TECHNOLOGICAL FIELD

The present invention is in the field of fault current limiter units and specifically relates to superconductor-based fault current limiter units.

BACKGROUND

Fault current limiter units are typically used for limiting overcurrent in an electrical system to avoid damage to various elements of the system. Generally, in an electrical grid or circuit, various fault types may result in high currents that may damage elements in the system. Circuit breaker units may be used for disconnecting the electric circuit or a part thereof in case of fault current, however such circuit breaker units might not be sufficiently fast in response time.

In some applications, Fault current limiters used are required for providing fast response to faulty high current. Fault current limiters configured to provide variable impedance are often used, enabling quick response to rise in current and limiting the current from damaging the circuit.

Superconducting fault current limiters utilize the extremely rapid loss of superconductivity when the conditions for superconductivity change. This may be a result of current increasing over a critical current density, increase of temperature, and/or and magnetic field. In normal operation, current flows through the superconductor without resistance and negligible impedance, and in case of rise in current breaking the superconducting conditions, it effectively develops high electrical resistance, limiting the current. GENERAL DESCRIPTION

As indicated above, superconducting fault current limiters (FCL) generally provide extremely fast response in limiting high fault current preventing damage to circuit elements. There is a need in the art for a novel FCL unit capable of supporting high power and maintaining its functionality after fault current event while allowing relatively high current and sustaining fields of several KV/m and more. In various configurations, fault current that arise due to fault in an electrical system may reach currents of 100A to 1000A and more. An FCL arrangement may thus be required to be capable of limiting such current while maintain its operability. In various configurations, the FCL arrangement may be formed of FCL units connected in series along electrical system. In some further configurations, the FCL arrangement may include two or more fault current limiter units connected in parallel between them to sustain even higher fault currents.

The present invention utilizes novel configuration of fault current limiter utilizing high temperature superconducting layer positioned on a ceramic substrate selected to provide efficient heat removal. In some configurations, the FCL described herein may also comprise a conducting shunt layer enabling limited current flow in case of fault current. The ceramic substrate according to the present invention may preferably be a relatively thick sapphire substrate. Selection of the substrate is directed at enabling increased removal of heat generated in case of fault current, thus enabling to increase supported power for given length, or enable decreased length for given power. Moreover, the use of thick, e.g. lmm or more, sapphire substrate formed by edge-define film-fed growth techniques, allows to provide substrate of selected dimension with reduced costs as compare to other available technique for providing sapphire substrates.

Diffusivity of material relates to the rate of diffusion within the material. More specifically, this material characteristic indicates the rate in which particles and/or heat can spread through the material or substrate. The present technique utilizes substrate material having high diffusivity, and in particular high thermal diffusivity, thus allowing efficient spreading of heat through the substrate.

In this connection, the present invention is based on the inventors’ understanding that thick substrate layer, formed by heat conducting material that has relatively high diffusivity at relevant temperature allowing superconducting state of the superconductor layer, e.g. greater than 40cm /s at 77K, enables greater heat dissipation as compared to thinner, and lower diffusivity substrate as generally described in the art. This allows the FCL according to the present technique to sustain high power by increase dissipation of heat generated in case of fault current thus preventing burnout of the device. Accordingly, the FCL may be shorter for given power, or sustain more power for given length of the superconducting layer. In this connection, the length of the FCL unit is determined by length of path of electrical current passing therethrough. The substrate may be formed by Sapphire ribbons produced by Edge-Defined Film-Fed Growth (EFG) technique enabling low cost production in relatively large scales as compared to conventional techniques for sapphire production. The super conducting layer may utilize YBCO superconducting material. Generally, YBCO may be grown on as-grown EFG substrate, typically using R-cut orientation, or alternatively on treated or polished EFG surface. Generally Sapphire grown by EFG may allow roughness below lnm enabling efficient growth of superconducting material thereon.

The use of EFG growing technique enables manufacturing of sapphire ribbons having high purity and selected physical dimensions such as thickness of l-5mm and length greater than 50cm. Generally, this technique enables to remove general limitations on length of the sapphire substrate, however for practical use the FCL of the present technique may be configured with length of several meters. The use of thick substrate enables the FCL according to the present technique to operate with increased power level at little or no increase in production cost. This is associated with the use of EFG process where raw material for production of sapphire substrate is provided in a very cheap form (liquid). By contrast thick sapphire wafers obtained by cutting them from bulk single crystals would be far more expensive.

Thus, according to a broad aspect, the present invention provides a fault current limiter unit comprising a ceramic substrate, superconducting layer located on the substrate, wherein said ceramic substrate is selected as a high diffusivity ceramic substrate and configured with thickness of lmm or more. Generally, the ceramic substrate is selected as having diffusivity greater than 40cm /s, and preferably greater than 80cm /s, at temperature below critical temperature of the superconducting layer (e.g. at 77°K or below). For example, the ceramic substrate may have diffusivity of over lOOcm /s at temperature of 65K, and higher values at lower temperatures.

