Date: 10-12 September 2012 Venue: Shah Alam Convention · PDF file•Cable Constructions...
Transcript of Date: 10-12 September 2012 Venue: Shah Alam Convention · PDF file•Cable Constructions...
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Electric Cables, by design
Presented By
Lokman A. Dahlan
Technical Advisor
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Contents
• Brief History on Cables
• Basic Design Principles
• Cable Constructions
• Common Design Options
• Cable Standards
• Tests on Cables
• Quality Assurance
• Special Topics
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Brief History on Cables
• 1880 Gutta Percha used as insulation
followed by natural rubber & bitumen
• 1890 Paper insulation introduced (Ferranti)
• 1914 Screening developed to increase voltage levels upto 33kv (Hochstadter)
• 1926 Pressurised oil filled type for voltages upto
66 kv (Emanueli)
• 1930s PVC insulation introduced (Germany)
• 1933 BS 480 published for PILC type
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Brief History on Cables (Cont.)
• 1943 First 132 kv oil pressured cable installed
• 1949 Introduction of mass-impregnated non-draining
compound for PILC cables
• 1950s PVC increased use for LV cables, thermosetting
(XLPE) insulation was under trials. Aluminium
conductors increasingly accepted as an economic
alternative to copper.
• 1954 First 275kv oil filled cable installed
• 1961 100kv DC link installed between France/England
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Brief History on Cables (Cont.)
• 1969 First 400kv oil filled cable installed • 1970 Metrication of British Standards • 1970s IEC 502 for extruded solid dielectric insulated cables upto 33kv was published. Thermosets, mainly XLPE were increasingly used upto 33kv. Trial installations were conducted for XLPE at higher transmission voltages. • 1980s Rapid development in process extrusion and cross-linking of polymerics
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Brief History on Cables (Cont.)
• 1990s XLPE widely used at LV/MV and increasingly
accepted at higher voltages.
Material type, cleanliness, handling and processing methods are becoming concerns of manufacturers & customers.
Qualification and test requirements are constantly reviewed to meet higher expectations for quality and reliability of supply.
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What Cable ?
• Aerial, underground, submarine,
• AC, DC, distribution, transmission
• Aluminium, Copper
• PILC, PVC, XLPE, Bare
• LV, MV, HV, EHV, 11kV etc.
• Screened, Belted, Armoured
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Design Requirements
• Satisfies power needs
• Flexible
• Reliable
• Has long life
• Minimal maintenance
• Economic
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Basic Design Elements
• CONDUCTOR
- determines base current ratings
• INSULATION
- determines voltage / stress levels
• PROTECTION
- determines installation conditions
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Conductor Metals
Metals VR @ 20oC
(.mm2/km)
Density
(gm/cm3)
Mass
(kg/km)
Price
(US$/kg)
Total
(US$/km)
Silver 16.4 10.5 172.2 169.6 29,205
Copper 17.2 8.89 152.9 1.667 254.9
Gold 24.4 19.3 470.9 8336 3,925,590
Aluminium 28.3 2.70 76.4 1.44 110
Tin 124.0 7.29 904.0 5.245 4741.5
Lead 214.0 11.4 2440 0.309 754
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Aluminium & Copper Conductors
• Copper
– Highly Conductive
– Good Mechanical Properties
– Relatively Easy to Process
– Usually Annealed
• Aluminium
– 60% conductance of copper at same size
– Half the weight of copper at the same conductance
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Insulation Types
• Common types
– PVC (Polyvinyl Chloride)
– PE (Polyethylene)
– XLPE (Cross linked PE)
– Rubber (EPR,EVA)
– PILC (Paper insulated Lead Covered)
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Insulation Type - PVC
• PVC resins with plasticizers, stabilizers, fillers & additives
• Applied by extrusion process
• Continuous operation at 70oC
• Good dielectric & thermal properties
• Operable up to 3.3kV only
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Insulation Type - XLPE
• Polyethylene resins with minor additives
• Applied by special chemically heat treated “vulcanized” extrusion process
• Continuous operation at 90oC
• Excellent dielectric, thermal and mechanical properties
• Widespread use at medium voltages
• Operable at EHV
• Sensitive to voids and moisture
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Protection
• Cables need to be mechanically protected against external damage & installed environments
• PRIMARY – by the insulation
• SECONDARY
• double insulation and/or sheaths • armour or reinforcement when exposed to potential
damage (direct in ground) • barriers to prevent ingression of moisture, oils,
solvents etc.
