Ready to Generate
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.
A metering and mixing unit introduces the low-viscosity liquid silicone rubber into the heatable mold, which remains clamped in the closed position for the duration of curing.
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.
A 145-kilovolt TE “Axicom” hollow-core insulator from TE Connectivity in front of a high-voltage mast.
Unlike a porcelain surface, a silicone surface is permanently hydrophobic or water-repellent. It is even capable of transferring its water-repellency to adhering particles of dirt. This is a huge advantage for silicone composite hollow-core insulators in areas of high airborne pollution, in intensively farmed regions and in coastal areas, says Dr. Hans-Jörg Winter, who has been in charge of an applied laboratory for the transmission and distribution market segment at WACKER SILICONES for over 20 years. “Rainwater and dew simply roll off the silicone sheds in beads,” adds Winter. “Even if the sheds are exposed to the elements for years outdoors, there is no risk of pollution flashovers, because a continuous, electrically conducting moisture film cannot form.” Silicone composite hollow-core insulators are unbreakable, earthquake-proof and weigh much less than their porcelain counterparts. They can withstand high internal pressure – up to 100 times atmospheric pressure – and will not shatter at higher internal pressures, i.e. they are explosion-proof.
Furthermore, composite insulators can be made much faster than porcelain insulators. Equipment makers, energy providers and grid operators are keen to exploit these advantages and are therefore turning more and more to composite hollow-core insulators, especially for high voltage levels. “Originally, silicone composite hollow-core insulators were mainly used at 220 kilovolts and above. They were too expensive for the lower voltage levels,” reports Rainer Röder. They have since been used more and more at the 72 kilovolt level. “The trend is clearly in favor of composite insulators,” says Röder, confidently.
Hollow-core insulators for high-voltage technology can assume vast dimensions. The higher the voltage level, the longer the insulator is, with every 100 kilovolts adding one meter to the length. Dimensions of this kind are not exactly conducive to cost-effective maximum-quality production, especially as the insulators are produced to order and each order is generally composed of just a few units.
Why Silicone Composite Hollow-Core Insulators?
For design and material reasons, silicone composite hollow-core insulators offer a number of advantages over their comparable porcelain counterparts. For one thing, they weigh only one third as much. This lower weight makes itself apparent during assembly, transport and installation of the electrical equipment in the plant. By virtue of their design, composite hollow-core insulators are explosion-proof and earthquake-proof and cannot break during transport. Thanks to their pronounced water repellency and ability to impart water-repellency to dirt deposits, silicone composite hollow-core insulators do not have to be cleaned in situ. To which must be added the typical aging resistance of silicones. All these factors combine to endow composite hollow-core insulators with a long service life. Finally, composite hollow-core insulators can be made in a much shorter time than porcelain models.
Silicone composite hollow-core insulators have been commercially available since the 1980s. “Back then, custom solutions involving tailored shed shapes were all the rage. Dedicated production methods were set up for these and this made them expensive,” recalls Röder. “That has changed over time. Nowadays, equipment makers are willing to tailor their designs to a portfolio of specified shed shapes and diameters.” This has paved the way for more efficient and thus more cost-effective industrial production methods.
Low-pressure diecasting at atmospheric pressure has since proved to be the best-practice method for industrial shielding with silicone. Although it is very similar to conventional injection molding, the mold-filling pressure is much lower. The principle is simple: Basically, the GRP pipe to be sheathed is inserted into a two-part negative mold of the shielding. After the mold has been closed, a pourable, liquid silicone rubber is automatically introduced into it and cured at elevated temperatures.
Gruppenbild mit Messwandlern (v. l.): Rainer Röder from Gardy, a consultancy; Martin Boss, vice president and technical head of Pfiffner; with Dr. Hans-Jörg Winter and Johan Dewitte from WACKER.
As the saying goes, “the devil is in the detail.” The viscosity of the liquid silicone rubber must be low enough to allow casting at atmospheric pressure without creating any bubbles in the mold. The rubber must also have an adequate processing life, yet still cure to a silicone elastomer as fast as possible in the heated mold at temperatures below 115 °C, as otherwise the GRP pipe would soften and buckle.
Conventional liquid silicone rubbers are unsuitable. “They are far too viscous, must be injected at high pressure and entail expensive injection molds,” explains Dr. Winter. In principle, a different rubber from the WACKER portfolio could be used, namely POWERSIL® 600, which cures at room temperature. It possesses the requisite low viscosity and cures to an elastomer which has excellent technical properties. “The only problem is that the mold needs to be cooled before each filling operation, as otherwise curing would start before it was completely filled,” adds the materials expert. This circumstance had the makers of the hollow-core insulators looking for an alternative.
