M.J. Given

The effects on polyetheretherketone and polyethersulfone of electron and /spl gamma/ irradiation

In this study, the effects of ionizing radiation on the aromatic polymers polyethersulfone (PES) and polyetheretherketone (PEEK) were investigated. UV light, /spl gamma/ rays and a high-energy electron beam were used as radiation sources. A range of techniques has been utilized to investigate changes in PES and PEEK as a result of exposure to these sources. Electron spin resonance (ESR) measurements showed the creation of radicals in PES by UV or /spl gamma/ radiation. These radicals decayed, but some were still present 24 h after irradiation at room temperature. In PEEK, radicals were formed as a result of UV at room temperature and by UV or /spl gamma/ irradiation at 77 K. After irradiation at room temperature, these radicals decayed totally within 24 h. UV-visible absorption measurements of PES samples showed a dose related change at the absorption edge; no such change was observed for PEEK. Phosphorescence after /spl gamma/ irradiation increased in intensity for PES at low temperature but decreased at higher temperatures. For PEEK, no such changes were observed after irradiation. The differential scanning calorimeter (DSC) measurements showed changes in the crystallization, glass transition and melt-point temperatures of both PES and PEEK after electron irradiation. Electrical changes induced by irradiation were investigated using thermally stimulated discharge current (TSDC) and low frequency dielectric responses, derived from transient current (TC) measurements. Dielectric changes were observed for PES exposed to either /spl gamma/ rays or an electron beam. For PEEK only minor changes in the TSDC spectrum were observed after exposure to /spl gamma/ radiation. More significant changes were observed after exposure to the electron beam. Correlations were found between the production of radicals and the changes in the structural and electrical properties of the materials. Since significant changes were observed in the dielectric spectra of PEEK and PES at low frequencies, dielectric spectroscopy may provide a measure of aging in both insulation systems.

Hydrodynamic modelling of transient cavities in fluids generated by high voltage spark discharges

Application of a voltage pulse having a rise time of tens of nanoseconds to electrodes immersed in water results in streamer development and the formation of a highly conductive plasma channel between the electrodes. The electrical resistance of such channels decreases rapidly from a few ohms to a few tens of milliohms due to Joule heating resulting from the high current which flows through the plasma. The dynamics of the plasma resistance depend on the parameters of the discharge circuit and the medium in which the discharge takes place. The resistance of the channel reaches a minimum value approximately at the moment of the peak current for under-damped current oscillations. During the resistance collapse, the pressure inside the channel rises to several GPa, causing a rapid expansion of the channel which forms a cavity in the liquid resulting in a high power ultrasound pulse. The cavity expands to a maximum size which is dependent on the circuit driving the discharge and the properties of the plasma discharge channel. The cavity then collapses producing a second acoustic pulse. In this paper the dynamic resistance of the spark channel is described using a phenomenological model based on the plasma channel energy balance equation used by Braginskii. The model which links the hydrodynamic characteristics of the channel and the resulting cavity with the parameters of the electric driving circuit allows the development of the plasma channel and cavity to be predicted. The peak high-power ultrasound (HPU) pressures calculated using this approach are compared with the pressure values estimated by an analytical model which uses a constant value of the spark channel resistance derived from experimental data. Comparisons are also made with direct measurements of HPU output made using a Pinducer sensor. Although the model is based on a phenomenological description of the plasma channel dynamics and its resistance and requires the value of the spark constant, the results obtained using this approach provide a reasonable agreement with experimental measurements and could therefore be used for the estimation of HPU pulse characteristics in practical applications of spark discharges in water.

Bactericidal Effect of Corona Discharges in Atmospheric Air

This paper explores the possibilities of using impulsive and steady-state corona discharges for biodecontamination operations. A high tension tubular corona electrode was stressed with positive or negative dc voltage with magnitude up to 26 kV, and a grounded mesh was used as an opposite electrode. Different operational regimes of this corona generator were investigated for the production of ozone in air flow and the inactivation of microorganisms. The test microorganisms used in this work were Escherichia coli and Staphylococcus aureus, populations of which were seeded onto agar plates. These bacterial plates were located behind the grounded mesh electrode to assess bactericidal efficacy. The results show that corona discharges have a strong bactericidal effect, for example, positive flashing corona discharges were able to reduce populations of the test microorganism by ~ 94% within a 30-60-s time interval. Negative steady-state corona discharges also produce noticeable bactericidal effect, reducing population of E. coli and S. aureus by more than 97% within a 120-s energization interval. The bactericidal efficiency of different corona discharge modes and its correlation with ozone levels produced by these discharges are discussed. The results obtained in this work will help in the design and development of compact plasma systems for environmental applications.

