Fault Current Limiter Elements Research Articles

Effect of inductors to mitigate the hot-spot problem in parallel-connected superconducting thin-film fault current limiting elements

We have been developing superconducting thin-film fault current limiter (FCL) elements, in which high-resistivity Au–Ag alloy shunt layers are used to protect YBa2Cu3O7 (YBCO) thin films deposited on CeO2-buffered sapphire substrates. The high resistance of the thin films enables the element to withstand high electric fields of more than 40 Vpeak cm−1 during the current-limiting period after quenching, thus greatly reducing the amount of YBCO thin film needed and, consequently, the cost of an FCL. We have succeeded in fabricating and testing 500 V/200 A FCL modules using two 20 cm long YBCO films connected in parallel. In the present study, we performed extensive switching experiments on FCL elements, in which two YBCO films are connected in parallel to achieve higher rated currents, and confirmed the previously observed phenomenon that the hot-spot problem causing film damage just after quench initiation becomes more severe when the total critical current of the thin films is higher. We have investigated the origin of this phenomenon and found that a rapid current transfer from the first-quenched film with lower critical current to the other film causes higher current in the secondly-quenched film that sometimes leads to hot spots. It is demonstrated that the serious hot-spot problem can be mitigated by the use of inductors when the high-resistance FCL elements are connected in parallel. Based on these findings we propose an appropriate architecture of a high electric-field superconducting thin-film FCL that can be used in a real power grid.

Study on stability evaluation and quench characteristics of non-stabilizing layer YBCO thin-film wire wound with the radius of curvature for application of SFCLS

The YBCO thin-film wire for superconducting fault current limiters shows promise with the following characteristics. (1) It has a high index value that enables a quick phase transition. (2) It has a high current density. (3) It experiences little change in the critical current in high magnetic fields. (4) It has varying quenching phenomena according to its stabilizing layers. Thus, it is being actively studied in many countries for application to power transmission and distribution systems. Based on current research results, this study examined the quenching occurrence trends in the YBCO thin-film wire that has been verified as a superconducting fault current limiter element and that has no stabilizing layers. This was done according to the wire’s radii of curvature and in terms of the increase in its resistance, initial limited current, and quenching occurrence time. Five kinds of spiral-shaped formers were produced with the radii increasing from the minimum radius by 25%, taking into account their changing critical properties due to bending stress. The YBCO thin-film wire without stabilizing layers was wound around these formers. Then an alternating over-current was applied to the current limiting elements that were produced according to the radii of curvature. The resulting trends in resistance enhancement were examined to evaluate the stability of the produced current limiting elements. Finally, the produced current limiting elements’ initial limited current and quenching occurrence time were observed at 90K, 180K, and 250K to evaluate the performance of the current limiting elements.

Methods to Increase Current Capacity of Superconducting Thin-Film Fault Current Limiter Using Au-Ag Alloy Shunt Layers

We have proposed a superconducting thin-film fault current limiter (FCL) that uses high-resistivity Au-Ag alloy shunt layers for the protection against ldquohot spotsrdquo. Its design enables reduction of the total length of FCL elements that can withstand high electric fields E > 40 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">peak</sub> /cm, thus greatly reducing the cost of FCLs. In this paper, two methods to increase current capacity of the FCLs were demonstrated. The first method protects the films from hot spots by using capacitors in addition to external resistors. The capacitors decrease rapid voltage rise in the films at the moment of quenching. The second method maximizes the rated current of two parallel-connected thin films by using parallel connections via superconducting tapes, even if the critical currents of these films are largely different. We then successfully tested two parallel-connected YBCO films prepared by a low-cost, metal organic deposition (MOD) method. Over-current tests showed that (a) superconducting transport currents up to 380 A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">peak</sub> flowed 5 cycles without quenching, which is more than 1.3 times larger than the peak value of the 200 A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> rated current and (b) after switching, E = 45.7 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">peak</sub> /cm was achieved without film degradation.

Characteristics of Resistive Fault Current Limiting Elements Using YBCO Superconducting Thin Film With Meander-Shaped Metal Layer

The authors have studied resistive-type fault current limiting (FCL) elements which have superconducting thin film deposited on almost whole area of the substrate and a gold layer deposited in the shape of meander on the superconductor. From experiments using YBCO thin film superconductor fabricated by MOD (metal organic deposition) method and FEM analysis, voltage generation at local areas of the FCL element before the S-N transition is compared. FEM analysis with critical current density (J C) distribution in the superconductor taken into account is also carried out to study the influence of inhomogeneous J C distribution on the current limiting characteristics of this type of FCL element. Electromagnetic phenomena in the FCL element with inhomogeneous J C distribution are clearly described.

