BSc_thesis_final

MARKUS OVASKAINEN

GRID CONNECTION FOR VARIABLE SPEED WIND TURBINES

Bachelor of Science Thesis

Examiner: D. Eng. Tuomas Messo

i

ABSTRACT

Markus Ovaskainen: Grid Connection for Variable Speed Wind Turbines Bachelor of Science Thesis, 36 pages April 2015 Bachelor’s degree programme in Electrical Engineering Major: Power Electronics Examiner: D. Eng. Tuomas Messo Keywords: wind power, variable speed wind turbine, power electronics, power converter, grid connection, grid converter, low voltage ride-through, islanding detection, grid code, converter control

In this thesis, wind power as an energy source is introduced in general. The power in

wind flow and the transformation to electrical power is briefly explained. The structure

of horizontal-axis wind turbines is analyzed. After introduction to wind turbines in

general, different types of variable speed wind turbines are discussed. The main variant

in the different types is the generator. The grid-connected converter types to achieve

the continuous variability of speed and at a wide range of speeds are reviewed. The

topologies are illustrated and the properties of the converters are discussed in

comparison to each other. The switching components utilized in the converters are also

briefly introduced.

Examples of converter control schemes are discussed on a fundamental level. Two

control schemes, field-oriented control and voltage-oriented control are introduced. The

requirements for the operation of a grid-connected wind turbine are reviewed, and

examples of fault-ride through methods are analyzed. These include the operation

during frequency variations, the low voltage ride-through capability and islanding

detection. Two examples of low voltage ride-through methods are explained. A core

concept in analyzing islanding detection methods, the nondetection zone, is introduced.

Finally, the thesis is concluded with a summary of the topics addressed and a discussion

of possible future trends.

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PREFACE

This thesis has been a good learning experience for me in scientific writing in English.

Furthermore, it has allowed me to gain a lot of knowledge of different power electronic

converter applications used in the industry and to create an understanding to the related

control structures.

I would like to thank my opponents, my thesis advisor and my friends for their helpful

comments along the way and my girlfriend for her support and advice in English during

the writing process.

Tampere, 27.4.2014

Markus Ovaskainen

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CONTENTS

1. INTRODUCTION .................................................................................................... 1

2. WIND POWER ......................................................................................................... 2

2.1 The Structure of HAWTs ............................................................................... 3

2.2 Transformation of Wind Flow Power ............................................................ 6

3. VARIABLE SPEED WIND TURBINE ................................................................... 8

3.1 Doubly-Fed Induction Generator ................................................................... 9

3.2 Wind Turbines with Full-Rated Conversion Systems .................................. 10

3.2.1 Induction Generator with a Full-Rated Converter ......................... 11

3.2.2 Synchronous Generator with a Full-Rated Converter .................... 11

4. FULL-RATED CONVERTERS IN VARIABLE SPEED WIND TURBINES ..... 13

4.1 Generator-side Converters............................................................................ 14

4.2 Grid-side- and Bidirectional Converters ...................................................... 16

4.2.1 Voltage-Source Grid Converters .................................................... 16

4.2.2 Current-Source Grid Converters .................................................... 17

4.3 Direct converters .......................................................................................... 18

4.4 Converter control.......................................................................................... 18

4.4.1 Reference Frame Transformation .................................................. 20

4.4.2 Field-Oriented Control ................................................................... 21

4.4.3 Voltage-Oriented Control .............................................................. 22

5. GRID REQUIREMENTS FOR WIND POWER SYSTEMS ................................ 24

5.1 Frequency Deviation and Control ................................................................ 25

5.2 Low Voltage Ride-Through ......................................................................... 27

5.2.1 The DFIG with a Crowbar ............................................................. 29

5.2.2 LVRT of a Full-Rated PMSG ........................................................ 30

5.3 Islanding Detection ...................................................................................... 31

6. CONCLUSIONS AND FUTURE TRENDS .......................................................... 33

REFERENCES ................................................................................................................ 35

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LIST OF SYMBOLS AND ABBREVIATIONS

A (Blade Sweep) Area

Cp Power Coefficient

J Inertia of the Generator

P Active Power

Po Output Power

Pm Mechanical Power

Pw Power Generated by the Generator

Te Electromagnetic Torque

Tm Torque Generated by the Turbine

v Wind Speed

xa An arbitrary a-phase variable in a 3-phase power system

xb An arbitrary a-phase variable in a 3-phase power system

xc An arbitrary a-phase variable in a 3-phase power system

xd An arbitrary d-axis variable in the dq-frame

xq An arbitrary q-axis variable in the dq-frame

ρ (Air) Density

ωm Rotational Speed of the Turbine

θ Angle Between the d- and the q- axes

λr Rotor Flux

λr Rotor Flux Vector

v

AC Alternating Current

DFIG Doubly-Fed Induction Generator

GC Grid Code

CSC Current-Source Converter

GWEC Global Wind Energy Council

DC Direct Current

DFOC Direct Field-Oriented Control

FOC Field-Oriented Control

HAWT Horizontal-Axis Wind Turbine

IGBT Insulated-Gate Bipolar Transistor

IGCT Integrated Gate-Commutated Thyristor

LVRT Low Voltage Ride-Through

MOSFET Metal-Oxide-Semiconductor Field-Effect Transistor

MPP Maximum Power Point

MPPT Maximum Power Point Tracking

NDZ Nondetection Zone

NPC Neural Point Clamped

PCC Point of Common Coupling

PI Proportional-Integral (controller)

PMSG Permanent Magnet Synchronous Generator

PWM Pulse-Width Modulation

SiC Silicon-Carbide

SCIG Squirrel Cage Induction Generator

TSO Transmission System Operator

TSR Tip Speed Ratio

VAWT Vertical-Axis Wind Turbine

VOC Voltage-Oriented Control

VSC Voltage-Source Converter

WECS Wind Energy Conversion System

WRIG Wound Rotor Induction Generator

abc-frame Three-phase stationary reference frame

αβ-frame Two-phase stationary reference frame

dq- frame Rotational reference frame

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1. INTRODUCTION

Wind energy, being clean and sustainable, is of considerable interest as a renewable

energy source. The installed wind energy capacity is constantly increasing, and

consequently plays a bigger role as a connected generating unit in the grid. The grid

connection of renewable energy sources is considerably different from traditional ones,

like water power plants which inherently provide the grid with stability, whereas with

renewable energy sources, additional measures are needed.

Variable speed wind turbines are the current workhorses of the wind energy industry,

and are characterized as state of the art applications of modern power electronics.

Power electronic converters enable the wind turbines to act in a more sophisticated

manner in the grid compared to traditional directly grid-connected wind turbines. For

example, they provide the generating unit with an enhanced ability to withstand faults.

This thesis gives an overview of the grid connection of variable speed wind turbines and

discusses a variety of power conversion configurations. The grid connection is

approached through the means of the wind turbine to meet the demands of the power

system. The crucial role of the power electronic converters as a part of a variable speed

wind turbine system is illustrated with various examples of different topologies. The

fundamentals of controlling a power converter system are also explored.

