Detailed information on energy research

Projektinfo 12/2011

Superconducting fault current limiters in a power plantSecond generation high temperature superconductors are used to control short circuits for the first time.

Superconducting fault current limiters can contribute significantly to increasing the safety, availability and reliability of electrical systems in power stations. According to experts, they also have an important role to play in expanding the power grid. In 2009 at the Boxberg power station, a superconducting fault current limiter was used for the first time to protect its own power supply. Soon a next-generation superconductor system with optimised characteristics will be tested at the same site.

Short circuits in power plants or electricity grids are expensive. First of all, high currents can damage system components and cause downtime. Then there are the additional costs involved in building the plant because every component needs to be able to withstand the maximum possible load in the event of a short circuit. It is particularly annoying when functioning components in existing systems are suddenly under-dimensioned due to an increased short-circuit power, and therefore have to be replaced. As power grids are upgraded for higher capacities, this is a situation that is likely to occur more frequently. Superconducting fault current limiters (SFCL) represent a new tool for containing these additional costs. They open up entirely new possibilities for designing power plants and grids. These fault current limiters work on a simple principle: the core of the device is a superconducting material which completely loses its electrical resistance to direct current below a material-specific temperature. With alternating current there is an extremely low residual resistance. Installed in a current path, the superconductor

This research project is funded by the:

Federal Ministry of Economics and Technology (BMWi)

normally does not affect the current flow at all. However, this is only the case up to a certain current density in the superconductor. It can be clearly seen on the current-voltage curve (see fig. 1). If the current exceeds a certain threshold, a “quench” occurs. Superconductivity ceases and instantaneously – within a matter of milliseconds – an electrical resistance occurs. As a result, the short-circuit current is automatically limited (see fig. 2). Since the current limitation effect is not only very fast but also very strong, the limited current may be lower than desired, e.g. to maintain particular operating states. However, the design of the SFCL can be adapted for the specific requirements. A parallel resistor to the superconductor – a shunt – allows the current characteristics to be controlled as desired in the event of a short circuit. The fault current limiter operates completely independently and is intrinsically safe. After just a brief cooling phase, it reactivates automatically without requiring any further maintenance. These fault current limiters utilise high temperature superconductors which have a transition temperature above 77 kelvins. Consequently they can be cooled simply using liquid nitrogen, which is an inexpensive industrial product. It can either be replenished or reliquefied directly in the system via a suitable cooling system.

Testing in the power plantThe first generation of fault current limiters are based on a ceramic superconductor material (bismuth-strontium-calcium-copper-oxide, or BSCCO for short). A number of prototypes are currently being tested in various distribution substations. The use of fault current limiters in power plants was pioneered at Boxberg in 2009, a state-of-the-art lignite-fired power plant operated by Vattenfall in Upper Lusatia. The system was developed, built and commissioned by Nexans SuperConductors GmbH. With its coal crushers, coal mills and induced draught fans, a lignite-fired power plant has relatively high own power requirements of around 8% of the total electrical power output. Hence, even in normal operation, currents of 800 amperes at the medium voltage level (12 kV) flow around the installation site via the complex and extensive own power supply network, with peak currents of around 4,000 A measured in tenths of a second. If there is a fault, currents can rise to 65,000 A. The power plant does not have fuses like a domestic installation, which simply interrupt the fault current. To protect against the strong mechanical and thermal stresses that occur in the event of a short circuit, the system components concerned are substantially over-dimensioned in relation to normal operation. This involves investment costs which could be minimised with a fault current limiter. Since there are no fuses in the conventional sense at the medium and high voltage level, in some cases medium voltage networks are fitted with systems of last resort for the worst case scenario which use explosive charges to disconnect the wiring and break the circuit. However, these devices are often not accepted by power plant operators because they depend on a trigger signal and are therefore not intrinsically safe. They also completely interrupt the circuit which means that the power plant’s internal protection is no longer operational. They then need to be repaired. In contrast, the superconducting fault current limiter is intrinsically safe, wear-free and maintenance-free. In particular, rather than completely interrupting the current flow, it only limits it. As a result,

selective short-circuit protection can be provided and existing protection concepts in the form of switches and isolators can be retained. For example, at the Boxberg power plant current is limited to the range of 6,500 to 7,000 A with a peak current of 65,000 A. The SFCL had to prove its capabilities in a test phase lasting more than one year. The limiter’s characteristics were extensively analysed during power-up procedures for transformers and large motors. Trouble-free operation was confirmed in all situations. This innovative technology won the 2010 Energy Masters Award. Another field test will be carried out to investigate the characteristics of second-generation superconductors.

