Documentation Full

A project Report on

MULTILEVEL INVERTER WITH FACTS CAPABILITY FOR RELIABLE POWER DISTRIBUTIONS USING SOURCE AS WIND ENERGY

Submitted towards the partial fulfillment of the requirements for the Award of the Degree of

Master of Technology In Power Electronics and Power Systems (PE&PS) Submitted by CH.YESURAJU (Roll No.156P1D9903) Under the Esteemed Guidance of Mr. M.SUDHEER KUMAR (M. Tech) Assistant Professor Dept of EEE, PCET

Department of Electrical and Electronics Engineering PRASIDDHA COLLEGE OF ENGINEERING AND TECHNOLOGY Approved by AICTE, Affiliated to JNTUK, Kakinada Anathavaram , Amalapuram , East Godavari district, Andhra Pradesh-533221 2017

CERTIFICATE This is to certify that the work presented in this project entitled “MULTILEVEL INVERTER

WITH

FACTS

CAPABILITY

RELIABLE

FOR

POWER

DISTRIBUTIONS USING SOURCE AS WIND ENERGY ”submitted in the partial fulfillment of the requirement for the award of the degree of Master of Technology in power electronics and power system (PE&PS) to the PCET, Anathavaram, A.P. Affiliated to the JNTUK, Kakinada is an authentic record of my own work carried out under the supervision of Mr. M.SUDHEER KUMAR, Assistant Professor. The matter exemplified in undertaking has not been submitted for the award of whatever other degree to any other University.

Date

CH.YESURAJU

Place: PCET, Anathavaram

(156P1D9903)

This is to certify that the above statement made by the candidate is correct and true to the best of my knowledge.

Project Guide

Principal

Head of the Department

External Examiner

ACKNOWLEDGEMENT I would like to express my deepest thanks to all those people who influenced my work. I would like to add a few heartfelt words for the people who gave their unending support with warm wishes. Firstly, I express my grave on account of Sri. M. VEERABHADRA RAO, Principal, Prasiddha College of engineering. I am feeling greatly obliged by Mr. K. V. RAMANA Associate professor and Head of the Department of EEE, for providing his encouragement and also full facilities for the execution of this work. I desire to express my deep appreciation to my guide Mr. M.SUDHEER KUMAR, Assistant Professor, Department of Electrical and Electronics Engineering, for supporting me at the conception of this work and helped me in resolving my problems in many direct and indirect ways. Their thoughtful and valuable reviews and suggestions have helped me to improve the work. There are no enough words to describe the debt I owe to my guide, for having created a stimulating atmosphere of academic excellence, the basic element of any long lasting endeavor. I heartily acknowledge the cooperation and moral support of my family, and to my friends who always supported me in the entire course of study and project work.

CH.YESURAJU (156P1D9903)

Page no. Abstract

i

List of figures

ii

List of tables

iv

CHAPTER 1: INTRODUCTION 1.1 Role of Power Electronics

1

1.2 Introduction to Wind Energy

4

1.2.1 Power from the wind

4

1.2.2 Wind Turbines

5

1.3 Why Induction Generator?

12

1.4 Literature Review

13

CHAPTER 2: DISTRIBUTION STATIC COMPENSATOR 2.1 Introduction to Wind Energy

15

2.2 Basic Configuration and Operation of D-STATCOM

16

2.3Compensation scheme of D-STATCOM

17

CHAPTER 3: MULTILEVEL INVERTER TOPOLAGIES 3.1 Cascaded H-Bridges Inverter

20

3.2 Diode-Clamped Multilevel Inverter

22

3.3 Flying Capacitor Multilevel Inverter

23

CHAPTER 4: MODULAR MULTILEVEL CONVERTER 4.1 Introduction

26

4.2 MMC variants

28

4.3 Structure of a single-phase MMC inverter structure

28

CHAPTER 5: PROPOSED CONTROL STRATEGY

31

CHAPTER 6: SIMULATION RESULTS 6.1 Eleven level inverter

38

6.2 Seventeen level inverter

45

6.3 Comparison statement

50

CHAPTER 7: CONCLUSION & FUTURESCOPE 7.1 Conclusion

51

7.2 Future scope

51

REFERENCES

50

ABSTRACT In this project, a new single-phase wind energy inverter (WEI) with flexible AC transmission system (FACTS) capability is presented. The proposed inverter is placed between the wind turbine and the grid, same as a regular WEI, and is able to regulate active and reactive power transferred to the grid. This inverter is equipped with distribution static synchronous compensators option in order to control the power factor (PF) of the local feeder lines. Using the proposed inverter for small-to-mediumsize wind applications will eliminate the use of capacitor banks as well as FACTS devices to control the PF of the distribution lines. The goal of this project is to introduce new ways to increase the penetration of renewable energy systems into the distribution systems. This will encourage the utilities and customers to act not only as a consumer, but also as a supplier of energy. Moreover, using the new types of converters with FACTS capabilities will significantly reduce the total cost of the renewable energy application. In this project, modular multilevel converter is used as the desired topology to meet all the requirements of a single-phase system such as total harmonic distortion (THD), efficiency, and total cost of the system. The proposed control strategy regulates the active and reactive power using power angle and modulation index, respectively. The function of the proposed 11 level inverter is to transfer active power to the grid as well as keeping the PF of the local power lines constant at a target PF regardless of the incoming active power from the wind turbine. And also we showing the use of 17 level inverter with power factor improvement and less THD compared to 11 level inverter. The simulations for an 11 and 17 level inverter have been done in MATLAB/Simulink.

I

List of Figures Fig No.

Name

Pg No.

1.1

Proposed system

2

2.1

Schematic Diagram of a D-STATCOM

15

2.2

Basic building blocks of the D-STATCOM

17

2.3

Vector diagrams of D-STATCOM

18

2.4

A radial distribution system with an unbalance load 19

and a D-STATCOM 3.1

Single-phase structure of a multilevel cascaded H21

bridges inverter 3.2

Output phase voltage waveform of an 11-level cascade inverter with 5 separate dc sources

3.3

Three-phase six-level structure of a diode-clamped 23

inverter 3.4

Three-phase six-level structure of a flying 24

capacitor inverter 4.1

21

Three phase MODULAR MULTILEVEL 26

CONVERTER 4.2

Structure of a single-phase MMC inverter structure

29

5.1

Schematic of the proposed controller system

31

5.2

CPWM waveforms for an11-level MMC inverter

33

5.3

Generatedoutput voltage levels

34

5.4

Selection of capacitors for different voltage levels

36

6.1

Simulation circuit diagram for 11 level inverter

38

6.2

Three phase multi module converter

39

6.3

Multi Module converter

39

6.4

Simulation circuit diagram for Submodule

40

6.5

Simulation circuit diagram for inverter control

40

6.6

Simulated output active power from wind turbine

41

6.7

Simulated active and reactive power of the inverter

42

6.8

Active and reactive power of the power lines

42

6.9

Simulated output voltage of an 11-level inverter

43

6.10

Simulated PF = .90 of the grid

43

II

6.11

Power angle of the 11-level inverter

44

6.12

Modulation index of 11 level inverter

44

6.13

Grid voltage and Current

45

6.14

THD analysis report

45

6.15

Simulation circuit diagram for 17 level inverter

46

6.16

3 Phase multi module converter

46

6.17

Multi Module Converter

47

6.18

Simulation circuit diagram for Submodule

48

6.19

Simulation circuit diagram for inverter control

48

6.20

Output voltage of 17 level inverter

49

6.21

Power factor of the grid

49

6.22

THD analysis report of 17 level inverter

50

III

List of Tables Tb. No.

Name

Pg No.