Surface flashover of oil-immersed dielectric materials in uniform and non-uniform fields

The applied electrical fields required to initiate surface flashover of different types of dielectric material immersed in insulating oil have been investigated, by applying impulses of increasing peak voltage until surface flashover occurred. The behavior of the materials in repeatedly over-volted gaps was also analyzed in terms of breakdown mode (some bulk sample breakdown behaviour was witnessed in this regime), time to breakdown, and breakdown voltage. Cylindrical samples of polypropylene, low-density polyethylene, ultra-high molecular weight polyethylene, and Rexolite, were held between two electrodes immersed in insulating oil, and subjected to average applied electrical fields up to 870 kV/cm. Tests were performed in both uniform- and nonuniform- fields, and with different sample topologies. In applied field measurements, polypropylene required the highest levels of average applied field to initiate flashover in all electrode configurations tested, settling at ~600 kV/cm in uniform fields, and ~325 kV/cm in non-uniform fields. In over-volted point-plane gaps, ultra-high molecular weight polyethylene exhibited the longest pre-breakdown delay times. The results will provide comparative data for system designers for the appropriate choice of dielectric materials to act as insulators for high-voltage, pulsed-power machines.

The effect of alternating electric fields on ion migration in solid dielectrics

A model has been developed that shows that an applied alternating electric field modifies the migration of ions in a solid dielectric material. In the case of a homogeneous alternating electric field, the components in the direction of the field of the ionic fluxes due to concentration gradients, when averaged over one cycle of the applied field, are enhanced irrespective of the polarity of the ions. In addition to this process if the applied field is inhomogeneous all ions will have a tendency to move to regions of lower electric field gradients.

Dielectric properties of natural ester, synthetic ester midel 7131 and mineral oil diala D

The insulating liquids used in industrial applications are typically mineral oils. In recent years however, significant attention has been paid to alternative insulating fluids, including synthetic and natural ester liquids. In order to expand their practical applications, it is important to have detailed information on their dielectric properties. In this present paper, the dielectric properties of synthetic ester, Midel 7131; mineral oil, Shell Diala D; and vegetable (rapeseed) oil have been investigated. It has been shown that Midel 7131 has a higher ac breakdown voltage (27.6 kV) as compared with Diala D oil (26.4 kV) and rapeseed oil (24.6 kV). However, the breakdown voltage of the Diala D oil has the smallest standard deviation (7%) amongst the tested liquids (13% for Midel 7131 and 11% for rapeseed oil). Statistical analysis of the breakdown voltages has been conducted and it has been shown that the ac breakdown voltages can be described by a normal distribution. dc I-V characteristics have been measured and the space charge saturation regime has been observed for all three liquids starting from ~9 kV for positive energisation and ~10 kV for negative energisation in the point-plane topology. Apparent mobilities of the charge carriers in the tested liquids have been obtained using I1/2-V curves; these mobilities can be used for calculation of the space charge influenced distribution of the electric field in liquid insulators stressed with dc voltage. Such analysis can be important for design and exploitation of HVDC power systems.

Effect of applied field and rate of voltage rise on surface breakdown of oil-immersed polymers

In sub-systems of high-voltage, pulsed-power machines, the introduction of a solid into bulk liquid insulation located between two conductors is often necessary to provide mechanical support. Breakdown events on or around the surface of the solid can result in permanent damage to the insulation system. Described in the present paper are experimental results pertaining to surface breakdown of five different solid dielectrics held between plane-parallel electrodes immersed in mineral oil. The effect of varying level of peak applied field from 200 kV/cm (dV/dt 70 kV/μs) to 1 MV/cm (dV/dt 350 kV/μs) is investigated, and the breakdown voltages and times to breakdown are compared to those for an open oil gap. The time to breakdown is shown to be reduced by the introduction of a solid spacer into the gap. Rexolite and Torlon samples suffered significant mechanical damage, and consistently showed lower breakdown voltage than the other materials ¿ average streamer propagation velocity up to 125 km/s was implied by the short times to breakdown. Although ultra-high molecular weight polyethylene yielded the longest times to breakdown of the five types of liquid-solid gap, breakdown events could be initiated at lower levels of applied field for spacers of this material than those with permittivity closely matched to that of the surrounding mineral oil. Polypropylene and low-density polyethylene are concluded to provide the most stable performance in mineral oil. Due to the similarity of the applied voltage wave-shape (1/6.5 μs) to short-tail lightning impulses, the results may also be of interest to high-voltage system designers in the power industry.

