National Renewable Energy Centre

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EWEA 2013 February, 2013, Vienna, Austria
OFFSHORE RENEWABLE PLANT HVDC POWER COLLECTOR AND
DISTRIBUTOR
National Renewable Energy Centre Chong Ng,
Principal Engineer Reliability
Validation Paul McKeever, RD Manager
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Narec Created by Government to stimulate the RE
industry, A Controlled and Independent Testing
Environment
Existing 50m blade test Still water tank Wave flume Simulated seabed Wind turbine training tower Electrical and materials laboratories New 3MW tidal turbine drive train - 2012 Offshore anemometry hub - 2012 100m blade test - 2012 15MW wind turbine drive train - 2013 99.9MW offshore wind demonstration site - 2013/14
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Presentation Contents

• Technical Paper Background
• Existing Systems
• HVAC transmission systems
• HVDC systems
• Proposed HVDC System
• Selected Challenges
• Conclusions
• Next Steps

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Technical Paper Background

• UK requires offshore wind to meet its renewable
  energy generation targets (2020, 2030, 2050)
  UK Energy Bill by 2020, 30 from Renewable
  Energy
• Likely to involve larger turbines (10MW? 20MW?)
  FP6 UpWind Project
• Offshore plant would benefit from an appropriate
  power collection, transmission and distribution
  technology
• HVDC potentially provides better efficiency,
  particularly over longer distances
• Benefits from power semiconductor and copper cost
  trends

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HVAC Transmission Systems

• Commonly used in many offshore wind farms
• Can suffer from excessive reactive current
• Increases cable losses
• Reduces power transfer capability
• Reactive power compensation required (extra
  equipment)
• Can suffer from high line losses and excessive
  voltage drops
• Extra cables required
• Inter-dependant characteristics need careful
  consideration
• Transmission voltage level, cable capacitance and
  charging currents

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Existing HVDC Systems

• Modern HVDC systems generally have advantages
  such as
• Lower transmission losses
• Fully controllable power flow
• No reactive power generation or absorption
  (cable only connections)
• Reduce/eliminate AC harmonic filter with the
  latest multilevel converter technologies (e.g.
  MMC HVDC)
• HVDC transmission systems can be categorised, by
  the converters used, into three categories
• Line-commutated Converters (LCC), Capacitor
  Commutated Converters (CCC) and Voltage Source
  Converters (VSC) as illustrated below
• Point to point HVDC power transmission Wind
  Farm Inter-array?
• What do we want?
• A dedicated high efficiency, robust, flexible and
  low cost power collection, transmission and
  distribution technology for use within the wind
  farm too

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Proposed HVDC System

• HVDC power transmission from the point of
  generation
• Reduce losses and components (i.e. make use of
  Turbine MV converter and availability of HVDC
  gird)
• Multi-terminal HVDC system
• Increase availability
• Offers flexibility and redundancy
• Reduce cost
• Removal of/minimise offshore substation
• Reduced cable losses (HV operation)

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Proposed HVDC System

• Hybrid HVDC Transformer (figure shows simplified
  circuit)
• Steps up MVDC to HVDC
• Reduced voltage stress on primary side and
  current stress on secondary side allows use of
  off the shelf force commutation devices
• Uses magnetic transformer to avoid high
  conversion ratio
• Potential to require less power capability from
  switches (30) when compared with conventional
  2-level 3-phase HVDC converter
• Many potential challenges that need full
  investigation (e.g. switching control, network
  stability, economic impact, protection and
  isolation)

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Proposed HVDC System

• Switching device comparison
• Proposed Hybrid HVDC Transformer vs.
  conventional HVDC converter (3-phase 2-level
  topology)

• Assumptions
• n number of series connected power switching
  devices in half of the bridge arm
• 6.5kV rated switching devices
• VSC-based HVDC converters use 3-phase, 2 (or
  multi) level converter topology
• Assumes 2 devices in series is sufficient to
  withstand the MV voltage stress
• 150kVdc example
• HVDC side needs n gt 30 devices in series
• For conventional VSC-based HVDC systems
• 6n gt 180 devices
• For hybrid HVDC transformer
• 4n 8 gt 128 devices
• 29 saving in power semiconductors used

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Selected Challenges

• The time to implement
• Dependent on development/readiness of the
  offshore wind industry
• Managing multi-vendor solutions
• Will this be a problem?
• Practical implementation (i.e. is it realistic?)
• Needs further investigation this is still a
  concept
• Will the subsea power cable size increase with no
  centralised collector?
• Shouldnt increase for similar voltage levels
  the overall power stays the same
• Would a platform still be required as a
  maintenance hub?
• A mobile platform could be used for this purpose
• Is there an operational impact?
• Turbine operation should be unaffected
• System optimum operation and control needs
  developing

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Conclusions

• Potential advantages for offshore wind farm
  applications
• An alternative to AC and point to point HVDC
  transmission topologies
• Suitable installation in every single power
  source
• Increases flexibility and redundancy of the
  entire HVDC system
• Positive impact on wind farm availability and OM
  costs
• Eliminates/minimises the need for a centralised
  offshore collection platform
• Potential lower component count at converter
  level
• Modular component sets across the system
• 100MW power block in centralised system vs. 20 x
  5MW power blocks in hybrid HVDC transformer
  system
• Increased component count at system level (due to
  de-centralisation)
• Balanced by no offshore substation and fewer
  components, e.g. fewer power semiconductors and
  filters

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Next Steps

• Investigate, in detail, the feasibility of this
  HVDC system concept
• Detailed study of the proposed hybrid HVDC
  transformer
• Explore the feasibility of the following
  advantages
• High flexibility leading to independent
  turbines
• Additional redundancy and high system
  availability (no centralised substation)
• High efficiency (power collection and OM
  efficiency)
• Cost reduction potential
• Installation in individual turbines
• Optimisation of materials (copper, semiconductor
  devices)
• Investigate the use of SiC switching devices
• Higher power density and heat tolerance

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Thank you for listening!

• Narec Contact Details
• Website www.narec.co.uk
• Technical Paper Authors
• chong.ng_at_narec.co.uk
• paul.mckeever_at_narec.co.uk