Mats Sjöberg

Electric field reduction due to charge accumulation in a dielectric-covered electrode system

Outlines for increased insulation performance of an air gap through the use of dielectric coatings are given. Theoretically, it is shown that the homogeneous electric field in a plane-parallel electrode system can be reduced if the electrodes are covered with a thick dielectric coating. Free charges will be affected by the electric field between the electrodes and are deposited at the dielectric surfaces. As a consequence, a counteracting electric field component results, which accordingly causes a reduction of the total electric field in the air gap and an enhancement of the field in the dielectric layers, i.e. the electric field is forced into the dielectric coatings by the charges. This effect has important implications in HV engineering. Introductory experiments supporting the idea have been carried out with promising results. It was confirmed that the withstand voltage of a plane-parallel electrode geometry with an open air gap, for dc as well as unipolar impulse voltage, could be increased considerably if the electrodes were covered with thick polymeric layers. Charge formation at the electrode surfaces as well as in the air gap is believed to be responsible for this effect. In todays insulation systems, this is known to work only for time-independent electric fields, i.e. generally dc voltages. Further experimental work is required and will be performed in order to scrutinize thoroughly and clarify the phenomenon, its capabilities and limitations.

Experimental study and numerical modelling of a dielectric barrier discharge in hybrid air-dielectric insulation

Results from experiments and numerical modelling of dielectric barrier discharges (or microdischarges) responsible for surface charging of a plane-parallel electrode arrangement covered with non-conducting solid dielectric coatings and comprising an air gap are presented. The purpose was to analyse the discharge progress and its influence on the surface charge build-up, which essentially is the cause of, from a high voltage insulation perspective, a beneficial electric field distribution with lowered electric field in the air gap and enhanced electric field in the dielectric coatings. The experimental investigation was performed in terms of high-voltage impulse testing, applying a crest voltage at the discharge inception level. Discharge current was measured and its progress in the air gap was photographed using an image intensified charged-couple device camera. Temporal and spatial development of an individual barrier discharge in a plane-parallel and rotational-symmetric geometry was numerically analysed by means of a two-dimensional diffusive-drift model. The idea of using limited barrier discharging to condition the hybrid insulation, which changes the electric field distribution and subsequently improves the insulation performance of the electrode system, is effectively demonstrated by the numerical simulation and supported by measured results. In a number of cases, measured and calculated discharge current patterns were found to agree structurally and to correctly display the predicted avalanche and streamer discharge phases.

Effects of charge accumulation in a dielectric covered electrode system in air

The effect of space and surface charges on the increased insulation performance of a dielectric-covered electrode system in air during negative impulse voltage stress has been investigated. Results obtained from high-speed photographing, using a CCD camera, and the corresponding current measurements of the discharge phenomena in an air-insulated, dielectric-covered electrode system during negative lightning impulse voltage are presented. The correlation between the measured current pulses and the photographs of light events in the air-gap was found to be clear. Measurements revealed two different phenomena: at the front (voltage rise) of the impulse voltage, a negative discharge pulse emerged; while at the tail (voltage decay) of the impulse voltage, a sequence of small positive discharge pulses occurred. It is believed that discharges at the front cause heavy and distinct ionization in the air-gap which deposits charge, surface as well as space charges, at the covered electrode interfaces. The discharges at the tail correspond to re-distributions of the accumulated charge when the applied voltage vanishes. The discharges are mainly of the barrier discharge type.

Electrical discharge in an air gap with dielectric-covered electrodes

Results of numerical simulations of discharge development in an air gap between electrodes covered with solid dielectric are reported. Temporal and spatial variations of gas discharge parameters, dynamics of surface charge accumulation on dielectric barriers and the corresponding behavior of electric field in the gas gap and in the dielectric layers are presented and discussed. Evaluation of calculated data allowed explaining phenomena observed experimentally in the similar electrode system with solid dielectric barriers, leading, in particular, to significant improvement of dielectric strength of the whole system.

On discharge phenomena in a covered electrode system in air

The process of surface charge deposition in a dielectric covered electrode system in air has qualitatively been investigated for a sequence of lightning impulses of negative polarity using high speed photographing and current measurements. It was found that at moderate electric stress in the air gap, a limited and voltage dependent number of impulses deposited a sufficient amount of surface charge at the coatings to quench further discharge activity. At higher electric stress, the surface charging process was of a more complex nature. The charge deposition continued throughout the entire sequence, but parts of the previously deposited charge relaxed when the stress in the air-gap was lowered due to the decay of the applied lightning impulse.

Surface charge accumulation in a dielectric-covered electrode system in air

This papers focus is an experimental study of the charging process in a plane-parallel disc shaped electrode system during applications of unipolar impulse voltages. Discharge current measurements together with surface charge assessments during single impulse as well as sequence of impulses of both polarities, and at various air gap widths are presented. The objective was to quantify the distribution of electric field in the combined dielectric structure in order to evaluate the general applicability of the method. Experimentally obtained results are compared with predictions from theoretical models.

Optical breakdown in fused silica and argon gas: application to Nd:YAG laser limiter

A gas cell filed with argon gas under pressure is placed in a tightly focused laser beam to provide a limiter for laser pulses above a certain peak power, corresponding to the optical breakdown threshold for the creation of a laser-induced plasma. Measurements of the threshold intensity as a function of argon gas pressure are given for a laser wavelength of 1.064 microm (Nd:YAG) and a pulse length of 6.4 ns. Threshold intensities for optical breakdown in fused silica were measured with the same optical system, enabling a relative comparison of breakdown thresholds, of interest for protecting fused-silica optical components in fiber-optic delivery systems for laser material processing applications. The threshold intensity was measured to 220 GW/cm2 in Ar at 1.0 x 10(5) N/m2 (1 atm), 80 GW/cm2 in Ar at 8.0 x 10(5) N/m2 (7.9 atm), and 55 GW/cm2 in fused silica. Even though the threshold in argon is higher than that in fused silica, the limiter will protect the optical components if the laser beam is focused to a tighter spot in the gas cell than at the input end of the fiber.

Power transmission limitations of Nd:YAG laser radiation in multimode fibers

Power transmission limitations of Nd:YAG laser radiation in multimode fibers were studied experimentally. To simulate materials processing CW lasers a Q-switched laboratory laser was used to generate kW pulses. The dominating non-linear power transmission limiting phenomenon is Stimulated Brillouin Scattering, SBS. The power threshold for the onset of SBS was determined as a function of the beam quality of the laser and as a function of the Q-pulse duration. The results of the study are relevant for the design of the next generation of fiber delivery systems.Power transmission limitations of Nd:YAG laser radiation in multimode fibers were studied experimentally. To simulate materials processing CW lasers a Q-switched laboratory laser was used to generate kW pulses. The dominating non-linear power transmission limiting phenomenon is Stimulated Brillouin Scattering, SBS. The power threshold for the onset of SBS was determined as a function of the beam quality of the laser and as a function of the Q-pulse duration. The results of the study are relevant for the design of the next generation of fiber delivery systems.