Chapter 2 provides insight on wind energy in general and introduces the structure of a

wind turbine. Chapter 3 introduces variable speed wind turbines with various examples

of the most common applications along with an analysis on their principle power

production scheme. Chapter 4 discusses in detail how variable speed wind turbines are

connected to the grid fully through a power conversion system with examples of

converter control schemes. Finally, in Chapter 5, the connection to the grid is analyzed

from the grid’s standpoint. In Chapter 6, the themes discussed are summarized and

concluded by elaborating on future trends and possibilities.

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2. WIND POWER

Wind can be defined as air in motion. The sun warms up the surface of the Earth

unevenly in a geographical sense, therefore causing balancing heat flow which

manifests in winds. Wind power production is the conversion of the kinetic energy of

flowing wind to a useful form of energy. The total amount of wind power available is

virtually inexhaustible, even considerably more than the present human use of power

from all other energy sources combined. [13, p. 227]

At the end of 2014, the total capacity of the wind power was approximately 370 GW

[4]. As illustrated in Figure 1, the growth of the capacity is slowly but constantly

accelerating. However, the proportion of wind power capacity of the total global

electricity demand is still small. In 2012, wind power accounted for about 2-3 % of the

global electricity supply [15, p. 643].

Figure 1. The global cumulative installed wind power capacity [4].

Wind power is used for the generation of electricity specifically in developed countries.

At the end of 2014, Europe and North America accounted for over 55 % of the global

total capacity. With China and India included, they accounted for almost 95 % of the

total capacity. However, according to the wind energy capacity growth statistics

provided by the Global Wind Energy Council (GWEC), the proportion from developing

countries has been rising. [4]

The reason for wind power still having such a small portion of the total production is

that the full economic costs of wind turbines are greater than the cost of traditional

energy sources. Despite the price, the growth is accelerating mainly driven by concerns

of climate change and energy supply security [15, p 643]. The European Union, for

example, aims to cover 20 % of the total energy production with renewable energy

sources by 2020 [9, p. 53]. In comparison to traditional energy sources, the

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environmental impact of wind power is minor, as wind power does not emit any air

pollution nor consume any fuel. [13, p 236]

There are two basic structure types of wind turbines, HAWTs (Horizontal-Axis Wind

Turbines) and VAWTs (Vertical-Axis Wind Turbines). Examples of these types are

shown in Figure 2. In HAWTs, the orientation of the rotational axis is parallel to the

ground, whereas in VAWTs, it is orthogonal.

Figure 2. Two Horizontal-Axis Wind Turbines (left) and a Vertical-Axis Wind

Turbine (right). [11, p. 3, 7, p. 168]

Of these two structure types, HAWTs are much more commonly used in the industry

[16, p. 11]. Other types of wind turbines are not commonly discussed in modern

literature [7, p. 2]. In this thesis, the wind turbines referred to throughout the text are

HAWTs.

2.1 The Structure of HAWTs

A HAWT captures the kinetic energy of wind with the turbine blades, which are

mounted on the rotor hub. The turbine blades transform the kinetic energy into

mechanical energy. The rotating shaft drives the generator which transforms the

mechanical rotational energy into electrical energy. The electrical energy is transformed

into a useful form in the power converter system, and finally the electrical power is fed

to the grid. The main components of a typical HAWT are shown in Figure 3.

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Figure 3. Main components of a HAWT. [16, p. 26]

The main variant in the layout shown in Figure 3 is the gearbox, which is not required

in all wind turbine configurations, depending on the generator type. Gearless wind

turbines are called direct-drive wind turbines, and they require a multi-pole synchronous

generator capable of operating efficiently at slow rotational speeds. Generators with a

high pole number can be driven with lower rotational speed of the rotor compared to

generators with lesser poles, still producing electrical power with the same frequency.

Figure 4 is a flowchart illustrating the power conversion in wind turbines. [16, p. 26]

Figure 4. The wind turbine power conversion process. [19, p. 1860]

Three-bladed wind turbines are considered the industrial standard for large wind

turbines. Wind turbines with fewer blades exist, and have some practical applications.

However, they rotate at higher speeds which creates more noise, and the power

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production is less symmetrical. A wind turbine with more than three blades is rare due

to increased material costs, the turbulence in air flow caused by the respective proximity

of the blades and also because the lower rotation speed requires higher gear ratio

therefore increasing costs.

The aerodynamical operation of the blades is based on a fluid-dynamical phenomenon

explained by Bernoulli’s principle [7, pp. 3-4]. The shape of the blade, referred to as the

blade profile, creates a difference between the wind flow speed above and below the

blade, which in turn creates a pressure difference that results in a lift force that causes

the blade to turn. [16, p. 27]

To keep the wind turbine aligned optimally in respect with the direction of the wind

flow, the yaw control system rotates the nacelle to face the wind so that the turbine

blade rotating area is swept by the wind as perpendicularly in respect with the blades as

possible. This is a fundamental mechanical means to maximize the energy captured

from the wind.

The height of the tower of the wind turbine is as high as possible within economical

reason, because the wind is more turbulent closer to the ground which interferes the

capturing the maximum power available. To avoid the turbulence caused by trees and

buildings, the tower must be at least 25 to 30 meters high. The winds higher above the

ground are also stronger and steadier. [16, p. 35, 13, p. 233]

The rotor of a three-bladed wind turbine is usually rotating at a relatively slow speed.

Therefore, for grid synchronization, wind turbine systems without full-rated power

conversion or a multi-pole synchronous generator require the generator to be driven

with rotational speed much higher than the low speed shaft seen in Figure 2. For this

reason, a gearbox is required in many wind turbine configurations. The disadvantages of

a gearbox are greater costs in initial investment and maintenance, lower efficiency, and

decreased reliability. [16, pp. 30-31]

Wind turbines can operate either on fixed or variable speed. In fixed-speed wind

turbines, the speed of the rotor stays approximately the same regardless of the wind

speed above the cut-in speed where the blades start turning, and below the rated

maximum speed where the wind turbine stops operating as a protective measure. The

nominal operating speed is called the rated speed. Fixed-speed wind turbines are

designed to operate at maximum efficiency at the rated speed. In variable speed wind

turbines the rotor speed can vary from the rated speed in a certain range continuously or

in steps in dual-speed pole-switching generators. [11, p. 161]

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Figure 5. Different wind turbine system configurations. [16, p. 154]

The generators mainly used in wind turbine systems are the Squirrel Cage Induction

Generator (SCIG), Doubly-Fed Induction Generator (DFIG), Wound Rotor Induction

Generator (WRIG), Wound Rotor Synchronous Generator (WRSG) and Permanent

Magnet Synchronous Generator (PMSG). They all can be used in variable speed

operation, but SCIG is the dominant generator choice for fixed-speed wind turbines.

Different generator types and their general operation are illustrated in the classification

in Figure 5.

2.2 Transformation of Wind Flow Power

Neglecting the mechanical efficiency, the average power of wind flow can be described

by the following equation [11, p. 8]:

𝑃 =1

2𝜌𝐴𝑣3𝐶𝑝, (1)

where 𝜌 is the air density, A is total disk area swept by the turbine blades, v is the wind

speed and Cp is the power coefficient of the wind turbine. If the power coefficient Cp is

neglected, the equation describes the wind power flowing through an arbitrary area. The

power coefficient indicates how much of the power of the wind can actually be captured

by the turbine blades. The power coefficient is a function of the wind turbine’s Tip

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Speed Ratio (TSR) which denotes the ratio between the tangential speed of the tip of the

blade and the actual velocity of the wind. Betz’s law sets the limit for the theoretical

maximum for the power coefficient which is 0.593. In practice, it is a little lower:

modern wind turbines can reach a power coefficient value of about 0.5 which is 84 % of

the theoretical maximum. [19, p. 1864, 11, p. 8]

It is worthwhile to note that the power of the wind flow is proportional to the third

power of the wind speed. Thus higher wind speeds produce considerably more power.

