the PMs a large flux leakage in the end region, i.e. end-effect, can be observed which could result in a large reduction in the electromagnetic performance. Therefore, the influence of end-effect on the torque-speed characteristics is investigated. 3D-finite element analyses (FEA) results are compared with their 2D-FEA counterparts in order to account for the end-effect influence.Findings– It has been concluded that due to end flux leakage, lower torque capability in the constant torque region is observed in the six machines. However, improved flux-weakening capability in the conventional machines can be exhibited at high current levels, whereas due to the large inductance lower power capability in the multi-tooth, E-core and C-core machines is obtained.Research limitations/implications– The influence of temperature rise on the performance is not included.Originality/value– This paper has analysed the influence of end-effect on the torque-speed characteristics of several SFPM machines.

EEG‐based brain source localization using visual stimuli

ABSTRACTElectroencephalography (EEG) is widely used in variety of research and clinical applications which includes the localization of active brain sources. Brain source localization provides useful information to understand the brain's behavior and cognitive analysis. Various source localization algorithms have been developed to determine the exact locations of the active brain sources due to which electromagnetic activity is generated in brain. These algorithms are based on digital filtering, 3D imaging, array signal processing and Bayesian approaches. According to the spatial resolution provided, the algorithms are categorized as either low resolution methods or high resolution methods. In this research study, EEG data is collected by providing visual stimulus to healthy subjects. FDM is used for head modelling to solve forward problem. The low‐resolution brain electromagnetic tomography (LORETA) and standardized LORETA (sLORETA) have been used as inverse modelling methods to localize the active regions in the brain during the stimulus provided. The results are produced in the form of MRI images. The tables are also provided to describe the intensity levels for estimated current level for the inverse methods used. The higher current value or intensity level shows the higher electromagnetic activity for a particular source at certain time instant. Thus, the results obtained demonstrate that standardized method which is based on second order Laplacian (sLORETA) in conjunction with finite difference method (FDM) as head modelling technique outperforms other methods in terms of source estimation as it has higher current level and thus, current density (J) for an area as compared to others.

Proposta e Análise de Dispositivo para Otimização do Destacamento da Gota na Soldagem MIG/MAG

Resumo Diversas forças atuam na soldagem MIG/MAG durante a transferência da gota fundida presente na extremidade do arame-eletrodo à poça de fusão, as quais variam de acordo com o modo de transferência adotado. No modo globular, o destacamento da gota ocorre quando a sua força peso supera a tensão superficial produzindo gotas maiores que provocam maior instabilidade durante sua transferência. Já no modo goticular, os altos valores de corrente intensificam a atuação da força de Lorentz. Esta força magnética proporciona o desprendimento da gota ainda em pequenas dimensões, resultando numa maior estabilidade. Entretanto, este modo de transferência caracteriza-se por um relativo grande aporte de energia, o que inviabiliza uniões de chapas finas ou fora de posição. Busca-se então neste trabalho, analisar a viabilidade da utilização de uma força magnética externa que auxilie o processo de destacamento da gota e propor um dispositivo para esta finalidade. O intuito é obter um modo de transferência semelhante ao goticular, porém, com baixos valores de corrente de soldagem sem, entretanto, a necessidade de emprego de fontes eletrônicas. Deste modo, espera-se obter uma maior estabilidade no processo aliada a uma menor quantidade de calor entregue a peça. Contudo, os resultados obtidos mostram a inviabilidade tecnológica quanto à utilização do dispositivo proposto, dada à necessidade de emprego de níveis de correntes extremamente elevados (da ordem de 100.000A) para sensibilizar a gota na ponta do eletrodo.

Dopaminergic-primed fetal liver mesenchymal stromal–like cells can reverse parkinsonian symptoms in 6-hydroxydopamine-lesioned mice

