• unlimited access with print and download
    $ 37 00
  • read full document, no print or download, expires after 72 hours
    $ 4 99
More info
Unlimited access including download and printing, plus availability for reading and annotating in your in your Udini library.
  • Access to this article in your Udini library for 72 hours from purchase.
  • The article will not be available for download or print.
  • Upgrade to the full version of this document at a reduced price.
  • Your trial access payment is credited when purchasing the full version.
Buy
Continue searching

Advanced control techniques for doubly fed induction generator-based wind turbine converters to improve low voltage ride-through during system imbalances

ProQuest Dissertations and Theses, 2009
Dissertation
Author: Murali Mohan Baggu
Abstract:
A doubly-fed induction generator (DFIG) applied to wind power generation is under study for low voltage ride-through application during system disturbances. Conventional dq axis current control using voltage source converters for both the grid side and the rotor side of the DFIG are analyzed and simulated. DFIG operation is investigated under balanced and unbalanced system disturbances. A conventional d-axis and q-axis control applied to a voltage source converter (VSC) during a system imbalance exhibits oscillations in the stiff DC link voltage as well as in the real and reactive powers of the converter. Multiple advanced control methods are explored and compared for imbalance operations. An advanced control technique utilizing both positive and negative sequence domain is evaluated. The approach demonstrates the stabilization of the DC link voltage to a greater extent during a disturbance but is more sluggish than the conventional control. An innovative control strategy that employs the technique of direct power control (DPC) is also investigated. This control achieves real and reactive power stability with simple active and reactive power control variables replacing the current control loops in the conventional case. A modified DPC algorithm is proposed to eliminate the current harmonics created by DPC during system disturbances. The DPC is further extended to the rotor-side converter of the DFIG thus controlling the complete system using this technique. The DPC is implemented using a three-phase converter designed on a PCB using Eagle ® . A Texas Instruments ® TMS320F2812 DSP is used to implement the control algorithm. The converter is tested for ride through capability using an industrial power corruptor. The results are compared to the simulation results for compliance with standard grid codes.

V TABLE OF CONTENTS Page ABSTRACT Hi ACKNOWLEDGMENTS iv LIST OF ILLUSTRATIONS viii LIST OF TABLES xii NOMENCLATURE xiii SECTION 1. INTRODUCTION 1 1.1 INTRODUCTION 1 1.2 DISSERTATION OUTLINE 3 1.3 GRID CODES 4 1.4 LITERATURE REVIEW 8 1.5 SUMMARY 11 2. CONVENTIONAL DFIG CONTROL AND OPERATION DURING GRID DISTURBANCES 12 2.1 INTRODUCTION 12 2.2 CONVENTIONAL DECOUPLED CONTROL 13 2.3 DFIG AND ROTOR-SIDE CONVERTER CONTROL 21 2.4 DFIG MODEL USING DIgSILENT® 27 2.5 SUMMARY 33 3. ANALYSIS OF UNBALANCED INPUTS AND SEQUENCE CONTROLLERS 34 3.1 INTRODUCTION 34 3.2 SEQUENCE COMPONENTS D-Q ANALYSIS 36

vi 3.3 SEQUENCE CONTROLLERS SIMULATION AND SYSTEM IMBALANCE 39 3.4 SUMMARY 47 4. DIRECT POWER CONTROL FOR DFIG-BASED WIND TURBINES 48 4.1 INTRODUCTION 48 4.2 DPC SWITCHING STATES AND DELTA MODULATION 49 4.3 SIMULATION OF VOLTAGE SOURCE CONVERTER USING DPC 54 4.4 MODIFIED DIRECT POWER CONTROL 60 4.5 DPC BASED ROTOR SIDE CONTROL 64 4.6 SUMMARY 74 5. HARDWARE IMPLEMENTATION OF DPC 75 5.1 INTRODUCTION 75 5.2 PCB DESIGN 76 5.3 2812 DSP ARCHITECTURE AND PROGRAMMING 81 5.4 INDUSTRIAL POWER CORUPTOR (IPC) 84 5.5 DPC IMPLEMENTATION AND RESULTS 85 5.6 SUMMARY 100 6. CONCLUSIONS AND FUTURE WORK 101 6.1 SUMMARY AND CONCLUSIONS 101 6.2 AUTHOR'S CONTRIBUTION 104 6.3 FUTURE WORK 104 APPENDICES A. SIMULATION AND PRACTICAL IMPLEMENTATION PARAMETERS 107 B. DESIGN LAYOUT AND CIRCUIT DIAGRAMS 110 C. DSP CODE FOR PRACTICAL IMPLEMENTATION OF DPC 113