Recovery in Superconducting Fault Current Limiters at Low Applied Voltages

We investigated recovery of superconductivity in superconducting fault current limiter elements based on Au/YBa2Cu3O7 (YBCO) films after the release of faults at low applied voltages. The elements were fabricated by patterning Au/YBCO films. They were subjected to simulated AC fault current, and then to small current for recovery measurements. The samples were immersed in liquid nitrogen during the experiment. After the fault, the resistance decreased monotonously to zero. The decrease was originated from the transition from the normal state to the superconducting state. The applied voltage before the fault release affected the recovery time, but not the decrease rate. The recovery speed depended on the size of samples and was faster in smaller samples. The experimental results were analyzed quantitatively with the concept of recovery propagation. A heat balance equation was derived and solved to explain the linear decrease of resistance during the recovery. The result agreed with data well.

Normal zone propagation in superconducting thin-film fault current limiting elements with Au-Ag alloy shunt layers

We have been developing a superconducting fault current limiter (FCL), in which YBCO superconducting thin films with Au-Ag alloy shunt layers are used. We have already achieved high electric fields (>40 Vpeak/cm), which enable the total length of FCL elements to be reduced drastically, thus greatly reducing the cost of FCLs. In this paper, we report the normal zone propagation velocity in our films when over-current was applied to the films at 50 Hz for 100 ms. The velocity plotted against the root-mean square values of the normalized film current showed a common curve or curves. The data were also discussed using the adiabatic theory. As the normal zone propagation velocity was not so fast, we divided one unit film of 120 mm length into two portions, to each of which an external resistance was attached. The test result showed that a high electric field of 45 Vpeak/cm and total voltage of 450 Vpeak were achieved in the first cycle after quenching, and the film withstood the voltage for five cycles. The temperature distribution along the length of the film was also shown.

Coated Conductor Based FCL With Controllable Time Response

Performance of fault current limiter demonstrators based on either a single coated conductor (CC) tape or CC assemblies tested in current limitation modus is reported. A primary goal of this work was to achieve a quick reaction of the fault current limiter (FCL) device, and to study possibilities to provide a reliable and controllable time performance of limiting devices. An improved homogeneity of coated conductors was achieved employing a new alternating beam assisted deposition technique. This technique was used for processing of biaxially textured buffer layers with enhanced quality. Typically, the tapes yielded a critical current of 200-300 A per cm width at 77 K and self field. In demonstrators based on single CC tape, a 10 mus transient time of quench was observed. A stack of coated conductors jointed in parallel was employed as an active FCL element. Two alternative cases are studied when (i) CC tapes exhibit an equal and homogeneously distributed critical current and (ii) the critical currents and contact resistances are different in different tapes. As a result, it was experimentally shown that the quench duration can be varied from 10 mus (in case (i)) to 2 ms (in case (ii)). Such artificially smoothed quenches with pre-determined time dependence should play an important role in AC and DC networks where current surges have to be reliably suppressed. We found that the quenching behavior of the entire electrical circuits is consistent with a finite element model that takes into account a non-linear type of resistivity of CC tape. Short recovery time of 0.5 ms was achieved in CC based fault current limiters.

Temperature Characteristics of Superconducting Thin-Film Fault Current Limiting Elements Using High-Resistivity Alloy Shunt Layers

We have been investigating a superconducting fault current limiter (FCL), in which YBCO superconducting thin films with Au-Ag alloy shunt layers are used. The ability of the films to withstand high electric fields (> 40 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">peak</sub> /cm) enables the total length of FCL elements to be reduced, thus greatly reducing the cost of FCLs. In this paper, we report the temperature characteristics of Au-Ag/YBCO composite films with sapphire substrates. Temperature of the film was estimated as average temperature from voltage and current in the films during the over-current period of 0.1 s using the relation between their resistance and temperature. The effect of thickness of the substrate was examined. Temperature data were analysed with the relation between the maximum temperature and joule heating during the over-current period. Data on recovery time are also shown.

Numerical analysis and design of fault current limiting elements using large-size YBCO thin films

This paper describes the characteristic analysis and design of large-capacity fault current limiting (FCL) elements using large-size YBCO thin film. The analysis was performed by a finite element method (FEM) based on current vector potentials. Effective patterns of superconducting and gold layers of the elements were designed, and their current limiting characteristics were analyzed. The analysis results indicated that the capacity of the FCL elements we designed was about twice as large as that of conventional elements.

