development of indigenous electrical insulation … · procedure and understood the critical issues...

R. SHARMA and V. L. TANNA

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AUTHOR et al.]

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DEVELOPMENT OF INDIGENOUS ELECTRICAL

INSULATION BREAKS FOR SUPERCONDUCTING

MAGNETS OF FUSION DEVICES

R. SHARMA

Institute for Plasma Research

Gandhinagar, Gujarat, INDIA

Email: [email protected]

V. L. TANNA

Institute for Plasma Research

Gandhinagar, Gujarat, INDIA

Abstract

Electrical insulation breaks (IB) are very critical component of large-scale fusion devices employing superconducting

magnets. The electrical insulation breaks developed for the requirement of up gradation of hydraulics for the

superconducting polodial field coils of SST-1 fusion machine. The electrical insulation breaks have been installed in the

hydraulic, validated and sustained the operational required temperature. It has performed in rigorous environment of many

thermal cycling from 300 K to 4. 2K of pressure 1-12 bar which induced more thermal stress in electrical insulation breaks.

Main function of such insulation break is to supply cold helium to superconducting magnets and to isolate the magnets

electrically from ground potential during the quench. The salient design features include bigger dimension of ½” size, break-

down voltage withstand capacity up to 5 kV, helium leak tightness ≤ 1 x 10-8 mbar-l/s at 4.5 K and needless to mention the

cryogenic compatibility and flexibility issues. Success rate is about 75 % as it is new attempt with Indigenous epoxy resin

system. The basic structural materials are stainless steel SS 316 L feed tube separated by a cryogenic grade G10 GFRP

insulation material which bonded with cryogenic epoxy resin. The failures causes have been identified, analyses that

considered and rectified during indigenous development of electrical insulation breaks. In past project, an imported

cryogenic epoxy resin system was used for fabrication of small size insulation breaks. The failure was observed after the

repeated 4.2 K cryogenic cycles which doubts the reliability of component and epoxy resin system. The real research &

development as well as challenge are to define and develop an adequate Cryo compatible epoxy. The electrical insulation

breaks and cryogenic epoxy resin are not commercially available items, not reliable, cost factor and failure was noticed after

cold thermal cycles. The In-house indigenous developed electrical insulation breaks can be used for future indigenous

superconducting magnet fusion machines, electrical isolation and for low temperature experiments purpose (up to 15 kV

applications), bonding and sealing of dissimilar materials at cryo temperature with very much cost effective. In this paper,

the design, development, fabrication, performance test at 300 K, 77 K and 4.2 K of electrical insulation breaks and highlight

on development of indigenous cryogenic epoxy resin system will be presented.

1. INTRODUCTION

Large numbers of electrical insulation breaks (IB) have used in the SST-1 machine to isolate the liquid helium

feeds which are at ground potential from the high voltage superconducting magnet system as shown in Fig. 1

The basic role of IB is to provide the required electrical isolation, Cryo compatibility as well as helium leak

tightness. These electrical breaks are located at the superconducting magnet inlet and outlet, isolation of bus bar

and current leads. As per the SST-1 requirement the crucial design incorporates 5 kV isolation as break down

voltage, helium leak tightness up to 1x10-8

mbar-l/s at 4.5 K and cryogenic compatibility. The maximum design

pressure is 40 bar (g) which is compatible with the maximum quench pressure of the magnet system. The basic

structural materials are two nos. of SS tubes separated by a high quality G-10 (FRP) tube. The bonding between

the two different materials, namely, the SS and FRP is achieved by in-house developed cryogenic compatible

epoxy. This component failure resulted the vacuum will be deteriorated in the machine that may resulted to have

to stop the fusion experiment and needs to be open. From the past experience of failure of electrical insulation

breaks, we have been able to establish the process right from the cryo-compatible material selection, fabrication

procedure and understood the critical issues involved in the design of electrical insulation breaks. For the

requirement of up gradation of hydraulics for the PF-3 and PF-5 SC (Superconducting) magnets of SST-1

machine, the bigger size (of ½ “ NB i.e. 21.3 mm Outer dia, OD) insulation breaks were in-house developed,

fabricated, tested as per operational requirement and installed in the circuit as shown in Fig. 2. The electrical

insulation breaks have been validated and performing as per the machine requirement. Since this component is

not commercially available locally or International and used in specific application as superconducting fusion

machines. Procurement from outsourcing and high cost factor influence us for development of this component.

