SESSION 12: MV UNDERGROUND CABLE SYSTEM … 12 - TNB...Hosted by: Organized by: 1 SESSION 12: MV UNDERGROUND CABLE SYSTEM – TNB’S EXPERIENCE by Ir. C. H. TAN • Development of

Hosted by: Organized by:

1

SESSION 12: MV UNDERGROUND CABLE

SYSTEM – TNB’S EXPERIENCE by Ir. C. H. TAN

• Development of TNB MV Underground Cable

Specification

• TNB MV Underground Cable Installation Practice

• Development of TNB Underground Cable Maintenance

Practices

• Future Development in TNB MV Underground Cable

System


2

Objective

• To enhance understanding on technical requirements

for TNB’s cables.

• To share TNB standard process & foster better

understanding.

• To inform public on TNB’s effort in developing high

reliability underground cable system


3

Development of TNB MV Underground Cable

Specification

• Types of cables used in TNB

– Low Voltage Cables

• PILC (4 cores)

• PVC (1 core)

• XLPE (4 cores)

– Medium Voltage (11, 22&33kV)

• PILC (3 cores)

• Oil Filled (3 cores)

• XLPE (1 core & 3 cores)


4

Development of TNB MV Underground Cable

Specification

• TNB uses two type of MVUG Cables

– PILCDSTA (PILC)

• Before mid 1980’s

• Ceased addition since 2007

– XLPE

• Starts mid 1980’s

• Now 100% new cables


5

Typical Construction of PILC Cable


6

Typical Construction of Single Core XLPE Cable


7

Typical Construction of Three Cores XLPE Cable


8

Specification Development Process

• Specification Development / Revision

– Major changes in reference standards

– New cable design is needed

• To keep up with technology advancement

• To introduce new requirements to enhance performance based on

feedback

– Diagnostic Test findings

– Failure investigation & analysis


9

Characteristic of TNB Technical Specification

• Functional requirement

• Refers International / National Standards

• Specific requirements to suit local

environmental & operational condition.


10

Functional Requirements

• System voltage

– Nominal voltage

– Maximum voltage

• System frequency

• Impulse withstand voltage level (BIL)

• Maximum continuous conductor operating

temperature

• 3 Phase Symmetrical Fault Level


11

Reference Standards

1. IEC – International Electrotechnical Commission

2. BS – British Standard (UK)

3. IEEE – Institute of Electrical and Electronics Engineers

(USA)

4. ICEA – Insulated Cable Engineering Association (USA)

5. NEMA – National Electrical Manufacturers Association

(USA)


12

Common Reference Standards

IEC 60502 -1 : Power cables with extruded insulation

and their accessories for rated voltage from 1kV

(Um = 1.2kV) up to 30kv (Um=36kV)

Part 1:

Cables for rated voltages of 1kV (Um = 1.2kV) and

3kV (Um = 3.6kV).


13


IEC 60502 -2 : Power cables with extruded insulation

and their accessories for rated voltage from 1kV

(Um = 1.2kV) up to 30kv (Um=36kV)

Part 2:

Cables for rated voltages of 6kV (Um = 7.2kV) and

30kV (Um = 36kV).


14


IEC 60228 : Conductors of insulated cables

BS 2627 : Wrought aluminum for electrical purposes.


15

Specific TNB’s Requirements

• Conductor screen

– Minimum thickness

– Minimum volume resistivity

• Insulation screen

– Minimum thickness

– Minimum volume resistivity

– Bonded or Striptable

• XLPE Insulation Free from voids & impurities


16


• Triple extrusion (Method of application)

– Conductor screen

– Insulation

– Insulation screen

Metallic screen

– Type (tape / wire)

– Thickness

– Method of application


17


• XLPE curing method (Dry)

• Type & grade of sheathing material

– MDPE ST7

– PVC ST2 (ABC)

– UV resistance

– Termite repellant

– Graphite outer coating (single core)


18


• Filler material

– Non hydroscopic

• Moisture barrier

– Longitudinal

• Water swell-able tape

– Radial

• Lead sheath (PILC & Submarine)

• Laminated aluminum foil


19

Quality Control Testing

• Type Test

– Design verification test

• Based on IEC Standard

• Tested by Independent Approved Laboratory

• Routine Test

– Manufacturing quality control test

• Based on IEC Standard

• 100% finished product


20

Quality Control Testing

• Sample Test

– Based on IEC Standard

– Every 10kM produced

– Commencement of contract & before Factory Acceptance

Test (FAT)

