DFIG-based Wind Power Conversion System Connected to Grid

Table of Content

DECLARATION

CERTIFICATE

ACKNOWLEDGEMENT

ABSTRACT

LIST OF TABLES

LIST OF FIGURES

LIST OF ABBREVIATIONS

LIST OF SYMBOLS

CHAPTER 1: INTRODUCTION
1.1 GENERAL
1.2 BACKGROUND
1.3 WIND ENERGY
1.4 STATE OF ART
1.4.1 Classification of Wind Turbine
1.4.2 Types of Wind Generators
1.5 DOUBLY-FED INDUCTION GENERATOR
1.5.1 Advantages of DFIGs
1.6 POWER CONVERTERS
1.7 ENERGY STORAGE SYSTEMS
1.7.1 Advantages of ESS
1.7.2 Types of ESS
1.8 BUCK-BOOST CONVERTER
1.8.1 Main Advantage of Buck-Boost Converter
1.9 POWER SYSTEM INTERCONNECTION
1.9.1 Transformer
1.9.2 Advantages of Transformer in Rotor Circuit of DFIG
1.10 WIND POWER CHALLENGES
1.11 TECHNICAL CHALLENGES
1.12 PROBLEM FORMULATION AND METHODOLOGY
1.13 OUTLINE OF THESIS

CHAPTER 2: LITERATURE REVIEW 20-25
2.1 GENERAL
2.2 LITERATURE REVIEW

CHAPTER 3: MODDELING, OPERATION AND CONTROL OF DFIG 26-39
3.1 GENERAL
3.1.1 DFIG Based Wind Energy Conversion System
3.1.2 Steady-State Equivalent Circuit
3.2 REACTIVE POWER CONTROL
3.2.1 Reactive Power Sources
3.2.2 Optimum Reactive Power Distribution
3.3 TRANSIENT MODELS AND CONTROL OF DFIG
3.3.1 Power Converter Controls
3.3.1.1 Rotor Side Converter Control
3.3.1.2 Current Regulator Control
3.3.2. Grid Side Converter Control
3.3.2.1 The Simulink Model of Current Regulator Control and Dc Voltage Regulator
3.4 SPEED CONTROL
3.5 AC VOLTAGE REGULATION
3.6. DECOUPLED CONTROL OF ACTIVE AND REACTIVE POWERS
3.7 CONCLUSIONS

CHAPTER 4: INTERCONNECTION OF DFIG WITH DIFFERENT SUB-SYSTEMS
4.1 INTEGRATION OF ENERGY STORAGE SUB-SYSTEM
4.1.1 Energy Storage Sub-System
4.1.2 DFIG with Energy Storage Sub-System
4.1.3Simulink Based Model of DFIG with Battery Energy Storage Sub-System
4.1.4 Energy Storage Control
4.1.5 Grid Side Converter
4.1.6 Energy Storage System Characteristics
4.1.6.1 Operating Characteristics at Constant Wind Speed
4.1.6.2 Operating Characteristics at Variable Wind Speed
4.2 DFIG WITH BUCK/BOOST CONVERTOR
4.2.1 Series Compensation Concept
4.2.2 Buck and Boost Mode
4.2.3Simulink Based Model of DFIG with Buck-Boost Converter Sub- System
4.2.3.1 Buck-Boost Converter Control
4.2.4 Buck-Boost Converter Characteristics
4.2.4.1 Operating Characteristics at Constant Wind Speed
4.2.4.2 Operating Characteristics at Variable Wind Speeds
4.3 DFIG WITH TRANSFORMER IN ROTOR SIDE CIRCUIT
4.3.1 Simulink Based Model of DFIG with Transformer in Between Machine and Rotor Side Converter
4.3.2 Transformer Characteristics
4.3.2.1 Operating Characteristics at Constant Wind Speed
4.3.2.2 OPERATING CHARACTERISTICS AT VARIABLE WIND SPEEDS
4.4 DFIG WITH 3-WINDING TRANSFORMER
4.4.1 Simulink Model of DFIG with 3-winding Transformer
4.4.2 3-Winding Transformer Characteristics
4.4.2.1 Operating Characteristics at Constant Wind Speed
4.4.2.2 OPERATING CHARACTERISTICS AT VARIABLE WIND SPEEDS
4.4 CONCLUSIONS

