the flashover characteristics of cylindrical stycast as a...

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)