CALCULATION OF THD IN VECTOR CONTROLLED

CALCULATION OF THD IN VECTOR CONTROLLED IGBT fed DOUBLY FED INDUCTION GENERATOR Project Submitted for the partial fulfilment of the requirement for the award of the degree of Bachelor of Technology in ELECTRICAL ENGINEERING Dr. A.P.J. Abdul Kalam Technical University, Lucknow Submitted by Ayush Gupta Rohit Tiwari Mirnank Gupta Ankitesh Bhragudev

1409120036 1409120087 1409120062 1409120022

Under the Guidance of Mr. Amit Kumar Roy Assistant Professor Electrical Engineering Department

2017-18 JSS MAHAVIDYAPEETHA JSS ACADEMY OF TECHNICAL EDUCATION, NOIDA DEPARTMENT OF ELECTRICAL ENGINEERING C-20/1, SECTOR – 62, NOIDA- 201301

DECLARATION I hereby declare that this submission is my own work and that, to the best of my knowledge and belief, it contains no material previously published or written by another person nor material which to a substantial extent has been accepted for the award of any other degree or diploma of the university or other institute of higher learning except where due acknowledgement has been made in the text.

Signature of Student: Student’s Name: Ayush Gupta Roll No: 1409120036 Date:

Signature of Student: Student’s Name: Rohit Tiwari Roll No: 1409120087 Date:

Signature of Student: Student’s Name: Mirnank Gupta Roll No: 1409120062 Date:

Signature of Student: Student’s Name: Ankitesh Bhragudev Roll No: 1409120022 Date:

i

CERTIFICATE This is to certify that the Project entitled ―Calculation of THD in Vector controlled IGBT fed Doubly fed Induction Generator‖ which is submitted by Ayush Gupta, Rohit Tiwari, Mirnank Gupta, Ankitesh Bhragudev in partial fulfillment of the requirement for the award of degree B. Tech in Department of Electrical Engineering of J.S.S Academy of Technical Education, Noida, affiliated to Dr. A.P.J. Abdul Kalam Technical University, Lucknow is a record of the candidates own work carried out by them under my supervision. The matter embodied in this thesis is original and has not been submitted for the award of any other degree.

Mr. Amit Kumar Roy

Dr. Md. Abul Kalam

Assistant Professor,

In-charge HOD & Associate Professor

Department of Electrical Engineering,

Department of Electrical Engineering,

JSSATEN

JSSATEN

Date:

Date:

Place: NOIDA

Place: NOIDA

ii

ACKNOWLEDGEMENT We would like to articulate our sincere gratitude towards all those who have contributed their precious time and helped us along in our project work. Without them it would have been a tough job to complete and understand this project work. We would especially like to thank Mr. Amit Kumar Roy, Assistant Professor, Electrical Engineering Department, JSSATEN and our project supervisor for his firm support and guidance and invaluable suggestions throughout the project work. We express our greatest appreciation to Dr. Md. Abul Kalam, HOD- In charge, Electrical Engineering Department, JSSATEN for the academic support throughout the course of this project work. Further, we express our indebtedness to all the faculty members and staff of the Department of Electrical Engineering, for their guidance and effort at appropriate times which has helped us a lot.

iii

ABSTRACT The project deals with the modelling and control of a doubly-fed induction generator (DFIG) based wind energy conversion system (WECS) operating in grid connected mode. A detailed modelling of the wind turbine, wind generator, converters used for the rotor side and stator side is performed. The proposed WECS is capable of working under variable wind speed. In order to efficiently transfer the wind power to the grid, back to back converter topology is utilised and the control of rotor side converter (RSC) and grid side converter (GSC) is performed in the synchronous reference frame coordinates. The function of the RSC is to capture the maximum power from the wind whereas the GSC transfers the electrical power generated at the stator to the grid. The WECS is interconnected with linear loads and with the utility grid at the point of common coupling (PCC). The objective of the project also lies to investigate the power quality aspects involved in a WECS, especially the poor current quality at the PCC due to the unwanted harmonics generated from the GSC. Total harmonic distortion(THD) analysis for the PCC current wave forms is also performed extensively and the role of LC filter to improve the current quality to in the WECS system is thoroughly showcased for various case scenarios. The vector control technique is used to generate the reference signals for RSC and GSC where the pulses are generated by comparing the reference and measured values of stator current and rotor currents which are fed to the IGBT controlled inverter to get controlled AC at the output of GSC. The performance of the entire DFIG based WECS is showcased by considering different case studies consisting of constant and variable wind speed and loading conditions. The entire system is simulated in Matlab/Simulink simulation environment where the critical performance parameters like DC link voltage, generator torque, DFIG stator current, active and reactive power delivered by the DFIG to the grid, PCC side voltage/ current profile and the PCC current THD are rigorously monitored, presented and discussed for each case studies.

iv

TABLE OF CONTENT DECLARATION

i

CERTIFICATE

ii

ACKNOWLEDGEMENT

iii

ABSTRACT

iv

LIST OF FIGURES

vii

LIST OF TABLES

ix

LIST OF ABBREVIATIONS

x

CHAPTER 1 INTRODUCTION

1

1.1

MOTIVATION

1

1.1.1

Advantages of using wind energy:

1

1.1.2

Relevant terms

1

1.2

LITERATURE REVIEW:

2

1.3

PROJECT OBJECTIVE:

3

1.4

ORGANIZATION OF THESIS:

4

CHAPTER 2 WIND TURBINES AND GENERATOR

5

2.1

WIND TURBINES

5

2.2

TYPES OF WIND TURBINES

6

2.2.1

Horizontal axis Wind Turbine

6

2.2.2

Vertical axis wind turbine

7

2.3

WIND GENERATORS

8

2.3.1

Synchronous Generator

8

2.3.2

Induction Generator

8

2.3.3

DFIG (doubly fed induction generator)

10

2.3.4

PMSG (Permanent magnet synchronous generator)

11

2.3.5

SCIG (Squirrel cage induction generator):

14

CHAPTER 3 CONVERTERS IN WIND ENERGY GENERATION SYSTEM 3.1

STRATEGY OF DFIG:

22

3.1.1

Pitch control

22

3.1.2

Scalar control

22

3.1.3

Direct torque control

23

3.1.4

Vector control

23 v

3.2

dq AXIS TRANSFORMATION (REFERENCE FRAME THEORY) 24 3.3

METHODOLOGY

25

3.3.1

GDC: Grid Side Converter

26

3.3.2

RSC:Rotor Side Converter

27

PULSE WIDTH MODULATION (PWM)

29

3.4

CHAPTER 4 RESULTS AND DISCUSSIONS

31

4.1

PROPOSED SYSTEM SCHEMATIC

31

4.2

SYSTEM COMPONENTS

31

4.3

SYSTEM RATING AND PARAMETERS

32

4.4

SIMULATION MODEL AND RESULTS

33

4.4.1

Steady State condition

33

4.4.2

No Wind Fluctuation

34

4.4.3

Under wind speed variations: Wind as step signal

37

4.4.4

Under wind speed variations: Wind as real time signal 41

4.4.5

Under load Variations: Only Load is Variable

45

4.4.6

Under load Variations: Load and wind is Variable

49

4.5

RESULT ANALYSIS

55

5.1

CONCLUSION

57

5.2

FUTURE SCOPE

58

REFERENCES

60

CHAPTER 5

vi

LIST OF FIGURES Fig 2.1: Schematic diagram of Horizontal axis and Vertical axis Wind turbine