CHAPTER 5: INTERCONNECTION OF DFIGS WITH GRID 75-86
5.1 GENERAL
5.2 INTERCONNECTION OF DFIGS
5.3 NORMAL WIND FARM OPERATION
5.3.1 Real Power Flow
5.3.2 Voltage Regulation
5.4 ANALYSIS OF TOTAL HARMONIC DISTORTION
5.5 EFFICIENCY OF DFIG WITH DIFFERENT SUB-SYSTEMS
5.6 CONCLUSIONS

CHAPTER 6: SUMMARY, CONCLUSIONS AND FUTURE WORK 87-88
6.1 SUMMARY AND CONCLUSION
6.2 FUTURE WORK

REFERENCES

Appendix A

Appendix B

Appendix C

List of Publications

ACKNOWLEDGEMENT

It gives me a great sense of pleasure to present the report of the M. Tech Project undertaken during M. Tech. Final Year. I owe special debt of gratitude to Dr. A. K. Rai, Department of Electrical Engineering, for his constant support and guidance throughout the course of my work. His sincerity, thoroughness and perseverance have been a constant source of inspiration for me. It is only his cognizant efforts that my endeavor has seen light of the day.

I also take the opportunity to acknowledge the contribution of Prof. M. P. Dave, Co-ordinator, M.Tech programme, Department of Electrical and Electronics Engineering, for his full support and assistance during the development of the project.

I am grateful to Prof. V. K. Paresar, Professor and Head, Dept. of Electrical and Electronics Engineering for providing necessary facilities in the department.

I also do not like to miss the opportunity to acknowledge the contribution of all faculty members of the department for their kind assistance and cooperation during the development of my project.

Finally, I would like to say thanks to my parents, whom I owe this degree most to. I would like to thank them for their patience, their understanding and their encouragement. Last but not the least, we acknowledge our friends for their contribution in the completion of the project.

Akshay Kumar

Date:

ABSTRACT

Wind generation has become the most important alternate energy source and has experienced increased progress in India during the past decade. While it has great potential as an alternative to less environmentally friendly energy sources, there are various technical challenges that cause wind to be considered negatively by many utilities. Wind energy conversion systems suffer from the fact that their real power generation is closely dependent on the local environmental conditions.

The Doubly Fed Induction Generator (DFIG) based wind turbine with variable-speed variable-pitch control scheme is the most popular wind power generator in the wind power industry. This machine can be operated either in grid connected or standalone mode.

In this thesis, a detailed electromechanical model of a DFIG-based wind turbine connected to power grid as well as separately operated wind turbine system with different sub-systems is developed in the MATLAB/SIMULINK environment and its equivalent generator and turbine control structure is realized. In this regard following configurations have been considered:

- DFIG with Battery storage sub-system
- DFIG with Buck-Boost converter
- DFIG with transformer
- DFIG with 3-winding transformer

Addition of battery storage and buck-boost converter sub-systems into the system enables not only dispatching of generator power but also decreases the variability in their reactive power requirements. The full control over both active and reactive power is possible by the use of transformer between DFIG and rotor side converter.

The steady state behavior of the overall wind turbine system is presented and the steady state reactive power ability of the DFIG is analyzed. It has been shown that major part of the reactive power should be supplied from rotor side converter to reduce the overall rating of the generator.

The DFIG with above mentioned sub-systems is connected to grid. The total harmonic distortion analysis and efficiency are carried out. It is found that DFIG with transformer in between machine and rotor side converter has lowest THD (2.29%) and DFIG with 3-winding transformer has maximum efficiency (above 93%).

LIST OF TABLES

Table 1.1 Variation of wind speed in Delhi

Table 1.2 The advantages and disadvantages of WTG

Table 1.3: Comparison of wind generator design characteristics and Costs

Table 3.1 Control objectives for converters and DFIG-WECS

Table 4.1 Energy storage system control for normal and limiting Operation

Table 5.1 Total harmonic distortion and efficiency of DFIG with different sub-systems

LIST OF FIGURES

Fig. 1.1 Types of wind turbine

Fig. 1.2 Savonius and Darrieus wind turbines

Fig. 1.3 Components of a modern horizontal-axis wind turbine

Fig. 1.4 Different wind generator topologies..

