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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 method, Active power method AE method （Acoustic Emission） Pick-up coil method

method

Voltage taps are directly attached to the coil, potential differences are measured Sound from the coil are measured Profile of magnetic fields are measured

Problem

Risk of spark over. Low reliability Low usability

There are not suitable monitoring systems for HTS devices.

• Fig. 3 New pick-up coil arrangement for non-contact

measurements of local Poynting’s vector. 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.

Our monitoring system

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

• peration.

 (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

Pick-up coil-E E1 E2

If E2 ~ 0 Pick-up coil-E can measure E1.

Principle of measuring Poynting’s vector

 

 

   

T T

t t d d H E P W

Poynting’s vector is composed

• f stored energy component

and loss component. In order to know conditions of winding, loss component must be measured. During AC operation or Excitation

At ac operations, phasor diagram corresponding to magnetic and electric fields Signal of pickup coil-H Signal of pickup coil-E We get this signal

Example of measurement by this method

When a area of winding was warmed up by nitrogen gas, the energy flows were measured. Operating current is AC.

Table 1 Parameters of superconducting coils Diameter 70 mm Length 64.3 mm Number of layers 1 Number of turns 15 Length of windings 3.30 m Inductance 11.8 µH Coil

Table 2 Parameters of pick-up coils Turn number 24 turn Length of inner arc 51.2 mm Length of outer arc 130 mm Turn number 80 turn Length of inner arc 51.2 mm Length of outer arc 55.0 mm Pickup coil-E Pickup coil-H

Result

(b) (c)

Results of Poynting’s vector measurements (1)

(c)

Monitoring system for HTS transformer as application of

• ur measuring method

That is pair of these pickup coil pair A and B. Poynting’s vectors of A is subtracted that of B. HTS transformer A B The reason of using two pair is to measure abnormalities even if load

• f transformer changed.

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

Measurment using two pick-up coil-H, A, B

A B

Result

Time [sec] Time [sec] Measured signal [μV] Temperature [K] Warm up time

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
• f Bi-2223 coil.
• The monitoring system for HTS transformer is proposed

and demonstrated.

• Even small pickup coils can measure normal transitions.