A generation change in record timeAs part of the ENSYSTROB project supported by the German Federal Ministry of Economics and Technology, researchers at the Karlsruhe Institute of Technology (KIT) are working on the second generation of superconducting fault current limiters in partnership with Nexans SuperConductors GmbH. They are using metal tapes coated with a thin layer, only a few micrometers thick, of an extremely high performance superconducting material (called

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Fig. 2 Limiting characteristics of a superconductor when a short circuit occurs (schematic depiction). Blue: short circuit (without limiter). Red: with limiter. Source: Nexans SuperConductors GmbH

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Fig. 1 Current-voltage curve for a superconductor (schematic depiction). Source: Nexans SuperConductors GmbH

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yttrium-barium-copper-oxide, or YBCO). This new conductor material has only become available recently in the required quality and quantity. Compared to the previous solid ceramic material it enables a more compact design, cools down again more quickly due to its lower thermal capacity, and offers better mechanical stability. Its alternating current (AC) losses are particularly relevant. Although these losses are less than 1 thousandth of the protected power, they are a dominant factor in the overall thermal balance. As waste heat, they play a significant role when it comes to cooling. Compared to ceramic, the new superconducting tapes have in the order of ten to twenty times lower AC losses, depending on the current. Operating costs directly reflect the reduced cooling requirements. Researchers also hope for investment cost benefits in the long term. At the present time, SFCL systems using the new material still cost appreciably more. However, the band conductors have a significantly higher cost reduction potential than solid ceramic conductors.A prototype is set to be tested soon. Before being field tested in a real application, it first has to be type tested at IPH (Institut “Prüffeld für elektrische Hochleistungstechnik” GmbH) in Berlin. The system’s electrical

Fig. 4 Testing the fault current limiter in the Boxberg power plant. The complete system fits inside a container. Source: Nexans SuperConductors GmbH

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Fig. 3 First-generation fault current limiters have a modular design with ceramic tubes. These are connected together in a module. The modular design allows the flexibility to adjust the current and voltage to the respective requirements. Source: Nexans SuperConductors GmbH

characteristics such as its switching behaviour and high voltage strength will be analysed. In autumn 2011 the new SFCL is to be installed at the Boxberg lignite-fired power plant in the same place that the first-generation superconductor system was used before. Since it was possible to reuse significant portions of the first system, the geometry and the external dimensions of the unit will be the same. Because it will also use the existing infrastructure, manufacturers and operators will be able to compare technological and efficiency aspects between the two systems. In both cases a particular challenge was the operator’s requirement for an unusually high secondary current of 6,500 to 7,000 A when the fault current limiter is activated. For safety reasons, this specification had to be met without an external parallel circuit. The internal shunt which was then developed independently for each conductor type – considerably increased the demands on the respective components and cooling concept.

DesignIndividual superconductor tapes have nowhere near the capacity to take the prototype’s required rated current of 800 A. Hence it was necessary to connect multiple tapes in parallel. To optimise production and allow flexibility for changing requirements, the researchers chose a modular design in which a large number of individual components are connected in series. Among various different versions that were tested, they decided on a “pancake” design with pairs of parallel conductors (twins) that are not mutually insulated (see cover image). With by far the lowest space requirements, this design could be conveniently integrated into the existing cryostats.

