Hulya Kirkici

Nano/Micro Dielectric Surface Flashover in Partial Vacuum

In this paper, we present experimental results of surface flashover characteristics of epoxy resin and epoxy resin cast with powdered Al2O3 (nano/micro-composite dielectric) samples in partial pressure of nitrogen. The flashover studies are conducted separately using DC and 20 kHz AC signals. The voltage, current, and light emission waveforms are obtained. The optical data is collected by a video camera and a photomultiplier tube (PMT). The nano/micro composite dielectric samples used in the experiments are produced in-house by mixing 1:10 weight ratio of AI2O3 powder with epoxy resin of known properties. Throughout this study, copper electrodes are used and are placed over the dielectric samples to initiate surface flashover events. The results of surface flashover voltages as a function of pressure are presented. Additionally, optical emission characteristics along with the voltage and current waveform data are also presented.

Optical beam-shaping design based on aspherical lenses for circularization, collimation, and expansion of elliptical laser beams

We present two optical system designs using aspherical lenses for beam circularization, collimation, and expansion of semiconductor lasers for possible application in lidar systems. Two different optical lens systems are investigated; namely, two aspherical lens and single aspherical lens systems. Software package programs of ZEMAX and MATLAB to simulate the optical designs are used. The beam reshaping results are presented for one specific laser beam output.

Breakdown characteristics of helium and nitrogen at kHz frequency range in partial vacuum for point-to-point electrode configuration

Power devices operating in partial vacuum, such as aerospace systems or space environment are susceptible to partial discharges, corona, or volume discharges due to the partial vacuum conditions. These power systems also operate under frequencies and waveforms quite different from those studied and developed for earthbound power distribution systems. In this paper, we present our studies on the breakdown characteristics of helium and nitrogen gases under positive square-pulsed voltage at frequencies varying from 50 kHz to 200 kHz in partial vacuum, for a point-to-point electrode configuration. It is observed that high frequency breakdown voltage is lower than the dc breakdown voltage under the same experimental conditions and electrode geometry. Also, breakdown voltage versus pressure curves exhibited a Pashcen like behavior. Breakdown voltages of helium and nitrogen as a function of signal frequency are also presented.

Nano-dielectric materials in electrical insulation application

High performance electrical insulation materials and structures must be free from unwanted and unpredictable dielectric breakdown through the insulator as well as surface flashover along the interface between the insulator and the ambient or vacuum. The last decade has witnessed significant developments in the area of nano-particulate and nano-dielectric materials, and significant effects of nano-scale fillers on electrical, thermal, and mechanical properties of polymeric materials have been observed. Surface flashover is one of the major issues in electrical insulation in power systems operating either in space (vacuum or partial vacuum) or atmospheric (earth-bound) environments. Thus, development of new and advanced materials to be used in power systems requires extensive studies on electrical insulation characteristics of these materials before they can be used in commercial systems. Most of the research in characterization of nano-dielectric materials has been for systems operating in atmospheric environment; there are limited or no data for systems using these materials in space environments. In this paper, we present experimental results of surface flashover characteristics of epoxy resin and epoxy resin cast with powdered Al/sub 2/O/sub 3/ in partial vacuum. The flashover studies were conducted separately using DC and AC signals. Surface flashover voltage and current waveforms of the samples and light emission waveforms and video images during the breakdown were recorded. Optical data collected by a video camera along with the PMT light emission data are analyzed and the results of the optical emission characteristics of surface flashover are presented along with the electrical data. The samples used in the experiments were produced by using either epoxy resin of known properties or by mixing known quantities of Al/sub 2/O/sub 3/ with epoxy. The electrode material placed over the dielectric samples was copper.

Gaseous breakdown at high frequencies under partial vacuum

High voltage devices and systems operating in space environment are usually subjected to partial discharges or corona discharges due to the partial vacuum environment, and these activities within a power system are usually considered as major design problems. Currently, space power systems operating, at voltages higher than the traditional 28 V systems are being considered. Furthermore, the availability of high power electronic devices operating at higher switching frequencies makes the corona and partial discharge problems more important than before. Our initial studies conducted in the laboratory suggest that the corona inception voltage levels at high frequency are indeed lower than the dc breakdown voltages for various pressures. Thus, this work will present the breakdown data obtained for ac-voltages at 10 kHz to 30 kHz in partial vacuum environment. Furthermore, results will be compared with the data available in the literature, and further research work in high frequency breakdown in partial pressure environment will be discussed.

