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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
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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
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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)
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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
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Typical Construction of PILC Cable
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Typical Construction of Single Core XLPE Cable
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Typical Construction of Three Cores XLPE Cable
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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
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Characteristic of TNB Technical Specification
• Functional requirement
• Refers International / National Standards
• Specific requirements to suit local
environmental & operational condition.
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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
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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)
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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).
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Common Reference Standards
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).
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Common Reference Standards
IEC 60228 : Conductors of insulated cables
BS 2627 : Wrought aluminum for electrical purposes.
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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
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Specific TNB’s Requirements
• Triple extrusion (Method of application)
– Conductor screen
– Insulation
– Insulation screen
Metallic screen
– Type (tape / wire)
– Thickness
– Method of application
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Specific TNB’s Requirements
• XLPE curing method (Dry)
• Type & grade of sheathing material
– MDPE ST7
– PVC ST2 (ABC)
– UV resistance
– Termite repellant
– Graphite outer coating (single core)
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Specific TNB’s Requirements
• Filler material
– Non hydroscopic
• Moisture barrier
– Longitudinal
• Water swell-able tape
– Radial
• Lead sheath (PILC & Submarine)
• Laminated aluminum foil
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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
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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
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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
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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
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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
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Direct Buried (Before 2008)
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Direct Buried (After 2008)
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Direct Buried (After 2008)
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Direct Buried
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Cross-section of direct buried LV, 11, 22kV Cables
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uPVC Cable Protection Slab
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Mechanical Protection for Cables
11& 22kV Cables 33kV Cables
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Mechanical Protection for Cables
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River Crossing – Cable Bridge
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Class B PVC Ducts
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Exposed Duct : PE Vs PVC
Source : BICC Electric Cables Handbook, 3rd edition, Pg 95
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HDD – Road, Railway, Small river crossing
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HDPE Grade PN10 Ducts
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Cable Laying (MEDINI – Iskandar Malaysia)
No Dig Policy
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Pre-Cast Concrete Trench Turf
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Cable Laying (Putrajaya)
No Dig Policy
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• 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
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• 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
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Sheath voltage α Load Current & Length of Cable
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Single core cable sheath Bonding
Cross Bonding System
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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
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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
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Do not lay single core cable in GI pipe !
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Flat formation with Core Transposition & Cross
Bonding
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Trefoil formation with cross bonding
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Cross Bonding Joint & Link Box
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Link Box
33kV Link Box
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Link Box
11&22kV Link Box
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Single Core Cable Maintenance Test
Sheath Voltage & Current Measurment
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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
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MVUG Cable Failure FY2010/2011
3299, 42%
2868, 36%
787, 10%
980, 12%
Cable Joint Termination Third Party
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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?
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TNB MVUG Cable Maintenance Practices
• Insulation integrity
• Sheath Integrity
• Connection Integrity
• Tier 1 (common test)
• Tier 2 (Special test)
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TNB MVUG Cable Maintenance Practices
• Insulation Integrity
– IR, PI , DAR – Tier 1
– Tan Delta – Tier 2
– PD Mapping –Tier 2
– Dielectric Spectroscopy – Tier 2
– HVDC* - Tier 1 (PILC)
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TNB MVUG Cable Maintenance Practices
• 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)
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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
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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
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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.
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Insulation Resistance Test (IR)
• 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
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Insulation Resistance Test (IR)
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Insulation Resistance Test (IR)
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.
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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.
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Insulation Resistance Test (IR)
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.
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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.
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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.
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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..
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Tan ∂ Measurement
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PD Mapping
High Voltage Part Operator Desk
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Joint
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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.
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Sheath Integrity Test
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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
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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
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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
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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
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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.
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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
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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
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Dielectric Spectroscopy
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Dielectric Spectroscopy (DS) Test
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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
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Thank You