Background aimsCell replacement therapy is considered a promising alternative in the treatment of degenerative diseases, and in this context, mesenchymal stromal cells (MSCs) have been proposed for transplantation in Parkinson disease (PD). Thus far, the results of animal studies are found to be inconsistent and inconclusive regarding the therapeutic ability of the cells. This study investigated the efficacy of fetal liver (FL)-MSC-derived dopaminergic (DA) neuronal primed cells for correction of parkinsonian symptoms in mice. MethodsFL-MSCs were differentiated for 21 days in the presence of a combination of neurotropic factors. The extent of cellular reprogramming was analyzed by quantitative polymerase chain reaction for DA-specific neuronal gene expressions and protein expressions by immuno-cytochemistry. The functionality of the cells was determined by electrophysiology and dopamine release assays. Ten-day-primed neuron-like cells or unprimed MSCs were transplanted into the 6-hydroxydopamine (6-OHDA)-lesioned striatum using a stereotaxic device. Dopamine-secreting properties and behavioral studies were used to assess improvement of parkinsonian symptoms. ResultsThe differentiated cells expressed DA-specific genes and proteins, while exhibiting a high level of voltage-gated potassium current. Furthermore, neuronal primed cells differentiated into tyrosine hydroxylase immunoreactive and dopamine-secreting functional neuron-like cells. Symptomatic correction of PD in the recipient mice within 2 months of transplantation was also observed. DiscussionFL-MSC-derived primed neuron-like cells integrated into the striatum of PD mice, improving parkinsonian symptoms. This study demonstrates an effective cell-based therapy for PD.

North-East Asian Super Grid for 100% renewable energy supply: Optimal mix of energy technologies for electricity, gas and heat supply options

In order to define a cost optimal 100% renewable energy system, an hourly resolved model has been created based on linear optimization of energy system parameters under given constrains. The model is comprised of five scenarios for 100% renewable energy power systems in North-East Asia with different high voltage direct current transmission grid development levels, including industrial gas demand and additional energy security. Renewables can supply enough energy to cover the estimated electricity and gas demands of the area in the year 2030 and deliver more than 2000TWhth of heat on a cost competitive level of 84€/MWhel for electricity. Further, this can be accomplished for a synthetic natural gas price at the 2013 Japanese liquefied natural gas import price level and at no additional generation costs for the available heat. The total area system cost could reach 69.4€/MWhel, if only the electricity sector is taken into account. In this system about 20% of the energy is exchanged between the 13 regions, reflecting a rather decentralized character which is supplied 27% by stored energy. The major storage technologies are batteries for daily storage and power-to-gas for seasonal storage. Prosumers are likely to play a significant role due to favourable economics. A highly resilient energy system with very high energy security standards would increase the electricity cost by 23% to 85.6€/MWhel. The results clearly show that a 100% renewable energy based system is feasible and lower in cost than nuclear energy and fossil carbon capture and storage alternatives.

On The Cost Implications of Technical Energy Losses on Nigerian 330-kV Transmission Grid System

Base on the author’s result of power line losses obtained for low, medium and high current levels as 146.73MW, 322.24MW and 738.28MW respectively, in his bid to evaluate the power line losses using symmetrical component theory of unbalanced fault, the annual energy (MWH) losses for year 2013 was calculated and validated in this study. The annual technical energy losses due to the low, medium and high power losses were respectively found to be 443.45GWH, 976.895GWH and 2231.230GWHbased on Load Factor and Load Loss Factor amounting to N8.4 billion, N18.6 billion and N42.4 billion respectively. The low power loss (steady-state) result of this work was validated by the result of load-flow obtained using the MATLAB and Power Word Simulator (PWS) while the annual MWH for the high power loss level compares favourably well with the normal practice of utility operator’s monthly energy balance thereby closing the gap between the practical information and the theoretical one.

On the avalanche ruggedness of optimized termination structure for 600 V punch-through IGBTs

In this paper, the current paths in avalanche conditions of a Floating Field Ring (FFR) termination for a Punch-Through (PT) Insulated Gate Bipolar Transistor (IGBT) are analyzed. The design of the termination region is achieved with two different optimization techniques, and both static and dynamic electrical behavior are analyzed by means of 2D TCAD simulations, up to high current density levels. A comprehensive analysis of the Unclamped Inductive Switching (UIS) operation of the proposed terminations is carried-out with electro-thermal simulations. Although the behavior of both structures at low current levels is different, results show the same current crowding effect at the main junction for high current levels, resulting in a reduced conduction area of the overall termination, hence, of the avalanche reliability. Finally, experimental confirmation of filamentary current conduction during UIS test are detected on 600V commercial devices by means of transient infrared thermography.

