Water Tree B54

09-11 Dec, 2012 Saudi Arabia-Jeddah

Probing Non-Destructive Techniques for Assessing Degradation Status of Water

Treed MV-XLPE Cables

Presented By

Dr. M.I. Qureshi

Saudi Aramco Chair in Electrical Power

King Saud University


Outline

1. Overview of Cable Degradation.

2. Non-destructive testing techniques.

3. MFA Set-up at KSU.

4. Results and Comparison.

5. Conclusions.


Overview

• XLPE medium voltage cables are extensively used worldwide.

• During service cables are subjected to several types of stresses.

– Electro-thermo-mechanical

– Chemical

– Moisture

• Dielectric properties of cable degrade.


• Most deleterious degradation process in XLPE insulation is water treeing phenomena.

• It initiates and propagates in the presence of electrical stress moisture, temperature, chemical species.

• Presence of chlorides and sulphates in sub-soil level dramatically increase their size and number density.


Water Trees in XLPE Cable

Vented Trees

Bow-Tie Tree


• MV-XLPE cable installed in several utilities operating in GCC countries are in service for almost 30 years.

• Utilities are faced to critically decide either to:

– Maintain

– Repair – rejuvenate OR

– Replace.


• Cable’s degradation does not depend on age alone as cables don't age uniformly with time.

• Replacement program should be based on their CONDITION assessment.

• Condition Based Maintenance (CBM) is successful only if utility has reliable diagnostic tools.


• Several non-destructive techniques are available commercially.

• Off-line.

• All work on the principle of assessment of dielectric response of insulation.

• Dielectric response of insulation under an applied electric field varies with the changing structure of insulation.

CBM


• In the past, HVDC methods Hi-pot / DC leakage current have

been applied by utilities in several countries, but they not only

provide insufficient diagnostic precision but on contrary can

lead to development of water trees in the cable insulation.

HVDC Methods


• Dissipation Factor (TD) measurements at

– 0.1 Hz using VLF set.

– 0.01 Hz to 10 Hz (DS).

• Residual charge measurement method. (RCM)

• Return voltage measurement method. (RVM)

• Isothermal Relaxation Current (IRC) method.

Successful Non-destructive Techniques


• Manufacturers generally claim superb performance of their equipment for detection of cable’s degradation.

• All techniques have certain measurement accuracy and also associated with disadvantages.

• Therefore, a comparison of performance of these techniques is the aim of this investigation.


• Two types of aged cables were tested using four detection methods.

• 220 m long 15 kV rated XLPE cable was subjected to long term accelerated aging for 7500 hours in the HV laboratory.

• A 50 m long 15 kV rated XLPE cable was removed from service from local industry.

• After non-destructive tests, destructive tests were also undertaken.

– BD

– Water tree parameters


MFA IRC-Technique

• 3 Uo

• 90 C • Cu2SO4


Non-Destructive Techniques 1. RVM-method

Tc Td


0

0.5

1

1.5

2

2.5

0.1 1 10 100 1000

Vo

lt

Tc (seconds)

Cable 1

Cable 2

‘RV-Tc’ spectra of two field aged cable segments.‎


Januz’s Model of Dielectric Spectrum

A model of dielectric response based on division spectra was introduced by Januz et al. in 2002.

Plot of [Tc/Td = 10] / [Tc/Td=2] as a function of ‘Tc’ should give flat response for new cable whereas aged cable should show a distinct peak.

Their simulation showed the peak to occur at ≤ 1s


0

0.5

1

1.5

2

2.5

3

3.5

0.1 1 10 100 1000

B/A

Tc (seconds)

Division spectra of field aged (cable 1) and laboratory aged (cable 2) cables.‎


2 & 3. TD in Time & Frequency Domain

Applied

Voltage (kVrms)

Tan 10-3

0.1 Hz 0.05 Hz 0.01 Hz

2.2 0.3 0.8 1.3

4.4 0.3 0.8 1.3

6.5 0.3 0.8 1.3

8.7 0.3 0.8 1.4

13.1 0.3 0.9 1.6

17.4 0.4 1.0 1.8

21.8 0.5 1.2 2.1

Field Aged Cable

TD = (TD at 2Uo – TD) TD = 0.1 10-3

TD 1 10-3 critical Havidson Technique works only on highly degraded cable.


Applied

Voltage

(kVrms)

Tan 10-3

0.1 Hz 0.05 Hz 0.01 Hz

Unaged 3000

Hr.

7500

Hr.

Unaged 3000

Hr.

7500

Hr.

Unaged 3000

Hr.

7500 Hr.

2.2 0.3 0.3 0.5 0.8 0.8 0.9 1.3 1.3 1.3

4.4 0.3 0.3 0.5 0.8 0.8 0.9 1.3 1.3 1.3

6.5 0.3 0.3 0.5 0.8 0.8 0.9 1.3 1.3 1.3

8.7 0.3 0.3 0.5 0.8 0.8 0.9 1.3 1.3 1.3

13.1 0.3 0.3 0.5 0.8 0.8 0.9 1.3 1.3 1.3

17.4 0.3 0.3 0.9 0.8 0.8 1.7 1.3 1.3 1.4

21.8 0.3 0.3 -- 0.8 0.8 -- 1.4 1.4 --

Laboratory Aged Cable

TD = 0.4 10-3 as IEEE 400.2 Good Insulation


i E (t)

u R (t)

3

A A

D

R M

3: Measurement (t M )

1 2

Conductor

LSI/LSA

XLPE- Insulation

Sheath

U F

R C

1: Forming (t F ) 2: Discharge (t D )

R D

Length

V / A

Shield

4. IRC Measuring Principle

30 s 5 s 1800 s


IRC – plots for new and field aged cables

Old

Critical

Aged New


Laboratory Aged Cable

12 Uo – 13 Uo

Mid Life


Field Aged Cable

7 Uo – 8 Uo

Critical


Water trees in laboratory aged cable.


Cable Sample TD at 0.1 Hz IEEE Std.

Status

Measured

U/Uo

IRC Prognosis

(kUo)

IRC-

Degradation

Status

Laboratory

Aged

0.4 10-3 Good 13.0 12 – 13 Mid Life

Field Aged 0.1 10-3 Good 8.5 7 – 8 Critical

Comparison of TD & IRC With Destructive Tests


Conclusions

1. RVM not suitable for XLPE insulation.

2. DS carried out in frequency sweep 0.01 – 0.1 Hz fails to give correct degradation status of mild-critical water tree degraded cable.

3. TD at 0.1 Hz provides reasonable good information on degradation but the IEEE set criteria that classifies the degradation status present misleading / conservative result.


4. Comparison of preceding results shows that software controlled IRC analytical tool is not only a potential technique for classifying the degradation level of aged XLPE Cable into ‘Mid-Life’, ‘Old’, and ‘Critical’ but also provides remaining life of the cable (RSP) and includes the effect of connected splices on the cable.

5. It does not stress the cable. It is safe, compact, quick and economical.

6. The utility personal can operate and get results with little training.


Thank you

Documents

Water Tree B54