Implementation of Ageing Laws and Cable Models to Estimate

Service Life for MV Cables using Laboratory Endurance Tests Essay Wen Shu, Nigel Hampton, Josh Perkel

10th International Conference on Insulated Power Cables

Background• Feedback from utility engineers consistently shows that anticipated longevity is the

top priority when deciding on a cable design to employ

• Accelerated Cable Life Test (ACLT) is a lab test to look at core endurance

• ACLT has a metric of time, thus adjustments are needed to bridge gaps between the test and actual service. The gaps include,• Operating Temperature / Voltage • Conductor Size / Cable Length• Environmental Conditions • Jacket• Performance Requirement

10th International Conference on Insulated Power Cables

Approach1. Collate the results of many public domain ACLT tests to model the impact of temperature,

electrical stress and some elements of cable design.

2. Algorithms constructed to scale test data on short cores to long cables in service using Life

Expansion and Reduction Factors.

3. Life Expansion Factors ↑1. Lower voltages in service compared to test voltages

2. Lower temperatures in service compared to temperatures used in the tests3. Use of jackets in service cables compared to jacketless cables used in tests

4. Lower load factors in service

5. Absence of water in the conductor interstices

4. Life Reduction Factors ↓1. Longer lengths installed in service compared to the short lengths employed in lab tests

2. Higher vol of insulation in service - large conductor vs small conductors used in tests

3. Lower critical risk levels (B1 or B5) for cable failures considered by utilities compared to

the mean lives (B50) considered by tests.

10th International Conference on Insulated Power Cables

Test ConditionsVoltage

Class

Conductor

Size

Wall

Thickness

Insulation

Type

15 kV

20 kV

35 kV

#2

1/0

4/0

175 mil

200 mil

220 mil

345 mil

HMWPE

XLPE

TRXLPE

EPR

JacketTest

Length

Test

Voltage

Test

Temperature

Jacketed

Jacketless

15 ft.

30 ft.

35 ft.

2Uo

3Uo

4Uo

30 °C

45 °C

60 °C

75 °C

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Results – selection of 32 from 124 used

10th International Conference on Insulated Power Cables

Impact of Temperatures and Stress

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Impact of Jacket

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Impact of Size

10th International Conference on Insulated Power Cables

Case Study

Case ACLT Service

1

αT,βT

@ ACLT (4,4) conditions from collated library

15kV, 1,000 kcmil100% XLPE wall, Jacketed300 ft. (100 m)Operated at 30 °C

Estimated service life – first failure: B50 = ?

B10 = ?

10th International Conference on Insulated Power Cables

Stress and TemperatureCase ACLT Service

1 αT,βT

@ ACLT (4,4) conditions from collated library

15kV, 1,000 kcmil100% XLPE wall, Jacketed300 ft. (100 m)Operated at 30 °C

Estimated service life – first failure: B50 B10

10th International Conference on Insulated Power Cables

Jacketing (water exclusion)Case ACLT Service

1 αT,βT

@ ACLT (4,4) conditions from collated library

15kV, 1,000 kcmil100% XLPE wall, Jacketed300 ft. (100 m)Operated at 30 °C

Estimated service life – first failure: B50 B10

10th International Conference on Insulated Power Cables

Length and VolumeCase ACLT Service

1 αT,βT

@ ACLT (4,4) conditions from collated library

15kV, 1,000 kcmil100% XLPE wall, Jacketed300 ft. (100 m)Operated at 30 °C

Estimated service life – first failure: B50 B10

10th International Conference on Insulated Power Cables

Life Estimation

ACLT Service

αT,βT

@ ACLT (4,4)

conditions from

collated library

15kV, 1,000 kcmil

100% XLPE wall,

Jacketed

300 ft. (100 m)

Operated at 30 °C

Estimated service life – first failure:

B50

B10

10th International Conference on Insulated Power Cables

Life EstimationACLT Service

αT,βT

@ ACLT (4,4)

conditions from

collated library

15kV, 1,000 kcmil

100% XLPE wall,

Jacketed

300 ft. (100 m)

Operated at 30 °C

Estimated service life – first failure:

B50 21 years

B10 16 years

”10% of 300 ft cable segments (15kV

1000kcmil jacketed XLPE 100% wall)

would be anticipated to experience

their first failure in service (operating

at 30 °C) on or before age 16 years”

10th International Conference on Insulated Power Cables

End of Useful Life• It is common for cable

systems to be repaired upon failure

• Thus End of Useful Life (EoUL) will be larger than the time to first failure

• Conditions for EoUL will depend upon Utility & Local appetite for Risk

• Impact of tolerance for repair upon EoUL can be modeled from the basic data

1st

Failure

2nd

Failure

3rd

Failure

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Conclusions• Collated extensive ACLT data (124 unique datasets) from nine organizations

(1,398 cable segments 32,000 feet (6.1 miles)).

• Model considers expansion/contraction factors including electrical stress,

operating temperature, cable length, cable jacket, and water presence

• The model and its process can be used to,

• Estimate cable life at a desired percentile,

• Establish the required test performance of a cable design with pre-

determined expected service life.

• Derive a useful Life Statement for cable segments

• A benefit of the model is that the impacts of each factor are considered

separately, enabling the user to better visualise the impacts of each choice of

system architecture and operation on the longevity.