LIGHTNING PROTECTION OF WIND TURBINES

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MSc in Electrical Power EngineeringEEPS05
Power System PlantCable Technology
By Dr. Jeff Robertson
24th November 2006
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Part 1 Introduction
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Cables Good Or Bad
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Part 2 Power Cable Components
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Power Cable Components
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CONDUCTORS

• Properties of good conductors
• Current Carrying Capability
• Required Voltage Regulation
• Required Conductor Losses
• Bending Radius, Flexibilty and Weight
• Jointing
• Cost

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DC Resistance

• Conductor resistance is important as it defines
• Heating of cable (i.e. energy losses)
• Voltage drop along cable (along with cable
  reactance)
• DC Resistance per metre of a conductor is
• Variation in conductor resistance with
  temperature must be considered

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Resistance
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Conductor Formations
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Examples Of Conductors

• Solid conductor, shaped to minimise wasted area
• Low flexibility and hence not used above approx.
  16mm2 for copper and 300mm2 for aluminium

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Examples Of Conductors

• Stranded - Good for flexibility, bad for overall
  size of cable
• Compacted stranded can maintain flexibility while
  reducing size penalty, can also reduce electric
  stresses
• Shapes of conductors are particularly determined
  by electric stresses at higher voltages

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AC Resistance Skin Proximity Effects

• Skin Effects
• Increase AC resistance of conductor as current
  density more highly concentrated on outside of
  conductor
• Proximity Effects
• Increases AC resistance of conductor due to
  magnetic flux coupling between adjacent cores

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Insulation Materials

• Properties of good insulation materials
• Sufficient electrical strength for a given
  insulation size
• Thin and flexible
• Free of voids
• Low dielectric losses
• Thermal, Mechanical and Electrical stability
  under cable operating conditions
• Low cost including ease of manufacture
• Ability to perform under steady state conditions,
  temporary overvoltages and transient voltages
• Long operating life

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Insulation Materials
Permittivity Dielectric Losses in Cable
Insulation
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Paper Insulation

• Paper, impregnated with oil/resin, has been used
  in the majority of power cables
• Paper is an old and declining technology but is
  still used at all voltage levels and so is very
  important still
• Paper is particularly used at transmission
  voltages
• The insulation on a paper cable consists of
  helically applied tapes which are laid up with an
  overlap

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Paper Lay-up

• Individual papers are placed around the conductor
  with an approximate 35-65 registration

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Impregnation

• Once the papers have been wrapped, the cable is
  placed into an oven for drying
• Drying process is carried out at temperature and
  vacuum
• Impregnant is introduced, this reduces future
  moisture absorption and fills in gaps / voids in
  insulation

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Polymeric Power Cables

• Polymeric cables were initially introduced at low
  voltages using polyethylene (PE) and polyvinyl
  chloride (PVC), and then at medium voltages by
  EPR and XLPE
• PE subsequently replaced by XLPE in the 1980s -
  its lower loss angle and lower thermal
  resistivity has pushed out EPR
• XLPE is widely used at distribution level and is
  becoming more common at transmission level
• EPR is still deployed in Italy and Spain, but is
  now mainly used for its chemical resistance and
  flexibility (i.e. in mines)

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XLPE Power Cables

• XLPE stands for Cross Linked Polyethylene
• The cross-linking process used to manufacture
  XLPE increases the maximum operating temperature
  to 90C. This compares to 65-70 C for paper
  cables and PE
• XLPE has a lower thermal resistivity than paper
  and EPR
• XLPE is less lossy than paper and EPR
• XLPE is cheaper to produce than paper and EPR
• XLPE cables made using an extrusion process

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Conductor/Insulator Shields

• Also referred to as screens or semicon layers
• Sharp / non-uniform edges around the outside of
  the conductor/outside of the insulation result in
  regions of elevated electric field
• Voids between the conductor/insulation and the
  insulation/sheath must be avoided
• A semiconducting conductor shield is used to
  prevent these from happening

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Effect Of Conductor Screen On Stress
Insulation
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EFFECT OF INSULATION SHIELD

• The insulation shield is earthed but not meant to
  carry current
• Used to provide an equipotential layer to
  insulation
• Paper Cables
• Carbon filled paper, metalised paper, metallic
  foils
• Polymeric Cables
• Carbon filled polymers bonded to insulation
• A capacitor is formed between the conductor and
  the insulation shield
• Arrangement of shields / screens in a 3core cable
  defines Screened and Belted core arrangements

