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Pictures of the Future
The Magazine for Research and Innovation

Power Transmission

Stabilizing the Grid with Superconducting Systems

Manfred Wohlfahrt ensures that an embedded ceramic superconducting material is precise to within one-one-hundredth of a millimeter when deposited on a substrate and coiled onto a bifilar winding.

Because more and more solar facilities and wind farms are feeding energy into the grid, a short circuit could cause extremely strong currents through power lines and destroy grid technology. Researchers at Siemens want to prevent this by developing superconducting fault current limiters. These systems would not only be reliable, they would also stabilize the grid. A prototype of such a system will soon be installed in Augsburg.

A debate is currently raging as to whether Germany will be able to achieve its climate target for 2020 of reducing emissions of the greenhouse gas carbon dioxide by 40 percent compared to 1990 levels. Meanwhile, the country continues to rapidly expand its use of renewable sources of energy. The German Association of Energy and Water Industries (BDEW) has calculated that renewables covered 28.5 percent of Germany’s electricity needs in the first half of 2014 — a new record. However, the increase in eco-electricity poses a challenge to grid operators, because every new wind or photovoltaic system is an additional electricity producer that has to be directly connected to the grid.

Although it wouldn’t be a problem for grid operators under normal conditions, the situation becomes tricky if a short circuit occurs because an excavator accidentally cuts through a power cable or a tree falls on a power line. The more electricity producers are directly connected to the grid, the more current could flow through the power lines. If short-circuit current flows unhindered into switchgear, it could tear the power lines out of their mountings and fry grid components.

To prevent this from happening, the power lines that lead to transformer substations and switchgear are equipped with short-circuit protectors known as circuit breakers. If a short-circuit current rapidly builds up, circuit breakers can generally interrupt this flow of electricity, provided it is not too large. Additional protection against especially powerful short-circuit currents is provided by series reactors, which damp short-circuit currents like a resistor. The problem with series reactors is that they not only act as resistors when there is a short circuit, but also during normal operation. This causes electricity to be continuously wasted. The power loss typically amounts to 25 kilowatts per series reactor coil. Experts estimate that up to 44,000 series reactors are installed worldwide. That translates into a global power loss of up to 1,100 megawatts, which is the equivalent of a large power plant’s output.

Siemens researcher Dr. Hans-Peter Krämer checks each bifilar superconductor winding to ensure that it has been precisely coiled.

Short Circuit

The traditional power grid is hierarchical. This means that big, centralized power stations feed electricity into the high-voltage level, from where the power flows down to the medium-voltage level, the low-voltage level, and, finally, to businesses and private households. These traditional voltage levels are separated by grid elements such as transformers so that energy can be safely transmitted and distributed from the producer to the consumer. Now, however, increasing numbers of biogas and solar facilities and wind farms are feeding energy directly into the medium-voltage level. Short circuits could thus cause strong currents that the medium-voltage level can’t cope with. That’s why grid operators have to upgrade their systems in line with the expanding use of renewable energy sources. Series reactors alone would not offer a solution — not only due to the power loss they cause, but also because the series reactors’ electrical resistance causes a voltage drop.

What is really needed is an alternative solution that, unlike series reactors, does not offer any electrical resistance in normal operation. This solution actually exists in the form of high-temperature superconductors, which consist of special ceramics made of yttrium-barium copper oxide. These superconductors can transport electricity with no resistance and almost no loss. However, they have to be cooled down to achieve these properties. This is normally done with liquid nitrogen at a temperature of minus 196 degrees Celsius. Although the cooling process also consumes energy, experts estimate that the energy needs of a superconducting fault current limiter are only about half as high as the power loss caused by a comparable series reactor.

The German Association of Energy and Water Industries (BDEW) has calculated that renewables covered 28.5 percent of Germany’s electricity needs in the first half of 2014 — a new record.
A superconducting winding embedded in metal is embedded in a substrate.

High-Temperature Superconductor

Scientists at Siemens Corporate Technology (CT) have been researching high-temperature superconductors for more than 20 years now. “We’re convinced that the increased use of renewable energy sources means the time is ripe for superconducting fault current limiters,” says Siemens researcher Peter Kummeth, who specializes in superconductors. Tabea Arndt, who heads the Superconducting Components and Applications research group, adds:  “Until a few years ago, the prices of superconducting materials were still dominated by the manufacturers’ development costs. The transition from a technology push to a market pull has made superconducting applications not only technologically appealing, but also economically attractive.” Superconductors offer substantial advantages. Because they do not have electrical resistance as is the case with series reactors, superconductors increase the grid’s stability and reliability. If grid operators used more series reactors, they would have to replace network components or install electrical components that strengthen the grids. Superconductors eliminate the need for such additional work.

