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Locating of normal transitions in high temperature superconducting (HTS) coils is important for protection and safety design of HTS apparatus. A general method to locate the normal transitions is to measure resistive voltages along HTS windings by many voltage taps directly soldered to the HTS coils. However, electrical insulation characteristics of the HTS coils are deteriorated because it is necessary to remove electrical insulations of the HTS wires for the soldering. It is a serious problem especially for AC HTS coils to which high voltages are applied. Therefore the authors have presented a non-contact voltage measurement method that can detect the resistive voltages without removing the insulations by voltage dividing capacitors. So far the authors have verified the principle of the non-contact method. In this paper, a method to locate the normal transitions in a Bi2223 HTS coil based on the non-contact method is proposed. The proposed method can not only detect the normal transitions but also locate their positions. It is experimentally confirmed that the proposed method is useful for locating the normal transitions.

AC high temperature superconducting (HTS) coils have been developed for transformers, motors and so on. Quench detection and protection system are essential for safety operations of the AC HTS facilities. The balance voltage method is universally used for the quench detection and protection, however especially for AC operations, the method has risks in terms of high voltage sparks. Because the method needs a voltage tap soldered to a midpoint of the coil winding and the AC HTS facilities generally operate at high voltages and therefore high voltage sparks may occur at the midpoint with no insulation. We have proposed the active power method for the quench detection and protection. The method requires no voltage tap on the midpoint of the coil winding and therefore it has in-built effectiveness for the AC HTS facilities. In this paper, we show that the method can detect the quench in an HTS transformer and moreover our proposed quench protection circuits which consist of thyristors are simple and useful for the AC HTS facilities.

AE (Acoustic Emission) measurements are well known methods to detect mechanical signals from superconducting coil The mechanical signals could be generated by micro cracks of epoxy resins, the motion of superconductors and the thermal expansion of superconductors, which were generated before and/or after a quench. We have presented a time-frequency visualization of AE signals as a method to detect the quench. We can detect very small AE signals regardless of lectromagnetic noises and can find the time of the AE occurrence and the frequency bands of AE signals by using this method. Recently it has been presented that YBCO superconductors are delaminated and degraded by a transverse tensile stress. The delamination is accompanied with AE signals. Also, it is known that amplitudes and frequency bands of AE signals vary with causes of AE occurrence. In this paper, we present time-frequency analyses of AE signa s caused by the delamination of a YBCO superconductor and the micro of epoxy resins.

High temperature superconducting (HTS) coils require reliable quench monitoring systems for safety operations. Also, locating of normal transition areas is important for developing reliable HTS coils. Resistive voltage measurement method is universally used for the quench detection and the location. The method needs many voltage taps soldered to superconducting windings and therefore insulations of superconducting wires have to be removed for the soldering and then it has high risks in terms of high voltage sparks. In this paper, the authors present a new detection method of normal transitions by using capacitor taps which can detect normal transitions regardless of the insulations (by no destruction) and has no risks about the high voltage sparks. In this method, two electro-conductive sheets are attached on a surface of a superconducting coil and are connected with an outside capacitor. Then two capacitors are created on the surface of the superconducting coil by sand iching the insulation between each electro-conductive sheet and the superconducting wire. The normal transitions can be detected as a voltage of the outside capacitor by voltage dividing with the three capacitors. A demonstration test was carried out for a Bi2223 coil and the quench detection and the location were successfully achieved.

In high temperature superconductors, local and excessive temperature rise occurs after normal transition because normal zone propagation velocities are slow at high operation temperature and then the superconductors may be degraded or burned. Therefore, it is essential to detect the temperature rise in high temperature superconducting coils and protect them as soon as possible. The authors have presented a detection method based on time-frequency visualization of AE signals. In this paper, the authors discuss detection and frequency analysis of AE signals accompanying with epoxy cracks caused by thermal expansion in an HTS coil by using the method.

The normal zone propagation velocities in high T c superconductors are slow at high operation temperature and therefore local and excessive temperature rise generates at quench occurrence, and then the superconductors are degraded or burned. Therefore it is essential to detect the temperature rise in high T c superconducting (HTS) coils as soon as possible and protect them. The authors have presented a quench detection method for HTS coils by time–frequency visualization of AE signals and have shown its usefulness for a HTS coil with height and outer diameter of 40–50 mm. In this paper, the authors present a quench detection/protection system based on superior method in quench detection time to the previous method and show its usefulness for a larger HTS coil (height and outer diameter: 160–190 mm) than the previous coil.

