UNIVERSITÉ LIBRE DE BRUXELLES Faculty of Applied Sciences Department of Electrical Engineering
Winding Short-Circuit Fault Modelling and Detection in Doubly-Fed Induction Generator based Wind Turbine
Systems
Thesis submitted for the degree of PhD in Engineering Sciences
by
Jawwad ZAFAR
October 13, 2011
Supervisor: Prof. Johan GYSELINCK
Abstract
This thesis deals with the operation of and winding short-circuit fault detection in a Doubly-Fed Induction Generator (DFIG) based Wind Turbine Generator System (WTGS). Both the faulted and faultless condition of operation has been studied, where the focus is on the electrical part of the system. The modelled electrical system is first simulated and the developed control system is then validated on a test bench. The test-bench component dimensioning is also discussed.
The faultless condition deals with the start-up and power production mode of operation. Control design based on the Proportional Integral (PI) control technique has been compared for power and torque control strategies against the Linear Quadratic Gaussian (LQG) control technique, at different operating points through the variable-speed region of WTGS operation following the maximum power curve of the system. It was found that the torque control strategy offered less degradation in performance for both the control techniques at operating points different for the one for which the control system was tuned.
The start-up procedure of the DFIG based WTGS has been clarified and simplified. The phase difference between the stator and the grid voltage, which occurs due to the arbitrary rotor position when the rotor current control is activated, is minimized by using a sample-and-hold technique which eliminates the requirement of designing an additional controller. This method has been validated both in simulation and experiments.
The faulted condition of operation deals with the turn-turn short-circuit fault in the phase winding of the generator. The model of the generator, implemented using the winding-function approach, allows the fault to be created online both in a stator and a rotor phase. It has been demonstrated that the magnitude of the current harmonics, used extensively in literature for the Machine Current Signature Analysis (MCSA) technique for winding short-circuit fault detection, is very different when the location of the fault is changed to another coil within the phase winding. This makes the decision on the threshold selection for alarm generation difficult. Furthermore, the control system attenuates the current harmonics by an order of magnitude. This attenuation property is also demonstrated through experiments. The attention is then shifted to the negative- sequence current component, resulting from the winding unbalance, as a possible fault residual. Its suitability is tested in the presence of noise for scenarios with different fault locations, fault severity in terms of the number of shorted-turns and grid voltage unbalance. It is found that due to the presence of a control system the magnitude of the negative-sequence current, resulting from the fault, remains almost the same for all fault locations and fault severity. Thus, it was deemed more suitable as a fault residual. In order to obtain a fast detection method, the Cumulative Sum (CUSUM) algorithm was used. The test function is compared against a threshold, determined on the basis of expected residual magnitude and the time selected for detection, to generate an alarm. The validation is carried out with noise characteristics different from the ones used during the design and it is shown that the voltage unbalance alone is not able to
trigger a false alarm. In all the scenarios considered, the detection was achieved within 40 ms despite the presence of measurement filters.
Acknowledgements
I would like to thank my promoter Prof. Johan Gyselinck for his useful comments on the structure of this thesis report and interest in the project. I am grateful to Prof. Michel Kinnaert for his expert advice on system control and fault detection issues. I would also like to thank the other members of my thesis committee namely Prof. Jean-Claude Maun and Prof. Raymond Hanus for useful comments during the annual project meetings. I am indebted to Prof. Torbjörn Thiringer of Chalmers University of Technology, Sweden for introducing me to the world of wind energy.
The financial support provided by ‘Ministère de la Communauté Française de Belgique – Direction Générale de l’Enseignement Non Obligatoire et de la Recherche Scientifique’ through the Concerted Research Action ARC no. 06/11- 344: ‘Advanced supervision and dependability of complex processes: application to power systems’, and the financial aid of the Belgian Research Fund F.R.S. – FNRS, are gratefully acknowledged.
