Wetting of a pollution layer by mist or light rain is inhibited, in the case of silicone rubber (SiR) insulators, by the migration of hydrophobic polymeric chains from the insulator to the layer surface. However, recent laboratory fog-chamber tests have shown that a salt/kaolin layer applied to the surface of an 11kV SiR insulator can reduce the specific creepage distance (SCD) at flashover to as low as 16mm/kV. Even for larger values of SCD, potentially damaging partial arcs can arise along the insulator surface. It has been shown that some mitigation of partial-arc activity and an increase of flashover voltage can be achieved by appropriate texturing of the SiR insulator housing. The present paper describes additional infrared (IR) recording which accompanied these previous tests. Although a reduction of the flashover voltage in polluted environments is generally surmised to be the result of the formation of dry bands in a conducting moistened surface layer, no direct observations of dry bands appear to have been previously demonstrated in the laboratory. Such observations are described here, where details of dry-band location and growth are revealed by IR recording. Dry bands are shown by close-up visual photography to be invariably bridged by small streamer/spark discharges which maintain current continuity in the pollution layer. Local surface heating by these discharges are the probable cause of the delayed rewetting of the bands. Partial-arc channels that may result in flashover develop from and across the dry-band streamers. It has become clear that clean-fog testing with infrared recording and leakage current measurements provide new possibilities for the modeling of dry band discharges and improvement of insulator design.

The development of polymeric materials for outdoor insulation has been much aided by existing standard laboratory tests such as inclined-plane procedures. The position is less satisfactory in respect of the optimization of insulator profiles and the choice of the minimum unified specific creepage distance (USCD) for a given site pollution severity (SPS). Standard tests currently under revision are stated to be 'not directly applicable to polymeric insulators'. Such tests are, therefore, urgently needed to discriminate the pollution performance of silicone-rubber insulators of different housing materials and profiles, to quantify the inverse relationship of pollution severity and flashover voltage and to assess the influence of wetting rate. A suitable test strategy would also avoid housing damage from excessive flashovers. A new test procedure is presented here to achieve these objectives. This is not confined to flashover voltage measurements. A high-resolution data acquisition system is used to monitor and store the waveforms of the applied test voltage and leakage current throughout tests on 11kV silicone-rubber insulators. This enhancement provides a further basis for the quantitative comparison of insulators in artificial pollution testing, because the results corroborate the inverse relationship between flashover voltage and pre-flashover leakage current. Part 2 demonstrates the pollution performance of 11kV silicone-rubber insulators when surface texturing is used to increase USCD values.

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.

The aim of this study is the presentation of the results of an in-lab comparative study of electrical and thermal monitoring of artificially polluted, HTV-textured silicone rubber insulators, with different pollution levels. This work is a preliminary study of an in-situ monitoring of 400 kV SiR textured in a polluted environment. The results showed that the rms leakage current magnitude and pulses, and the average dissipated power depended on the pollution levels and the dry-bands formation. The discharge activity and their nature are governed by the pollution level and the voltage. A differentiation and a quantification between dry-band discharge onset and dry-band arc inception is highlighted.

Pollution flashover occurs when soluble and nonsoluble materials cover the surface of an insulator, and this may ultimately cause a reduction in its performance. In this paper, the common type of sodium chloride (NaCl) was used as a soluble pollutant (ESDD) and kaolin as a nonsoluble pollutant (NSDD). Samples of silicone rubber (SiR) insulators were selected for this study and fabricated at the Advanced High Voltage Engineering Research Centre (AHIVEC) at Cardiff University. The samples were preconditioned and polluted according to standard specifications. Additionally, the AC voltage ramp technique was used to achieve flashover (FOV) voltage with different pollution levels. The aim of this work was to investigate the effect of nonsoluble materials on flashover characteristics to understand their interaction with dry-band arcs by using FOV electrical equations and experimental data. The test results show that the FOV voltage of the silicone rubber insulator substantially decreased with the increase in both ESDD and NSDD values. It was also identified from these results that the dry-band arcs were considerably influenced by both ESDD and NSDD levels. This impact can be quantified by determining the variation of discharge parameters (N, n). Based on the FOV equations and experimental data, a mathematical model was suggested, taking into account the effect of both ESDD and NSDD.

In High Voltage Direct Current (HVDC) systems, composite outdoor insulators play a critical role and focus is needed on the research of their failure. In the case of a pole to ground fault of a bipolar scheme, the healthy pole experiences an overvoltage similar to a Switching Impulse (SI) superimposed on the operational DC voltage. However, there is no standard test for such voltage superimposition. This paper investigates the performance of a composite insulator stressed with such superimposition under dry and rain conditions. To determine the direct voltage influence on the flashover voltage, the results have been compared with the SI only case. Four different rain conductivities have been used in the test programme. Both vertical and horizontal orientations have been studied. Most cases show that direct voltage pre-energisation has a negative impact on the flashover performance. The increase in rain conductivity leads to lower flashover values, and the horizontal orientation outperforms the vertical configuration. Under these test conditions, the flashover voltages were all larger than a typical DC operational switching surge. Thus, in the case of pole to ground fault, the healthy pole failure risk seems small. Further research is needed on higher voltages and polluted conditions.