D.L. Schweickart

An evaluation of diagnostic techniques relevant to arcing fault current interrupters for direct current power systems in future aircraft

Arc fault circuit interrupters (AFCI) are a relatively new development in the area of circuit protection. Typical circuit breakers are designed to detect an overload current in the protected circuit and trip open within a specified period of time. The AFCI is a device that detects and interrupts an arcing fault when it occurs. An arc fault is a dangerous situation with very high temperatures in the arc. These high temperatures are sufficient to ignite wire insulation and other combustibles in the vicinity of the arc. Standard circuit breakers and ground fault circuit breakers generally will not trip in the event of an arc fault since there is sufficiently high impedance in the circuit to limit current below the trip level of its characteristic curve. Numerous arc faults that have been observed on aging aircraft, due to failing insulation, have spurred an interest in AFCIs for both commercial and military aircraft. Such AFCIs assess the AC current waveform to distinguish that an arc is occurring in the circuit. When this occurs, the AFCI will open to mitigate damage to the wiring system. In the AC case, the opening device will generally take advantage of the natural current zero in the AC wave form to clear the circuit. Protective devices for DC systems cannot exploit this characteristic. In addition, the charge transport properties (conductivity) of the DC arc are affected by the gas density (i.e., altitude) surrounding the arc. This paper presents experimental results intended to help evaluate the potential applicability of the typical diagnostics used in AC AFCIs as a valid means of detecting a series DC arc. This is particularly relevant to future aircraft power systems employing 270 Volt DC distribution. Also, these DC arcs have been characterized at low atmospheric pressures to simulate high altitude operation

Development of procedures for partial discharge measurements at low pressures in air, argon and helium

Partial discharge (PD) characteristics in air, argon and helium at pressures between 101 kPa (760 Torr) to about 0.27 kPa (2 Torr) under 60 Hz AC energization were studied with various electrode arrangements. Measurements are presented for two representative electrode configurations, (1) needle-plane, with 20 mm spacing and a dielectric barrier, and (2) a twisted pair of insulated conductors. Typical PD current pulse waveforms are presented. Difficulties in adhering to measurement guidelines defined by the IEC 60270 standard are described, and suggested modifications of the standard procedures are presented for measurement and calibration for low-pressure PD.

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.

High frequency breakdown characteristics of various electrode geometries in air

In airborne and spacecraft electronic applications, minimum weight and volume constraints have always influenced electrical component and cabling designs. Insulation system specifications must include the total operating environment, voltage excursions, current capacity, mechanical integrity and aging. This is true for both power source equipment as well as special purpose electronics such as radar or high power transmitters, where bare or insulated conductors are often used for high frequency signal transmission, as well as power transmission. A brief review of relevant data is presented to exemplify the effects of high frequency content on breakdown voltages in air, for specific electrode configurations. At high frequencies, the spacing between electrodes or conductors becomes a deterministic parameter for the breakdown phenomenon known as multipactor. When the mean free path of an electron exceeds the order of electrode spacing or relative dimension of the high frequency system, multipactor breakdown may occur, usually at pressures less than 10/sup -2/ torr. Multipactor is prevalent within or around components of the high frequency system such as the waveguide, antenna, cabling, connectors and switches. Information that may be useful to designers of components and interconnections includes high frequency initiation phenomena and multipactor breakdown characteristics. Data concerning low pressure corona initiation on antennae is also presented.

Polyamideimide-alumina nanocomposites for high-temperatures

Metal oxide nanoparticles have been shown to improve the dielectric breakdown strength of insulating polymers. Polyamideimide is a material of practical use for high temperature insulation, and nanoparticle fillers show promise in improving the high temperature stability and dielectric breakdown strength. This work demonstrates the effect of 40–50 nm alumina nanoparticles on the improvement in AC breakdown strength of polyamideimide at elevated temperature and characterizes related electrical properties to gain insight into the mechanisms leading to this improvement. Breakdown tests were performed at temperatures up to 300 °C. The buildup and movement of space charge as a function of filler loading was measured at room temperature using pulsed electroacoustic analysis. Dielectric spectroscopy was used to measure the permittivity at temperatures up to 300 °C. In addition to improving the dielectric breakdown strength, alumina nanoparticles also improved the resistance to thermal degradation, which should allow operation of the composites at temperatures above the operating temperature of the base resin.