6

Fig 2.2: Schematic diagram of synchronous generator in WECS

8

Fig 2.3: Schematic diagram of induction generator in WECS

9

Fig 2.4: Schematic diagram of DFIG in wind power applications

11

Fig 2.5: Schematic diagram of PMSG

12

Fig 2.6: Schematic diagram of rotor field rotation in PMSG

13

Fig 2.7: Schematic diagram showing field interaction in PMSG

13

Fig 2.8: Phasor diagram of PMSG

14

Fig 2.9: Schematic diagram of SCIG

15

Fig 2.10: Stator Circuit

16

Fig 2.11: Rotor Circuit at slip frequency

16

Fig 2.12: Rotor Circuit at Stator Frequency

17

Fig 2.13: Complete Equivalent Circuit (at Stator Frequency)

17

Fig 2.14: Torque-Speed Characteristics Curve for varying external resistance

18

Fig 2.15: Rotor Injection

19

Fig 2.16: Simplified Equivalent circuit to find I2

20

Fig 2.17: Comparison of various generators

21

Fig-3.1: Stator flux oriented reference frame

23

Fig 3.2: Schematic diagram of Vector Control technique

24

Fig 3.3: Axis in dq frame

25

Fig 3.4: Cp v/s λ curve for different values of β

28

Fig 3.5: Sinusoidal Pulse Width Modulation

29

Fig 4.1: Schematic diagram of Grid connected DFIG

31

Fig 4.2: Simulation Model of DFIG with GSC and RSC

33

Fig 4.3: DC Link voltage

34

Fig 4.4: Power consumed and delivered by DFIG

34

Fig 4.5: DFIG characteristics

35

Fig 4.6: Grid side voltage and current (actual values)

36

Fig 4.7: Grid side voltage and current (pu)

36

Fig 4.8: THD in GSC current

37

Fig 4.9: Wind input to the turbine

37

Fig 4.10: DC Link voltage

38 vii

Fig 4.11: Power Consumed and Delivered by DFIG

38

Fig 4.12: DFIG characteristics

39

Fig 4.13: Grid side voltage and current (actual values)

40

Fig 4.14: Grid side voltage and current (pu)

40

Fig 4.15: THD in GSC current

41

Fig 4.16: Wind input to the turbine

41

Fig 4.17: DC Link voltage

42

Fig 4.18: Power Consumed and Delivered by DFIG

42

Fig 4.19: DFIG characteristics

43

Fig 4.20: Grid side voltage and current (actual values)

44

Fig 4.21: Grid side voltage and current (pu)

44

Fig 4.22: THD in GSC current

45

Fig 4.23: DC Link voltage

45

Fig 4.24: DFIG Characteristics

46

Fig 4.25: Power Consumed and Delivered by DFIG

46

Fig 4.26: Grid side voltage and current (actual values)

47

Fig 4.27: Grid side voltage and current (pu)

47

Fig 4.28: THD in GSC current

48

Fig 4.29: Simulation model of DFIG with variable load and wind

49

Fig 4.30: Wind input to the turbine

50

Fig 4.31: DC link voltage

50

Fig 4.32: Circuit breaker parameters load parameters

51

Fig 4.33: Power Consumed and Delivered by DFIG

51

Fig 4.34: DFIG characteristics

52

Fig 4.35: Grid side voltage and current (actual values)

53

Fig 4.36: Grid side voltage and current (pu)

53

Fig 4.37: THD in GSC current

54

viii

LIST OF TABLES Table 4.1: System parameter and rating

33

ix

LIST OF ABBREVIATIONS TSR

Tip Speed Ratio

THD

Total Harmonic Distortion

IGBT

Insulated Gate Bipolar Transistor

DFIG

Doubly-Fed Induction Generator

WECS

Wind Energy Conversion System

MATLAB

Matrix Laboratory

VSC

Voltage Source Converter

FFT

Fast Fourier Transform

DC

Direct Current

AC

Alternating Current

D axis

Direct axis

Q axis

Quadrature axis

PWM

Pulse Width Modulation

RSC

Rotor Side Converter

GSC

Grid Side Converter

HAWT

Horizontal Axis Wind Turbine

VAWT

Vertical Axis Wind Turbine

PMSG

Permanent Magnet Synchronous Generator

PMA

Permanent Magnet Alternator

PMG

Permanent Magnet Generator

MMF

Magneto Motive Force

SCIG

Squirrel Cage Induction Generator

RPM

Revolutions Per Minute

DTC

Direct Torque Control

FOC

Field Oriented Control

WT

Wind Turbine

x

CHAPTER 1 INTRODUCTION 1.1

Motivation

There is a general acceptance that the burning of fossil fuels is having a significant influence on the global climate. Effective changes in climate change will require deep reductions in greenhouse gas emissions. The electricity system is viewed as being much easier to transfer to low-carbon energy sources than more challenging sectors of the economy such as surface and air transport and domestic heating. Hence the use of costeffective and reliable low-carbon electricity generation sources, in addition to demand-side measures is becoming an important objective of energy policy in most countries. Over the past few years, wind energy has accounted for the fastest rate of growth of any form of electricity generation with its development stimulated by concerns over climate change, energy diversity and security of supply by national policy makers. The maximum extractable energy from the 0-100 meters layer of atmosphere has been estimated to be around 1012 kWh per annum, which is of the same order as hydro-electric potential. 1.1.1 Advantages of using wind energy: 1. Since it is powered by wind, it is a clean fuel source and doesn't have harmful effects on the environment unlike fossil fuels which rely on combustion of coal, natural gas etc. It also doesn’t produce emissions such as greenhouse gases and doesn't cause acid rain. 2. Its available in abundance and its sustainable, just needs to be harnessed. 3. It relies on the renewable power of wind which is in fact a form of solar energy. Winds are caused by non-uniform heating of atmosphere by sun, the rotation of earth and earth's surface irregularities. 4. Its low-priced costing just between 4-6% per kWh and can be built on farms or ranches benefiting the rural economy where the best wind sites are located. 1.1.2 Relevant Terms: Power Contained in Wind: This is the same as the kinetic energy of the flowing air mass per unit time given by:

1 P0= 𝜌AV3 2

1

(1.1)

Betz limit: It gives the maximum energy which can be extracted from the wind and is given by:

8

16

Pmax=27 P0=

27

𝜌AV3

(1.2)

Tip Speed Ratio: The tip speed ratio (TSR) of a wind turbine is defined as

λ=

1.2

2 ㄫ𝑅𝑁

(1.3)

𝑉

Literature Review:

This project involves two back to back converter system connected by a capacitor. As our machine is doubly fed induction generator, it can generate the power and can supply to grid also. The purpose of the project is to reduce the THD of grid side converter by making use of vector control technique so that the pulses generated by the comparison of reference and measured values of stator current are fed to the IGBT controlled inverter.[1] This project deals with the control strategy using the combined reactive power compensation from both back-to-back power converters for their optimized lifetime distribution. The detailed review of DFIG-based WECS can be divided into three parts namely, modelling and simulation of DFIG-based WECS, intelligent controllers used to control the DFIG in wind power applications and different controlling techniques of DFIG in power quality improvement. Finally, it concludes with a scope for the research work that can be carried out in future.[3] The power output of a conventional power plant can be controlled, whereas the power output of a WECS depends on the wind. The dynamic steady-state simulation model of the DFIG was developed using MATLAB. Simulation analysis was performed to investigate a variety of DFIG characteristics, including torque-speed, real and reactive-power over speed characteristics. From the simulation analysis, it is clear that the DFIG characteristics are affected by its rotor injected voltage. By varying the amplitude and phase angle of the rotor injected voltage, the DFIG torque-speed characteristics are shifted from the oversynchronous to sub-synchronous speed range, to generate electricity and also increase the DFIG pushover torque, thereby improving the stability of operation. For both motoring and generating modes, the DFIG sends additional real power through its rotor to the grid. Unlike the stator power, the characteristics of rotor power are mainly influenced by the rotor injected voltage [6].Along with conventional control schemes for wind turbine, an 2