5.1

Operatingregions for an11-level MMC inverter

38

6.1

Parameters used for the simulation

41

6.2

Comparative statement

50

IV

CHAPTER I INTRODUCTION 1.1 Role of Power Electronics The role of power electronics in distribution systems has greatly increased recently. The power electronic devices are usually used to convert the non conventional forms of energy to the suitable energy for power grids, in terms of voltage and frequency. In permanent magnet (PM) wind applications, a back-to-back converter is normally utilized to connect the generator to the grid. A rectifier equipped with a maximum power point tracker (MPPT), converts the output power of the wind turbine to a dc power. The dc power is then converted to the desired ac power for power lines using an inverter and a transformer. With recent developments in wind energy, utilizing smarter wind energy inverters (WEIs) has become an important issue. There are a lot of single-phase lines in the United States, which power small farms or remote houses. Such customers have the potential to produce their required energy using a small-to-medium-size wind turbine. Increasing the number of small-to-medium wind turbines will make several troubles for local utilities such as harmonics or power factor (PF) issues. A high PF is generally desirable in a power system to decrease power losses and improve voltage regulation at the load. It is often desirable to adjust the PF of a system to near 1.0. When reactive elements supply or absorb reactive power near the load, the apparent power is reduced. In other words, the current drawn by the load is reduced, which decreases the power losses. Therefore, the voltage regulation is improved if the reactive power compensation is performed near large loads. Traditionally, utilities have to use capacitor banks to compensate the PF issues, which will increase the total cost of the system. The modern ways of controlling the PF of these power lines is to use small distribution static synchronous compensators (DSTATCOMs). The D-STATCOMs are normally placed in parallel with the distributed generation systems as well as the power systems to operate as a source or sink of reactive power to increase the power quality issues of the power lines. Using regular STATCOMs for small-to-medium-size single-phase wind applications does not make economic sense and increase the cost of the system significantly. This is where the idea of using smarter WEIs with FACTS capabilities shows itself as a new idea to 1

meet the targets of being cost-effective as well as compatible with IEEE standards. The proposed inverter is equipped with a D-STATCOM option to regulate the reactive power of the local distribution lines and can be placed between the wind turbine and the grid, same as a regular WEI without any additional cost. The function of the proposed inverter is not only to convert dc power coming from dc link to a suitable ac power for the main grid, but also to fix the PF of the local grid at a target PF by injecting enough reactive power to the grid. In the proposed control strategy, the concepts of the inverter and the D-STATCOM have been combined to make a new inverter, which possesses FACTS capability with no additional cost. The proposed control strategy allows the inverter to act as an inverter with D-STATCOM option when there is enough wind to produce active power, and to act as a D-STATCOM when there is no wind. The active power is controlled by adjusting the power angle δ, which is the angle between the voltages of the inverter and the grid, and reactive power is regulated by the modulation index m.

Fig 1.1 Proposed system

There are a large number of publications on integration of renewable energy systems into power systems. A list of complete publications on FACTS applications for grid integration of wind and solar energy was presented in some publications. In some published papers, new commercial wind energy converters with FACTS capabilities are introduced without any detailed information regarding the efficiency or the topology used for the converters. In one published paper, a complete list of the most important multilevel inverters was reviewed. Also, different modulation methods such as sinusoidal pulse width modulation (PWM), selective harmonic elimination, optimized harmonic stepped waveform technique, and space vector 2

modulation were discussed and com-pared. Among all multilevel topologies, the cascaded H-bridge multilevel converter is very well known for STATCOM applications for several reasons. The main reason is that it is simple to obtain a high number of levels, which can help to connect STATCOM directly to medium voltage grids. The modular multilevel converter (MMC) was introduced in the early 2000s. With reference to some published papers describes a MMC converter for high voltage DC (HVDC) applications. This project mostly looks at the main circuit components. Also, it compares two different types of MMC, including H-bridge and full-bridge sub modules. In a publication new single-phase inverter using hybrid-clamped topology for renewable energy systems is presented. The proposed inverter is placed between the renewable energy source and the main grid. The main drawback of the proposed inverter is that the output current has significant fluctuations that are not compatible with IEEE standards. The authors believe that the problem is related to the snubber circuit design. Several other applications of custom power electronics in renewable energy systems exist, including an application of a custom power interface where two modes of operation, including an active power filter and a renewable energy STATCOM. Another application looks at the current-source inverter, which controls reactive power and regulates voltage at the point of common coupling (PCC). Varma et al, an author propose an application of photovoltaic (PV) solar inverter as STATCOM in order to regulate voltage on three-phase power systems, for improving transient stability and power transfer limit in transmission systems. The authors called their proposed system PV-STATCOM. Similar to wind farms (when there is no wind), solar farms are idle during nights. We proposed a control strategy that makes the solar farms to act as STATCOMs during night when they are not able to produce active power. The main purpose of the PVSTATCOM system is to improve the voltage control and the PF correction on threephase transmission systems. In this project, the proposed WEI utilizes MMC topology, which has been introduced recently for HVDC applications. Replacing conventional inverters with this inverter will eliminate the need to use a separate capacitor bank or a STATCOM device to fix the PF of the local distribution grids. Obviously, depending on the size of the power system, multiple inverters might be used in order to reach the desired PF. The unique work in this project is the use of MMC topology for a single-phase voltage-source inverter is able to control the PF of 3

the grid regardless of the wind speed Fig.1.1 shows the complete grid-connected mode configuration of the proposed inverter. The dc link of the inverter is connected to the wind turbine through a rectifier using MPPT and its output terminal is connected to the utility grid through a series-connected second-order filter and a distribution transformer. 1.2 Introduction to wind energy Wind is abundant almost in any part of the world. Its existence in nature caused by uneven heating on the surface of the earth as well as the earth’s rotation means that the wind resources will always be available. The conventional ways of generating electricity using non renewable resources such as coal, natural gas, oil and so on, have great impacts on the environment as it contributes vast quantities of carbon dioxide to the earth’s atmosphere which in turn will cause the temperature of the earth’s surface to increase, known as the green house effect. Hence, with the

advances in science and technology, ways of generating electricity using renewable energy resources such as the wind are developed. Nowadays, the cost of wind power that is connected to the grid is as cheap as the cost of generating electricity using coal and oil. Thus, the increasing popularity of green electricity means the demand of electricity produced by using non renewable energy is also increased accordingly. 1.2.1 Power from the Wind Kinetic energy from the wind is used to turn the generator inside the wind turbine to produce electricity. There are several factors that contribute to the efficiency of the wind turbine in extracting the power from the wind. Firstly, the wind speed is one of the important factors in determining how much power can be extracted from the wind. This is because the power produced from the wind turbine is a function of the cubed of the wind speed. Thus, the wind speed if doubled, the power produced will be increased by eight times the original power. Then, location of the wind farm plays an important role in order for the wind turbine to extract the most available power form the wind. The next important factor of the wind turbine is the rotor blade. The rotor blades length of the wind turbine is one of the important aspects of the wind turbine since the power produced from the wind is also proportional to the swept area of the rotor blades i.e. the square of the diameter of the swept area. 4

Hence, by doubling the diameter of the swept area, the power produced will be fourfold increased. It is required for the rotor blades to be strong and light and durable . As the blade length increases, these qualities of the rotor blades become more elusive. But with the recent advances in fiber glass and carbon-fiber technology, the production of light weight and strong rotor blades between 20 to 30 meters long is possible. Wind turbines with the size of these rotor blades are capable to produce up to 1 megawatt of power. Thus, in selecting wind turbine available in the market, the best and efficient wind turbine is the one that can make the best use of the available kinetic energy of the wind. Wind power has the following advantages over the traditional power plants.     



Improving price competitiveness,



Modular installation,



Rapid construction,



Complementary generation,



Improved system reliability, and



Non-polluting.

1.2.2 Wind Turbines: There are two types of wind turbine in relation to their rotor settings. They are: 



Horizontal-axis rotors, and



Vertical-axis rotors.