vii BIBLIOGRAPHY 126 VITA 131

viii LIST OF ILLUSTRATIONS Figure Page 1.1. LVRT requirements for emergency low voltage per FERC order No. 661 [3] 5 1.2. LVRT requirements at the point of intersection FERC Order 661 A [5] 6 1.3. Definition of Fault ride through (FRT) requirement in Germany [6] 7 2.1. DFIG model 12 2.2. PWM converter 14 2.3. Conventional current control block diagram for grid-side converter 16 2.4. PLECS® Circuit diagram for grid-side converter 17 2.5. Simulation of a three phase fault 18 2.6. Simulation of a three phase fault 19 2.7. Simulation of a single line to ground fault 20 2.8. Simulation of a single line to ground fault 21 2.9. Rotor side converter and induction machine implementation 24 2.10. DFIG operation above rated speed 25 2.11. Rotor Side converter at rated speed 26 2.12. DFIG Model in DIgSILENT® 28 2.13. Normal Operation 29 2.14. Simulation of a SLG fault 31 2.15. Simulation of a three phase fault 32 3.1. Sequence components and park's co-ordinates during a voltage imbalance 34 3.2. Voltage vs. dq components for a voltage disturbance 35 3.3. Block diagram of the VSC controller using sequence controllers 41

ix 3.4. Voltage imbalance simulation for conventional controller 42 3.5. Voltage imbalance simulation for sequence controllers 42 3.6. Voltage imbalance simulation for conventional controller 43 3.7. Voltage imbalance simulation for sequence controller 44 3.8. Single line fault simulation for sequence controllers 45 3.9. Single line fault simulation for sequence controller 46 4.1. Single line diagram of the PWM converter 49 4.2. PWM Converter switching states 50 4.3. Voltage vector plane divided into 12 sectors 50 4.4. DPC structure for voltage source converter 55 4.5. VSC using DPC for a voltage dip of 60% 56 4.6. Simulation of the VSC using DPC for a voltage dip of 60% 56 4.7. Simulation of conventional controller for voltage dip of 60% 57 4.8. Voltage imbalance simulation of VSC for DPC 58 4.9. Voltage imbalance simulation of VSC using DPC 58 4.10. Simulation of the voltage source converter using DPC for at 5 KHz 60 4.11. Partial simulation diagram for the modified DPC showing unbalanced powers.... 62 4.12. Simulation of the modified DPC for voltage dip of 60% 63 4.13. Simulation of the modified DPC for voltage dip of 60% 63 4.14. Stator flux vector plane divided into six sectors 66 4.15. DFIG circuit diagram including the back-to-back converters 67 4.16. DPC structure of the grid side and rotor side converter of DFIG 68 4.17. DFIG simulation results 69 4.18. DFIG simulation results 71

X 4.19. DFIG simulation results 71 4.20. DFIG simulation results 72 4.21. DFIG simulation results 72 4.22. DFIG simulation results 73 5.1. Circuit diagram of the converter for practical implementation 75 5.2. Schematic for the IGBT bridge module using Eagle® 76 5.3. Schematic of the converter designed in Eagle® 78 5.4. Mounted PCB of the three phase converter 79 5.5. Circuit to convert gate outputs to open collector configuration 80 5.6. Phase A voltage and DC link voltage sensed by the ADC 82 5.7. Theta and sector values calculated by the ADC 82 5.8. Front view of the IPC 85 5.9. Picture of the experimental setup 87 5.10. Experimental results for normal operation 88 5.11. Experimental results for normal operation 89 5.12. Real and reactive powers for normal operation 89 5.13. Simulated result for normal operation 90 5.14. Experimental result for 70% voltage for 2 seconds 91 5.15. Experimental results for 70% voltage for 2 seconds 91 5.16. Real and reactive powers for 70% voltage for 2 seconds 92 5.17. Simulated result for 70% voltage starting at 0.3 s 92 5.18. Experimental result of 50% voltage for 12 cycles 93 5.19. Experimental results for 50% voltage for 12 cycle starting at 0 s 94 5.20. Real and reactive powers for 50% voltage for 12 cycles 94