innovative voltage control scheme is proposed that manipulates dynamically the reactive power from the Voltage Source Converter (VSC),taking into account its operating state and limits. The goal of investigation was to make use of available wind-turbine technology, namely the variable speed DFIG with power electronic converters, to take an active part in improving the voltage control at a remote location where the wind turbines are connected to a grid.[2,5] The vector dynamic approach has been used to model the DFIG. It has been shown that choice of the reference frame affects the waveforms of all d-q variables and also the simulation speed and, in certain cases, the accuracy of the results. In this project, an estimation of the Total Harmonic Distortion (THD) levels of the inverter output voltage has been presented using Fast Fourier Transform (FFT).The proposed algorithm is applied to a DFIG with a stator directly connected to the grid and a rotor connected to the grid through a back-to-back AC-DC-AC PWM converter [7]. The main goal of the proposed control strategy is to operate the RSC at its full capacity, without any over-rating, in terms of reactive power compensation and active filtering capability. Elsewhere, the GSC is controlled in such a way to guarantee a smooth DC voltage and to ensure sinusoidal current in the grid side. Simulation results show that the wind turbine can operate at its optimum power point for a wide range of wind speeds and that power quality can be improved.[9]

1.3

Project Objective: ● To develop simulation model and analysis of control of a Doubly-fed Induction Generator (DFIG) driven by wind turbine. ● To implement control techniques using the Grid Side converter and Rotor Side converter to regulate DC link voltage constant and limit torque pulsations respectively. ● To implement vector control scheme in order to achieve higher performance under torque and speed changes. ● To determine the Total Harmonic Distortion and do the Harmonic Analysis.

1.4

Organization of Thesis:

The thesis is divided into five parts where each chapter focuses on an independent theory required to proceed further. 3

Chapter1:Introduction and literature review Chapter2: It deals with the fundamentals of the wind turbine and generators. It presents different types of turbines present and shows the advantages of the using wind turbine for energy generation purpose. It also compares the vertical and horizontal types of wind turbines along with the aerodynamics present.

Chapter3It deals with the doubly fed induction generator in detail and the basics of DFIG and operation principles. It then focuses on finding out the equivalent circuit model for the same and the power and torque relations leading to four modes of operations. It deals with the vector control theory and the d-q transformation to convert the three phase parameters into two phase so that further modelling may be achieved. It describes about the GSC, RSC and PWM technique used to achieve vector control.

Chapter4: It describes the various simulation models of DFIG incorporated with vector control scheme under various conditions, like variable load, variable wind and simultaneous input of variable load and variable wind. And presents the results.

Chapter5: It deals with the conclusion of the project and the future scope and areas of improvement.

4

CHAPTER 2 WIND TURBINES AND GENERATOR 2.1

Wind Turbines

A wind turbine is a device that converts the wind's kinetic energy into electrical energy. Wind turbines vary vastly in size, shape, and orientation. Small turbines are used for a variety of rudimentary applications, while larger turbines are often found in wind farms and provide power to the electric grid. Wind was shown to have the "lowest relative greenhouse gas emissions, the least water consumption demands and cause far less environmental damage than other current energy providers, such as fossil fuels.[1] The energy in the wind turns two or three propeller-like blades around a rotor. The rotor is connected to the main shaft, which spins a generator to create electricity. A wind turbine works the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity. When the wind strikes the rotor blades, blades start rotating. The rotor is directly connected to a high-speed gearbox. Gearbox converts the rotor rotation into high speed which rotates the electrical generator. An exciter is needed to give the required excitation to the coil so that it can generate required voltage. The exciter current is controlled by a turbine controller which senses the wind speed based on that it calculates the power what we can achieve at that particular wind speed. Then output voltage of electrical generator is given to a rectifier and rectifier output is given to line converter unit to stabilize the output AC that is feed to the grid by a high voltage transformer. An extra units is used to give the power to internal auxiliaries of wind turbine (like motor, battery etc.), this is called Internal Supply unit. ISU can take power from the grid as well as from wind. Chopper is used to dissipate extra energy from the RU for safety purpose. Conservation of mass requires that the amount of air entering and exiting a turbine must be equal. Accordingly, Betz's law gives the maximal achievable extraction of wind power by a wind turbine as 16/27 (59.3%) of the total kinetic energy of the air flowing through the turbine.[3] The maximum theoretical power output of a wind machine is thus 16/27 times the kinetic energy of the air passing through the effective disk area of the machine.

5

2.2

Types of Wind Turbines

There are generally two types of wind turbines. The horizontal axis and vertical axis. The horizontal axis is divided as upwind and downwind whereas vertical axis is divided as a drag based and lift based. 

Horizontal Axis Wind Turbine or HAWT - Up wind



Horizontal Axis Wind Turbine or HAWT - Down wind



Vertical Axis Wind Turbine or VAWT - Drag based



Vertical Axis Wind Turbine or VAWT - Lift based

Fig 2.1: Schematic diagram of Horizontal axis and Vertical axis Wind turbine

2.2.1 Horizontal Axis Wind Turbine In Horizontal Axis up Wind turbine, the shaft of turbine and alternator both are aligned horizontally, and the turbine blades are placed at the front of the turbine that means air strikes the turbine blades before the tower. In the case of Vertical Axis, Down Wind turbine the shafts of the rotor and generator are also placed horizontally, but turbine blades are placed after the turbine that means the wind strikes the tower before the blades. Large three-bladed horizontal-axis wind turbines (HAWT), with the blades upwind of the tower produce the overwhelming majority of wind power in the world today. These turbines have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator. Some turbines use a different 6

type of generator suited to slower rotational speed input. These don't need a gearbox, and are called direct-drive, meaning they couple the rotor directly to the generator with no gearbox in between. While permanent magnet direct-drive generators can be more costly due to the rare earth materials required, these gearless turbines are sometimes preferred over gearbox generators because they eliminate the gear-speed increaser, which is susceptible to significant accumulated fatigue torque loading, related reliability issues, and maintenance costs. Most horizontal axis turbines are have their rotors upwind of its supporting tower. Downwind machines have been built, because they don't need an additional mechanism for keeping them in line with the wind. In high winds, the blades can also be allowed to bend which reduces their swept area and thus their wind resistance. Despite these advantages upwind designs are preferred, because the change in loading from the wind as each blade passes behind the supporting tower can cause damage to the turbine.

2.2.2 Vertical Axis Wind Turbine Vertical-axis wind turbines (or VAWTs) have the main rotor shaft arranged vertically. One advantage of this arrangement is that the turbine does not need to be pointed into the wind to be effective, which is an advantage on a site where the wind direction is highly variable. It is also an advantage when the turbine is integrated into a building because it is inherently less steerable. Also, the generator and gearbox can be placed near the ground, using a direct drive from the rotor assembly to the ground-based gearbox, improving accessibility for maintenance. However, these designs produce much less energy averaged over time, which is a major drawback. The key disadvantages include the relatively low rotational speed with the consequential higher torque and hence higher cost of the drivetrain, the inherently lower power coefficient, the 360-degree rotation of the aerofoil within the wind flow during each cycle and hence the highly dynamic loading on the blade, the pulsating torque generated by some rotor designs on the drive train, and the difficulty of modelling the wind flow accurately and hence the challenges of analysing and designing the rotor prior to fabricating a prototype.

7

2.3

Wind Generators

2.3.1 Synchronous generator: Synchronous generator are widely used in standalone WECS where the synchronous generator can be used for reactive power control in the isolated network. To ensure the wind turbine connection to the grid a back-to-back PWM voltage source inverters are interfaced between the synchronous generator and the grid. The grid side PWM inverter allows for control of real and reactive power transferred to the grid. The generator side converter is used for electromagnetic torque regulation.