The breakthrough for the pouring method came in the form of a two-component rubber launched by WACKER in 2000 under the name POWERSIL® XLR® 630. “This product embodies all the advantages of the two different rubber systems,” says Dr. Winter. The two-component, extra-low-viscosity liquid silicone rubber – XLR® stands for extra-liquid rubber – became the product of choice for low-pressure diecasting of the protective silicone and was instrumental in having the method adopted by the industry. Röder says that 80 percent of composite hollow-core insulators in Europe are now made by low-pressure diecasting. “Nearly all companies that have installed production lines for composite hollow-core insulators in recent years have opted for this process and use XLR® silicone from WACKER”, explains the sector expert.
Birthplace of the Method
Low-pressure casting was pioneered by TE Connectivity – formerly Cellpack AG – at its Wohlen site in Switzerland. The company is a member of the TE Connectivity Group, which is headquartered in Schaffhausen, Switzerland. “This site is where silicone shielding was first produced by low-pressure diecasting – it was here that the method was brought to production readiness and first used for industrial shielding in 1988,” comments Röde, a former managing director at the site. Thanks to its plant in Wohlen, TE Connectivity has become the world-leading maker of composite insulators, sold under the name Axicom. The thousands of composite hollow insulators for high-voltage equipment that are made yearly are all produced by low-pressure diecasting. For more than ten years, we have used only POWERSIL® XLR® 630 to sheathe the pipes. With this silicone, we get flawless, cost-effective production results,” says Dr. Robert Strobl, market director of high-voltage products at TE Connectivity.
Not only does the company apply the silicone shielding, it also winds the GRP pipes itself. Over the years, it has accumulated a vast store of expertise. This is hardly surprising, given that the Wohlen site has so far manufactured several hundred thousand composite hollow-core insulators for voltages ranging from 72 kilovolts to 1.1 megavolts. Dr. Strobl believes that collaboration with WACKER has been a key success factor: “The technical service engineers from WACKER were always on hand to resolve our technical queries and problems. What's more, the silicone has always been of the same consistently high quality over the years.”
Stucture of a Hollow-Core Insulator
Typical Application: Instrument Transformer
Switzerland is also home to Pfiffner Messwandler AG, in Hirschthal. This mid-sized company is a reputable maker of voltage and current converters for all voltage levels used in power transmission and distribution. These devices – small, specialty transformers – are capable of converting high voltages or currents to a measurable level so that they can be automatically further processed by electronic control devices.
This is necessary for two reasons. First, every power producer, every grid operator and every energy provider wants to know exactly how much electrical energy has been transported – even if it is only for billing purposes. The transformers employed for this must meet high accuracy specifications. Second, transformers are essential for protecting the grids. They are designed to detect any over-voltages or short-circuit currents, and the evaluation electronics are programmed to issue a signal to shut down the affected grid section as necessary. Instrument transformers are therefore vital elements of power transmission and distribution, and hundreds of thousands of them are used around the world – they can be found in the terminals of every cable, from power plants through to substations.
“We are convinced that silicone composite hollow-core insulators will become the standard for transformer external insulation.”
Owner of Pfiffner Messwandler AG
Instrument transformers for high-voltage equipment are always custom-made. “Our expertise lies in correctly dimensioning the transformers for the intended application and in making them so that they will do their job totally reliably and without any maintenance for about forty years,” says Fritz Hunziker, owner of Pfiffner Messwandler AG. “We are convinced that silicone composite hollow-core insulators will become the standard for transformers' external insulation. We already use composite hollow-core insulators in half of our high-voltage equipment. Their practical benefits are so huge that our customers are increasingly ordering transformers with composite hollow-core insulators, even though they can be more expensive.”
Silicone elastomers are rubber-elastic solids derived from polyorganosiloxanes. They are made by the curing of silicone rubber, a process in which the polymer chains of the organosilicon macromolecules are chemically crosslinked to each other. Silicone elastomers do not absorb water and have a highly hydrophobic (water-repellent) surface. They are flame-resistant, dampen mechanical vibrations and are very good electrical insulators. Silicone elastomers are typified by excellent resistance to a wide range of physical and chemical stresses. For instance, their chemical, physical and technical properties remain constant over the range -45 to 180 °C. They also exhibit extraordinary aging resistance: withstanding permanent thermal, mechanical and electrical loads and continuous exposure to oxygen, ozone and UV radiation – without showing any change in their dielectric characteristics. Owing to this property set, silicone elastomers are ideal insulators for power transmission and distribution. They make a great contribution to the high operational reliability of outdoor electrical equipment.
Demand for instrument transformers and other electrical operating equipment is growing as power grids around the world are being expanded. And with it, demand for composite hollow-core insulators. Low-pressure diecasting has a proven manufacturing track record, the machines are available and, in POWERSIL® XLR® 630, WACKER has a silicone which is tailored precisely to both the process and the machines. WACKER developers are currently working on further developing low-viscosity liquid silicones for even faster processing, with a view to lowering production costs for silicone composite hollow-core insulators even further.