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

Global Wind Energy Almost 12 GW added between 2004 and Almost 12 GW added between 2004 and Source: Global Wind Energy Outlook 2006, Global Wind Energy Council

New Installations Most of new installations continue to be in US and Europe. Most of new installations continue to be in US and Europe. Source: Global Wind Energy Outlook 2006, Global Wind Energy Council

Wind Energy Overview Germany US Spain Denmark India Germany US Spain Denmark India

US Installed Projects Because of slow Midwest growth, the US still has huge potential. Because of slow Midwest growth, the US still has huge potential. Source: American Wind Energy Association,

Wind Energy Overview Wind generators and farms are getting larger. Wind generators and farms are getting larger. 5 MW wind generators are now available with 7 MW in the works. 5 MW wind generators are now available with 7 MW in the works. (graphic from Vestas.com)

Wind Generator Topologies Direct connected. Direct connected. Simplest. Simplest. Requires switch to prevent motoring. Requires switch to prevent motoring. Draws reactive power with no reactive control. Draws reactive power with no reactive control.

Wind Generator Topologies Doubly-fed. Doubly-fed. The doubly-fed topology is the most common for high power. The doubly-fed topology is the most common for high power. Rotor control allows for speed control of around 25% of synchronous. Rotor control allows for speed control of around 25% of synchronous. Rotor converter rating is only around 25% of total generator rating. Rotor converter rating is only around 25% of total generator rating. Reactive power control. Reactive power control.

Wind Generator Topologies Full-rated converter connected. Full-rated converter connected. Lower cost generator than DFIG. Lower maintenance. Lower cost generator than DFIG. Lower maintenance. Converter must be full-rated. Converter must be full-rated. Full-rated converter allows for complete speed and reactive power control. Full-rated converter allows for complete speed and reactive power control. Could also be used with a synchronous generator. Could also be used with a synchronous generator.

Wind Generator Topologies Direct-drive. Direct-drive. Eliminate the gearbox by using a very-high pole synchronous generator. Eliminate the gearbox by using a very-high pole synchronous generator. Resulting generator design is relatively wide and flat. Resulting generator design is relatively wide and flat. No gearbox issues. No gearbox issues. Full-rated converter is required. Full-rated converter is required. Full speed and reactive power control. Full speed and reactive power control.

Wind Energy Issues Wind is intermittent Wind is intermittent –Limits wind’s percentage of the energy mix Wind energy is often located in rural areas 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 Unbalanced grid voltages cause many problems for induction generators –Torque pulsations –Reactive power pulsations –Unbalanced currents

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

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 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 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: 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 torque pulsations Reduce reactive power pulsations Reduce reactive power pulsations Balance stator currents Balance stator currents

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

DFIG Overview - Topology Rotor control allows for speed and reactive power control. (Cage IG are fixed.) Rotor control allows for speed and reactive power control. (Cage IG are fixed.)

DFIG Overview – Variable Speed Control Higher C p means more energy captured Higher C p means more energy captured Maintain tip-speed ratio at nominal value Maintain tip-speed ratio at nominal value (graphic from Mathworks)

DFIG Overview – Reactive Power Control

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

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

DFIG Control – Machine Flux Oriented q-axis controls reactive power (flux) q-axis controls reactive power (flux) d-axis controls torque d-axis controls torque

DFIG Control – Grid Flux Oriented Align d-axis with voltage, instead of flux Align d-axis with voltage, instead of flux Easier, more stable Easier, more stable d-axis -> torque d-axis -> torque q-axis -> reactive power (Q s ) q-axis -> reactive power (Q s )

DFIG Control d-axis controls torque, hence speed d-axis controls torque, hence speed

DFIG Control q-axis controls reactive power (Q s ) q-axis controls reactive power (Q s )

DFIG Control – Stability DFIGs naturally have complex poles near the RHP, near the grid frequency DFIGs naturally have complex poles near the RHP, near the grid frequency (i rd /v rd transfer function)

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

3 Phase Voltage Unbalance Causes torque puslations, reactive power pulsations, unbalanced currents, possible over heating Causes torque puslations, reactive power pulsations, unbalanced currents, possible over heating Unbalance can be seen as the addition of a negative sequence 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 factor (VUF, IUF) is the magnitude of the negative sequence over the magnitude of the positive sequence

Unbalance – Second Harmonic Therefore, compensate for the second harmonic in the dq system Therefore, compensate for the second harmonic in the dq system balanced unbalanced

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

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

d-axis Inner Loop Compensation controller looks like a bandpass and lead- lag filter Compensation controller looks like a bandpass and lead- lag filter

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

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

Hardware Pictures

Hardware Results (15 kW) Transient activation of compensation Transient activation of compensation VUF = 0.04 VUF = 0.04

Hardware Results (15 kW)

Steady state Steady state Reduction, Simulation: Torque -> 11.5 Qs -> 17.7 IUF -> 7.4 Reduction, Hardware: Torque -> 29.1 Qs -> 22.8 IUF -> 5.5

Thank You! Questions?