IAEA-FIP/P8-12

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FIG. 1. Electrical insulation breaks in FIG. 2. In Polodal SC coils FIG. 3. In-house developed IB

Superconducting (SC) coils

magnets

2. TECHNICAL REQUIREMENT

As per the needs the electrical insulation breaks have developed for SC hydraulics and 10 kA current feeder

systems as helium vapour insulation breaks. The in-house developed electrical insulation breaks as final product

having features of optimized dimension by mechanical analysis, contour design and electrical field

consideration in Paschen condition, in-house uses of cryogenic resin system, manufacturing process

optimization by filament winding process and rigorous stage wise performance testing of component. The

function requirement of IB listed below. Fig. 3 shows the schematic of developed electrical insulation breaks.

(a) Working condition (inside): 4 bar LHe flow at 4.5 K and with vacuum (outside): 10-5

mbar

(b) Helium leak rate at 300 K and after 5 numbers of 77 K thermal shocks: ≤ 10-8

mbar l/s

(c) Helium acceptable leak rate: ≤ 10-8

mbar l/s at 4.5 K with liquid helium flow to vacuum

(d) Compatibility design pressure: 40 bar (a)

(d) Electrical Isolation: > 2 kV DC

(e) Size: 5 mm ID/ 8 mm OD, ½” NB, ¾”, 1” (SS conductor end tube OD)

FIG. 3. Schematic of electrical insulation breaks

3. WORK CARRIED OUT TOWARDS THE DEVELOPMENT OF ELECTRICAL INSULATION

BREAKS

Different type of dissimilar materials used for bonding as Stainless steel (SS) to GFRP with cryogenic resin

system, Teflon to SS with resin system etc. The joints have been fabricated and tested for fabrication of IB. The

cryogenic resin system is not readily available, its reliability, higher cost factor and dependency on and

reliability of imported resin induced us for in-house development. The various tasks have been carried during

development of component from materials selection to final component acceptance as listed below

(a) Selection of raw materials as SS 316L for conductor materials, cryogenic resin system, S-glass fibre roving

comparable with E-glass.

(b) Electrical design analysis for optimizing design parameters and electrical field strength has been analyzed

at different surface and contours along with gaps considering Paschen condition.

(c) Manufacturing process of inner insulation tubes, winding process based on glass fibre content and resin

ratio, fibre angle and its feed that strongly influence the final component performance. Wet Filament

automatic winding process is selected for fabrication of inner insulation tubes and outer final winding over



AUTHOR et al.]

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insulation breaks.

(d) Physical, microstructure and chemical test of raw materials.

(e) Performance tests on epoxy, composites samples and complete insulation breaks as per ISO/ASTM

standards

(f) The first stage bonding with SS conductor and inner insulation tubes with selected cryogenic resin, with

helium leak tightness of < 1x10-8

mbar-l/s at 300 K and after 5 cycle of thermal shock at 77 K, at

helium pressure 10 bar (g), 77 K of passed joints will be considered for the 2nd

stage outer final filament

winding.

(g) In-house cryogenic resin development, optimized DGEBA epoxy resin system with aromatic amine

hardener and silane coupling agent used for fabrication of IB.

(h) Controlled pulsed type TIG welding of connectors and vacuum coupler to IB for vacuum and pressure test.

