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R. SHARMA and V. L. TANNA
[Left hand page running head is author’s name in Times New Roman 8 point bold capitals, centred. For more than two authors, write
AUTHOR et al.]
1
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
R. SHARMA and V. L. TANNA
[Left hand page running head is author’s name in Times New Roman 8 point bold capitals, centred. For more than two authors, write
AUTHOR et al.]
3
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
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(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%
R. SHARMA and V. L. TANNA
[Left hand page running head is author’s name in Times New Roman 8 point bold capitals, centred. For more than two authors, write
AUTHOR et al.]
5
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
[Right hand page running head is thepaper number in Times New Roman 8 point bold capitals, centred]
(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
R. SHARMA and V. L. TANNA
[Left hand page running head is author’s name in Times New Roman 8 point bold capitals, centred. For more than two authors, write
AUTHOR et al.]
7
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
[Right hand page running head is thepaper number in Times New Roman 8 point bold capitals, centred]
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)