Fig. 1.5 Energy storage system technologies..

Fig. 1.6 Connection of the DFIG to the MV network using (a) 2-winding transformers (b) 3-winding transformer..

Fig. 1.7 Wind farm interconnections (a) ac interconnect (b) LVDC interconnect

Fig. 3.1 DFIG wind energy conversion system

Fig. 3.2 Wound rotor induction machine equivalent circuit.

Fig. 3.3 Reactive power options for wind generators (a) switched capacitors (b) SVC (c) STATCOM (d) DFIG

Fig. 3.4 Reactive power sources in the DFIG

Fig. 3.5 Rotor side converter control

Fig. 3.6 Current control structure of rotor side.

Fig. 3.7 Grid side converter control .

Fig. 3.8 Current regulator control of grid side..

Fig. 3.9 DC Voltage regulator .

Fig. 3.10 Step response of dc voltage regulator using MATLABSimPowerSystems..

Fig. 3.11 Speed control loop for generation of Ps,ref

Fig. 3.12 Slip and rotor currents for operation from 1.1 wsyn to 0.9 wsyn using MATLAB – SimPowerSystems

Fig. 3.13 Response for Ps and Qs using MATLAB SimPowerSystems

Fig. 4.1 Energy storage system in DC link

Fig. 4.2 MATLAB based model of Battery energy storage system in DC-link of DFIG.

Fig. 4.3 Performance of DIFG with battery storage system at constant wind speed (10m/s)..

Fig. 4.4 Performance of DIFG with battery storage system at variable wind speeds

Fig. 4.5 Grid side converter control for DFIG with ESS..

Fig. 4.6 Power flows for DFIG with Battery energy storage system

Fig. 4.7 Reactive power flows for DFIG with Battery energy storage system.

Fig. 4.8 Flow of power with battery energy storage system in variable speed characteristics.

Fig. 4.9 Buck-boost converter system in DC link.

Fig. 4.10 Conventional Buck-Boost convertor.

Fig. 4.11 MATLAB based model of Buck-boost converter in DC-link of DFIG

Fig. 4.12 Performance of DIFG with buck-boost converter at constant wind speed (10m/s)..

Fig. 4.13 Performance of DIFG with buck-boost converter at variable wind speed

Fig. 4.14 Buck boost converter control circuit.

Fig. 4.15 Power flows for DFIG with Buck-boost converter ..

Fig. 4.16 Reactive power flows for DFIG with Buck-boost converter.

Fig. 4.17 Flow of power with buck-boost converter in variable speed characteristics.

Fig. 4.18 Energy storage system in DC link.

Fig. 4.19 MATLAB based model of Transformer in between DFIG and rotor side converter

Fig. 4.20 Performance of DIFG with transformer in rotor circuit at constant wind speed (10m/s)

Fig. 4.21 Performance of DIFG with transformer in rotor circuit at variable wind speeds..

Fig. 4.22 Power flows for DFIG with transformer in rotor circuit

Fig. 4.23 Reactive power flows for DFIG with transformer in rotor circuit

Fig. 4.24 Flow of power with transformer in variable speed characteristics.

Fig. 4.25 Connection of the DFIG to the MV network using (a) 2-winding transformers (b) 3-winding transformer..

Fig. 4.26 Schematic diagram of WECS with 3-winding transformer

Fig. 4.27 Simulation model of 3-Winding Transformer

Fig. 4.28 Performance of DIFG with 3-Winding transformer at constant wind speed (10m/s)

Fig. 4.29 Performance of DIFG with 3-Winding transformer at variable wind speeds

Fig. 4.30 Power flows for DFIG with 3-Winding transformer.

Fig. 4.31 Reactive power flows for DFIG with 3-Winding transformer .

Fig. 4.32 Flow of power with 3-Winding transformer in variable speed characteristics..

Fig. 5.1 Interconnection of small wind farm with power system for transient system level studies

Fig. 5.2 Interconnection of DFIG through transmission line with GRID.

Fig. 5.3 Power supplied to the system for the cases of identical wind conditions, different wind conditions, and (a) DFIGs with energy storage, (b) DFIG with buck-boost converter, (c) DFIG with transformer in rotor circuit and (d) DFIG with 3-winding transformer....