(i) Mechanical, electrical performance, quality assurance and quality control tests of each stages during

development and fabrication of IB have been carried out as presented in performance test section.

4. MANUFACTURING PROCEDURE OF SAMPLES

The complete electrical insulation breaks consists of SS 316L stubs which is separated by glass fibre tubes of G-

10 CR grade as shown in Fig. 4. Each stage of fabrication and testing, proper QA/QC aspects [2] followed, the

following are the procedure for the fabrication of the complete IB. The stage wise snaps during fabrication is

shown in Fig. 5(a) to Fig. 5(g).

(a) The fabricated seamless SS 316L and GFRP tubes of optimized dimension in 10 numbers of each batch

immerse into liquid nitrogen for more than 10 hr.

(b) The helium leak tightness test of each SS 316 L and GFRP tubes, acceptable leak rate < 1x10E-8 mbar –l/s.

(c) Cleaning with isopropyl alcohol the tubes samples.

(d) Prepare the resin system as per the ratio manually; the entrapped air removed by vacuum evacuation upto 1

mbar in Desiccators vessel.

(e) Apply the resin system on both external and internal mating threads of SS 316 L and G-10 tubes.

(f) Assemble SS316L tube and G-10 tube, the excess resin would be removed by applying S-glass roving at

transient joint at 2-3 overlap rotation.

(g) Cure the 1st stage bonded part as per resin system curing schedule for 24 hrs at 300 K and followed by @70

⁰C, 3 hrs in oven.

(h) Performance test carried out of 1st stage bonded assembly as per acceptance test criteria; the passed boded

assembly of respective batches is ready for outer final filament winding.

(i) The outer winding done by same resin system with optimized wet winding parameters.

(j) The complete component will be accepted after carried out and passing of all performance tests done in

samples.

(k) From the measurement and test result, the density and mass of insulation tube using glass fibre roving 70-65

% is higher than using glass fibre tape of 50-65%. This significantly influences the thermal contraction rate

of insulation tubes.

FIG. 4. Internals of electrical insulation breaks

The wet filament winding method parameters for fabrication of inner insulation tubes and outer winding

samples of IB have been shown in Table 1.

TABLE 1. WET FILAMENT WINDING PARAMETERS

IAEA-FIP/P8-12


(a) (b) (c) (d)

(e) (f) (g)

FIG. 5. (a) Sand blasting of SS stubs (b) Inner insulation tube fabrication by fiber tape (c)1st stage bonding of smaller size

IB (d) Bigger size IB bonding (e)Filament winding by manual method (f) Filament winding by automatic machine

(g) Complete electrical insulation breaks

5. PERFORMANCE AND ACCEPTANCE TEST RESULTS

The following tests were carried out from 1

st stage bonded assembly to final component IB according to the

technical requirement and acceptance criteria of SST-1 machine. The various performance test results are

summarized in Table 2.

TABLE 2. TEST RESULT OF ELECTRICAL INSULATION BREAKS

Test Acceptance Criteria Complete electrical

insulation breaks

Observations

/Remarks

Helium leak test at 300 K ≤ 1x10-8

mbar-l/s ≤ 5.0 x10-9

Average reading

Thermal shock test 300-77 K (European

Standard ANSI/EIA-364-32 C-200)

5 cycles, no crack, visual

inspection

No cracks found

Helium leak test at 300 K after thermal ≤ 1x10-8

mbar-l/s ≤ 6.0 x10-9

Accepted

Helium leak test @ 12 bar (g) 300 K ≤ 2.0x10-6

mbar-l/s ≤ 3.2x10-9

Sniffer mode

Parameters Inner insulation tubes

(10 Nos.)

Outer winding

(2 Nos.)