Fig 2.2: Schematic diagram of synchronous generator in WECS

Synchronous generators of 500 kW to 2 MW are significantly more expensive than induction generators with a similar size. One should note that the use of a multipole synchronous generator (large diameter synchronous ring generator) avoids the installation of a gearbox as an advantage but a significant increase in weight will be accepted in counterpart. Indeed, the industry uses directly driven variable speed synchronous generators with large-diameter synchronous ring generator. The variable, directly driven approach avoids the installation of a gearbox, which is essential for medium and largescale wind turbines. Permanent magnet synchronous generator is a solution that is appreciated in small wind turbines but it cannot be extended be extended to large-scale power because it involves the use of big and heavy permanent magnets.

2.3.2 Induction generator Induction generators are increasingly used these days because of their relative advantageous features over conventional synchronous generators. These features are brushless and rugged construction, low cost, maintenance and operational simplicity, selfprotection against faults, good dynamic response, and capability to generate power at varying speed. The later feature facilitates the induction generator operation in stand8

alone/isolated mode to supply far flung and remote areas where extension of grid is not economically viable; in conjunction with the synchronous generator to fulfil the increased local power requirement, and in grid-connected mode to supplement the real power demand of the grid by integrating power from resources located at different sites. The reactive power requirements are the disadvantage of induction generators. This reactive power can be supplied by a variety of methods, from simple capacitors to complex power conversion systems. Induction generators were used for a long time in a constant speed WECS, where the pitch control or active stall control are dictated for power limitation and protection, a soft starter is also used to limit transients when the generator is connected to the grid.

Fig 2.3: Schematic diagram of induction generator in WECS

For variable speed WECS, back to back PWM inverters are used, where the control system of the inverter in the generator side regulates the machine torque and consequently the rotor speed, therefore keeping the frequency within defined limits. On the other hand, the inverter in the grid side controls the reactive power at the coupling point. In this case, the doubly-fed induction generator is widely used. Indeed, amongst many variable speed concepts, WECS using doubly-fed induction generators have many advantages over others. For example, the power converter in such wind turbines only deals with rotor power, therefore the converter rating can be kept fairly low, approximately 20% of the total machine power. This configuration allows for variable speed operation while remaining more economical than a series configuration with a fully rated converter. Other features such as the controllability of reactive power help doubly-fed induction generators play a similar role to that of synchronous generators.

9

2.3.3

DFIG (DOUBLY FED INDUCTION GENERATOR)

DFIG for Double Fed Induction Generator, a generating principle widely used in wind turbines. It is based on an induction generator with a multiphase wound rotor and a multiphase slip ring assembly with brushes for access to the rotor windings. It is possible to avoid the multiphase slip ring assembly, but there are problems with efficiency, cost and size. A better alternative is a brushless wound-rotor doubly-fed electric machine. The principle of the DFIG is that rotor windings are connected to the grid via slip rings and back-to-back voltage source converter that controls both the rotor and the grid currents. Thus rotor frequency can freely differ from the grid frequency (50 Hz). By using the converter to control the rotor currents, it is possible to adjust the active and reactive power fed to the grid from the stator independently of the generator's turning speed. The control principle used is either the two-axis current vector control or direct torque control (DTC). DTC has turned out to have better stability than current vector control especially when high reactive currents are required from the generator. Wind turbines use a doubly-fed induction generator (DFIG) consisting of a wound rotor induction generator and an AC/DC/AC IGBT-based PWM converter. The stator winding is connected directly to the 50 Hz grid while the rotor is fed at variable frequency through the AC/DC/AC converter. The DFIG technology allows extracting maximum energy from the wind for low wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on the turbine during gusts of wind. The optimum turbine speed producing maximum mechanical energy for a given wind speed is proportional to the wind speed. Another advantage of the DFIG technology is the ability for power electronic converters to generate or absorb reactive power, thus eliminating the need for installing capacitor banks as in the case of squirrel-cage induction generator.The AC/DC/AC converter is basically a PWM converter which uses PWM technique to reduce the harmonics present in the wind turbine driven DFIG system. The schematic diagram presenting various components has been given below

10

Fig 2.4: Schematic diagram of DFIG in wind power applications

The stator is directly connected to the AC mains, whilst the wound rotor is fed from the Power Electronics Converter via slip rings to allow DFIG to operate at a variety of speeds in response to changing wind speed. Indeed, the basic concept is to interpose a frequency converter between the variable frequency induction generator and fixed frequency grid. The DC capacitor linking stator and rotor-side converters allows the storage of power from induction generator for further generation. To achieve full control of grid current, the DClink voltage must be boosted to a level higher than the amplitude of grid line-to-line voltage. The slip power can flow in both directions, i.e. to the rotor from the supply and from supply to the rotor and hence the speed of the machine can be controlled from either rotor- or stator-side converter in both super and sub-synchronous speed ranges. As a result, the machine can be controlled as a generator or a motor in both super and sub-synchronous operating modes realizing four operating modes. Below the synchronous speed in the motoring mode and above the synchronous speed in the generating mode, rotor-side converter operates as a rectifier and stator-side converter as an inverter, where slip power is returned to the stator. Below the synchronous speed in the generating mode and above the synchronous speed in the motoring mode, rotor-side converter operates as an inverter and stator side converter as a rectifier, where slip power is supplied to the rotor. At the synchronous speed, slip power is taken from supply to excite the rotor windings and in this case machine behaves as a synchronous machine.

2.3.4 PMSG (Permanent Magnet Synchronous Generator) A permanent magnet synchronous generator is a generator where the excitation field is provided by a permanent magnet instead of a coil. The term synchronous refers here to the 11

fact that the rotor and magnetic field rotate with the same speed, because the magnetic field is generated through a shaft mounted permanent magnet mechanism and current is induced into the stationary armature. In the majority of designs the rotating assembly in the centre of the generator—the "rotor"—contains the magnet, and the "stator" is the stationary armature that is electrically connected to a load.

Fig 2.5: Schematic diagram of PMSG

The load supplied by the generator determines the voltage. If the load is inductive, then the angle between the rotor and stator fields will be greater than 90 degrees which corresponds to an increased generator voltage. This is known as an over excited generator. The opposite is true for a generator supplying a capacitive load which is known as an under excited generator. A set of three conductors make up the armature winding in standard utility equipment, constituting three phases of a power circuit—that correspond to the three wires we are accustomed to see on transmission lines. The phases are wound such that they are 120 degrees apart spatially on the stator, providing for a uniform force or torque on the generator rotor. The uniformity of the torque arises because the magnetic fields resulting from the induced currents in the three conductors of the armature winding combine spatially in such a way as to resemble the magnetic field of a single, rotating magnet. This stator magnetic field or "stator field" appears as a steady rotating field and spins at the same frequency as the rotor when the rotor contains a single dipole magnetic field. The two fields move in "synchronicity" and maintain a fixed position relative to each other as they spin. They are known as synchronous generators because f, the frequency of the induced voltage in the stator (armature conductors) conventionally measured in hertz, is directly proportional to RPM, the rotation rate of the rotor usually given in revolutions per minute (or angular speed). If the rotor windings are arranged in such a way as to produce the effect of more than two magnetic poles, then each physical revolution of the rotor results in more magnetic poles moving past the armature windings. Each passing of a north and South Pole 12

corresponds to a complete "cycle" of a magnet field oscillation. By increasing the torque on the prime mover, a larger electrical power output can be generated. In practice, the typical load is inductive in nature. In a permanent magnet generator, the magnetic field of the rotor is produced by permanent magnets. Other types of generator use electromagnets to produce a magnetic field in a rotor winding. The direct current in the rotor field winding is fed through a slip-ring assembly or provided by a brushless exciter on the same shaft. Permanent magnet generators (PMGs) or alternators (PMAs) do not require a DC supply for the excitation circuit, nor do they have slip rings and contact brushes. A key disadvantage in PMAs or PMGs is that the air gap flux is not controllable, so the voltage of the machine cannot be easily regulated. A persistent magnetic field imposes safety issues during assembly, field service or repair. High performance permanent magnets, themselves, have structural and thermal issues. Torque current MMF vectorially combines with the persistent flux of permanent magnets, which leads to higher air-gap flux density and eventually, core saturation. In this permanent magnet alternators the speed is directly proportional to the output voltage of the alternator.As shown in the diagram, the perpendicular component of the stator field affects the torque while the parallel component affects the voltage.

