Maximum power point tracking and power

smoothing in wind Energy conversion system using

fuzzy logic pitch controller

A Thesis submitted

in partial fulfillment of the requirement for the award of the degree

of MASTER OF TECHNOLOGY

in POWER ELECTRONICS & ASIC DESIGN

by

Pankaj Shukla (Reg. No. 2008PE19)

Under the Guidance of

Dr. S.R.Mohanty Asst. Professor, EED

DEPARTMENT OF ELECTRICAL ENGINEERING

MOTILAL NEHRU NATIONAL INSTITUTE OF TECHNOLOGY

(DEEMED UNIVERSITY) ALLAHABAD-211004

JUNE 2010 MOTILAL NEHRU NATIONAL INSTITUTE OF TECHNOLOGY

ALLAHABAD

CERTIFICATE This is to certify that the thesis entitled, Maximum power point tracking and power

smoothing in wind Energy conversion system using fuzzy logic pitch controller

submitted by Mr. Pankaj Shukla in partial fulfillment of the requirement of the award of

the degree of Master of Technology in Electrical Engineering with specialization in

Power Electronics & ASIC Design to the Motilal Nehru National Institute of Technology,

Allahabad (Deemed University) during the academic year 2009-10. The results

embodied in this thesis have not been submitted for the award of any other degree. We

approve his submission for the above mentioned degree.

Date: June 2010 (Dr. S.R.Mohanty) Place: Allahabad (U.P.) Assistant Professor, EED

CANDIDATESS DECLARATION

I, Pankaj Shukla hereby submit the thesis, as approved by the thesis supervisors Assistant Professor, Dr.S.R.Mohanty, Assistant Professor, Electrical Engineering Department, MNNIT, Allahabad. I hereby declare that the work presented in this thesis is an authentic work carried out by me during July 2009-June- 2010. I have read and understand the Institutes rule relating to the thesis, inventions, innovations and other work and agree to be bound by them. I also declare that, to the best of my knowledge and belief, this work has not been submitted earlier for the award of any other degree or thesis.

June, 2010 (Pankaj shukla) Allahabad Reg.No.2008PE19

Dedicated

To

My parents

ACKNOWLEDGEMENTS

I would like to express my sincere thanks and deepest to my honorable Thesis Supervisors Dr.S.R.Mohanthy, Assistant Professor, Department of Electrical Engineering, Motilal Nehru National Institute of Technology, Allahabad, for their invaluable guidance, motivation, support, advice and supervision during the entire period of this thesis. Their meticulous guidance, constructive and valuable suggestions, timely discussions and clarifications of my doubts increased my cognitive awareness and helped me for making a deeper analysis of the subject under study.

I also express my sincere thanks to my Head of the Department, Prof. Dinesh Chandra, for his invaluable support and encouragement throughout the thesis.

I also express my heartfelt gratitude to the Department of Electrical Engineering MNNIT Allahabad for giving us this opportunity, which has enriched our knowledge and experience immensely.

Lastly, I wish to express thanks to my parents, family members and friends for their patient encouragement and cooperation, which has gone along way in making this report a success.

(Pankaj Shukla) Reg. No. 2008PE19

ABSTRACT

In recent years, there has been a growing interest in wind energy as it is a potential source for electricity generation with minimal environmental impact. With the advancement of aerodynamic designs, wind turbines, which can capture hundreds of kilowatts of power, are readily available. When such wind energy conversion systems (WECS) are integrated to the grid, they produce a substantial amount of power, which can supplement the base power generated by thermal, nuclear, or hydropower plants. The purpose of this work is to develop a maximum power tracking control strategy for variable speed wind turbine systems. In this thesis, four different methods of tracking the peak power in a wind energy conversion system (WECS) is discussed. The algorithms search for the peak power by varying the speed in the desired direction. The generator is operated in the speed control mode with the speed reference being dynamically modified in accordance with the magnitude change of active power. The peak power points in the P curve correspond to dP/d =0. This fact is made use of in the optimum point search algorithm. The generator considered is a wound rotor induction machine whose stator is connected directly to the grid and the rotor is fed through back-to-back pulse-width-modulation (PWM) converters. Pitch angle control is the most common means for adjusting the power output of the wind turbine when wind speed is above rated speed and various controlling variables may be chosen, such as wind speed, generator speed and generator power. As conventional pitch control usually use PI controller, the mathematical model of the system should be well known. A fuzzy logic pitch angle controller is developed in this thesis, in which it does not need well known about the system. The design of the fuzzy logic controller and the comparisons with conventional pitch angle control strategies with various controlling variables are carried out. The simulation results show that the fuzzy logic controller can achieve better control performances than other three methods of maximum power point control strategies. The output power of WECS is also effectively smoothened using the proposed method.

CHAPTER-1 INTRODUCTION

1.1 INTRODUCTION

Wind energy is one of the most available and exploitable forms of renewable energy.

Wind blows from a region of higher atmospheric pressure to one of the lower

atmospheric pressure. The difference in pressure is caused by:

(A) The fact that earths surface is not uniformly heated by the sun and

(B) The earths rotation.

The global electrical energy is rising and there is a steady rise of the demand on power

generation, transmission, distribution and utilization. The maximum extractable energy

from the 0-100m layer of air has been estimated to be the order of 1012

KWh/annum,

which is of the same order as hydroelectric potential.

Wind Energy, energy contained in the force of the winds blowing across the earths

surface.When harnessed, wind energy can be converted into mechanical energy for

performing work such as pumping water, grinding grain, and milling lumber. By

connecting a spinning rotor (an assembly of blades attached to a hub) to an electric

generator, modern wind turbines convert wind energy, which turns the rotor, into

electrical energy [1].

Since earliest recorded history, wind power has been used to move ships, grind grain and

pump water. This is the evidence that wind energy was used to propel boats along the

Nile River as early 5000 B.C. within several centuries before Christ; simple windmills

were used in china to pump water.

In the United States, millions of windmills were erected as the American West was

developed during the late 19th century. Most of them were used to pump water for farms

and ranches. By 1900, small electric wind systems were developed to generate current,

but most of these units fail into disuse as inexpensive grid power was extended to rural

areas during the 1930s. By 1910, wind turbine generators were producing electricity in

many European countries.

Wind turbines are available in a variety of size, and therefore power ratings. The largest

machine, such as the one built in Hawaii, has propellers that span the more than the

length of a football field and stands 20 building stories high, and produces enough

electricity to power 1400 homes. A small home-sized wind machine has rotors between 8

and 25 feet in diameter and stands upwards of 30 feet and can supply the power needs of

an all-electric home or small business.

All electric-generating wind turbines, no matter what size, are comprised of a few basic

components: the (the part that actually rotates in the wind), the electrical generator, a

speed control system, and a tower. Some wind machine have fail- safe shutdown system

so that if part of the machine fails, the shutdown system turn the blades out of the wind or

puts brakes.

In Fig.1.1, the data showing the present situation of installed units in different countries

of the world. It shows that the maximum units is installed in U.S.A (31.62%), then in

China (23.83%) and then in India (6.57%) [44].

Fig.1.1.Installed units of wind power in different countries in percentage.

1.1.1- Benefits of wind power

A wind energy system can provide a cushion against electric power price increases. If

you live in a remote location, a small wind energy system could help you avoid the high

USA, 31.62

CHINA, 23.83

INDIA, 6.57

GERMANY, 6.3SPAIN, 0.9

ITALY, 0.82

FRANCE, 3.59

PORTUGAL, 3.29REST OF THE

WORLD, 14.88

costs of having utility power lines extended to your site. Although wind energy system

involves a significant initial investment, they can be competitive with conventional

energy sources when you account for a lifetime of reduced or altogether avoided utility

costs. The length of the payback period the time before the savings resulting from your

system equal the cost of the system itself- depends on the system you choose the wind

resource on your site, electricity costs in your area, and how you use your wind system.

Wind energy is the world's fastest-growing energy source and will power industry,

businesses and homes with clean, renewable electricity for many years to come. In India,

wind power plants have been installed in Gujarat, Orissa, Maharashtra and Tamil Nadu,

where wind blows at speed of 30 km/h during summer. On the whole, the wind power

potential of India has been estimated to be around 20,000 MW [2].