Stronger winds can appear more briefly compared to slow wind speeds, but much

energy is available. If the wind turbine is operating at a fixed speed rated for steady

slow-speed conditions where the wind speed is mostly steady, but greatly stronger wind

speeds appear briefly but often, much of the available energy is not captured. From this

it can be deduced, that to maximize the efficiency of the wind turbine, capturing a wide

range of different speeds is required.

The mechanical regulation of the wind turbine rotational speed is done by controlling

the pitch of the turbine blades, i.e. rotating the wind turbine blades on their vertical axis.

The pitch control changes the angle of attack of the wind which means the direction of

the wind flow in respect to the turbine blade. By changing the angle, the power captured

by the wind can be controlled to a certain degree. This provides protection over

excessive wind speeds, and also creates the possibility to maintain more or less the same

rotor rotational speed on different wind speeds which is required for fixed-speed wind

turbines. [16, p. 29]

Another important aerodynamical power control method is called stall control which is

important especially in fixed-speed wind turbines without pitch control. The idea of stall

control is that above the rated maximum wind speed, with increased angle of attack, the

strong wind causes turbulence on the surface of the blade which causes the lift force

applied to the blade decrease significantly, and with even stronger wind, eventually

disappear. This phenomenon, called stalling, can be used to protect the wind turbine

from winds too strong. Stall control can be categorized into passive and active control.

In passive stall, the blade profile is designed so that stalling occurs only when the wind

speed is above the rated maximum. In active stall, the angle of attack can be adjusted by

the pitch control mechanism, consequently changing the wind speed where stalling

occurs. [16, pp. 39-40]

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3. VARIABLE SPEED WIND TURBINE

The main advantage of a variable speed wind turbine compared to a fixed-speed wind

turbine is that it can capture maximum available energy at different wind speeds which

makes it more efficient and therefore increases the production of energy. The

mechanical stress on the blades and the whole drive train is also reduced when it is not

required to maintain a fixed rotor speed in different wind conditions, resulting in a

longer lifespan of the system. [10, p. 752]

The main goal in controlling a variable speed wind turbine is to maximize the power

capture at different wind speeds. The mechanical power in rotating machines can be

expressed by the following equation:

𝑃𝑚 = 𝑇𝑚𝜔𝑚, (2)

where Pm is the mechanical power, Tm is the torque generated by the turbine shaft and

𝜔𝑚 is the rotational speed of the turbine. Therefore the product of the torque and the

rotational speed should be kept at the maximum at all times. This point of operation is

called the Maximum Power Point (MPP). Figure 6 illustrates the MPP characteristics

with an ideal angle of attack assumed. Each curve stands for a different wind speed. [16,

pp. 43-44]

Figure 6. The maximum power point of a wind turbine at different wind speeds. [16,

p. 44]

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Below the rated wind speed, the MPP is followed by controlling the generator. Above

the rated wind speed the power is kept at the rated maximum by controlling the pitch of

the blades, so that the turbine speed does not increase above the safety limit.

MPP Tracking (MPPT) requires a feedback control scheme where the generated power,

the turbine rotational speed or the torque generated by the generator is measured and the

control input is adjusted accordingly. The control is based either on the power curve

provided by the manufacturer of the wind turbine or the rated parameters of the

generator. [16, pp. 44-46]

In variable speed wind turbines, the speed may be varied fully on a wide range or only

partially. The wide range of variability is achieved by completely decoupling the

generator from the grid through power electronic converters, and the partial variability

by varying the slip of an induction generator, or using an induction generator with

switchable pole number.

Only wind turbines with continuous variability of speed are discussed, except for the

WRIG with a variable rotor resistance. This configuration does not employ power

converters to achieve the variability of speed.

3.1 Doubly-Fed Induction Generator

The DFIG is one of the most popular wind turbine configurations [16, p. 158]. The

DFIG is based on a WRIG where the stator is directly connected to the grid and the

rotor is connected to the grid through a power conversion system which enables the

variable speed operation. A typical DFIG power conversion system is illustrated in

Figure 7.

The rotor windings are connected to the converter through slip rings. The generator can

feed the grid with power with a variance of about 30% below and above the rated speed

[19, p. 1862]. The idea of feeding only a part of the power produced through the

converter instead of full-rated conversion is that the nominal power of the conversion

system can be considerably smaller than the nominal power of the wind turbine. Thus

the power conversion system is considerably less costly.

10

Figure 7. A typical DFIG power conversion system.[19, p. 1862]

The DFIG normally uses a back-to-back converter system with a DC-link which can be

seen in Figure 7. In a back-to-back converter, the power can flow in both directions, and

voltage-source converters, introduced in Chapter 4, are employed on both sides.

Induction generators require reactive power flow to the rotor for field excitation. The

converters in this configuration also have the ability of controlling both active and

reactive power independently which is advantageous because no reactive power

compensation on the grid-side is needed for the excitation of the rotor. The converters

provide the rotor with the required reactive power, but the reactive power flow does not

appear grid-side. [19, p. 1862]

Other advantages of the DFIG are that the noise created by the turbine can be reduced

since it can operate at a slower speed when the wind speed is slower and the converter

can be utilized to start the wind turbine in a more controlled manner compared to

connecting the generator to the grid directly. [19, p. 1862]

3.2 Wind Turbines with Full-Rated Conversion Systems

In wind turbines with full-rated power conversion, all the electrical energy created by

the generator is fed to the grid through a power conversion system. The converters

usually have back-to-back functionality, meaning that the power flow is bidirectional.

Since the generator is fully decoupled from the grid, the generator can operate at a wide

range of rotational speeds and the torque can be controlled. The synchronization with

the grid is managed by the grid-side converter. The grid-side converter can also control

the active and reactive power supply independently which improves the dynamic

response of the wind turbine. [19, p. 1863]

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3.2.1 Induction Generator with a Full-Rated Converter

A further improvement compared to the DFIG is an SCIG with full power conversion.

In comparison, the dynamics and the grid-side behavior are enhanced, because even the

stator of the generator is not directly connected to the grid. A typical configuration of a

SCIG with a full-rated back-to-back conversion system is shown in Figure 8.

Figure 8. A typical full-rated SCIG wind turbine. [19, p. 1863]

A gearbox is always required in this configuration because standard SCIGs do not

operate efficiently at low speeds. The SCIG always requires reactive power flow in the

direction of the generator for rotor magnetization. Therefore, no diode-bridge can be

used on the generator-side. However, because of the complete decoupling from the grid,

no additional reactive power compensation is needed. [14, p. 127]

3.2.2 Synchronous Generator with a Full-Rated Converter

Synchronous generators have many more available configurations than full-rated

induction generators. This is due to the fact that diode-bridges can be used in power

conversion and multipole generators can be implemented more easily and cost-

effectively than with induction generators [16, p. 163]. An example of a full-rated

synchronous generator configuration is shown in Figure 9.