The superconducting layer may be of thickness in the range of 50nm to 5000nm, or preferably between lOOnm and lOOOnm. The length of the FCL unit, along direction of transmission of current therethrough, is selected to be greater than 20cm and may be up to a few meters. It should be noted that the use of EFG technique for manufacturing of the substrate enables providing long and thick sapphire substrates with reduced costs as opposed to the conventional single crystal sapphire, which are generally costly and limited in size.

The fault current limiter may further comprise a conducting shunt located on said superconducting layer, enabling transmission of certain current in case of faulty current. The conducting shunt may be formed by gold, silver or copper layer.

According to some embodiments, the ceramic substrate is sapphire. In some configurations the sapphire ceramic substrate is formed by Edge-Defined Film-Fed Growth (EFG) technique.

According to some embodiments, the thickness of said ceramic substrate is in the range of 1-5 millimeters.

In some embodiments, the present invention provides a fault current limiting system comprising two or more fault current limiter units connected between them in parallel and/or in series with respect to general direction of propagation of current through the system.

According to one other broad aspect, the present invention provides a fault current limiter unit comprising a ceramic substrate, superconducting layer located on the substrate, wherein said ceramic substrate is selected as a high diffusivity ceramic substrate and configured with thickness of lmm or more.

The ceramic substrate may be formed of sapphire substrate, and preferably formed by Edge-Defined Film-Fed Growth (EFG). This allows formation of relatively large sapphire elements allowing selection of substrate thickness and length. According to some embodiments, the substrate may be configured with rectangular cross section and oriented with R-cut orientation direct to said superconducting layer located thereon.

In some embodiments, the ceramic substrate (e.g. sapphire substrate) may be heat treated prior to formation of said superconducting layer thereon. Additionally or alternatively, surface of said ceramic substrate is polished prior to formation of said superconducting layer thereon. In some preferred embodiments said superconducting layer may be deposited on as-grown surface of said ceramic substrate.

According to some embodiments, the fault current limiter further comprises a buffer layer between said ceramic substrate and said superconducting layer. The buffer layer may be used to improve lattice matching between the substrate and superconducting layer and avoid chemical contamination.

According to some embodiments, the fault current limiter may further comprise a conducting shunt located on said superconducting layer, enabling transmission of certain current in case of faulty current. The conducting shunt may be formed by gold, silver or copper layer or alloys of these metals.

According to some embodiments, the fault current limiter may have length greater than 50cm along general direction of current transmission through the fault current limiter.

According to some embodiments, the thickness of said ceramic substrate may be in the range of 1-5 millimeters.

According to some embodiments, the ceramic substrate may have diffusivity greater than 40cm /s or in some examples, greater than 80cm /s. Generally, this diffusivity level relates to operation temperature of the FCL, below T _c of the superconducting layer.

According to some embodiments, the superconducting layer is formed of high T _c superconductor.

According to some embodiments, the fault current limiter may be configured for operation in liquid nitrogen environment. In some configurations the fault current limiter may be configured for operation at temperature bellow 77K, in some configurations, the FCL may be configured for operation at temperature below 65K or below 50K.

According to one other broad aspect, the present invention provides a fault current limiter unit comprising superconductor layer located of sapphire substrate, wherein said sapphire substrate is formed by Edge-Defined Film-Fed Growth (EFG) and has thickness exceeding 1 millimeter. The fault current limiter may further comprise a metallic conducting shunt located on said superconducting layer. Additionally or alternatively, the fault current limiter may further comprise a lattice matching buffer layer located between said superconducting layer and said sapphire substrate. BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 schematically illustrates a fault current limiter unit according to some embodiments of the invention;

Figs. 2A and 2B show experimental results of resistance vs. time measurements for fault current using thinner substrate and thicker substrate according to the present technique respectively; and

Fig. 3 shows a comparison between the results of Figs. 2A and 2B illustrating substantially similar time propagation of the resistance buildup;

Fig. 4 shows diffusivity values of sapphire at temperature below 400K; and

Fig. 5 shows surface profile of sapphire sample produced by EFG technique measured by AFM microscopy.

DETAILED DESCRIPTION OF EMBODIMENTS

As indicated above, various superconducting materials may be beneficially used in fault current limiters. This is due to properties shared by many of the superconducting materials including e.g. second-generation high temperature superconductors (HTS). Generally, in the superconducting state, these materials allow electric currents to be transmitted with no resistivity. However, if the superconducting state breaks down in response to one or more of the following events: temperature increase over critical temperature, current density increases over critical current, or increase in magnetic field, these materials develop high electrical resistivity and effectively cut current flow. Reference is made to Fig. 1 illustrating a fault current limiter (FCF) unit 100 configured according to the present invention.