In this project, only the horizontal-axis wind turbine will be discussed since the modeling of the wind driven electric generator is assumed to have the horizontal-axis rotor. The horizontal-axis wind turbine is designed so that the blades rotate in front of the tower with respect to the wind direction i.e. the axis of rotation are parallel to the wind direction. These are generally referred to as upwind rotors. Another type of horizontal axis wind turbine is called downwind rotors which has blades rotating in back of the tower. Nowadays, only the upwind rotors are used in large-scale power generation and in this project, the term horizontal-axis wind turbine refers to the upwind rotor arrangement. The main components of a wind turbine for electricity

5

generation are the rotor, the transmission system, generator, and the yaw and control system. The main components of a wind turbine can be classified as    



Tower



Rotor system



Generator



Yaw



Control

 (i) Tower: It is the most expensive element of the wind turbine system. The lattice or tubular types of towers are constructed with steel or concrete. Cheaper and smaller towers may be supported by guy wires. The major components such as rotor brake, gearbox, electrical switch boxes, controller, and generator are fixed on to or inside nacelle, which can rotate or yaw according to wind direction, are mounted on the tower. The tower should be designed to withstand gravity and wind loads. The tower has to be supported on a strong foundation in the ground. The design should consider the resonant frequencies of the tower do not coincide with induced frequencies from the rotor and methods to damp out if any. If the natural frequency of the tower lies above the blade passing frequency, it is called stiff tower and if below is called soft tower. (ii) Rotor: The aerodynamic forces acting on a wind turbine rotor is explained by aerofoil theory. When the aerofoil moves in a flow, a pressure distribution is established around the symmetric aerofoil. A reference line from which measurements are made on an aerofoil section is referred to as chord line and the length is known as chord. The angle, which an aerofoil makes with the direction of airflow measured against the chord line is called the angle of attack  . The generation of lift force L on an aerofoil placed at an angle of attack  to an oncoming flow is a consequence of the distortion of the streamlines of the fluid passing above and below the aerofoil. When a blade is subjected to unperturbed wind flow, the pressure decreases towards the center of curvature of a streamline. The consequence is the reduction of pressure (suction) on the upper 6

surface of the aerofoil compared to ambient pressure, while on the lower side the pressure is positive or greater. The pressure difference results in lift force responsible for rotation of the blades. The drag force D is the component that is in line with the direction of oncoming flow. These forces are both proportional to the energy in the wind. To attain a high efficiency of rotor in wind turbine design is for the blade to have a relatively high liftto-drag ratio. This ratio can be varied along the length of the blade to optimize the turbine’s energy output at various wind speeds. The lift force, drag force or both extract the energy from wind. For aerofoil to be aerodynamically efficient, the lift force can be 30 times greater than the drag force. Cambered or asymmetrical aerofoil’s have curved chord lines. The chord line is now defined as the straight line joining the ends of the camber line and  is measured from this chord line. Cambered aerofoil is preferred to symmetrical aerofoil because they have higher lift/drag ratio for positive angles of attack. It is observed that the lift at zero angle of attack is no longer zero and that the zero lift occurs at a small negative angle of attack of approximately 4 o. The center of pressure, which is at the ¼ chord position on symmetrical aerofoil has at the ¼ chord position on cambered aerofoil and moves towards the trailing edge with increasing angle of attack.

Arching or cambering a flat plate will cause it to induce higher lift force for a given angle of attack and blades with a cambered plate profile work well, under the conditions experienced by high solidity, multi bladed wind turbines. For low solidity turbines, the use of aerofoil section is more effective. The characteristics of an aerofoil, the angle of attack, the magnitude of the relative wind speed are the prime parameters responsible for the lift and drag forces. These forces acting on the blades of a wind turbine rotor are transformed into a rotational torque and axial thrust force. The useful work is produced by the torque where as the thrust will overturn the turbine. This axial thrust should be resisted by the tower and foundations.

7

(a) Rotor speed: Low speed and high-speed propeller are the two types of rotors. A large design tip speed ratio would require a long, slender blade having high aspect ratio. A low design tip speed would require a short, flat blade. The low speed rotor runs with high torque and the high-speed rotor runs with low torque. The wind energy converters of the same size have essentially the same power output, as the power output depends on rotor area. The low speed rotor has curved metal plates. The number of blades, weight, and difficulty of balancing the blades makes the rotors to be typically small. They get self-started because of their aerodynamic characteristics. The propeller type rotor comprises of a few narrow blades with more sophisticated airfoil section. When not working, the blades are completely stalled and the rotor cannot be self-started. Therefore, propeller type rotors should be started either by changing the blade pitch or by turning the rotor with the aid of an external power source (such as generator used as a motor to turn the rotor). Rotor is allowed to run at variable speed or constrained to operate at a constant speed. When operated at variable speed, the tip speed ratio remains constant and aerodynamic efficiency is increased.

b) Rotor alignment: The alignment of turbine blades with the direction of wind is made by upwind or downwind rotors. Upwind rotors face the wind in front of the vertical tower and have the advantage of somewhat avoiding the wind shade effect from the presence of the tower. Upwind rotors need a yaw mechanism to keep the rotor axis aligned with the direction of the wind. Downwind rotors are placed on the lee side of the tower. A great disadvantage in this design is the fluctuations in the wind power due to the rotor passing through the wind shade of the tower which gives rise to more fatigue loads. Downwind rotors can be built without a yaw mechanism, if the rotor and nacelle can be designed in such a way that the nacelle will follow the wind passively. This may however include gyroscopic loads and hamper the possibility of unwinding the cables when the rotor has been yawing passively in the same direction for a long time, thereby causing the power cables to twist. Upwind rotors need to be rather inflexible to keep the rotor blades clear of the tower, downwind rotors can be made more flexible. The latter implies possible savings with respect to weight and may

8

contribute to reducing the loads on the tower. The vast majority of wind turbines in operation today have upwind rotors. c) Number of rotor blades: The three bladed rotors are the most common in modern aero generators. Compared to three bladed concepts, the two and one bladed concepts have the advantage of representing a possible saving in relation to cost and weight of the rotor. However, the use of fewer rotor blades implies that a higher rotational speed or a larger chord is needed to yield the same energy output as a three bladed turbine of a similar size. The use of one or two blades will also result in more fluctuating loads because of the variation of the inertia, depending on the blades being in horizontal or vertical position and on the variation of wind speed when the blade is pointing upward or downward. Therefore, the two and one bladed concepts usually have socalled teetering hubs, implying that they have the rotor hinged to the main shaft. This design allows the rotor to teeter in order to eliminate some of the unbalanced loads. One bladed wind turbines are less widespread than two bladed turbines. This is because they in addition to a higher rotational speed, more noise and visual intrusion problems, need a counter weight to balance the rotor blade. (iii) Generator: Electricity is an excellent energy vector to transmit the high quality mechanical power of a wind turbine. Generator is usually 95% efficient and transmission losses should be less than 10%. The frequency and voltage of transmission need not be standardized, since the end use requirements vary. There are already many designs of wind/ electricity systems including a wide range of generators. The distinctive features of wind/electricity generating systems are: 1) Wind turbine efficiency is greatest if rotational frequency varies to maintain constant tip speed ratio, yet electricity generation is most efficient at constant or near constant frequency. 2) Mechanical control of turbine to maintain constant frequency increases complexity and expense. An alternative method, usually cheaper and more efficient is to vary the electrical load on the turbine to control the rotational frequency. 3) The optimum rotational frequency of a turbine in a particular wind speed decreases with increase in radius in order to maintain constant tip speed ratio. Thus, only small

9

turbines of less than 2 m radius can be coupled directly to generators. Larger machines require a gearbox to increase the generator drive frequency. 4) Gearboxes are relatively expensive and heavy. They require maintenance and can be noisy. To overcome this problem, generators with a large number of poles are being manufactured to operate at lower frequency. 5) The turbine can be coupled with the generator to provide an indirect drive through a mechanical accumulator (weight lifted by hydraulic pressure) or chemical storage (battery). Thus, generator control is independent of turbine operation. The generators used with wind machines are i) Synchronous AC generator ii) Induction AC generator and iii) Variable speed generator a) Synchronous AC generator: The Synchronous speed will be in the range of 1500 rpm – 4 pole, 1000 rpm – 6 pole or 750 rpm, - 8 pole for connection to a 50 Hz net work. The ingress of moisture is to be avoided by providing suitable protection of the generator. Air borne noise is reduced by using liquid cooling in some wind turbines. An increase of the damping in the wind turbine drive train at the expense of losses in the rotor can be obtained by high slip at rated power output. Synchronous generators run at a fixed or synchronous speed, N s . We have N s  120 f p , where p the number of poles is, f is

the electrical frequency and N s is the speed in rpm. b) Induction AC generator: They are identical to conventional industrial induction motors and are used on constant speed wind turbines. The torque is applied to or removed from the shaft if the rotor speed is above or below synchronous. The power flow direction in wires is the factor to be considered to differentiate between a synchronous generator and induction motor. Some design modifications are to be incorporated for induction generators considering the different operating regime of wind turbines and the need for high efficiency at part load, etc. c) Variable speed generator: Electrical variable speed operation can be approached as: All the output power of the wind turbine may be passed through the frequency converters to give a broad range of variable speed operation. A restricted speed range may be achieved by converting only a fraction of the output power. 10