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Figure 9. A Permanent-Magnet Synchronous Generator with a full-rated converter.

[19, p. 1863]

In Figure 9, the generator is excited with permanent magnets. The generator can also be

field-excited, as in the WRSG, at which time the generator needs a small power

converter for the excitation. In such a case the converter is fed from the output of the

grid-side converter, so that the wind turbine uses its own power production for the

excitation. Separate excitation of the rotor allows control over the ratio of the active and

the reactive power generated in the generator, but this feature is not important in full-

rated systems, where the control of the active and reactive power is taken care of by the

converter system. Synchronous generators can be used either in direct-drive or with a

gearbox, depending on the number of poles of the generator. Larger diameter multipole-

generators can be used gearlessly. [1, p. 69]

The PMSG is a very attractive wind turbine configuration because of the high overall

efficiency, reliability and the power-density compared to field-excited generators [19, p.

1873]. However, the materials used for producing permanent magnets and the

manufacture process are expensive. The magnet pole system of PMSGs can have salient

poles, usually in larger-diameter slow-speed generators, or can be cylindrical. One

major disadvantage in PMSGs is the temperature sensitivity of the magnets. The

magnets may lose their magnetic qualities during a fault situation creating excess heat.

[1, p. 70]

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4. FULL-RATED CONVERTERS IN VARIABLE

SPEED WIND TURBINES

Full-rated power conversion combines the rectification on the generator-side and the

inversion on the grid-side into a frequency converter. In full-rated conversion, the

generator is always fully decoupled from the grid, and depending on the converter, the

grid-side and the generator-side can be viewed independently.

The power converters in a variable speed wind turbine have various advantages to both

the grid and the wind turbine itself. Power converters can provide the system with

controllable frequency and traditional power plant characteristics. The latter means that

the wind turbine can become an active element in the grid, participating in frequency

control and allowing the decision to be made on how much power to inject and when,

within the limits of the rated power of the wind turbine and the performance of the

power electronic components. [14, p. 123]

Other advantages include reduced noise, improved power quality and the optimal

operation to capture maximum energy from the wind. On the other hand, the

disadvantages are extra costs, additional losses, and the injection of high harmonic

currents, that create additional losses, to the grid created by the converters. [1, p. 59]

Variable speed wind turbines can utilize two types of converters which use different

components. These two types of converters can be characterized as self-commutated or

grid-commutated devices. Grid-commutated devices mainly consist of thyristors. Self-

commutated consist of either Gate Turn-Off-thyristors (GTOs), Integrated Gate-

Commutated Thyristors (IGCTs) or transistors. Thyristor is a cheap solution with low

losses, but the drawbacks are the consumption of reactive power and the production of

large harmonics. The most common type of transistor used in wind turbine converter

applications is the Insulated-Gate Bipolar Transistor (IGBT). Self-commutated

converters have high switching frequencies, which allows the harmonics to be filtered

out more easily resulting in reduced disturbances in the grid. IGCTs and GTOs both are

able to handle more power than IGBTs, but they require more complex control schemes,

and have relatively low switching frequencies. The disadvantages of both IGCTs and

IGBTs are their high price and high losses. [1, pp. 61-74, 9, p. 86]

Self-commutated converters are either Voltage-Source Converters (VSCs) or Current-

Source Converters (CSCs), which both can control frequency and voltage. VSCs

produce a defined voltage waveform according to the modulation method, whereas

CSCs produce a defined current waveform. Full back-to-back VCSs are the most

14

common option for a converter to fully control active and reactive power [19, p. 1863].

In VSCs, the voltage in the link between the generator-side terminal and the grid-side-

terminal is kept constant, whereas in CSCs, the current is kept constant. The voltage can

be maintained in the link using a capacitor, and the current using an inductor. [1, p. 62]

Furthermore, the converters can be characterized as rectifiers and inverters based on the

direction of the power conversion. Rectifiers convert AC to DC, and inverters DC to

AC. However, many converters can act as both.

4.1 Generator-side Converters

The generator-side converter can work uni- or bidirectionally. In systems with

unidirectional power flow, a diode bridge is used. A diode rectifier is a simple and a

cheap solution that can be utilized with synchronous generators. A diode rectifier bridge

rectifies the voltage nonlinearly, and consequently, creates harmonic currents. A diode

rectifier is not able to control the generator-side voltage nor the current, and therefore is

a passive element in the system. [1, p. 73]

Not all converter solutions can be applied to all generators. The generator and the

generator-side converter have to be chosen as a combination, whereas the grid-side

converter can be chosen almost independently. Diode-bridges and thyristor converters

can only be used with synchronous generators, because they do not allow the reactive

magnetizing current flow to the direction of the generator which is required for

asynchronous generators i.e. SCIG [16, p. 164]. With asynchronous generators, the

converter bridge consists of GTOs and IGBTs which allow the control of reactive

power. This feature increases costs and losses compared to a simple diode bridge. [1,

pp. 73-74]

With synchronous generators, a boost converter is often used together with a diode

rectifier. A boost converter is a basic type of a power electronic converter that can

elevate the voltage level from input to output. The boost converter is placed between the

generator-side diode-bridge and the grid-side converter, as shown in Figure 10.

Figure 10. A synchronous generator with a boost converter. [16, p. 88]

15

The boost converter is used for tracking the MPP of the generator and boosting the DC-

voltage to an appropriate level for the grid-side converter which is important to ensure

the delivery of the maximum power available to the grid at different wind speeds

efficiently [16, p. 97, 14, p. 129]. The boost converter can have a single channel or

multiple channels. Single-channel boost converter can be used in low and medium

power wind turbines, but in high-power applications, the voltage can increase beyond

what one switching device can handle. As a solution, several switching devices can be

connected in parallel or in series. In a multi-channel boost converter, several power

converters are connected in parallel. The basic topology of a multi-channel boost

converter is presented in Figure 11. In this figure, the power flow is from left to right.

Figure 11. A multi-channel boost-converter topology. [16, p. 109]

Interleaved multi-channel boost converters are used in low-voltage high-power wind

turbines to handle the high currents in the system. Interleaving is realized by phase

shifting the gating signals for each of the parallel converters. This solution brings the

advantage of a higher equivalent switching frequency which results in lower input

current ripple, output voltage ripple, faster dynamic response and better power handling

capacity. [16, p. 98]

IGBTs are normally used in interleaved boost converters instead of Metal-Oxide-

Semiconductor Field-Effect Transistors (MOSFETs) which are used in common boost

converters because the IGBT has better voltage and power capacity. MOSFETs have the

capability for a higher switching frequency.

16

4.2 Grid-side- and Bidirectional Converters

Some converter topologies allow bidirectional power flow and can operate either on the

grid- or the generator-side. However, regardless of the topology used, the control of the

grid-side converter is more delicate. The grid-side converter controls the balance

between active and reactive current flow between the generator and the grid. The grid

synchronization is also the grid converter’s responsibility. [14, p. 123]

4.2.1 Voltage-Source Grid Converters

A typical bidirectional converter, used both on the generator- and the grid-side, is the 2-

level VSC. This converter, which is shown in Figure 12, is a proven, widely used

converter in different applications in industry, and the literature and documentation

available for this type of converter is unmatched by all other converter types [5, p. 58].

