Grids That Communicate
Ever more power is being generated from renewable energy sources. However, because the supply from wind or the sun can fluctuate greatly, depending on the weather, operators must always have up-to-date information on the current condition of their grids. Composite hollow-core insulators filled with silicone gels and equipped with fiber-optic cables facilitate the installation of the required measuring instruments.
Europe’s electricity supply is changing – the electricity market is largely liberalized, cross-border power trading is on the rise and ever more wind and solar power units are joining the grid. In Germany, the government is promoting the switch to renewable energy sources with an ambitious energy-transition program. The amount of electricity derived from wind, the sun or hydropower is increasing in other regions of the world, too. The International Energy Agency (IEA) estimates that, by 2050, renewable energy sources will supply over a quarter of the electricity in the Middle East and more than 15 percent in China.
However, because wind turbines and photovoltaic systems depend on the weather in producing their power, it’s getting more and more difficult to balance electricity supply and demand. This influences grid stability and thus also supply security – the risk that the existing grids will collapse is increasing. In extreme-case scenarios, power failures pose a threat.
In order to guarantee high supply security, grid operators control and regulate the power supply. To do this, they continuously require extensive information on the immediate condition of their systems – they always have to know what’s happening in the grid at any given time. “Their information needs will continue to increase as a result of the renewable energy revolution,” predicts Renate Glowacki, who is responsible for WACKER SILICONES’ technical support for silicones in the transmission and distribution industry.
“The grids will be more flexible in the future than they are today and they will increasingly be able to control and regulate themselves,” continues Glowacki, who studied industrial engineering with a focus on electrical engineering. Industry visionaries are speaking of smart grids. However, even today, the grids are not entirely stupid, at least not high-voltage ones. Their switching stations contain measuring and sensor technology that continuously supplies up-to-date information – for example about the voltage, current or power line frequency. A digital information and communication system evaluates the data supplied, automatically initiates switching operations if necessary and forwards the information to the control room. There, technicians can take additional control and regulation measures.
Protecting Sensitive Measurement Heads
At the applications laboratory in Burghausen, the quality of POWERSIL® Gel C 670 is checked before each batch is dispatched. The gel can be poured readily and penetrates into even the smallest of cavities. This helps avoid air bubbles and increases the dielectric strength. Furthermore, in its cured state, the gel adheres very well and can be stretched freely.
Like everything else in high-voltage technology, the equipment used in the metrological monitoring of the grid depends on reliable electrical insulation. All of the equipment’s live parts – usually the entire measurement head – must be insulated from the ground and grounded equipment and are often positioned on a hollow-core insulator. “Where measuring instruments are operated outdoors, which is the case in many high-voltage substations, operators are increasingly relying on silicone composite hollow-core insulators,” says Dr. Armin Merten, head of development at Reinhausen Power Composites GmbH (RPC). This Bavarian company, headquartered in Regensburg, is one of the world’s leading manufacturers of silicone composite hollow-core insulators.
New-generation measurement heads deliver an optical signal; many measurement heads also require light in order to create information. Fiber-optic cables transport the light. They connect the measurement head to the connection point of the communication system. Up to now, the fiber-optic cables have been guided via a so-called insulator string – rod insulators joined together in a chain-like manner. The insulator string is an additional component alongside the post insulator that carries the measurement head. “The sensitive fiber-optic cables cannot simply be suspended freely, but must be mechanically supported to protect them against damage or breaking,” explains Merten. “At the same time, the support must not create an electrically conductive path to the ground or the grounded junction box. That’s why the fiber-optic cables are guided with the aid of an insulator string.”
Hollow core insulators
A hollow-core insulator is rather like an empty, electrically insulating pipe, around the outside of which are arranged annular, umbrella-like sheds. Hollow-core insulators particularly find use as exterior insulation for various kinds of electrotechnical equipment such as bushings, instrument transformers, circuit breakers and over-voltage protection devices. Hollow-core insulators have traditionally been made from a dark-brown or gray hard porcelain. Composite hollow-core insulators, on the other hand, consist of a glass-fiber-reinforced plastic (GRP) pipe, fitted with flanges at both ends, that carries sheds made of an elastic polymer. The material of choice for the sheds is a silicone elastomer. The GRP pipe provides the composite hollow-core insulator with the requisite mechanical strength and impermeability. The silicone sheds reliably confer electrical insulation. The flanges are used for assembling the insulator on location. Composite hollow-core insulators for high-voltage technology can assume large dimensions. As a rule of thumb, the length is around 1 meter per 100,000 volts.
However, this kind of insulator string represents additional costs for electricity grid operators, which they want to avoid, if possible. “A few years ago, a customer asked us whether it would be possible to guide the fiber-optic cables through the interior of the post insulator and so do without the insulator string,” remembers Merten. This question inspired RPC to develop a composite hollow-core insulator in which the fiber-optic cables are already integrated and guided safely. This kind of composite hollow-core insulator can act as a post insulator for a measurement head or sensor that no longer requires the additional insulator string.
Integrated Fiber-Optic Cables
This type of composite hollow-core insulator contains several ready-to-connect, plastic-coated fiber-optic cables. Slightly curved, they are guided through the glass-fiber-reinforced plastic (GRP) pipe – which is closed with metal lids and makes up the basic structure of the hollow-core insulator – and protrude from the insulator on both sides. The cavity of the GRP pipe is completely filled with a compressible silicone gel, so that the fiber-optic cables are embedded in the gel.
“The cavity must be filled with a highly insulating medium – air is insufficient in high-voltage applications, as its dielectric strength is too low,” explains Merten. The dielectric strength defines the maximum voltage at which no arc develops that could pass through the entire insulation medium.