Fig. 5.4 Reactive power supplied by wind farm (a) with and without Battery energy storage system, (b) with and without Buck-boost, (c) With and without transformer and (d) with and without 3-winding transformer.

Fig. 5.5 DFIG-BESS voltage waveform and THD

Fig. 5.6 DFIG with Buck-Boost converter voltage waveform and THD..

Fig. 5.7 DFIG with transformer, voltage waveform and THD.

Fig. 5.8 DFIG with 3-winding transformer, voltage waveform and THD

Fig. 5.9 (a) Efficiency of DFIG with BESS, (b) Efficiency of DFIG with Buck-Boost Converter,

(c) Efficiency of DFIG with transformer in between machine and rotor side converter and (d) DFIG with 3-winding transformer....

LIST OF ABBREVIATIONS

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LIST OF SYMBOLS

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CHAPTER 1

INTRODUCTION

1.1 GENERAL

With the development of societies and comfort life style, the utilization of electric power is increased day-by-day and the gap between demand and supply is increasing around the world. Electric power generation through non renewable power plants leads to polluting the air and also decreasing the fossil fuels. Due to limited fossil fuel resources and large environmental problems caused by them, renewable energy sources like solar power, wind power in particular are developing quickly in the world. Wind energy is one of the fastest growing renewable sources of energy in the world. The generation of wind power is clean and non-polluting; it does not harm the environment. During the last decade use of wind energy has raised substantially, and its share in total energy production has increased to a great extent. There are many countries around the world that are spending on wind projects from the research to installation .Wind energy is a very clean and suitable solution to be one the answers for increasing energy demand.

1.2 BACKGROUND

In the starting of 1000 AD, wind turbine came as a horizontal axis windmill for mechanical power generation in Persia, Tibet and China. In between 1100 and 1300, mechanical windmill technology transfer from the Middle East to Europe took place, followed by advance development of the technology in Europe. During the 19th century various tens of thousands of modern windmills with rotors of 25 meters in diameter were operated in Netherlands, Germany and the France, most part of the mechanical power used in industry was based on wind energy.

In the year 1191, the traditional windmill operated at the Abbey of Bury St Edmunds, in Suffolk. This windmill replaced animal power for drawing water from well and other farm activity such as grinding grain was carried out.

Introduction of DC electric power in 1882, and 3-phase AC power production in 1890s, provided a technical base for creating wind turbines that generate electricity. The most widely known pioneer of electricity generation using wind power is the Danish scientist and engineer Poul La Cour. In 1891, he introduced a four shuttle sail rotor design for generating approximately 10kW of DC electric power. He also utilized the hydrogen gas for gas lamps to light up the local school grounds and applied the DC current for water electrolysis. Europe gained its leadership role in wind energy electricity generation because of La Cour's efforts, development and commercialization of wind electricity in Europe. In 1888, Charles F. Brush introduced the first automatically operating wind turbine generator in Cleveland Ohio, operated for 20 years (12kW, 17-meter-diameter machine).

The Global wind energyCouncil's 2012 market data show continued growth of the market, with annual market growth of almost 10%, and increasing capacity growth of about 19%. A record installation year for US whereas slower market in China, which means that the two countries tied for the first spot in year 2012 [1].

1.3 WIND ENERGY

The energy that can be extracted from the wind is called wind energy and it is directly proportional to the cube of the wind speed. The characteristics of the wind are too critical to understand, from the identification of suitable location to predication of the economic capability. The annual variability of wind speed in Delhi is shown in Table 1.1 [2].

Table 1.1 Variation of wind speed in Delhi

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The wind is highly variable, depends upon both temperature and geological nature. Moreover this variability occurs over a very wide range of scales, both in space and time. This is important because extractable wind energy varies with the cube of wind velocity. This variation is due to different climatic conditions in the different parts of world, also the incline of earth on its axis and its own revolving results in different wind distributions across the world. Within any climatic region, there is a big variation on a smaller scale, which depends upon several factors such as relation of land and water, presence of mountains etc. Depending on location, there may also be significant variations with the time of day, and day of the year. These variations are too important and they can affect production of large scale wind energy, consequent integration into grid. These short term turbulences cause variations in the quality of power to be delivered.

