A Novel Control Scheme for a DoublyFed Induction

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 Unbalance and Induction Machines DFIG Unbalance Compensation Hardware Results

Global Wind Energy • Almost 12 GW added between 2004 and 2005. Source: Global Wind Energy Outlook 2006, Global Wind Energy Council

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 • Germany US Spain Denmark India

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

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 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 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 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

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

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

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

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

DFIG Overview – Reactive Power Control

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

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

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

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 Control • d-axis controls torque, hence speed

DFIG Control • q-axis controls reactive power (Qs)

DFIG Control – Stability • DFIGs naturally have complex poles near the RHP, near the grid frequency (ird/vrd transfer function)

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

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

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

Unbalance Compensation • 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 leadlag filter

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

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

Hardware Pictures

Hardware Results (15 k. W) • Transient activation of compensation • VUF = 0. 04

Hardware Results (15 k. W)

Hardware Results (15 k. W) • 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?