Control of a multiterminal VSC transmission scheme for connecting offshore wind farms

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Control of a multi-terminal VSC transmission
scheme for connecting offshore wind farms
European Wind Energy Conference 2007, Milan
Ralph L. Hendriks (R.L.Hendriks_at_tudelft.nl), G.
C. Paap, W. L. Kling
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Contents

• Offshore wind farms and interconnectors
• Voltage-sourced converter model
• Voltage-sourced converter controls
• Simulation study
• Conclusions

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Offshore wind farms and interconnectors

• Offshore wind has a good potential as a source of
  renewable energy
• Current offshore wind farms located relatively
  close to shore (12-miles zone)
• Future farms will be located further offshore
• Grid connection costs will dominate wind farm
  project budgets

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Capacity factor

• Transmission system capacity utilization is
  dictated by wind stochastics
• Capacity factor offshore wind farm 3545
• Transmission costs per unit energy are relatively
  high

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Offshore wind farms and interconnectors

• Since the liberalization of electricity markets,
  a growing demand for interconnection capacity
  exists
• Cross-border interconnectors have been build to
  share regulating and reserve power
• Cross-border interconnectors are now the backbone
  for international trade

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Priority axes for interconnector capacity
development
Source TEN-E project documentation, DG TREN
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Combination of interconnector and wind farm

• Proposed interconnector routes cross promising
  wind farm sites
• Maximization of cable utilization lowers
  transmission costs
• Use same infrastructure for grid connection and
  energy trade
• DC technology unavoidable, how to control and
  dispatch power?

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Combination of interconnector and wind farm
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AC versus DC transmission

• AC transmission through submarine cables has
  technical limitations ? maximum 100 km
• DC transmission ? only resistive losses ?
  unlimited transmission distance
• Line-commutated converter (LCC) technology versus
  voltage-sourced converter (VSC) technology

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Voltage-sourced converter (VSC)

• Independent control of active and reactive power
• Provides local voltage support
• Power flow can be reverted by voltage polarity or
  current direction

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VSC equivalent model
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Vector control

• Fast inner loop controls id and iq
• D-Q-Z type PLL tracks the utility voltage phase
  angle
• Decoupling iq controls P, id controls Q
• Current references obtained by slower outer-loop
  controllers

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DC-voltage control

• DC voltage is a measure for unbalance in the DC
  system
• At least one controller in the scheme should
  regulate the DC voltage
• No additional signals or telecommunications are
  required balancing
• Compare power/frequency control in ac grids

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DC voltage control

• DC voltage control by straightforward
  PI-regulator
• Only P-control when more than two terminals are
  present
• Controller can be offset by P0
• power dispatch on the link, centralized
  controller with telecommunication links

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Simulation study

• System modelled in Matlab/Simulink
• Three identical converters rated 300 MW
• Total link length 200 km (purely resistive)
• 300 MW wind farm connected to middle terminal

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Simulation study-50 MW step in wind farm infeed
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Simulation studyControlled shut-down of wind farm
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Conclusions

• Combination of wind farm grid connection and
  interconnector is a promising cost-saving
  alternative
• Proof-of-concept first step in development of
  pan-European offshore grid
• DC voltage is a measure for unbalance
• No telecommunications-in-the-loop required
• Simplified VSC model that can be included in
  power system simulation software

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Control of a multi-terminal VSC transmission
scheme for connecting offshore wind farms
European Wind Energy Conference 2007, Milan
Ralph L. Hendriks (R.L.Hendriks_at_tudelft.nl), G.
C. Paap, W. L. Kling