Fig 2.6: Schematic diagram of rotor field rotation in PMSG

Fig 2.7: Schematic diagram showing field interaction in PMSG

13

Fig 2.8: Phasor diagram of PMSG[8]

f(Hz) =

𝑅𝑃𝑀∗𝑝𝑜𝑙𝑒 120

Pg= Tg*RPM Ia=

(2.2)

|𝐸|∠𝛿−|𝑉𝑡|

P= Q=

(2.1)

(2.3)

𝑗𝑋 𝑑 𝑉𝑡 𝐸𝑔 𝑆𝑖𝑛𝛿

(2.4)

𝑋𝑑 𝑉𝑡 (|𝐸|𝑐𝑜𝑠𝛿 −|𝑉𝑡|) 𝑋𝑑

S=P+jQ;

(2.5) (2.6)

2.3.5 SCIG (Squirrel Cage Induction Generator): A SCIG based WECS for fixed speed concept. This type of WECS is used in conventional concept i.e. an upwind, stall regulated and three bladed wind turbine concept of WECS development. In this configuration rotor of SCIG is directly connected to the turbine through the multistage gearbox. Stator is connected to the grid through the coupling transformer.SCIG based variable speed concept WECS uses a full scale back to back power converter in place of capacitor bank and soft starter.

14

Fig 2.9: Schematic diagram of SCIG

Various equations involved for the power calculation and simplification for analysis are as follows:

Pm=Tm*ωr

(2.7)

Ps=Tem*ωs

(2.8)

𝑑𝑊𝑟

J

𝑑𝑡

=Tm – Tem

(2.9)

Pm = Ps + Pr

(2.10)

By using Equation (2.7) (2.8) (2.11) we get

Pr = Pm + Ps=Tmωr –Temωs Pr=Tem(ωr –ωs)=

𝑇𝑒𝑚 𝜔𝑠

ωs(ωr – ωs)

𝜔 𝑟 −𝜔 𝑠

Pr=(ωs Tem) (

𝜔𝑠

(2.11) (2.12)

)

(2.13)

Pr = -sPs

(2.14) 𝜔 𝑟 −𝜔 𝑠

Slip(s) =

(2.15)

𝜔𝑠

Usually the magnitude of slip s is below 1, so Pr is small compared to Ps, the mechanical torque Tm is positive (during generation), synchronous speed ωsis positive and fixed (for constant frequency at grid), and therefore the sign of Pr depends on the sign of slip. It’s positive when slip is negative (for rotational speeds above the synchronous speed) and negative when slip is positive (for rotational speeds below the synchronous speed). 15

During super synchronous mode, Pr is sent to the DC link capacitor which raises the DC voltage. During sub synchronous mode, Pr is extracted from the capacitor lowering the DC voltage. The grid converter then extracts or delivers the grid power to keep the dc voltage fixed. During steady state, Pgc is equal to Pr, also the turbine speed can be found out from Pr extracted by or fed to Crotor. The phase sequence of AC voltage produced by Crotor depends on rotor speed and is positive when rotor speed is less than synchronous speed and negative when rotor speed exceeds the synchronous speed. The magnitude of the frequency of this AC voltage is slip times the frequency of the grid.

Fig 2.10: Stator Circuit

V1=E1+I1(Rs+jXs)

(2.16)

E1=4.44fKw1T1φm

(2.17)

Fig 2.11: Rotor Circuit at slip frequency

E2s=4.44dfKw2T2φm= E2s=

4.44 𝑚

sfkw1T1φm

(2.18)

𝑠𝐸1

(2.19)

𝑚

16

Where, m= 𝐸2𝑠

I2s=

𝑘 𝑤 1 𝑇1 𝑘 𝑤 2 𝑇2 𝑠𝐸1

=

(2.20)

𝑅𝑟 +𝑗𝑠𝑋 𝑟 𝑚 (𝑅𝑟 +𝑗𝑠𝑋 𝑟 )

Fig 2.12: Rotor Circuit at Stator Frequency

I’2=

I2 𝑚

𝐸

I’2=𝑅′ 𝑟 1 𝑠

(2.21)

𝑗𝑋 𝑟

Where, R’r=m2Rr X’r=m2Xr 𝑠𝑋𝑟

Φ2=tan-1 Φ2=tan

𝑅𝑟 -1 𝑋𝑟

𝑚 2𝑋

𝑠𝑋𝑟

-1 -1 𝑟 𝑅′ 𝑟 =tan 𝑚 2 𝑅 𝑟 =tan 𝑠

𝑠

(2.22)

𝑅𝑟

Fig 2.13: Complete Equivalent Circuit (at Stator Frequency

Pri=3E1I’2cosφ2 Pri=

3I′ 2 𝑅′ 𝑟 𝑠

17

𝑅′ 𝑟

Prl=3I22sRr=3(mI’2)2(

𝑚2

)=3I’22R’r

(2.23)

⸪Prl=sPri Pm=Prσ= Pri-Prl Pm=3I’2R’r(

1−𝑠 𝑠

)

Pm=(1-s)Pri

(2.24)

Torque 𝑃

Tem= 𝑚 =3I’22R’r(

1−𝑠

𝜔𝑟

Tem= Tem=

3I′ 22 R ′r 𝑠 3 𝜔𝑠

∗

1

* =

𝑠

)*

1

𝜔𝑟

𝑃 𝑟𝑖

𝜔𝑠 𝜔𝑠 𝐾 2 𝑉12 (𝑅𝑟′ +𝑅𝑥′ )/𝑠

(2.25)

𝑅1 +(𝑅𝑟′ +𝑅𝑥′ )/𝑠]2+[𝑋1 𝑋𝑟′ ]2

Fig 2.14: Torque-Speed Characteristics Curve for varying external resistance

18

Fig 2.15: Rotor Injection

sE2=I2z2s + Ej sE2cosφ2=Ejcos(φ2+β)+I2Rr sE2I2cosφ2=I22Rr+EjI2cos(φ2+β) sE1I’2cosφ2=I’22R’r+E’jI’2cos(φ2+β)

(2.26)

E1I’2cosφ2=Pri=Pag Also,

I’22R’r=Prl

(2.27)

E1I’2cos(φ2+β)=P2 sPag=Prl+P2 (1-s)Pag=Pm Pag=Prl+P2+Pm

(2.28)

Pm=(1-s)Pag Pm=3

1−𝑠 𝑠

𝐼′22 𝑅𝑟′ +3

𝑘𝑤 1 𝑇1

M1=

𝑘𝑤 2 𝑇2

Pm=3

1−𝑠 𝑠

1−𝑠 𝑠

E’jI’2cos(φ2+β)

=1

[𝐼22 𝑅𝑟 + 𝐸𝑗 𝐼2 cos(φ2+β)] 19

𝑃

Tem= 𝑚 =

𝑃𝑚

=

3

𝜔 𝑟 (1−𝑠)𝜔 𝑟 𝜔 𝑠 𝑠

[I22Rr+I Ej II I2 I cos(φ2+β)]

(2.29)

Fig 2.16: Simplified Equivalent circuit to find I2 𝐸 𝑉1 − 𝑗 𝑠 I2 = 𝑅 (𝑅𝑠 + 𝑠 𝑠 )+𝑗 (𝑋𝑠 𝑋𝑟 )

│I2│ =

𝐼 𝑠𝑉1 −𝐸𝑗 𝐼

(2.30)

𝑠𝑅𝑠 +𝑅𝑟 2+ 𝑋𝑠 +𝑋𝑟 2

The SCIG is a very popular machine due to its mechanical simplicity and construction. Methods of control include uncontrolled rectifier with inverter full power frequency converter, fixed capacitor bank, thyristor and static VAR controller, matrix converter.