In October 2014 CT launched a cooperation project with the Augsburg municipal utility company. The project is being conducted as part of the BayINVENT program for innovative energy technologies and energy efficiency, and receives support from the Bavarian Ministry of Economics, Media, Energy and Technology. The project's partners want to build a prototype superconducting fault current limiter by the end of 2015. The current limiter will be installed between the grid of the Augsburg municipal utility company and a facility operated by MTU onsite energy (MTU), which manufactures cogeneration plants. MTU regularly tests the cogeneration plants on its premises. Like a wind farm, the company feeds electricity from its cogeneration plants into the Augsburg grid. These tests sometimes achieve peak outputs of 15 megawatts, which is more or less equivalent to the energy produced by five large wind turbines.

Superconductive windings are rewound at -192 C in a kryostaten vacuum, resulting in virtually zero electrical resistance.

Superconducting Fault Current Limiter

The superconducting current limiter in Augsburg will be combined with a series reactor. During normal operation, the current will flow through the superconductor without any resistance or loss. In such situations, the superconductor will be in a sense “invisible” to the electricity. However, if a short circuit occurs, the high short-circuit current will cause the superconductor to lose its superconducting properties and suddenly turn into a resistor. As a result, the superconductor will heat up like the a wire in a toaster. Moreover, this will trigger a switch that reroutes the current through the series reactor, which would then act as a resistor as usual. The superconductor could also operate without a series reactor, of course. However, while the series reactor is limiting the current, the superconductor can cool off and regenerate itself so that it will automatically be usable again a short time later.

Expulsion fuses are another alternative to series reactors. Like the good old ceramic fuses in household fuse boxes, they burst when the electricity network experiences a short circuit so that it cannot cause any damage. “However, such fuses have to be replaced by hand, which is a pretty time-consuming process,” says Kummeth’s colleague Christian Schacherer, who is responsible for the development of current limiters at Corporate Technology. “By contrast, the fuse in our facility goes back to its starting condition on its own as soon as the superconductor has cooled down again.” Another advantage of the CT current limiter is that it is intrinsically safe, which means it cannot fail. Short circuits cause it to heat up and turn into a resistor all by itself. “An expulsion fuse, on the other hand, can be defective and fail to blow,” says Schacherer.

In order to efficiently intercept short circuits, a superconducting current limiter will be installed between the city of Augsburg’s electrical grid and a combined heat-and-power plant owned by MTU.

A Market that's Ready for Take off

The CT researchers are being assisted in their work by Siemens’ Energy Management Division. Peter Menke, who heads Power Transmission’s Innovation unit, has analyzed the business potential of the new superconducting fault current limiter for his division. “Although the superconducting material is of high quality and the system has to be cooled, the superconductor’s price is getting close to that of existing switchgear using series reactors, which have the familiar drawbacks,” he says. Despite these advantages, it is unlikely that grid operators will replace all of their existing systems and buy thousands of the new superconducting fault current limiters. “The device is ideal for a growing niche market,” says Kummeth. “I think it will initially be mainly used in the manufacturing industry, which has to protect its plant power networks against short circuits. This is especially the case with chemical and petrochemical processes and reactors that handle large amounts of materials. A breakdown at such a facility generates huge costs.” In principle, superconducting fault current limiters can be used to combine virtually any number of subnetworks with each other. For example, the medium-voltage supply network could be connected to company networks or to solar facilities and wind farms.

According to Peter Menke, it’s remarkable how comparatively inexpensive the development of the new superconducting fault current limiter was, as it only cost a few million euros to create. By comparison, the predecessor that CT developed in the late 1990s with the help of the German Research Ministry had a total budget of 50 million deutschmarks (slightly more than €25 million). One reason why the old system was much more expensive is that the superconducting materials had to be specially developed for this purpose. Today, developers at Siemens exploit synergies with other superconductor applications and use commercially available materials. In a great accomplishment, the researchers have used these applications and materials to create a compact and powerful fault current limiter, which is now being installed in the Augsburg grid.

Tim Schröder