Small wind energy systems can be used in connection with grid-connected systems, or in

stand-alone application that are not connected to the utility grid. A grid-connected wind

turbine can reduce consumption of utility-supplied electricity for lighting, appliances, and

electric heat. If the turbine cannot deliver the amount of energy you need, the utility

makes up the difference. When the wind system produces more electricity than the

household requires, the excess can be returned to the grid. With the interconnection

available today, switching takes place automatically. Stand-alone wind energy systems

can be appropriate for homes, farms, or even entire communities that are far from the

nearest utility lines.

Either type of the system can be practical if the following condition exist

Some few requirements of wind generation system [4]

1. Wind generation is dependent on the quality and quantity of the wind hitting the

blades. The better the wind you have the more power you will generate.

2. The power available in wind increases by the cube of the wind speed if wind

speed doubles, power output increases by eight.

3. Turbulent wind (from obstruction, geographical features, etc.) will reduce the

power output as the turbine swings back and forth hunting for the wind.

4. These are the few requirements of site for wind generation system:

5. The higher a turbine, the more power is generated, the better quality the wind.

6. A wind turbine should be at least 40 ft above any object within a 400 ft radius.

Note there is often exception to this rule depending on your site.

7. It is often more economical to install a higher tower than purchasing a larger

turbine.

8. Space: generally locations with an acre or more will be suitable. A guyed tower

requires the height of the tower as a radius at a minimum for location of anchor

points. Space is also required for ground assembly and erection of the tower.

Lattice towers require less surface area, but are more complex and expensive to

install.

Wind energy has been the subject of much recent research and development. In order

to overcome the problems associated with fixed speed wind turbine system and to

maximize the Wind energy capture, many new wind farms will employ variable speed

wind turbine. DFIG is one of the components of Variable speed wind turbine system.

DFIG offers several advantages when compared with fixed speed generators including

speed control. These merits are primarily achieved via control of the rotor side converter.

Many works have been proposed for studying the behavior of DFIG based wind turbine

system connected to the grid. Most existing models widely use vector control Double Fed

Induction Generator. The stator is directly connected to the grid and the rotor is fed to

magnetize the machine [3].

Large wind farms have been installed or planned around the world and the power ratings

of the wind turbines are increasing. Wind energy generation equipment is most often

installed in remote, rural areas. These remote areas usually have weak grids, often with

voltage unbalances and under voltage conditions. [5].

Many places also do not have the potential for generating hydel power. Nuclear power

generation was once treated with great optimism, but with the knowledge of the

environmental hazard with the possible leakage from nuclear power plants, most

countries have decided not to install them anymore [42]. It is, however, only since the

1980s that the technology has become sufficiently mature to produce electricity

efficiently and reliably from the wind. Over the last two decades, a variety of wind

energy systems have been developed. Power extracted from wind energy contributes a

significant proportion of consumers electrical power demands. In recent years, many

power converter techniques have been developed for integrating with electrical grid [2].

The first wind turbines were probably simple vertical-axis, such as those used in

Persia as early as about 200 B.C for grinding grain. The use of these vertical-axis mills

subsequently spread throughout the Islamic world Later, horizontal-axis windmills,

consisting of up to ten booms, rigged with jibs sails, were developed. In middle ages,

by the eleventh century A.D, windmills were in extensive use in the Middle East and

were introduced to Europe in the thirteen century by returning crusaders. By the fourteen

century the Dutch had taken the lead in improving the design of windmills, and used

them extensively thereafter for draining the marshes and lakes of the Rhine River delta.

Between 1608 and 1612, Beemster Polder, wetland area which was about 10 feet below

sea level, was drained by 26 windmills of unto 50 hours power (hp) each, operating in

two stages .The first oil mill was built in Holland in 1582 and paper mill 1586. By the

middle of the nineteenth century, some 9000 windmills were being used in the

Netherlands [1, 3].

By 1960, fewer than 1000 were still in working condition due to introduction of

steam engine. The Dutch introducing many improvements in the design of windmills and

particular, the rotors, large industrial mills could deliver up to 90 hp in high winds.

Industrialization, first in Europe and later in America, led to a gradual decline in the use

of windmills. The steam engine replaced European water-pumping windmills. In the

1930s, the Rural Electrification Administration's programs brought inexpensive electric

power to most rural areas in the United States [1, 3].

Since the end of the 19th

century the wind power used to generate electricity. In

1888, Charles F. Brush built the first automatically operating wind turbine of 12 KW with

a rotor diameter of 17 meter and 144 rotor blades made of cedar wood for electricity

generation. The Danish Poul la Cour (1846-1908), another pioneer of electricity

generating wind turbines, discovered fast rotating wind turbines with few rotor blades in

1897 in Askov (Denmark). It was more efficient for electricity production than the slow

moving ones.

Modern wind turbine technology has been accomplished with the help of many areas,

such as material science, computer science, aerodynamics, analytical methods, testing,

and power electronics. Without the help of these areas the rapid development of new

technologies would not be possible. A relatively new area for wind turbines is power

electronics based variable speed drives. Power electronic systems allow

synchronization between the wind turbine system and the utility grid and operate the

wind turbine at variable speeds, increasing the energy production of the system. In

addition, power electronics provide a means to transfer energy to and from storage

units, which can allow the storage of excess energy generation for later use[2].

It is important to find an alternative form of energy before the worlds fossil fuels

are depleted. It is predicted that oil and gas reserves will be depleted by 2032. Due to the

combustion of fossil fuels, carbon dioxide is released into the atmosphere causing the

atmosphere to trap solar radiation that then leads to global warming or the green house

effect.

1.2 LITERATURE REVIEW

A lot of research work has been carried out in the area of wind power

technologies in power systems which led to the development of different methodologies

and approaches. Both grids connected and stand-alone operation is feasible. A lot of

research work has been reported in the area of wind energy conversion systems,

Since wind availability is sporadic and unpredictable. A brief literature review of these

methodologies and approaches is present below.

J.G. Slootweg et al. [41] presented dynamic model (d-q frame) of wind turbine

concept namely a doubly fed (wound rotor) induction generator with a voltage source

converter feeding the rotor. Thus wind turbine concept is equipped with rotor speed, pitch

angle and terminal voltage controllers. The wind turbine response is simulated in this

paper.

In [1], the authors focused on future concepts to increase the penetration of wind

power in power system, where Offers broad coverage ranging from basic network

interconnection issues to industry deregulation and future concepts for wind turbines and

power system. Discusses wind turbine technology, industry standards and regulations

along with power quality issue. [1] presents models for simulating wind turbines in power

system.

The [2, 4] are added with almost all existing machines, but they introduced new control

concepts on different motors and its drives. Introduced matlab/simulink model for The

doubly fed (wound rotor) induction generator control through a rotor connected

bidirectional a.c., d.c., a.c. PWM converter that is used for pump storage hydro and wind

energy conversion today.All the renewable sources, non renewable sources and other

energy sources are discussed in [3].

F. Mei and B. Pal [5] investigated the modal analysis of a grid connected doubly fed

induction generator (DFIG). The change in modal properties for different system

parameters, operating points, and grid strengths are computed and observed. The results

offer a better understanding of the DFIG intrinsic dynamics, which can also be useful for

control design and model justification. L. Szabo et al. [1] presented simulation tool for

induction generators. In this paper, a mathematical model of doubly-fed induction

generator was built to control active and reactive power in wind power plants. In this

model parks transformation is used where three phased stator and rotor symmetrical

windings are transformed in orthogonal axis systems to improve power quality, high

energy efficiency and controllability.

The wind farm power collection system, grounding of wind farms against power system

faults and transient over voltages and Wind turbine lightning protection systems are

discussed in [5]. The Embedded wind generation, Electrical distribution networks and the

impact of dispersed generation, the per-unit system, power flows and voltages in simple

radial distribution networks, connection of embedded wind generation, power system

studies, Power (voltage) quality. Voltage flicker, harmonics from variable speed wind

turbines, measurement and assessment of power quality of grid connected wind turbines

also be mention[5].[6]This book devoted to wind power and solar photovoltaic

technologies, their engineering fundamentals, conversion characteristics, operational

considerations to maximize output, and emerging trends also includes new and

specialized technologies and explore the large-scale energy storage technologies, overall

electrical system performance[6].