Advantages: 1. It is cheap and easy to construct. 2. Since it is being used at fixed wind speed so it provides stable control frequency.

Disadvantages: 1. Gearbox maintenance and its cost is a problematic issue. 2. A soft-starter is required for smooth grid connection. 3. SCIG always draw reactive power from the grid so reactive power compensation is required.

20

Fig 2.17: Comparison of various generators[6]

21

CHAPTER 3 CONVERTERS IN WIND ENERGY GENERATION SYSTEM 3.1

Strategy of DFIG:

Since DFIG maximizes the output power at low wind speeds and limits the power at high wind speeds is the most complete solution in high power applications. To do so effective control of DFIG is very significant and several methods have been developed over the years to benefit from the advantages of DFIG. Because, the sudden changes in wind speed may result in fluctuations in output signals and undesired operation of machine. There are two ways to divide the complete control strategy of the machine, one is scalar control and the other is vector control. The limited uses of scalar control makes way for vector control. Although it is easy to execute the scalar control strategy, but the inherent coupling effect present gives slow response. Various control techniques have been employed namely:

3.1.1 Pitch Control The Pitch Control method is different than the other methods because it is more related to control of the turbine than generator. This method can be used together with any of other control techniques. The objective of pitch control is to adjust the angle of the blades so that maximum power can be extracted from the wind or system disturbances can be lowered in case of very high wind speeds. Sensors on the top of the wind turbines always monitor the wind speed for a proper pitch control [3]

3.1.2 Scalar Control Scalar control, also known as V/f control, can be used in control process of DFIG to keep the magnitude and frequency of the voltage induced on stator constant. In DFIG, flux created by rotor which also cuts the stator windings rotate due to both rotor movement and AC voltage supply. That means changing the magnitude and frequency of AC rotor supply can be used to adjust stator voltage and frequency. Mathematically, frequency of the current injected to rotor to keep stator flux constant is as follows: 𝑁𝑟𝑜𝑡𝑜𝑟 ×𝑝

frotor= fgrid౼

120

(3.1)

Where frotor and fgrid are frequency of rotor current and stator current respectively, Nrotor is rotational speed of rotor in rpm and p is number of poles. Similarly, to maintain constant 22

magnitude voltage on stator, the voltage value given to the rotor should satisfy the selected V/f ratio.[3]

3.1.3 Direct Torque Control Direct Torque Control was introduced in 1985. The main objective of DTC, as its name suggests, is to provide proper control of torque. The robustness of DTC comes from the precise estimation of torque and flux. These estimated values are compared with the values in the predefined look-up table. This method provides the following advantages: current regulators, PWM signal generators and transformation elements are not needed. It provides simple and fast operation.[3]

3.1.4 Vector Control Vector control, also called field-oriented control (FOC) invented in the 1970s. Hasse and Blaschke developed this technique by inspiring from DC motor drives. The method allows separate controlof flux and torque. The first step of FOC is to convert phase currents to coordinate system and then to d-q coordinate system with Clark and Park transformations, respectively. The difference between those two coordinate systems is that 𝛼 − 𝛽 system varies with time where d-q system does not. In d-q system, there is a 90° difference between d(direct) axis and q(quadrature) axis. Direct component of the current represents flux whereas quadrature component represents torque. The field orientation could be based on flux or voltage. In flux orientation, flux is aligned with d-axis which means d-axis component of voltage is zero. For voltage oriented reference frame stator voltage is aligned with the q-axis.[2,3]

Fig-3.1: Stator flux oriented reference frame[5]

The figure above also proves that in stator-flux orientation direct component of rotor current controls reactive power and quadrature component of rotor current controls active power. FOC makes control process much easier as the controllers have to handle with DC components instead of AC signals. However, elements responsible from the signal conversion in the system are critical, 23

because they must operate very fast. An Induction Motor can be executed like a dc machine with the help of vector control. [2,3]

Fig 3.2: Schematic diagram of Vector Control technique[2]

Vector control is employed to achieve a decoupled control of the active and reactive power. The basis of the vector control theory is d-q axis theory. Study of the d-q theory is essential for vector control analysis.

3.2

d-q axis transformation (Reference Frame theory):

dq0 or direct-quadrature-zero transformation is a mathematical transformation employed to simplify the analysis of three phase circuits, where three AC quantities are transformed to two DC quantities. The mathematical calculations are performed on the imaginary DC quantities and the AC quantities are again recovered by performing an inverse transformation of the DCquantities. It is similar to Park’s transformation, and it also solves the problem of AC parameters varying with time.[4] 24

Owing to the smooth air-gap in the induction motor, the self-inductance of both the stator and rotor coils are constant, whereas the mutual inductances vary with the rotor displacement with respect to the stator. Therefore the analysis of the induction motor in real time becomes complex due to the varying mutual inductances, as the voltage is not linear. A change of variables is therefore employed for the stator and rotor parameters to remove the effect of varying mutual inductances. This leads to an imaginary magnetically decoupled two phase machine.

Fig 3.3: Axis in dq frame[3]

The orthogonally placed balanced windings, called d- and q- windings can be considered as stationary or moving relative to the stator. In the stationary frame of reference, the ds and qs axes are fixed on the stator, with either ds or qs axis coinciding with the a-phase axis of the stator. In the rotating frame, the rotating d-q axes may be either fixed on the rotor or made to move at the synchronous speed.

3.3

Methodology

The DFIG based Wind Turbine (WT) has two main control systems inside: grid-side converter (GSC) and rotor-side converter (RSC) control system.DFIG based WECS with the vector control. The variations in rotor current d and q components directlyreflected to their corresponding stator active and reactivepowers. Therefore, rotor currents are used to control the statoractive and reactive powers. The rotor side converter controlsthe DFIG, while the grid side converter’s function is tomaintain the DC link voltage constant and the flow of thereactive power between the rotor winding and the grid. Thevector control on DFIG is implemented in two steps.