The cooperation of the staff of the electrical engineering department namely Mme Ariane Ducornez-Germain, Mr Christophe Reyntiens and Mr Pascal Provot deserves to be mentioned here.
I would like to thank my colleagues Alicia Valero, Farid Fodil-Pacha, Jacques Warichet, Manuel Ricardo Gálvez Carrillo and Michaël Hurtgen for good companionship both within and outside of the university. I would especially mention the useful and enjoyable collaboration with Manuel and Farid during laboratory experiments.
I would like to thank my friends Chethan Krishnan, Nathaly Schipper, Santhosh Jayaraju, Sven Van Den Meerssche and Vijay Kumar Verma for memorable times spent together.
Most importantly, I would like to thank my family for their support and especially my wife Maria for being an exceptional companion.
Table of Contents
Page
Abstract iii
Acknowledgements v
Table of Contents vii
Chapter 1 Introduction 1 – 1
1.1 The Wind Power Business……… 1 – 1 1.2 Key Areas of Wind Energy Research……….. 1 – 2 1.3 Faults in Wind Turbine Generator Systems………... 1 – 2 1.4 Condition Monitoring Systems Available in Industry……….. 1 – 4 1.5 Motivation……….. 1 – 5 1.6 Contributions……… 1 – 8 1.7 List of Publications……… 1 – 9 1.8 Structure of the Thesis Report……… 1 – 10
Chapter 2 Wind Turbine Generator Systems 2 – 1 2.1 Wind Energy Extraction………. 2 – 1 2.2 Control Objectives for Wind Turbine Generator Systems………. 2 – 3 2.2.1 Maximum Power Point Tracking Control……… 2 – 3 2.2.1.1 Torque Control Strategy……… 2 – 4 2.2.1.2 Speed Control Strategy………... 2 – 5 2.2.2 Power Control Strategy……….. 2 – 6 2.3 Aerodynamic Power Control………... 2 – 6 2.3.1 Stall Control………... 2 – 6 2.3.2 Pitch Control………. 2 – 7 2.4 Types of Wind Turbine Generator Systems………... 2 – 7 2.4.1 Fixed-Speed WTGS……… 2 – 8 2.4.2 Limited Variable-Speed WTGS………... 2 – 9 2.4.3 Variable-Speed WTGS………... 2 – 10 2.4.3.1 Full-Scale Power Converter based WTGS………. 2 – 11 2.4.3.2 Partial-Scale Power Converter based WTGS……….. 2 – 12 2.5 Improvements for Wind Turbine Generator Systems……….. 2 – 12 2.6 Conclusions………. 2 – 14
Chapter 3 Control of the Doubly-Fed Induction Generator based
Wind Turbine Generator System 3 – 1 3.1 Operating Regions of a DFIG based WTGS……….. 3 – 1 3.2 System Overview……… 3 – 2 3.3 A Note on Model Complexity………... 3 – 3 3.4 Electrical System Components Modelling………... 3 – 5 3.4.1 dq Reference Frame Transformation………. 3 – 5 3.4.2 Induction Machine Model………. 3 – 5 3.4.3 Converter Model………. 3 – 6 3.4.4 DC-link Model………... 3 – 7 3.4.5 Inductance Filter Model………. 3 – 7 3.4.6 Controller Model………. 3 – 8 3.4.7 Phase-Locked Loop………... 3 – 8 3.4.8 Measurement Filter Model………... 3 – 9 3.4.9 Pulse-Width Modulation Strategy………. 3 – 10 3.5 Control Design for the DFIG system………. 3 – 12
3.5.1 Selecting the Angular Velocity and Alignment of the dq