A.Tapia et al. [33] described the modeling of the machine considers operating conditions

below and above synchronous speed, which are actually achieved by means of a double-

sided PWM converter joining the machine rotor to the grid. In order to decouple the

active and reactive powers generated by the machine, stator-flux-oriented vector control

is applied. The wind generator mathematical model developed in this paper is used to

show how such a control strategy offers the possibility of controlling the power factor of

the energy to be generated.

In [6], a new control scheme implemented for the variable speed grid connected

wind energy generation system, that helps a induction generator driven by an emulated

wind turbine with two back to back voltages fed PWM inverters to interface the generator

and grid. The machine currents are controlled using an indirect vector control technique

[6]. The generator torque is controlled to drive the machine to the speed for maximum

wind turbine aerodynamic efficiency [6]. In order to implement the separated positive

and negative sequence controllers of DFIG, two methods to separate positive and

negative sequence in real time are compared [7]. The features of each generator

converter configuration are considered in the context of wind turbine systems [8].

H. Karimi-Davijani et al. [34] presented fuzzy logic control of Doubly Fed Induction

Generator (DFIG) wind turbine in a sample power system.. Fuzzy logic controller is

applied to rotor side converter for active power control and voltage regulation of wind

turbine. Wei Qiao et al. [35] presented an approach to use the particle swarm

optimization algorithm to design the optimal PI controllers for the rotor-side converter of

the DFIG. A new time-domain fitness function is defined to measure the performance of

the controllers. Simulation results show that the proposed design approach is efficient to

find the optimal parameters of the PI controllers and therefore improves the transient

performance of the WTGS over a wide range of operating conditions.

Rohin M. Hilloowala, and Adel M. [35] presented a rule-based fuzzy logic controller to control

the output power of a pulse width modulated (PWM) inverter used in a standalone wind energy

conversion scheme (SAWECS).

C. A. M. Amendola, and D. P. Gonzaga [36] presented the energy capture control is

made applying a fuzzy-logic controller directly on the turbine pitch-angle and the speed

control is made by a field-oriented fuzzy-logic controller, that acts on DFIG

electromotive torque so that to follow the reference value generated by an optimum

angular speed estimator. Yongchang and Z. Zhengming [38] presented a conventional

PI controller, sliding mode controller (SMC) and fuzzy logic controller (FLC) for rotor

field oriented controlled (RFOC) induction motor drives are studied comparatively. PI is

simple but sensitive to parameter variations. SMC provides strong robustness to

parameters variations, disturbance rejection and system order reduction. FLC does not

need exact system mathematical model and can handle intricate nonlinearity, but its

implementation is more complicated than that of PI and SMC. Comparative study of PI,

SMC and FLC are carried out from four aspects: dynamic performance and steady-state

accuracy, parameter robustness, and complexity of implementation.

In the [10, 11] developed a 30kW electrical power conversion system for a

variable speed wind turbine system.. As the voltage and frequency of generator output

vary along the wind speed change, a dc-dc boosting chopper is utilized to maintain

constant dc link voltage. The input dc current was regulated to follow the optimized

current reference for maximum power point operation of turbine system. Line side PWM

inverter supply currents into the utility line by regulating the dc link voltage. The active

power was controlled by q-axis current whereas the reactive power can be controlled by

d-axis current. The phase angle of utility voltage was detected using software PLL

(Phased Locked Loop) in d-q synchronous reference frame[9, 10] .Proposed scheme

gives a low cost and high quality power conversion solution for variable speed WECS.

A switch-by-switch representation of the PWM converters with a carrier-based

Sinusoidal PWM modulation for both rotor- and stator-side converters has been

proposed. Stator-Flux Oriented vector control approach is deployed for both stator- and

rotor-side converters to provide independent control of active and reactive power and

keep the DC-link voltage constant [12]. In order to set synchronous vector controllers,

decoupled design based on Internal Model Control approach is applied, where dynamics

of the PWM converters is taken into account [12, 14].

After controlling method for the power of variable speed DFIG a method of

tracking the peak power proposed which is independent of turbine parameters and air

density is proposed. The algorithm searches for peak power points by varying the speed

in desired direction. The generated is operated in speed control mode with the speed

reference being dynamically modified in accordance with the magnitude and direction of

change of active power [14, 15, 16]. But this method is rotor speed dependent, then a

method proposed that doesnt depend on wind generator speed and rotor speed ratings

nor the dc/dc power converter rating [17, 19]. The two methods utilize the turbine

characteristics (torque, power and power coefficient curves) to determine the

operating point that results in maximum power capture [20, 22]. The only difference

between the two methods presented is that one requires an anemometer so that the wind

speed is physically measured while and the second method calculates the wind speed

using electrical parameters [ 22].

These methods are advantageous for fast optimum point determination and

easy implementation since all the physical characteristics of the turbine are programmed

directly and optimum operation point is determined by simply examining the

characteristics. A disadvantage of these strategies however, is that they are customized

for a particular turbine.. Another drawback of this algorithm is that it cannot take into

account the atmospheric changes in air density, since for all its calculations, it assumes a

certain value. But for overall efficiency improvement and to reduce the cost PWM

converters were used with reduced switch count power converters [23].

In [41] The complete system is modeled and simulated in the MATLAB Simulink

environment in such a way that it can be suited for modeling of all types of induction

generator configurations. The model makes use of rotor reference frame using dynamic

vector approach.

1.3 OBJECTIVE AND OGANIZATION OF THESIS

Objective

The objective is to develop a model and control methodology for a Doubly Fed

Induction Generator and maximum power point tracking for this model that can be

achieved.

Thesis Outline

Chapter -2 deals with the types of wind energy conversion system and configuration.

This chapter also deals with wind energy back ground and wind turbine characteristics.

Chapter-3 deals with induction machine with basic dynamic d-q model, axes

transformation and also describe dc drive analogy and vector control of induction

machine in brief.

Chapter-4 deals with modeling of wind turbine, pitch angle control, rotor side

controller, grid side controller of DFIG and also deals with detail modeling of wind

turbine coupled with DFIG.

Chapter-5 deals with DFIG under Maximum Power Point Tracking (MPPT) and power

smoothing using fuzzy pitch controller.

Chapter-6 deals with simulation model and parameter initializations.

Chapter-7 deals with simulation results and discussion between different results.

Chapter-8 deals with conclusion and future work related to DFIG.

CHAPTER-2 TYPES AND CONFIGURATIONS OF WIND ENERGY CONVERSION SYSTEMS

In this chapter various types and configurations of wind energy conversion

systems are discussed i.e. the fixed speed wind energy conversion systems and

variable-speed wind energy conversion systems. Also wind turbine characteristic

which are specific to each turbine and depends on the aerodynamic design of the

turbine and the site location of wind power plant are discussed .But in this thesis only

variable speed wind turbines will be considered [26].

2.1 General

A special type of induction generator, called a doubly fed induction generator (DFIG), is

used extensively for high-power wind applications. DFIGs ability to control rotor

currents allows for reactive power control and variable speed operation, so it can operate

at maximum efficiency over a wide range of wind speeds. The Doubly-Fed Induction

Generator (DFIG) is widely used for variable-speed generation, and it is one of the most

important generators for Wind Energy Conversion Systems (WECS). Both grid

connected and stand-alone operation is feasible. For variable speed operation, the

standard power electronics interface consists of a rotor and stator side PWM inverters

that are connected back-to-back. These inverters are rated, for restricted speed range

operation, to a fraction of the machine rated power. Applying vector control techniques

yields current control with high dynamic response. In grid-connected applications, the

DFIG may be installed in remote, rural areas where weak grids with unbalanced voltages

are not uncommon. As reported in induction machines are particularly sensitive to

unbalanced operation since localized heating can occur in the stator and the lifetime of

the machine can be severely affected. Furthermore, negative-sequence currents in the

machine produce pulsations in the electrical torque, increasing the acoustic noise and

reducing the life span of the gearbox, blade assembly and other components of a typical

WECS. To protect the machine, in some applications, DFIGs are disconnected from the

grid when the phase-to-phase voltage unbalance is above 6%.

