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The design principles and prototype manufacture of 11kV silicone-rubber composite textured insulators are described. The moulded surface texture increases the maximum creepage length from 375 mm to 503 mm for the four-shed profile used. The trunk sections of the insulator are patterned with a square-section array of hemispherical protuberances, and the shed undersides with interlaced logarithmic spirals of hemispherical ridges. Both textures are sufficiently shallow to facilitate fabrication, to allow effective washing and to avoid an increased salt deposit density (SDD) in artificial pollution tests. The test procedures which were developed in Part 1 of the paper are applied here to the textured prototypes. These procedures had been shown to quantify reliably the performance of conventional non-textured insulators of the same profile: the variation of flashover voltage and leakage current due to the silicone-rubber formulation, SDD and wetting rate could be measured. For textured insulators, it is found that the increased creepage distance improves the flashover voltage by up to 27%, with a corresponding reduction of pre-flashover leakage current. Comparative tests for two silicone-rubber formulations of different hydrophobicity and pollution performance were performed. Although very severe pollution causes a large reduction in flashover voltage, surface texturing mitigates this reduction effectively. This is particularly relevant to recent concerns expressed in IEC-TS 60815-3 (2008) relating to the selection and dimensioning of high-voltage a.c. polymeric insulators for use in heavily polluted conditions. [ABSTRACT FROM AUTHOR]

The established model of the flashover of an insulator, whose surface is covered with a moist, conducting layer of pollution, envisages that it develops from the formation and bridging of a dry band by a partial-arc pre-discharge. This pre-discharge is assumed to possess arc characteristics, so that the electric field across the dry band will fall as the partial arc current increases. Observations of insulators which are only lightly polluted, however, show that pre-discharges are of low luminosity and rich in ultraviolet, and resemble the spark leader inception and extension sequence in air gaps. This is because the current limitation by low values of surface-layer conductance can inhibit the transition to an arc at the pre-discharge stage. An alternative model using simplified voltage-current equations is developed to represent these spark properties rather than an arc characteristic. The predictions of the partial-arc model and this new approach are both tested here, using published experimental data from light-pollution fog tests in the authors' laboratory. For this purpose, the partial-arc theory is further developed in order to reveal its implied prediction for the increase of pre-discharge length with applied voltage before flashover. A corresponding relationship is obtained for the increase of spark leader length using the new model. It is found that for light pollution, the dry-band spark model better represents the test data than the partial-arc model. Consequent predictions are made for the variation of flashover voltage with pollution severity in this regime, which may account for anomalous insulation failures. [ABSTRACT FROM AUTHOR]

Electric field strength and leakage current density in the shank region of a polluted polymeric insulator may cause local dry-banding, with a risk of long-term degradation from partial-arc discharges. In a novel approach to dry-band control, the characteristics of silicone rubber surfaces with a textured finish are investigated and, depending on the geometry chosen for the texture, two useful objectives can be achieved. First, the increase of surface area can both reduce the leakage current density in the vulnerable shank region, and also increase the longitudinal creepage distance. Secondly, the damage arising from surface discharges can be mitigated. The principles of the proposed anti-fog design are described in detail. While complete prototype insulators are now being specified for fabrication, this paper reports preliminary results from tests using rectangular samples, which show an improvement in performance. The tests are of two kinds: a) inclined-plane, to assess erosion and tracking on rectangular samples of insulation materials. Such samples have been prepared both with a conventional plane surface and with a textured surface for comparative tests; b) clean-fog tests that use an initially dry pollution layer of known salinity. Here, plane and textured samples have been used for comparative tests, where leakage current and discharge activity are monitored during the generation of the fog. [ABSTRACT FROM AUTHOR]

In the design of high-altitude high-voltage transmission systems and in the optimization of the reliability of power sources in aeronautical environments, it is important to have reliable data concerning the effect of air density and humidity on reducing air insulation performance under local conditions. This is helpful also in the modeling of discharges and lightning in mountainous regions. This paper describes a systematic laboratory investigation of the combined effects of humidity and gas density on the breakdown strength of a 0.2 m rod/plane air gap that is subjected to positive-polarity lightning impulses, using a test chamber to reproduce the conditions commonly found in such regions. The results show that there are appreciable deviations between the measured humidity and air density correction factors and those of the 1973 and 1989 I.E.C. Standards when extended to low air density. The paper proposes the adoption of a radically different correction procedure, based upon the influence of density and humidity on streamer propagation field, and including the effect of the high gradient in the anode region. [ABSTRACT FROM AUTHOR]

We report for the first time the monolithic integration of a micromechanical modulator and a p-n photodiode on a silicon substrate yielding a versatile optoelectronic device. Because both devices are monolithically integrated on a silicon substrate, the combination is compact with minimal parasitic elements. We demonstrate that such a device combination fields a transistor-like element with positive and negative small-signal voltage amplification. The maximum small-signal voltage gain achieved is 500 while the modulated current of the device exhibits a maximum ON-OFF ratio of 3:1. In addition, while the theoretical current gain of the device is infinite, a 10-pA noise level limited the measured dc current gain to 106 [ABSTRACT FROM PUBLISHER]

Inclined-plane tests of silicone-rubber samples prepared with textured patterns of hemispherical protuberances are described. These are compared with the results obtained for conventional non-textured plane-surface samples of the same material. A remarkable improvement is achieved by such texturing: a material which normally fails the test is enabled to satisfy the standard test criteria, and the tracking and erosion of the samples caused by the tests are significantly reduced. These texturing techniques are currently being extended to prototype composite insulators. It is shown that this improvement for textured samples is a result of the formation of a stable, controlled dry band on the polluted surface. Although discharge inception occurs across the band, these discharges result in a fall, rather than an increase, of the leakage current. The transition to a higher current arc phase is thus inhibited, compared with non-textured samples for which stable dry bands did not occur. These observations of reproducible dry band breakdown also provide an opportunity to characterize experimentally the electrical properties of these discharges. A recent proposed model represents the dry-band discharge as a streamer-spark structure, in contrast to the standard model of an arc-like interdependence of current and voltage. These two models are tested against the present new data, and the discharge is shown to have streamer characteristics. [ABSTRACT FROM PUBLISHER]