Reference Frame……….. 3 – 12 3.5.2 Selecting the Sampling and Switching Frequency……….. 3 – 13 3.5.3 Grid-Side Converter Control………. 3 – 13 3.5.3.1 The Control Scheme………... 3 – 14 3.5.3.2 Inductance Filter Current Control……… 3 – 14 3.5.3.3 DC-link Voltage Control………... 3 – 15 3.5.4 Rotor-Side Converter Control……….. 3 – 17 3.5.4.1 The Control Scheme………... 3 – 18 3.5.4.2 Rotor Current Control……….. 3 – 18 3.5.4.3 Stator Voltage Control……….. 3 – 19 3.5.4.4 Stator Power Control……… 3 – 20 3.5.4.5 Electromagnetic Torque Control………... 3 – 21 3.6 Comparison of Control Techniques………. 3 – 22 3.6.1 Steady-State Characteristics of the 2 MW DFIG……… 3 – 23 3.6.2 Open-Loop Analysis………... 3 – 25 3.6.3 Closed-Loop Analysis……… 3 – 27
3.6.3.1 Reference Tracking and Disturbance Rejection
for Current Loops……… 3 – 28 3.6.3.2 Power and Torque Control Strategy……… 3 – 30 3.6.3.3 LQG and PI Controller Performance Comparison……… 3 – 31 3.7 Start-Up of the DFIG System………. 3 – 33 3.7.1 Charging the DC-link Capacitor……….. 3 – 34 3.7.2 Activating the Rotor Current Control………. 3 – 36 3.7.2.1 Rotor Position Error Detection and Correction………… 3 – 36 3.7.3 Grid Connection and Power Generation……… 3 – 38 3.7.4 Simulation Results……….. 3 – 39 3.8 Conclusions………. 3 – 41
Chapter 4 Fault Detection in the Doubly-Fed Induction Generator
based Wind Turbine Generator System 4 – 1 4.1 Modelling of the Machine……….. 4 – 1 4.1.1 Winding Functions……… 4 – 1 4.1.2 Mutual Inductance of Coils………... 4 – 4 4.1.3 Specifications of the Modelled Machine………... 4 – 7 4.1.4 Software Code……….. 4 – 8 4.2 Simulation of the Machine………. 4 – 10 4.2.1 State-Space Model………... 4 – 10 4.2.2 Singly-Fed Induction Generator – Model Validation………. 4 – 12 4.2.3 Doubly-Fed Induction Generator……….. 4 – 15 4.2.3.1 Frequency Harmonic Analysis……… 4 – 17 4.2.3.1.1 Stator Faults………... 4 – 17 4.2.3.1.2 Rotor Faults……… 4 – 19 4.2.3.2 Symmetrical Component Analysis……… 4 – 21 4.2.3.3 The CUSUM Algorithm………. 4 – 23 4.2.3.4 Validation………. 4 – 25 4.3 Conclusions………. 4 – 27
Chapter 5 Test Bench for the Doubly-Fed Induction Generator
based Wind Turbine Generator System 5 – 1 5.1 Test Bench Layout………. 5 – 1 5.2 Power Handling Components………. 5 – 3
5.2.1 Back-to-Back Converter………. 5 – 3 5.2.1.1 Component Description………. 5 – 3 5.2.1.1.1 IGBT Modules………... 5 – 3 5.2.1.1.2 IGBT Drivers……….. 5 – 3 5.2.1.1.3 Snubber Capacitors………... 5 – 4 5.2.1.2 Component Dimensioning……… 5 – 4 5.2.2 DC-link Capacitor………... 5 – 5 5.2.2.1 Component Description………. 5 – 5 5.2.2.2 Component Dimensioning……… 5 – 5 5.2.3 Inductance Filter………. 5 – 6 5.2.3.1 Component Description………. 5 – 7 5.2.3.2 Component Dimensioning……… 5 – 7 5.2.4 Emulator Motor………... 5 – 8 5.2.4.1 Component Description………. 5 – 9 5.2.4.2 Component Dimensioning……… 5 – 9 5.2.5 Generator………. 5 – 10 5.2.6 The Encoder……… 5 – 13 5.3 Control Hardware………... 5 – 14 5.3.1 Current Transducers………. 5 – 14 5.3.2 Voltage Transducers………. 5 – 14 5.3.3 DS1104 Rapid Prototyping System……….. 5 – 14 5.3.3.1 Analogue to Digital Converters (ADC)……… 5 – 15 5.3.3.2 Digital to Analogue Converters (DAC)……… 5 – 15 5.3.3.3 Master PPC Digital Input/Output……….. 5 – 15 5.3.3.4 Slave PWM Digital Input/Output………... 5 – 15 5.3.3.5 Incremental Encoder Interface………... 5 – 15 5.3.3.6 Serial Interface……….. 5 – 15 5.3.4 Measurement Filter……… 5 – 15 5.4 Conclusions………. 5 – 17
