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A Novel Control Scheme for a Doubly-Fed Induction Wind Generator Under Unbalanced Grid Voltage Conditions. Ted Brekken, Ph.D. Assistant Professor in Energy Systems Oregon State University. Outline. Wind Energy Overview Research Objectives DFIG Overview DFIG Control

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slide1

A Novel Control Scheme for a Doubly-Fed Induction Wind Generator Under Unbalanced Grid Voltage Conditions

Ted Brekken, Ph.D.

Assistant Professor in Energy Systems

Oregon State University

outline
Outline
  • Wind Energy Overview
  • Research Objectives
  • DFIG Overview
  • DFIG Control
  • Unbalance and Induction Machines
  • DFIG Unbalance Compensation
  • Hardware Results
global wind energy
Global Wind Energy
  • Almost 12 GW added between 2004 and 2005.

Source: Global Wind Energy Outlook 2006, Global Wind Energy Council

new installations 2005
New Installations - 2005
  • Most of new installations continue to be in US and Europe.

Source: Global Wind Energy Outlook 2006, Global Wind Energy Council

wind energy overview
Wind Energy Overview
  • GermanyUSSpainDenmarkIndia
us installed projects
US Installed Projects
  • Because of slow Midwest growth, the US still has huge potential.

Source: American Wind Energy Association, www.awea.org/projects

wind energy overview7
Wind Energy Overview
  • Wind generators and farms are getting larger.
  • 5 MW wind generators are now available with 7 MW in the works.

(graphic from Vestas.com)

wind generator topologies
Wind Generator Topologies
  • Direct connected.
  • Simplest.
  • Requires switch to prevent motoring.
  • Draws reactive power with no reactive control.
wind generator topologies9
Wind Generator Topologies
  • Doubly-fed.
  • The doubly-fed topology is the most common for high power.
  • Rotor control allows for speed control of around 25% of synchronous.
  • Rotor converter rating is only around 25% of total generator rating.
  • Reactive power control.
wind generator topologies10
Wind Generator Topologies
  • Full-rated converter connected.
  • Lower cost generator than DFIG. Lower maintenance.
  • Converter must be full-rated.
  • Full-rated converter allows for complete speed and reactive power control.
  • Could also be used with a synchronous generator.
wind generator topologies11
Wind Generator Topologies
  • Direct-drive.
  • Eliminate the gearbox by using a very-high pole synchronous generator.
  • Resulting generator design is relatively wide and flat.
  • No gearbox issues.
  • Full-rated converter is required.
  • Full speed and reactive power control.
wind energy issues
Wind Energy Issues
  • Wind is intermittent
    • Limits wind’s percentage of the energy mix
  • Wind energy is often located in rural areas
    • Rural grids are often weak and unstable, and prone to voltage sags, faults, and unbalances
  • Unbalanced grid voltages cause many problems for induction generators
    • Torque pulsations
    • Reactive power pulsations
    • Unbalanced currents
outline13
Outline
  • Wind Energy Overview
  • Research Objectives
  • DFIG Overview
  • DFIG Control
  • Unbalance and Induction Machines
  • DFIG Unbalance Compensation
  • Hardware Results
research objectives
Research Objectives
  • Research was carried out from 2002 to 2005 at the U of M and at NTNU in Trondheim, Norway on a Fulbright scholarship
  • Doubly-fed induction generators are the machines of choice for large wind turbines
  • The objective is to develop a control methodology for a DFIG that can achieve:
    • Variable speed and reactive power control
    • Compensation of problems caused by an unbalanced grid
      • Reduce torque pulsations
      • Reduce reactive power pulsations
      • Balance stator currents
outline15
Outline
  • Wind Energy Overview
  • Research Objectives
  • DFIG Overview
  • DFIG Control
  • Unbalance and Induction Machines
  • DFIG Unbalance Compensation
  • Hardware Results
dfig overview topology
DFIG Overview - Topology
  • Rotor control allows for speed and reactive power control. (Cage IG are fixed.)
dfig overview variable speed control
DFIG Overview – Variable Speed Control
  • Higher Cp means more energy captured
  • Maintain tip-speed ratio at nominal value

(graphic from Mathworks)

outline19
Outline
  • Wind Energy Overview
  • Research Objectives
  • DFIG Overview
  • DFIG Control
  • Unbalance and Induction Machines
  • DFIG Unbalance Compensation
  • Simulation Results
  • Hardware Results
dfig control
DFIG Control
  • Control is done by transforming three-phase to two-phase
dfig control machine flux oriented
DFIG Control – Machine Flux Oriented
  • q-axis controls reactive power (flux)
  • d-axis controls torque
dfig control grid flux oriented
DFIG Control – Grid Flux Oriented
  • Align d-axis with voltage, instead of flux
  • Easier, more stable
  • d-axis -> torque
  • q-axis -> reactive power (Qs)
dfig control23
DFIG Control
  • d-axis controls torque, hence speed
dfig control24
DFIG Control
  • q-axis controls reactive power (Qs)
dfig control stability
DFIG Control – Stability
  • DFIGs naturally have complex poles near the RHP, near the grid frequency

(ird/vrd transfer function)

outline26
Outline
  • Wind Energy Overview
  • Research Objectives
  • DFIG Overview
  • DFIG Control
  • Unbalance and Induction Machines
  • DFIG Unbalance Compensation
  • Hardware Results
3 phase voltage unbalance
3 Phase Voltage Unbalance
  • Causes torque puslations, reactive power pulsations, unbalanced currents, possible over heating
  • Unbalance can be seen as the addition of a negative sequence
  • Unbalance factor (VUF, IUF) is the magnitude of the negative sequence over the magnitude of the positive sequence
unbalance second harmonic
Unbalance – Second Harmonic

balanced

unbalanced

  • Therefore, compensate for the second harmonic in the dq system
outline29
Outline
  • Wind Energy Overview
  • Research Objectives
  • DFIG Overview
  • DFIG Control
  • Unbalance and Induction Machines
  • DFIG Unbalance Compensation
  • Hardware Results
unbalance compensation
Unbalance Compensation
  • Intentionally injecting a disturbance with an auxiliary controller to drive the disturbance to zero
d axis inner loop
d-axis Inner Loop
  • Compensation controller looks like a bandpass and lead-lag filter
compensation controller design
Compensation Controller Design

(Cd,comp)

(d-axis loop gain)

outline33
Outline
  • Wind Energy Overview
  • Research Objectives
  • DFIG Overview
  • DFIG Control
  • Unbalance and Induction Machines
  • DFIG Unbalance Compensation
  • Hardware Results
hardware results 15 kw
Hardware Results (15 kW)
  • Transient activation of compensation
  • VUF = 0.04
hardware results 15 kw37

Reduction, Simulation:Torque -> 11.5Qs -> 17.7IUF -> 7.4

Reduction, Hardware:

Torque -> 29.1Qs -> 22.8IUF -> 5.5

Hardware Results (15 kW)
  • Steady state
thank you
Thank You!

Questions?