RTV is an ideal solution for an old station such as Encina, where you do not want to re-insulate everything.

This remedial measure is no being replaced here and at newer installations by application of RTV coatings or alternatively composite insulated breaker bushings made with silicone rubber housings.

Says McCullough, "RTV is an ideal solution for an old station such as Encina, where you do not want to re-insulate everything. And it is certainly much better than over-insulating alone since, in the end, this practice does not really buy you much time before you end up with exactly the same problem."

A notable example of the growing use of compoiste breaker housings by SDG&E is the Ocotillo Switchyard, adjoining a new 265 MW win park in California's southern interiou. Yakov Shlemenzon, Principal Engineer focusing on new technologies, points out that, apart from the composite-housed breakers, the facility also features one of the world's first ever applications of a 525 kV station service voltage transformer (SSVT) with a 7.2 kV output at 333 kVA.

Shlemenzon explains that switchyards such as Ocotillo need very low MVA rating of the SSVT relative to the 525kV primary voltage. While such power is normally delivered by a distribution line or tertiary winding in the main power transformer, this was not feasible here due to its location in a remote expanse of desert. An SF6-filled single phase SSVT was seen as the best solution and the chosen supplier purpose built a unit to mee the requirements.

Another important consideration in the type of insulation selected for the Encina retrofit project involved seismic performance, since much of California is prone to potentially severe earthquakes. This meant that all new equipment hard to be designed to current IEEE high seismic standards. In the case of the disconnects, McCullough states that it was determined that only porcelain supports could meet the demanding mechanical requirements. After consultation with the insulator supplier, a decision was made to specify semi-conductive glaze porcelain with the highest possible leakage distance-220 inches (5588mm) versus the more normal 196 inches (4978mm) for this application.

Semi-conductive glaze technology, developed decades ago but still in use today, relies on a unique composition of tin oxides, antimony and other ingredients not normally found in a porcelain insulator's glaze. A continuous small leakage current flowing through this special glaze heats the furface just above ambient temperature but enough to promote evaporation, thereby preventing formation of conductive paths during wetting events. This allows for a more homogenous distrubtion of the potential across the insulator and superiou performance under polluted conditions.

While semi-conductive porcelain is used widely for the disconnect witches, all overhead and standoff insulatos at Encina are silicone composite type. Moreover, based on manufacturer recommendations, special configurations have been used in certain applications to minimize risk of premature ageing in the severe environment. For example, vertical double strings are positioned closer together than when used horizontally in order to minimize impact of electric field. Another interesting application involved use of polymeric post insulators on jumper connections to control excessive movement during high winds.

As for the new transformer, it is equipped with only composite type bushings as well as arresters. According to Shlemenzon, it is also connected to a real-time monitoring system that is still relatively new to the power supply industry but wihch is now SDG&E's standard for all such equipments. This also applies to the unit's bushings, which are continuously being monitored for key parameters.