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The Flashover Characteristics of
Cylindrical Stycast as a Function of
Temperature
Horatio Rodrigo and Dong Soon Kwag
11 Oct. 2011
Florida State University, Center for Advanced Power Systems
2000 Levy Ave. Tallahassee, FL, USA
10th EPRI Superconductivity Conference
• Motivation & Objectives
• Introduction
• Method
• Results
• Conclusions
Contents
Motivation & Objectives
• To design and fabricate suitable terminations for a 30 m helium gas
cooled 3 MVA power cable (1 kV dc, 3 KA)
• Comparison of properties of Stycast at different temperatures
• Comparison of the reliability of Stycast under operating conditions
of the cable
Why gaseous Helium?
Temperature below 63K possible
Higher critical current in HTS
Lower risk of asphyxiation in confined spaces
Air
Air
Air
Air
From: A. Kübler, Hochspannungstechnik, Springer 2005
• Helium gas at room temperature
(RT) has low dielectric strength
• Properties improve with higher
density
• Higher pressure (0.44...2.17 Mpa)
• Lower temperature (40...70 K)
• All parts at RT are especially prone
to partial discharge (e.g. bushing)
• Tests necessary
Dielectric Properties of Helium Gas
Introduction
• Flashover voltages used in device design
• Flashover characteristics at fault conditions
• Flashover prevention to increase lifetime
• Flashover voltage in LN2 is the highest for intermediate
diameter of test samples
• Breakdown voltage of gaseous helium improves
significantly with change in density
• Marked improvement in PDIV was achieved by modifying
the electrical feedthrough using Stycast
Experiment
Experimental Setup
Transformer
Pressure vessel H.V.
Transformer0~100kV150kVA
RegulatingTransformerComputer
VoltageDivider
Oscillo-scope
TESTCELL
Electrode Systems
Carousel type
In Gaseous Helium In LN2 & Oil
Thicknesst1=3mmt2=5mm
Diametersd1=9.5mmΦd2=12.7mmΦd3=19mmΦ
12.7
10
37
25.4
R6.35
12.7
Cylindrical
Stycast
d
t
• Samples held between electrodes
• Immersion in gaseous Helium, LN2 and transformer oil
• High voltage source connected as shown
• Voltage ramped up at 0.5 kV/s until flashover
• Flashover voltage and waveform were recorded
Experimental Method
Breakdown Strength of Gaseous Helium
Results
0
1
2
3
4
5
6
7
8
0 0.5 1 1.5
Bre
akd
ow
n s
tre
ngt
h (
kV/m
m)
Pressure (MPa)
IMP 77K
D C 77K
A C 77K
IMP 293K
D C 293K
A C 293K
Plane to plane(25mmΦ) electrode, 5mm gap
AC Flashover Voltage of Stycast in gHe (77K / 1.5MPa)
Thick.
[mm]
Dia.
[mmΦ]
Scale
(V63.2%)
[kV]
Shape
(β)
3
9.5 18.43 48.39
12.7 18.15 21.61
19 24.29 33.58
5
9.5 25.58 24.81
12.7 24.37 11.87
19 25.95 9.964
Thick.
[mm]
Dia.
[mmΦ]
Scale
(V63.2%)
[kV]
Shape
(β)
3
9.5 5.066 80.2
12.7 5.305 49.02
19 7.338 20.17
5
9.5 8.476 87.58
12.7 8.572 121.8
19 9.694 55.88
AC Flashover Voltage of Stycast in gHe (293K / 1.5MPa)
Thick.
[mm]
Dia.
[mmΦ]
Scale
(V63.2%)
[kV]
Shape
(β)
3
9.5 48.15 16.17
12.7 40.08 8.424
19 42.65 6.27
5
9.5 52.48 7.755
12.7 64.54 5.327
19 50.71 29.32
AC Flashover Voltage of Stycast in LN2 (77K)
Thick.
[mm]
Dia.
[mmΦ]
Scale
(V63.2%)
[kV]
Shape
(β)
3
9.5 32.85 9.178
12.7 44.71 5.018
19 39.31 7.269
5
9.5 51.74 11.3
12.7 47.94 16.49
19 49.31 17.99
AC Flashover Voltage of Stycast in Oil (293K)
0
10
20
30
40
50
60
70
8 9 10 11 12 13 14 15 16 17 18 19 20
Scal
e p
aram
eter
V6
3.2
%(k
V)
Diameter of Stycast cylinder [mm]
3mm in GHe(77K, 1.5MPa)
5mm in GHe(77K, 1.5MPa)
3mm in GHe(293K, 1.5MPa)
5mm in GHe(293K, 1.5MPa)
3mm in LN2(77K)
5mm in LN2(77K)
3mm in oil(293K)
5mm in oil(293K)
Comparisons of Scale Parameters V63.2%
Comparisons of Shape Factor β
0
20
40
60
80
100
120
8 9 10 11 12 13 14 15 16 17 18 19 20
Shap
e fa
cto
r β
Diameter of Stycast cylinder [mm]
3mm in GHe(77K, 1.5MPa)
5mm in GHe(77K, 1.5MPa)
3mm in GHe(293K, 1.5MPa)
5mm in GHe(293K, 1.5MPa)
3mm in LN2(77K)
5mm in LN2(77K)
3mm in oil(293K)
5mm in oil(293K)
LHe GHe
LN2 GN2
Temperature sensor
High voltage (AC 60Hz)
H.V.
Bushing
Temp.
monitoring
Heat
exchanger
High Pressure Cryostat
High Pressure Cryo-Feedthrough(original)
Comparison of E-field before and after modification
•Voltage: 9 kV
•Max. E-field in Helium: 1.0 kV/mm
Original with Stycast and Corona ring
•Voltage: 9 kV
•Max. E-field in Helium: 0.5 kV/mm
Stycast 2850FT
Degassed in vacuum
Cured in high-pressure nitrogen
SST 20855CA
Ceramics feedthrough
High Pressure Cryo-Feedthrough(with Stycast)
Partial Discharge Measurement System
Transformer Filter
Voltagedivider
PD capacitors
Conductor tothe cryostat
Groundingswitch
0
5
10
15
20
25
30
35
40
45
50
0 5 10 15 20
Par
tial
dis
char
ge [
pC
]
Voltage [kV]
0.62MPa (original)
1.14MPa (original)
2.17MPa (original)
0.62MPa (with Stycast)
1.14MPa (with Stycast)
2.17MPa (with Stycast)
HV Bushing PD Test (before & after modification)
PD inception voltage: 5 kV 16 kV (peak) at 2.17 MPa
Conclusions
• Breakdown field rises as the temperature gets colder
• Reliability rises with falling temperature
• Breakdown voltage of gaseous Helium improves
significantly with increase in pressure and decrease in
temperature (change in density)
• Modified high voltage bushing is suitable for the
measurement of the model cables (PDIV 5kV16kV)
Future work
DC characterization
Test the 30 m long prototype cable
Acknowledgement
We would like to thank the following colleagues for their
contributions to the work reported in this presentation.
Dr. Lukas Graber
Bianca Trociewitz
Steve Ranner
Danny Crook
Florian Salmhofer
THANK YOUFOR YOUR ATTENTION
This work was supported by:
US Department of Energy (DE-FC26.07NT43221)
Office of Naval Research (N00014-08-1-0080)