xi 5.21. Simulated result for 50% voltage starting at 0.3 s 95 5.22. Experimental result of 30% voltage for 3 cycles 96 5.23. Experimental results for 30% voltage for 3 cycles starting at 0 s 96 5.24. Real and reactive powers for 30% voltage for 3 cycles 97 5.25. Simulated result for 30% voltage starting at 0.3 s 97 5.26. Experimental result of 50% voltage for 1 second 99

XII LIST OF TABLES Table Page 4.1. Optimal switching table for voltage source converter 52 4.2. Comparison of ripple in DC link Voltage, real power and reactive power for voltage imbalance 59 4.3. Comparison of ripple in DC link Voltage, real power and reactive power for 60% voltage dip 64 4.4. Optimal switching table for rotor side converter of DFIG 66 4.5. Stator, rotor and total real and reactive powers for subsynchronous and supersynchronous operations 70 5.1. Compare registers values of voltage vectors 84 5.2. Comparison of ripple in DC link Voltage, real power and reactive power for different grid code events 98 6.1. Summary of the performance of different controllers during system disturbance 103

NOMENCLATURE XIII Symbol lor Ios E R L Va vb Vc la lb Ic Val Vbl Vcl n vd Vq id Description Rotor side current Stator side current DC link voltage Series line resistance Series line inductance A phase line to neutral voltage B phase line to neutral voltage C phase line to neutral voltage A phase line current B phase line current C phase line current A phase line to neutral converter voltage B phase line to neutral converter voltage C phase line to neutral converter voltage Neutral point d-axis voltage q-axis voltage d-axis current lq C0r q-axis current Electrical synchronous speed Electrical rotor speed

xi v Qsiip Electrical slip speed 6e Angul ar position of t he vol tage va Stati onary reference f rame alpha vol tage vp Stati onary reference f rame beta vol tage Xs Stator fl ux linkages X& Stator d-axis fl ux linkages U Mutual i nductance per phase U Stator leakage i nductance per phase Lr Rotor leakage i nductance per phase Lm Magnet i zi ng i nduct ance per phase i ms Stator magneti zi ng current ids Stator d-axis current idr Rotor d-axis current iqs Stator q-axis current iqr Rotor q-axis current vdr Rotor d-axis vol tage vqr Rotor q-axis vol tage a Leakage coeffi ci ent of i nducti on machi ne Rs Stator resistance Rr Rotor resistance Te Ai r gap or el ectromechani cal t orque Xas Flux linkage of alpha phase in stati onary reference f rame Xfc Flux l i nkage of beta phase in stati onary reference f rame

XV vas Stator alpha vol tage in stati onary reference f rame v ^ Stator beta vol tage in stati onary reference f rame l<*s Stator al pha phase current p" Stator beta phase current Xa A phase component s (vol tage/current) Xb B phase component s (vol tage/current) Xc C phase components (vol tage/current) xmp Positive sequence components (vol tage/current) xmn Negative sequence component (vol tage/current) xmo Zero sequence component (Vol tage/current) ap Positive sequence angl e an Negative sequence angl e a0 Zero sequence angl e Xde d-axis component synchronous reference f rame Xqe q-axis component synchronous reference f rame Xdpe Positive sequence d-axis component synchronous reference f rame Xqpe Positive sequence q-axis component synchronous reference f rame Xdne Negative sequence d-axis component synchronous reference f rame Xqne Negative sequence q-axis component synchronous reference f rame T32 A, B and C phase t o dq transformati on Xabc A, B and C phase component s Xqdpe q-axis sequence component s in positive synchronous reference f rame Xqdne d-axis sequence component s in negati ve synchronous reference f rame