Dielectric Integrity of High-Temperature Nanocomposites

The addition of nanoscale metal oxide fillers to polymers has been shown, in many cases, to lead to an improvement in the dielectric breakdown strength and voltage endurance. In this paper, dielectric properties for silica- and alumina-filled polyamideimide (PAI) thin films are reported as a function of particle loading. The dispersion and thermal behavior are quantified. Experiments were also conducted using particulates which were functionalized with Aminopropyltriethoxysilane in order to augment the chemical bonding to the host matrix. The glass transition temperature and decomposition temperature are reported as a function of nanoparticle type and loading. The dielectric strength is provided for both AC and DC voltages. It was found that the enhancement in breakdown strength for a nanocomposite formulation is greater under DC conditions than AC. In addition, alumina filled PAI was found to exhibit greater electrical breakdown strength than silica filled PAI. A discussion of possible reasons is included.

Improved Electrical Properties of Epoxy Resin with Nanometer-Sized Inorganic Fillers

Nanometer-sized inorganic fillers are increasingly used as reinforcing materials for mechanical or thermal property improvement of polymers. Improvements in mechanical modulus or heat deflection temperature are often realized. These fillers may also improve some electrical properties such as corona endurance or dielectric breakdown voltage in polymers. In compact high voltage power supplies, epoxy resins are often the potting material of choice in manufacturing processes. This is often true for applications requiring a robust or position-insensitive insulation system design, such as mobile communications equipment or aerospace flight vehicles. Nanometer-sized inorganic fillers in epoxy resins can result in improved mechanical and electrical performance, without affecting the processes for component manufacturing. In our previous work, polyhedral oligomeric silsesquioxane (POSS) was selected as the nanometer-sized inorganic filler of interest. POSS-filled epoxies showed a five times improvement in ac corona lifetime for selected POSS-epoxy blends compared to unloaded epoxy. In the current study, the average dielectric breakdown voltage of POSS-filled epoxy was increased thirty-four percent compared to unloaded epoxy. Additionally, scanning electron microscopy showed uniform dispersion of the POSS filler down to a level of 10-100 nm. Dispersion uniformity appears to be a critical parameter in obtaining the desired property enhancements.

Inorganic fillers for corona endurance enhancement of selected polymers

In high voltage applications, polymer insulation can be exposed to very high electrical field stress, resulting in long term exposure to corona. The electrical field stress may be much below dielectric breakdown threshold. Eventually the exposure to corona can lead to failure of the high voltage component. Nanometer sized inorganic fillers are increasingly used as reinforcing materials for mechanical or thermal property improvement of polymers. Improvements in mechanical modulus or heat deflection temperature are often realized. These fillers may also increase some electrical properties such as corona endurance in polymers. Polyhedral oligomeric silsesquioxane (POSS) loaded polymers are tested for corona endurance. POSS is a synthesized silicon based cage structure with an intermediate oxidation between SiO and SiO/sub 2/. Results suggest at least a five times improvement in ac corona. lifetime of selected POSS-polymer composites.

Breakdown Voltage of Thermoplastics with Clay Nanometer-Sized Fillers

The addition of fillers, such as talc, mica and carbon black, are used commonly in industry to improve physical properties of polymers, such as stiffness, hardness, wear, heat distortion temperature or electrical conductivity, or to reduce the overall raw material cost of a part. Not withstanding these opportunities, the addition of micron-sized fillers to a polymer may have detrimental effects on its dielectric characteristics, such as dielectric loss, breakdown strength and dielectric durability. Recently, the addition of nanometer-sized fillers, or nanofillers, has shown potential for improving the polymers dielectric breakdown voltage in conjunction with augmentation of its mechanical properties. Five different sets of thermoplastics were tested between opposed cylindrical rod electrodes of 6.4 mm diameter with rounded edges of 0.8 mm radius. The applied voltages were at 60 Hz. All polymers in this study showed an increase in the average dielectric strength from five to fifty-six percent with the nanoscale dispersion of 1-5 wt% organically modified montmorillonite (nanoclay). Most of these increases exhibited statistically significant margins. The tested thermoplastic polymers include nylon-6, low-density polyethylene, low-density polyethylene/ethylene-vinyl acetate copolymer, and polyester. The percent composition of nanofiller was confirmed by thermogravimetric analysis and nanofiller distribution was analyzed using transmission electron microscopy.

Partial discharge measurements-frequency related considerations

Partial discharge (PD) test circuits have been developed for various test series in different gases in the 13.3 Pa to 101.3 kPa pressure range with a needle-plane electrode arrangement. (The purpose of the test series is to establish results in terms of PD inception voltage, waveforms and rise times.) The paper describes features of the initial and improved test circuits, in terms of components, circuit layout, grounding and safety aspects. Impedance vs. frequency measurements performed in the 1 kHz-200 MHz frequency range are reviewed and discussed. PD current waveform measurement procedures are also reviewed. Waveform measurements are analyzed in terms of the initial rise time and the discharge rate of the coupling capacitor.