A New Detection Method of Normal Transition in HTS Coils by using Poynting’s Vector Method Kagoshima University, Japan Akifumi Kawagoe
Contents Background Principle Example of detection Example of Application of this method Miniature pickup coil pair Possibilities of application to quench detection of accelerator magnet ?
Introduction High temperature superconducting coil - High stability. High specific heat due to high operating temperature - Low thermal diffusivity may lead to generate hotspot. It is important that monitoring system for HTS devices can safely detect narrow normal transitions in the winding
Old monitoring System Contact type Non-contact type Balance voltage AE method Pick-up coil method, method （ Acoustic Emission ） Active power method method Voltage taps are Sound from the coil Profile of magnetic directly attached to the are measured fields are coil, potential measured differences are measured Problem Risk of spark over. Low reliability Low usability There are not suitable monitoring systems for HTS devices.
Our monitoring system Poynting’s vector as electromagnetic energy flows around the coil are measured. If conditions of winding change by some disturbance, Poynting’s vector also change. Our method can measure Poynting’s vector around the coil by pick up coil pairs composed of two kind of pick up coil. Fig. 3 New pick-up coil arrangement for non-contact measurements of local Poynting ’ s vector.
Advantages of this method (1) It is safe because the method can measure the winding abnormalities of superconducting coils without contact, (2) It is highly reliable because the electrical method is accurate. (3) The method can measure local abnormalities and their propagation in superconducting coils during operation. (4) This method can measure abnormalities at room temperature outside cryostats, so maintenance of both superconducting devices and monitoring systems is made easy.
Principle of measuring electric fields
Principle of measuring electric fields If E2 ~ 0 Pick-up coil-E can measure E 1 . E 1 E 2 Pick-up coil-E
Principle of measuring Poynting’s vector During AC operation or Excitation Poynting’s vector is composed of stored energy component and loss component. In order to know conditions of winding, loss component must be measured. W P d E H d t t T T
Signal of pickup coil-H Signal of pickup coil-E We get this signal At ac operations, phasor diagram corresponding to magnetic and electric fields
Example of measurement by this method Table 1 Parameters of superconducting coils Coil Diameter 70 mm Length 64.3 mm Number of layers 1 Number of turns 15 Length of windings 3.30 m 11.8 µ H Inductance Table 2 Parameters of pick-up coils Pickup coil- E Turn number 24 turn Length of inner arc 51.2 mm Length of outer arc 130 mm Pickup coil- H Turn number 80 turn Operating current is AC. Length of inner arc 51.2 mm Length of outer arc 55.0 mm When a area of winding was warmed up by nitrogen gas, the energy flows were measured.
Result (b) (c)
Results of Poynting’s vector measurements (1) (c)
Monitoring system for HTS transformer as application of our measuring method That is pair of these pickup coil pair A and B. Poynting’s vectors of A is subtracted that of B. A B The reason of using two pair is to measure abnormalities even if load of transformer changed. HTS transformer
Results Blue and Red points are corresponded to measured data by one pick- up coil pair Green plots are subtracting result.
Possibilities to apply quench detector for accelerator (1) This method measure magnetization of winding. Our experiment is under ac operation. Therefore, even small change magnetization can be measured. Under dc operation, if temperature of winding quickly rise up, the temperature change might be measured because magnetization is rapidly changed
Possibilities to apply quench detector for accelerator (2) If more sensitive measurement is required, pick-up coil pairs put near magnets. In this case, it's better to change coil shape. Because magnetization change equal to current distributions change, our method applied to detect current distribution change.
Example of small pickup coil pairs A B Measurment using two pick-up coil-H, A, B
Result Warm up time Measured signal [ μ V] Temperature [K] Time [sec] Time [sec]
Summary A new non-contact measuring method using pick-up coils set at room temperature, which can measure accurately the local normal transitions such as hot spots in HTS coils has been proposed. -Our measuring method can measure normal transitions of Bi-2223 coil. -The monitoring system for HTS transformer is proposed and demonstrated. -Even small pickup coils can measure normal transitions.
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