XVI R(cot) Ref erence f r ame t r ansf or mat i on vdpe Posi ti ve sequence d-axi s synchronous ref erence f r ame vol t age vqPe Posi ti ve sequence q-axi s synchronous ref erence f r ame vol t age vdne Negat i ve sequence d-axi s synchronous ref erence f r ame vol t age Vqne Negat i ve sequence q-axi s synchronous ref erence f r ame vol t age idpe Posi ti ve sequence d-axi s synchronous ref erence f r ame cur r ent iqpe Posi ti ve sequence q-axi s synchronous ref erence f r ame cur r ent i dne Negat i ve sequence d-axi s synchronous ref erence f r ame cur r ent iqne Negat i ve sequence q-axi s synchronous ref erence f r ame cur r ent P Real power P0 Average real power Pc2 Real power cosi ne t er m coef f i ci ent Ps2 Real power si ne t er m coef f i ci ent Q Reacti ve power Qo Average reacti ve power Qc2 Reacti ve power cosi ne coef f i ci ent QS2 Reacti ve power si ne coef f i ci ent Vaibici Convert er A, B and C phase t o neut ral vol t age iabc A, B and C phase source current s Vabc A, B and C phase t o neut ral vol t ages U0 t o U7 space vect ors V source phase vol t age Vi conver t er phase vol t age XL Li ne i nduct ance

Angle between source voltage and converter voltage Real power due to system imbalance Reactive power due to system imbalance digitalized real power output digitalized reactive power output Real power reference Reactive power reference 6 vector position

1. INTRODUCTION 1.1 INTRODUCTION Wind energy has gained popularity in recent years all over the world, mainly because wind energy is renewable and eco-friendly. There are, however, many challenges associated with harnessing this type of energy for grid application mostly due to its intermittent nature. This dissertation addresses some of the problems associated with the grid integration aspect of wind energy. Generally, wind power generation uses either fixed speed or variable speed turbines which are classified in to four major types by the Western Electricity Coordinating Council (WECC) to develop industry-standard wind turbine models [1]. These types are briefly described below: 1. Pitch regulated squirrel cage induction generator directly coupled to the grid. 2. Variable slip squirrel cage induction generator directly coupled to the grid. 3. Wound rotor induction generator with an AC/DC/AC power converter connected between the rotor terminals and grid and is pitch regulated. 4. Synchronous or asynchronous machine connected to the grid using full rated AC/DC/AC power converter and is pitch regulated. Type I and II use the simplest topology where the turbine is directly connected to the grid. This type of machine needs a switch to prevent motoring operation during low wind speeds and also suffers a major drawback of reactive power consumption as there is no reactive power control. Type II uses an electronically modulated rotor resistance to effect dynamic changes in the machine torque speed characteristics. Type III is the most common for high power grid applications. This topology allows the complete control of reactive power using rotor and grid side converters and around 25% of the

2 speed control using rotor side converter. The converters used for this topology need to be rated at only 25% to 30% of overall machine rating as they are used on the rotor side to supply the slip power which is 25% to 30% of the overall machine power. Type IV topology uses a full rated converter with either a synchronous or an asynchronous machine. This type of topology has full control of the reactive power and speed. The major advantages associated with Type III wind turbines is that it has four quadrant active and reactive power flow capabilities, with constant frequency power output and low power converter ratings making it the most widely used topology. Hence this dissertation focuses attention on the integration of Doubly Fed Induction Generator (DFIG) - Type III based wind turbines to the power grid. The major issues concerning integration of DFIG wind turbines are that the best wind regimes are located in rural areas and the power grid in these areas is often weak and prone to voltage sags, faults and unbalances. Unbalanced grid voltages can cause many problems for DFIG wind turbines such as torque pulsations, unbalanced currents, and reactive power pulsations. These problems may be attributed to the instability in the power converter's DC link voltage during system disturbances. This issue with the DC link voltage is mainly due to the distorted control in the back to back PWM converters during system unbalance. A review of the pertinent literature reveals several papers addressing the effect of unbalance associated with the DFIG and rotor side converter. This is discussed later in the literature review. The effect of unbalance on the grid side converter is studied in detail in this dissertation and possible remedies for voltage ride through during disturbances are addressed.

