Large capacitance enhancement and negative compressibility of two-dimensional electronic systems at LaAlO3_3/SrTiO3_3 interfaces

Novel electronic systems forming at oxide interfaces comprise a class of new materials with a wide array of potential applications. A high mobility electron system forms at the LaAlO$_3$/SrTiO$_3$ interface and, strikingly, both superconducts and displays indications of hysteretic magnetoresistance. An essential step for device applications is establishing the ability to vary the electronic conductivity of the electron system by means of a gate. We have fabricated metallic top gates above a conductive interface to vary the electron density at the interface. By monitoring capacitance and electric field penetration, we are able to tune the charge carrier density and establish that we can completely deplete the metallic interface with small voltages. Moreover, at low carrier densities, the capacitance is significantly enhanced beyond the geometric capacitance for the structure. In the same low density region, the metallic interface overscreens an external electric field. We attribute these observations to a negative compressibility of the electronic system at the interface. Similar phenomena have been observed previously in semiconducting two-dimensional electronic systems. The observed compressibility result is consistent with the interface containing a system of mobile electrons in two dimensions.Comment: 4 figures in main text; 4 figures in the supplemen

Investigation into effectiveness of a downwire insulator and insulated bonding wire as mitigation measures for pole-top fires in woodpole structures

Woodpole structures are widely used for overhead medium voltage (MV) distribution lines in South Africa. The problem is that wood burning, also known as pole-top fires, occurs on such structures, resulting in undesirable power outages, replacement costs and safety risks to humans and animals. The feasibility of a modified partially bonded woodpole structure with a small downwire insulator instead of the 500 mm gap along the wood in reducing the risk of pole-top fires and bird safety was investigated. Tests were conducted in a natural heavily polluted environment at the Koeberg Insulator Test Pollution Station (KIPTS), where several structures were erected and energised at 22 kV (phaseto- phase). Laboratory tests were also performed to confirm the findings obtained at KIPTS. The downwire insulators have been proven to effectively conduct leakage current to earth under polluted and wet conditions, with very little current flowing through the woodpole. Furthermore, no tracking and/or burning was observed on the woodpoles of all the test structures. The findings are promising and indicate that a downwire insulator may be effective in reducing the risk of pole-top fires. The downfall is that the impedance of the downwire insulators and insulation levels provided by the downwire insulators under such conditions may not be adequate to ensure acceptably low risk of bird electrocution. It may be feasible to design such an insulator that only conducts leakage current when necessary to prevent pole-top fires, while maintaining sufficiently high impedance to prevent bird electrocution. The preliminary specifications of the new downwire are provided

Correlation analysis between ceramic insulator pollution and acoustic emissions

La mayoría de los estudios relacionados con la contaminación de aisladores son desarrollados normalmente con base en un análisis individual entre la corriente de fuga, la humedad relativa y la densidad de los depósitos de sal equivalente (ESDD). Este trabajo presenta un análisis de correlación entre la corriente de fuga y las emisiones acústicas medidas en una subestación eléctrica de 230 kV, en la ciudad de Barranquilla, Colombia. Además, se consideraron variables atmosféricas para desarrollar un modelo de caracterización del proceso de la contaminación de los aisladores eléctricos. Este modelo fue utilizado para demostrar que los niveles de emisión de ruido son un indicador fiable para detectar y caracterizar la contaminación en aisladores de alta tensión. La correlación encontrada entre las variables atmosféricas, eléctricas y las de sonido permitieron determinar las relaciones para el mantenimiento de los aisladores cerámicos en zonas de alta contaminación. En este artículo se muestran los resultados de la investigación sobre el comportamiento de la corriente de fuga en aisladores cerámicos y el sonido que se producen ante diferentes condiciones atmosféricas, lo cual permite evaluar el mejor momento para limpiar el aislador en la subestación. Además, mediante experimentación in situ y utilizando modelos estadísticos, se obtuvo la correlación entre las variables ambientales y la corriente de fuga de la subestación. Algunos de los inconvenientes que trae el ruido externo en la medición fueron superados utilizando varios micrófonos y software especializado que permitían filtrar correctamente el sonido y lograr una mejor medición de las variables.Most of the studies related to insulator pollution are normally performed based on individual analysis among leakage current, relative humidity and equivalent salt deposit density (ESDD). This paper presents a correlation analysis between the leakage current and the acoustic emissions measured in a 230 kV electrical substations in the city of Barranquilla, Colombia. Furthermore, atmospheric variables were considered to develop a characterization model of the insulator contamination process. This model was used to demonstrate that noise emission levels are a reliable indicator to detect and characterize pollution on high voltage insulators. The correlation found amount the atmospheric, electrical and sound variables allowed to determine the relations for the maintenance of ceramic insulators in high-polluted areas. In this article, the results on the behavior of the leakage current in ceramic insulators and the sound produced with different atmospheric conditions are shown, which allow evaluating the best time to clean the insulator at the substation. Furthermore, by experimentation on site and using statistical models, the correlation between environmental variables and the leakage current substation was obtained. Some of the problems that bring the external noise were overcome using multiple microphones and specialized software that enabled properly filter the sound and better measure the variables

Effect of insulator application on the leakage current performance of woodpole distribution line structures