• Factory Acceptance Test

– Based on IEC Standard (Routine Test)

– 10% sample


21

Request for New Tech Spec

Prepare Draft

Get Feedback

Feedback Prepare Final Draft

Technical Working

Committee

Comments

Distribution Technical

Committee Approve

Distribute to

End Users

Yes

Yes

No

No

END

Yes

No


22

TNB UG Cable Installation Practice

• Direct buried

– Open trench

• Laid in Duct

– Busy & congested areas (HDPE / PVC pipes)

– Road / railway / stream / river crossing (HDD)

• Laid in Duct bank

– No dig policy

– Putrajaya


23

TNB UG Cable Installation Practice

• Laid in dedicated precast concrete trench

– No Dig Policy

• Medini – Iskandar Malaysia

• Utility tunnel

– Laid on cable tray

– No Dig Policy

• Putrajaya


24

Direct Buried (Before 2008)


25

Direct Buried (After 2008)


26

Direct Buried (After 2008)


27

Direct Buried


28

Cross-section of direct buried LV, 11, 22kV Cables


29

uPVC Cable Protection Slab


30

Mechanical Protection for Cables

11& 22kV Cables 33kV Cables


31

Mechanical Protection for Cables


32

River Crossing – Cable Bridge


33

Class B PVC Ducts


34

Exposed Duct : PE Vs PVC

Source : BICC Electric Cables Handbook, 3rd edition, Pg 95


35

HDD – Road, Railway, Small river crossing


36

HDPE Grade PN10 Ducts


37

Cable Laying (MEDINI – Iskandar Malaysia)

No Dig Policy


38

Pre-Cast Concrete Trench Turf


39

Cable Laying (Putrajaya)

No Dig Policy


40

• Single core cable installation

– LV cable

• Limitation on length allowed

• Safety / Fault current return path

– MV cable

• Single point bonding

• Both end bonding

• Cross bonding


41

• Single core cable installation

– MV cable

• Single point bonding

– Sheath induce voltage - <50V

– Fault current return path

• Both end bonding

– Sheath circulation current

– De-rating of cable current carrying capacity

• Cross bonding

– Link box maintenance

– Sheath voltage management

– Sheath current monitoring


42

Sheath voltage α Load Current & Length of Cable


43

Single core cable sheath Bonding

Cross Bonding System


44

Effect of Bonding Types Cable Laid in Trefoil Formation

Trefoil Direct buried in backfill (native soil = 1.5, backfill=1.2), cable depth = 3.5 ft, trench width = 1.64 ft

No Cable Bonding typeAmpacity

(A)

Induced

sheath

current

(A)

Induced

sheath

voltage (V/km)

Both end 485 28.4 -

Single end 493 - 28.6

Both end 488 278.6 -

Single end 570 - 32.6

Perfect cross bond 570 - 0

11 kV 1C 500

33 kV 1C 630

1

2


45

Laying of Single Core Cables

Flat Formation Trefoil Formation

•Higher Ampacity (Flat non

touching)

•Voltage unbalance must kept

<2%

•Need to transpose core at each

cross bonding joint

•Lower Ampacity

•No voltage unbalance

•No need for core transpose at

cross bonding joint

•Simple and easy to manage

•No confusion during fault

location


46

Do not lay single core cable in GI pipe !


47

Flat formation with Core Transposition & Cross

Bonding


48

Trefoil formation with cross bonding


49

Cross Bonding Joint & Link Box


50

Link Box

33kV Link Box


51

Link Box

11&22kV Link Box


52

Single Core Cable Maintenance Test

Sheath Voltage & Current Measurment


53

TNB’s MV System Breakdown Statistic

1613, 12%

3880, 29% 7935, 59%

Overhead Substation Underground

MV INTERRUPTIONS BY COMPONENTS FOR FY 0910

Underground System contributes 59% of MV Interruptions


54

MVUG Cable Failure FY2010/2011

3299, 42%

2868, 36%

787, 10%

980, 12%

Cable Joint Termination Third Party


55


56

TNB MVUG Cable Maintenance Practices

• Maintenance required for MVUG cable?

– 3 cores cable

– 1 core cable

• What maintenance do you do for MVUG cable

system?