25

3.3.1 GSC: Grid Side Converter The objective of the GSC control system is to keep the voltage of DC link between two converters constant so that the power produced in the rotor can be supplied to the grid in constant voltage magnitude and frequency. Control of DC voltage is also needed for the operation of RSC control system. What GSC control system does is that it compares the actual voltage value with the reference value all the time. From the difference between those two values with the help of a PI controller it obtains the reference value for direct component of stator current. Reference value for quadrature component of stator current is given as zero, manually. Then it compares the measured stator current values with the reference values. Reference voltage values are derived from this difference by a PI controller, again. Finally control signals are provided and given to the GSC after a mathematical Operation [1]. RSC control system is more complicated than GSC control system as it handles more than one variable: active power (frequency) and reactive power (output voltage). For a grid connected DFIG-WT system q-axis component of rotor current controls active power and d-axis component of rotor current controls reactive power in flux oriented reference frame. In this section active and reactive power control is explained step by step. Measured values of active power and reactive power are compared with the respective reference values.[2] In the grid side control, mainly DC link voltage is compared with a reference DC link voltage and error is fed to a PI controller to maintain constant DC link voltage. It corresponds to the PI controller, a reference current I*gcd is generated and q-axis current component I*gcq is set as zero, to maintain the flow of reactive power zero from the grid. 𝐾𝑖4

I*gcd=(Kp4+

𝑆

)(V*dc-Vdc)

(3.2)

Now, the reference grid converter currents I*gcd and I*gcq are compared with the actual grid currents in order to generate the control signals for grid converter as, 𝐾𝑖5

Vgcd=(Kp5+

𝑆

)(I*gcd-Igcd)

𝐾𝑖5

Vgcq=(Kp5+

𝑆

)(I*gcq-Igcq)

(3.3) (3.4)

The required reactive power for the DFIG is to be generated by the GSC and reactive power drawn from the grid is set zero.

26

3.3.2 RSC: Rotor Side Converter The rotor-side converter is used to control the wind turbine output power and the voltage measured at the grid terminals. The power is controlled in order to follow a predefined power-speed characteristic, named tracking characteristic. The main task of the RSC is to control the machine to track the maximum power point and to maintain the stator at unity power factor.The main purpose of RSC is to extract maximum power with independent control of active and reactive powers. Here, the RSC is controlled in voltage oriented reference frame. So the active and reactive powers are controlled by controlling direct and quadrature axis rotor currents (Idr and Iqr) respectively. Direct axis reference rotor current is selected such that maximum power is extracted for a particular wind speed. This can be achieved by running the DFIG at a rotor speed for a particular wind speed.The speed error (ωr) is obtained by subtracting sensed speed (ωr) from the reference speed (ωr*). Reference rotor speed (ωr*) is estimated by optimal tip speed ratio control for a particular wind speed. Normally, the quadrature axis reference rotor current (Iqr*) is selected such that the stator reactive power (Qs) is made zero. In this DFIG, quadrature axis reference rotor current (Iqr*) is selected for injecting the required reactive power.[1,2] To maximize the turbine output power, DFIG must be controlled through the control of Ird and Irq. However, based on this is not a straightforward task since Ψsd and Ψsq are also *

functions of Ird and Irq. To simplify the control and calculateI rd, the stator flux component Ψsd is set to zero Current control loops are taken for control of actual direct and quadrature axis rotor currents (Idr and Iqr) close to the direct and quadrature axis reference rotor currents (Idr* and Iqr*). The rotor currents Idr and Iqr are calculated from the sensed rotor currents (Ira , Irb and Irc) from the bus selector. Direct and quadrature axis rotor voltages (Vdr՛ and Vqr՛ ) are obtained from direct and quadrature axis rotor current errors.[2,3] To maintain the stator at unity power factor (UPF), the flow of reactive power from the stator side is maintained at zero, by generating a q-axis reference rotor current I*rq as,

Irq*=(Kp1+

𝐾𝑖1 𝑆

)(Qs*-Qs)

(3.5)

The reference value of the reactive power is set as zero to maintain the stator at UPF. The DFIG torque is given as

27

𝐿𝑚

Te=1.5

𝐿𝑠

(𝜓sd Irq-𝜑sqIrd)

(3.6)

To maximize the turbine output power, DFIG must be controlled through the control of Ird and Irq. However, this is not a straightforward task since Ψsd and Ψsq are also functions of Ird and Irq. To simplify the control and calculate I*rd, the stator flux component Ψsdis set to zero. Now the eq. in reference reduces as,

T*e=-1.5

𝐿𝑚 𝐿𝑠𝑠

𝜑sqI*rd

Te* can be calculated by the processing the error between reference rotor speed (wr*) and actual rotor speed (ωr) at any instant and it is given as, 𝐾𝑖2

T*e=(Kp2+

𝑆

)(𝜔𝑟∗ − 𝜔r)

(3.7)

For the above equation 𝜔𝑟∗ is calculated using the Cp v/s λ curve

Fig 3.4: Cp v/s λ curve for different values of β

In case of the pitch control the maximum value of Power coefficient (Cp) was achieved for blade pitch angle (β) = 0 degree at λ=8.1. Power coefficient (Cp) is defined as the power output of the wind turbine to the available power in the wind regime. The value of the maximum power coefficient is dependent on the tip speed ratio as, λ= ωR/v. The reference rotor speed (wr*) can be fixed accordingly to the available wind speed to track the maximum power point. The above realization can be done using a look-up table. Now, the actual rotor currents Ird and Irq are compared with the reference rotor currents I*rd and I*rq to generate the control signals for the rotor side converter as, 28

𝐾𝑖3

Vrd=(Kp3+

𝑆

𝐾𝑖3

Vrq=(Kp3+

𝑆

)(I*rd-Ird)

(3.8)

)(I*rq-Irq)

(3.9)

The loss components of the stator and rotor are added to the above control signals.

3.4

PULSE WIDTH MODULATION (PWM)

The above generated signals of voltage (d,q) on rotor side and grid side are converted into voltage (a,b,c) using the inverse park transform. The signals the generated are fed into the three phase PWM generator, which generates the gating signals for the IGBT Variable voltage and frequency supply for Adjustable Speed Drives (ASD) is invariably obtained from a three-phase VSI. In power electronics, converters and motors, the PWM technique is mostly used to supply AC current to the load by converting the DC current and it appears as a AC signal at load or can control the speed of motors that run at high speed or low. [5]

Fig 3.5: Sinusoidal Pulse Width Modulation[7]

The duty cycle of a PWM signal varies through analog components, a digital microcontroller or PWM integrated circuits. Figure 2 shows the comparator gets the inputs as reference waveform (square wave) and a carrier wave (triangular wave) is supply to the comparator to obtained PWM waveform. Triangular wave is formed by op-amp driver. Triggering pulses are produced at the instant of the carrier signal magnitude is greater than the reference signal magnitude. To turn-on the IGBT switches, firing pulses are produced, the output voltage during the interval triangular voltage wave stipulated the square modulating wave.[6]

29

CHAPTER 4 RESULTS AND DISCUSSIONS

4.1

Proposed System/ Schematic

Fig 4.1: Schematic diagram of Grid connected DFIG[1]

4.2

System Components



Wind Turbine



DFIG (Doubly Fed Induction Generator)



RSC: Rotor Side Converter



GSC: Grid Side Converter



Three Phase PWM generator



Grid



DC Link Capacitor



Wind Signal Generator



Park Transform and Inverse Transform blocks

30

4.3

System Rating and Parameters

Parameter

System Rating β =0 Wind Speed(Vw)=12 m/s

Wind Turbine

Output Power=15KW λ = 8.1 Power = 15KW Voltage(L-L) = 415V

DFIG

Pole Pairs = 3 Inertia Constant = 0.385 RSC (Rotor Side Converter) Control

Kp = 0.6 , Ki = 8

VSC (Grid Side Converter) Control

Kp = 350 , K i= 3500

Grid

Voltage(L-L) = 415V , frequency = 50Hz

DC Link (Reference Voltage)

830V

DC Link Capacitor

9000µF

Table 4.1: System parameter and rating

31

4.4

Simulation Model and Results

4.4.1 Steady state condition

Fig 4.2: Simulation Model of DFIG with GSC and RSC

32

4.4.2 No Wind fluctuation

Fig 4.3: DC Link voltage

Fig 4.4: Power Consumed and Delivered by DFIG

33

Fig 4.5: DFIG Characteristics

34

Fig 4.6: Grid side voltage and current (actual values)

Fig 4.7: Grid side voltage and current (pu)