The major advantage of wind is that it is an environmentally friendly form of energy production. Without this characteristic, its growth would be significantly delayed since government encouragements have helped to make it modest in Europe and in North America [3-4]. Compared with other alternate energy sources such as photovoltaic (PV), and combined heat and power (CHP) units, wind is also non-dispatchable, it has the advantage of high capacity and is more well suited for large installations. Wind may serve as one of the larger capacity forms of Distributed generation (DG) while it may be enhanced by more controllable forms of DG either in the form of diesel or PV. Modeling issues for wind and other forms of DG have been addressed [5-6]. Small CHP and Photovoltaic will likely required wind and other larger capacity forms of DG like small hydropower in order to make the case for DG as a means of changing transmission improvements an accurate solution.

1.4 STATE OF ART

A wind turbine is a machine for converting the kinetic energy in the wind into mechanical energy and mechanical energy is then converted into electricity. The machine which converts mechanical energy into electrical energy is called wind generator or aero generator. If the mechanical energy is used directly by machinery, such as a pump or grinding stones, the machine is called a windmill.

1.4.1 Classification of Wind Turbine

There are mainly two types of wind turbines: horizontal axis wind turbine and vertical axis wind turbine, as shown in Fig. 1.1.

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Fig. 1.1 Types of wind turbine

Further these are classified into following categories:-

1) Horizontal axis wind turbine (HWAT)

i) Slow speed (multi-blade), used for mechanical power,
ii) Fast speed (2 or 3 blades), used for electrical power.

2) Vertical axis wind turbine (VAWT)

i) Slow speed (Savonius rotor), used for mechanical power,
ii) Fast speed (Darrieus), used for electrical power.

Savonius and Darrieus wind turbines are shown in Fig. 1.2.

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Fig. 1.2 Savonius and Darrieus wind turbines

The major components of a modern horizontal-axis wind turbine are following and shown in Fig. 1.3.

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Fig. 1.3 Components of a modern horizontal-axis wind turbine

1) Rotor Blade: - The turbine blades are made of high density wood or GFRP (glass fiber reinforced polyester), modern wind turbines have 2 or 3 blades. Two blade design are most cost effective than 3 blade design but the 2 blade design face difference of vibration during orientation of yaw. To over came this difference teething control are necessary. Tether is required to balance the wheel, we know that wind speed increased with height, when the rotor is vertical the blade in upper position experience the high pressure than the lower position, without such teething a additional fatigue from many shaft could effect the life of two blade. Three blades rotor have the smooth power output there is no need to tether the rotor.

2) Hub: - It is a central solid part of the wheel over which blades are mounted. It is made of cast iron which is bolted on a low speed shaft.

3) Gear box: - Two types of gear box are used in wind turbines

i) Parallel shaft:- It is used in small turbines, design is simple, maintenance is easy, high mass material and low power.
ii) Planetary:- It is used in large turbines, complex design, skilled person required.

4) Generator: - The conversion of mechanical power of the wind turbine into the electrical power can be accomplished by one of the following types of the electrical machine:-

i) Synchronous machine
ii) Induction machine
iii) Direct machine.

5) Brake’s: - Two independent set are incorporated on the rotor low speed shaft and high speed shaft, the low shaft brakes are hydraulic where as high speed shaft are self adjusted and spring loaded.

6) Nacelle: - All the major sub-systems of wind generator such as gear box, generator with accessories, brakes are kept in the nacelle. Its primary function is to provide a great support to the system components and provide environment protection for the sub-systems. Nacelle is kept on steel tower by special bearing.

7) Yaw control: - The yaw control continuously orients the rotor in the direction of wind. Most wind turbines are yaw acting i.e. as the wind direction changes, the motor rotates the turbine slowly about the vertical axis, so as to face the blade into the wind. The area of wind steam is swept by the rotor is maximum.

8) Tower: - The wind tower supports the turbine and the nacelle. For large and medium size (20m-50m), the tower is slightly toller than the rotor diameter. Small turbines are generally mounted on tower, otherwise they suffer due to poor wind speed near the ground surface. Both steel and concrete are available and are rarely used, the construction could be tubular or lattice. Lattice tower are cheap and easily transportable and can be protected against corrosion, the main dis-advantage is poor visual appeal. Tubular tower are expensive than the lattice tower.