3 1.2 DISSERTATION OUTLINE The later part of this section deals with major codes followed by US and European grid networks during grid disturbances. A brief literature review of the existing remedies associated with the PWM converters that can be possibly adapted for DFIG based wind turbine generators is also presented in this section. The detailed operation of DFIG control using the conventional current control techniques are discussed in section two using a mathematical model in Matlab Simulink® and also with a more detailed model in DIgSILENT®. The effect of grid imbalances are also analyzed using these models. The effect of unbalance on the grid side converter is analyzed in detail in section three. An advanced control technique using sequence controllers to stabilize the operation of the grid-side converter during grid imbalance is also presented in detail in this section. A novel control technique using direct active and reactive power control called Direct Power Control (DPC) is discussed in section four. This controller eliminates the conventional current loops and uses delta modulation comparators, hence has faster response. The switching of the converter is done using a simple optimum switching table. A Grid-side controller is simulated and the results are compared with the conventional and sequence controllers. A complete DFIG including the rotor side and grid-side converters is simulated using DPC and is tested for ride through during system and grid disturbances. Section five discusses the hardware implementation of a grid side converter using DPC. A 2kW converter is designed using Eagle® for PCB design. The control is coded in C and implemented in TMS320F2812 DSP. The converter using DPC is tested

4 for system unbalance conditions created by an Industrial Power Corrupter (IPC) [2] in the laboratory. The laboratory results are compared with the simulation results for accuracy errors and verification. The concluding remarks and summary of the work along with some future developments are discussed in section six. The Appendix presents the system parameters, circuit diagrams and the C code used for hardware implementation. 1.3 GRID CODES Grid codes determine the requirement of the wind power systems to stay connected to the grid during grid disturbances. The codes are typically described on a time voltage diagram as in Figure 1.1, Figure 1.2 and Figure 1.3. These diagrams contain safe limits or the border of operation of wind power plants. The system should remain connected during the fault inception and post fault recovery time according to the border limits and the times mentioned in the grid code diagrams. According to the Low Voltage Ride Through (LVRT) requirement for wind generation facilities per Federal Energy Regulatory Commission (FERC) order No. 661 [3] the grid code standard requires that the machine stay connected for emergency low voltages at the terminals for as low as 15% of nominal per unit voltage for approximately 0.6 seconds. This time increases as the terminal voltage requirement increases. For example during the post-fault recovery, the wind power system should be able to withstand a low voltage of 60% at the terminal for a time period of two seconds as shown in Figure 1.1. In practice, the high voltage drop during grid disturbance gives rise to rotor protection which acts to short circuit the dc link capacitor (via a crowbar) in order to protect the converter and the associated components from high currents [4].

5 o = o g> °- £:- g> c as o > .ffi 12 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -1.0 Minimum Required Wind Plant Response to Emergency Low Voltage | Beginning of Emergency Low Volt^ne <** #* Wind Ham Required to Remain Online "Wind Plant Not Required to Remain unime 0.0 0.625 1.0 2.0 3.0 4.0 Time (s) * Per Unit = Ratio of Actual to Nominal Voltage Figure 1.1. LVRT requirements for emergency low voltage per FERC order No. 661 [3] The North American Electrical Reliability Corporation (NERC) and American Wind Energy Association on behalf of FERC had discussions to resolve the issues with FERC order 661 [5]. According to these discussion, "the new language stated that wind generating plants are required to remain in service during three phase faults with normal clearing (which is a time period of approximately four to nine cycles) and single line to ground faults with delayed clearing, and subsequent postfau/t voltage recovery to prefault voltage unless clearing the fault effectively disconnects the generator from the system" [5]. According to this new practice the old voltage versus time curve is no longer applied and the code is to go with post-fault clearing time as show in Figure 1.2.

