Short-circuit protection to a fault: Superconducting fault current limiters

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Dr. Joachim Bock, Nexans Superconductors, Germany

Nexans has commissioned the world’s first superconducting fault current limiter to protect a power plant’s internal power supply at the Boxberg lignite plant in Germany. Joachim Bock, CEO of Nexans Superconductors, explains how this technology can help to increase personnel and plant safety while simultaneously reducing investment costs.

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Vattenfall Europe Generation AG’s Boxberg power station in Germany is piloting the new SCFL based high temperature superconductor technology developed by Nexans Superconductor

An increasingly decentralized supply of power, higher power flows and the present backlog of investment in equipment are all developments that will require stronger adaptations to the power network in the coming years. In this context, high short-circuit currents play an essential role.

For example, in power networks short circuits can arise because of lightning strikes or failures of system components and power lines, resulting in high fault currents. These cause extremely high dynamic and thermal loads that all system components of the power network must resist.

Due to their modular construction, superconducting fault current limiters (SCFLs) can be used for various nominal voltages and currents, and can be adapted to particular limiting characteristics in case of short circuits. Electrical equipment that controls high fault currents can increase the security of the network and allow power equipment to be designed more cost effectively.

The SCFL is such a device. In contrast to a high-voltage fuse it does not disconnect the line in case of a short circuit but limits the very high currents to defined values. In addition, it allows electrical interconnections of existing systems, which would not be possible without limiters.

During operation, the superconductors of the limiter are cooled to a temperature of around -200 à‚°C, which is easily and cost effectively accomplished by means of liquid nitrogen. At these temperatures, the materials used have virtually no electrical resistance ” even at nominal power loads.

However, if the so-called ‘critical current’, i.e. above the nominal current, is exceeded, the material suddenly loses its superconducting properties and behaves like a ‘normal’ resistor. This relatively high resistance limits the current to a predefined value. These material properties only make superconductors ideal self-actuating current limiting elements.

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SCFL’s active Module

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The active part of the SCFL consists of 48 superconducting elements per phase connected in series and immersed in liquid nitrogen in a cryogenic vessel. The element assemblies are connected to the outside of the vessel through high-voltage bushings and current leads designed to cope with the temperature gradient between the liquid nitrogen and the exterior of the vessel. The liquid nitrogen is cooled by an external cryocooler.

A circuit breaker in series with the limiter is tripped when the fault current has been reduced. The SCFL is thoroughly instrumented to continually monitor its operation and the series circuit-breaker will be tripped if abnormal conditions (e.g. cryocooler failure) occur.

Communication with the DNO telecontrol systems covers operational items and ancillary information such as failure of auxiliary electricity supplies.

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Maintenance-free operation

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After a short circuit, the limiter must be powered off for a short period so that it can automatically return to the operational state by means of cooling. After a few seconds or minutes, depending on the design, the limiter can again accept the nominal power and is ready for the next short circuit event.

The compactly designed current limiter provides close to ideal operating conditions. In normal operation, it is virtually ‘transparent’ to the network; in case of a fault, it limits the short circuit current automatically and reliably ” independent of its level.

By using a SCFL as a protection device, networks can be coupled without causing the short circuit currents to be added. A short circuit current is limited to a defined value by the current limiter, which is within the realms of milliseconds.

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potential uses for SFCLs

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Typical uses of SCFLs are:

  • Busbar coupling while retaining the switching equipment
  • Transformer in-line protection
  • Protection of local networks in industrial areas or chemical parks
  • Protection of the house load in power plants
  • Coupling of networks for the reduction of harmonics

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Additional advantages of SCFLs include:

  • Due to the passive limiting characteristics of the superconductor, the current is reliably limited within the first half cycle in the presence of a short circuit.
  • The SCFL is inherently safe and is free from wear and maintenance; on-site activation is not required. Thus, no additional operating costs arise from a short circuit.
  • Considerable cost savings can be attained within the initial installation or revision of switching equipment because the equipment can be scaled down to lower short circuit power.
  • The peak limited current and the symmetrically limited current can be defined independently of each other.
  • In case of a short circuit, the power flow is not interrupted completely, so that existing protection designs can be retained.
  • During a short circuit, system components are subject to less mechanical and thermal stresses. This can significantly increase their lifetime.

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Boxberg power plant is fueled by lignite and has a capacity of 1900 MW

In addition to the design, production and installation of current limiters, Nexans can also provide for their maintenance and service. Thus, the continuous security and reliability of the equipment is guaranteed.

