Electrical Insulation for Superconducting
Applications
C. M. Rey and M. J. Gouge
Applied Superconductivity Group
Oak Ridge National Laboratory
Oak Ridge, TN, 37931, USA
Research sponsored by the U.S. Department of Energy, Office of Electric Transmission and Distribution,
Superconductivity Program for Electric Power Systems, under contract No. DE-AC05-00OR22725 with UTBattelle, LLC
1
Why Cryogenic Dielectrics R&D?
z
Power has two components: P = I V
z
HTS carries the current– only half the story.
z
Cryogenic dielectric must carry the voltage.
z
z
z
Survival of an HTS quench also requires moderate to high
voltage capability for short times.
AC grid applications require high voltage standoff for the
component lifetime
Without viable cryogenic dielectrics, there is no
transmission or conversion of power!
2
Why Cryogenic Dielectrics R&D?
• Grid-based HTS devices a single failure can remove the component
from the grid.
• Generally HV voltage breakdowns are not self-healing (unless in
flowing LN2)
• Vacuum not reliable and subject to the Paschen breakdown as
vacuum degrades.
• Thermal, structural, insulation requirements limit materials selection
(some epoxies, G-10/11, cable tapes, ultem..)
– Need more materials to make engineering tradeoffs in real
devices
• Better characterization
– partial discharge (P.D.) and impulse (lightning)
– P.D.Æ aging mechanismÆfailure over lifetime
3
Why Cryogenic Dielectrics R&D?
Magnet Failures*
Insulation
29
Mechanical
25
System Performance
21
Conductor
17
Coolant
7
SPI ProjectsÆ Insulation
3
Conductor
1
*ÆY. Iwasa, Case Studies in Superconducting Magnets, New York: Plenum Press, 1994.
4
Impact of failures – high current vs. high voltage
Gradual degradation after
intentional damage to single-phase
HTS cable.
epoxy puncture
inside of outer copper shell
0.45
Ic before damage = 4730 A
Ic after damage = 4360 A
0.40
0.35
Voltage (mV)
0.30
0.25
0.20
0.15
0.10
Breakdown!
0.05
0.00
-0.05
0
1000
2000
3000
4000
5000
6000
Current (A)
Lose 4 tapes in a layer-cable
remains on line at reduced Ic
Single void in epoxy
electrical insulation – HV breakdown
Single-phase trip---then
other 2 phases trip off
5
HTS Insulation Methods
• Vacuum
• Gases (Nitrogen and Helium or perhaps Hydrogen) at several bar
• Liquids (LN and LHe)
• Tapes (e.g. PPLP, polyimides Kapton™, Apical™ with pre-preg)
• Epoxy impregnation (s-glass, insulating paper, etc.)
• Chemical Coatings (Formvar™, oxides, etc.)
• Solids (G-10, G-11, phenolic, etc.)
6
Breakdown Voltage of Typical Insulants
under DC Uniform Fields
From J. Gerhold, Cryogenics, Vol. 38, No. 11, pp. 1063-1081, 1998
7
Breakdown Strength (Es) as a Function of
Temperature for Selected Polymers
• Non-polar materials:
- Es nearly constant at low T
- Es decreases above glass
transition temp (Tg)
• Polar materials:
- Es increases at low T
- More gradual decrease at
higher T
• Breakdown mechanisms:
- Electronic (avalanche, field
emission, intrinsic)
- Thermal processes
- Electromechanical
Refs.: M. Kosaki, IEEE Elect. Insul.
Mag., Vol. 12, No. 5, pp. 17-24, 1996;
M. Ieda, IEEE Trans. EI, Vol. EI-15, No. 3,
pp. 206-224, 1980
Recessed Specimen
DC Voltage
8
Major Issues for Cryogenic Coil Insulation
• Mechanical Properties
– CTE compatibility with conductors
– Tensile and compressive strength or others
• Electrical Insulation Properties
–
–
–
–
–
Es w/ and w/o mechanical loading (t-t, l-l)
P. D.
εR
Tan δ or dielectric loss
Aging (lifetime) characteristics
• Thermal Conductivity
– Filled epoxies are higher
9
HV issues on recent projects: volume scaling
•
Project 1
–
–
–
–
•
“All 3 phases exhibited PD inception at very low voltages”
“Dielectric failure at less than rated voltage”
“All three phase sets failed in different places”
“Epoxies generally lose strength for large stressed volumes; problem is worse when
defects such as bubbles are present; scaling with volume generally not known for
most materials”
Data from Project 2:
10
What can we learn from LTS?
