Flashover of smooth and knurled dielectric surfaces in dry air

In pulsed power engineering, solid spacers are used to insulate high voltage parts from extraneous metal parts, providing electrical insulation as well as mechanical support. The breakdown/flashover voltage, at which a discharge process initiates across the solid/air interface, is important in the design process, as it informs designers of specific threshold ‘failure’ voltages of the insulation system. In this paper, a method to potentially increase the failure voltage, tested under multiple environmental conditions, without increasing the length of the solid spacer, was investigated. Three dielectric materials: HDPE (high-density polyethylene), Ultem (polyetherimide) and Delrin (polyoxymethylene), were tested under a 100/700 ns impulse voltage. Cylindrical spacers made of these materials were located in the centre of a plane-parallel electrode arrangement in air, which provided a quasi-uniform electric field distribution. Breakdown tests were performed in a sealed container at air pressures of -0.5, 0 and 0.5 bar gauge, with a relative humidity (RH) level o

Continuous monitoring of aerial bioburden within intensive care isolation rooms and identification of 'high risk' activities

Background: The spread of pathogens via the airborne route is often underestimated and little is known about the extent to which airborne microbial contamination levels vary throughout the day and night in hospital facilities. Aims: This study aims to evaluate variability in airborne contamination levels within ICU isolation rooms over extended time periods to improve understanding of the extent to which ward activities, and consequential increases in airborne bioburden, may contribute to cross-infection of patients. Methods: Environmental air monitoring was conducted within occupied and vacant inpatient isolation rooms. A sieve impactor sampler was used to collect 500 L air samples every 15 minutes over 10-hour (08:00-18:00 h) and 24-hour (08:00-08:00 h) periods. Samples were collected, room activity logged, and the bacterial contamination levels were recorded as cfu/m3 of air. Findings: A high degree of variability in levels of airborne contamination was observed across all scenarios in the studied isolation rooms. Air bioburden increased as room occupancy increased, with air contamination levels highest in rooms occupied for the longest time during the study (10 days) with a mean value of 104.4 cfu/m3 and a range of 12–510 cfu/m3. Counts were lowest in unoccupied rooms, with an average value of 20 cfu/m3 and during the night. Conclusion: Peaks in airborne contamination showed a direct relation to an increase in activity levels. This study provides first clear evidence of the extent of variability in microbial airborne levels over 24-hour periods in ICU isolation rooms and directly correlates microbial load to ward activity

Mixtures of midel 7131 and THESO insulating liquids for pulsed power applications

The main objective of the present work is the measurement and comparison of the dielectric properties of mixtures of two insulating liquids: synthetic ester liquid Midel 7131, used in the power industry, and THESO high permittivity oil developed by Tetra Corp. for pulsed power applications. It is shown that these two liquids can form stable mixtures with tailored dielectric properties. The dielectric characteristics of Midel 7131/THESO liquid mixtures such as their low frequency dielectric permittivities, DC conductivities and breakdown volt-time characteristics have been measured and will be discussed

Apparatus and method for electric spark peening of gas turbine components

Peening provides compression of component (6, 46, 56) surfaces in order to create residual surface compressions to resist crack propagation in components such as aerofoils. Previously peening techniques have had problems with respect to achieving adequate treatment depths, speed of treatment and with respect to effectiveness. By the present method arrangement an electrical conductor (1, 41, 51) in the form of a wire is subject to electrical pulses to cause evaporation and subsequent breakdown with high power ultrasound (HPU) propagation in a volume of dielectric fluid towards a component and so peening. The electrical conductor (1, 41, 51) ensures that there is limited possibility of electrical discharge to the component (6, 46, 51) surface whilst the positioning of the wire (1, 41, 51) relative to the surface can be adjusted to achieve best effect particularly if reflector (5) devices are utilised to concentrate (HPU) pulse presentation to the component (6, 46, 56). Furthermore, the component (6, 46, 56) can be surface treated in order to provide protection from potentially damaging emissions from evaporation and electrical discharge to the wire (1, 41, 51)

Mechanisms of impulse breakdown in liquid: the role of Joule heating and formation of gas cavities

The impulse dielectric behaviour 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 which could be responsible for dielectric breakdown by sub-microsecond and microsecond impulses. While for short, sub-microsecond impulses ionisation (plasma streamer) is likely to be the main breakdown mechanism, for longer impulses, thermal effects associated with Joule heating start to play an important role. The present paper is provides a theoretical analysis of the latter mechanism in dielectric liquids of different degrees of purity stressed with high voltage impulses with duration sufficient to cause local heating, evaporation and formation of pre-breakdown 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. In order 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 a good agreement when compared with the experimental data available in the literature