The FCF unit 100 is formed by a superconducting layer 110, located on a substrate 130. In the example of Fig. 1 FCF unit 100 also includes a metallic shunt layer 120 located on other side of the superconducting layer 110. The FCF unit 100 includes input and output connections exemplified by connection 140. In some configurations the FCF 100 may include a buffer layer 150 located between the substrate 130 and the superconducting layer 110. Generally, for proper operation the FCL 100 is cooled below the critical temperature T _c of the superconducting layer 110 to maintain the layer 110 at its superconducting state. The metallic shunt 120 may be used and configured as a stabilizing layer allowing some current flow when the superconductivity state breaks down. The FCL unit 100 is typically characterized by length dimension l along direction of flow of electrical current there through, thickness and width. Generally, the thickness and width of the superconducting layer 110 together with its critical current density, determine the maximal current that may flow through the FCL in normal operation condition before the current density increases beyond critical current supported by the superconducting layer 110, and thus providing current limit of the FCL unit 100. Thickness d of the substrate 130 determines maximum heat-dissipation per unit area of the FCL 100 during a fault event. As indicated above, the superconducting layer 110 is preferably selected as a high temperature superconductor (HTS) such as ReBCO based superconductors (e.g. Rare Earth Barium Copper Oxide such as YBCO).

The inventors of the present invention have understood that growth of superconductors may be advantageous on dielectric substrates having high thermal diffusivity, and in particular on substrates such as Sapphire. Moreover, the inventors have found that the use of dielectric substrate having high diffusivity (such as sapphire) having selected thickness d greater than 1 millimeter provides substantially improved FCL performance for a given length l of the FCL unit 100.

More specifically, the use of thick substrate enables increased heat removal from superconducting layer 110 of the FCL 100 in case of fault current and establishes that the maximum power per unit area of the superconducting layer 110 that can be dumped during a fault, without burning out the FCL element 100, is proportional to the substrate thickness. More specifically, the inventors have found that the maximal power may increase with the thickness between thickness of 0.3 mm to 1 mm and further provide increased functionality of the FCL at greater thickness. The inventors have measured maximum power dissipation greater than 2kW/cm and electric fields of more than 2 kV/m. Higher maximal power dissipation can be achieved with increase in substrate 130 thickness d , as well as with operation of the FCL 100 at lower temperatures, due to sharp increase of the diffusivity of the substrate 130. This is different with respect to the use of commercially available 2G CC (coated conductor) layers that generally use thin Hastelloy substrates. In some experimental examples, superconducting YBCO layers were grown by sputtering on 1 cm wide sapphire ribbons having the R-cut orientation. The sapphire ribbons were selected having thickness of 0.3 mm, 0.5 mm and 1 mm. An YSZ buffer layer having a thickness of about 20 nm was first grown, followed by a template YBCO layer grown at low oxygen pressure having a thickness of 50 nm. Then a fully oxygenated YBCO layer was grown, followed by the deposition of gold or silver shunt layer, having thicknesses ranging from 50 nm to 150 nm, providing FCL units as illustrated in Fig. 1.

These FCL elements were tested following a procedure by which increasing AC voltages were applied in successive steps for durations of the order of lOOms to 200ms, during which the current was measured. The resistance (voltage/current) of the sample was then calculated and plotted as a function of time. Fig. 2A and 2B show results obtained for 0.3 mm and 1 mm thick sapphire ribbons respectively. The procedure was repeated at increasing voltage values, until the sample was burned out. At the last measurement before burning out, the resistance of the element reached its room temperature value after about 50ms, and a temperature of about 400K was reached after l50ms. As shown, the use of lmm thick sapphire ribbon as substrate allows the FCL to sustain power of 2000Watt/cm before burning, while the 0.3mm thick substrate cannot support powers greater than 700Watt/cm and in some configurations, greater than 800 Watt/cm ² .

The values of maximum power and voltages reached before burning out are shown in Table 1 for a number of measurements.

Fig. 3. Shows a comparison between the resistance (temperature) evolution of two elements grown respectively on 0.3mm and lmm thick substrates near maximum power. It is clearly seen that the temperature evolution is similar for both cases.

Table 1

According to the above results, it can be understood that maximal power that can be supplied to the FCL while avoiding burnout depends on substrate thickness, and the use of thick sapphire substrates enables increase of maximal power by at least a factor of 2 as compared to use of 0.5mm thick sapphire substrates. This is in contrary to the conventional understanding that the maximum power that can be applied on the FCL element might depend mainly on the shunt thickness.

As also shows in Fig. 3, the normalized resistance increases between the two samples (i.e. 0.3mm thick substrate at 700W/cm and lmm thick substrate at 2000W/cm ) is almost similar. This scaling indicates that up to substrate thickness of at least lmm, the temperature is substantially uniform across the entire thickness. Temperature gradient might be formed only at greater thickness. This is due the high diffusivity of sapphire enabling fast heat removal. Further, this indicates that thicker substrates may typically allow greater power.