Capacitance roll‐off and frequency‐dispersion capacitance–conductance phenomena in field plate and guard ring edge‐terminated Ni/SiO2/4H‐nSiC Schottky barrier diodes

In this work, field plate and guard ring edge‐terminated Ni/4H‐nSiC Schottky barrier diodes (SBD) were fabricated using standard photolithography process. Strange peaks in capacitance–conductance curves, capacitance roll‐off, and a high value of ideality factor ( = 1.3) in fabricated SBD were seen as a signature of interface trap states (Nss) at the residual oxide (2.2 nm)/4H‐nSiC interface and series resistance (Rs). Schottky capacitance spectroscopic, High–low capacitance–voltage (C–V) and forward‐bias current–voltage (I–V) techniques, in the frequency range from 100 Hz to 1 MHz, determines Nss of the order of 1012 cm−2 eV−1 and were found exponentially distributed in the bandgap of SiC. Using Hill–Coleman's method, the density Nss was calculated to be 1.15 × 1015 cm−2 eV−1 at 100 Hz and 7.81 × 1012 cm−2 eV−1 at 1 MHz, which explains the larger value of capacitance at low frequencies. Relaxation times and capture cross sections of Nss were also estimated. Calculated values of Nss were used in a Silvaco simulation that emphasize that bulk level defects present in the SiC also contributes in the experimentally observed strange peaks in C–V characteristics of fabricated SBD. At higher current levels, calculated values of Rs (V, ), confirm an increase of leakage current through residual oxide and describes the capacitance roll‐off phenomena in the fabricated SBD.

Performance of the cold powered diodes and diode leads in the main magnets of the LHC

During quench tests in 2011 variations in resistance of an order of magnitude were found in the diode by-pass circuit of the main LHC magnets. An investigation campaign was started to understand the source, the occurrence and the impact of the high resistances. Many tests were performed offline in the SM18 test facility with a focus on the contact resistance of the diode to heat sink contact and the diode wafer temperature. In 2014 the performance of the diodes and diode leads of the main dipole bypass systems in the LHC was assessed during a high current qualification test. In the test a current cycle similar to a magnet circuit discharge from 11 kA with a time constant of 100 s was performed. Resistances of up to 600 μΩ have been found in the diode leads at intermediate current, but in general the high resistances decrease at higher current levels and no sign of overheating of diodes has been seen and the bypass circuit passed the test. In this report the performance of the diodes and in particular the contact resistances in the diode leads are analysed with available data acquired over more than 10 years from acceptance test until the main dipole training campaign in the LHC in 2015.

Self-Heating Effects In Polysilicon Source Gated Transistors.

Source-gated transistors (SGTs) are thin-film devices which rely on a potential barrier at the source to achieve high gain, tolerance to fabrication variability, and low series voltage drop, relevant to a multitude of energy-efficient, large-area, cost effective applications. The current through the reverse-biased source barrier has a potentially high positive temperature coefficient, which may lead to undesirable thermal runaway effects and even device failure through self-heating. Using numerical simulations we show that, even in highly thermally-confined scenarios and at high current levels, self-heating is insufficient to compromise device integrity. Performance is minimally affected through a modest increase in output conductance, which may limit the maximum attainable gain. Measurements on polysilicon devices confirm the simulated results, with even smaller penalties in performance, largely due to improved heat dissipation through metal contacts. We conclude that SGTs can be reliably used for high gain, power efficient analog and digital circuits without significant performance impact due to self-heating. This further demonstrates the robustness of SGTs.

Nanoscale Bubble Valves on MWCNT Membranes for Chemical Energy Storage

Nanoscale Bubble Valves on MWCNT Membranes for Chemical Energy Storage

Protection Scheme for Island Operated Medium-Voltage Microgrid

During island operation, high fault currents from the utility grid are not present. Also, most of the distributed generation units that are connected to microgrid, are likely to be inverter interfaced and have limited fault current feeding capabilities. Due to that, during microgrid island operation, conventional distribution system protection schemes which assume a single path for the fault current and a high fault current level when compared to the load current, can be less efficient, may have slower fault clearing time and be less sensitive and selective. Several solutions have been proposed and many of the proposed solutions rely on the use of high-speed communication links to send tripping or blocking signals between relays. This paper proposes a new protection scheme, including both short-circuit and earth-fault protection, for island operated medium-voltage distribution network (microgrid) having multiple protection zones without the need for high-speed communication. The proposed protection scheme is not dependent from fault current contribution of the units and takes into account and is selective with low-voltage-ride-through curves of the units.

