Composite insulators are considered mature products and a broad range of insulator types is nowadays available, including line insulators, hollow-core insulators and apparatus insulators, for applications ranging from distribution voltages to the highest transmission voltages. This development has created a need to elaborate diagnostic methods and tools for determining the quality of composite insulators. A review on recognized insulator faults and methods for their detection is provided.

Diagnostic Methods for Outdoor Polymeric Insulators

This paper surveys the properties which give silicone rubber its distinctive and highly desirable performance characteristics for use as outdoor insulation. The methods of assessing these properties arediscussed and this is followed byadetailedsurveyof whatis knownof agingaud recovery mechanisms. Corona is identified as the principle electrical aging agent and evidence is presented for irreversible loss of polymer as a result of aging. However, the surface recov. ery mechanisms are robust and, provided there is an adequate resting period, the hydrophobic properties recover, This supports the view, from service experience, that there is no substantial evidence to show that normal aging, unless it results in the exposure of the glass fiber core, detracts from the performance of insulators.

Review of aging and recovery of silicone rubber insulation for outdoor use

Water droplets on hydrophobic surfaces of polymeric insulators oscillate under applied high voltage electric field stress. Deformation of the droplets increases local electric field stress in the triple zones, causing lower partial discharge inception voltage and consequently affecting the ageing performance of the insulator. For this contribution dominant modes of single water droplet oscillation on hydrophobic surfaces were investigated in a wide range of droplet volume and frequency of applied electric field using a high speed camera. 3D frame analysis showed for all volumes an increasing trend of mode number with frequency in the range of 20 to 100 Hz. Experimentally found resonance frequencies for water droplets on a hydrophobic surface approximately correlate with published frequencies of free water droplets. Conductivity showed no effect on the pattern of water droplet oscillation. Partial discharge inception voltages on two different hydrophobic surfaces (silicone rubber and epoxy resin) with conductive and non-conductive (distilled) water droplets were measured. These measurements showed that inception voltage makes a step whenever the oscillation mode of water droplet changes. Conductivity of the water droplets was found to have no effect on the inception voltage on silicone rubber surfaces having a high degree of hydrophobicity, whereas conductive droplets on epoxy resin with lower hydrophobicity lead to lower inception voltages.

Experimental investigations on water droplet oscillation and partial discharge inception voltage on polymeric insulating surfaces under the influence of AC electric field stress

This paper presents a review discussing methods of determination the resistance to corona discharges and ozone of polymeric materials for applications in outdoor insulation. It also describes a part of international activities within CIGRE WG D1.01 aiming to develop suitable methodology for the testing, which includes designing the necessary test arrangement, defining test conditions and finally exemplifying results of the treatment by monitoring changes of electrical, mechanical and structural properties of some materials.

Evaluating resistance of polymeric materials for outdoor applications to corona and ozone

On the way of making efforts to standardize the selection of polymeric materials for high voltage outdoor environments, the resistance to corona and ozone has been recognized as one among twelve parameters to be evaluated within the OGRE Working Group D1.14. Our previous related work reported on the evaluation of the resistance of five commercially available polymeric materials to ac corona-ozone treatment, according to the test procedure recommended by this Working Group. With the same test arrangement in use, this report focuses on comparing the influences of ac and dc corona-ozone exposures on high temperature vulcanized (HTV) silicone rubbers. For this purpose, two types of HTV materials were used, each one characterized by different resistivity levels and tensile strength. For defining the treatment conditions, the ac and dc coronas were characterized by measuring the associated discharge current as well as the power released from the electrode. The intensity of ac corona discharge was found to be higher than that of dc corona, the latter being controlled by electrostatic charging of the treated polymeric surfaces, which also resulted in significant differences of the resulting ozone concentration. Experimental investigations after the treatments concentrated on evaluating dielectric properties (e.g. surface and volume resistivities, dielectric response (DR)), mechanical properties (e.g. tensile strength and elongation at break), as well as chemical and structural analyses (e.g. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR)). Results obtained from the measurements of dielectric properties indicated a strong sensitivity of surface resistivity to the treatments, though the impact of the dc corona remained less pronounced. On the other hand, the dielectric response and volume resistivity did not exhibit a high susceptibility to the exposure. No substantial differences were also found among the ac and dc corona treated samples as regards the mechanical properties. The XPS and FTIR analyses revealed proofs of surface oxidation for the samples treated by ac corona, while the samples treated by dc corona showed much less tendency for oxidation.

AC and DC zone-induced ageing of HTV silicone rubber

This paper identifies corona and local arcing as the major ageing influences on silicone rubber surfaces. The hydrophobic properties of silicone rubber lead to the formation of discrete water droplets on the surface and these can lead to local enhancement of the electric field and corona activity. This activity leads to surface ageing and a consequent loss of hydrophobicity. Once wetting takes place, surface currents flow and dry bands form. Arcing across dry bands also leads to ageing but this is not as deleterious as corona ageing. Experimental evidence on corona ageing and the stages in the ageing process, in service, are presented.

http://ieeexplore.ieee.org/iel5/6616/17709/00821219.pdf

I.R. Jandrell

Papers

Surface ageing mechanisms and their relationship to service performance of silcone rubber insulation