57


• Insulation integrity

• Sheath Integrity

• Connection Integrity

• Tier 1 (common test)

• Tier 2 (Special test)


58


• Insulation Integrity

– IR, PI , DAR – Tier 1

– Tan Delta – Tier 2

– PD Mapping –Tier 2

– Dielectric Spectroscopy – Tier 2

– HVDC* - Tier 1 (PILC)


59


• Sheath Integrity

– Sheath test – Tier 1

• Commissioning

• Maintenance (In service)

• After repair

• Connection Integrity

– Contact resistance – Tier 1

• New & after repair (connector)

• Thermography (Bad contact)


60

Condition Monitoring

Technologies XLPE PILC LV

Purpose Technologies 1Commissio

ning

2In-

Service

3After

Repair

Commission

ing

In-

Service

After

Repair

Commission

ing

In-

Service

After

Repair

Insulation

Integrity

Insulation

Resistance

PI test

DAR test

Tan-delta

Partial

Discharge

Mapping

Dielectric

Spectroscopy

HVDC 4


61

Condition Monitoring

Technologies XLPE PILC LV

Purpose Technologies 1Commi

ssioning

2In-

Service

3After

Repair

Commi

ssioning

In-

Service

After

Repair

Commis

sioning

In-

Service

After

Repair

Sheath

Integrity5

HVDC Sheath

Test 6

Integrity of

Connections

Contact

Resistance Test

Metallic Sheath

Continuity

Thermography


62

Insulation Resistance Test (IR)

This test is done on cables at regular interval of 24

months for MV cables and 60 months for LV cables

under normal condition. This test results are considered

for condition assessment while the cable is in service.


63


• Insulation Resistance Test

is the specific resistivity of the cable insulation

(Ωm)

is the length of cable (m)

is the external radii of insulation

is the internal radii of insulation

•Insulation resistance is temperature sensitive

•Long test time is needed to allow for charging current to ceased

•Reading is length dependent

•Difficult to make decision


64



65


PI = Polarization Index = R10 / R1

Insulation resistance measurement values are recorded at 1

minute and 10 minutes of voltage application.

The ratio at 10 minutes to 1 minute will determine the

polarization index value.


66

Results Classification Action

PI value ≥ 3.0

Excellent

Normal. The monitoring periodicity of 24

months for MV cables and 60 months for

LV cables can be maintained.

1.5< PI value < 3.0

Good

The monitoring periodicity should be

revised to 6 months for MV cables and 24

months for LV cables.

1.0< PI value < 1.5

Moderate

The monitoring periodicity should be

revised to 3 months for MV cables and 12

months for LV cables.

PI value < 1.0

Bad / Dangerous

Remove the Cable from service and

perform Tier 2 tests (if applicable)

immediately for MV cable. Replace LV

cables.


67


DAR = Dielectric Absorption Ratio = R60 / R30

Insulation resistance measurement values are recorded at 30

second and 60 second of voltage application.

The ratio at 60 Second to 30 second will determine the

Dielectric Absorption Ratio value.


68

Results Classification Action

DAR value ≥ 1.6

Excellent

Normal. The monitoring periodicity

of 24 months for MV cables and 60

months for LV cables can be

maintained.

1.< DAR value <

1.5

Good

The monitoring periodicity should

be revised to 6 months for MV

cables and 24 months for LV cables.

1.0 < DAR value <

1.1

Moderate

The monitoring periodicity should

be revised to 3 months for MV

cables and 12 months for LV cables.

DAR value < 1.0

Bad / Dangerous

Remove the Cable from service and

perform Tier 2 tests (if applicable)

immediately for MV cable. Replace

LV cables.


69

Tangent Delta (Tan ∂) Test

This test is done on cables at regular interval of 24 months

under normal condition. This test results are considered for

condition assessment while the cable is in service.


70

Results Action

XLPE:

tan δ (2 U0) < 1.2 E-3 and

[tan δ (2 U0) - tan δ (U0)] < 0.6 E-3

PILC:

tan δ (50Hz) < 2.3 E-3

Normal. The monitoring periodicity of all Tier 1 tests

can be maintained at 24 months. Practice Tan delta

test if necessary.

XLPE:

1.2 E-3 ≥ tan δ (2 U0) < 2.2 E-3 and

0.6 E-3 ≥ [tan δ (2 U0) - tan δ (U0)] < 1.0 E-3

PILC:

2.3 E-3 < tan δ (50Hz) < 3.0 E-3

Retest the cable for Tan delta test after 6 months.