Controller design parameters for the operation of induction generators in unbalanced

grids have been reported in, where it is proposed to inject compensating current in the

DFIG rotor to eliminate or reduce torque pulsations. The main disadvantage of this

method is that the stator current unbalance is not eliminated. Therefore, even when the

torque pulsations are reduced, the induction machine power output is rerated, because the

machine current limit is reached by only one of the stator phase. Compensation of

unbalanced voltages and currents in power systems are addressed in where a STATCOM

is used to compensate voltage unbalances.

However, the application of the control method to DFIGs is not discussed. No formal

methodology for the design of the control systems is presented and only simulation

results are discussed in. In this thesis, a controller design is specified, which compensates

the stator current unbalance in grid-connected and stand-alone DFIG operation.

The strategy uses two revolving axes theory (rotating synchronously at to obtain the d

q components of the negative and positive-sequence currents in the stator and grid/load.

The unbalance is compensated by the rotor side converter. The positive-sequence current

is conventionally controlled to regulate the dc link voltage, whereas negative-sequence

current is regulated to reduce or eliminate the grid voltage unbalance.

2.2 Type of Wind turbines

Wind turbine converts mechanical energy into generator torque and the generator

converts this torque into electricity and feeds it into the grid as other generation processes

does. The only difference from other generation processes is that the mechanical energy

is from wind. There are currently three main types of wind turbines available as shown in

Fig.2.1.[20]

Gear Box Grid IG

(a)Fixed speed wind turbine with an induction

generator

Gear Box Grid IG

(b)Variable-speed wind turbine with a doubly-fed induction

generator

RSC GSC

Gear Box Grid PM

(c)Variable-speed wind turbine with a permanent magnet synchronous

generator

RSC GSC

Blades

converters

Fig. 2.1 General structures of three different types of wind turbines

Fig.2.1 shows the structures of three different types of wind turbines in Fig.2.1 (a), (b)

and (c) shows as:

(a) Fixed speed wind turbine with an asynchronous squirrel cage induction generator (IG)

directly connected to the grid via a transformer.

(b) Variable speed wind turbine with a doubly fed induction generator (DFIG) and blade

pitch control.

(c) Variable speed wind turbine using a permanent magnet synchronous generator that is

connected to the grid through a full-scale frequency converter. This is called direct

drive (DD) wind turbine.

However, indirect grid connected wind turbines still need many improvements to

compete with other conventional electricity generation technologies. Firstly, as Fig. 2.1

shows, the indirect grid connected wind turbines will need a rectifier and two inverters,

one to control the stator current, and another to generate the output current, but it may

change as the cost of power electronics decreases. Secondly, there are energy losses

associated with AC/DC/AC conversion process, and harmonic distortions of the

alternating current may be introduced in the electrical grid by power electronics devices,

thus reducing power quality.

To improve the performance of wind turbines, different technologies are being applied to

them. Now two types of indirect grid connected wind turbines dominate the market. The

DD type of wind turbines is mainly built by Enercon (Germany). This type of wind

turbines is combined with synchronous permanent magnet generator and AC/DC/AC

converter with a rating of 100% of the rated wind turbine power. Since it does not need

the gear box, the weight at the hub height can be lowered a lot, and the operation and

maintenance of the gear box are not needed. But because the capacity of the converter has

to match the maximum output power of the generator, its cost is highest among all types

of wind turbines. Also the generator is bigger than other types of wind turbines. In the

long term, the operation and maintenance costs of the gear box can be saved.

The other type of indirect grid connected wind turbine is a variable speed wind turbine

with DFIG, which dominates the market with their total share to be around 84.5%-86%.

The wind turbine with DFIG is combined with gear box, induction generator, and

AC/DC/AC converter with a rating of only 20%30% of the rated wind turbine power..

The cost of DFIG system is lower than the direct drive system because its power

converter is approximately one-third the size of the direct drive system. But the control

system of a DFIG is more complex than that of a DD.

2. 3 TYPES OF WIND ENERGY CONVERTION SYSTEMS

Wind electric conversion systems can be broadly classified as;

Constant speed constant frequency (CSCF);

Variable speed constant frequency (VSCF);

Variable speed variable frequency (VSVF);

2.3.1 Constant speed constant frequency (CSCF)

In the CSCF scheme, the rotor is held constant by continuously adjusting the

blade pitch and/or generator characteristics. For synchronous generators, the requirement

of constant speed is very rigid and only minor fluctuations of about 1% for short duration

could be allowed [5].As the wind fluctuates, a control mechanism becomes necessary to

vary the pitch of the rotor so that the power derived from the wind system is held fairly

constant. Such a control system is necessary since wind power varies with the cube of

wind velocity. During gusty periods, the machine is subjected to rapid changes in the

input power. The control mechanism must be sensitive enough to damp out these

transient so that the machine output does not become unstable. Such a mechanism is

expensive and adds complexity to the system [21].

2.3.2 Variable speed constant frequency (VSCF)

The variable speed operation of wind electric system yields higher output for both

low and high wind speeds. This results in higher annual energy per rated installed

capacity. Both horizontal and vertical axis wind turbines exhibit this gain under variable

speed operation [17]. In this scheme, the need for a costly blade control mechanism is

avoided. Generation schemes involving speed rotors are more complicated than constant

speed systems. Variable frequency power must be converted to constant frequency

power, and this can be done by using power electronics [21].

2.3.3 Variable speed variable frequency (VSVF)

General, resistive heating loads are less frequency sensitive. Synchronous

generators can be affected at variable speed, corresponding to the changing drive speed.

For this purpose, self-excited induction generator can be conveniently used. This scheme

is gaining importance for standalone wind power applications [5, 18, 21].

2.4 WIND GENERATORS

According to the turbine position the wind generators are divided into two axes

that are horizontal axis and vertical axis generators.

2.4.1 Horizontal axis wind generators

Horizontal axis wind generators have the main rotor shaft and electrical generator

at the top of a tower, and must be pointed into the wind. Small generators are pointed by

a simple wind vane or tail. Large generators often use a wind sensor coupled with a

servomotor. Most large wind generators use a gearbox, which turns the slow rotation of

the blades into a quicker rotation that is more suitable for generating electricity [4, 5].

2.4.2 Vertical axis wind generators

Vertical axis wind generators have the main rotor shaft running vertically. The

advantages of this configuration are that the generator and/or gearbox can be placed at the

bottom, near the ground, so the tower doesn't need to support the additional weight, and

that the generator doesn't need to be pointed into the wind. They generally also operate at

lower wind speeds. However, they are not as efficient at extracting energy from the wind

[4, 5].

2.5 CHOICE OF GENERATORS

Basically, a wind turbine can be equipped with any type of 3 phase generator.

Today, the demand for grid-compatible electric current can be met by connecting

frequency converters, even if generator supplies AC of variable frequency or DC. Several

general types of generators may be used in WT [4, 5, 21].

1. Permanent magnet generators,

2. Caged rotor induction generators,

3. Synchronous generators,

4. Doubly fed induction generators.

2.5.1 Permanent magnet synchronous generators

Permanent magnet excitation is generally favored in newer smaller scale turbine

designs, since it allows for higher efficiency and smaller wind turbine blade diameter.

While recent research has considered larger scale designs, the economics of large

volumes of permanent magnet material has limited their practical application. The

primary advantage of permanent magnet synchronous generators (PMSG) is that they do

not require any external excitation current. A major cost benefit in using the PMSG is the

fact that a diode bridge rectifier may be used at the generator terminals since no external

excitation current is needed. Flexibility in design allows for smaller and lighter designs

and higher output level may be achieved without the need to increase generator size.

Lower maintenance cost and operating costs, bearings last longer, there is no significant

losses generated in the rotor and the Generator speed can be regulated without the need

for gears or gearbox .Very high torque can be achieved at low speeds and Eliminates the

need for separate excitation or cooling systems[5].

But some disadvantages are there in PMSG that Higher initial cost due to high

price of magnets used and Permanent magnet costs restricts production of such generators

for large scale grid connected turbine designs. High temperatures and sever overloading

and short circuit conditions can demagnetize permanent magnets. Use of diode rectifier in

initial stage of power conversion reduces the controllability of overall system [5, 17].

2.5.2 Induction generators

The use of induction generators (IG) is advantageous since they are relatively

inexpensive, robust and they require low maintenance. The nature of IG is unlike that of

PMSG, Lower capital cost for construction of the generator and Known as rugged

machines that have a very simple design. Higher availability especially for large scale

grid connected designs and Excellent damping of torque pulsation caused by sudden wind

gusts, relatively low contribution to system fault levels [5].

