In another case, one of the silicone bushings developed ‘alligatoring’ on the surface after operating from 4 to 5 years in an area of heavy contamination. Some of these bushings, installed on the primary and secondary side of power transformers, had lost most of their hydrophobicity (HC of 6) after only 5 years in service. Figs. 5 and 6 show two different bushing surfaces with moderate degradation after 5 and 4 years of service and HC 3 and 4 respectively on the upper surfaces of sheds. On lower surfaces, hydrophobicity was HC 1 but there was evidence of light cracking. Based on the performance of these bushings, it has been deemed necessary to conduct detailed examination of them every year to assure their safe operation.
In another case, a 400 kV air to GIS bushing (Fig. 7) showed a HC 6 on the upper side of sheds. The bushing shed material had become rigid, brittle and easily torn, with deep cracks and loss of insulating material. An IR spectrum of this silicone material revealed critical degradation compared with a typical polydimethylsiloxane (PDMS) spectrum and the manufacturer was asked to replace the bushing.
A silicone rubber RIP bushing was installed in 2006 near a 946 MW power plant in an area of central Mexico with heavy industrial pollution (Fig. 8). The characteristics of this bushing are shown in Table 1. After 7 years of service, with annual cleaning using a cloth, visual inspection revealed no surface degradation, even though it had been contaminated with combustion deposits. Performance under wet conditions was good and corona inspection showed no evidence of damage to the insulation along the bushing. There was only limited corona activity observed around the high voltage conductor (see Fig. 9), attributed to high electric field strength in that region.
Silicone housings were also applied to an instrument transformer installed in Mazatlan. The silicone-housed current transformer, shown in Fig. 10, was ordered with specifications based on the electrical system requirements and it main characteristics are listed in Table 2. During installation, one of the main cautions was to exercise care when installing the current transformer column because damage could occur if pressure was applied over the sensor during erection. Another precaution was to avoid anything striking the column to ensure no nitrogen would leak from its interior. Installation proved easier compared to a porcelain-housed unit due to the much lower weight. This was seen as an opportunity for additional installations of silicone housed CTs because of the lower amount of civil work and structural requirements needed. In this case, the silicone rubber CT was installed over a metallic post, with connection box at the column’s base.
Connections of the fiber optic cable, modulator cable and temperature sensor were done in the connection box at the base of the column. The silicone-housed CT required two connections to the control center – one at the base of the column and the other in the control room. One of the connections was repeated due to problems in the optical transmission. After completion of the connections, the system was energized and it was realized that a stabilization period of 20 minutes is necessary for the silicone CT to reach full precision. Measurements from the silicone-housed CT were compared to a conventional bushing type CT using an OPH instrument and the difference in measurements was less than 3%.