Daniel Schweickart

Partial discharge measurements in air and argon at low pressures with and without a dielectric barrier

Partial discharge (PD) characteristics in air and argon under low pressures down to 13.3 Pa (0,1 Torr) and 60 Hz AC energization are studied in an energized needle-plane electrode arrangement. The electrode configuration, vacuum chamber, facilities, and electrical connections for the experimental setup are described. Two cases are studied for each of two gases, air and argon, with 20 mm spacing between the two electrodes: (1) with and (2) without a Teflon/spl reg/ cap (dielectric barrier). Results for the four series of experiments and analysis of the discharge current pulse waveforms are presented. Topics discussed are the typical waveforms of the discharge current pulses at different pressures, and discharge current pulse rise time vs. pressure relationships.

Dielectric Breakdown of Polymeric Insulations Aged at High Temperatures

Most of electric insulation materials used in power generation and energy storage are polymeric dielectric materials: winding insulations in power transformers, electric starter/generators, electric actuators, high frequency resonators, and power converters; dielectric films in power capacitors or energy storage devices; encapsulation in electronic components and devices. These polymeric dielectric materials have limited continuous use temperatures because of their electrical properties deteriorating at high temperatures. The temperature dependence and thermal aging effect on their dielectric breakdown are of prime concern for analyzing their failure mechanism to prevent catastrophic breakdown of the power system in aviation and military platforms. In this paper, the thermal aging effect on breakdown strength, mechanical properties and physical features of various polymeric insulations, such as polyimide (PI), polyetherimide (PEI), polyetheretherketones (PEEK), and perfluoroalkoxy (PFA) films, will be presented and discussed.

Dielectric breakdown of polymeric insulation films under AC, DC and pulsed voltages

Polymeric insulation materials are widely used in power generation and energy storage apparatuses. Several examples are: dielectric films for capacitors and resistors; encapsulations for electronic components and devices; insulations for power cables and electrical machines. The dielectric breakdown behaviors of these insulation materials are dependent of electrical stress conditions, chemical structures and their application environment, such as temperature. In this paper, the electrical stress and chemical structure effects on breakdown strength of some typical insulation films are presented and discussed.

Improved performance of polyhedral oligomeric silsesquioxane epoxies

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 improve some electrical properties such as corona endurance in polymers. In the current study, polyhedral oligomeric silsesquioxane (POSS) loaded epoxies were produced and tested for corona endurance. Results suggest a five times improvement in ac corona lifetime of selected POSS-epoxies compared to unloaded epoxy.

Failure mechanisms of polyimide and perfluoroalkoxy films under high frequency pulses

Breakdown behaviors of polyimide and perfluoroalkoxy high temperature films under unipolar and bipolar repetitive pulses are investigated. A bipolar 20kV, 20 kHz pulse generator with fast dV/dt pulse risetime has been designed and built to study the impact of pulse frequency, pulse rise time, and pulse polarity and pulse duty cycles on breakdown strength of these films films. Space charge injection and decay processes are also investigated. Possible failure mechanisms are discussed.

Helium Breakdown Characteristics under 100 kHz Range Pulsed Voltages in Partial Vacuum for Point-to-point Electrode Geometry

In this paper we present our work on breakdown studies conducted in helium at partial vacuum conditions for a point-to-point electrode setup. A high frequency pulsed voltage signal is applied across the electrodes and the voltage-current characteristics are observed. The applied signal consists of a train of square pulses in the frequency range of 50 to 200 kHz with 50% duty cycle and rise/fall times in the range of 20-30 ns. These studies were conducted to understand and compare the role of the pulse repetition rate in electrical breakdown initiation in low pressure conditions. Preliminary data of voltage and current waveforms, along with the light emission data are presented. The optical data collected by a PMT (photo multiplier tube) as a function of the time is presented in comparison to the varying voltage

Low-Pressure Partial-Discharge Measurements: Monitoring the Insulation Integrity of Aircraft Power Wiring Systems

Partial-discharge (PD) characteristics in three gases (air, argon and helium) at pressures between 101 kPa (760 Torr) and about 0.27 kPa (2 Torr) under 60-Hz ac energization were studied. Measurements for two representative electrode configurations showed that the electrical characteristics of PD (specifically, electric pulse waveforms) at low pressures differ significantly from those at sea-level pressures. Difficulties in obtaining accurate measurements when adhering to guidelines defined by the IEC 60270 standard are described. Suggested modifications of the standard procedures are presented for measurement and calibration for low-pressure PD.

Partial discharge characteristics at low pressures in dry air and argon

Characteristics of partial discharges (PD) in two gases (air and argon) under pressures in the range of 0.1 to 760 Torr, and 60 Hz AC energization are discussed for an energized needle grounded plane electrode arrangement. The most significant conclusion of the study is that the frequency content of PD current pulses for all the tests conducted is much lower at low pressures than at atmospheric pressure. In general, the frequency content of PD current pulses at low pressures is outside the detection range of typical commercially available PD equipment.

Corona testing of prototype avionics connectors

Corona tests were run on three prototype avionics connectors intended for use in a 270-V direct-current aircraft electrical system. The corona performance was evaluated in an altitude simulation chamber. The control parameters for the testing included pressure, temperature, and applied voltage. A sophisticated test stand was developed to provide control of pressure gradients to better simulate airframe mission profiles. Test results indicate that gas desorption or outgassing from insulating surfaces subjected to pressure excursions may play a major role in the level of discharge activity.<<ETX>>

Flashover and breakdown investigations of aircraft wiring in low pressure environment

The flashover and breakdown characteristics of (a) electrical wiring and (b) areas between traces of printed circuit (PC) boards used in aircraft are a function of the environment in which these systems operate. As the altitude of an aircraft increases, the pressure decreases and the flashover and breakdown characteristics change. This change, combined with the trend to apply higher voltages (270 VDC), and the possibility of spikes superimposed on the 270 VDC supply voltage due to power-electronic-based controllers, makes it imperative that the standards used to manufacture and test these wiring components be revisited and updated as appropriate.