Competing systems

At the present time, three manufacturers in Germany are developing superconducting fault current limiters. In the resistive type presented here, current from the electrical grid being protected flows directly through the limiter. With high intrinsic safety, its features include a compact design, a high fault current limiting capacity, and low AC resistance in normal operation. In the inductive type of limiter, the superconductor is connected via an iron-core transformer. Of these there are two types, one with a shielded and one with a saturated iron core. The advantage of the inductive fault current limiter is its lower cooling load; the cooled superconductor is not connected in series and the power from the electrical grid does not flow through it. A significant disadvantage of this fault current limiter concept is the high cost of materials and resultant weight and size, as well as a relatively low fault current limiting capacity (saturated iron core).

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Fault current limiters for grid expansion The blueprint for the future of Germany’s power supply has been unveiled: a number of large-scale power plants will be replaced by a network of comparatively small generation plants in conjunction with energy storage systems. In addition to high voltage power lines which will carry wind power from the north to the south of the country, for example, a close-knit medium voltage grid will balance regional supply and demand. As a result of the new structure, higher short-circuit power may occur in the power grids. This in turn means that considerable investments are necessary. Superconducting fault current limiters are a valuable new tool for building the new grid as they allow better utilisation of existing grid structures as well as improving the grid’s quality and stability. In many places they may reduce the costs of expanding transmission line and plant capacities, or even make this step unnecessary. The protection provided by SFCLs allows grids to be connected without adding together the short-circuit currents. To give a specific example, this may mean that a wind turbine can be connected directly to the local medium voltage grid instead of having to feed into the high voltage grid via a new transmission line which may have to be constructed, and making a long detour via an expensive transformer substation. In future, using SFCLs will make it possible to integrate small decentralised power generators safely into grids that have a high short-circuit power. Researchers from various EU countries are also developing a multi-purpose fault current limiter system for various different applications and installation sites (in an EU project called ECCOFLOW).SFCLs will also play an important role when superconducting cables are used to massively expand medium voltage grid capacity in urban areas. For example, there is a planned pilot project in Essen where a superconducting cable approximately one kilometre long is to be laid with a transmission capacity of 40 MW. Then a 10 kV high temperature superconductor (HTS) cable will replace a conventional 110 kV high voltage (HV) cable. This could also be achieved with five conventional 10 kV lines, but would involve significantly higher losses and require more space. The SFCL which also forms part of a HTS system of this kind prevents the cable heating as the result of a short circuit. If this happened it could take hours before it cooled down sufficiently to be operational again. An SFCL will therefore also increase safety and improve availability in future urban power supply networks.Development of even higher performance conductors for transporting electricity is also making progress based on the new generation of band conductors. One example is the project supported by the German Federal Ministry of Economics and Technology which has the acronym “Highway”. In this project, an alliance of businesses and research institutes is conducting research into arrangements of YBCO band conductors for use as high current conductors with particularly low AC losses.

Project organisationFederal Ministry of Economics and Technology (BMWi) 11019 Berlin Germany

Project Management Organisation Jülich Research Centre Jülich Dr. Claus Börner 52425 Jülich Germany

Project number 03KP102A

ImprintISSN0937 - 8367

Publisher FIZ Karlsruhe · Leibniz Institute for Information InfrastructureHermann-von-Helmholtz-Platz 1 76344 Eggenstein-LeopoldshafenGermany

AuthorDr. Franz Meyer

Cover imageKIT, Karlsruhe , Germany

CopyrightText and illustrations from this publication can only be used if permission has been granted by the BINE editorial team. We would be delighted to hear from you.

Project participants >> Project coordination: Nexans SuperConductors GmbH, Dr. Joachim Bock, 50354 Hürth, Germany,

www.nexans.de>> Component development: Karlsruhe Institute of Technology (KIT) / Institute for Technical Physics

(ITEP), Dr. Wilfried Goldacker, 76344 Eggenstein-Leopoldshafen, Germany, www.itep.kit.edu

Links and literature (in German)>> www.Nexans.com>> Karlsruher Institut für Technologie. Institut für Technische Physik (Hrsg.):

Entwicklung eines neuartigen supraleitenden YBCO-Tape-Strombegrenzers. ENSYSTROB: Netzintegration und Feldtests. Förderkennzeichen 03KP102B. Abschlussbericht , 2011

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Publications/Projektinfos. Additional information in German, such as other project addresses and links, can be found under “Service”.

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