Disadvantages of this generator is Increased converter cost since converter must

be rated at the full system power then Results in increased losses through converter due

to large converter size needed for IG Generator requires reactive power and therefore

increases cost of initial ACDC conversion stage of converter and May experience a

large in-rush current when first connected to the grid .it also increased control complexity

due to increased number of switches in converter [5, 17].

2.5.3 Synchronous generators

The major advantages of synchronous generator is that its reactive power

characteristics can be controlled, and therefore such machine can be used to supply

reactive power to systems that require reactive power. The application of synchronous

generators (SG) in wind power generation has also been researched. A brief description

of one possible converter-control scheme is given for a small wind energy conversion

system. The use of a diode rectifier along with a DC/DC boosts stage and inverter as a

power electronic interface for grid connection. It possesses Minimum mechanical wear

due to slow machine rotation. Due to direct drive applicable further reducing cost since

gearbox not needed. it allow for reactive power control as they are self excited machines

that do not require reactive power injection and Readily accepted by electrically isolated

systems for grid connection. It Allow for independent control of both real and reactive

power [5].

Disadvantages are typically having higher maintenance costs again in comparison

to that of an IG and magnet used which is necessary for synchronization is expensive. But

magnet tends to become demagnetized while working in the powerful magnetic fields

inside the generator. It requires synchronizing relay in order to properly synchronize with

the grid [5].

2.5.4 Doubly fed induction generators

As the PMSG has received much attention in wind energy conversion, the doubly

fed induction generator has received just as much consideration, if not more. If a wound

rotor induction machine is used, it is possible to control the generator by accessing the

rotor circuits. A significant advantage in using doubly fed induction generators (DFIG) is

the ability to output more than its rated power without becoming overheated. It is able to

transfer maximum power over a wide speed range in both sub- and super-synchronous

modes. The DFIG along with induction generators are excellent for high power

applications in the MW range. More importantly, converter power rating is reduced since

it .is connected to the rotor, while the majority of the power flows through the stator [5].

Fig. 2.2: Typical wind generators.

2.6 MODELING OF DFIG SYSTEM

2.6.1 Blade modeling (wind modeling)

An aerodynamic model of the wind turbines is a common part of the dynamic models of

the electricity-producing wind turbines. The captured aerodynamic power is given by:

=1

2

2 , (2.1)

where is the captured power from wind, is the air density, v is the wind speed, A

is the swept area of the blade, ( ) is the power coefficient, is the ratio between

blade tip speed and wind speed at hub height, is the pitch angle. (, ) can be

obtained from wind turbine manufacturers.

2.6.2 Drive train modeling

The mechanical construction of the wind turbines is simply modeled as a lumped-mass

system with the lumped combined inertia constant of the turbine rotor and the generator

rotor. The shaft dynamic equation is [15]:

2

= ( ) (2.2)

2

= ( ) (2.3)

= ( ) (2.4)

where JT and JG are the inertia constant of the turbine rotor and the generator rotor,

respectively, Ks and Ds are the shaft stiffness and damping constant respectively, TG is

the electrical twist angle of the shaft, o is the base value of angular speed, T and G are

the angular speeds of shaft at the ends of turbine and generator, respectively, TT and TE

are the mechanical and electrical torque, respectively.

2.6.3Generator modeling

As mentioned earlier, there are three types of generators used in wind turbines: one is

induction generator, the second one is doubly fed induction generator, and the other is

permanent magnetic synchronous generator.

1) Induction generator (IG)

The equivalent circuit of the induction generator is shown in Fig.2.3, and the electric and

magnetic equations of the model are described by equations (2.5)-(2.10) [20].

Fig.2.3 Equivalent circuit of the induction generator

Stator Voltage is given by:

= +dds

dt (2.5)

= + +dqs

dt (2.6)

Rotor Voltage is given by:

= +ddr

dt (2.7)

= + +dqr

dt (2.8)

Flux Linkage is given by:

=

=

=

= (2.9)

+

-

jws +rdq

Vsdq

Rs Rr Ls Lr

Lm V+rdq

J(s-r)+

rdq

Electronicmagnetic Toque is:

= (2.10)

where vs, is and s are stator voltage, current and flux respectively; vr, ir and r are rotor

voltage, current and flux respectively; s is the angular velocity of the chosen frame of

reference; d and q represent d and q axis, respectively. Lm is the mutual inductance; Lsl

and Lrl are the stator and rotor leakage inductances, respectively.

2) Doubly fed induction generator (DFIG)

Doubly fed induction generator is a modified version of IG where two rotor windings

receive electrical excitation from external sources. As a result, the rotor equations are

modified as presented in section D below. Rests of the equations are same as IG.

3) Permanent magnet synchronous generator

The generator for a direct drive wind turbine is different from the other types. It is a

permanent magnet synchronous generator, and using Parks transformation, can be

expressed by the following equations [20] and [18].

= +dds

dt

= +dqs

dt (2.11)

= + +

= + (2.12)

where r is the mechanical angular velocity of the rotor at any instant, d and q represent d

and q axis respectively, m is the flux produced by the permanent magnets.

Electronicmagnetic Toque is given by:

= (3

2 )(

2 )( ) (2.13)

Where p is the number of poles.

2.6.4 Converter modeling and control

With the assumption that the converters are lossless, the equations of converters are as

follows:

1) DFIG converter

The power at the rotor side (also called slip power) is given by:

= +

= (2.14)

And the power at the stator side is given by:

= +

= (2.15)

So the total output power is:

= + = + + +

= + = + (2.16)

2) Frequency converter

For a direct drive system, all the power produced by the generator goes from the stator

and pass through the converter.

= = +

= =

2.7 WIND ENERGY BACKGROUND

The amount of power captured from a wind turbine is specific to each turbine and

is governed by [1].

(2.17)

Where:

Pt = the turbine power(W),

= the air density (kg/m),

A= the swept turbine area (m^3),

CP = the coefficient of performance

vw = is the wind speed(m/s).

The coefficient of performance of a wind turbine is influenced by the tip-speed to

wind speed ratio or TSR given by

(2.18)

Where w is the turbine rotational speed and r is the turbine radius. A typical

relationship, as shown in Fig. 2.4, indicates that there is one specific TSR at which the

turbine is most efficient. In order to achieve maximum power, the TSR should be kept at

the optimal operating point for all wind speeds. The turbine power output can be plotted

versus the turbine rotational speed for different wind speeds, an example of which is

shown in Fig. 2.4. The curves indicate that the maximum power point increases and

decreases as wind speed rises and falls [4, 22, 23],

3

2

1wpt vACP

,wv

wrTSR

0.4

0.3

0.2

0.1

0.0

Cp

12 10 8 6 4 2 0

Tip Speed Ratio

Fig. 2.4: Typical coefficient of power curve

P1 max

P2 max

P (W)

(rad/s)

V2 > V1

Fig. 2.5: Turbine output power characteristic

CHAPTER -3

PRNCIPLE OF DOUBLY FED INDUCTION GENERATOR

3.1 INTRODUCTION

Variable speed ac drives have been used in the past to perform relatively

undemanding roles in application which preclude the use of dc motors, either because of

the working environment limits. Because of the high cost efficient, fast switching

frequency static inverter. The lower cost of ac motors has also been a decisive economic

factor in multi motor systems. However as a result of the progress in the field of

power electronics, the continuing trend is towards cheaper and more effective power

converters, and a single motor ac drives complete favorably on a purely economic basis

with a dc drives. Among the various ac drive systems, those which contain the cage

induction motor have a particular cost advantage. The cage motor is simple and rugged

and is one of the cheapest machines available at all power ratings. Owing to their

excellent control capabilities, the variable speed drives incorporating ac motors and

employing modern static converters and torque control can well complete with high

performance four quadrant dc drives [27].

