A Novel Control Scheme for a Doubly-Fed Induction Wind Generator
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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 l.jpg

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 l.jpg
Outline Generator

  • Wind Energy Overview

  • Research Objectives

  • DFIG Overview

  • DFIG Control

  • Unbalance and Induction Machines

  • DFIG Unbalance Compensation

  • Hardware Results


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Global Wind Energy Generator

  • 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 Generator

  • 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 Generator

  • GermanyUSSpainDenmarkIndia


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US Installed Projects Generator

  • 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 Generator

  • 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 Generator

  • Direct connected.

  • Simplest.

  • Requires switch to prevent motoring.

  • Draws reactive power with no reactive control.


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Wind Generator Topologies Generator

  • 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 Generator

  • 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 Generator

  • 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 Generator

  • 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


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Outline Generator

  • Wind Energy Overview

  • Research Objectives

  • DFIG Overview

  • DFIG Control

  • Unbalance and Induction Machines

  • DFIG Unbalance Compensation

  • Hardware Results


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Research Objectives Generator

  • 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 Generator

  • 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 Generator

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


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DFIG Overview – Variable Speed Control Generator

  • Higher Cp means more energy captured

  • Maintain tip-speed ratio at nominal value

(graphic from Mathworks)



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Outline Generator

  • 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 Generator

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


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DFIG Control – Machine Flux Oriented Generator

  • q-axis controls reactive power (flux)

  • d-axis controls torque


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DFIG Control – Grid Flux Oriented Generator

  • Align d-axis with voltage, instead of flux

  • Easier, more stable

  • d-axis -> torque

  • q-axis -> reactive power (Qs)


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DFIG Control Generator

  • d-axis controls torque, hence speed


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DFIG Control Generator

  • q-axis controls reactive power (Qs)


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DFIG Control – Stability Generator

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

(ird/vrd transfer function)


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Outline Generator

  • 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 Generator

  • 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 Generator

balanced

unbalanced

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


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Outline Generator

  • Wind Energy Overview

  • Research Objectives

  • DFIG Overview

  • DFIG Control

  • Unbalance and Induction Machines

  • DFIG Unbalance Compensation

  • Hardware Results


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Unbalance Compensation Generator

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


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d-axis Inner Loop Generator

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


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Compensation Controller Design Generator

(Cd,comp)

(d-axis loop gain)


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Outline Generator

  • Wind Energy Overview

  • Research Objectives

  • DFIG Overview

  • DFIG Control

  • Unbalance and Induction Machines

  • DFIG Unbalance Compensation

  • Hardware Results



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Hardware Results (15 kW) Generator

  • Transient activation of compensation

  • VUF = 0.04



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

Reduction, Hardware:

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

Hardware Results (15 kW)

  • Steady state


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Thank You! Generator

Questions?


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