This converter is composed of six switches with an antiparallel diode for each.

Figure 12. A 2-level VSC connected to the grid. [14, p. 130]

The switches used in the 2-level VSC are either IGBTs or IGCTs usually controlled

with Pulse Width Modulation (PWM), depending on the power and the voltage rating of

the converter. The converter is mainly used in low voltage, low- and medium-power

systems, up to 2 MW. On high power, switching losses increase substantially and the

components may not be able to handle the higher voltage levels. This converter type

requires high-order output filtering to reduce harmonics in currents injected to the grid.

[14, pp. 129-131]

The 2-level VSC is typically utilized in full-rated SCIG wind turbine systems as a back-

to-back converter, as shown in Figure 8, and as an inverter in synchronous generator

wind turbine systems either as part of a back-to-back converter, or to reduce costs, with

a diode rectifier as shown in Figure 9. This is also the main converter type in the DFIG,

having reduced-scale conversion, as shown in Figure 7.

17

With higher output power, it may be more cost effective to utilize medium voltage (3-4

kV) to decrease losses. With low voltage and high power, the significant current

increase is directly proportional to cable losses. However, on higher voltages, the stress

for the components in the 2-level VSC may be too much to handle. In applications with

higher power rating, i.e. over 2 MW, the multilevel Neutral Point Clamped VSC (NPC

VSC) can be used. This topology is illustrated in Figure 13. [16, p. 161]

Figure 13. A SCIG with a 3-level NPC VSC system. [16, p. 161]

This converter can reach a power rating of 6 MVA. The components need to be able to

withstand high voltages, so IGCTs or special high-voltage IGBTs are used. However, in

this topology, the relative voltage stress to an individual switch is reduced in

comparison to the 2-level VSC. Regardless of Figure 13, where a SCIG is included in

the topology, in 2011, this converter was widely used only in medium-voltage

synchronous generator wind turbine systems. Generally, NPC VSC can be configured as

a 3-, 4-, or 5-level topology, with the 3-level topology being the dominant one in

practical applications. [16, pp. 125-162]

The main advantages of NPC-converters are reduced harmonics and reduced dv/dt

which consequently reduces the switching losses. In NPC-converters, the switching

devices are not required to connect in series to withstand higher voltages. The main

drawback is increased cost in comparison to 2-level VSC since it has a large number of

components. [16, p. 126]

4.2.2 Current-Source Grid Converters

CSCs are of relatively simple design, and feature a reliable short circuit protection. The

short-circuit protection is inherent for the design because the inductor in the DC-link

restrains the current transients produced by a short circuit. CSCs are particularly

suitable for high-power wind turbine applications. Generally speaking, CSCs can be

applied where VSCs can, but the power rating is higher. A typical back-to-back CSC

configuration is shown in Figure 14. [16, pp. 131-162]

18

Figure 14. A 2-level back-to-back CSC. [14, p. 132]

The converters both generator- and grid-side are identical. Due to the large stress on the

components, IGCTs, GTOs, high-voltage IGBTs and thyristors are used. CSCs also

require a capacitor on each phase to assist the commutation of the switching devices and

to reduce harmonics produced by switching. [14, p. 131, 16, p. 162]

4.3 Direct converters

In this thesis, the generator-side converter and the grid-side converter are discussed as

separate entities, which is reasonable with, for example, back-to-back converters, where

the generator- and the grid-side control is decoupled by the capacitor in the DC-link.

The back-to-back converter is currently the dominant topology in wind power

applications. [1, p. 74, 19, p. 1863]. However, it is worthwhile to note that in some

converters, like in the matrix converter and in the multilevel converter, the whole

conversion process from generator to the grid is controlled jointly [8]. In such a case,

the conversion is called direct. [1, p. 75]

The main advantages of direct converters are smaller thermal loads of the power

devices, lesser switching losses and a better harmonic performance in comparison to, for

example, the 2-level VSC. The absence of the DC-link also makes direct converters

attractive due to reduced costs when no large capacitor nor inductor are required. Direct

converters are also smaller in size and more reliable than conventional converters. The

disadvantages are the higher number of components required hence producing more

losses, and more complex control. Direct converter applications in wind turbines may

challenge back-to-back converters in the future, but still require more research. [8, 14, p.

128]

4.4 Converter control

The wind turbine system controls the power injected to the grid by means of both

mechanical and electrical control. Both control loops are able to limit the power

injected, since redundancy is specifically requested by standards for safety reasons [14,

p. 135]. The mechanical control loop tracks and limits the maximum power captured

19

from the wind by varying the pitch angle. The electrical control loop controls active and

reactive power balance on the grid-side and aims to keep the DC-link voltage or current

constant, depending on the converter type. The aim of the control of the generator-side

converter is to extract the maximum available power at all times. The general wind

turbine system control scheme is shown in Figure 15. [14, pp. 135-137]

Figure 15. The control structure for a wind turbine system. [14, p. 136]

On the grid-side the control system may participate in regulating the voltage and the

frequency of the grid by means of active and reactive power control. This is important

especially during grid faults. The control of the converters is more complex during

faults, and careful design is required to aid the fault-ride through. However, in-depth

analysis of these control structures during faults is beyond the scope of this thesis.

The converters may be controlled by several different schemes. One thing that all

control schemes have in common, is that the estimation and tracking of the variables

indicating the state of the grid is important for proper operation and fast dynamics of the

grid-side converter. Accurate information of phase angles and the amplitudes of grid

voltages is needed. [14, p. 244]

The control scheme used for the generator control varies according to the generator type

and its characteristics. The grid-side converter, however, is decoupled and therefore not

tied to the generator type. To elaborate the control of a variable speed wind turbine

system, Field-Oriented Control (FOC) method for generator control and Voltage

Oriented Control (VOC) method for grid inverter control are analyzed as examples. To

understand these control principles, the concept of reference frame transformation is

introduced.

20

4.4.1 Reference Frame Transformation

Different reference frames can be used to simplify the analysis of electrical machines,

and to make the digital implementation of control schemes easier. Reference frames are

essentially different coordinates in observing the electrical phasor quantities in three-

phase systems. The most commonly used reference frames are the three-phase

stationary frame (abc-frame), the two-phase stationary frame (αβ-frame) and the

synchronous frame (dq-frame). [16, pp. 50-51]

In abc-frame, the three-phase axes are stationary in space, and there is a generic

electrical variable (for example, voltage, current or flux) representing the magnitude in

each of the three phases. Let these variables be xa, xb and xc. In dq-frame, the two axes,

d and q, that are always perpendicular to each other, rotate at an arbitrary speed. Both

axes rotate at the same speed. In the dq-frame, let the variables expressing the three-

phase quantities be xd and xq. If the two reference frames are drawn on top each of other

in such a way that their respective origos are at the same point of space, as in Figure 16,

and assuming, that at any given time, the angle between the d-axis (in the dq-frame) and

the a-axis (in the abc-frame) is known, we can derive xd and xq from xa, xb and xc by

orthogonal projection of the latter quantities to the d and q axes. Thus the reference

frame transformation from abc-frame to dq rotational frame is achieved. [16, pp. 51-52]

Figure 16. The abc/dq reference frame transformation. [16, p. 52]

21

Utilizing trigonometrics, the abc/dq reference frame transformation can be expressed in

matrix form as follows [16, p. 52]:

[𝑥𝑑𝑥𝑞] =

2

3[cos 𝜃 cos (𝜃 −

2𝜋

3) cos (𝜃 −

4𝜋

3)

−𝑠𝑖𝑛𝜃 − sin (𝜃 −2𝜋

3) − sin (𝜃 −

4𝜋

3)] [

𝑥𝑎𝑥𝑏𝑥𝑐],

(3)

where xa, xb and xc are the abc-frame variables, xd and xq are the dq-frame variables, and

𝜃 is the angle between the d- and the a- axes.