Novel bidirectional DC–DC converters based on the three-state switching cell

ABSTRACTIt is well known that there is an increasing demand for bidirectional DC–DC converters for applications that range from renewable energy sources to electric vehicles. Within this context, this work proposes novel DC–DC converter topologies that use the three-state switching cell (3SSC), whose well-known advantages over conventional existing structures are ability to operate at high current levels, while current sharing is maintained by a high frequency transformer; reduction of cost and dimensions of magnetics; improved distribution of losses, with consequent increase of global efficiency and reduction of cost associated to the need of semiconductors with lower current ratings. Three distinct topologies can be derived from the 3SSC: one DC–DC converter with reversible current characteristic able to operate in the first and second quadrants; one DC–DC converter with reversible voltage characteristic able to operate in the first and third quadrants and one DC–DC converter with reversible current and voltage characteristics able to operate in four quadrants. Only the topology with bidirectional current characteristic is analysed in detail in terms of the operating stages in both nonoverlapping and overlapping modes, while the design procedure of the power stage elements is obtained. In order to validate the theoretical assumptions, an experimental prototype is also implemented, so that relevant issues can be properly discussed.

Single-system differential cutoff of power transformers of rural power networks constructed from aperiodic magnetizing inrush current

At the present moment, the possibility of a distribution transformer suffering heavy damage because of short circuit current in its tank is still very high. Thus, it is very important to determine any coil damage inside the tank of a transformer with the highest possible speed and sufficient sensitivity. The problem of magnetizing inrush current must be solved to set up a system of protection that meets all these requirements. With digital protection devices, this problem is solved mostly by using second harmonics and a high level of cutoff operation current. All this may affect the system of protection negatively. At the same time, there is an opportunity to sustain very efficient and not overly complicated systems of protection with an appropriate level of current tripping in a distributied electrical network with low short circuit currents, such as rural power networks may be. The possibility of creating simple and effective (as well as reliable) differential cutoff is considered in this paper. The elaborated differential cutoff is more sensitive to the two- or threephase short circuits. The scheme of the differential cutoff is shown with a numerical example, which improves its efficiency. There are also oscillograms of two regimes. The first oscillogram is the regime of inrush current, and the second one is the regime of short circuit current. The elaborated device is blocked in the regime of magnetizing inrush current, and works in the short circuit regime, which disconnects the damaged transformer from the network.

Optical-cell model based on the lasing competition of mode structures with different Q-factors in high-power semiconductor lasers

A model describing the operation of a completely optical cell, based on the competition of lasing of Fabry-Perot cavity modes and the high-Q closed mode in high-power semiconductor lasers is proposed. Based on rate equations, the conditions of lasing switching between Fabry-Perot modes for ground and excited lasing levels and the closed mode are considered in the case of increasing internal optical loss under conditions of high current pump levels. The optical-cell operation conditions in the mode of a high-power laser radiation switch (reversible mode-structure switching) and in the mode of a memory cell with bistable irreversible lasing switching between mode structures with various Q-factors are considered.

Characterization and Implementation of Dual-SiC MOSFET Modules for Future Use in Traction Converters

Silicon (Si) insulated-gate bipolar transistors are widely used in railway traction converters. In the near future, silicon carbide (SiC) technology will push the limits of switching devices in three directions: higher blocking voltage, higher operating temperature, and higher switching speeds. The first silicon carbide (SiC) MOSFET modules are available on the market and look promising. Although they are still limited in breakdown voltage, these wide-bandgap components should improve traction-chain efficiency. Particularly, a significant reduction in the switching losses is expected which should lead to improvements in power–weight ratios. Nevertheless, because of the high switching speed and the high current levels required by traction applications, the implementation of these new modules is critical. An original method is proposed to compare, in terms of stray inductance, several dc bus-bar designs. To evaluate the potential of these new devices, a first set of measurements, based on a single-pulse test-bench, was obtained. The switching behavior of SiC devices was well understood at turn-off and turn-on. To complete this work, the authors use an opposition method to compare Si-IGBT and SiC-MOSFET modules in voltage source inverter operation. For this purpose, a second test-bench, allowing electrical and thermal measurements, was developed. Experimental results confirm the theoretical loss-calculation of the single-pulse tests and the correct operation of up to three modules directly connected in parallel. This analysis provides guidelines for a full SiC inverter design, and prospects for developments in traction applications are presented.