(iv) Yaw system: It turns the nacelle according to the actuator engaging on a gear ring at the top of the tower. Yaw control is the arrangement in which the entire rotor is rotated horizontally or yawed out of the wind. During normal operation of the system, the wind direction should be perpendicular to the swept area of the rotor. The yaw drive is controlled by a slow closed- loop control system. The yaw drive is operated by a wind vane, which is usually mounted on the top of the nacelle sensing the relative wind direction, and the wind turbine controller. In some designs, the nacelle is yawed to attain reduction in power during high winds. In extremity, the turbine can be stopped with nacelle turned such that the rotor axis is at right angles to the wind direction. One of the more difficult parts of a wind turbine designs is the yaw system, though it is apparently simple. Especially in turbulent wind conditions, the prediction of yaw loads is uncertain. (v) Control systems: A wind turbine power plant operates in a range of two characteristic wind speed values referred to as Cut in wind speed uin and Cut out wind speed uo u t . The turbine starts to produce power at Cut in wind speed usually between 4 and 5 m/s. Below this speed, the turbine does not generate power. The turbine is stopped at Cut out wind speed usually at 25 m/s to reduce load and prevent damage to blades. They are designed to yield maximum power at wind speeds that lies usually between 12 and 15 m/s. It would not be economical to design turbines at strong winds, as they are too rare. However, in case of stronger winds, it is necessary to waste part of the excess energy to avoid damage on the wind turbine. Thus, the wind turbine needs some sort of automatic control for the protection and operation of wind turbine. The functional capabilities of the control system are required for:   



Controlling the automatic startup



Altering the blade pitch mechanism



Shutting down when needed in the normal and abnormal condition



Obtaining information on the status of operation, wind speed, direction and power production for monitoring purpose

As can be seen in figure 1 (c), the nacelle consists of several components. They are the generator, yaw motor, gearbox, tower, yaw ring, main bearings, main 11

shaft, hub, blade, clutch, brake, blade and spinner. Other equipment that is not shown in the figure might include the anemometer, the controller inside the nacelle, the sensors and so on. The generator is responsible for the conversion of mechanical to electrical energy. Yaw motor is used power the yaw drive to turn the nacelle to the direction of the wind. The gearbox is used to connect the low-speed shaft (main shaft in the figure) to the high-speed shaft which drives the generator rotor. The brake is used to stop the main shaft from over speeding. The blades are used to extract the kinetic power from the wind to mechanical power i.e. lifting and rotating the blades. The tower is made from tubular steel or steel lattice and it is usually very high in order to expose the rotor blades to higher wind speed. An induction generator is a type of electrical generator that is mechanically and electrically similar to a polyphase induction motor. Induction generators produce electrical power when their shaft is rotated faster than the synchronous frequency of the equivalent induction motor. Induction generators are often used in wind turbines and some micro hydro installations due to their ability to produce useful power at varying rotor speeds. Induction generators are mechanically and electrically simpler than other generator types. They are also more rugged, requiring no brushes or commutators. Induction generators are not self-exciting, meaning they require an external supply to produce a rotating magnetic flux. The external supply can be supplied from the electrical grid or from the generator itself, once it starts producing power. The rotating magnetic flux from the stator induces currents in the rotor, which also produces a magnetic field. If the rotor turns slower than the rate of the rotating flux, the machine acts like an induction motor. If the rotor is turned faster, it acts like a generator, producing power at the synchronous frequency. In induction generators the magnetizing flux is established by a capacitor bank connected to the machine in case of standalone system and in case of grid connection it draws magnetizing current from the grid. 1.3 Why Induction Generator? Induction generator is commonly used in the wind turbine electric generation due to its reduced unit cost, brushless rotor construction, ruggedness, and ease of 12

Maintenance. Moreover, induction generators have several characteristics over the synchronous generator. The speed of the asynchronous generator will vary according to the turning force (moment, or torque) applied to it. In real life, the difference between the rotational speed at peak power and at idle is very small approximately 1 percent. This is commonly referred as the generator’s slip which is the difference between the synchronous speed of the induction generator and the actual speed of the rotor. This speed difference is a very important variable for the induction machine. The term slip is used because it describes what an observer riding with the stator field sees looking at the rotor which appears to be slipping backward. A more useful form of the slip quantity results when it is expressed on a per unit basis using synchronous speed as the reference. 1.4 Literature review Kuo-Ching Tseng and Chi-Chih Hung [1] proposed a High step-up HighEfficiency Interleaved Converter with Voltage Multiplier Module for Renewable energy system. The configuration of the proposed converter not only reduces the current stress but also constrains the input current ripple, which decreases the conduction losses and lengthens the lifetime of the input source. Y. P. Hsieh, J. F. Chen, T. J. Liang, and L. S. Yang [2] presented a comprehensive review on a Novel high step-up DC-DC converter for distributed generation system. High step-up dc–dc converters that do not require isolation. Some dc–dc converters can provide high step-up voltage gain, but with the penalty of either an extreme duty ratio or a large amount of circulating energy. DC–DC converters with coupled inductors can provide high voltage gain, but their efficiency is degraded by the losses associated with leakage inductors. Converters with active clamps recycle the leakage energy at the price of increasing topology complexity. A family of highefficiency, high step-up dc–dc converters with simple topologies is proposed. Y.Zhao, X. Xiang, W. Li, X. He, and C. Xia, [3] worked on a combined operation of the Advanced symmetrical voltage quadruple rectifiers for high step-up and high output-voltage converters. Two-inductor, interleaved power-factor corrected (PFC) boost converter that exhibits voltage-doubler characteristic when it operates 13

with a duty cycle greater than 0.5 is introduced. The voltage-doubler characteristic of the proposed converter makes it quite suitable for universal-line (90–264 RMS) PFC applications. Because the proposed PFC boost rectifier operates as a voltage doubler at low line, its low-line range efficiency is greatly improved.

14

CHAPTER 2 DISTRIBUTION STATIC COMPENSATOR 2.1 INTRODUCTION OF D-STATCOM

Fig 2.1 Schematic Diagram of a DSTATCOM.

A D-STATCOM (Distribution Static Compensator), which is schematically depicted in Fig 2.1, consists of a two-level Voltage Source Converter (VSC), a dc energy storage device, a coupling transformer connected in shunt to the distribution network through a coupling transformer. The VSC converts the dc voltage across the storage device into a set of three-phase ac output voltages. These voltages are in phase and coupled with the ac system through the reactance of the coupling transformer. Suitable adjustment of the phase and magnitude of the D-STATCOM output voltages allows effective control of active and reactive power exchanges between the DSTATCOM and the ac system. Such configuration allows the device to absorb or generate controllable active and reactive power. It may be mentioned that the effectiveness of the D-STATCOM in correcting voltage sag depends on the value of Zth or fault level of the load bus. When the shunt injected current Ish is kept in quadrature with VL, the desired voltage correction can be

achieved without injecting any active power into the system. On the other hand, when the value of Ish is minimized, the same voltage correction can be achieved with minimum apparent power injection into the system. The control scheme for the DSTATCOM follows the same principle as for DVR. The switching frequency is set at 475 Hz. The VSI structure is designed to make use of a three-level pole structure, also called neutral point clamped (NPC), instead of a standard two-level six-pulse inverter 15

structure This three-level inverter topology generates a more sinusoidal output voltage waveform than conventional structures without increasing the switching frequency. The additional flexibility of a level in the output voltage is used to assist in the output waveform construction. In this way, the harmonic performance of the inverter is improved, also obtaining better efficiency and reliability respect to the conventional two-level inverter. A drawback of the NPC inverters is that the split dc capacitor banks must maintain a constant voltage level of half the dc bus voltage. Otherwise, additional