Following similar principles, transformations can be done between other reference

frames as well, but understanding the abc/dq reference frame transformation is

sufficient to analyze the FOC and the VOC schemes.

4.4.2 Field-Oriented Control

FOC is a control scheme to control the rectifier on the generator-side and consequently

the generator. The field orientation control can be classified in to stator flux, air-gap

flux and rotor flux orientations, with the last being one of the most used schemes wind

turbine systems, being suitable specifically to full-rated SCIG wind turbine systems.

[16, p. 192]

The idea of FOC is to control the reference variables, rotor flux λr and the

electromagnetic torque Te, independently. In rotor flux orientation, the stator current can

be broken down to two components with abc/dq reference frame transformation: a flux-

producing component, producing the rotor flux λr and a torque-producing component

producing the electromagnetic torque Te. In rotor flux orientation, the rotor flux vector

λr is aligned with the d-axis of the dq-frame. Consequently, the flux-producing stator

current component is aligned to the same direction, and the torque-producing

component in the direction of the q-axis. [16, pp. 192-193]

If the rotor flux is kept constant, the developed electromagnetic torque can be directly

controlled by controlling the stator current aligned with the q-axis. In FOC, the flux-

producing current component is normally kept at its rated value whereas the torque-

producing component is controlled independently. A type of a FOC scheme is

illustrated in Figure 17. [16, pp. 192-193]

One key issue in FOC is the estimation of the angle 𝜃 for the field orientation and

consequently for the abc/dq transformation. In Direct Field-Oriented Control (DFOC),

as in Figure 17, the angle is obtained by measuring generator terminal voltages and

currents. [16, p. 193]

It is worthwhile to note that the control itself is done in the synchronous reference frame

by the Proportional-Integral (PI) controllers, where all the variables are of DC

22

components in a steady state due to the abc/dq-transformation. This makes the design of

the control system easier. The control ultimately comes down to generating the desired

PWM signal controlling the switches in the generator-side converter.

Figure 17. Direct Field-Oriented Control scheme for a full-rated SCIG wind

turbine system. [16, p. 194]

As can be seen from the figure, the electromagnetic torque Te is set in accordance with

the rotational speed of the wind turbine to stay at MPP. The MPPT may be carried out

as discussed in Chapter 3.

4.4.3 Voltage-Oriented Control

The grid-side converter can be controlled with various schemes. On the grid-side, the

generator is not a factor in choosing the control scheme. VOC is a control scheme for

VSCs, where the power flow is bidirectional. Like in FOC in the generator-side, the

control algorithm is based on the abc/dq reference frame transformation, where the

three grid phase voltages are transformed to two variables in the dq-frame. The phase

angle of the grid voltage is carefully detected for the voltage orientation. Various

methods are available for the angle detection. [16, p. 144]

23

There are three main control loops in VOC. The d- and the q- axis currents, which are

the active and the reactive current components of the three-phase line-currents,

respectively, are controlled separately in inner loops, and the DC-link voltage is

controlled in accordance to the desired value. The separate control of the currents allows

accurate and independent control of the active and the reactive power injected to the

grid. The VOC scheme and the control loops are illustrated in Figure 18. In the figure,

the generator-end of the DC-link is replaced by a voltage source and a resistor.

Figure 18. Voltage-Oriented Control of the grid-side converter. [16, p. 144]

The reference control variables in this scheme are the desired reactive power injected to

the grid and the desired DC-link voltage. The desired reactive power can also be zero

for unity power factor operation. When the converter is operating in a steady state, the

DC-link voltage is kept constant. By varying the DC-link voltage level, the amount of

the power injected to the grid can be restricted. [16, pp. 143-146]

24

5. GRID REQUIREMENTS FOR WIND POWER

SYSTEMS

Traditionally wind turbines have not provided a considerable portion of the power

supplied to the grid and they have not been required to participate in frequency and

voltage control [19, p. 1865]. However, in recent years, the wind energy capacity has

been rapidly growing, and therefore the wind turbine participation in maintaining the

stability of the grid has become of greater importance.

For example, in the worst case, voltage sags caused by a momentary grid faults can

drive the wind turbine to disconnect from the grid. Voltage sag stands for a sudden

reduction of the grid voltage, where the voltage level generally drops to 10–90 % of the

nominal voltage [14, p. 177]. This type of operation is undesirable in the grid, as it

further contributes to the instability and prevents the grid from recovering from the

fault. Therefore regulation is needed to ensure proper operation and stability.

Transmission System Operators (TSOs) have introduced Grid Codes (GCs) as

requirements for grid-connected power generation systems. The GCs define the

requirements for both normal operation and operation during grid faults. These

requirements may include, but are not limited to, fixed minimum time that the wind

turbine system has to stay connected to the grid even if the voltage goes down to zero

and supporting the grid recovery by injecting reactive current to the grid during faults.

[14, p. 158]

Since the generating units are required to comply with the GCs, the stability of the

power dispatch is maintained regardless of the generation technology used. The GCs

also lay ground rules for wind turbine manufacturers on what the requirements are for

the function of their equipment. The GCs vary between countries but the general

message is the same: it is expected for wind power systems to behave in the same way

as large synchronous machines in traditional power generation as much as possible.

Large synchronous generators are well-established technology, and they have various

attractive features contributing to the transient stability to the grid; for example, they

offer inertia, resynchronizing torque, oscillation damping, reactive power generation

and fault ride-through capabilities. [14, p. 146]

Frequency control and the operation during a voltage sag are among the most important

qualities for a grid-connected wind turbine to have in order to be able to act as an active

unit in the grid. Islanding detection is an important advanced feature for all renewable

25

energy sources to avoid damage to the equipment or personnel when unintentionally

operating on an island.

5.1 Frequency Deviation and Control

The frequency of the grid is an indicator of the imbalance between production and

consumption of the electric power. In a normally operating grid, the actual frequency

should be close to its nominal value. In European countries, the frequency in normal

operation varies between 49.9–50.1 Hz [1, p. 123]. One requirement for grid-connected

units is to be able to increase or decrease the power production in accordance to the

frequency of the grid, therefore participating in grid frequency control. Since wind may

not be controlled, the power production in wind turbines is intentionally kept lower than

the possible maximum so that at under-frequencies, the power production may be

increased. The power output of a wind turbine may vary up to 15 % of the installed

capacity. [9, p. 273, 1, p. 124]

Wind turbines may participate in frequency control only in a certain frequency range.