• Most LTS applications are LV/non-continuous coils (MRI,
NMR, R&D, mag. sep., accelerator, fusion):
– Quenches (rare) & voltages typically < few kV
– Partial discharge as a damage (aging) mechanism not an
issue even if voids are present
– Only limited data on continuous HV ac applications with
LTS conductor
• BNL ac cable
• LTS conductor is typically sold as in insulated
system
11
Implications for HTS-2G
• Should HTS tape be offered as an insulated
product?
– SEI offers PPLP and PVF-coated BSCCO
– SUPERCON, Inc. offers FORMVAR (polyvinyl
enamel), polyester-enamel, polyimide enamel and Kapton
insulation with NbTi conductor
– QC tests in factory environment
• HTS should be insulation friendly
– Tape edges not sharp, no burrs
– Deliberate electrical connection between substrate and
YBCO/silver/copper
12
Cryogenic Dielectrics Workshop
• Workshop planned for October 2005 in
conjunction with 2005 CEIDP in Nashville
– ORNL will be the local host
– CEIDP is the Conference on Electrical Insulation and
Dielectric Phenomena and attracts the world’s experts in
electrical insulation
– Will bring in HV people working on HTS applications
together with dielectrics experts, needed for success of
future HTS HV projects
13
HV properties of materials
• HV properties of commonly used materials such as
G10
–
–
–
–
AC, impulse, PD, aging, surface flashover
Scaling issues (volume, gap and surface effects)
electrode geometry
room temp and 77 K, perhaps 30 K
• Survey of broad range of materials, using simple
breakdown test for screening
• Need design rules
– Make data available to others
14
Non-destructive diagnostics
• Improved diagnosticsÆ voids or other defects in
solids during casting and before assembly of
insulator parts
• Some kind of combination of x-ray and P.D.
diagnostics
• R&DÆ improved techniques beneficial to future
projects, especially in achieving higher voltage
capability
15
Conclusions and Future Needs
•
Insulation materials must not degrade under thermal contraction or cycling
•
Es/unit thickness generally decreases with increasing thickness, volume, or
area due to inherent defects – more studies needed
•
Primary aging mechanism is P.D.
– Voids or trapped air gaps can result in PD
– Turn-to-turn and layer-to-layer systems need to be tested
– What are PD characteristics for a trapped air void as T is lowered: effect
of condensation and ice surface??
•
Pulsed aging studies needed (application dependent)
•
More systematic R&D needed to fill in the gaps from earlier studies
•
•
Nano-particle fillers will be important for tailoring future dielectrics
Based on continuing issues with the performance of dielectric materials at
cryogenic temperatures and at high voltage, more emphasis is needed on
R&D and design guidelines in this area for the grid-based SPI projects.
16
Thermal Compatibility
• Thermal contraction of
dielectric must match
conductor
• Conductors range from
0.2% - 0.4% (incl. Bi2223
HTS)
• Polymers 1.3% - 2.4%
(some are lower)
• Filled epoxies and GFRP
0.2% - 0.7%
• Should verify thermal
compatibility and
dielectric strength for
candidate materials
Ref.: M. Kosaki, IEEE Elect. Insul. Mag., Vol. 12, No. 5, pp. 17-24, 1996;
17
Effect of Compressive
Stress on Breakdown
• Breakdown strength
increases to yield stress
• Strength declines due to
voids or cracks induced
• GFRP lower due to filler resin interfaces
S. Nishijima and M. Hara, Cryogenics, Vol. 38, No. 11, pp. 1105-1113, 1998
18
Effect of Tensile Stress
on Breakdown
• Cracks propagate
easily perpendicular
to tensile stress
• Breakdown strength
under tensile less
than compressive
• LN2 fills cracks
before field applied
in Method M
S. Nishijima and M. Hara, Cryogenics, Vol. 38, No. 11, pp. 1105-1113, 1998
19
Various Properties as Function of Temperature
• Dielectric constant ε does not change significantly with T
• Tan delta decreases significantly from 293 K to 77 K
• Yield strength generally increases at lower T
From E. B. Forsyth, Proc. IEEE, Vol. 79, pp. 31-40, 1991
20