distortion

will

be

contributed

to

the

output

voltage

of

the

DSTATCOM/BESS. In this work, the use of battery energy storage in an arrangement with neutral point (NP) permits to independently contributing to the charge of the capacitors C1 and C2, and thus to maintain the voltage balance of the dc capacitors without using additional control techniques. The connection to the utility grid is made by using low pass sine wave filters in order to reduce the perturbation on the distribution system from high-frequency switching harmonics generated by PWM control. The total harmonic distortion (THD) of the output voltage of the inverter combined with a sine wave filter is less than 5 % at full rated unity power factor load. Typically, leakage inductances of the step-up transformer windings are high enough as to build the sine wave filter simply by adding a bank of capacitors in the PCC. In this way, an effective filter is obtained at low costs, permitting to improve the quality of the voltage waveforms introduced by the PWM control to the power utility and thus meeting the requirements of IEEE Standard 519-1992 relative to power quality. 2.2 Basic Configuration and Operation of D-STATCOM The D-STATCOM is a three-phase and shunt connected power electronics based device. It is connected near the load at the distribution systems. The major components of a D-STATCOM are shown in Figure 2.2. It consists of a dc capacitor, three-phase inverter (IGBT, thyristor) module, ac filter, coupling transformer and a control strategy. The basic electronic block of the D-STATCOM is the voltagesourced inverter that converts an input dc voltage into a three-phase output voltage at fundamental frequency.

16

Fig.2.2 Basic building blocks of the d-statcom The D-STACOM employs an inverter to convert the DC link voltage Vdc on the capacitor to a voltage source of adjustable magnitude and phase. Therefore the DSTATCOM can be treated as a voltage-controlled source. The D-STATCOM can also be seen as a current-controlled source. Figure shows the inductance L and resistance R which represent the equivalent circuit elements of the step-down transformer and the inverter will is the main component of the D-STATCOM. The voltage Vi is the effective output voltage of the D-STATCOM and δ is the power angle. The reactive power output of the D-STATCOM inductive or capacitive depending can be either on the operation mode of the D-STATCOM. Referring to figure 2.1, the controller of the DSTATCOM is used to operate the inverter in such a way that the phase angle between the inverter voltage and the line voltage is dynamically adjusted so that the D-STATCOM generates or absorbs the desired VAR at the point of connection.

The three basic operation modes of the D-STATCOM output current, I, which varies depending upon Vi. If Vi is equal to Vs, the reactive power is zero and the DSTATCOM does not generate or absorb reactive power. When Vi is greater than Vi, the D-STATCOM shows an inductive reactance connected at its terminal. The current I, flows through the transformer reactance from the D-STATCOM to the ac system, and the device generates capacitive reactive power. If Vs is greater than Vi, the DSTATCOM shows the system as a capacitive reactance. Then the current flows from the ac system to the D-STATCOM, resulting in the device absorbing inductive reactive power. 2.3 COMPENSATION SCHEME OF D-STATCOM The D-STATCOM is a DC/AC switching power-converter composed of an air-cooled voltage source converter. Basically, the D-STATCOM is used to suppress 17

voltage variations and control reactive power in phase with the system voltage. The D-STATCOM produces phase- synchronized output voltage, therefore, it can compensate for inductive and capacitive currents linearly and continuously. Active and reactive power trade between the power system and the D-STATCOM is accomplished by controlling the phase angle difference between the two voltages.

Fig.2.3 Vector diagrams of D-STATCOM

If the output voltage of the D-STATCOM VI is in phase with the bus terminal voltage VT, and Vi is greater than VT, the D-STATCOM provides reactive power to the system. If Vi is smaller than VT, the D-STATCOM absorbs reactive power from the power system. Ideally, VT and Vi have the same phase, but actually VT and Vi have a little phase difference to compensate for the loss of transformer winding and inverter switching, so it absorbs some real power from system. Fig. 2.3 shows the DSTATCOM vector diagrams, which show the inverter output voltage Vi, system voltage Vi, reactive voltage Vl and line current I in correlation with the magnitude and phase α. Fig.2.1 a and Fig. 2.1 b explain how Vi and VT produce inductive or capacitive power by controlling the magnitude of the inverter output voltage Vi in phase with each other. Fig. 2.1 c and Fig. 2.1 d show that the D-STATCOM produces or absorbs real power with Vi and VT having a phase difference ±α 18

Figure 2.4 shows a radial type electric power distribution system feeding an unbalanced load. A D-STACOM is installed in parallel with the unbalance load for on-site load compensation. The reactive power output of the D-STATCOM in each phase, which is inductive or capacitive, can be independently controlled by the controller of the D-STATCOM for real-time load compensation. The method of symmetrical components is used in the project for deriving the compensation scheme of the D-STATCOM.

Fig.2.4 A radial distribution system with an unbalance load and a D-STATCOM

The D-STATCOM is now treated as a current-controlled source to locally supply the needed compensation current for on-site load compensation. In the implementation, a current-regulated PWM (CRPWM) inverter is used as the power stage of the D-STATCOM for generating the compensation current, as shown in figure. In order to keep the dc-link voltage of the inverter in the D-STATCOM at an assigned level during operation, the D-STATCOM needs to absorb active power from the power source to supply the power losses and charge the dc-link capacitor in the DSTATCOM. Hence, use of a P-I type feedback controller in the D-STATCOM controller regulates the active current of the D-STATCOM. The overall compensation scheme of the D-STATCOM is now completed.

19

CAHPTER 3 MULTILEVEL INVERTER TOPOLAGIES An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits. Static inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility highvoltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries. The electrical inverter is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters was made to work in reverse, and thus were "inverted", to convert DC to AC. The inverter performs the opposite function of a rectifier. Types in multilevel inverter are discussed below. 3.1 Cascaded H-Bridges inverter A single-phase structure of an m-level cascaded inverter is illustrated in fig.3.1. Each separate dc source (SDCS) is connected to a single-phase full-bridge, or H-bridge, inverter. Each inverter level can generate three different voltage outputs, +Vdc, 0, and –Vdc by connecting the dc source to the ac output by different combinations of the four switches, S1, S2, S3, and S4. To obtain +Vdc, switches S1 and S4 are turned on, whereas –Vdc can be obtained by turning on switches S2 and S3. By turning on S1 and S2 or S3 and S4, the output voltage is 0. The ac outputs of each of the different full-bridge inverter levels are connected in series such that the synthesized voltage waveform is the sum of the inverter outputs. The number of output phase voltage levels m in a cascade inverter is defined by m = 2s+1, where s is the number of separate dc sources. An example phase voltage waveform for an 11level cascaded H-bridge inverter with 5 SDCSs and 5 full bridges is shown in figure. The phase voltage van = va1 + va2 + va3 + va4 + va5.

20

Fig 3.1 Single-phase structure of a multilevel cascaded H-bridges inverter

Fig 3.2 Output phase voltage waveform of an 11-level cascade inverter with 5 separate dc sources.

The conducting angles, θ1, θ2, ..., θs, can be chosen such that the voltage total harmonic distortion is a minimum. Generally, these angles are chosen so that predominant lower frequency harmonics, 5th, 7th, 11th, and 13th, harmonics are eliminated. More detail on harmonic elimination techniques will be presented in the next section.

21

The main advantages and disadvantages of multilevel cascaded H-bridge converters are as follows Advantages:



 The number of possible output voltage levels is more than twice the number of dc sources (m = 2s + 1).  The series of H-bridges makes for modularized layout and packaging. This will enable the manufacturing process to be done more quickly and cheaply. Disadvantages:  Separate dc sources are required for each of the H-bridges. This will limit its application to products that already have multiple SDCSs readily available. 3.2 Diode-Clamped Multilevel Inverter The neutral point converter proposed by Nabae, Takahashi, and Akagi in 1981 was essentially a three-level diode-clamped inverter. In the 1990s several researchers published articles that have reported experimental results for four-, five-, and six-level diode-clamped converters for such uses as static VAR compensation, variable speed motor drives, and high-voltage system interconnections. A three-phase six-level diode-clamped inverter is shown in Figure 3.3. Each of the three phases of the inverter shares a common dc bus, which has been subdivided by five capacitors into six levels. The voltage across each capacitor is Vdc, and the voltage stress across each switching device is limited to Vdc through the clamping diodes. State condition 1 means the switch is on, and 0 means the switch is off. Each phase has five complementary switch pairs such that turning on one of the switches of the pair require that the other complementary switch be turned off. The complementary switch pairs for phase leg are (Sa1, Sa’1), (Sa2, Sa’2), (Sa3, Sa’3), (Sa4, Sa’4), and (Sa5, Sa’5). Below table shows that in a diode-clamped inverter, the switches that are on for a particular phase leg is always adjacent and in series. For a six-level inverter, a set of five switches is on at any given time.

