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

1
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

2
Outline

• Wind Energy Overview
• Research Objectives
• DFIG Overview
• DFIG Control
• Unbalance and Induction Machines
• DFIG Unbalance Compensation
• Hardware Results

3
Global Wind Energy

• Almost 12 GW added between 2004 and 2005.

Source Global Wind Energy Outlook 2006, Global
Wind Energy Council
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New Installations - 2005

• Most of new installations continue to be in US
  and Europe.

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

• GermanyUSSpainDenmarkIndia

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US Installed Projects

• Because of slow Midwest growth, the US still has
  huge potential.

Source American Wind Energy Association,
www.awea.org/projects
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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)
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Wind Generator Topologies

• Direct connected.
• Simplest.
• Requires switch to prevent motoring.
• Draws reactive power with no reactive control.

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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.

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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.

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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.

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Wind Energy Issues

• Wind is intermittent
• Limits winds 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

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Outline

• Wind Energy Overview
• Research Objectives
• DFIG Overview
• DFIG Control
• Unbalance and Induction Machines
• DFIG Unbalance Compensation
• Hardware Results

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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

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Outline

• Wind Energy Overview
• Research Objectives
• DFIG Overview
• DFIG Control
• Unbalance and Induction Machines
• DFIG Unbalance Compensation
• Hardware Results

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DFIG Overview - Topology

• Rotor control allows for speed and reactive power
  control. (Cage IG are fixed.)

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DFIG Overview Variable Speed Control

• Higher Cp means more energy captured
• Maintain tip-speed ratio at nominal value

(graphic from Mathworks)
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DFIG Overview Reactive Power Control
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Outline

• Wind Energy Overview
• Research Objectives
• DFIG Overview
• DFIG Control
• Unbalance and Induction Machines
• DFIG Unbalance Compensation
• Simulation Results
• Hardware Results

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DFIG Control

• Control is done by transforming three-phase to
  two-phase

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DFIG Control Machine Flux Oriented

• q-axis controls reactive power (flux)
• d-axis controls torque

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DFIG Control Grid Flux Oriented

• Align d-axis with voltage, instead of flux
• Easier, more stable
• d-axis -gt torque
• q-axis -gt reactive power (Qs)

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DFIG Control

• d-axis controls torque, hence speed

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DFIG Control

• q-axis controls reactive power (Qs)

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DFIG Control Stability

• DFIGs naturally have complex poles near the RHP,
  near the grid frequency

(ird/vrd transfer function)
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Outline

• Wind Energy Overview
• Research Objectives
• DFIG Overview
• DFIG Control
• Unbalance and Induction Machines
• DFIG Unbalance Compensation
• Hardware Results

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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

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Unbalance Second Harmonic
balanced
unbalanced

• Therefore, compensate for the second harmonic in
  the dq system

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Outline

• Wind Energy Overview
• Research Objectives
• DFIG Overview
• DFIG Control
• Unbalance and Induction Machines
• DFIG Unbalance Compensation
• Hardware Results

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Unbalance Compensation

• Intentionally injecting a disturbance with an
  auxiliary controller to drive the disturbance to
  zero

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d-axis Inner Loop

• Compensation controller looks like a bandpass and
  lead-lag filter

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Compensation Controller Design
(Cd,comp)
(d-axis loop gain)
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Outline

• Wind Energy Overview
• Research Objectives
• DFIG Overview
• DFIG Control
• Unbalance and Induction Machines
• DFIG Unbalance Compensation
• Hardware Results

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Hardware Pictures
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Hardware Results (15 kW)

• Transient activation of compensation
• VUF 0.04

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Hardware Results (15 kW)
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Hardware Results (15 kW)

• Steady state

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Thank You!

• Questions?