1.4.2 Types of Wind Generators

There are three types of typical generator systems exist for large wind turbine, as shown in Fig.1.4, the first type is fixed speed wind turbine system using a multi-stage gear box and standard squirrel cage induction generator. Second type is a variable speed wind turbine system with a multi-stage gear box and a doubly fed induction generator. In this case, the power electronic converter feeding the rotor winding has a power rating of 30% of generator capacity. Stator winding of the doubly feed induction generator is directly connected to grid. Third type is a variable speed wind turbine but it is a gear less wind turbine system with a direction driven generator. Normally, a low speed high scale power electronic converter is used.

Synchronous generators are, however, sometimes implemented and variable speed operation is possible when the generator is connected to the system using a back-to-back voltage source converter (VSC). Permanent magnet synchronous machines (PMSM) are the most common choice and are typically implemented in stand-alone systems. PMSM are often used since no field winding control is necessary and the gear-box can be eliminated, making it low maintenance and therefore ideal for operation in remote locations.

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a) Directly coupled synchronous or asynchronous machine

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b) Back-to-back VSC connected generator

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Fig: - c) Doubly fed induction generator

Fig. 1.4 Different wind generator topologies

The advantage with the back-to-back VSC topology is that the generator side converter can control the speed of the generator while the line side converter can adjust the reactive power delivered to the system. Thereby the wind generator system emulates a synchronous machine at the PCC, with the exception that the output power is not completely controllable.

When the wind generator is directly coupled, it is typically a squirrel cage induction machine because it does not need to be synchronized with the system. However, addition of reactive power sources is usually required since the generators consume VArs under all modes of operation, and hence impact the voltage at the PCC, especially during transients.

Finally, the generator may be of the doubly-fed induction machine type, also known as a wound-rotor induction machine, where a back-to-back VSC is connected between the rotor and stator windings. This allows control of the reactive power from both the rotor and supply side converters, with a reduced converter rating. Again the machine speed can be controlled as well which enables peak power tracking or adjustment of the real power output. However, the cost of the machine is greater than the case of a squirrel Cage induction machine and therefore, the additional controls come with an additional price. Compression of advantages and disadvantages are shown in Table 1.2 [14].

Table 1.2 The advantages and disadvantages of WTG

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1.5 DOUBLY-FED INDUCTION GENERATOR

A doubly fed induction machine is basically a standard, wound rotor induction machine with its stator winding directly connected to the gird and its rotor windings connected to the gird through a converter. The power converters are divided into two components, the rotor side converter and the grid side converter. These converters are voltage sourced converters that use force commutated power electronic devices to synthesize an AC Voltage from a DC source. Capacitor is connected on the DC side which acts as the DC voltage source.

The DFIG is currently the most popular machine topology for wind power applications. The majority of large capacity machines (>1 MW) available from manufacturers such as Vestas and General Electric-Wind are all DFIGs (although some producers such as Entercon are favoring PMSM). The power electronic converters enable control over the generator operating characteristics such as speed and reactive power, features which are lacking or limited in squirrel cage induction machine. This allows for variable speed operation for peak power point tracking or output power regulation. In addition, the converter rating is significantly reduced compared with the stator connected converter system.

1.5.1 Advantages of DFIGs

The main reasons that make the DFIG a popular choice for wind power applications include its ability for variable speed operation, reactive power control, and reduced converter ratings. Due to the fact that the rotor side presents voltages, which are at most 20% of the stator side voltage, the minimum kVA rating of the converter is approximately 20% that of the case of a back-to-back converter connected machine. Table 1.3 summarizes the differences between the three main wind generator topologies.