Lifetime estimation and monitoring of power transformer considering annual load factors

This paper focuses on the realistic remaining service life prediction and monitoring system of power transformers, using an algorithm developed by the authors. The power transformer is one of the vital components of power transmission and distribution substations. It is important to know the loading capacity and remaining service life of a power transformer to ensure its economical and secure operation. This information is directly related to the condition of cellulosic insulation. According to classical method of prediction, the power transformer is considered to be operated under constant load for the remainder of its lifespan. However, in reality the load factor varies continuously, and this is a key factor affecting the service life of power transformers. In this study, a realistic method for monitoring and predicting the service lifetime of power transformers is presented by considering annual load factors. The proposed method is applicable to any power transformer and can be updated by considering relevant standards for different liquid and solid insulation materials. As a case study, life-loss calculation and the remaining service life predictions are carried out using a thermal model that has been created considering a 250 MVA mineral oil-immersed power transformer.

Partial vacuum breakdown characteristics of helium at 20 kHz for inhomogeneous field gap

In general, power devices and systems operating in vacuum or space environment are more susceptible to partial discharges, corona, or volume discharges due to the partial vacuum conditions. Additionally, high frequency operation of a power system is a contributing factor in lowering the breakdown voltage of insulation. In this paper we present our studies on the breakdown characteristics of helium operating in DC and 20 kHz AC field in partial vacuum, for a point-to-point and point-to-plane electrode configurations. Breakdown voltage as a function of pressure in the range of 27 to 400 Pa (0.2 to 3 torr) for both the DC and 20 kHz AC cases is presented. Voltage and current waveforms along with the optical emission waveform of the breakdown events are also presented. A variable DC power supply for DC and an in-house built variable DC-offset-AC power supply for the high frequency breakdown experiments are used. A high voltage probe and a Pearson current sensor are used for the voltage and current detection, and a photo-multiplier-tube with a digital pico-ammeter and a video camera are used for the optical signal detection of this set-up. The breakdown voltage as a function of pressure for both the AC and DC experiments, along with voltage breakdown waveforms for both electrodes are presented.

Carbon-Nanotube-Triggered Pseudospark Switch

Pseudospark switches are fast-closing low-pressure gas discharge switches with a hollow-cathode geometry. They can be triggered optically or by electron injection, where electrons emitted from a cold cathode emitter are accelerated in the electric field and initiate breakdown. The efficiency of charge carrier accumulation in the hollow-cathode structure determines the performance of the switch operation. The advantages of such switches include high hold-off voltage, high conduction current, fast current rise time, low delay and jitter time, and long lifetime. In this paper, the operating characteristics of a pseudospark switch, triggered by a carbon-nanotube (CNT)-coated electron emitter, are presented. The electron emitter is fabricated by coating randomly oriented CNTs on silicon substrate using chemical vapor deposition method. Field emission characteristics of CNTs are determined and then used as part of the trigger electrode which is integrated into an in-house constructed pseudospark switch. The operating characteristics of the switch are tested under different background pressures and different trigger voltages. Delay and jitter time under different working conditions are analyzed. Hold-off voltage and current rise time are also presented.

Operating characteristics of a segmented hollow cathode over a wide pressure range

A segmented hollow cathode has been designed, constructed and operated over a wide pressure range from 1 mbar to 200 mbar to determine optimum operating characteristics for a series of unique applications. The device is designed for use as a plasma source for molecular gas lasers operating in the UV and visible spectra. The cathode consists of 1 mm thick nickel disks and 0.1 mm tungsten foil disk segments stacked alternately, forming a cylindrical geometry on its outer surface and a near-spherical geometry inside. The tungsten foil disk inner diameters vary along the longitudinal axis of the cathode. With this special geometric arrangement one hollow cathode structure is embedded within another. The spacing between the segments (tungsten foil disks) creates one hollow cathode geometry, and the spherical geometry formed by the inner diameter of the segments is a second hollow cavity. This design permits one to operate the device at an expanded pressure range. The devices voltage and current characteristics are studied at different operating pressures and the optimum operating parameters such as pressure, voltage, current, and foil thickness are investigated. Helium, argon or a helium-argon mixture is used as the fill gas throughout the series of experiments. >

Three-Dimensional FDTD Simulation of Nonlinear Ferroelectric Materials in Rectangular Waveguide

Nonlinear transmission lines have numerous applications in the communications and defense industries due to their ability to form and propagate short-duration ultrawideband pulses. This paper simulates a short-duration Gaussian transient exciting low-order TEm,0 modes in a nonlinear ferroelectric-filled conducting waveguide. A 3-D finite-difference time-domain simulation is employed in the analysis, and the ferroelectric-filled waveguide model is based on a nonlinear polarization relationship extrapolated from measurements. A small portion of the frequency band operates in the nonlinear polarization region. These components will propagate at higher velocity than lower amplitude components, and this effect counteracts dispersion and results in compression of the pulses into solitons as they propagate.