22

Fig 3.3 Three-phase six-level structure of a diode-clamped inverter.

Advantages: 

All of the phases share a common dc bus, which minimizes the capacitance requirements of the converter. For this reason, a back-to-back topology is not only possible but also practical for uses such as a high-voltage back-to-back

 

inter-connection or an adjustable speed drive. 

The capacitors can be pre-charged as a group.



Efficiency is high for fundamental frequency switching.

Disadvantages: 

Real power flow is difficult for a single inverter because the intermediate dc levels will tend to overcharge or discharge without precise monitoring and



control. 

The number of clamping diodes required is quadratically related to the number of levels, which can be cumbersome for units with a high number of levels.

3.3 Flying Capacitor Multilevel Inverter Meynard and Foch introduced a flying-capacitor-based inverter in 1992. The structure of this inverter is similar to that of the diode-clamped inverter except that instead of using clamping diodes, the inverter uses capacitors in their place. The 23

circuit topology of the flying capacitor multilevel inverter is shown in Figure 3.4. This topology has a ladder structure of dc side capacitors, where the voltage on each capacitor differs from that of the next capacitor. The voltage increment between two adjacent capacitor legs gives the size of the voltage steps in the output waveform.

Fig 3.4 Three-phase six-level structure of a flying capacitor inverter.

One advantage of the flying-capacitor-based inverter is that it has redundancies for inner voltage levels; in other words, two or more valid switch combinations can synthesize an output voltage. In addition to the (m-1) dc link capacitors, the m-level flying-capacitor multilevel inverter will require (m-1) × (m-2)/2 auxiliary capacitors per phase if the voltage rating of the capacitors is identical to that of the main switches. One application proposed in the literature for the multilevel flying capacitor is static var generation. The main advantages and disadvantages of multilevel flying capacitor converters are as Advantages:





Phase redundancies are available for balancing the voltage levels of the capacitors.



Real and reactive power flow can be controlled.

24



The large number of capacitors enables the inverter to ride through short duration outages and deep voltage sags.

Disadvantages: 

Control is complicated to track the voltage levels for all of the capacitors. Also, precharging all of the capacitors to the same voltage level and startup

 

are complex. 

Switching utilization and efficiency are poor for real power transmission.



The large numbers of capacitors are both more expensive and bulky than clamping diodes in multilevel diode-clamped converters. Packaging is also more difficult in inverters with a high number of levels.

25

CHAPTER 4 MODULAR MULTILEVEL CONVERTER 4.1 Introduction

Fig 4.1 Three phase MODULAR MULTILEVEL CONVERTER (MMC) for hvdc

First proposed for HVDC applications in 2003 by Marquardt and first used commercially in the Trans Bay Cable project in San Francisco, the Modular MultiLevel Converter (MMC) is now becoming the most common type of voltage-source converter for HVDC. Operating principle of Modular Multi-Level Converter (MMC) for HVDC, with four series-connected submodules per valve. For clarity only one phase of the three is shown. Like the two-level converter and the six-pulse linecommutated converter, a MMC consists of six valves, each connecting one AC terminal to one DC terminal. However, where each valve of the two-level converter is effectively a high-voltage controlled switch consisting of a large number of IGBTs connected in series, each valve of a MMC is a separate controllable voltage source in its own right. Each MMC valve consists of a number of independent converter submodules, each containing its own storage capacitor. In the most common form of the circuit, the half-bridge variant, each submodule contains two IGBTs connected in series across the capacitor, with the midpoint connection and one of the 26

two capacitor terminals brought out as external connections. Depending on which of the two IGBTs in each submodule is turned on, the capacitor is either bypassed or connected into the circuit. Each submodule therefore acts as an independent two-level converter generating a voltage of either 0 or Usm (where Usm is the submodule capacitor voltage). With a suitable number of submodules connected in series, the valve can synthesize a stepped voltage waveform that approximates very closely to a sine-wave and contains very low levels of harmonic distortion. The MMC differs from other types of converter in that current flows continuously in all six valves of the converter throughout the mains-frequency cycle. As a result, concepts such as “on-state” and “off-state” have no meaning in the MMC.

The direct current splits equally into the three phases and the alternating current splits equally into the upper and lower valve of each phase. A typical MMC for an HVDC application contains around 300 submodules connected in series in each valve and is therefore equivalent to a 301 level converter. Consequently the harmonic performance is excellent and usually no filters are needed. A further advantage of the MMC is that PWM is not necessary, with the result that the power losses are much lower than those of the 2-level converter, at around 1% per end. Finally, because direct series-connection of IGBTs is not necessary, the IGBT gate drives do not need to be as sophisticated as those for a 2-level converter. The MMC has two principal disadvantages. Firstly, the control is much more complex than that of a 2-level converter. Balancing the voltages of each of the submodule capacitors is a significant challenge and requires considerable computing power and high-speed communications between the central control unit and the valve. Secondly, the submodule capacitors themselves are large and bulky. A MMC is considerably larger than a comparable-rated 2-level converter, although this may be offset by the saving in space from not requiring filters. As of 2012 the largest-capacity MMC HVDC system in operation is still the 400 MW Trans Bay Cable scheme but many larger schemes are under construction, including an underground cable interconnection from France to Spain consisting of two 1000 MW links in parallel at a voltage of ±320 kV.

27

4.2 MMC variants A variant of the MMC, proposed by one manufacturer, involves connecting multiple IGBTs in series in each of the two switches that make up the submodule. This gives an output voltage waveform with fewer, larger, steps than the conventional MMC arrangement. This arrangement is referred to as the Cascaded Two Level (CTL) converter. Functionally it is exactly equivalent to the conventional half-bridge MMC in every respect except for the harmonic performance, which is slightly inferior – although still claimed to be good enough to avoid the need for filtering in most instances. Another alternative replaces the half bridge MMC submodule described above, with a full bridge submodule containing four IGBTs in an H bridge arrangement, instead of two. The full-bridge variant of MMC allows the submodule capacitor to be inserted into the circuit in either polarity. This confers additional flexibility in controlling the converter and allows the converter to block the fault current which arises from a short circuit between the positive and negative DC terminals (something which is impossible with any of the preceding types of VSC). Furthermore it allows the DC voltage to be of either polarity (like a LCC HVDC scheme), giving rise to the possibility of hybrid LCC and VSC HVDC systems. However, the full-bridge arrangement requires twice as many IGBTs and has higher power losses than the equivalent half-bridge arrangement. 4.3 Structure of a single-phase MMC inverter structure MMC has gained increasing attention recently. A number of papers were published on the structure, control, and application of this topology, but none has suggested the use of that for inverter + D-STATCOM application. This topology consists of several half-bridge (HB) submodules (SMs) per each phase, which are connected in series. An n-level single-phase MMC consists of a series connection of 2(n−1) basic SMs and two buffer inductors. Each SM possesses two semiconductor

switches, which operate in complementary mode, and one capacitor. The exclusive structure of MMC becomes it an ideal candidate for medium-to-high-voltage applications such as wind energy applications.

28

Fig. 4.2 Structure of a single-phase MMC inverter structure.