With slight deviation, the wind turbine has to operate continuously, and within a certain

small frequency range, the wind turbine has to stay connected in the grid for a defined

time. If the frequency deviates even more, the wind turbine disconnects from the grid

for security reasons. Different countries have different requirements (GCs) for the

frequency control of wind turbines. The requirements are illustrated in Figure 19. [9. p.

273]

The TSO companies in Figure 19 represent different countries and their respective

requirements for the operation in Europe. It can be seen that the strictness of the

requirements differ a great deal from country to country. The strictest limits for

continuous operation are in the British grid (NGET).

The frequency (x-axis) and the voltage (y-axis) range where the wind turbine is

expected to stay in continuous operation is illustrated in black in the graphs. The gray

shades represent time intervals of how long the wind turbine needs to stay connected to

the grid on a defined range. The lengths of the time intervals, the voltage range and the

frequency range vary from country to country, but it can be seen that the wind turbine

needs to stay in continuous operation at least between 49.5–50.1 Hz. However, the

limits set by the British grid operator are between 47.5–52.0 Hz which is a relatively

broad operation range.

26

Figure 19. Requirements for operation during frequency deviations in

European countries. [14, p. 152]

There are two types of frequency controls based on the duration and the goal of the

control, namely primary and secondary. The primary control varies the generation of

active power for a time span of 1–30 seconds or until the balance between consumption

and production is restored. The aim of the primary control is to stabilize the frequency

so that the deviating will stop. The secondary control is employed to restore the

frequency to its nominal value for a time span of 1–15 minutes. The secondary control

varies the power generation more slowly which results in slower decrease or increase in

the frequency. Depending on the country, the secondary control may be automatic, or

employed by request of the system operator. Wind turbines can participate in the

primary control, but additional devices, called the secondary reserve, connected to the

grid are usually used for the secondary control. [1, pp. 123-146]

27

5.2 Low Voltage Ride-Through

Low Voltage Ride-Through (LVRT) stands for the capability of the wind turbine to

withstand a voltage sag caused by a temporary fault in the grid. As discussed earlier, in

a voltage sag, the grid voltage reduces quickly and suddenly. A voltage sag, also called

a voltage dip, is usually caused by short circuits, faults to ground, transformers

magnetizing and connection of large induction motors in the grid [14, p. 177]. Full-rated

variable speed wind turbines have great advantage in satisfying the LVRT requirement

due to the ability to fully control active and reactive power in the grid-side [12, p. 97].

Voltage sags manifest in several types depending on the cause of the phenomenon. A

voltage sag caused by a three-phase short circuit or a three-phase to ground fault is

illustrated in Figure 20. The phasor graph on the left side of the figure shows the

symmetrical nature of this type of fault; all phase voltages in the grid are reduced in

equal proportion. On the right side of the figure, the reduction of the voltage can be seen

as a function of time.

Figure 20. A symmetrical voltage sag caused by a three-phase short-circuit or

a three-phase to ground fault. [14, p. 178]

Disconnecting from the grid during grid faults due to voltage sags is a problem

especially for induction generator based wind turbines. As the voltage drops down from

the nominal value at the Point of Common Coupling (PCC), the electromagnetic torque

normally significantly decreases which leads to instability between the mechanical

torque applied by the wind turbine and the electromagnetic torque applied by the grid.

The PCC is the point where the wind turbine is connected to the grid, for example, in a

28

transformer. This may lead to uncontrollable acceleration of the wind turbine rotor

because not enough countering torque is applied. [19, p. 1870]

The situation aforementioned does not necessarily yet lead to disconnection from the

grid. After the fault in the grid has been cleared, and the voltage in the system is

recovering, induction machines create big reactive inrush currents to recover the

magnetic air gap flux between the stator and the rotor, which creates the countering

electromagnetic torque. These big inrush currents can cause further voltage drop in the

grid which at this point makes it necessary that enough reactive power is available in the

grid for the recovery. If the grid voltage is recovered and the wind turbine rotor speed is

not yet too high, the electromagnetic torque may be re-established thus slowing down

the rotor speed and the normal operation of the wind turbine may be restored. [19, p.

1870]

If the grid voltage could not be recovered close to the nominal value, the

electromagnetic torque will not be able to balance the mechanical torque which results

in uncontrollable acceleration of the turbine rotor. In this situation the wind turbine is

tripped by the overspeed protection devices in the wind turbine and the wind turbine is

disconnected from the grid. [19, p. 1870]

The time that the wind turbine is required to stay connected to the grid during faults

depends on the magnitude of the voltage decrease. Generally the wind turbine should

stay connected to the grid at least for 150 milliseconds even if the voltage goes down to

zero [14, p. 158]. The demands vary from country to country, but as an example, the

requirements on staying connected to the grid according to German GC are illustrated in

Figure 21.

Figure 21. LVRT requirements in the German GC. [14, p. 159]

29

A small amount of voltage deviation is allowed in normal operation, as can be seen

from the figure. For a short duration, the wind turbine should stay connected to the grid

even on relatively low voltages compared to the nominal. Within the black area, the

wind turbine should not disconnect from the grid, and within the light grey area, the

wind turbine may disconnect in accordance with a previously established agreement

with the TSO. The wind turbine may disconnect from the grid regardless of the TSO if

the fault lasts more than 1.5 seconds. [14, p. 160]

The capability for LVRT is achieved through several methods according to the wind

turbine configuration type. The capability is better and the operation more

straightforward in full-rated wind turbines because the generator is completely

decoupled from the grid. Examples of ride-though strategies are discussed in the

following.

5.2.1 The DFIG with a Crowbar

During a voltage sag, the high transient currents generated by the SCIG may cause

damage to the power electronics in the DFIG. Therefore protective measures for the

power conversion system are needed. To avoid the acceleration and resulting overspeed

of the wind turbine rotor, additional load connected to the rotor windings is needed to

balance the electromagnetic and the mechanical torque. The most common option for a

protective measure is a crowbar where the rotor is disconnected from the conversion

system and short-circuited to the crowbar. Figure 22 is an example DFIG topology with

a crowbar protection measure. [19, p. 1871]

Figure 22. DFIG with a passive crowbar. [19, p. 1873]

The thyristor in the crowbar circuit is turned on when the DC-voltage in the DC-link in

the conversion system reaches its given limit value, and simultaneously the rotor is fully

30

disconnected from the converter. From this point, the following action of the crowbar

falls in the either active or passive.

In the passive crowbar, the rotor stays connected to the crowbar until the main circuit

breaker disconnects the stator from the grid. After the fault in the grid has been cleared,

the rotor-side converter is reconnected, and after synchronization, the stator is also

reconnected to the grid. However, this operation only protects the converter devices in

the wind turbine system, and the disconnection from the grid still occurs, and the wind

turbine has failed to ride-through the fault. [19, p. 1871]

In the active crowbar, the thyristor is replaced by an actively controlled component such

as an IGBT, and the crowbar may be switched on and off actively during abnormal

transient currents or overvoltages. This provides control over the scale of how much

power is transmitted to the crowbar, and the wind turbine can stay connected to the grid.

