Providing Grid Frequency Support Using Variable Speed Wind Turbines With Augmented Control

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Providing Grid Frequency Support Using Variable
Speed Wind Turbines With Augmented Control

• Adam Stock and Bill Leithead
• University of Strathclyde

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Overview

• A technique for varying the power output of
  variable speed wind turbines has been developed
• An additional outer feedback loop
• Generic
• No modification to the inner controller required
• One use for this controller is to provide
  synthetic inertia
• Quick response
• No effect on normal operation

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The Power Adjusting Controller (PAC)

• Allows the power output of the turbine to be
  altered
• A hierarchical structure is chosen
• An inner, faster layer acts on demanded torque
• An outer, slower layer acts on demanded pitch
  angle

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Synthetic Inertia

• Variable speed wind turbines are asynchronous and
  so do not naturally boost grid inertia
• Instead of increasing the inertia value, the
  supplied power could be increased
• The additional power could be supplied by
  converting some of the kinetic energy in the
  rotating blades into electrical energy

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Application of the PAC for Synthetic Inertia

• Higher power output is achieved by implementing a
  positive ?Td value
• - The pitch angle of the blades is already at its
  optimum so additional power cannot be maintained
  indefinitely.
• This therefore results in a reduction in the
  rotational speed of the rotor.
• The ?ßd output is therefore no longer required
• The controller can be split into
• two sections, CT and C?

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The CT controller

• The CT controller, in response to ?P and ?g,
  determines ?Td.
• In part (1) the additional torque, Tad,
  corresponding to ?P is determined.
• In part (2) a correction, T, is added to the
  constrained value of Tad to compensate for the
  change in rotor speed caused by the PAC.
• In part (3) the additional torque is limited to
  constrain the rotor speed within an allowed range.

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The C? controller

• The C? controller, in response to ?Td, Td, ?g and
  ßd, determines ??g, the dummy generator speed.
• The C? controller utilises a first order
  approximation of the dynamics between torque
  demand and generator speed G(s) to provide the
  dummy speed value ??g.
• Switching is used so that no ??g value is
  generated when the pitch angle of the blades is
  above ßdupper when ?P is originally requested
  unless the pitch demand subsequently falls below
  ßdlower.
• A transfer function H(s) with suitable switching
  is used to vary the speed of the recovery when ?P
  is no longer required.
• An addition to the controller is made that
  prevents operation above a set
• torque TL for any given generator speed, where
  TLA(?g).

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Constant Wind Speed Simulations

• The controller was developed and tuned in
  Simulink
• Converted to GL Bladed for the simulations
• Constant wind speed simulations shown with a
  large frequency drop (from 50Hz to 49.2Hz in
  approximately 8 seconds before recovery)
• Additional power output is not affected by the
  wind speed

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Variable Wind Speed Simulations

• Variable wind speed simulations with a turbulence
  of 5 were conducted
• Additional power is not affected by the wind
  speed
• At high wind speeds no recovery period is
  required
• Even at 7m/s there is enough energy available to
  provide sufficient additional power

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Preventing Operation Outside of a Safe Envelope

• The Controller prevents operation outside of a
  set operational envelope
• If the operating point moves outside of the set
  envelope then additional power production is
  curtailed

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Changes in Effectiveness Due to Turbine Design

• Turbine design greatly alters the effectiveness
  of synthetic inertia
• Larger wind turbines have greater inertial
  resource

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Conclusions

• A Power Adjusting Controller (PAC) is developed
  that allows an alteration in the power output of
  a wind turbine to be made.
• One use for the PAC is as a synthetic inertia
  controller
• The controller is capable of producing a fast
  response to a drop in grid frequency across a
  broad range of wind speeds.
• The total energy lost in the process is very low
  and the aerodynamic power loss does not exceed
  1.
• The controller is shown to be effective at
  keeping the operating point within a set
  operational envelope.

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