Mechanisms of Impulse Breakdown in Liquid: The Role of Joule Heating and Formation of Gas Cavities

The impulse dielectric behavior of insulating liquids is of significant interest for researchers and engineers working in the field of design, construction, and operation of pulsed power systems. Analysis of the literature data on transformer oils shows that potentially there are several different physical processes that could be responsible for dielectric breakdown by submicrosecond and microsecond impulses. While for short submicrosecond impulses ionization (plasma streamer) is likely to be the main breakdown mechanism, for longer impulses, the thermal effects associated with Joule heating start to play an important role. This paper provides a theoretical analysis of the latter mechanism in dielectric liquids of different degrees of purity stressed with high-voltage (HV) impulses with duration sufficient to cause local heating, evaporation, and formation of prebreakdown gas bubbles. The proposed model is based on the assumption that dielectric breakdown is developed through percolation channels of gas bubbles, and the criterion of formation of these percolation chains is obtained. To test the developed model, the breakdown field-time characteristics have been calculated for the liquid with chemical composition close to that of transformer oils but with known thermodynamic characteristics (n-hexane). Its dielectric strength has been obtained as a function of externally applied pressure and temperature. The analytical results show good agreement when compared with the experimental data available in the literature.

Surface Discharge Propagation: The Influence Of Surface Charge

Surface discharges are of considerable importance, because of their influence on the reliability of high voltage pulsed power systems. It is possible to achieve long discharges with relatively low voltages particularly in the presence of surface charge. The theory of streamer development in gases has been the subject of major experimental studies into the precise mechanisms of their initiation and development. However, surface discharge development is complicated by the proximity of the underlying insulating surface and the coupling of the discharge to that surface. This leads to modifications of the electric field surrounding the streamer, particularly at the head of the discharge. Photons generated at the discharge head can interact with the surface, producing photo-emission thus changing the surface charge distribution. Ablation of the surface results in local modifications of the gas atmosphere. The present experimental study has investigated the influence of surface charging on guided streamers propagating over insulating surfaces. The streamers were initiated from a surface mounted electrode excited by a high voltage pulse, the duration of which was much shorter than the time taken for the streamer to propagate across the surface. It was found that the velocity and distance of propagation is strongly influenced by the presence and magnitude of the surface charge and that discharge propagation can occur in the absence of the HV pulse. Measurements have shown that the deposited surface charge decays with time and that there are two distinct decay rates. These changes in the surface charge distribution could have a bearing on the period of time that should elapse before the subsequent reapplication of HV pulses to produce consistent discharge behaviour or to avoid surface flashover

Diagnostic dielectric spectroscopy methods applied to water-treed cable

Considerable effort has gone into developing polymer formulations and cable designs to minimize failures through water tree growth. However, diagnostic techniques still are required to enable the estimation of the level of damage present within a service cable. This paper reports on progress regarding the application of dielectric spectroscopy to cable diagnostics. A 40 kV, crosslinked polyethylene (XLPE) insulated coaxial cable was used as a model power cable. Sample lengths were immersed in a potassium chloride solution and some of these were subjected to AC electrical stress. After an 8 week duration, a high density of tress was found in the electrically stressed cable. Dielectric spectra have been measured for both sample types in the frequency range of 10/sup -5/ to 10/sup 5/ Hz. Insertion loss measurements were also carried out in the frequency range of 3/spl times/10/sup 5/ to 3/spl times/10/sup 9/ Hz. From both types of measurement, it was possible to distinguish between the cables containing water trees and those that were free from water tree structures. These approaches could therefore be developed in order to provide diagnostics for the detection of water tree damage in electrical power cables.