The induction motors were evolved from being a constant speed motors to a

variable speed. In addition, the most famous method for controlling induction motor is by

varying the stator voltage or frequency. To use this method, the ratio of the motor voltage

and frequency should be approximately constant. With the invention of Field Orientated

Control, the complex induction motor can be modeled as a DC motor by performing

simple transformations. In a similar manner to a dc machine, in induction motor the

armature winding is also on the rotor, while the field is generated by currents in the stator

winding. However the rotor current is not directly derived from an external source but

results from the emf induced in the winding as a result of the relative motion of the rotor

conductors with respect to the stator field. In other words, the stator current is the source

of both the magnetic field and armature current. In the most commonly used, squirrel

cage motor, only the stator current can directly be controlled, since the rotor winding is

not accessible. Optimal torque production condition are not inherent due to the absence of

a fixed physical disposition between the stator and rotor fields, and the torque equation is

non linear. In effect, independent and efficient control of the field and torque is not as

simple and straightforward as in the dc motor [27, 28].

The concept of the steady state torque control of an induction motor is

extended to transient states of operation in the high performance, vector control ac drive system

based on the field operation principle defines condition for decoupling the field control

from the torque control. A field oriented induction motor emulates a separately exited dc motor

in two aspects [27].

I - Both the magnetic field and torque developed in the motor can be controlled

independently.

II - Optimal condition for the torque production, resulting in the maximum torque per unit

ampere, occurs in the motor both in steady state and in transient condition of operation.

3.2 DC MOTOR ANALOGY

Fig-3.1 DC motor analogy

Where torque (T) Ia.If

And where Ia represents the torque component and If the field.

The orthogonal or perpendicular relationship between flux and mmf axes is

independent of the speed of rotation and so the electromagnetic torque of the dc motor is

proportional to the product of the field flux and armature current. Assuming negligible

magnetic saturation, field flux is proportional to field current and is unaffected by armature

current because of the orthogonal orientation of the stator and rotor field. Thus in a

separately excited dc motor with constant value of field flux, torque is directly proportional

to armature current [27, 28].

Ia

If

Ia If

Fig. 3.2: Separately excited

The principle behind the field oriented control or the vector control is that the

machine flux and torque are controlled independently, in a similar fashion to a separately

excited DC machine. Instantaneous stator currents are transformed to a reference frame

rotating at synchronous speed aligned with the rotor stator or air gap flux vectors, to

produce a d-axis component current and a q-axis component current. (SRRF).In this work,

SRRF is aligned with rotor mmf space vector, the stator current space vector is split into

two decoupled components, one controls the flux and the other controls the torque

respectively [27, 28].

3.3 INDUCTION MOTOR ANALOGY

An induction motor is said to be in vector control mode, if the decoupled

components of the stator current space vector and he reference decoupled components

defined by the vector controller in the SRRF match each other respectively. Alternatively,

instead of matching the two phase currents (reference and actual) in the SRRF, the close

match can also be made in the three phase currents (reference and actual) in the stationary

reference frame. Hence in spite of induction machines non linear and highly interacting

multivariable control structure [28].its control has becomes easy with the help of FOC.

Therefore FOC technique operates the induction motor like a separately excitedly DC

motor.

The transformation from the stationary reference frame to the rotating reference

frame is done and controlled by with reference to specific flux vector (stator flux linkage,

rotor flux linkage) or magnetizing flux linkage). In general, there exits three possibilities

for such selection and hence, three vector controls. They are stator flux oriented control,

rotor flux oriented control and magnetizing flux oriented control. As the torque producing

component in this type of control is controlled only after transformation is done and is not

the main input reference, such control is known as indirect torque control. The most

challenging and ultimately, the limiting feature of field orientation is the method whereby

the flux angle is measured or estimated. Depending on the method of measurement, the

vector control is sub divided into two sub categories: direct vector and indirect vector

control. In direct vector control, the flux measurement is done by using flux sensing coils

or the hall devices [27, 28].

FOC uses a d-q coordinates having the d-axis aligned with rotor flux vector that

rotates at the stator frequency. The particular solution allows the flux and torque to be

separately controlled by the stator current d-q components. The rotor flux is a flux of the d-

axis component stator current ids .The developed torque is controlled by the q axis

component of the stator current iqs. The decoupling between torque and flux is achieved

only if the rotor flux position is accurately known. This can be done using direct flux

sensors or by using a flux estimator [28].

3.3.1 Vector control techniques of induction motor

The synchronously rotating reference frame (SRRF) can be aligned with the stator

flux or rotor flux or magnetizing flux (field flux) space vectors respectively. Accordingly,

vector control is also known as stator flux oriented control or rotor flux oriented control or

magnetizing flux oriented control. Generally in induction motors, the rotor flux oriented

control is preferred. This is due to the fact that by aligning the SRRF with the rotor flux,

the vector control structure becomes simpler and dynamic response of the drive is observed

to be better than any other alignment of the SRRF.

The vector control can be classified into (i) Direct vector control and (ii) indirect vector

control [28].

Fig. 3.3: Vector controlled induction motor

In vector control the dynamic performance of the induction motor improves to a

great extent. The squirrel cage induction motor behaves similar to a separately excited dc

motor with control of field and torque being independent of each other. Therefore the drive

exhibits quick starting response, fat reversal response and quick change over from one

operating point to another. With proper choice of speed controller, the drive can be further

improved in terms of performance indices such as starting time, reversal time, and dip in

speed on load application, overshoot in speed on load removal, steady state speed error on

load etc [27, 28].

3.3.2 DYNAMIC DQ MODEL

R.H. Park in 1920's proposed a model for synchronous machine with respect to

stationary reference frame. H.C. Stanley in 1930's proposed a model for induction

machine with respect to stationary reference frame. Later G. Bryons proposed a

transformation of both stator and rotor variables to a synchronously rotating reference

frame that moves with the rotating magnetic field. Lastly Krause and Thomas proposed a

model for induction machine with respect to stationary reference frame.

Transformation: - the stator winding axes as-bs-cs with voltage with respect

to stationary reference frame, the voltages are referred as [29].

csbsas vvv &,

qsds vv &

Fig. 3.4: Stationary frame a-b-c to dq.

Fig. 3.5: Stationary frame to synchronous rotating frame

3.3.2.1 Synchronously rotating reference frame-Dynamic model (Kron's equation)

The dynamic model of DFIG is derived from the two-phase synchronous

reference frame in which the q-axis is 90 ahead of the d-axis with respect to the direction

of rotation. The electrical model of DFIG in the synchronous reference frame, here the

quantities on the rotor side have been referred to the stator side. The model is composed

of two groups, i.e. the first one is the voltage equations and the other is the flux ones. The

general model for wound rotor induction machine is similar to any fixed-speed induction

generator [12].

The DFIG system consists of stator, rotor and turbine. So the model design according

these, the followings parameters are used to modeling the DFIG:

3.3.2.2 Voltage equations

Stator Voltage Equations:

(1)

(2)

Fig. 3.6: d-axes transform

Rotor Voltage Equations:

(3)

(4)

Fig. 3.7: q-axes transform

3.3.2.3 Power Equations:

(5)

(6)

3.3.2.4 Torque Equation:

qssdsqsqs iRPV

dssqsqsds iRPV

qrrdrqrqr iRrPV )(

drrqrdrdr iRwrwPV )(

)(2/3 qsqsdsdss iViVP

)(2/3 qsdsdsqss iViVQ

(7)

3.3.2.5 Flux Linkage Equations:

Stator Flux Equations:

(8)

(9)

Rotor Flux Equations:

(10)

Then, the d-axis of reference frame to be along the stator flux linkage (stator flux oriented

control) will be

(11)

And hence from stator flux equation:

(12)

Substituting for in torque equation will result in:

(13)

For to remain unchanged at zero, must be zero. Substituting for using

stator voltage equation we get

(14)

Neglecting stator resistance will lead to ; substituting this, the power equation

simplified as

)(22

3dsqsqsdse ii

p

qrmqsmlsqs iLiLL )(

drmdsmlsds iLiLL )(

dsmdrmlrdr

qsmqrmlrqr

iLiLL

iLiLL

)(

)(

0eqs

e

qr

mls

me

qs iLL

Li

e

qsi

e

qr

e

ds

mls

m

e iLL

Lp

22

3

e

dse

dspe

dsp

e

dss

e

ds irV

0edsV

(15)

Therefore, the above equations show that active and reactive powers of the stator can be

controlled independently.