Winding machine Tech Specification 4 Axis FWM 4 Axis FWM

Max spindle speed RPM 80 80

Number of layers 21 No. of layers in total 9 sections

In Hoop winding: 80, In Helical : 5

Type of winding Helical Hoop and Helical

Speed of fiber and Angle used 0.679 m/min, 35⁰ 4.930 m/min, 35⁰

Total fiber used 0.507 Kg 0.163 Kg

Total resin mixed/used 1.310 Kg 0.646 Kg

Tension in fiber during winding ~ 2 Kg ~ 2 Kg

Temperature and humidity 27-30 ⁰C, 40-52% 28-19 ⁰C, 46-47%



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Helium leak test @ 12 bar (g) 77 K ≤ 2.0x10-6

mbar-l/s ≤ 3.0x10-6

Sniffer mode

Helium leak test @ 0-20 bar (g) 4.2 K ≤ 2.0x10-6

mbar-l/s ≤ 2.5.0x10-6

Sniffer mode

Helium leak testing under mechanical

loading at 0-20 bar(g) helium pressure

(a) Tensile and compressive load

2000N at 300 K, 77 K

(b) Bending load 100 N-m

(c) Torsion Load 100 N-m

≤ 2.0x10-6

mbar-l/s

5 cycles, 15 minute

holding time, 10 samples

Avg. Helium leak

rate mbar -l/s

(a): 4.0x10-09

(b) 5.0x10-09

(c) 2.0x10-09

Load applied

(a) (0-400 Kg),

(b) (0-300 Kg)

(c) (0-102 Kg),

Electrical test at 0-2 bar (g) He gas, in

Paschen condition

(1000 to10-5

mbar vacuum)

(a) DC test

(b) AC test

(100 kV Transformer)

At 300 K, 77 K

(a) 0-5 kV

(b) 0-30 kV

Leakage current:

7.5x10-9

Amp

Insulation

Resistance:

> 200 Giga-Ohm

No Flashover in

DC

Flashover in

insulation surface

at 35-38 kV range

AC Test

Tensile test

(a) Composite sample

(S-glass and DGEBA resin system)

At 300 K

ASTM D 638-1991

Deflection: 6-9 mm

Average tensile

strength: 350 MPa

5 samples, Average

Breaking load: 15

kN

Tensile test of complete insulation

breaks

At 300 K

ASTM D 638-1991

Breaking load :

6523 kN

Insulation break

failed at outer

winding and inner

insulation tube

Lap shear strength of epoxy resin

(composite to composite) and breaking

load Resin Systems (a) I (b) II and (c)

III

At 300 K

ASTM D 5868-01,

(a) 5.16 MPa

(b) 5.28 MPa

(c) 4.71 MPa

5 samples,

Average

breaking load

2.8 to 3.0 kN

Other than above, the various tests were performed on raw materials, epoxy resin bonded joint of SS316L and

G-10 and complete electrical insulation breaks to confirm the material property, optimize the ratio of composite

that resulted in enhancement of final component performance. The snaps during performance test shown below

in Fig. 6(a) to 6(j).

(i) Glass content: (68-75 %) in roving tube, (55-65%) in fiber glass tape tube, Specific gravity: 1.87 (roving),

1.63 (glass fiber tape, 10 mm width and 0.1 mm thickness) as per IS 13360 Part 8 Sec 8-A and void fraction

as per ASME Sec V-2015

(ii) Chemical and microstructure of SS316L material as per ASTM 479-11 and E 562, E 407 (ASTM grain size

9-10) respectively from the Government approved laboratory. The various performance tests snaps

(iii) The composite sample consists of S-glass yarn of 9 µ, 360 Tex, breakdown strength of 13 kV/mm and three

component epoxy resin system of viscosity 1500-3500 cps, > 17.5 MPa shear strength, 77 K.

