1

Estimating the impact of VLF Frequency on Effectiveness of

VLF Withstand Diagnostics for MV Cable Systems

Nigel Hampton1, Joshua Perkel1, JC Hernandez2, Marina Kuntsevich3, and Vivek Tomer3

1NEETRAC2Universidad de los Andes, Merida

3Dow Chemical

2

Outline

• Background• Length Issues • Test Protocols• Water Tree Lengths• Breakdown Strengths• Conclusions

3

Introduction

• .

• VLF is used in Simple Withstand, Monitored Withstand and Tan Delta Tests

• A range of frequencies are permitted: 0.01 to 1 Hz

VLF is the waveform most widely used by utilities who employ diagnostic tests

Examples of VLF Sources

VLF Waveforms

SEBAKMT VLF40 @5kv RMS with 280feet XLPE Load

-8000

-6000

-4000

-2000

0

2000

4000

6000

Volta

ge (k

V)

HVA30 @5kV RMS with 280feet XLPE Load

-8000

-6000

-4000

-2000

0

2000

4000

6000

8000

Volta

ge (k

V)

Sinusoidal

Cosine-Rectangular

Preferred Test Protocol

HOLD

RAMP

Time

Voltage

“Real Time decision here if you wish to curtail or

extend Withstand

“Real Time decision here if you wish to continue Withstand

30 Mins

VLF Frequency

• A range of frequencies are permitted: 0.01 to 1 Hz

• The frequency changes when the length of the cable system being tested increases – longer cable require lower frequencies

Concern that the withstand result may dependuponfrequency

Malaysia 11 & 33kV System, Moh, CIRED 20030.1Hz 0.05Hz 0.02Hz

Survival 87% 75% 74%Fail On Test (FOT) 10% 19% 20%

Fail In Service (FIS) 3% 6% 6%

Lengths Tested with VLF

8

Circuit Length [Conductor ft]

Perc

ent

840007200060000480003600024000120000

30

25

20

15

10

5

0

Median Length = 3500 ft

9

Time on Test [Minutes]

Failu

res

onTe

st[%

ofTo

talT

ests

]

100.010.01.00.1

20

10

5

3

2

1

1000 Feet500 FeetNONE

AdjustmentLength

Length Effect on Failures on Test

Time on Test [Minutes]

Failu

res

onTe

st[%

ofTo

talT

ests

]

100.010.01.00.1

20

10

5

3

2

1

1000 Feet500 FeetNONE

AdjustmentLength

Time on Test [Minutes]

Failu

res

onTe

st[%

ofTo

talT

ests

]

100.010.01.00.1

20

10

5

3

2

1

4.1%

2.4%

17.2%

30

1000 Feet500 FeetNONE

AdjustmentLength

Length Effects

• Comparison of withstand failure on test rates must include length adjustments

• Lower test frequencies come from the longer lengths

15 kV 0.1Hz L=1

0.05Hz L=2

0.02HzL=5

35 kV 0.02HzL=3

0.1Hz 0.05Hz 0.02HzFail On Test 10% 19% 20%

Length Adj FOT 10% 20 – 30% 30 – 50%

• Observed effect is likely not an effect of frequency• Observed effect is likely an effect of length which in turn

impacts frequency

• Is there an effect of frequency?

• To investigate will need 1. consistent defects2. forced frequency

Effect of Frequency

Ashcraft Test

• Ashcraft test is a way to reproducibly grow water trees in the laboratory

• Water trees grow from a water needle

• Tree inception & growth are accelerated by

• Field enhancement

at the water needle

• Ionic solution

• High AC frequency

• After 30 days we end up with consistently treed cells that can be VLF tested at selected conditions

Test Program

• Grow water trees to consistent lengths in selected materials: EPR, WTRXLPE, XLPE

• Step Test groups (4) to failure using sinusoidal VLF at selected frequencies (0.1 & 0.05Hz)

– Establish VLF strength– Water Tree Length

• If low frequency VLF is less effective then there should be a measurable increase in the VLF breakdown strength

Water Tree Lengths very Comparible

40

5

01

51

02

52

0 01 02 03 0

W

tnecreP

)noitalusnI fo %( htgneL eerT reta

0qerF FLV

01.050.

VLF Breakdown Strength – water treed

10987654

99

90

8070605040

30

20

10

5

3

2

1

Estimated Mean Breakdown Strength (kV/mm)

Perc

ent

0.050.10

Freq Weibull

Low VLF frequency is not to the right (higher) than common 0.1 Hz VLF

Weibull Confidence Limits

Confidence limits overlap for low VLF frequency and common 0.1 Hz VLF

Breakdown Strength & Water Trees

100806040200

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

Water Tree Length - Long Trees (% of ins)

Bre

akdo

wn

Stre

ngth

- E

arly

Fai

lure

s (k

V/m

m)

0.05 Hz0.1 Hz

FrequencyVLF

Conclusions

• The reported VLF frequency effect on simple withstand is consistent with being due to the increased length of the circuits tested not VLF frequency

• VLF frequency is correlated with increased FOT & FIS but does not cause the effect

• Controlled tests, on very similarly degraded samples, does not show the hypothesised increase in breakdown strength with decreasing frequency

• VLF tests at lower frequencies are likely to be, wrt common 0.1 Hz, either – as effective or – marginally more effective (resulting in lower

breakdown strengths)