The SCFL is perceived to be a low-risk fail-safe device, utilizing a nonlinear ‘high temperature’ superconducting ceramic rather than electronic, electromechanical, mechanical or pyrotechnic components. When the superconducting material is operated at below its critical temperature it loses all electrical resistance, thereby allowing normal load current to flow with negligible losses.

Either the increased current density caused by the passage of fault current or the loss of the liquid nitrogen cooling medium cause the temperature of the superconducting material to rise with the result that the material reverts to a normal resistive state.

This added resistance has the effect of reducing the fault current to a lower, more acceptable level. This process is referred to as ‘clamping’ because it effectively sets a limit above which the fault current will not rise. The SCFL operates in a few milliseconds, after which its resistance remains high until the fault current is cleared by a circuit breaker.

The SCFL’s operation is sufficiently fast to ensure that the first peak of the fault current is limited; this is vitally important when considering the closing of a circuit breaker onto a section of faulty network. The degree to which the subsequent current is limited can be set at the design stage to suit a specific application. It will, in many cases, be convenient to choose this level such that existing protection arrangements do not need to be adjusted.

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Strategic deplayment onto networks

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Trials will result in the development of commercially available devices, that are capable of clamping fault levels to within network design limits. This can bring a number of benefits. SCFLs could be strategically deployed onto the network in areas either with existing high fault level issues, or experiencing a high degree of distributed generation connection activity (e.g. urban combined heat and power generation systems). In this application, SCFL could provide a method of deferring the replacement of switchboards or reconfiguration of networks, whilst ensuring fault levels are maintained within safe limits.

Where fault levels are generally high, there may be operational benefits associated with minimizing the often complicated switching required to ensure that equipment operates within its fault rating during network reconfiguration and outages. This could reduce the risk of incurring customer interruptions and customer minutes lost arising from either network switching or from operating parts of the network temporarily on a single circuit. An improvement in staff safety may also be delivered.

If network fault current magnitudes are restricted equipment will be subjected to reduced electrodynamic and thermal stresses ” these are both proportional to the square of the current, so a modest reduction in fault level results in a considerable reduction in these stresses ” potentially reducing the probability of follow-on faults and prolonging the asset life.

SCFLs may, subject to resolution of protection issues, allow radial circuits to be interconnected, with associated improvements to customer supply continuity and power quality (i.e. flicker and harmonics). This could facilitate a radical change in the way networks are designed and operated.

In addition to the above, there are specific benefits associated with the type of design being implemented here, which uses the superconducting to normal transition.

In its normal state, the SCFL does not add significant reactance to the network and therefore does not affect the upper threshold of impedance envelope that DNOs need to stay within to ensure that they do not exceed voltage levels in the event of sudden loss of load etc. It will also not increase system losses.

A significant issue for DNOs today is the increase in X/R ratio which increases the DC component and therefore the asymmetrical current at break time. This also (to a lesser effect) impacts the peak making current under fault conditions. Although a series reactor will reduce the AC component of short circuit current it may make the X/R ratio rise, and although it reduces the overall asymmetrical current, the DC component can be made greater. Circuit breakers are not tested for this increased DC component and associated longer arcing times.

Due the superconducting/normal transition, the SCFL adds resistance to the fault path. This reduces the AC and DC components of current and the level of asymmetry dramatically. This provides much easier making and breaking duties for a circuit-breaker and additionally, greatly reduces the peak voltage generated (transient recovery voltage) at the point of current interruption.

The level of fault contribution from connected inductive load is now calculated according to IEC909 and DNOs are finding sites where the peak making currents of circuit breakers are being exceeded. The fast clamping of the SCFL reduces these peaks to within circuit breaker ratings. This eliminates the need to reconfigure networks before closing operations.

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Choice of HTS material

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‘High-temperature’ superconductors (HTS) were first discovered in 1986 when Màƒ¼ller and Bednorz at IBM in Switzerland found a lanthanum- barium-copper oxide ceramic exhibiting superconductivity at 30 K. Since then, the numerous discoveries of materials displaying superconductivity at temperatures above 77 K have increased the potential for exploitation of superconductivity in a variety of applications.

Prior to these discoveries it was necessary to use helium, which liquefies at 4 K, to cool the metallic superconductors sufficiently. Helium is very expensive and cooling to below 4 K is also costly in terms of equipment and the energy consumed. The discovery of HTS promised to reduce the costs of cooling and to deliver new applications including high-capacity power cables, more compact transformers, motors and fault current limiters.

However, it has taken longer than expected for manufacturers to develop ways to produce HTS materials suitable for use in high-power applications. Ceramics are brittle by nature and susceptible to shattering when stressed which is undesirable in practical applications such as cables.

