Solid core, one piece HTV silicone insulators were installed as bus supports near a 252 MW combined cycle power plant in the southern part of Mexico where both temperature and humidity are high. The insulators (Fig. 11) required no maintenance and are still in good conditions after 10 years of service.
Fig. 12 shows a 115 kV bushing after 10 years in operation at the Nizuc GIS substation. This location is near the Caribbean coast and service conditions include high humidity and sea contamination.
E. Baja California
A polymeric bushing with 12 years in operation near the Pacific coast required washing at least once a year. It was installed as part of 230 kV GIS located beside a 496 MW power plant (see Fig. 13). The polymeric insulation is still performing well in the local harsh environment and there have been no reports of failure.
During the early 1980s, both silicone rubber and EPDM were commonly used for line insulators but field experience soon showed that silicone was the better option. HTV silicone rubber has since become the most popular type of material for this application.
In order to improve manufacturing processes for hollow core composite insulators, a number of different techniques have been developed, including helical extrusion with HTV silicone as well as casting and injection with liquid silicone rubber (LSR). Some of the latest LSR 1:1 formulations require up to 70°C for vulcanization, however silicone is but one component of the system and therefore typically has to meet equipment characteristics instead of defining them. Often, the best silicone formulations may not be feasible for manufacturing due to equipment limitations. In addition, any change in silicone materials can require major process modifications, including in the equipment itself and also new design and type tests for the modified component.
As is the case for insulators as a proportion of the total investment to build a new line, the cost of a bushing is relatively low compared to the value of the equipment where it is installed, i.e. usually from 5 to 10 percent. But its performance can affect the safe operation of that equipment or substation and thereby affect many customers. A decision to try even a relatively mature technology may therefore be different in a small power company versus a utility with more than 3000 substations from 69 kV to 400 kV.
In Mexico, the CFE was motivated to begin use of polymeric-housed bushings by factors such as easier handling, better pollution performance and outstanding seismic performance. Another reason was reduced safety risk, both for personnel and other apparatus located near arresters and other equipment capable of explosive failure from high internal pressure. However, experience with premature failure of certain of these units has made the utility concerned that application of polymeric insulation technology to bushings may have been made without sufficient study of their behavior outdoors under high pollution and intense radiation and based mainly on service experience with silicone line insulators.
Introduction of this technology into the market has come with several serious failures that risk jeopardizing its continued use. For example, premature failure of polymeric bushings at a GIS substation began the process of imposing restriction on these materials in the Mexican transmission system and delayed realization of all the benefits that this advanced technology can offer. Based on failure analysis, the materials involved were identified as silicone rubber with a minimum PDMS content of 30% by weight. It is interesting to note that much the same process of delayed introduction occurred with transmission line insulators in the 1980s due to several highly publicized brittle fracture failures.
Even in the activities of IEC WG D1.27, it is not easy to identify the composition of a polymeric material and it is difficult for utilities to identify the process and validate the equipment against previous testing. The standards for HV equipment normally refer to the polymeric insulation standards so as to fulfill the technical requirements. For example, IEC 62231-2006, which applies to composite post insulators for substations with AC voltages, refers to IEC 62217 for several electrical tests, e.g. on interfaces and end fitting connections. IEC 62231 indicates that pollution tests should be as specified in IEC 60507 but the standard mentions that such pollution tests, performed on composite insulators, do not correlate with experience obtained from service. Moreover, these pollution tests are still under consideration. On the other hand, IEC 60137-2008, applicable for insulated bushings and including composite bushings, indicates that if artificial pollution tests are required, they shall be performed according to IEC 60507, which is the standard for ceramic and glass insulators exposed to polluted environments.