A novel design for a multistage corona stabilized closing switch

The possibility of controlling the voltage distribution across the gaps of a closing switch based on a cascade of corona stabilised electrodes has been examined. When operating in a corona stabilised mode the corona current flowing in each gap must be equal. By altering the corona characteristics of the gaps, through changing electrode separation and the area of corona emission, it should therefore be possible to control the voltage distribution across the elements of the cascade. A simple theoretical model of the behaviour of corona emission for an electrode with a protruding cylindrical corona generation element has been developed. Experimental tests have confirmed the broad validity of the model. The experimental data for individual electrode geometries has been used to predict the expected self break behaviour of a two gap cascade and the predictions are compared with experimental data. The voltage distribution across the cascade has also been measured and compared with the values predicted from the mode

An investigation of spark discharge parameters for material processing with high power ultrasound

High power ultrasound (HPU) generated using pulsed-power techniques provides an alternative method for treating ores and minerals prior to extraction processes and for the comminution of waste materials as part of a recycling process. In an earlier publication [Wilson, M.P., Balmer, L., Given, M.J., MacGregor, S.J., Mackersie, J.W., Timoshkin, I.V., 2006. Application of electric spark generated high power ultrasound to recover ferrous and non-ferrous metals from slag waste. Minerals Engineering 19, 491–499], preliminary results using the HPU technique to treat stainless steel slag and waste bottle glass were reported. This paper describes further work performed on stainless steel slag to determine the likely energy costs associated with HPU processing. In industrial applications of HPU it is important to optimise the parameters of the high-voltage (HV) spark discharge causing the shock wave in the working liquid to maximise the efficiency of the treatment. However, because of the high intensity of the shock wave it is difficult to measure its output close to the HPU source. Experiments have therefore been performed using Pinducer sensors to measure the pressure waves produced by the source at distances relatively far from the spark discharge. These measurements made in the far field have been correlated with the electrical energy provided to the discharge and treatment rates for stainless steel slag and bottle glass obtained under identical conditions

Breakdown characteristics of plasma closing switches filled with different gases

When operating gas-filled plasma closing switches, the most commonly used dielectric gas is sulphur hexafluoride (SF6) due to its desirable and advanced dielectric properties. In recent years, concerns over the impact of SF6 on the environment and its contributions to the global greenhouse effect [1], as well as the continual rise in the cost of acquiring SF6 [2] has led to a desire within the pulsed power community to find an alternative switching medium to replace SF6, [3]. The main concern over replacing SF6 is that plasma closing switches filled with any replacement gas must have similar performance parameters to the SF6–filled switches, e.g. parameters such as the DC breakdown voltage and the triggered breakdown voltage, jitter and breakdown voltage spread, the recovery of the switch and the energy losses [4]. This paper presents results obtained for DC breakdown tests in a self-breakdown switch with an adjustable inter-electrode gap that has been filled with argon, nitrogen, atmospheric air and an argon/oxygen mixture at different pressures. As a result of the experimental work conducted, the characteristics of the plasma closing switches filled with different gases have been obtained and compared to previous work carried out [5] as well as compared with the characteristics of commonly used SF6–filled switches. Initial development of the model of a plasma closing switch has been developed which will be developed so that the model can be used for characterising the performance of different pulsed power systems and for optimization of current systems in order to improve their performance parameters e.g. decrease jitter and pre-fire rate, generate tailored wave-forms and improve overall stability of operation

Modelling of the temperature gradient across biological cell membranes stressed with pulsed electric fields

Membranes of microorganisms stressed with pulsed electric fields (PEF) of sufficient intensity and duration can be permanently damaged by irreversible electroporation. Such PEF-induced damage of a biological cell membrane can lead to the death of the microorganism; this process facilitates practical applications of PEF for microbial inactivation in liquids and lysis. PEF treatment is considered a “non-thermal” inactivation process: typically the global temperature of liquid samples treated with impulsive electric fields remains below the thermal inactivation threshold. However, intense electric fields may result in the development of local temperature gradients across biological membranes. Thus, it is important to investigate these local heating effects for further understanding and optimisation of PEF treatment of microorganisms. Pore formation also happens in biological membranes during PEF treatment, with simulation results showing that local heating also exists in these pores. These heating effects may help enhance the process of the formation of pores during PEF treatment. This local heating in a pore is shown to be influenced by the position and dimension of the pore