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Cable Sheath / Screen Tapes / Wires

• Function to carry charging, fault and circulating
  currents
• Different arrangements in paper and polymeric
  cables
• Paper
• Full metallic sheath
• Lead (PILS) or Corrugated Aluminium (PICAS)
• Polymeric
• Copper wire / tape
• Other
• Full metallic sheath designed to protect against
  moisture ingress

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Oversheath

• Used to protect sheath and armour from corrosion
• Material used is PVC, PE, MDPE, Bitumen/ Hessian
• Must be resistant to moisture ingress and
  abrasion
• Oversheath damage has led to sheath corrosion and
  failure of fluid filled cables due to the loss of
  hydraulic pressure

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Armour

• To provide extra mechanical tensile strength,
  wire or metal tape armour can be placed around
  the sheath
• Armour can be used to increase fault level
  current rating of the cable
• Separated from sheath by bedding so must be
  bonded to it
• Commonly used materials include steel tape, steel
  wire and aluminium wire
• Steel should not be used on single core cables

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Part 3 Power Cable Assemblies
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Screened and Belted 3 Core Cables
Lead / Corrugated Aluminium Sheath
Belted Cable
Filler
Belt Insulation
Oversheath
Electrical Flux in a Belted Cable
Bitumen
Conductor Shield / Screen
Core Insulation
Armour
Belt Shield / Screen
Armour Bedding
Lead / Corrugated Aluminium Sheath
Filler
Oversheath
Bitumen
Core Inner Semicon Screen
Core Insulation
Screened Cable
Armour
Core Insulation Shield / Screen
Armour Bedding
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Advantages / Disadvantages of Belted Cables
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Low Voltage Cables
600/1000V 4-core copper conductor, PVC insulated
SWA cable with extruded bedding and SNE
600V/1000V 3-core solid sectorial aluminium
conductor, PVC insulated, copper wire screen,
unarmoured CNE waveform cable
4 core 70mm2, 600/1000V paper insulated lead
sheathed cable with steel tape armour, bitumen
impregnated hessian oversheath and SNE
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Paper Insulated Medium Voltage Cables
3-core 11kV belted PICAS cable with a corrugated
aluminium sheath
3-core 150mm2 6.35/11kV screened paper insulated
lead sheathed (PILS) cable with a PVC oversheath
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MV / HV Polymeric Cables
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Medium Voltage Polymeric Cables

• Pirelli 33kV Single Core Cable
• Stranded or solid Al / Cu conductor
• Extruded inner conductor shield
• Extruded XLPE insulation
• Extruded outer insulation sheild
• 5 Copper wire screen
• 6 Bindings
• 7 MDPE sheath

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Pirelli 11kV Three Core Cable

• 1 Conductor,
• Inner conductor shield,
• XLPE insulation
• Insulation shield,
• Fillers,
• Semiconducting paper tape,
• Collective wire sheath/ screen

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Part 4 GIL Gas Insulated Lines
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Gas Insulated Lines
Flexible SF6 insulated cable for 220kV
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Some advantages/disadvantages of GIL
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Part 5 Electrical Characteristics of Power
Cables
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Conductor Resistance and Power Dissipation

• AC and DC resistance calculations defined earlier
• Power losses in conductors in AC systems must use
  AC resistance values
• Heat produced in conductor per meter of cable

W/m
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Conductor Inductance

• The inductances of an arrangement of 1-core cable
  or of a 3 core cable is complex to derive
• Values for inductance are generally given in
  tables
• Inductance made from 2 parts
• Self inductance of current carrying conductor
• Mutual inductance between conductor and sheath
• For a single core cable

H/m
K 0.05 for circular conductors
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Conductor to Sheath Capacitance

• Hard to calculate for belted cable arrangements
• Results in a charging current always flowing into
  the cable this has implications for maximum
  current carrying capability of cable / maximum
  cable length
• Capacitance largely determines no-load losses
  (I2R and dielectric losses)
• For screened and single core cables use co-axial
  geometry calculations

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Capacitance

• No capacitor is perfect
• As current flows through the capacitance there is
  a small dielectric loss

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Power Losses due to Capacitance

• Power Dissipated in dielectric

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Electric Field Distribution

• The electric field at the conductor is greater
  then that at the outer insulation
• The electric field will be increased at local
  defects (sharp edges, voids etc)

