Work Done So Far

1
RESOLVING PERFORMANCE CONFLICTS IN NEW WIND
TURBINE BLADE DESIGNS
By Vidyadhar Peesapati Electrical Energy and
Power Systems Group
Payam Jamshidi Northwest Composites Centre
Laith Rashid Microwave and Communication Systems
2
Aim of Present Work

• To find a possible solution that answers
  conflicts among the following areas in wind
  turbine design,
• Lightning Protection
• Radar Interference
• Material Processing and Increase In Fatigue
  Loading
• Groups Involved ,
• Electrical Power Systems Group (National Grid HV
  Research Centre), University Of Manchester.
• The Microwave Communications Systems Group,
  University Of Manchester.
• Northwest Composites Centre, University Of
  Manchester.

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Issue (1) Radar Interference

• Scatter and Ghost Targets created by large RCS of
  Wind Turbines.
• Scatter created by
• Tower
• Blades
• Nacelle
• Doppler signature of moving blades cause false
  track-initiation.
• Considerable RCS reduction on tower and nacelle
  can be achieved by shaping
• Blades cannot be re-shaped.
• Introduction of Radar Absorbing Material into
  blade is a possible solution.

4
Issue (2) Lightning and Wind Turbines

• What does a lightning strike require to form?
• 1. A tall object capable of developing streamers
  or leaders
• 2. Sharp edges where the field enhancement is
  high.
• Add these together, and a wind turbine, tall
  and its blades are the perfect set up for
  streamers that can attach themselves to incoming
  leaders.
• The blades are the most vulnerable parts to be
  damaged.
• All new blades are now protected with an inbuilt
  lightning protection system.

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Lightning Protection Methods
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Lightning and Wind Turbines - Damage

• Not all lightning strikes end up being
  intercepted by the receptor or the lightning
  protection points. Local damage to the area
  around receptors is still common.
• The main mechanism of damage is when the
  lightning current penetrates the blade and forms
  an arc inside.
• The pressure/shock wave could damage the blade,
  or cause cracks on the surface.

(Source Global Wind Power)
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Issue (3) Fatigue
(Source DFVLR)

• Stress on blades and towers induce fatigue
  loading e.g. Tension-tension, torsional
  fatigue and tension compression. Self weight of
  big blades becomes a significant issue.
• RAM solution increases blades weight
• Materials solutions
• - Reduce weight
• Use more fatigue resistant materials

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Blades Materials solution

• Needs lightweight and high stiffness, hence
  composites

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The Conflict of Interest.

• The idea of introducing new materials into the
  blade laminate causes issues for the lightning
  protection systems and also the blade laminate
  manufacturing and fatigue life.
• Efficiency of lightning protection (Attachment
  and Conduction).
• Introduction of new materials to the blade
  laminate brings forth problems such as durability
  and integrity of the laminate.
• Also the feasibility of adding this material in
  existing blade manufacturing process

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Preliminary RAM Solution

• Model consists of 2 additional layers to be
  included with the blade laminate.
• A fabricated metallic pattern of capacitors and
  resistors. (resonance absorbers)
• A perfect conducting sheet acting as a reflecting
  ground plane.
• A composite Nickel and Copper coated carbon fiber
  layer is tested which can be used as the
  reflective layer.

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Materials SolutionMetal coated carbon fibres
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Vacuum Infusion
Metal coated carbon veils Epoxy resin
Skins moulded in female moulds
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Lightning Impact Of RAM

• Two lightning protection solutions exist mesh
  and receptor
• Present model is based on the receptor lightning
  protection solution (mesh solution would only
  work in terms of radar should a specific
  dielectric layer be placed above it)
• Initial model shows a significant change in the
  field enhancement on the down conductor (i.e. the
  internal blade field). This is a positive result
  as the chances on streamers inside the blade are
  minimised.
• However, the field around the receptors is
  significantly distorted and reduced
• Once lightning attachment has been established,
  the lightning protection system will have to
  conduct the lightning current without damage,
  this is tested by high current tests.

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Impact on Lightning Attachment

• A normal blade would show a high field
  intensification on the receptor.
• The field is quite high on the surface of the
  down conductor as well.
• The presence of a copper inner layer inside the
  fiber glass blade would decrease the blade
  internal field and, it seems, that around the
  receptor.
• This would decrease the inception efficiency of
  the receptor.
• An enhanced electric field inside the fiber glass
  increases the risk of breakdown of this
  dielectric as well.

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Impact on Lightning Current Conduction

• All lightning current carrying components needs
  to conduct and dissipate the energy without
  causing damage
• Existing lightning protection systems rated to
  carry lightning current and are tested to confirm
  this.
• The new composite layer will have to carry part
  of the lightning current (IEC standards require
  current conduction of 200 kA).
• All conducting materials will have to be
  electrically bonded.
• High Current tests were performed on the new
  composite layer.
• Composite tested as itself and also as a sheet
  infused with epoxy.

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High Current Tests

• Samples of different lengths and widths were
  tested.
• Ideally layer should distribute strike energy
  evenly thus avoiding damage to other layers.
• Unfortunately it does not, even at currents
  levels around 15kA.

After
Before
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Results and Conclusions

• Feasible with existing manufacturing processes of
  blades.
• Affects the lightning attachment.
• Field distortion in around the receptor might be
  improved by selective layering.
• Present composite layer is not capable of
  carrying rated lightning current.

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Authors

• Mr Vidyadhar Peesapati
• Dr Payam Jamshidi
• Mr Laith Rashid
• Dr Ian Cotton
• Prof Paul Hogg
• Prof Anthony Brown
• Thanks to
• Dr Alan Nesbitt (Northwest Composites)
• Dr Nikolaos Kokkinos (Elemko SA, Greece)
• Research has been part of the Supergen Wind
  Consortium funded by the EPSRC.