Chapter 6 Experimental Results 6 – 1
6.1 Parameter Estimation Tests……… 6 – 1 6.1.1 Grid Impedance Estimation………. 6 – 1 6.1.2 Machine Parameter Estimation………. 6 – 2
6.1.3 Converter Delay……….. 6 – 4 6.2 Grid-Side Converter Operation……….. 6 – 8 6.2.1 Reference Tracking……… 6 – 10 6.2.2 Disturbance Rejection……….. 6 – 12 6.2.3 Reactive Power Generation……….. 6 – 14 6.3 Rotor-Side Converter Operation……… 6 – 16
6.3.1 Generation of Stator Voltage and Synchronization with
the Grid……….. 6 – 17 6.3.1.1 Generation of Stator Voltages and Angle Correction……. 6 – 19 6.3.1.2 Stator Voltage Control……….. 6 – 19 6.3.1.3 Harmonic Suppression………. 6 – 19 6.3.2 Stand-Alone Generator Operation……… 6 – 20 6.3.2.1 Frequency Harmonic Analysis……… 6 – 23 6.4 Tests to Determine the Cause of the RSC Problem………. 6 – 24 6.4.1 DFIG Rotor Balance……… 6 – 24 6.4.2 Open-Loop Test……… 6 – 25 6.4.3 Closed-Loop Test………. 6 – 26 6.4.4 Conclusion of the Investigation……….. 6 – 27 6.5 Frequency Harmonic Analysis – Motor Mode……… 6 – 29 6.6 Conclusions………. 6 – 31
Chapter 7 Conclusions and Future Work 7 – 1 7.1 Conclusions……… 7 – 1 7.2 Future Work……….. 7 – 2
References R – 1
Symbols and Abbreviations S – 1
Appendix A dq Reference Frame Transformation A – 1 A.1 Transformation from Three-Phase to dq Currents………... A – 1 A.2 Derivation of the Relation between the DC-Link Voltage and
the Converter Output Voltage in Power Invariant dq Transformation……. A – 3 A.3 Derivation of Expressions for the Active and Reactive
Power in Power Invariant dq Transformation………. A – 4
Appendix B Linearization of the DFIG and Control System Models B – 1 B.1 Principle of Linearization………. B – 1 B.2 Linearization of the DFIG Model……….. B – 2 B.3 Linearization of the Rotor Current Control System………. B – 4 B.4 Linearized Model of the Closed-Loop Rotor Current Control……… B – 5 B.5 Plotting the Response of an Input Output Pair for the Closed-Loop
System………... B – 7
Appendix C, Paper-1 Comparison of Classical and State-Space
Control Techniques of Doubly-Fed Induction Generator
based Wind Energy Conversion Systems C – 1
Appendix C, Paper-2 CUSUM based Fault Detection of Stator Winding Short Circuits in Doubly-Fed Induction
Generator based Wind Energy Conversion Systems C – 11
Appendix C, Paper-3 Effect of Winding Short-Circuit Fault Location and Control Action on Currents in a Doubly-Fed Induction Generator based Wind Energy Conversion
System C – 17
Appendix D User’s Manual – Semiteach based Converter
System for Electrical Machines D – 1
