Although porcelain insulators have been used widely at all voltage levels for many years now, they have not performed entirely without problems. These typically tend to occur when pollution and moisture combine to form a highly-conductive layer on their surfaces. In such a situation, surface resistance can decrease by up to 10,000 times compared to the dry and clean state. Surface discharges then occur and bridging dry bands are formed. This dry band arcing can quickly develop into a complete flashover of the insulator. Different solutions aimed at combating such dry band arcing were developed over the years and included oil-bath insulators, water-repellent insulators, insulators with built-in heaters and insulators having a semi-conducting glaze. However, few of these concepts survived the test of time. Although use of semi-conducting glazes was not found practical when the concept was first proposed, it re-appeared once more reliable new materials had become available. The principle behind the improved performance lies in the ability of such glazes to conduct low current (~1 mA) thereby promoting heating of the insulator surface to a few degrees above ambient. Therefore, any existing pollution layer is kept dry, even under conditions of dew or fog. Moreover, the glaze also acts as an alternative path for the leakage current flowing in the pollution layer. As such, dry band arcing formed when the surface of the insulator does not dry homogeneously is prevented. This lowers the risk of flashover. In addition, the layer of semi-conducting glaze also stabilizes the voltage distribution along the insulator. In spite of these advantages, however, there have been concerns expressed in regard to ageing of the glaze with a corresponding reduction in its conductivity over time. This INMR article from 2004, by Prof. Stanislaw Gubanski of Chalmers University of Technology in Sweden, reviewed research performed on ageing of antimony-doped, tin oxide semi-conducting glaze under both AC and DC voltage stress. The objectives were to study how ageing mechanisms differ depending on type of stress and to establish whether semi-conducting glaze insulators can be considered equally reliable to those having ordinary, non-conducting glaze.
Semi-conducting glazes are obtained by introducing a conducting phase into the glassy matrix. This conducting phase is based on metal oxides which are mixed with ordinary base glaze. The conducting properties of the glaze are then created during the firing process. The first such glazes were based on iron oxide. Attempts were also made to use other metal oxides, for example, nickel, zinc, cobalt, chromium and copper oxides among others. These glazes, however, demonstrated poor resistance to corrosion as well as thermal instability, often leading to thermal runaway. Glazes containing partially-reduced titanium oxide were found to offer better thermal behaviour as well as superior resistance to corrosion. However, these tended to lose their conductivity easily when exposed to spark discharges. Finally, it was discovered that glazes with antimony-doped tin oxide satisfied most of the basic requirements for use on high voltage insulators. Insulators having such tin oxide-based, semi-conducting glazes have been produced commercially now since the 1970s. Great improvements have been made over the years and service experience with such insulators under exposure to AC voltages has been quite promising. The major concern, despite the proven benefits, has remained ageing of the glaze surface over time, leading to deterioration as well as reduction in its conductivity. In addition, field experience of such insulators under DC voltage has been quite sparse and this is especially relevant since laboratory investigation seems to indicate that ageing in such applications can be much more severe than for AC.
An important part of the research involved investigating the performance of full-scale, long-rod porcelain insulators having semi-conducting (SC) glazes. In this regard, special insulators manufactured for this project were exposed to a natural marine environment at an outdoor test station and placed under both AC and DC electric stresses. Applied voltages were 180 kV and 130 kV (rms phase-to-ground) on the DC and AC lines respectively.
The test insulators were covered with two types of semi-conducting glaze and were manufactured in different lengths. All had core diameters of 70 mm and outer diameters of 130 mm. At the same time, insulators with non-conducting (NC) glaze as well as composite insulators with a similar design were installed on the same test lines. Relevant data for these various insulators are provided in Table 1.
During exposure, the electrical performance of these insulators was monitored in terms of leakage current and incidence of flashover. For insulators exposed to DC, the DC component of the current was recorded, while under AC it was the mean value of the half-wave rectified value. When the insulators were first energized, they were clean and dry. Therefore the ‘base’ leakage current could be measured and was found to be between 0.04 mA and 0.15 mA for semi-conducting glaze insulators under DC voltage. This same base value was between 0.16 mA and 0.36 mA for those insulators installed under AC voltage. For insulators with non-conducting glaze and for the composite insulators, the amount of current was lower. Figs. 2 and 3 show typical examples of the leakage currents recorded over the test period. Along the time axis, rainfall is indicated by a line of crosses. Leakage currents varied only slightly over time. However, during rainfall, a clear increase could be seen on ceramic insulators with both semi-conducting and non-conducting glaze. This behaviour was less pronounced on composite insulators.