An RPC employee testing an insulator.
In theory, a better insulating gas or insulating oil could be used, but this would introduce a high risk of leakage. The insulation medium could escape into the environment and, at the same time, air could enter the cavity. Thus, if a gaseous or liquid insulation medium were to be used, the system would have to be inspected and serviced regularly, which would be time-consuming and expensive for the operator. These problems do not exist with a solid insulation medium, such as a cured silicone gel. “With a silicone gel, a composite hollow-core insulator can be operated maintenance-free for decades,” says Merten.
Following extensive testing with different solid insulation media, Reinhausen Power Composites decided to go with WACKER’s POWERSIL® Gel C 670. This product is a pourable, addition-curing silicone rubber that, unlike conventional silicone rubber, doesn’t cure to form an elastomer, but a soft and highly tacky gel. “The cured rubber is loosely crosslinked, which makes it very compliant and flexible and allows it to snugly fit the form of solid surfaces,” says Glowacki. “In addition, POWERSIL® Gel C 670 is formulated with a special filler that makes the gel compressible and, at the same time, gives it a low density.” Added to this are the typical silicone advantages – the gel is aging resistant and highly electrically insulating, does not chemically attack other materials and has no known health risks.
Ideal Inherent Tack
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 are one third lighter. This lower weight makes itself apparent during assembly, transport and installation of the electrical equipment in the plant. By virtue of their elasticity, composite hollow-core insulators are earthquake-proof and cannot break during transport or explode as a result of willful damage or under fire. Since silicone elastomers are highly hydrophobic, or water repellent, and because they are even able to impart water-repellency to dirt deposits, silicone composite hollow-core insulators do not have to be cleaned in situ. This minimizes maintenance. 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 at minimum maintenance costs. Another advantage is that composite hollow-core insulators can be made in a shorter time than porcelain models.
The silicone gel’s property profile won over Reinhausen Power Composites, and head of development Merten is particularly impressed with the material’s inherent tack: “The silicone gel adheres incredibly well to the GRP pipe, the fiber-optic cables and the metal flanges. This is important for our application, because the gel must not detach from the other components, even if the materials differ in their thermal expansion.” If the filler were to detach, air bubbles would form – but high-voltage technicians try to avoid air bubbles like the plague. Merten explains why: “Partial discharges can occur on air bubbles, which increasingly damage the insulation medium and eventually cause the insulator to fail.”
Since the silicone gel is compressible, it counteracts its own thermal expansion in the sealed GRP pipe – it adjusts its volume to the given space during the expansion. This is important in practice, because the insulator is always exposed to daily and seasonal temperature fluctuations and, moreover, can heat up during operation. If the temperature rises, thanks to its compressibility, the gel, enclosed in the pipe, only expands so slightly that it doesn’t cause the pipe to burst. If the temperature falls, the gel practically doesn’t shrink, but again occupies the given volume without detaching from the other components – compressibility and adhesive properties harmonize perfectly here.
Testing a composite hollow-core insulator made by Reinhausen Power Composites.
In order to fill the cavity without bubbles, RPC injects the liquid POWERSIL® Gel C 670 into the composite hollow-core insulators from below. To do this, the gel is added under slight excess pressure and is thus already compressed to some extent. “With a precisely metered excess pressure, we provide additional assurance that the gel will always snugly fit the form of the pipe wall, even in the depths of a Siberian winter,” explains Merten. After the gel is filled, it cures at room temperature within approximately eight hours.
As a soft and compliant material, the compressible gel offers the embedded fiber-optic cables optimum protection against external mechanical influences. For Merten, one thing is certain: “In our new, special composite hollow-core insulators, the fiber-optic cables are locked up safe and sound. In POWERSIL® Gel C 670, we have found the ideal potting compound to suit our needs.” Under its POWERSIL® trademark, WACKER offers special silicones for transmission & distribution applications – alongside silicone gels, these include elastomers and silicone coatings.
“In POWERSIL® Gel C 670, we have found the ideal potting compound to suit our needs.”
Dr. Armin Merten, Head of Development, Reinhausen Power Composites GmbHFirst Batch Production
An RPC employee testing an insulator.
Reinhausen Power Composites also uses a WACKER silicone rubber for the manufacture of the composite hollow-core insulators’ sheds: the two-component liquid silicone rubber POWERSIL® XLR® 630. RPC sprays this low-viscosity silicone onto the GRP pipe under relatively low pressure. POWERSIL® XLR® 630 was designed specifically for this procedure, which is widely used in the sector.
“We have much more than a simple supplier relationship with WACKER,” explains Merten. “We particularly value that WACKER SILICONES’ technical service engineers are always on hand to resolve our technical queries and problems. We can rely on expert technical support in the development of new products, too.”
By now, the first batch of these composite hollow-core insulators with integrated fiber-optic cables is in operation. As part of a modernization project, they are used together with optical instrument transformers in 110-kilovolt substations in New Zealand. The hollow-core insulators used there are approximately 1.5 meters tall and have an internal diameter of around 15 centimeters.
Composite hollow-core insulators with integrated fiber-optic cables can do even more: the cables’ optical fibers can also be used as light and temperature sensors. So, since fiber-optic cables do not just transport signals, but also deliver information themselves, the new, special composite hollow-core insulators are in principle suited to monitoring other electrical equipment. They make it possible to detect arcs that occur if there is a malfunction, for example. This can help in identifying faulty equipment, which can then be switched off automatically – another step toward smart grids. WACKER will continue to support the development with tailor-made silicone products in the future, because the application potential of the new composite hollow-core insulators is by no means exhausted yet.