35

Fig 4.8: THD in GSC current

4.4.3 Under wind speed variations: Wind as Step Signal

Fig 4.9: Wind input to the turbine

36

Fig 4.10: DC Link voltage

Fig 4.11: Power Consumed and Delivered by DFIG

37

Fig 4.12: DFIG Characteristics

38

Fig 4.13:Grid side voltage and current (actual values)

Fig 4.14:Grid side voltage and current (pu)

39

Fig 4.15: THD in GSC current

4.4.4 Under wind speed variations: Wind as real time signal

Fig 4.16: Wind input to the turbine

40

Fig 4.17: DC Link voltage

Fig 4.18: Power Consumed and Delivered by DFIG

41

Fig 4.19:DFIG Characteristics

42

Fig 4.20:Grid side voltage and current (actual values)

Fig 4.21:Grid side voltage and current (pu)

43

Fig 4.22: THD in GSC current

4.4.5 Under load Variations: Only Load is Variable

Fig 4.23:DC Link voltage

44

Fig 4.24:DFIG Characteristics

Fig 4.25:Power Consumed and Delivered by DFIG

45

Fig 4.26:Grid side voltage and current (actual values)

Fig 4.27:Grid side voltage and current (pu)

46

Fig 4.28: THD in GSC current

47

4.4.6 Under Load variations: Variable load and wind:

Fig 4.29: Simulation model of DFIG with variable load and wind

48

Fig 4.30:Wind input to the turbine

Fig 4.31: DC Link voltage

49

Fig 4.32: Circuit BreakerParameters Load Parameters

50

Fig 4.33: Power Consumed and Delivered by DFIG

Fig 4.34:DFIG Characteristics

51

Fig 4.35:Grid side voltage and current (actual values)

Fig 4.36:Grid side voltage and current (pu)

52

Fig 4.37: THD in GSC current

53

4.5 Result analysis: A doubly-fed induction machine is a wound-rotor doubly-fed electric machine and has several advantages over a conventional induction machine in wind power applications. First, as the rotor circuit is controlled by a power electronics converter, the induction generator is able to both import and export reactive power. This has important consequences for power system stability and allows the machine to support the grid during severe voltage disturbances (low voltage ride through, LVRT). Second, the control of the rotor voltages and currents enables the induction machine to remain synchronized with the grid while the wind turbine speed varies. A variable speed wind turbine utilizes the available wind resource more efficiently than a fixed speed wind turbine, especially during light wind conditions. Third, the cost of the converter is low when compared with other variable speed solutions because only a fraction of the mechanical power, typically 2530%, is fed to the grid through the converter, the rest being fed to grid directly from the stator. The model of the WECS shown in Fig 4.2 and Fig 4.32 are developed in the MATLAB/Simulink and the results are presented to demonstrate its behaviour. 

No wind fluctuation:Fig 4.2 shows that the input to the wind turbine is constant speed of 12m/s with a linear load applied to the grid.It can be inferred from Fig 4.3 that the DC link voltage remains constant around 830V, and the result of Fig 4.4 shows that the power fed by the DFIG to the grid is in the range of 8-10KW. The grid voltage is maintained at around 600V or 1pu as can be seen in Fig 4.6 and Fig 4.7.



Variable Wind (Stepped):The wind input signal shown in Fig 4.9 is in the form of step signal and is minimum in the interval 16-21s at 9m/s, when it is fedto the turbine, under normal operation is observed that the rotor speed varies in accordance to it in steps. From Fig 4.11 it can be seen that active power fed by DFIG to grid is stepped and alternating and it is minimum in the interval where the wind speed was minimum. The grid voltage is maintained constant at around 600V and the current and voltage waveform is nearly sinusoidal as seen in Fig 4.13, thus the THD observed using the FFT analysis settles around 0.31 as shown in Fig 4.15.



Variable Wind (Real time):The next case under study is wind signal as in real time systems with gradual changes in speed as shown in Fig 4.16.It is observed

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from Fig 4.19 that the rotor speed corresponds to wind speed variation and as seen in Fig 4.18 the power fed by DFIG to grid alternates around 10-12 KW. 

Variable Load, Constant Wind:When the WECS load is time varying by operating the circuit breaker while the wind is constant throughout at 12m/s, then the power output of DFIG as observed from Fig 4.25 is alternating around 10KW. While the DC link voltage as seen in Fig 4.22 remains constant around 830V. Fig 4.26 also explains to us that the voltage and current waveform are nearly sinusoidal and is confirmed by Fig 4.28 as the THD content in the Grid side current Ipcc under one cycle of is 0.31.

 Variable load & Variable Wind: The simulation model shown in Fig 4.29 is incorporated with variable wind input to the turbine and loads of different magnitudes for different duration of time. Fig 4.30 and Fig 4.32 gives us a clear picture that the wind input is time varying as encountered in real time and the loads are of different magnitudes 3.5e3W, 6e3W linear in nature and are present in the system for different duration of time which can be observed from the CB parameters. The voltage waveform at Grid side is nearly sinusoidal with magnitude maintained at 600V as shown in Fig 4.35. The results were further observed and Fig 4.33 reveals that the power output of the DFIG was in discrete steps in the range of 6-7KW.

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CHAPTER 5 5.1

CONCLUSION

The modelling, the control and the simulation of an electrical power conversion system based on a DFIG connected directly to the grid by the stator using vector control technique as control algorithm has been presented in this project. As a result of all the information provided in the previous sections and the simulation results it is now observed that just by making some mathematical operations three-phase alternating current and voltage signals can be converted into two-component signals which are in a form close to DC signals using the Park transformation blocks. The signals then obtained were converted back from dq to abc and ged as input to three phase PWM generator so that gating pulses for the controllers can be obtained. Vector control technique adopted in the project helps PI controllers work properly and provides separate control of flux and torque and hence active and reactive power exchanged between the stator of the DFIG and the grid can be controlled.The model from the base paper was taken as reference and changes were made in the PI controller constants Kp and Ki so that results obtain were accurate. The simulation models and results were used to observe generated power, DC voltage, wind speed load variation and grid side voltages and currents. Due to the advances in power electronics the RSC and GSC controller use devices like IGBTs, and it makes advantageous to use the doubly fed induction generator system with variable speed connected to the electrical grid through these AC-DC-AC converter for improving the efficiency of the power conversion. The DFIG is able to provide a considerable contribution to grid voltage. The DFIG has been operated at unity power factor, by making an assumption that reactive power is zero so more power can be extracted from the stator side and 25-30% power can be extracted for the rotor side also, so 30-35% reduction in themachine rating, i.e. rotor is able to supply 30% of its nominal value. Due to this reduction in the machine rating cost of the overall system is reduced. The rating of converters is only 25-30% of the total power, so losses in converters are reduced because the power handled by the converters is only a fraction of the total power. This improves the efficiency and the cost is reduced in practical purposes. Therefore, the DFIG-based wind energy conversion systems are best to harness the energy from the available wind regime. There were challenges faced during the 56

modelling of the controllers of GSC and RSC. Certain calculations were made to get the desired output and inbuilt simulink blocks were used as well in order to get better results.

5.2

FUTURE SCOPE

Vector control technique uses PI controllers andsatisfactory performance is obtained around the operating point at which they are tuned. The loads used in the project are linear and satisfactory results were obtained but when the loads become non-linear the control becomes difficult.The majority of the physical systems are non-linear and are prone to parametric variation therefore need automatic control. Non Linear vector control is an area to look for in the future. Fuzzy Logic Controller is not restricted by these limitations. It is independent of the process model. It has self-tuning property which can adapt to the variations from the previous error signals. Thus it would be better to say that Fuzzy Logic Controller is the next technology which can improve the power quality and provide desired output.

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