Electrical Stress (kV/mm)
Distance across insulation (mm)
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Induced Sheath Voltages and Power Dissipated in
the Sheath / Armour
N.B. Currents in the sheath / armour of a THREE
CORE cable will normally be minimal as the
induced sheath current should be approximately
zero for a balanced three phase system. As such
losses due to induced voltage and currents in
cable sheaths / armour are only a consideration
in single core cables layed up in a trefoil or a
flat formation
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Induced Sheath Voltages and Power Dissipated in
the Sheath / Armour

• Depends on method of bonding
• Voltage induced in sheath
• Where
• So

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Induced Sheath Voltages and Power Dissipated in
the Sheath / Armour

• Sheath losses expressed as a ratio of conductor
  losses

• Similar for Armour Losses (?2)

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Induced Sheath Voltages and Power Dissipated in
the Sheath / Armour

• For Cables Layed in Flat Formation

• For Cables Layed in Trefoil

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Part 6 Thermal Characteristics of Power
Cables
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Thermal Characteristics

• Electrical characteristics determine heat
  production
• Thermal characteristics determine temperature
  rise
• Thermal resistance and resistivity are analogous
  to electrical resistance and resistivity

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Sources Of Heat Production

• Conductor losses (I2R)
• Dielectric losses (2?f E2 C tan?)
• Sheath losses (?1)
• Armour losses (?2)

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Thermal Resistances
A typical form for single core thermal resistance
is
Where rT thermal resistivity of the
material t1 material thickness dc source
diameter
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Thermal Resistances
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Temperature Rise
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Cable Temperature Rise / Current Rating
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Increasing Current Carrying Capability

• Reduce losses
• Reduce thermal resistances
• Place cable in an alternative location
• Increase cable spacing

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Cable Earth Bonding

• In single core cable arrangements bonding
  arrangements of cable sheaths / screen wires can
  determine magnitude of induced voltages /
  circulating currents
• 3 Possibilities
• Single point end bonding
• Mid point bonding
• Cross Bonding

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Cable Earth Bonding

• How not to do i!

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Cable Earth Bonding
Single Point End Bonding
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Cable Earth Bonding
Mid Point Bonding
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Cable Earth Bonding

• Cross Bonding
• Earth cross bonding
• Phase cross bonding

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Cross Bonding Of Cable Sheaths
The sheaths are cross bonded in a box such as
that above
A number of 72kV joints in a trench
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Part 7 Cable Accessories
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What About The Ends Of The Cables?

• Cables are made and installed in finite lengths
• Joints and terminations are generally made in
  situ
• These are the most common point of failure
• Design can easily be controlled but processes are
  more difficult
• Inspection after assembly impossible

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Cable Accessories

• Cable accessories include joints and terminations
• Accessories must be as reliable as the cable
• Accessories are typically assembled in
  uncontrolled conditions
• Accessories are areas of high electric fields and
  therefore increased discharge levels

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Termination Technologies
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Electric Stresses At Terminations
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Screened Cable Connector 24kV
1 screened body 2 Inner screen 3 Compression
fitting 4 Stress cone adaptor 5 Earthing eye and
ground lead 6 Threaded pin 7 Rear plug 8 Test
point 9 Conductive screen
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Jointing Technologies
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Field Management In A Joint
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Cross-bonding break
Shear head bolts used for mechanical connection
of conductors
Stress is controlled by conductive coatings
Grounding leads
Heat-shrink tube with holt melt adhesive on its
inner surface are used for insulation and overall
sheathing
72 kV heat-shrink Joint
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Heat Shrink

• Often has hot melt adhesive on the inside

Low voltage joint
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Cold Shrink Joint For Up To 400kv
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11kV Resin Filled Hybrid Joints
This joint is prepared and ready for filling with
a high permittivity material
Filling with the pre-packed encapsulating resin
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Plumbed Oil Rosin Filled Cast Iron Jointing
Systems
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Fluid Filled Joints

• A fluid filled joint
• The joint is pressurised so care must be taken
  for leaks

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150kV Cable Prefabricated Joint

• Mechanical connector
• Silicone rubber adaptor including stress cones
• Main body inc faraday cage electrodes for stress
  relief
• Fixing ring
• Smooth shielding clamp
• Shield connection
• Conductive tubing
• Sealing sleeve

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Low Voltage T Joints

• Indoor joints and components may require flame
  retardant and halogen free compounds

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Inspection Of The Trench
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Cable Jointing Process
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Removal Of Sheath
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Connector Crimping
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Applying High Permittivity Tape
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Placement Of Joint Casing
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Screen Bonding
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Fitting Protective Jacket
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Complete Joint