Generally, diffusivity of the substrate may provide an important role in operation of the FCL according to the present technique. The rate at which a heat front propagates is governed by the thermal diffusivity D = K/C _p where K is the thermal conductivity and C _p is the heat capacity. The heat front will reach a length scale x after a time t according to the law:

X ² = Dt

Over the switching time scale T _S the heat front moves a distance d given by:

5 = (Dr _s ) ^m

Sapphire is a specific example of a dielectric substrate having a high diffusivity. At 65K, below the critical temperature of typical high T _c superconductors (such as YBCO), the sapphire diffusivity value is about l40cm /s. Thus, if a hot spot develops because of the presence of some defect in the HTS layer, within lms the temperature increase of the substrate is smoothed out over the length scale

d=3 mm, which is of the order of a typical HTS layer width. The heat front also penetrates to that depth into the substrate. By comparison the thermal diffusivity of Hastealloy is only 0.03cm /s, and d = 50mih at a temperature of 77K. Fig. 4 shows the rise in Sapphire diffusivity values at temperature below 400°K. The heat penetration depth d determines the maximum effective thermal thickness of the SFCL element, and hence the maximum effective heat capacity and the lowest possible rate of temperature increase for a given power p. The larger the distance d, the thicker the substrate that can be effectively used and the slower this rate can be. A SFCL element based on sapphire may heat up less at a given time than one based on Hastealloy, and as a result is capable to withstand a larger power. Generally, d is not strongly time dependent. This is because at long times the element heats up and the diffusivity reduces. For sapphire d remains close to 3mm from lms, the time scale that characterizes the transition to the normal state, to lOOms which is the time at which a mechanical device will cut the fault current and heating will stop. Therefore, the time evolution of the restored resistance scales with thickness from 0.3 mm to 1 mm thick sapphire thickness.

When a superconductive fault current limiter (SFCL) element is submitted to an applied ac voltage V over a length scale equal to the width of the HTS layer, the power dissipated per unit area upon return to the normal state is given by:

p = V _rm f/ RrtT)

Where Ro relates to resistance per square (sheet resistance measured by Ohms per square) of the shunt. The rate of temperature increase is given by:

dT/dt = p(C _p h) ¹

where h is limited to the value of d calculated above. The maximum power allowed p _M is:

P _M = C _p d ( DT/t _m )

where AT is the maximum allowed temperature increase and t _m is a time scale that can range from x _s if immediate bum out is to be avoided, to the time at which the mechanical switch is operated. In fact, the shortest time scale at which the ac power is defined is half a cycle, or lOms. In what follows similar time scale is used, assuming AT=300K.

In the case of sapphire, the heat capacity is strongly temperature dependent, ranging from 0.4J/cm around 90K to 2.8J/cm at room temperature. Taking a value of lJ/cm 3 , and d = 0.3cm, provides maximum power of 9,000W/cm 2.

For comparison for a Hastealloy substrate, using a room temperature heat capacity value 2.7J/cm 3 and d =50 pm provides a maximum power of 400 W/cm 2. These values compare reasonably well with published data. For epi-polished sapphire Kraemer l al a maximum power of l,200W/cm , Yamasaki et al. -1,600 W/cm (lmm thick epi-polished sapphire) while for Hastelloy based layers reported values are on the order of few 100 W/cm . It should be noted that thick sapphire substrates obtained by Edge-Defined Film-Fed Growth (EFG) technique show increase efficiency in heat dissipation, as well as allow formation of increased length of the FCL unit as described herein.

Reference is made to Fig. 5 showing surface profile of sapphire sample produced by EFG technique coated with an YSZ buffer layer and measured using AFM microscope. As shown, the roughness of the surface is lower than lnm for unpolished sample. This allows growing high temperature superconductor (HTS) such as ReBCO based superconductors on as grown EFG sapphire substrate. The HTS layer is typically grown on a buffer layer such as yttria- stabilized zirconia (YSZ) as mentioned above.

Thus, the present technique provides a novel fault current limiter unit utilizing superconducting layer located on a relatively thick substrate enabling the FCL to sustain greater power while limiting fault current in an electrical system. Accordingly, the use of Edge-Defined Film-Fed Growth (EFG) technique enables growth of sapphire substrates that are larger, typically longer, than what can be achieved using the conventional techniques for growing single crystal sapphire elements. This enables formation of thicker substrates FCL as described herein, providing great improvement in maximal power to length ratio and efficiency of the FCL. Thus, the present technique provides novel FCL units enabling to maintain greater power al lower costs, utilizing high diffusivity ceramic substrate having thickness greater than lmm.