MMC has gained increasing attention recently. A number of papers were published on the structure, control, and application of this topology, but none has suggested the use of that for inverter + D-STATCOM application. This topology consists of several half-bridge (HB) submodules (SMs) per each phase, which are connected in series. An n-level single-phase MMC consists of a series connection of 2(n−1) basic SMs and two buffer inductors. Each SM possesses two semiconductor switches, which operate in complementary mode, and one capacitor. The exclusive structure of MMC becomes it an ideal candidate for medium-to-high-voltage applications such as wind energy applications. Moreover, this topology needs only one dc source, which is a key point for wind applications. MMC requires large capacitors which may increase the cost of the systems; however, this problem is offset by the lack of need for any snubber circuit. The main benefits of the MMC topology are: modular design based on identical converter cells, simple voltage scaling by a series connection of cells, simple realization of redundancy, and possibility of a common dc bus. Fig. 4.2 shows the circuit configuration of a single-phase MMC and the structure of its SMs consisting of 29

two power switches and a floating capacitor. The output voltage of each SM (V o) is either equal to its capacitor voltage (Vc) or zero, depending on the switching states. The buffer inductors must provide current control in each phase arm and limit the fault currents. To describe the operation of MMC, each SM can be considered as a two-pole switch. If Sui, which is defined as the status of the ith sub module in the upper arm, is equal to unity, then the output of the ith SM is equal to the corresponding capacitor voltage; otherwise it is zero. Likewise, if Sli which is defined as the status of the ith submodule in the lower arm, is equal to unity, then the output of the i th lower SM is equal to the corresponding capacitor voltage; otherwise it is zero. Generally, when Sui or Sli is equal to unity, the ith upper or lower SM is ON; otherwise it is OFF.

30

CHAPTER 5 PROPOSED CONTROL STRATEGY The proposed controller consists of three major functions. The first function is to control the active and reactive power transferred to the power lines, the second function is to keep the voltages of the SMs’ capacitors balanced, and the third function is to generate desired PWM signals.

Fig 5.1 Schematic of the proposed controller system.

Fig. 5.1 shows the complete proposed controller system. The aim of the designed inverter is to transfer active power coming from the wind turbine as well as to provide utilities with distributive control of volt-ampere reactive (VAR) compensation and PF correction of feeder lines. The application of the proposed inverter requires active and reactive power to be controlled fully independent, so that if wind is blowing, the device should be working as a normal inverter plus being able to fix the PF of the local grid at a target PF (D-STATCOM option), and if there is no wind, the device should be only operating as a D-STATCOM (or capacitor bank) to regulate PF of the local grid. This translates to two modes of operation: 1) when wind is blowing and active power is coming from the wind turbine: the inverter plus DSTATCOM mode. In this mode, the device is working as a regular inverter to transfer active power from the renewable energy source to the grid as well as working as a normal D-STATCOM to regulate the reactive power of the grid in order to control the PF of the grid and 2) when wind speed is zero or too low to generate active power: the D-STATCOM mode. In this case, the inverter is acting only as a source of reactive 31

power to control the PF of the grid, as a D-STATCOM. This option eliminates the use of additional capacitor banks or external STATCOMs to regulate the PF of the distribution feeder lines. Obviously, the device is capable of outputting up to its rated maximum real power and/or reactive power, and will always output all real power generated by the wind turbine to the grid. The amount of reactive power, up to the design maximum, is dependent only on what the utility asks the device to produce. Generally, (5.1) and (5.2) dictate the power flow between a STATCOM device and power lines == −

5.1 − 2

5.2 where X is the inductance between the STATCOM (here as inverter) and the grid which is normally considered as output filter inductance added to the transmission line inductance. The root mean square (RMS) voltage of the STATCOM (= inverter) is given as E s and is considered to be out of phase by an angle of δ to the RMS line voltage E 1. . In the proposed control strategy, active and reactive power transferred between the inverter and the distribution grid is controlled by selecting both the voltage level of the inverter and the angle δ between the voltages of inverter and grid, respectively. The amplitude of the inverter voltage is regulated by changing the modulation index m and the angle δ by adding a delay to the firing signals which concludes =−

5.3 −

2

5.4

=− In this project, m is the key factor to control the reactive power compensation and its main task is to make the PF of the grid equal to the target PF. δ is the control parameter to adjust the active power control between the inverter and the grid. Several assumptions should be considered for the proposed controller which are as: 1) The load on the feeder line should be considered fixed for a small window of time and there is no change in the load during a cycle of the grid frequency; 2) The feeder line 32

can be accurately modeled as a constant P, Q load. This means that the power produced by a wind turbine will displace other power on the feeder line and not add to it; and 3) Although making a change in m or δ has effect on both (5.3) and (5.4), it is assumed that a change in the modulation index will predominantly affect Q, while a change in delta will predominantly affect P. Any effect on Q from a small change in delta is thus ignored. This results in controlling P and Q independently.

Fig. 5.2 CPWM waveforms for an11-level MMC inverter,

33

Fig 5.3 Generated output voltage levels.

Equation (5.5) shows the relation between the target reactive power and the target PF = √2+

2

×

(5.5) where PG is the amount of active power on the grid, Q T is the target amount of reactive power, and PF T is the target PF desired by the utility. So, QT can be calculated as

= √2 ×

2

(5.6) Using (5.5) and (5.6), the target reactive power for the grid is determined and is compared with the actual value of the reactive power of the grid. Using a PI compensator will determine the desired value for the modulation index. The power angle is also determined by comparing the actual dc voltage of the inverter with a reference value. A PI compensator determines the desired value for the power angle. The second function of the controller systems is to keep the capacitors’ voltages balanced. In order to do this, a carrier-based pulse width modulation (CPWM) method is used. The graph Fig 5.1 shows the reference signal and the carrier waveforms for an 11-level MMC inverter using CPWM technique. The graph of Fig. 5.2 shows the output voltage levels generated based on Table 5.1.

34

Voltage Status

nUpperArm

nlowerArm

Vout

Vr≥Vc1,Vc2, Vc3, Vc4, Vc5, Vc6, Vc7, VC8, VC9, Vc10

0

10

5Vdc/10

1

9

4Vdc /10

2

8

3Vdc /10

3

7

2Vdc /10

4

6

Vdc /10

5

5

6

4

-Vdc /10

7

3

-2Vdc /10

8

2

-3Vdc /10

9

1

-4Vdc /10

10

0

-5Vdc /10-

level 1

Vr
2

Vr≥Vc2, Vc3, Vc4, Vc5, Vc6, Vc7, VC8, VC9, Vc10 Vr
3 4 5 6 7 8 9 10 11

Vr≥Vc3, Vc4, Vc5, Vc6, Vc7, VC8, VC9, Vc10 Vr < Vc1,Vc2, Vc3 Vr≥ Vc4, Vc5, Vc6, Vc7, VC8, VC9, Vc10 Vr < Vc1,Vc2, Vc3, Vc4 Vr≥ Vc5, Vc6, Vc7, VC8, VC9, Vc10 Vr < Vc1,Vc2, Vc3, Vc4, Vc5 Vr≥ Vc6, Vc7, VC8, VC9, Vc10 Vr < Vc1,Vc2, Vc3, Vc4, Vc5, Vc6 Vr≥ Vc7, VC8, VC9, Vc10 Vr < Vc1,Vc2, Vc3, Vc4, Vc5, Vc6, Vc7 Vr≥ VC8, VC9, Vc10 Vr < Vc1,Vc2, Vc3, Vc4, Vc5, Vc6, Vc7, VC8 Vr≥ VC9, Vc10 Vr < Vc1,Vc2, Vc3, Vc4, Vc5, Vc6, Vc7, VC8, VC9 Vr≥ Vc10 Vr < Vc1, Vc2, Vc3, Vc4, Vc5, Vc6, Vc7, VC8, VC9, Vc10

0

Table 5.1 Operating regions for an 11-level MMC inverter

In an 11-level CPWM technique, ten carrier signals are compared with a reference sinusoidal signal. In Fig 5.1, based on the phase of the reference signal (V r), there are 11 operating regions where each region defines a voltage level in the output nupperArm+ nlowerArm = 10

(5.7)

Where nupperArm and nlowerArm are the numbers of SMs which are in the upper arm or lower arm, respectively. In an 11-level MMC inverter, there are ten upper and ten lower SMs where each SM has a capacitor. For instance, in voltage level 1 of Table 5.1, all the upper SMs should be OFF and all the lower SMs should be ON, which translates to the fact that the main switches S m of all upper SMs and the auxiliary switches of all lower SMs have to be ON and all the other switches have to be OFF . In this case, the input 35

dc voltage is applied only to the ten lower capacitors, so that the output voltage is V DC/2.

Fig. 5.4 Illustrates selection of capacitors for different voltage levels shown in Table 5.1.

