the accelerated water treeing test (awtt) · 2016. 12. 9. · treeing). • the cec already...
TRANSCRIPT
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
Spring 2002 ICC Education Program – AEIC AWTT
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.
Spring 2002 ICC Education Program – AEIC AWTT
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
Spring 2002 ICC Education Program – AEIC AWTT
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.
Spring 2002 ICC Education Program – AEIC AWTT
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.
Spring 2002 ICC Education Program – AEIC AWTT
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.
Spring 2002 ICC Education Program – AEIC AWTT
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.
Spring 2002 ICC Education Program – AEIC AWTT
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).
Spring 2002 ICC Education Program – AEIC AWTT
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.)
Spring 2002 ICC Education Program – AEIC AWTT
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
Spring 2002 ICC Education Program – AEIC AWTT
Electrical MeasurementsSample 7
Electrical MeasurementsSample 1
Test No. 1-High Voltage Time
Test, Samples 1, 2, 3
Physical MeasurementsSample 1
Test No. 3-High Voltage TimeTest, Samples 7, 8, 9
Physical MeasurementsSample 7
Electrical MeasurementsSample 13
Test No. 5-High Voltage TimeTest, Samples 13,14,15
Physical MeasurementsSample 13
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
Spring 2002 ICC Education Program – AEIC AWTT
AWTT - Procedure
Spring 2002 ICC Education Program – AEIC AWTT
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
Spring 2002 ICC Education Program – AEIC AWTT
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
Spring 2002 ICC Education Program – AEIC AWTT
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)
Spring 2002 ICC Education Program – AEIC AWTT
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)
Spring 2002 ICC Education Program – AEIC AWTT
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
Spring 2002 ICC Education Program – AEIC AWTT
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
Spring 2002 ICC Education Program – AEIC AWTT
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!
Spring 2002 ICC Education Program – AEIC AWTT
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.
Spring 2002 ICC Education Program – AEIC AWTT
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