6 •I r> u.) ri. o CD I 0.8- lnt« > 2 aB" c o CD 0.4" <5 | 0 2 - > ft - .»-:«^. s • //1m / / HITPlant Must Be D e s'9 n e d to >/ ^ ^ • $ B r" Remain Online During Posl- / / ^ Disturbance Recovery Period; \/ Specific Requirements Will Depend on System Characteristics. y Fault Cleared Normally / Within Nine Cycles {150 ms) / U T T — 1 1 1 1 - 1 /0 1 2 3 4 Begi nni ng—' Time (s) of Fault Figure 1.2. LVRT requirements at the point of intersection FERC Order 661 A [5] Fault Ride Through (FRT) requirements of Germany are based on the time voltage diagram shown in Figure 1.3 [6]. These requirements state that the wind turbine has to connect to the grid within areas 1 and 2. Possible Short Term Interruption (STI) of less than 2 seconds can occur within area two due to over load or stability concerns or other kinds of technical problems. Short disconnection of the turbine is allowed in area 3 whereas resynchronization of the turbine is necessary after two seconds. Step wise tripping of the wind turbine is allowed in area 4.

7 Maximum L-L Voltage UtUN 100%' 99%- 70%- 45% 0%' No Tripping i 1 : No Tripping r"\ ' 2 Possibly STI -.J.J ^^i - « « • • **" - ^% • * * ^v y- • m w r - T -*-^i • ^ i * i # * * * -, Stepwise Tripping by System .•.•.•.".".•.•." *• Automatic Safeguard II • *'!*!- I' !v!*! .* after 1.5 ... 2.4s ".•.'.-.*.V.-. m * * » • * * m i ft t> /^. STI Resynehronization " • • * a J Before Primary Cont r ol 1.-.•.-.*.-.".".*.'.•.*.*.*,•.*.*.•..*. 0 150 + s|s 700 1,500 Time fins) 3,000 Figure 1.3. Definition of Fault ride through (FRT) requirement in Germany [6] Looking at different grid codes, it can be concluded that DFIG controllers should be capable of overcoming temporary voltage disturbances. Hence, this work deals with the design of controllers that would keep the wind turbine in stable operation during a fault causing a low voltage of up to 40% for two seconds. This type of fault is of importance because the rotor need not be protected by crowbar during this time and the back-to-back PWM converters work normally even with the unbalance. The proposed controllers are tested for different fault conditions for ride through on the grid side as well as the turbine side.

8 1.4 LITERATURE REVIEW Advanced control techniques for power converter control are reviewed in this literature review. Some of the controllers are adopted for implementation with DFIG to counter system imbalances. The control techniques are broadly divided into three major methods based on their operation. The first type is based on decomposing the system voltage and currents into symmetrical components and using different controllers in positive and negative sequence components [7] through [13]. Enjeti and Choudhury [7] propose a technique which counter balances the gating signals so that the imbalance in the supply is rectified. The technique is a feed-forward approach where the sequence components of the unbalanced input supply are calculated and are used to counter-unbalance the PWM gating signals of the converter switches to cancel the generated abnormal harmonics. This method is suitable for high power GTO-type PWM ac to dc converters. Rioual et al. [8] propose a cascade regulation of PWM rectifier in the Park's d-q frame. The system is distributed with unbalanced voltage; the regulation is calculated with a positive sequence network voltage as well as a negative sequence network voltage separately. This control achieves desired positive and negative sequence current commands for a constant dc link voltage and average reactive power. Kim et al. [9] propose a PWM converter where decomposition into symmetrical components of input voltage is needed. The negative sequence current component is commanded to be zero. It is observed that negative sequence is transformed into the second harmonic component on the synchronous reference frame and the positive sequence can be detected using a band pass filter with center frequency of 120 Hz.