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Protection
• Cables need to be electrically protected against damage to adjacent cables, connecting equipment and for safety of users against electrical hazards
• PRIMARY – by the insulation & sheaths
• SECONDARY
- by conductive layer(s) for the safe transfer of leakage currents, to be appropriately sized to meet system ratings and/or suitable grounding or bonding methods against leakages, transients and lightning
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Protection from Pest Attack
Method Features
Mixing termite repellent in surrounding earth
Environmental problem
Coating oversheath with termite repellent
Difficult to apply Health problem
Aldrin & Dieldrin Effective but use is however banned in many countries
Mixing termite repellent into cable sheaths Copper naphthanate,
Cypermethrin Alternative to Aldrin & Dieldrin, reasonably effective
Polyethylene Sheath Slows down attack
Common metal sheaths Slows down attack
Tape armour Good protection from large insects and rodents
Nylon Sheath Resistant to termites, but difficult to manufacture
SINGLE CORE XLPE CABLE (Foil Laminate)
Conductor Conductor
Screen
XLPE
Insulation Insulation
Screen Semi-conducting
Water swellable
tape
Copper Wire
Screen
Copper Equalising
Tape
Semi-con Water
Swellable Tape
Laminated Metal
Tape
Oversheath
THREE CORE XLPE CABLE (Lead Sheathed)
Conductor
Conductor
Screen
XLPE
Insulation
Insulation
Screen
Copper Tape
Screen
Binder Tape
Fillers
Separation
Sheath
Lead Sheath
Bedding
sheath
Armour Wires
Oversheath
Preferred for waterlogged
environments
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Voltage rating LV MV HV
Application Home & indoor
Others (industrial, distribution & transmission)
Conductor
Copper Copper or Aluminium
Single or Multicores Single core
Circular Circular or
Sector Circular
Flexible or fixed
Fixed Class II
Conductor screen N/A Required
Insulation PVC, Rubber, XLPE XLPE
Colourable Natural
Insulation screen N/A Required
Metallic screen N/A Required
Moisture barrier N/A Optional Required
Fillers For Multicores N/A
Armour N/A Optional
Jacketing Optional Required
Cable Constructions
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Cable Construction
• Conductors
– Copper conductors are commonly used for home & light industrial applications
– Aluminium conductors are optionally used by utilities for distribution & transmission
– BS 6360/IEC 60228 of varying Class depending on usage
– Single wire “solid” type for small conductors or Stranded Class 5 for flexible installations
– Stranded Class II for fixed installations
– Sector shaped for LV 50-500sqmm multi-cores
– Circular stranded can be compacted
– Circular segmental for 1000sqmm and above
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Cable Construction
• Conductor Screen (MV/HV)
– semi-conductive
– thermosetting
– extruded type only
– thickness not normally specified
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Cable Construction
• PVC & LV XLPE Insulation – thickness as stipulated in standards – Consistent in thickness and finish – Colourable
• MV/HV XLPE Insulation – thickness stipulated in standards, specified by
customer or proven design – Natural and clean – triple extruded with adjacent screens (one step
preferred) – handling and processing more critical at higher
voltages
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Cable Construction
• Insulation Screen (MV/HV)
– semi-conductive
– thermosetting
– extruded types (thermosetting) preferred
– thickness not normally specified
– strippable for MV cables only
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Cable Construction - XLPE
• Metallic Screen (MV/HV)
– applied immediately over or in electrical
contact with insulation screen
– non-corrosive metals preferred, in the form of (copper) tapes, wires or (lead) sheath
– can be sized accordingly to meet specified
earth fault rating
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Cable Construction - XLPE
• Fillers (For Multicores)
– applied when combining individual cores together to fill interstices between cores
– suitable materials in the form of strings, twines, pre-formed or extruded fillers
– completely removable
– overall binder tape applied to maintain circular construction (except extruded type)
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Cable Construction - XLPE
• Bedding (For Armoured Cable)
– also known as inner or separation sheath
– usually extruded (PVC or Pe)
– tapes may be applied instead of or in addition to extruded sheath
– thickness as specified or calculated from Standards
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Cable Construction - XLPE
• Armour
– for multicores, a layer of steel wire or double layer of steel tape normally applied
– galvanised steel to reduce corrosion
– for single core cables, non-magnetic materials applied (i.e. aluminium)
– wire armour can provide additional path for short circuit currents
– armour dimensions specified by standards
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Cable Construction
• Outer Covering(s)
– also known as over/outer sheath or Jacket
– extruded sheath or compounded fibrous tapes or twines
– preferably black for extruded sheaths
– thickness specified in standards or higher
– may consists of several layers for multiple functions
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Common Design Options
- Swellable yarns, compounds, powders applied
strategically in conductor for water blocking
- Semi-con tapes applied over conductor, in
addition to extruded screen
- Extruded metallic sheath (preferred) or foil
laminate applied as radial moisture barrier
- Swellable tapes applied between extruded layers
as transverse (longitudinal) moisture barrier
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Common Design Options
– Double layer of wire armour for additional protection (e.g. submarine type)
– Application of tinned copper wires between
wire armour to increase fault rating
– Anti - Termite protection using additives, steel
armour tapes, brass tapes, nylon sheath
– Conductive (graphite) layer applied on oversheath for periodic sheath integrity test
– Fire or Flame retardant tapes or sheaths
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Property PVC Polyethylene LSOH
Tensile (N/sqmm) 15 25 10
Elongation 150% 300% 100%
Density 1.3 - 1.5 0.91 - 0.96 1.4 - 1.6
Physical Soft and flexible Hard and rigid Semi-hard and rigid
Abrasion Resistance Poor Excellent Acceptable
Hot indentation Acceptable Excellent Good
Impact Resistance (thick slab) Good Poor Poor
Stress cracking Resistant Variable (dependent on
molecular weight i.e.