(a) (b) (c) (d)

(e) (f) (g)

IAEA-FIP/P8-12


(h) (i) (j)

FIG. 6. (a) Helium leak test at 300 K (b) helium leak test at 10 bar (g) 77 K (c)Thermal shock test (d) Helium leak test at

4.2 K (e)Schematic of 77 and 4.2 K test (f) Mechanical test set up (g)Tensile test (h) Tensile test of IB (i) DC electric test (j)

4.2 K test cooling cycle of IB

6. MECHANICAL AND ELECTRICAL ANALYSIS

6.1 Mechanical analysis

The mechanical design and analysis was carried out considering the staic yield criteria (Von Mises stress limit)

for SS 316 L material.The desin stress intensity Sm = minimum (2/3 Sy, 1/3 SU), where, Sy= yield strength and SU

is ultimate strength respectively. Whereas, the yield criteria for insulation material applied to the maximum

allowable compressive stress for yield stress. The compressive stress in normal whiles the maximum allowable

shear stress parallel to the glass reinforcement plane of insulation.

Mechanical analysis result of electrical insulation breaks shown in Fig. 7 and Fig. 8 for the following boundary

conditionusin ANSYS engineering software:

(a) Tensile force of 2000 N along axial direction

(b) Thermal stress analysis for Temperature decreasing from 300 to 4.2 K

The analysis result shows that the design is safe in working conditions of IB in machine.

6.2 Electrical analysis

The electrical analysis of insulation breaks was done using the electrostatic solid axi-symmetric model as shown

in Fig. 9 by COMSOL 4.1 multiphysics software with boundary criteria [3]

(a) Design high voltage to ground: 2 kV

(b) Designed to withstand maximum voltage: > 5 kV

(c) Relative permittivity of GFRP : 3 and air and in vacuum: 1.0

(d) The maximum design electrical field strength of the insulation materials: not more than 3 kV/mm

(e) Breakdown strength of GFRP (Glass Fiber Reinforced Plastic: 20-30 kV/mm (in vacuum) and nearly 10

kV/mm (in air medium ) [1]

From the test result the maximum electric field found at the arc of SS metallic tubes is 220-230 V/mm and in the

gap between the metallic conductors is about 450- 500 V/mm. This is acceptable value as per the design

requirement. The result of electric field distribution is shown below in Fig. 10 and Fig. 11.

FIG. 9. Schematic of profile distribution of electrical insulation breaks interface



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FIG. 7. Maximum Von Stress 94.7 MPa (SS Stubs) FIG. 8. Maximum Shear Stress 34.9 MPa (SS Stubs)

FIG. 10. Maximum. Electric field: 230 V at arc of SS conductor FIG. 11. Maximum Electric field: 500 V

between two SS conductors

7. TECHNICAL CHALLENGES, PROBLEMS, FAILURE AND EXPERIENCE LEARNT

(a) In first phase of SST-1 machine assembly house developed and fabricated (~500 numbers) and tested at

77 K and 4.2 K for LHe and LN2 services electrical IB were installed in SST-1 machine.

(b) 2-3% failure rate was reported in electrical insulation breaks.

(c) In 2nd

phase of assembly of SST-1 machine, LHe and LN2 insulation breaks were procured from IPP,

China and replaced, the IPP breaks also observed helium leak in 6 numbers of bigger size (24 mm SS

metal OD) at high pressure and cryogenic temperature whereas the smaller size (8 mm SS metal OD)

insulation breaks also found leakage after cold cycle at 4.2 K. in 5 numbers, around 5 numbers breaks

were reported for cold leak at 120 -130 K temperature as shown in Fig. 12.

(d) Due to the requirement in SST-1 machine SC coil hydraulic, 10 kA current lead system, supply and

delivery hydraulic cryogenic transfer line to magnet system and other low temperature experiments, in-

house electrical breaks were developed. From the past experience, the reasons which responsible for

failure were considered more attentive, namely, the bigger size IB is more prone to leaks as induced

thermal radial stress occurs is comparatively more than small sizes IB during cool down cycles,

Arc Length at SS Stub (mm)

Ele

ctri

c F

ield

Str

eng

th

(V/m

)

Arc length at SS Stub (mm)

Ele

ctri

c F

ield

Str

eng

th (

V/m

)

IAEA-FIP/P8-12


flexibility in hydraulic and induced thermal stress during cool down to 300-4.2 K and the temperature at

the transition adhesive point of SS metal and insulation material during welding. (should be < 70 °C)

(e) The availability and limited vendors experience in low temperature, interest in research and development

work for formulating the epoxy resin system for cryogenic application.