The monitoring periodicity of all Tier 1 tests should

be revised to 6 months.

XLPE:

2.2 E-3 ≥ tan δ (2 U0) < 2.8 E-3 and

1.0 E-3 ≥ [tan δ (2 U0) - tan δ (U0)] < 1.5 E-3

PILC:

3.0 E-3 < tan δ (50Hz) < 3.5 E-3

Retest the cable for Tan delta test after 3 months.

Arrange for replacement of defective section(s).

XLPE:

tan δ (2 U0) ≥ 2.8 E-3 or

[tan δ (2 U0) - tan δ (U0)] ≥ 1.5 E-3

PILC:

tan δ (50Hz) ≥ 3.5 E-3

Indicates serious problem requiring immediate

evaluation, additional testing and consultation with

experts. Recommendation is to remove from service

immediately and replace the cable..


71

Tan ∂ Measurement


72

PD Mapping

High Voltage Part Operator Desk


73


74

Joint


75

PD Mapping

Results Action

Severity Index < 2 Normal. The monitoring periodicity of all Tier 1 tests can be maintained

at 24 months. Practice partial discharge test if necessary.

2 < Severity Index < 5 Retest the cable for partial discharge after 6 months. The monitoring

periodicity of all Tier 1 tests should be revised to 6 months.

5 < Severity Index < 7 Retest the cable for partial discharge after 3 months. Arrange for

replacement of defective section(s).

Severity Index > 7 Indicates serious problem requiring immediate evaluation, additional

testing and consultation with experts. Recommendation is to remove from

service immediately and replace the cable.


76

Sheath Integrity Test


77

VLF Withstand Test

• Replacement test system for HV DC for XLPE cables

• Destructive test

• To proactively flush out weak spot in cable system

• Also used as High Voltage Source for PD Mapping


78

VLF Withstand Test

Before 2007

• Commissioning Test &

• Routine Maintenance Test

– Test Voltage 3Uo

– Test Duration 60 min

– Test Frequency 0.1Hz, 0.05Hz, 0.02Hz


79

VLF Withstand Test

Since 2007/8

• Commissioning Test

– New cables

• Test voltage 2.0Uo

• Test duration 30min

• Test Frequency 0.1Hz

– After fault repaired

• Test voltage 1.6Uo

• Test duration 15min

• Test Frequency 0.1Hz

Alternative to VLF Withstand

Test – 24 hour system voltage

SOAK Test


80


81


82

TNB’s Experience

Duration

(Minutes)

Nos. of failure %

0-12 1472 67.62

13-30 496 21.54

31-45 129 5.93

46-60 107 4.92


83

IEEE Std 400.2–2004

IEEE Guide for Field Testing of Shielded Power Cable Systems Using

Very Low Frequency (VLF)

• The results of field tests on over 15000 XLPE cable circuits

tested showed that ~68% of the recorded failures occurred

within 12 minutes, ~89% within 30 minutes, ~95% after 45

minutes, and 100% after 60 minutes (Moh [B17]).

• The recommended testing time varies between 15 and 60

minutes, although the data in Moh [B17] suggest a testing

time of 30 minutes.


84

Future Development

• Cable Design & Construction

– Tree Retardant (TR-XLPE)

– Stringent requirement on insulation compound cleanliness

– Strict requirement on eccentricity of cable core

– Fully bonded insulation screen

– Water tight cable construction


85

Future Development

• Cable Operation & Maintenance

– Sheath Integrity test for 3 core cables

– Advance CBM diagnostic test

• On-line PD Detection & Location

– Monitoring of Water Tree in MVUG Cables

• Dielectric Spectroscopy test

• Polarization & Depolarization Current Measurement


86

Dielectric Spectroscopy


87

Dielectric Spectroscopy (DS) Test


88

Conclusion

• TNB continuously seek to improve its MVUG cable system performance via

– Improve quality of cable and its accessories

– Adopt new Accessories technology

– Adopt new methodology for

• Installation method

• Condition based maintenance practice

• New diagnostic tools


89

Thank You

Documents

SESSION 12: MV UNDERGROUND CABLE SYSTEM … 12 - TNB...Hosted by: Organized by: 1 SESSION 12: MV UNDERGROUND CABLE SYSTEM – TNB’S EXPERIENCE by Ir. C. H. TAN • Development of