density)
Variable (dependent on base
compound and mix)
Moisture Absorbs moisture with
prolonged contact
Negligible absorption Absorbs and retains moisture
within a short time
Vapour permeability Reasonably permeable Resistant Permeable
High temp. performance Increased ageing at higher
temps.
Improved thermal & ageing
performance
Generally stable
Low temp. performance Brittle at sub zero Stable at sub zero Generally stable
Resistance to chemicals Good Excellent Poor
Fire Performance Flame retardant, emits toxic
fumes & smoke
Low OI, burns without
toxic fumes
Flame retardant, low smoke &
no toxic fumes
Processability Readily extrudable Extrudable Extrudable with special tools
Compound Compounded with
additives and fillers
Homogeneous Highly filled base compound
with additives and fillers
InstallationConditions :
Direct in Ground - Dry Excellent Excellent Good
Direct in Ground - Wet Good (short term only) Excellent Not Recommended
Exposure to UV light Resistant Good (require UV resistant
additives)
Variable (dependent on base
compound and mix)
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Cable Standards
International
• IEC 60227, IEC 60502, IEC 60840
National
• MS, BS, AS/NZS, JIS, VDE
Associations
• AEIC, ASTM, UL, EEMUA
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Cable Standards - International
• IEC 60227 Polyvinyl Chloride insulated cables 450/750V
• IEC 60502 Cables with extruded insulation 1-33kV
• IEC 60055 Paper Insulated 1-33kV • IEC 60840 Tests for cables with extruded
insulation >33 - 150kV • IEC 60811 Common test methods for
cable insulation and sheaths • IEC 60287 Calculation of Current Rating
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Cable Standards - National
• BS 6480 Paper Insulated 1-33kV
• BS 5467 Polymeric cables 1-3.3kV
• BS 6622 Polymeric cables 6.6-33kV
• AS 1429-1 Polymeric cables 3.3-33kV
• AS 1429-2 Polymeric cables >33-132kV
• VDE 0272 XLPE cables 1kV
• VDE 0273 XLPE cables 10-30kV
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Cable Standards by Associations
• AEIC CS5 XLPE cables 5-46kV
• AEIC CS7 XLPE cables 69-138kV
• ICEA S66-524 XLPE cables for transmission & distribution of energy
• IEEE 575 Application & Calculation for sheath bonding methods
• ESI 09-16 Test Methods for XLPE cables 66-132kV
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Publications & Reports
• Neher/McGrath Calculation of load capabilities of cable systems
• ERA 69-30 Current ratings to BS 6480
• CIGRE (1986) Transients & Cyclic Loads
• BICC C55/4 Engineering Recommendation for Insulated Sheath Systems
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wire drawing
WD1, WD2,
WD3
Stranding
ST1,ST2,ST3,
ST4
XLPE
insulation
TM2,CV1,CV2
Curing/
degassing
Core screen
SC1,SC2,LP5
Lay up
LU1,LU2,LU3
Bedding
QX1,QX2,QX3
Armouring
AR1,AR3,AR4,
AR5
Sheathing
QX1,QX2,QX3
Testing
XT1,XT2
Delivery
RL1,RL2
Lead sheath
LS1,LS2
drawing of aluminum/copper
wire rod to smaller diameter
twisting of wires together and the
formation of a compact strand
conductor
screen,insulation core
screen
the crosslinking of the product using
heat and moisture. to provide
mechanical strength and resistance to
high temperature
copper tape,
copper wires
to screen individual cores and improve
electrical performance
fillers, tapes2 or more cores twisted together to form
a circular cable
PE, PVCapplied for cables used
in oil and gas industry
steel, aluminum wires,
steel tapes
covering of the cable which provide
some electrical protection
PE, PVC
partial
discharge,
resistance
final covering for further protection
cable
inspection
FLOW CHART FOR MANUFACTURE OF XLPE CABLES
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Cable Tests & Inspection
• ROUTINE TESTS - final inspection on all completed cables according to standards
• SAMPLE TESTS - special tests on selected samples as required by customer
• TYPE TESTS - comprehensive test regime on a cable type to demonstrate the ability of manufacturer to meet the requirements
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Routine Tests – Conductor Resistance
• Confirm (electrical) size of conductor
• Accurate measurement by a suitable resistance bridge, corrected to 20oC
• Corrected measurement per kilometre shall not exceed the maximum ohm/km value in the standard for conductor type and size
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Routine Tests – Voltage Withstand
• Determine ability of cable insulation to withstand a fixed voltage of higher rating.