Table 1.3: Comparison of wind generator design characteristics and Costs

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1.6 POWER CONVERTERS

The converters that are connected between the rotor and the stator of the machine are typically back-to-back VSC, however a matrix converter could be used alliteratively. The advantages compared with the VSC connected machine is that the converter rating is reduced by about factor of between 2 and 5, since the rating is now based upon rotor voltages (Abbildung in dieser Leseprobe nicht enthaltenwhich are related to the speed range of the machine. The rotor voltages are related to the stator voltages (Abbildung in dieser Leseprobe nicht enthaltenby:

Abbildung in dieser Leseprobe nicht enthalten (1.1)

Where s is machine slip and s is a measure of the rotor speed relative to synchronous speed, which is given by:

Abbildung in dieser Leseprobe nicht enthalten (1.2)

The converter rating is then defined by the maximum speed and the maximum stator Current and voltage at that speed. The tums ratio which exists across the machine has been assumed to be 1 for simplicity and therefore, the rotor Current magnitude is equal to the stator Current magnitude. If the upper speed limit is taken to be 1.2 of synchronous speed then the minimum converter rating will be 20% that of the machine's rating. However, for practical purposes a rating of 30-50% might be used taking into account transient operation and the ability to deliver reactive power from the stator.

1.7 ENERGY STORAGE SYSTEMS

Storage of energy is used extensively in low power applications, viable, low-cost energy storage devices. In the conventional utility, power is produced at one location in the system and consumed at another, connection between the two points being accomplished by the transmission network. Still, many benefits can be understood through application of a short term energy storage device and energy storage systems (ESS) have been employed in various applications, despite their consistently higher costs [10,15-17].

1.7.1 Advantages of ESS

Energy storage has been used for various applications inc1uding transportation, uninterruptible power supplies (UPS) and flexible ac transmission systems (FACTS). The main advantages are following: -

- Enables both storage and supply of energy
- Smoothing of the output power from non-dispatchable energy sources
- Short term supply prevents multiple switching of back-up generators
- Instantaneous exchange of real power with the system using power electronic interface.

1.7.2 Types of ESS

There are various types of ESS, and each is modeled slightly differently. Fig. 1.5 presents the four most common systems which are available today. Each of the systems realizes the same goal, where, the manner in which they store energy is different, as well as their modeling and control differs significantly.

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Fig. 1.5 Energy storage system technologies

Many factors need to be considered when selecting the type of energy storage system including: size, rating, speed, and cost. Some storage devices are better suited for larger ratings and the speed of exchange of energy also typically differs. As is expected the cost differs greatly between the various elements.

1.8 BUCK-BOOST CONVERTER

Recently, most WECS uses capacitor or battery as power source in dc link. The efficiency of DC/DC converters becomes an important issue, in order to maintain long working times for batteries in WECS. To over came this problem, buck-boost converter is used.

A general conventional buck-boost DC/DC converter uses an inverting chopper or a combination chopper, which consists of a buck copper and a boost chopper. The inverting chopper stores output energy in storage device, such as reactor or capacitors. Therefore, the converter efficiency is decreased since the power loss occurs in the storage devices. On the other hands, because the combination chopper has two stages for conversion process, the converter efficiency decreases.

From the viewpoint of the battery application, the input voltage i.e. battery voltage, is almost constants under normal operation. The battery voltage becomes markedly higher than normal voltage in the initial condition or overcharge operation, and lower than the nominal voltage in the over-discharge. Therefore the efficiency for voltage at normal condition is very important.

1.8.1 Main Advantage of Buck-Boost Converter

- High efficiency
- Downsizing for use in applications
- Increases the life of energy storing system.

1.9 POWER SYSTEM INTERCONNECTION

The DFIG is connected to the medium voltage (MV) level of the power system by a step-up transformer. Since the 1ine side converter typically requires an additional transformer to match the converter output voltage to the line voltage, either two, 2-winding transformers or one, 3-winding transformer may be used Fig. 1.6.

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(a)

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(b)

Fig. 1.6 Connection of the DFIG to the MV network using (a) 2-winding transformers (b) 3-winding transformer

Typically in a wind park, numerous wind generators would be connected to the MV bus which would then be connected to the high voltage feeder via another transformer. High voltage dc (HVDC) transmission systems have been considered for integration of wind with the ac system, however, it seems to be only economically feasible for very large installations, located far offshore [9],[10]. Recently low voltage dc transmission (LVDC) has received interest as a possible compromise between HVDC and ac interconnection, with the advantages of HVDC at a reduced cost [11]. However, in most cases an ac intertie is the preferred method of interconnection. These wind park arrangements are presented in Fig. 1.7.

