Silicone rubber materials for high voltage transmission and distribution insulators.

Since the early 1970s, composite insulators have become increasingly recognized as an alternative to the traditional porcelain insulators and glass cap and pin insulators. The use of composite insulators has accelerated more recently as the benefits of composites are becoming more widely recognized. These advantages include the lighter weight of the composite insulators with greater mechanical strength, less breakage during transit or installation and greater seismic resistance.

In substation applications, composite surge arrestors, breakers and insulators, due to the non-brittle housing, can overcome the damage to neighboring installations that occurs due to fracture of porcelain-housed apparatus.

In areas of moderate to high contamination, there has been a strong trend to the use of silicone housed composite insulators due to the increased performance in the suppression of leakage currents, flashover resistance and the reduced maintenance of the line. Areas such as those affected by coastal or industrial contamination, together with agricultural areas benefit from the unique surface ability of silicone-housed insulators to retain and recover their hydrophobicity. This ability to produce and retain a non-wetting surface interrupts the normal contamination-induced flashover mechanism, leading to improved system reliability and lower cost to the utility companies.

In recent years, the focus has been on improving electrical performance of the silicone housing material, but developments in production technology have reduced the production cost of the composite insulators.

Silicone materials for high voltage insulators

Essentially, two types of silicone materials exist for high voltage insulator application, and their relative viscosities characterize these materials:

* High consistency silicone rubbers (HCR) are solid materials suitable for high pressure molding applications or extrusion applications. They are cured by heat, hence they are sometimes called HTV (high temperature vulcanizing) silicone rubbers. There are several different cure systems for these materials, depending on fabrication process, but the two main systems are peroxide cure and, more recently, addition cure.

* Liquid silicone rubbers, both high temperature vulcanizing (typically called LSR) and room temperature vulcanizing (RTV), are low viscosity polymer systems that range in viscosity from pourable water-like viscosities to thick pastes. These material are usually molded in a closed system (with or without pressure) and may be cured at room or elevated temperatures.

There is also a range of primers and adhesives designed specifically for high voltage applications to enable the fabricator to use a variety of production techniques.

Fabrication of silicone composite insulators

Many processes have been described in the literature for the processing of silicone rubber into composite insulators. These include direct molding onto the epoxy rod or fabrication using pre-molded sheds fitted to a coated rod. These techniques rely on the processing parameters of the material, such as flow-ability, cure speed and demolding capability of the materials. Some of these properties have been overlooked in the past, as the focus has been on improving the electrical performance of the materials.

The latest generation of materials has addressed many of the processing concerns, while also improving the electrical performance of the materials. It is on the processing and end part performance that this article will concentrate This review will be split by technology: HCR and LSR.

Improvements in processing of HCR silicone rubber

Rheology

In the past, with the constraints of the formulations to meet the electrical end-performance requirements, many of the HCR products were difficult to process due to high plasticity and limited flowability. Flowability can be measured using rheological methods, such as capillary rheometry, to differentiate between material that may appear to be similar in plasticity or green strength but behaves very differently in processing in injection or compression molding. Ease of flow in the mold and good release from the mold after cure are essential to be able to efficiently produce high voltage insulators at reduced cost.

By using the truncated power law and calculating temperature shifts using an Arrehenius method, we can predict the viscosity at different temperatures and shear rates. This can be used to differentiate between materials that have the same initial plasticity but exhibit different flow behaviors at higher shear rates.

In figures 1 and 2, a material that shows an apparently higher viscosity at low shear is shown to flow better in the sprue and runners in an injection molding process.

[FIGURES 1-2 OMITTED]

In those examples, the calculated viscosities (initial viscosity and viscosity at 10,000 [s.sup.-1]) are: New HV material, 20,514 initial and 19.7; first generation material, 11,433 initial and 28.0.

Even though the new HV material appears to have higher plasticity at low shear rates, it is shown to flow better in the shear range typical for an injection molding process.

Vulcanization 

The cure of the material, as previously described, can be via two principal routes, including peroxide cure or addition cure. The addition cure system can offer considerable advantages in terms of cure speed and the overall efficiency of the molding operation. In switching from a peroxide cure system to an addition cure system, the cure cycle is usually reduced by at least 50%, with no loss in electrical or mechanical performance. In fact, several mechanical parameters can be improved, such as the hot tear strength that aids the demolding of the finished insulator.

An example of the change in cure speed and, hence, productivity and overall part cost, can be seen in figure 3.

[FIGURE 3 OMITTED]

Due to the fast cure speed of the addition cured material, it is usual to reduce the mold temperature to obtain time to fill the mold while still giving a 50% reduction in cure time over a standard peroxide cured rubber.