Low-cost and high-power-density resistive fault-current limiting elements using YBCO thin films and Au-Ag alloy shunt layers

We propose a new design for the high-temperature superconducting thin-film faultcurrent limiter (FCL), which uses high-resistivity Au-Ag alloy shunt layers instead of the pure gold (or silver) shunt layers conventionally used. An FCL element (5 mm wide and 40 mm long) with a YBCO thin film (THEVA) and a parallel inductively-wound shunt resistor successfully withstood very high electric field (> 44 Vpeak/cm) for 5 cycles (0.1 sec) after switching, and achieved a very high switching power density, ∼2.0 kVA/cm2. We confirmed similar maximum tolerable electric field (>40 Vpeak/cm, limited by power supply) in a larger sample (1 cm × 6 cm). The composition of our FCL element is very simple, and the achieved power density is more than five times higher than conventional devices, which leads to a dramatic reduction in the amount of expensive superconducting thin films. We made a conceptual design and cost estimation of our FCL elements used in a typical 6.6 kV FCL.

超電導薄膜限流器とテープ巻線型限流器の導体コスト比較

We recently proposed a new design for a high-temperature superconducting thin-film fault-current limiter (FCL), which uses high-resistivity Au-Ag alloy shunt layers instead of the pure gold (or silver) shunt layers conventionally used. Due to the much larger resistance of the Au-Ag alloy layers, the FCL elements withstood very high electric fields (> 40 Vpeak/cm), and realized a very high switching power density, ∼2.0 kVA/cm2. The composition of our FCL element is very simple, and the achieved power density is more than five times higher than conventional devices, which leads to a dramatic reduction in the amount of expensive superconducting thin films required. Similarly, Kinder et al. recently proposed a new coated-conductor-based FCL element, which achieved a relatively high electric field of 2.7 Vpeak/cm. We estimated the cost of our thin-film FCL elements used in a typical 6.6 kV FCL that is introduced in a distributed power supply site. We also estimated the cost of coated conductors used to produce the FCL, and compared the two.

Distribution of Inductive&amp;lt;tex&amp;gt;$J_c$&amp;lt;/tex&amp;gt;in Two-Dimensional Large-Size YBCO Films Prepared by Fluorine-Free MOD on&amp;lt;tex&amp;gt;$rm CeO_2$&amp;lt;/tex&amp;gt;-Buffered Sapphire

Distribution of J/sub c/ in the two-dimensional large-size YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) film was investigated by an inductive method for the application to fault current limiter elements. The YBCO films were prepared by metal-organic deposition (MOD) using a fluorine-free acetylacetonate-based complex solution on R-plane sapphire with an evaporated-CeO/sub 2/ buffer layer. The average J/sub c/ values of 1-3 MA/cm/sup 2/ at 77 K were obtained for 1 cm /spl times/ 12 cm-, 3 cm /spl times/ 12 cm-, and 10 cm /spl times/ 30 cm-rectangular YBCO films as well as on 2-inch-diameter films. In contrast with the TFA-MOD method via BaF/sub 2/ precursor, the fluctuation of J/sub c/ in the present films was fairly small; no significant size effects or shape effects were recognized on the fluctuation of J/sub c/. Instead, fluctuation of J/sub c/ depended on the average J/sub c/ values; higher-J/sub c/ films tended to exhibit more uniform J/sub c/ distribution.

Current Limiting and Normal Operation Characteristics of Parallel Connected Fault Current Limiting Elements with YBCO Films

Fault current limiting experiments were performed for parallel connected fault current limiting elements (FCL) which were arranged polygonal and on plane. It is found that the arrangement didn’t depend on the current limiting characteristics. The current re-distribution was observed at early stage of quench, and it is found that the current re-distribution accelerate the quench. Moreover, the normal operation tests were carried out for the parallel connected FCL, and the loss of the polygonal arrangement was about 1/5 comparing that of the on plane arrangement. Therefore, the advantage of the polygonal arrangement was confirmed.

Transition into dissipative state of superconducting YBa 2Cu 3O 7− x/Au films and fault current limitation: switching and recovering properties

Properties of the transition into dissipative states of YBa 2Cu 3O 7− x /Au thin films are studied during sweep current and fault current limitation (FCL) experiments. Dissipation mechanisms associated with power laws behaviour E≈( J/ J c) n are observed and lead to a transition into the normal state by a thermal runaway. FCL experiments show that bilayer devices limit the current at about 2.5 J c and allow one to develop electric fields of about 3 kV/m without any damage. After the default regime, a recovery of the superconducting state under nominal current I n is for the first time reported. These results demonstrate the strong potential of YBCO/Au thin films as the superconducting fault current limiter elements that could be used autonomously in case of transient over-currents.