the accelerated water treeing test (awtt) · 2016. 12. 9. · treeing). • the cec already...

The Accelerated Water Treeing Test (AWTT)

Background, Summary, Example Data, Pros & Cons

Rick Hartlein

Georgia Tech NEETRAC

Presented at ICC Education Session

Spring, 2002

Spring 2002 ICC Education Program – AEIC AWTT

Background

• In early 1970’s extruded cables began to failunexpectedly.

• The reasons for the unexpected early failuresare varied.– Improper installation practices– Inadequate cable designs (bare neutrals, inadequate neutrals,

poor shield designs)– Inadequate insulation and shield compound cleanliness– Poor manufacturing and material handling practices


Background

• This sometimes-poor performance history led the electric utilityand cable industries to develop accelerated test programs toevaluate cable performance.

• These tests are generally conducted at elevated temperatures andvoltages over a period of several months. The idea is to establish,in a relatively short period, whether or not a cable can be expectedto operate reliably in service.

• While no direct correlation between performance in the lab andperformance in the field is established, the test conditions werebelieved to be sufficient to provide a general indication of theability of a cable to survive in a moist environment.


Background

• Materials, manufacturing techniques and constructions forextruded underground power cables have improved significantlyover the past 10 to 15 years.

• However, utilities are still cautiously uncertain about theirexpected life and rely heavily on the results of acceleratedlaboratory tests. This reliance is particularly important in view ofthe new cable insulation and shield materials that are beingintroduced into the marketplace.

• Cable manufacturers and insulation compound manufacturers use

the results from accelerated aging tests to:– Evaluate– Qualify– Promote


Background

• At this point, there is no satisfactory method for using acceleratedaging test data to accurately predict cable life in service:

– Aging factors are incredibly complex– Aging mechanisms not completely understood– Field conditions vary significantly

• The most commonly employed accelerated test in North America isthe AWTT. In this test, full size, extruded distribution cables areexposed to elevated temperatures and voltages in the presence ofmoisture.

• Primary performance indicators are impulse and ac breakdown

data.


AEIC AWTT Background

• The group within the AEIC that deals with technical issues relatedto cable, the Cable Engineering Committee (CEC), developed theAWTT in the early 1970’s.

• While the reason for the premature failures was not fullyunderstood, they are believed to be the result of moisturemigration into the insulation, which caused degradation (watertreeing).

• The CEC already published a variety of cable specifications thatcontained qualification and production test requirements so theaddition of a test to address this premature cable failure problemseemed like an appropriate task to undertake.


AEIC AWTT Background

• The AWTT procedure was published in AEIC CS5 and CS6 formany years.

• AWTT procedure now published in:– ANSI/ICEA S-94-649, “Standard for Concentric Neutral Cables

Rated 5 Through 46 kV”

– ANSI/ICEA S-97-682, “Standard for Utility Shielded Power CablesRated 5 Through 46 kV”

– Supplements (increased tree counting requirements) are published inAEIC CS8.


AWTT ObjectiveThe primary objective of the AWTT test is to provide a standardized qualification testmethod that will give reasonable assurance that an extruded, medium voltage cable

design made by a given manufacturer will meet minimum performance requirementsfor operation in a wet environment.

To meet this objective, the test is designed to:

– Accelerate operating conditions normally found in the field without creating failuremechanisms that do not occur in service.

– Be relatively easy and economical to set up and run.

– Provide results in a reasonable period of time (1 year).

– Help assure that the conductor shield, insulation and insulation shield materials are compatible.

– Help assure that the manufacturer is capable of manufacturing a reasonably well made cableusing the materials being qualified.


AWTT - Procedure

• A total of 21 samples are tested (1/0 AWG conductor, 175 mils of insulation, no jacket)– Six without aging– Six with thermal aging only– Nine with thermal and wet aging–

• Thermal conditioning is conducted in PVC tubes with the conductor at 130 Deg. C usingconductor current. (Thermal load cycle for 14 days)

• New (unaged) and thermally conditioned samples subjected to ac breakdown and hotimpulse tests.

• Dissipation factor measured on selected samples.

• Wet aging is accomplished with 3X rated voltage applied and conductor current sufficientto achieve approximately 60 Deg. C conductor temperature in water. Aging is conductedin tap water filled PVC tubes with tap water in the conductor strands. All wet agingsamples are subjected to the thermal load cycle test before wet aging to thermallycondition the samples (drive off excessive volatiles in the insulation).


AWTT - Procedure

• Three samples are removed after each aging period - 120, 180 and 360 days and subjectedto an ac breakdown test.

• Comments:

– Test conditions are generally well defined.

– Primary performance indicator is ac breakdown test results. (Impulse breakdown test alsoperformed on new and thermally aged samples.)