• Upto 33kv : 3.5 Uo for 5 minutes
• Above 33kv : 2.5 Uo for 30 minutes
• Pass criteria: No breakdown of insulation
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Routine Tests – MV/HV Partial Discharge
• Detection of discharges caused by “defects” in the cable insulation at a fixed voltage level
• Upto 33kV : 1.73 Uo
• Above 33kv : 1.5 Uo
• Pass criteria : 10 pC maximum
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Sample Tests
• Hot set test (XLPE)
• Thickness and Diameters
• Overall construction
• Capacitance
• DC sheath test (graphited sheaths)
• Bending test
• Flame retardant test
• Water penetration test (water-blocked designs)
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Special Sample Tests
• Material Tests
– Tensile and elongation
– Ageing test (insulation and sheaths)
– Galvanising tests (zinc coating, adhesion)
– Oxygen index (PVC and special sheaths)
– Shrinkage tests (PE)
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Type Tests – Electrical (MV/HV)
• Partial discharge
• Bending & partial discharge
• Tan delta measurement
• Heat cycle test
• Impulse (lightning) test
• 4hr Voltage withstand
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Type Tests – Non-Electrical
• Dimensional checks
• Mechanical test
• Ageing test
• Compatibility test
• Special properties (hot set, screen resistivity, strippability, heat shock, loss of mass, shrinkage etc.)
• Flame propagation, water penetration test (as designed)
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Type Tests - definition
• “Tests made before supplying on a general commercial basis, a type of cable covered by this standard, in order to demonstrate satisfactory performance characteristics to meet the intended application. NOTE. These tests are of such a nature that, after they have been made, they need not be repeated, unless changes are made in the cable materials or design or manufacturing process which might change the
performance characteristics.” (IEC 60502-2 : 2005 Clause 3.2.3)
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Quality Assurance
• Verification on design & construction
• In-coming material tests
• In-process control & inspection
• Final tests & inspection
• Documented system
• Internal quality audit
• External Registrar or Customer Audits
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Special Topics
• Short circuit ratings
• Water-treeing in cables
• Moisture barrier
• Conductor ratings (ampacity)
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Short Circuit Ratings
• Symmetrical faults
– On phase conductors
– Determined by conductor size and temperature rating of insulation
• Asymmetrical faults
– On external metallic coverings
– Dependant on effective area of metallic coverings and their respective temperature limits
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Short Circuit Ratings - Asymmetrical
• Copper Tape (not for high currents)
• Copper Wires
• Lead Sheath
• Aluminium Sheath
• Armour Wires
– Unreliable in the event of corrosion
In the event of a fault a low resistance return path
to earth is required.
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Critical Issues for HV XLPE Cables
• “SuperClean” insulation
• High grade bonded screens
• Special extrusion control
• Strict material handling methods
• Moisture barrier is essential
• Well proven construction
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Water Treeing in Cables
• Formation of “tree-like” structures in polyolefins such as PE and XLPE.
• Reduces the breakdown strength of insulation resulting in “electrical trees”
• Degrades with time, electrical stress, frequency and water pressure
• “Bow ties” within insulation due to voids and contaminants
• “Vented trees” from screen interface protrusions and imperfections
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Moisture Barrier - Radial
• Extruded Lead Sheath (preferred)
• Extruded Aluminium Sheath
• Metallic Foil Laminate (most economical but least effective)
A radial moisture barrier is required to prevent
the initial ingress of moisture into the cable
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Moisture Barrier - Longitudinal
• Using tapes and yarns loaded with swellable material or “solid” compounds
• Strategically positioned underneath and between extruded layers, within metal screens, armour or conductor
In the event of damage to the radial moisture barrier
longitudinal water blocking is essential
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Cable Current Ratings
• Temperature
• Cable Design
• Installation Conditions
• Neighbouring Cables
Important factors affecting ratings
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Circuit Analogy – Heat Transfer
Temperature Difference
Conductor
Insulation
Metal Sheath
Armour
Bedding Protective Finish
Soil
Cable Surface
Ground Surface