9 Seok and Nam [10] propose a dual converter scheme where two reference frames are used for measuring the positive sequence in the positive synchronous reference frame (SRF) by eliminating the negative sequence with the 120 Hz and negative sequence in negative SRF. Separately measured currents are used for two feedback PI controllers called as the dual current controller. One regulates the positive sequence current and the other regulates the negative sequence current thus allowing the control of the negative sequence in its own frame. Giuseppe et al. [11] propose two different controllers to deal with unbalanced conditions. The first controller is implemented in the positive synchronous reference frame. Active and reactive currents are controlled independently of each other with a high bandwidth. The second controller uses a feed forward approach for sequence separation. The DC link voltage controller used in this case is the same as that for the first controller. A third controller similar to the one proposed in [10] is considered; however the transient performance of the controller is not analyzed in [10]. Lie Xu et al. [12] proposed the use of a main controller using decoupled control without positive and negative sequence decomposition and an auxiliary controller in negative sequence with inclusion of cross coupling for negative sequence components. Lie Xu [13] Proposed complete modeling of the DFIG in positive and negative sequence to deal with network disturbances. The second method proposed by Slavomir et al. [14-15] discusses a finite element modeling (FEM) of the magnetic field in the generator in two dimensions. This model takes into account magnetic saturation of the stator and the rotor leakage inductances. The large difference from the conventional model is that, during an

10 unsymmetrical fault, the FEM model represents unsymmetrical magnetic saturation better than the conventional analytical models. The third method is based on a novel control technique that directly controls the active and reactive power of the systems. This method more suits the controllers used to connect the utility as this directly achieves the unity power factor requirement of the utility. References [16] to [18] discuss the concept as applied to the grid side and rotor side converters of the DFIG. In references [19] and [20], the authors modify the control and the switching table to accommodate for system disturbance and include the power due to imbalance. Noguchi et al. [16] propose direct power of a three-phase Pulse Width Modulated (PWM) converter without line voltage sensors. In this control the real and reactive powers are calculated as the sum of power consumed by the inductance and the converter. The voltage angle is calculated from the powers and the currents. Mariusz et al. [17] propose direct power control of three-phase pulse width- modulated rectifiers without line voltage sensors. The control is based on assuming the three phase grid as a virtual induction machine and the flux in the virtual machine is defined as virtual flux (VF). It is shown that the VF-DPC exhibits several advantages, particularly providing sinusoidal line current when the supply voltage is not ideal. Rajib Datta et al. [18] proposed a method of direct decoupled control of active and reactive powers. The algorithm extends the switching logic of Direct Torque Control (DTC) to the rotor side of a doubly-fed wound rotor induction machine. The direct power control algorithm uses only stator quantities for active and reactive power measurements and is inherently position-sensorless. It is computationally simple and

Full document contains 150 pages
Abstract: A doubly-fed induction generator (DFIG) applied to wind power generation is under study for low voltage ride-through application during system disturbances. Conventional dq axis current control using voltage source converters for both the grid side and the rotor side of the DFIG are analyzed and simulated. DFIG operation is investigated under balanced and unbalanced system disturbances. A conventional d-axis and q-axis control applied to a voltage source converter (VSC) during a system imbalance exhibits oscillations in the stiff DC link voltage as well as in the real and reactive powers of the converter. Multiple advanced control methods are explored and compared for imbalance operations. An advanced control technique utilizing both positive and negative sequence domain is evaluated. The approach demonstrates the stabilization of the DC link voltage to a greater extent during a disturbance but is more sluggish than the conventional control. An innovative control strategy that employs the technique of direct power control (DPC) is also investigated. This control achieves real and reactive power stability with simple active and reactive power control variables replacing the current control loops in the conventional case. A modified DPC algorithm is proposed to eliminate the current harmonics created by DPC during system disturbances. The DPC is further extended to the rotor-side converter of the DFIG thus controlling the complete system using this technique. The DPC is implemented using a three-phase converter designed on a PCB using Eagle ® . A Texas Instruments ® TMS320F2812 DSP is used to implement the control algorithm. The converter is tested for ride through capability using an industrial power corruptor. The results are compared to the simulation results for compliance with standard grid codes.