In terms of rotor current component

(16)

Where =

CHAPTER-4 CONTROLLER FOR DOUBLY FED INDUCTION GENERATOR

4.1 DFIG WITH BACK TO BACK CONVERTER

A double fed induction generator is a standard, wound rotor induction machine

)(2

3

)(2

3

e

ds

e

qs

e

s

e

qs

e

qs

e

s

iVQ

iV

)(

)(

s

s

e

drme

m

e

s

e

qr

s

me

m

e

s

L

iLVQ

iL

LV

sL mls LL

with its stator windings is directly connected to grid and its rotor windings is

connected to the grid through an AC/DC/AC converter. AC/DC converter connected to

rotor winding is called rotor side converter and another DC/AC is grid side converter.

Doubly fed induction generator (DFIG) ability to control rotor currents allows for

reactive power control and variable speed operation, so it can operate at maximum

efficiency over a wide range of wind speeds [43].

Fig. 4.1: Wind Energy System.

In modern DFIG designs, the frequency converter is built by self-commutated

PWM converters, a machine-side converter, with an intermediate DC voltage link.

Variable speed operation is obtained by injecting a variable voltage into the rotor at slip

frequency. By controlling the converters, the DFIG characteristics can be adjusted so as

to achieve maximum of effective power conversion or capturing capability for a wind

turbine and to control its power generation with less fluctuation. The DFIG is a WRIG

with the stator windings connected directly to the three phases, constant-frequency grid

and the rotor windings connected to a back-to-back voltage source converter. Thus, the

term doubly-fed comes from the fact that the stator voltage is applied from the grid and

the rotor voltage is impressed by the power converter [5, 41].

Vector control of a doubly fed induction generator drive for variable

speed wind power generation is described. The control scheme uses stator flux-oriented

GEAR BOX

3~

DOUBLY FED INDUCTION GENERATOR

AC DC

DC AC

TRANSFORMER

GRIDDDD

control for the rotor side converter bridge control and grid voltage vector control for the

grid side converter bridge. The purpose of the grid side converter is to maintain the dc

link voltage constant. It has control over the active and reactive power transfer between

the rotor and the grid, while the rotor side converter is responsible for control of the flux,

and thus, the stator active and reactive powers. A complete simulation model is

developed for the control of the active and reactive powers of the doubly fed generator

under variable speed operation [6, 9, 43].

Fig. 4.2: DFIG with converter control signal.

The wind turbine and the doubly-fed induction generator (DFIG) is shown in the

Fig. 4.2. The AC/DC/AC converter is divided into two components: the rotor-side

converter (Crotor) and the grid-side converter (Cgrid).A capacitor connected on the DC side

acts as the DC voltage source. A coupling inductor L is used to connect Cgrid to the grid.

The three-phase rotor winding is connected to Crotor by slip rings and brushes and the

three-phase stator winding is directly connected to the grid. The power captured by the

wind turbine is converted into electrical power by the induction generator and it is

transmitted to the grid by the stator and the rotor windings. The control system generates

the pitch angle command and the voltage command signals Vr and Vgc for Crotor and Cgrid

respectively in order to control the power of the wind turbine, the DC bus voltage and the

reactive power or the voltage at the grid terminals.[6, 9, 43].

4.2 POWER-SPEED CHARACTERISTIC

control

Sign Vc,Vg

Pitch angle

From previous discussion it is clear that the controller, i.e Crotor and Cgrid have the

capability of generating or absorbing reactive power and control the reactive power or the

voltage at the grid terminals. The power is controlled is follow the power-speed

characteristic (Fig. 4.3).

Fig. 4.3: Power-speed characteristic.

The above ABCD curve shows the power characteristics. The actual speed of the

turbine r is measured and the corresponding mechanical power of the tracking

characteristic is used as the reference power for the power control loop. The tracking

characteristic is obtained over four points. From zero speed to speed of point A the

reference power is zero. Between point A and point B the characteristic is a straight line,

the speed of point B must be greater than the speed of point A. Between point B and

point C the tracking characteristic is the locus of the maximum power of the turbine. The

tracking characteristic is a straight line from point C and point D. The power at point D is

1 pu and the speed of the point D must be greater than the speed of point C. Beyond point

D the reference power is a constant equal to 1 pu [43].

4.3 WIND-TURBINE MODEL.

Wind turbines convert aerodynamic power into electrical energy. In a wind

turbine two conversion processes take place. The aerodynamic power is first converted

into mechanical power. Next, that mechanical power is converted into electrical power.

Wind energy conversion systems are systems that generate electrical power from

mechanical power derived from the wind. The major components of a typical wind

energy conversion system include a wind turbine, a generator and control systems as

shown in Fig. 4.2.

Cp is the power coefficient which, in turn, is a function of tip speed ratio and

blade angle .i.e. Cp = Cp (, ) and = (*r)/ v; One common way to control the

active power of a wind turbine is by regulating the value of the rotor turbine. In the

model, the value of the turbine rotor is approximated using a non-linier function [7,

15].

(4.1)

Where the tip is speed ratio and is the pitch angle. The value is given

according to the following relation.

(4.2)

The maximum value of can be found using a graphical method, this tip speed value is

assigned as the optimum tip speed. Based on this value, the optimum turbine speed curve

at any given wind speed can be obtained. This curve is then used as a reference in the

active power control. The variation of Cp as a function of assuming constant pitch

angle = const [43].

The out put power from turbine: (4.3)

Torque is

pc

pc

reCi

P

5.12

)54.116

(22.0),(

i

1

035.

08.

112

i

pc

3

2

1wpt vACP

rtm PT /

Fig. 4.4: simulation model of turbine.

4.4 PITCH ANGLE CONTROL

The pitch angle control is a common control method to regulate the aerodynamic

power from the turbine. Pitch angle controller controls the wind flow around the wind

turbine blade, thereby controlling the toque exerted on the turbine shaft. If the wind speed

is less than the rated wind speed of the wind turbine, the pitch angle is kept constant at its

optimum value. It should be noted that the pitch angle can change at a finite rate, which

may be quite low due to the size of the rotor blades. Small change in pitch angle can have

a dramatic effect on the power output. The maximum rate of change of the pitch angle is

in the order of 3 to 10 degrees/second. In this controller a slight over-speeding of the

rotor above its nominal value can be allowed without causing problems for the wind

turbine structure. The relationship between the pitch angle and the wind speed is shown

Tm (pu)1

wind_speed 3^

u(1) 3^

pu->pu

-K-

pu->pu

-K-

lambda_nom

-K-

cp(lambda,beta)

lambda

betacp

Scope1

Scope

Product

Product

-K-

Avoid divisionby zero

Avoid divisionby zero

1/wind_base

-K-

1/cp_nom

-K-

Wind speed(m/s)

3

Pitch angle (deg)2

Generator speed (pu)1

Pwind_puPm_pu

lambda

cp_pu

cp_pulambda_pu

wind_speed_pu

in Figure 4.6.[11,43].

Fig. 4.5: Pitch Angle control

Fig. 4.6: Relationship between Pitch Angle and Wind Speed

The pitch angle controller employs a PI (proportional integral) controller as

shown below.

In Fig.4..5. When the wind turbine output power Pmeasured is lower than the rated power

Pref of the wind turbine, the error signal is negative and pitch angle is kept at its optimum

value. When the wind turbine output power Pmeasured exceeds the rated power Pref, the

error signal is positive and the pitch angle changes to a new value, at a finite rate, thereby

reducing the effective area of the blade resulting in the reduced power output. The PI

controller inputs are in per-unit.

4.5 ROTOR SIDE CONVERTER

The rotor-side converter is used to control the wind turbine output power and the voltage

or reactive power measured at the grid terminals.

Fig. 4.7: Rotor side and Grid _side converter control circuit.

The actual electrical output power, measured at the grid terminals of the wind turbine, is

added to the total power losses (mechanical and electrical) and is compared with the

reference power obtained from the tracking characteristic. A Proportional-Integral (PI)

regulator is used to reduce the power error to zero. The output of this regulator is the

reference rotor current Iqr_ref that must be injected in the rotor by converter Crotor. This is

the current component that produces the electromagnetic torque Tem. The actual Iqr is

compared to Iqr_ref and the error is reduced to zero by a current regulator (PI). The

output of this current controller is the voltage Vqr generated by Crotor. The current

regulator output is Vqr [8, 13].Reactive and Active Power at grid terminals is controlled

by the reactive and active current flowing in the converter Crotor

The output of the voltage regulator or the var regulator is the reference d-axis

current Idr_ref that must be injected in the rotor by converter Crotor. The same current

regulator as for the power control is used to regulate the actual Idr. The output of this

regulator is the d-axis voltage Vdr generated by Crotor. The current regulator output is

Vdr. Vdr and Vqr are respectively the d-axis and q-axis of the voltage Vr [8, 30].