Bismuth strontium calcium copper oxide (BSCCO) and similar materials have a layered structure and the superconductivity occurs in the layers containing mainly copper and oxygen atoms. It is very difficult to manufacture large single crystals of these materials so superconducting components made from bulk material, as well as tapes and wires where the HTS material is deposited on or in a metallic strip, incorporate the HTS in a polycrystalline form.

The orientation of the individual crystals with respect to each other has to be controlled so that the superconducting planes are substantially parallel and much work has been done on production methods to achieve this. SCFL prototypes have been designed using Bi-2212 and YBCO in a variety of guises.

A Bi-2212 sheet, bonded to a metal shunt and cut into a meander to provide a low inductance current path provided the basis for ABB’s singe-phase demonstrator reported in 2002. At around the same time, thin-film YBCO on a sapphire substrate was used by Siemens for a high-voltage prototype. A Bi-2212 tube, bonded to a tubular shunt and cut into a bifilar helical meander, manufactured by Nexans Superconductors, was used for the first major live network trial, known as ‘CURL 10’, hosted by RWE at Siegen in Germany in 2004. Consideration of the costs, availability and suitability of the various candidate materials for application in fault current limiting led to bulk
Bi-2212 in tubular form emerging as the most attractive material to use in a commercial SFCL.

The SCFL will provide short circuit protection for Boxberg’s internal medium voltage supply for its coal mills and crushers

Based on the experience gained in providing the superconducting elements for the CURL 10 demonstrator, Nexans was clearly in a good position to supply the superconducting components for the UK trial.The company has developed a Bi-2212 component for use in the first UK pilot. The component is made from a tube of melt-cast
Bi-2212 soldered to the interior of a copper-nickel-manganese tube, which provides a metallic shunt to prevent the formation of hot spots in the HTS.

This arrangement is then cut into a helix giving an effective length of 3 metres for each component. The components are supported from the interior by a tube of fibre-reinforced plastic. Pairs of tubes are joined end-to-end and 24 of these connected in series provide the current limiting function for each of the three phases.

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Deployment of first SFCL

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The site for the first pilot was selected in 2006. It is in a semi-urban location in Lancashire and was chosen for two reasons. Firstly, there is plenty of space for the installation and secondly, the site provides an example of where an SFCL might be installed in response to real need.

The two 33/11 kV transformers feeding the substation were recently upgraded, with the result that the fault level increased to above the making and breaking capacities of the existing circuit breakers. It was therefore necessary to build a new substation and install a new 11 kV switchboard of primary distribution circuit breakers comprising ten feeders, two incomers and one bus-section.

Thus, while the fault level problem was addressed in a conventional manner, the situation has allowed the design of the SFCL to be determined according to realistic criteria, as though it were being used to provide a solution to the fault level issue. The rating of the old switchgear was taken as the basis on which the operating characteristics of the SFCL would be determined.

The old switchgear was rated at 11 kV with a short circuit capability of 150 MVA. This equates to a breaking capacity of 7.87 kA and a making capacity of 19.7 kA.The fault contribution of each of the two transformers is calculated to be 11 kA peak, 4.2 kA rms. It was decided that the SFCL should limit the fault current seen by any circuit breaker to 95 per cent of the old breaker rating, i.e. 7.48 kA break and 18.72 kA.

The SFCL, which is deployed in a bus-section configuration, effectively in parallel with the existing bus section circuit breaker (which of course will be left open during the trial) must limit the fault contribution from the healthy to the faulted busbar when a fault occurs on an outgoing circuit.

This contribution, together with the contribution from the transformer directly feeding the faulted bar, must be no more than 95 per cent of the rating of the old switchgear. The limiting, or clamping performance of the SFCL is thus defined by the network characteristics and the rating of the available plant.

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FIRST POWER PLANT INSTALLATION

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In a pilot project for Vattenfall Europe Generation AG, the SFCL will provide short circuit protection for the internal medium voltage power supply that feeds coal mills and crushers in the Boxberg brown coal power plant in Saxony, Germany. Vattenfall’s SFCL, based on Nexansà‚´ HTS technology and designed for a rated current of 800 A, is undergoing live testing by daily routine operation in a feeder bar of the 12 kV power supply for rebound hammer mills (used for crushing coal).

For the SFCL used in Boxberg, Nexans Superconductors designed and built the device according to the specifications from Vattenfall and the Brandenburg Technical University in Cottbus (Germany), which is providing scientific support for this project. The device can limit a 63 kA prospective short circuit current to less than 30 kA immediately and to about 7 kA after 10 milliseconds.

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