When the quench occurs in the superconducting coil, the excessive joule heating in the normal region may damage the coil. It is necessary to detect the quench in the coil as soon as possible and discharge the magnetic energy stored in the coil. We have presented a quench protection system based on the active power method which detects a quench regardless of the self-inductive and mutual-inductive voltage for multi-layer superconducting coils. In this paper, the usefulness of the developed system is confirmed for two-layer superconducting coils wound with Bi2Sr2Ca2Cu3Ox HTS tapes.

The authors have been developing a small and light power source with a hybrid single-phase Bi2223 high temperature superconducting (HTS) transformer. The transformer consists of a primary copper coil and a secondary Bi2223 superconducting coil and can output current of 500 A. For safe operation of the transformer, it is important to detect normal transitions in the transformer and to protect it from excessive heating in the normal area. The authors have presented a protection system based on the active power method and magnetic flux detection coil. However, in the conventional method, the active power does not become zero in a superconducting state due to AC loss and therefore incorrect recognition may be caused. In this paper, the authors propose a method to reduce the loss signals by configuration and mounting position of the magnetic flux detection coil. Through experimental results for a hybrid single-phase Bi2223 superconducting transformer, the authors show that the proposed method can detect the normal transitions more accurately than the conventional one.

It is important to locate positions of normal transitions in a high temperature superconducting coil for identifying its design and fabrication weakness. The authors have proposed capacitor type voltage terminals as a contactless method to measure voltages of the coil windings through insulation of the windings. This method was useful for locating the positions of the normal transitions. Higher resolution of the location is achieved by attaching many terminals to the coil. Then shorter electro-conductive sheets of capacitor type voltage terminals are needed. However, the shorter sheets make the measured voltage signals small and the signals are supposed to be incorrectly measured. In this paper, the authors propose a method to achieve high resolution locating by long electro-conductive sheets of capacitor type voltage terminals. Numerical values of the measured voltage by the terminals depend on the positions of the normal transitions and therefore analysis of the values can achieve high resolution locating without increasing the number of the terminals. Through experimental results for a Bi2223 HTS coil, it was confirmed that the proposed method could achieve the high resolution locating.

When the quenching occurs in a superconducting coil, excessive joule heating in normal area may damage the coil. It is necessary to detect quenching in the coil as soon as possible and discharge the magnetic energy stored in the coil. Therefore, we propose a superconducting coil protection system based on an active power method. The system is highly resistant to the noise and does not require cancel voltage taps, so it is useful for both AC and DC coils. We have presented the effectiveness of the system using some test coils cooled in LN 2 or LHe. However, we have not discussed the effectiveness of the proposed system for helium-free cryocooled magnets, in which a larger temperature rise occurs after quenching than in liquid-cooled magnets. In this paper, we verify the effectiveness and practicality of the proposed system through coil protection tests for a cryocooled Nb 3 Al LTS coil.

Recently, HTS coils have been developed for electric power apparatuses. In superconducting coils, local and excessive joule heating may give damage to the superconducting windings when a quench occurs and therefore it is essential that the quench is detected quickly and precisely so that the coils can be safely discharged. Resistive voltage measurement method is universally used for the quench detection, however, it is vulnerable to an electromagnetic noise which causes insufficient quench detection and at least needs a central voltage tap in windings. In a large superconducting coil, a lead-wire from the central voltage tap may cause a short-circuit when high voltage will be applied. In this paper, we present a quench protection system based on the active power method which detects a quench by measuring the instantaneous active power generated in a superconducting coil. The protection system based on this method is very strong against to the noise and no more needs a central voltage tap. The performance of system developed by us is confirmed by using test coil wound with Bi-2223 HTS tapes.