Wind parks are typically only connected at a single point although reliability could be improved if connection to multiple feeders was done. Particularly in the case of offshore installations, the cost associated with multiple interties compounded with the strict environmental regulations make the connection of a radial wind farm at a single point to the transmission system the norm.

The interconnection of the wind farm is often to a weak system, which amplifies many of the technical difficulties associated with wind generators [8]. In this case, fluctuating output power results in a highly variable voltage at the PCC and therefore, reactive power control is required to help regulate the system voltage. The response of the system following faults in this case is also of great concern and the under-voltage and over-speed protections must be carefully chosen [12].

Future wind park projects have been limited by a number of factors, most importantly, issues related to transmission [1]. Since new installations are usually located away from central generation and load centers, transmission problems have slowed their progress. Although many sites have access to rural distribution networks, the transmission system is usually very weak or inadequate to support large amounts of generation. The problem is complicated further when the generation is unpredictable, resulting in similar changes in the bus voltage. This emphasizes the need for reactive power control but also the ability to smooth the power fluctuations due to the wind dynamics.

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Fig. 1.7 Wind farm interconnections (a) ac interconnect (b) LVDC interconnect

1.9.1 Transformer

Mostly, WESC uses the ESS, or as proposed above buck-boost converter to increase the life of energy storing system. Rotor side converter only provides controlled power according to input supply which means that it provides only to its peak value. As we know that rotor power of DFIG is very low as compared to stator side. If, the rotor side voltage is increased then rotor side converter provides more voltage across the ESS which means that system is highly capable for controlling the reactive power and voltage regulation.

To increase the voltage across rotor side converter, introduce a step-up transformer in between DFIG and rotor side converter.

1.9.2 Advantages of Transformer in Rotor Circuit Of DFIG

- High active and reactive power control
- Voltage regulation is easy
- Support of rotor power at grid is increased.

1.10 WIND POWER CHALLENGES

The challenges associated with operation of a wind farm lead to a greater concern for the operation under disturbances within the nearby power system. Since wind farms are typically composed of only induction machines, loss of stability is no longer a concern. However, tripping of the generators due to under-voltage and over-speed of the generators can result in voltage stability problems and even small disturbances may lead to widespread tripping and associated instabilities. Reactive power compensation can help to improve transient stability and the integration of energy storage into variable reactive power sources has shown to provide a further increase in the transient stability of the system [13].

1.11 TECHNICAL CHALLENGES

When the penetration of wind increases, the mode in which it interacts with the power system becomes more important. DGs are normally required to trip during under-voltages and other system disturbances and are not required to support in voltage or frequency regulation [7]. But, for high penetrations of DG, these requirements are likely unreasonable and in some cases may even be harmful to the stability of the system. For instance, without voltage regulation controls, the voltage may regularly fluctuate outside the tolerable operating range [4], and fast tripping of wind generators following under-voltages have been shown to lead to voltage failure [5].

The problem of reactive power and voltage regulation control is even bigger in the case of a weak connection. The large source impedance results in major fluctuations in the voltage at the point of inter-connection (Point of Common Coupling) due to changes in power flows and in the case of reactive compensation has been shown too critical [8]. Propagation of harmonics in the system is moderately unimportant compared with voltage stability issues. The capability to smooth the power oscillations will result in a more stable terminal voltage. However, reactive compensation would still be a strict requirement in steady-state but even more importantly during transients.

The response of systems with increased levels of wind energy to several transients has been examined [9, 5, 10]. Different conventional systems, the loss of synchronism is not a concern since the generators are usually all asynchronous, however over speed and under-voltage protection causes tripping of the turbines, which can frequently lead to voltage instability or failure. From these studies, it was shown that wind parks composed of DFIGs typically demonstrate a higher level of stability due to the ability for reactive power compensation and a better control of the generator speed. Effect of additional energy storage may further improve the DFIGs tolerance to power system disturbances. However, definition of the various protections for this system and an assessment of the system performance under transients are required.

Major factors that have fast-tracked the wind-power technology growths are as follows:

1. Development of fiber composites high-strength, large, low-cost blades.
2. Improved plant operation, pushing the availability up to 95 percent
3. Variable-speed operation of electrical generators to capture maximum energy.

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