The most critical issue to control MMC is to maintain the voltage balance across all the capacitors. Therefore, the SMs’ voltages are measured and sorted in descending order during each cycle. If the current flowing through the switches is positive, so that capacitors are being charged, n upper Arm and n lower Arm and of the SMs in upper arm and lower arm with the lowest voltages are selected, respectively. As a result, ten capacitors with lowest voltages are chosen to be charged. Likewise, if the current flowing through the switches is negative, so that capacitors are being discharged, n upper Arm and n lower Arm of the SMs in upper arm and lower arm with highest voltages are selected, respectively. As a result, ten capacitors with highest voltages are chosen to be discharged. Consequently, the voltages of the SMs’ capacitors are balanced. Considering Table I and based on the direction of the current flowing through the switches, the proper algorithm will be selected to maintain capacitor balance. The third function of the controller system is the PWM generation block. In this block, based on the desired modulation index, power angle, voltages of the capacitors, 36

direction of the current flowing through the switches and using Table 5.1, the controller generates the PWM signals in order to meet all the system requirements.

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CHAPTER 6 SIMULATION RESULTS This is divided into two parts. First shows the design of an 11-level MMC inverter was carried out in MATLAB/Simulink. The simulation time is 20 seconds and contains severe ramping and de-ramping of the wind turbine. The goal is to assess the behavior of the control system in the worst conditions. Second part shows the design of 17 level inverter showing the improved power factor and less THD. 6.1 Eleven Level inverter The following figures describe the 11 level inverter and its sub components. Fig 6.1 shows the simulation circuit diagram for 11 level inverter. As inverter used is a 3 phase system each phase has a multi module converter. Depending on the level of the inverter number of sub modules is presented. Fig 6.2 shows 3 phase multi module converter. Fig 6.3 shows multi module converter. Fig 6.4 shows circuit diagram for submodule.

Fig 6.1 Simulation circuit diagram for 11 level inverter.

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Fig 6.2 Three phase multi module converter

Fig 6.3 Multi Module converter

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Fig 6.4 Simulation circuit diagram for submodule

Fig 6.5 Simulation circuit diagram for inverter control

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Parameter

Value

Lline Rline Lfilter Transformer primary voltage Transformer secondary voltage Switching frequency Load active power Load reactive power Target PF DC link voltage

15mh 1 ohm 5 mh 12000 V 600 V 2 KHz 50 KW 34.8 KVAR 0.90 2000V

Table. 6.1 Parameters used for the simulation

Table 6.1 shows the values of the parameters used for the simulation. Before t = 6 s, there is no wind to power the wind turbine; therefore, the dc link is opencircuited. At t = 6 s, the input power of the inverter is ramped up to 12 kW in 5 s, and then ramped down to 3.5 kW 4 s later.

Fig. 6.6. Simulated output active power from the wind turbine.

Fig 6.6 shows the output active power from the wind turbine. In the simulation, the local load makes the PF 0.82. When the simulation starts, the inverter provides enough compensation to reach the target PF 0.90.

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Fig 6.7 Simulated active and reactive power of the inverter

Fig. 6.8 Active and reactive power of the power lines

Fig. 6.7 and Fig 6.8 show the output active and reactive power from the wind turbine and the power lines. After t = 6 s, the output power of the wind turbine is increased, and as a result the level of active power provided by the feeder line is decreased by the same amount.

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Fig. 6.9 Simulated output voltage of an 11-level inverter.

The simulated output voltage of the inverter before the filter is shown in Fig. 6.9 and Fig. 6.10 shows the PF of the grid.

Fig. 6.10 Simulated PF = .90 of the grid.

The PF of the grid is constant at 0.90 regardless of the active power from the wind turbine, showing that the main goal of the inverter is achieved. The set-point for dc link voltage of the inverter is 2000 V and the RMS value of the output ac voltage is 600 V. The power angle (delta) and modulation index graphs are shown in Fig. 6.11 and Fig 6.12.

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Fig. 6.11 Power angle of the 11-level inverter.

Fig 6.12 modulation index of 11 level inverter

As soon as the active power comes from the wind turbine, the controller system increases the value of the power angle in order to output more active power to the grid. Therefore, the active power provided from the feeder lines to the load is decreased, and as a result the reactive power from the feeder lines is decreased. Consequently, the modulation index is increased by the controller system to inject more reactive power needed by the load. And fig 6.13 shows the grid voltage and current. 44

Fig 6.13 Grid voltage and Current

Fig 6.14 THD analysis report

Fig 6.14 shows THD analysis report. By this can say that THD of the system 10.87%. 6.2 Seventeen level inverter The simulation circuit diagram of 17 level inverter is the same but number submodules presented in the inverter are different. The following figures describe the 17 level inverter and its sub components. Fig 6.15 shows the simulation circuit diagram for 17 level inverter. As inverter used is a 3 phase system each phase has a multi module converter. Depending on the level of the inverter number of sub modules is presented. Fig 6.16 shows 3 phase multi module converter. Fig 6.17 shows multi module converter. Fig 6.18 shows circuit diagram for submodule. Fig 6.19 45

shows the inverter control diagram. Fig 6.20 shows the Output voltage of 17 level inverter giving clear view of the voltage levels in the system.

Fig 6.15 Simulation circuit diagram for 17 level inverter

. Fig 6.16 3 Phase multi module converter

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Fig 6.17 Multi Module Converter

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Fig 6.18 Simulation circuit diagram for submodule

Fig 6.19 Simulation circuit diagram for inverter control

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Fig 6.20 Output voltage of 17 level inverter

In the figure the output voltage is from -1700 to 1700 voltage level clearly saying that it is alternating. By keeping the filters at the output of the inverter we convert into the smoother waveform.

Fig 6.21 Power factor of the grid

Fig 6.21 shows power factor = 0.95 of the grid with improved. As the power factor is the major consideration the small change will increase system performance. Finally we say that the inverter transfers the whole active power of the wind, excluding its losses, to the grid. The amount of reactive power is dictated by the target PF. When the active power from the wind turbine increases, the controller increases the power angle δ in order to output more active power to the grid in order to decrease the dc link voltage. The modulation index m is also increased when the inverter is supposed to inject more reactive power to the grid. The transient response of the PI controllers used to control the modulation index. Fig 6.22 shows THD analysis report. By this can say that THD of the system 2.39%. 49

Fig 6.22 THD analysis report of 17 level inverter

6.3 Comparison statement

THD 11 level inverter 17 level inverter

Power Factor

10.87%

0.9

2.39%

0.95

Table 6.2 Comparison statement

By seeing above table 6.2 we can say THD was reduced and power factor is increased. In seventeen level inverter, what happens was that the inverter transfers the whole active power of the wind, excluding its losses, to the grid. The amount of reactive power is dictated by the target PF. When the active power from the wind turbine increases, the controller increases the power angle δ in order to output more active power to the grid in order to decrease the dc link voltage.

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CHAPTER 7 CONCLUSION AND FUTURE SCOPE 7.1 Conclusion In this project, the concept of a new multilevel inverter with FACTS capability for small-to-mid-size wind installations is presented. The proposed system demonstrates the application of a new inverter with FACTS capability in a single unit without any additional cost. Replacing the traditional renewable energy inverters with the proposed inverter will eliminate the need of any external STATCOM devices to regulate the PF of the grid. Clearly, depending on the size of the compensation, multiple inverters may be needed to reach the desired PF. This shows a new way in which distributed renewable sources can be used to provide control and support in distribution systems. The proposed controller system adjusts the active power by changing the power angle (delta) and the reactive power is controllable by the modulation index m. The simulation results for an 11-level inverter and 17 level inverter with D-STATCOM capability are simulated in MATLAB Simulink. Active power, reactive power, power factor and THD of 11 level inverter with facts capability are presented which within specified ranges. Power factor and THD of 17 level inverter with D-STATCOM capability are improved as compared with corresponding parameters of 11 level inverter with facts capability. 7.2 Future scope In 17 level inverter needs 8 H-bridges, Each H-bridge 4 switches. The total switches used in designing 17 level inverter was 32 switches in which circuits become complicated, cost and size increases. So I decided to create a reduced switch count i.e., creating 17 level inverter with 17 switches in which circuit less complicated, reduced cost and size

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