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The growing decentralization of energy supplies through the hooking up of wind farms and solar energy plants to power grids is stimulating demand for the construction of new electric transmission lines and transformers. These, in turn, require hollow-core insulators, which are readily made by low-pressure diecasting a low-viscosity liquid silicone rubber from WACKER. Silicones can withstand extreme weather conditions – even those which prevail out to sea.
For modern societies, a dependable power supply is as important as air itself. Yet, there are many regions where the installed electrical infrastructure – power plants, electricity grids and switching stations – is no longer up to the job. Emerging economies need more efficient grids to cope with surging electricity consumption. And industrialized nations are finding it increasingly important to modernize and expand their synchronous grids to ensure security of supply, especially as they switch to renewable energies. In Germany, more extensive harnessing of wind power has necessitated the construction of additional, low-loss electric transmission lines extending from north to south. The reason is that the turbines are nearly all located in the windswept north, whereas the largest consumers – such as industry – are mostly in the south. Not only that, but changes are also needed to the grid topology, because power is increasingly being generated at a regional level and at intermittent, unpredictable times. Aside from the wind turbines, numerous solar energy plants need to be hooked up to the grid.
Wherever power plants, power lines and switching stations are built or modernized, there is a demand for high-voltage insulators and equipment – and that demand has been growing for years all around the world. As current-carrying components must be insulated in the interests of safety, power grid operators and energy providers must have reliable insulators. Insulators that fail can give rise to flashovers and short-circuits, triggering an automatic shutdown of the affected component within the power grid. In the worst-case scenario, an entire region might experience a power failure. High-voltage grids require different kinds of insulators: long-rod and hollow-core types. The latter are always used wherever an active electrical component or device requires external insulation. A hollow-core insulator is rather like an empty, electrically insulating pipe, around the outside of which are arranged annular, umbrella-like sheds. High-voltage, hollow-core insulators provide external insulation for bushings, instrument transformers, circuit breakers, over-voltage protection devices, cable terminations and other electrotechnical equipment.
Open-air insulators are frequently exposed to high moisture levels. When the dirt on their surfaces is moistened by fog, dew or rain, an electrically conducting pollution layer can form. Where this gives rise to continuous, conductive paths, a leakage current flows that will cause the layer to heat up to different extents, creating dry zones. If the voltage is high enough, small, unstable partial discharges occur initially across the dry zones – and can be seen as a bluish glow at night time. This can pose a threat to the power grid itself if the dry zones keep expanding and eventually a large, stable arc or spontaneous gas discharge of high current density occurs. This electrical arcing is known as flashover because it flashes across the entire insulator. Pollution flashovers generate very high short-circuit currents and must be avoided at all costs. A permanently hydrophobic surface of the kind afforded by silicone shielding is beneficial because it prevents a continuous electrically conducting pollution layer from forming.
The insulators used in overhead power lines connected to high-voltage grids are exposed to the elements throughout their service life, which is roughly forty years. During all that time, they must never fall into an unpredictable operating condition, whether as a result of temperature changes, UV radiation, moisture or contamination. Dirt on insulators poses a special threat – moisture might render it electrically conducting. Consequently, dirt can lead to pollution flashovers.
Particularly Critical: Outdoor Use
High-voltage insulators have traditionally been made from a dark-brown or gray hard porcelain. “This ceramic material has excellent insulation properties,” says Rainer Röder, an engineer with Gardy Technology AG, Erlinsbach, Switzerland, which has been providing technical advice to the hollow-core insulator industry for years. “The problem is that water will wet a porcelain surface. This means that dirty porcelain insulators are particularly prone to pollution flashovers. “Added to which, porcelain insulators are heavy and fragile.”
Silicone composite hollow-core insulators do not suffer from these disadvantages. They consist of a glass-fiber-reinforced plastic (GRP) pipe, fitted with aluminum flanges at both ends, and sheds made of silicone elastomer. The GRP pipe with its firmly mounted and bonded flanges provides the requisite mechanical strength and impermeability, while the sheds confer electrical insulation by virtue of the typical silicone properties. The flanges are used for assembling the insulator on location.
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