Test Protocol Summary

Test Characteristic AWTT Well Defined Protocol? Yes Aging Period 1 Year Water Type Tap Voltage 3 Times Vg Maximum Aging Temperature 45 r 3 ºC on Ins. Shld. in water (60 qC on conductor) Aging Environment PVC Tubes Time at Elevated Temperature 8 hours out of 24 hours Number of Labs that Perform the Test? Many Primary Performance Indicator AC Breakdown Defined Performance Limits? Yes Severity of Test Parameters Moderate


Electrical MeasurementsSample 7


Test No. 1-High Voltage Time

Test, Samples 1, 2, 3

Physical MeasurementsSample 1

Test No. 3-High Voltage TimeTest, Samples 7, 8, 9



Test No. 5-High Voltage TimeTest, Samples 13,14,15


Test No. 2 – Hot Impulse TestSamples 4, 5, 6

Load Cycle – 14 DaysSamples 7-21

Test No. 4 – Hot Impulse TestSamples 10, 11, 12

Accelerated Water Treeing (AWTT)Samples 13-21

Test No. 6Continue AWTT for180 Days of Aging

High Voltage Time Test

Samples 16, 17, 18

Test No. 7Continue AWTT for 360 Days of Aging

High Voltage Time Test

Samples 19, 20, 21

21 Samples

120 Days of Aging

Tree Count

Test ProtocolOverview


AWTT - Procedure


14 D ay Load Cycle Cable Tempe ratures

In Conduit - N o Water

Co nductor Current = 27 0 Ampere s

102.3 100.1

129.9 128

28.3 27 .2

0

50

100

150

Te

mp

erat

ure

- D

eg

. C

Ins l. Sh ld .

Cond.

Am bie nt Air

1 /0 Al C o n d u c to r , 17 5 m il w a ll T R XL P E - No Jac ke t

O uter TubesInner Tubes


T ypical Aging Load Cycle

T herm al Profile

0

20

40

60

80

100

0 1 2 3 4 5 6 7 8

Lo ad Cyc le Hou rCen ter Co n du it

Te

mp

era

ture

- D

eg

. C

Cond. (air) Ins. S hld . (air) Cond. (H2O)

Ins. Shld. (H20) (H20) Am b. A ir


Examp le TRXLPE DataAC Bre akd o wn Stre ng th Ov e r Time

(B a se d o n N om ina l W a l l Th ic kne ss)

0

200

400

600

800

1000

1200

1400

1600

0 2 0 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380

Aging Days

AC

Bre

ak

do

wn

Str

en

gth

(V/m

il)


Exam ple XLPE DataAC Bre akd o wn Stre ng th Ov e r Time

(B a se d o n N om ina l W a ll Th ic kne ss)

0

200

400

600

800

1000

1200

1400

1600

0 2 0 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380

Aging Days

AC

Bre

ak

do

wn

Str

en

gth

(V/m

il)


D is s ip a tio n F ac to r M e a s u r em e n ts

0.0000

0.0200

0.0400

0.0600

0.0800

0.1000

Des ign 1 Des ign 2 Des ign 3 De s ign 4 Des ig n 5 Des ign 6

Dis

sip

ati

on

Fa

cto

r (%

)

Sam ple 1- Unaged Sam ple 7 - Load Cyc led Sam ple 13 - Aged 120 Days

Example Dissipation Factor Data


Example Impulse Breakdown Data

Im p ulse B reakdo w n S treng th - U nag ed

(Based on Nominal Wall T hickne ss)

0

500

1000

1500

2000

2500

3000

3500

Imp

uls

e B

rea

kd

ow

n S

tre

ng

th (

V/m

il)

S4

S5

S6

Des ign 1 Des ig n 2 Des ign 3 Des ign 4 Des ign 5 De s ign 6


Performance Requirements• Impulse Breakdown Strength

– New: XLPE: 1,200 V/mil, EPR: 800 V/mil– After thermal aging: XLPE: 1,200 V/mil, EPR: 800 V/mil–

• AC Breakdown Strength– New: XLPE: 620 V/mil, EPR: 500 V/mil– After thermal aging: XLPE: 620 V/mil, EPR: 500 V/mil– 120 days: XLPE: 260 V/mil, EPR: 300 V/mil– 180 days: XLPE & EPR: no requirement!– 360 days: XLPE & EPR: no requirement!


Interpreting AWTT Data

• Assure that all appropriate cable construction parameters are the same whencomparing results from the AWTT.

• For AWTT, results from different labs should be comparable.

• Be aware that the accelerated tests conditions are truly accelerated.

• Current performance requirements for qualification using the AWTT need tobe increased.

• Examine multiple data sets – one data set does not establish a firm benchmark.


Pros and Cons - AWTTTable 2

Pros and Cons for AWTT Pros Cons

9 Test protocol well established, allowing for good confidence when comparing different data sets

9 Provides for basic, tangible performance

requirements for ac and impulse dielectric strength in a relatively short period of time

9 The test is conducted by a wide variety of

laboratories so a broad data base of test results is available

9 Does a relatively good job of identifying poor

insulation or shield compounds or poor manufacturing processes in a relatively short period of time

9 The number of ac breakdown samples for each time period is relatively small

9 The total aging time is one year which may be somewhat short when projecting life in service

9 The primary data provided is ac breakdown strength over time, which is only indirectly related to cable life

the accelerated water treeing test (awtt) · 2016. 12. 9. · treeing). • the cec already...

Documents