Capacitive-graded porcelain bushings used in high voltage power transformers and circuit breakers have traditionally been considered one of the weakest elements in Mexico’s power network, with a demonstrated 26% risk of failure. This statistic is based on failures of 230 kV and 400 kV equipment recorded between 1983 and 1998 by the country’s power grid operator, Comisión Federal de Electricidad (CFE).
Polymeric materials have been used in line insulators now for well over 35 years and are regarded as a mature technology. However, in the case of substation equipment such as arresters, breakers, bushings and column insulators, widespread application of polymeric housings started much later. The advantages of these materials versus porcelain in the case of bushings include lighter weight and greater flexibility, hydrophobicity and lower cost for transport and installation.
Several bushings and current transformers (CTs) with silicone housing were installed at three 400 kV substations in Mexico. But failure occurred on one of the bushings only about 5 years after installation. This focused attention on the need to better evaluate behavior of such equipment, particularly in highly polluted environments. Preliminary analysis of the incident concluded that special contamination issues, lack of sufficient standards, equipment design being mainly for application in countries with lower average ambient temperatures than in Mexico and deficiencies in installation based on insufficient supplier information are all factors that can adversely impact proper selection of such equipment. These issues are discussed in this article based on a paper at the 2013 INMR WORLD CONGRESS by Manuel Guzmán V., Sergio Corrales S., Héctor Lara C. and Isaías Ramírez-Vázquez of the Comisión Federal de Electricidad and the IIE.
Several failures of porcelain-housed bushings occurred in the wake of two severe earthquakes in Mexico: the first, in October 1995, a 7.9 Richter scale event along the Colima coast and the second, in January 2003, which registered 7.6 on the Richter scale. These areas were the location of two major power plants, Manzanillo I and II, and both suffered severe damage with loss of 1900 MW to the interconnected system for periods of 45 and 30 days respectively. Loss of equipment was due mainly to the vulnerability of porcelain when it comes to withstanding earthquakes.
Apart from these seismic events, additional problems in Mexico have been caused by environmental factors – particularly in areas of high pollution, where external flashovers can occur during strong wind. The end result has been damage to bushings that can sometimes include complete destruction of affected equipment (see Fig 1).
Because of damage to power transformers caused by the first earthquake (i.e. from leaks in 52 of the 230 kV and 400 kV bushings), the two generating plants and substations remained disconnected from the grid for weeks. During the second earthquake, experience gained from the earlier event helped in limiting the damage and resulted in only 2 days of plant downtime. However, the cost of repairing or replacing bushings proved significant and included cost of cranes, materials, manpower as well as lack of availability.
Given this experience, attention was focused on polymeric housings, which can be an attractive alternative to porcelain since they make the bushing lighter and facilitate both transport and installation. These type of bushings also offer other advantages, including surface hydrophobicity that assures good performance under polluted conditions. Their lower weight, easier handling and higher mechanical stability together make them an ideal solution for application in seismic areas where the silicone external insulation also assures that there is no hazard due to explosion, no damage to adjacent equipment in the event of failure and greater resistance to bending motion. For example, the greater flexibility of polymers compared with inorganic porcelain means that the silicone rubber housing withstands up to 10 times more tensile stress without failure. This suggests that polymeric bushings can better withstand earthquakes of more than 1.0 g. Finally, the same polymeric material with high flexibility helps seal in insulating fluids, preventing oil leaks if displacements take place. There is continuity of service even with damage to some sheds.
Damage to porcelain skirts from bullets or other projectiles, such as stones, is common in rural areas of Mexico. While in the case of porcelain housings any fracture causes leaks and consequent failure of the equipment, this type of failure mode is much less likely for silicone rubber housings, representing an important advantage for HV equipment operation.
Resistance to Environmental Pollution
One of the factors when evaluating a polymeric material to ensure proper performance as a housing for HV equipment is resistance to environmental pollution. Such pollution can be semi-conductive or insulating but, when wetted by rain or other moisture, the bushing’s field distribution and capacitive grading can change. When contamination becomes deposited onto the sheds (skirts) of a bushing, electrical field becomes concentrated in wetted areas. Leakage current then causes the moisture to evaporate, resulting in appearance of dry bands. In areas with dry band arcing, localized ionization causes heating, tracking and erosion. A hydrophobic surface avoids the presence of dry bands and wetted areas. In the case of a silicone rubber housing, it has been found that hydrophobicity is maintained even over long periods in operation and, if temporarily lost, requires only about 8 hours without dry band arcing to restore its initial hydrophobicity. Another important point is that even if the surface is contaminated and the pollution layer not too thick, the polymeric surface remains hydrophobic.
The first CFE project with these types of bushings was in the Western Transmission Area, specifically the Manzanillo Substation, where, a total of 10 power transformers had their porcelain bushings replaced (Fig. 3).
New silicone rubber bushings using resin-impregnated paper (RIP) technology replaced oil-impregnated paper bushings (OIP) with ceramic insulation. The objective was to reduce the risk of damage due to earthquakes and pollution. Another potential benefit was reducing the need for hot line washing. With the previous porcelain bushings, washing was done during 7 months of the year whereas this was reduced to only once per year or less for the new silicone-housed units. At the same time, the 400 kV conventional air-insulated substation was replaced by a gas insulated substation that used 21 silicone rubber bushings to interconnect with transmission lines, making this the largest installation of these types of bushings in Mexico.
However, failures have since occurred. In one case, the silicone sheds of a bushing were damaged due to mechanical impact of debris thrown out from an exploding nearby porcelain-housed instrument transformer. Fig. 4 shows a close-up of one damaged section where it can be seen that the impact of debris caused a cut in the rubber to extend all the way to the resin core. Since this would eventually allow moisture to penetrate into the core and cause failure, this equipment was immediately replaced. This experience caused a modification in the criteria to install non-ceramic bushings and it is now recommended that installation not be too close to porcelain-housed equipment in order to increase bushing reliability.