In-plane electrical bistability of photochromic diarylethene/Cu composite film

Electrical bistability is an essential property for memory devices. We report here the in-plane electrical bistability of photochromic diarylethene (DAE)/Cu composite film, which is prepared by Cu vapor deposition on the DAE surface with a low glass-transition temperature. The low-current level around 10−8A was switched to a high-current level of ca. 10−4A at a low threshold voltage (Vth) in the first voltage sweep. Once this switching occurred, the high-current level was kept in the second voltage sweep, and electrical bistability was achieved for the in-plane current. Vth was distributed in a wide range of voltages (0.5–10V), and the colored sample obtained by the UV irradiation showed a relatively higher Vth than the colorless sample. The highest ON–OFF ratio in current was ca. 106. The origin of the bistability attributed to the electrical breakdown in the insulated lines that was consisted of DAE in Cu film. The in-plane bistability of the DAE/Cu composite film has good retention time (>60min) and readout-cycle endurance (>106 cycles), indicating that it is suitable for write-once organic semiconductor memory characteristics.

Effect of laser cavity parameters on saturation of light – current characteristics of high-power pulsed lasers

We report an experimental study of power characteristics of semiconductor lasers based on MOVPE-grown asymmetric separate-confinement heterostructures with a broadened waveguide as functions of cavity length, stripe contact width and mirror reflectivities. It is shown that at high current pump levels, the variation of the cavity parameters of a semiconductor laser (width, length and mirror reflectivities) influences the light – current characteristic saturation and maximum optical power by affecting such laser characteristics, as the current density and the optical output loss. A model is elaborated and an optical power of semiconductor lasers is calculated by taking into account the dependence of the internal optical loss on pump current density and concentration distribution of charge carriers and photons along the cavity axis of the cavity. It is found that only introduction of the dependence of the internal optical loss on pump current density to the calculation model provides a good agreement between experimental and calculated characteristics for all scenarios of variations in the laser cavity parameters.

Improved linearity in AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors with nonlinear polarization dielectric

We demonstrate highly improved linearity in a nonlinear ferroelectric of Pb(Zr0.52Ti0.48)-gated AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor (MIS-HEMT). Distinct double-hump feature in the transconductance-gate voltage (gm-Vg) curve is observed, yielding remarkable enhancement in gate voltage swing as compared to MIS-HEMT with conventional linear gate dielectric. By incorporating the ferroelectric polarization into a self-consistent calculation, it is disclosed that in addition to the common hump corresponding to the onset of electron accumulation, the second hump at high current level is originated from the nonlinear polar nature of ferroelectric, which enhances the gate capacitance by increasing equivalent dielectric constant nonlinearly. This work paves a way for design of high linearity GaN MIS-HEMT by exploiting the nonlinear properties of dielectric.

A Comprehensive Study of Transistors Based on Conductive Polymer Matrix Composites

A comprehensive study is conducted on the electron transport in conductive polymer matrix composites (CPMCs), employing the nonequilibrium Green's function formalism. This paper provides a microscopic insight into the electron tunneling through the potential barriers existing between conducting sites. It is shown that Wentzel-Kramers-Brillouin approximation as well as other models with simple barrier shapes, which are widely used in literature, can lead to inaccurate results in comparison with the quantum mechanical approach using a hyperbolic barrier. In this paper, unlike most previous ones, percolation-related effects are disregarded for further focus on electron transport through the polymer potential barriers. It is assumed that a tunneling-conductive channel exists between the electrodes. This can be created either by applying electric field alignment or using a filler volume fraction higher than the percolation threshold. A two electrode resistive device is studied and the results indicate that a conductor-insulator transition occurs at a barrier thickness of ~1.7 nm and the barrier thickness should be larger than several angstroms. Next, a novel tunneling field-effect structure based on CPMCs is introduced and its characteristics are comprehensively investigated. This device features a remarkably simple structure, an extremely high channel to gate coupling, a large transconductance, and a high current level. Besides, it has the advantage of being based on polymers. This ensures favorable physical properties, ease of fabrication, and low-cost processing techniques.