Fig. 4.8: Rotor side converter control.

4.5.1 MATLAB SIMULATION MODEL

VAR

MEASUREMENT

TRACKING CHA

POWER

MEASUREMENT

Var

CURRENT

POWER

REGULATOR

CURRENT

REGULATOR

V

I

V

I

Wr

P

Pref

Q

Qref

I Id

Iq

Id ref

Iq ref

+

-

+

-

+

-

-

+

Fig. 4.9: Simulation Model of Rotor-Side Controller.

4.6 GRID SIDE CONVERTER

The converter Cgrid is used to regulate the voltage of the DC bus capacitor. In this

thesis , this model Cgrid converter to generate or absorb reactive power. In this control

system ,measuring the d and q components of AC currents to be controlled as well as the

DC voltage Vdc. The output of the DC voltage regulator is the reference current Idgc_ref

for the current regulator. The current regulator controls the magnitude and phase of the

voltage generated by converter Cgrid (Vgc) from the Idgc_ref produced by the DC voltage

regulator and specified Iq_ref reference. The current regulator give the Cgrid output

voltage [6, 43].

The magnitude of the reference grid converter current Igc_ref is equal to

.The maximum value of this current is limited to a value

defined by the converter maximum power at nominal voltage. When Idgc_ref and Iq_ref

rotor side control voltage

Vabc_r

1

turbine power charecteristics

wr

idqs

Vdqs

Freq

Iqr *

reactive power

Q_ref

QIdr *

dq 2 abc

Vdq*

Vdc

Angle

Uctrl_r

abc_dq

Theta

Iabc _s

Idq _s

abc to dqr

In1

angle _rotor

Iabc _r

Idq _r

r_angle

abc to dq

Theta

Vabc

Vdq

Vq calculation

f(u)

Vd calculation

f(u)

1/z1/2

F

50

PI

Discrete

3-phase PLL

Vabc (pu )

Freq

wt

Sin _Cos

Demux

Demux

Demux

Demux

Vdc

8

angle _rotor

7

Q

6

Iabc_r1

5

Iabc_s

4

wr

3

Q_ref

2

Vabc

1

Idr*

w-wr

Idr

Iqr

vd'

vq'Iqr*

22 __ refIqrefIdgc

are such that the magnitude is higher than this maximum value the Iq_ref component is

reduced in order to bring back the magnitude to its maximum value [9].

Fig. 4.10: Grid side converter control.

4.6.1 MATLAB SIMULATION MODEL

DC VOLTAGE

REGULATOR

CURRENT

MEASUREMENTCURRENT

REGULATORIg Idg

Iqg

Idg ref

Iqg ref

Vq

+

+

-

-

+

-

Vd

Vdc ref

Fig. 4.11: Simulation Model of Grid-Side controller and Power.

dq2abc converter voltage

P3

Q

2

Vabc_g

1

curent to volatge tf

Idq

Idq_ref

vdq ref

active and reactive power

Vabc

Iabc

T_F

Q

P

abc to dq 1

Iabc

Theta1

Idq

abc to dq

Theta

Vabc

Vdq

Vdcref

Vdc_nom

1/z

Theta

Vdq*_g

Vdc

control _g

-K-

Discrete

3-phase PLL

Vabc (pu)

Freq

wt

Sin_Cos

Demux

Demux

DC to Idq ref

Vdc_ref

Vdc

Iq_ref

Idq_ref

Iabc

5

Iq_ref

4Vdc

3

Iabc_g

2

Vabc

1vd'

vq'

Icr

10

Ibr

9

Iar

8

Ics

7

Ibs6

Ias

5

Q1

4

P1

3

Te

2

Nr

1

dq to abc 1

Iq

Id

We-Wr

Ia

Ib

Ic

dq to abc

Iq

Id

We

Ia

Ib

Ic

abc 2 dq

Va

Vb

Vc

Vq

Vd

We

Subsystem

Vds

Ids

Vqs

Iqs

P1

Q1

IM model

Vqs

Vds

We

Vqr

Vdr

TL

Iqs

Ids

Iqr

Idr

Wr

Te

Gain

-K-

Constant 1

0

Constant

0

Add

Vcs

4

Vbs

3

Vas

2

TL

1

Fig. 4.12: Induction machine model

CHAPTER-5 MAXIMUM POWER POINT TRACKING AND POWER SMOOTHING

5.1 INTRODUCTION

In this thesis different way to track the maximum power were implemented. All

these tracking characteristic process are previously implemented, but here these processes

are compared and new one is implemented in different way. The variable speed control is

Te

6

Wr

5

Idr

4

Iqr

3

Ids

2

Iqs

1

Subsystem 2

Iqs

Ids

Iqr

Idr

TL

Te

Wr

Subsystem 1

Fqs

Fds

Fqr

Fdr

Iqs

Ids

Iqr

Idr

Subsystem

Vqs

Vds

Vqr

Vdr

Iqs

Ids

Iqr

Idr

We

Wr

Fqs

Fds

Fqr

Fdr

TL

6

Vdr

5Vqr

4

We

3

Vds

2Vqs

1

done based on the optimal power curve that shows the relation between the maximum

output of the system (output) and the generator speed (input), namely maximum power

point tracking (MPPT). The wind speed control or the generator speed control is adopted

for MPPT.

At a given wind velocity, the mechanical power available from a wind

turbine is a function of its shaft speed. To maximize the power captured from the wind,

the shaft speed has to be controlled. For a given shaft speed turbine power increases with

increase in wind velocity v. Also peak power points of turbine power occurs at

different turbine speed for different wind velocity and shaft speed corresponding to

maximum power increases with increase in wind speed. To trap maximum power from

the wind some control algorithm should be incorporate such that rotational speed of

the wind turbine adapts the to the wind speed v automatically leading to maximum

power point operation. This is known as maximum power point operation of wind

turbine, and the process of keeping track of peak Power points with change in wind speed

is Maximum Power Point Tracking MPPT [17, 22].

The conventional method is to generate a control law to produce the target

generator torque Te, which provides wind turbine with sufficient acceleration or

deceleration torque to attain particular angular velocity leading to maximum power point

operation. Irrespective of the generator used for a variable speed wind energy

conversion system the output energy depends on the method of tracking the peak

power points on the turbine characteristics due to fluctuating wind. The generator is

operated in speed control mode with the speed reference being dynamically modified in

accordance with the magnitude and direction of change of active power. If we operate

at a peak power point a small increase or decrease in turbine speed would result in

no change in output power because necessary condition for the speed to be a

maximum power point is dP/dw =0 [14].

5.2 First Method using Power Point Tracking Characteristics

The ABCD curve shows the power characteristics (Fig. 5.1). The actual speed of

the turbine r is measured and the corresponding mechanical power of the tracking

characteristic is used as the reference power for the power control loop. The tracking

characteristic is obtained over four points. From zero speed to speed of point A the

reference power is zero. Between point A and point B the characteristic is a straight line,

the speed of point B must be greater than the speed of point A. Between point B and

point C the tracking characteristic is the locus of the maximum power of the turbine. The

tracking characteristic is a straight line from point C and point D. The power at point D is

1 pu and the speed of the point D must be greater than the speed of point C. Beyond point

D the reference power is a constant equal to 1 pu [43].

Fig. 5.1: Power Point Tracking Characteristics.

5.2.1 MATLAB MODEL

Fig. 5.2: Simulation of Power Point Tracking Characteristics.

5.3 Second Method using MPPT curve implemented as look-up table

From the above discussion it can be conclude that for the maximum power

characteristic divided in different region, then using slop equation manipulates the value

of power which is used as reference power for the simulation. Here same characteristics

is used as look-up table ,where the power only measured only some few wind velocity

like at A,B,C & D. at other poin