Reducing AC loss in a superconducting apparatus is one of the most important issues, and the precise measurement and estimation of AC losses are essential to reduce them. The four-terminal method is universally used as an electric measurement method of AC losses for superconducting tapes and wires. In this method, noise and inductive voltage superposed on the terminal voltage of the superconductor are eliminated by a lock-in amplifier and cancel coil, respectively, and then measurement of very small resistive voltage is achieved. However, using this conventional method, a plurality of measuring instruments and apparatuses are needed, and therefore the measuring system becomes complicated and much time is consumed in the calibration process. In this paper, we present a simple and precise measurement system based on an active power detection method, which is proposed as a quench detection method. The proposed system consists of a small number of instruments and apparatus and is less susceptible to noise. Its usefulness is verified by comparing the proposed method and the conventional four-terminal method in measuring the AC transport current loss of a Bi2223/Ag tape.

It is known that current sharing effects of the stabilizer are limited in cryocooling conditions being different from poolboiling. Therefore we have studied stability under it, focusing on minimum quench energy (MQE) of Nb/sub 3/Sn superconductors under various conditions of the applied magnetic field, temperature, and the transport current, for various volume fractions and configurations of Cu stabilizer. From the experimental results, the larger the external Cu volume fraction, the higher the MQE was obtained. All MQE data were summarized by only one line in characteristics of normalized MQE and normalized B(B/B/sub c2/). The obtained MQE behavior was to be characterized by only one empirical equation. It is thought that the obtained formula is useful for a standard of designing the Nb/sub 3/Sn superconductor and its coil under cryocooled condition.

In this paper, we present an electric quench detection method without a central voltage tap which may cause the short-circuit of the lead-wires from the voltage taps in the quench detection of a large AC superconducting coil. In this method, an inductive voltage detection coil is used instead of the central voltage tap. The inductive voltage detection coil is electrically insulated from a superconducting coil and therefore the lead-wires do not break down. Through the quench detection tests for a Bi-2223/Ag HTS coil, we show the feasibility of the proposed method for detecting the quench of the large AC superconducting coil.

The authors have been developing a compact power supply with a single-phase Bi2223 high temperature superconducting (HTS) transformer. The conventional transformer has an iron-core for enhancing magnetic coupling between its primary coil and secondary one. However, the iron-core has great majority of size and weight of the transformer and therefore it is desirable to be removed for a smaller and lighter transformer. In this paper, the authors propose an air-core HTS transformer for a more compact power supply than the conventional one. As experimental results, it is shown that appropriate design of the air-core transformer has a possibility to decrease the weight and volume of the large AC current supply.

The authors have been developing a hybrid single-phase Bi2223 high temperature superconducting (HTS) transformer used in the AC current source with rated current of over 500A. Its primary coil is a copper coil and secondary coil is a Bi2223 HTS coil. In this paper, the authors propose a new detection method of normal transitions in the secondary coil by using the active power method and a magnetic flux detection coil attached on the inside of the secondary coil. In the proposed method, the normal transitions are detected by measuring active power dissipated in the secondary coil, and induced voltage of the magnetic flux detection coil by a primary and leakage flux of the secondary coil enables to calculate the active power dissipated in only the secondary coil. As experimental results for a hybrid single-phase Bi2223 HTS transformer, it was found that the proposed method enabled to detect the normal transitions in its secondary superconducting coil.

The normal zone propagation velocities in the high TC superconductors are slow at the high operation temperature and therefore the local and excessive temperature rise generates when the superconducting-to-normal transition occurs, and then the superconductors are degraded or burned. Therefore it is essential to detect the temperature rise in the high TC superconducting coils as soon as possible to protect the coils. In general, the temperature rise can be detected by many voltage taps and thermocouples, however, the taps and thermocouples make the electric insulation of the windings degraded. The author has presented the time–frequency visualization of AE signals as a method to detect the temperature rise with non electric contact and the usefulness for Bi2Sr2Ca2Cu3Ox (Bi2223) superconductors has been presented. In this paper, the usefulness of the proposed method for YBa2Cu3Ox (YBCO) superconductors is verified and the experimental results show that the proposed method can detect the temperature rise in a YBCO coil.

This paper presents the results of the quench detection tests of a high Tc superconducting coil (HTS coil) wound with Bi-2223 tape, impregnated by epoxy resin and cooled by LN/sub 2/ using a partial active power detecting method we have proposed as a new type quench detecting method. The tests were carried out for DC and AC current. The results showed that the proposed method was useful for the quench detection of the HTS coil.