(f) During development, 3 numbers of resin system failed at 77 K and pressure condition, however it works

at 300 K. We learnt that at cryogenic temperature, the epoxy resin should have quality of high crack

resistance, high toughness, long usable life, low viscosity for good flow and penetration and very low

shrinkage on cure.

FIG. 12. IPP China LHe insulation breaks leakage in SC magnet

8. DISCUSSION AND CONCLUSION

In this development work, the electrical insulation breaks fabricated and tested in house, three kind of two and

three component epoxy resin system have been used. Form the test results it found that the thermal contraction

is lower in radial direction, fibre content is more than in fibre tape filament winding that results the more resin

content in the composites which developed cracks and leaks at cryo temperature. Electrical insulation breaks (8

mm to ≥ ½”NB SS stub OD) for superconducting coils hydraulic have been installed, validated and performing

as per the acceptance criteria of SST-1 machine. Several cool down cycles to 4.2 K have been carried out, no

failure was reported in developed component. For the fabrication of IB, in-house developing epoxy resin system

have used for bonding and filament winding. The developed electrical insulation breaks and resin system costs

significantly less than from outside available Industries as this item is not commercially available. The

insulation materials and resin system have experimentally validated upto 1017

n/m2

neutron fluence irradiation

environment and it is under process for 1022

n/m2

radiation dose in FBTR fission Reactor. Further, more

fabrication of IB in batch wise project is in-line for the repeatability of acceptance criteria. The rigorous

performance tests, analysis and stringent quality controls in each stages of fabrication of IB would be the main

aspects of success rate. The developed IB can be used for future indigenous superconducting magnet fusion

machines, electrical isolation, dissimilar materials joining and sealing at cryogenic temperature.

ACKNOWLEDGEMENTS

The authors acknowledge the extensive work carried by M/s Uniglass Industries Ltd., M/s CNC Technics Pvt

Ltd and M/s Hy-Vol Fibre Glass work for manufacturing of electrical insulation breaks by filament winding

process, and support for performance test on composites samples. The authors also acknowledge M/s Atul

Polymer division for supporting in development of epoxy resin system at cryogenic temperature, resin samples

testing at 300 K and 77 K. For the electrical analysis parts of insulation breaks by COMSOL software, authors

deeply acknowledge Mr. Amardas for his significant contribution.

REFERENCES

[1] Wanjiang P., Pierre B., David C., Jean J., Yinfeng Z. Cheng W., Nannan H., “ Design of the Room

Temperature Insulation Breaks for ITER Correction Coil Feeders”, (IEEE Transactions on Applied

Superconductivity, MT-24 Special Issue, MT24-4OrBB-04.R1, pp. 1-4 (2016)

[2] Eun-nam B., Keun-Soo L., Young-Min P., Young-Ju L., Ju-Sik B and Kyong-Soo L, “Electrical

Breaks for KSTAR In-Cryostat Helium Line”, (Journal of the Korean Physical Society, Vol 49,

December 2006, pp S232-235 (2005)

[3] Sharma R., Tanna V. L., Amardas A., Pradhan S. and Chandramouli S., “Electrical Design Analysis and

Breakdown Voltage Test Aspects of Indigenously Developed Electrical Breaks at Cryo Temperature”,

(Proceeding of the 26th

International Symposium on Discharges and Electrical Insulation in Vacuum

(ISDEIV-2014) in Vol. 1 pp 61-64)

development of indigenous electrical insulation … · procedure and understood the critical issues...

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