A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the Degree of Master of Science in Engineering, 29 February 2016.Medium voltage distribution networks use woodpole structures extensively. Woodpole structures are susceptible to burning, referred to as pole-top fires which can result in a loss of supply and electrocution hazard to the public and animals in the area. Leakage current flowing inside or on the surface of the wood is the cause of pole-top fires. Leakage current measurements on woodpole structures were conducted for varying insulator material, insulator shape and positioning on the cross-arm. This comprised laboratory measurements on a reduced scale woodpole structure that was artificially polluted to obtain a baseline. To substantiate the results, measurements on full scale woodpole structures exposed to natural pollution are presented. Leakage current performance of a woodpole structure was found to be most impacted by the choice of insulator material followed by insulator profile for silicone rubber insulators and insulator orientation for porcelain insulators. Structures with silicone rubber insulators recorded low leakage current magnitudes. The structure with Room Temperature Vulcanized (RTV) silicone rubber coated insulators yielded the most improved structure leakage current performance provided hydrophobicity is retained. The structure insulators require reapplication of the silicone rubber coating after a certain period in service. Therefore, the use of High Temperature Vulcanised (HTV) silicone rubber insulators with a wide and short profile and alternating sheds was identified to be the most attractive solution for reducing the risk of pole-top fires occurring. For cases when only porcelain insulators can be used, mounting the insulators horizontally results in less leakage current flow on the structure compared to mounting the insulators vertically. The classic woodpole distribution structure has a combination of unfavourable insulator material and orientation, close proximity to sources of pollution and critical wetting can therefore lead to severe burning at the insulation coordination gap during light pollution as shown from visual inspections. The evaluated structure cases all exhibited a voltage at the insulation coordination gap implying an existing risk of burning at the gap. Suggestions for insulator application for improved structure leakage current performance to reduce the risk of pole-top fires are offered

Modelling the Leakage Current Behaviour of Polluted Ceramic Insulators by Using Acoustic Emissions and Relative Humidity

Understanding the behaviour of leakage current (LC) is not an easy task for engineers and new techniques are needed to identify the outage risk of some electrical circuits. Therefore, this paper presents a Generalized Linear Model (GLM) to characterise the LC variations in 230 kV ceramic insulators located in outdoor electrical substations subject to high pollution levels. The model uses data such as LC, acoustic emission (AE), and environmental variables (EVs). We found that the model represents the LC variation of electrical insulators of high voltage circuits. This model is useful for designing detection systems that represent the contamination levels and predict the behaviour of changes related to EV. With this model, we can determine the risk indicators for failure of electrical insulators in high-pollution areas

Diamond Detectors for the TOTEM Timing Upgrade

This paper describes the design and the performance of the timing detector developed by the TOTEM Collaboration for the Roman Pots (RPs) to measure the Time-Of-Flight (TOF) of the protons produced in central diffractive interactions at the LHC. The measurement of the TOF of the protons allows the determination of the longitudinal position of the proton interaction vertex and its association with one of the vertices reconstructed by the CMS detectors. The TOF detector is based on single crystal Chemical Vapor Deposition (scCVD) diamond plates and is designed to measure the protons TOF with about 50 ps time precision. This upgrade to the TOTEM apparatus will be used in the LHC run 2 and will tag the central diffractive events up to an interaction pileup of about 1. A dedicated fast and low noise electronics for the signal amplification has been developed. The digitization of the diamond signal is performed by sampling the waveform. After introducing the physics studies that will most profit from the addition of these new detectors, we discuss in detail the optimization and the performance of the first TOF detector installed in the LHC in November 2015.Comment: 26 pages, 18 figures, 2 tables, submitted for publication to JINS

Laboratory investigation of insulator contamination problems

Includes bibliographical references.This thesis deals with the practical aspect of the laboratory examination of contaminated insulators. A technique which gives a pollution curve for cylindrical insulator models is described. The technique is later extended to apply to practical insulator shapes, giving curves which can be used to assess insulator performance. A method of applying these curves to results obtained in the field is suggested. The method takes into account the effect of insulator shape and weather conditions, on the accumulation of contaminant at a given test site. In addition, a system is described whereby leakage currents from contaminated insulators can be measured and recorded. Typical results obtained are given as examples of system application

High Voltage Insulation Surface Condition Analysis Using Time Frequency Distribution

This paper presents an analysis of high voltage insulation surface condition under high contamination level using experimental test of tracking and erosion according to BS EN 60587 standard. Surface condition on glass and polymeric types of insulation is investigated. The leakage current patterns on these types of insulation are applied for surface condition classification. Due to the limitation fast Fourier transforms (FFT), a new approach of time frequency distributions (TFD) analysis is employed. Spectrogram and s-transform as time frequency distribution (TFD) are then applied to represent and analysis of the leakage current (LC) in time frequency representation (TFR) in temporal and spectral. These techniques extract relevant information from leakage current (LC) signal, and then leakage current (LC) parameters are estimated to identify its characteristics. These include root mean square current (RMS), total harmonic distortion (THD), total non harmonic distortion (TnHD) and total current waveform distortion (TWD). Leakage current’s instantaneous root mean square current per unit value and total current waveform distortion percentage are useful to reveal the insulator surface condition. Referring to rules based value, the classification of material surface state could be determined instantaneously. Instead of fast Fourier transform (FFT), it was found that time frequency distribution (TFD) is an appropriate tool for surface condition classification. Then, the unsymmetrical discharge pattern on polymeric insulation material indicated that erosion is occurred on the surface of the insulator

Galileo lithium/SO2

The current status of the Galileo lithium SO2 battery is described. The following general requirements of the battery are discussed: (1) electrical characteristics, (2) storage, (3) reliability, and (4) performance