This reduction in mold temperature has several other advantages, including:

* Allows the use of epoxy rod formulations that exhibit lower Tg values and are less expensive; * reduction in energy cost;

* reduction in mold changeover time; and

* improved demolding characteristics.

Normally, a reduction of 15-35[degrees] C allows easy injection and fast cure, while still benefiting from an overall reduction in cure time of up to 100%.

Aging characteristics

For outdoor electrical applications, the environmental performance of the insulating material is extremely important. Silicone materials perform exceptionally well in outdoor environments. The excellent UV and weathering resistance of silicone elastomers is demonstrated in figure 4 in comparison with EPDM organic polymer. A similar trend is seen in the change in elongation and tensile strength with a reduction in elastomeric properties of the housing material.

[FIGURE 4 OMITTED]

The inherent stability of silicone is due to the backbone structure, comprised of the silicone-oxygen polymeric bond, which has higher bond energy than a carbon-carbon bond that is the backbone of organic polymers.

Electrical characteristics

In addition to the UV resistance the newer generation silicone materials have improved wet electrical performance as compared with first generation materials.

The changes in electrical properties, such as volume resistivity, dielectric strength, dielectric constant and dissipation factor, are shown in comparison with a first generation silicone rubber for high voltage electrical applications (figures 5-8). It can be noted that the changes after water immersion for the new generation materials are significantly less than with previous materials.

[FIGURES 5-8 OMITTED]

Volume resistivity, as determined by ASTM D257, is a measure of bulk electrical resistance. It is the resistance to leakage current through the body of an insulating material. The higher the volume resistivity, the lower the leakage current and the less conductive the material (figure 5).

The dielectric strength is a measure of the ability of the material to withstand large field strength without electrical breakdown. It is the value of the electric field strength after which the discharge happens (figure 6).

Measurements using ASTM D 149 yield values are obtained under controlled test conditions and may not accurately reflect actual field performance. Factors such as corona discharge, frequency, temperature and humidity can significantly affect the long term insulating characteristics of a material

The effectiveness of dielectrics is measured by their relative ability, when compared to a vacuum, to store energy, and is expressed in terms of a dielectric constant, with the value for a vacuum taken as unity. This is the measure of the ability of a material to resist the formation of an electric field within it. Good insulators have low DK (figure 7).

Dissipation factor (figure 8) is a measure of the degree of electrical loss due to the imperfect nature of an insulation material. AC loss should be small, both in order to reduce the heating of the material and to minimize its effect on the rest of the network.

As can be seen from these results, the latest generation HCR silicone materials show better environmental electrical performance than those previously available, while also adding processing advantages. All of these benefits will increase the competitive nature of silicone housed composite insulators.

Processing characteristics of LSR materials

LSR materials can have similar issues with viscosity, as do HCR silicone rubbers, depending on the process. When using a low pressure casting process, the initial viscosity of the material and its viscosity characteristics over a range of shear rates can have a major impact on the processing of the material.

If the LSR material increases in viscosity in the region of low shear, such as found in pumping equipment, then this can lead to problems of transporting the material without air inclusion, especially if no follower plate is used in the pumping equipment. Also, the filling of the mold may be very different for materials that exhibit different viscosity behavior.

We can see in figures 9 and 10 that three different LSR materials show very different viscosity behavior when subjected to shear. These materials demonstrate very different processing characteristics.

[FIGURES 9-10 OMITTED]

In figure 9, we can see that LSR 1 shows an increase in viscosity at low shear rate before showing the characteristic shear thinning effect that we normally associate with LSR materials. LSR 3 shows a more traditional viscosity/shear behavior. The different behaviors of these LSR materials can change processing parameters.

Even though the two LSR materials in figure 10 show a very similar viscosity at a shear rate of 10 [s.sup.-1], which is traditionally used to quote the viscosity, the low shear behavior of the materials is very different.

In fact, the behaviors of the two components of LSR 2 are significantly different, and this can affect the processing performance. Part B is inherently more viscous than part A at very low shear rate such as encountered in low power pumping equipment. If no follower plate is used in the pumping equipment, this high viscosity may lead to a well being formed in the material and the pump cavitating due to air being introduced into the equipment. This may also lead to increased waste due to the inability of the equipment to pump the material from the bottom of the drum.

Conclusions

The processing performance of both HCR and LSR silicone rubbers has been significantly improved with the latest generation silicone materials designed for high voltage applications. This allows a more efficient production process leading to lower overall total part costs for the fabricator. The end user also benefits from the improvement in electrical performance and weathering behavior compared with previous silicone materials, and especially when compared to organic polymers.

All of these advantages combine to make silicone rubber composite insulators the insulators of choice for the next generation power transmission and distribution systems.

Philip J. Rogal and Tatyana Collins, Dow Corning