This requires detailed design on the control of when the crowbar is connected and when

it is not. [2]

5.2.2 LVRT of a Full-Rated PMSG

During a voltage sag at the PCC caused by a fault in the grid, the maximum active

power that the wind turbine can inject to the grid is quickly reduced. However, the

power that the generator is feeding to the converter is not reduced as quickly. This leads

to the excess power being stored in the DC-link capacitor, resulting in voltage increase.

This can cause damage to both the generator-side converter and the grid-side converter.

Therefore the main control objective during a voltage sag is to keep the DC-link voltage

constant. [3, p. 623]

Keeping the DC-link voltage constant results in a momentary increase of speed in the

wind turbine. Because of the large mass of the turbine and the PMSG technology used,

this does not pose a problem, because the speed doesn’t increase dramatically. The

power flow of the wind turbine can be described by the following equation [3, pp. 623-

624]:

𝑃𝑤 − 𝑃𝑜 = 𝑃𝑚 = 𝜔𝑚𝐽𝑑𝜔𝑚

𝑑𝑡, (4)

where Pw is the power created by the turbine, Po is the output power fed to the grid, Pm

is the power stored as mechanical energy, 𝜔𝑚 is the rotational speed of the wind turbine

and J is the inertia of the generator. In normal operation, Pw and Po should be equal so

that no mechanical energy is stored in the system. During a voltage sag, the power fed

to the grid can be reduced to maintain the DC-voltage constant, therefore resulting in

the growth of the speed of the wind turbine. Because of the large inertia of the turbine,

the rotor speed will not increase uncontrollably during a short fault. Another way to

help maintain the DC-link voltage constant is to employ a chopper or a crowbar (similar

31

to previously discussed) to the DC-link, where the excess power is fed to resistor and

the power balance therefore maintained. [3, p. 624, 17, pp. 2091-2095]

5.3 Islanding Detection

Islanding is a condition where a single generative unit or small part of the network is

disconnected from the grid but still remains energized by the power generation. If

unintentional, this can cause hazards to utility line workers, because the voltage remains

in the islanded part, and furthermore, it may damage the generating equipment and

interfere with the restoration of the normal operation of the grid. [6, p. 1]

Therefore it is important for a wind turbine system and all other renewable energy

generating units to recognize the islanded mode of operation. As discussed earlier,

during temporary grid faults, wind turbines should generally not disconnect from the

grid. It may still be advantageous for the wind turbine to recognize the islanding

condition so it can switch between modes of standalone and grid-connected operation.

In standalone operation, the wind turbine has more responsibility in the voltage and the

frequency control of the islanded part of the grid. However, not a lot of research has

been done on islanding detection designed specifically for wind turbines. [6, p. 2, 14, p.

93]

The reliability of an islanding detection method can be defined as the ability to

recognize the islanding condition regardless of the cause and the grid conditions. An

islanding detection method should also be selective: it should be able to discriminate

between islanding and a simple perturbation in the grid. [14, p. 93]

Islanding detection methods can be categorized to passive and active methods. Passive

methods are slower to detect the islanding condition, and less accurate. Passive methods

are based on the detection of a change in the parameters of the power system such as the

amplitude of the voltage, frequency, phase or harmonics in the voltage. Active islanding

detection methods intentionally generate disturbances to force a change of a power

system parameter. The change in the grid is then detectable by the passive islanding

detection methods. The drawback of this feature is that the generated disturbance can

affect the power quality and generate further instability in the grid. [14, pp. 97-98]

Nondetection Zone (NDZ) represents the reliability of islanding detection methods.

NDZ is defined by the active and reactive power mismatch between the generating unit

and the grid at the PCC, where the islanding is not detectable and therefore the system is

prone to a fault [14, p. 94]. To elaborate, assume that in normal operation both the grid

and the generating unit are feeding power to a local load, as in Figure 23. If all the

power fed to the load is generated by the generating unit, disconnection from the grid at

the PCC is theoretically not visible from the generating unit point of view.

32

Figure 23. The generating unit and the grid are both feeding power to a local

load. The grid is disconnected by the utility breaker. [6, p. 2]

If the load is partially fed by the grid at the time of disconnection, or islanding, the drop

in the power balance can be seen as frequency deviation or voltage deviation at the

PCC. The voltage is proportional to the active power and the frequency is proportional

to the reactive power [14, pp. 94-95]. The NDZ, illustrated in Figure 24, is the space of

the power mismatch at the PCC that is too small for the generating unit to detect.

Figure 24. The nondetection zone. [14, p. 95]

Ideally the nondetection zone is negligibly small. The NDZ is typically large when

passive islanding detection methods are used. The NDZ can be significantly reduced by

using active islanding detection methods. [14, pp. 94-95]

33

6. CONCLUSIONS AND FUTURE TRENDS

The grid connection of a wind turbine is a very timely issue for the wind turbine

industry. Wind power is constantly representing a bigger proportion of the total

generated electrical power, and consequently, the requirements for wind turbine

operation are becoming stricter. These requirements are met by modern variable speed

wind turbines employing power electronic applications giving wind turbines enhanced

qualities such as better power quality and better dynamics in comparison to older fixed-

speed applications.

A number of different combinations of generators and converter topologies that have

been discussed in this thesis are used in the wind turbines today. Different combinations

employ different control schemes. Substantial amount of documentation exists of the

traditional and currently popular wind turbine configurations, like the DFIG. More

advanced technologies based on full-rated conversion are not as proven yet, and as the

initial investment costs are also higher, they have not penetrated the market with their

full potential. There is a great demand for renewable energy sources, especially for wind

power. However, the proportion in the total power production of a power system may

not increase arbitrarily without the technology capable of meeting the demands of a

power system. Therefore it can be concluded that the new installations likely employ

modern technology.

The most important concern in the grid connection of a wind turbine is its ability to

withstand faults and to contribute to the stability of the grid, rather than impairing it.

Grid-connected wind turbines are required, not only to operate reliably, but also to aid

in the fault ride-through during faults in the grid. The full-rated variable speed wind

turbines reviewed in this thesis are able to meet these demands and act as a reliable and

stable part of the grid. Some issues regarding the grid-connection of a wind turbine,

such as the islanding detection, still require more research. More research on converter

types not as commonly used in the wind turbines, such as the direct converters, is also

needed.

In the future, the wind turbines may be improved even further not only by improving

the power conversion, but also the generator. Superconducting generators are an

interesting choice for wind turbines due to their reduced size and weight. However, this

experimental technology is not yet sufficiently proven, and the economic costs of this

type of generator surpass the feasible limit. This technology could effectively reduce the

size of the nacelle and the tower, therefore reducing the overall cost of the wind turbine.

[9, pp. 78-79]

34

The material traditionally used in composing semiconductor switching devices is

silicon. The performance and the reliability of the power switching devices is directly

proportional to the performance and the reliability of the whole power conversion

system in wind turbines. Therefore constant research and development is needed in this

area.

Using SiC (Silicon-Carbide) materials as a substitute for silicon can substantially

improve the switching devices. Theoretically the performance can be improved 10 times

better in areas such as thermal conductivity, reverse voltage blocking capability and

reverse recovery characteristics related to switching the polarity in respect to the

switching device. SiC-based switching devices can also reach much higher switching

frequencies [18, p. 21]. These properties consequently result in improvements in

reliability, physical size and efficiency. Simulations have been made to support this

conclusion [9, p. 91-106]. The use of SiC materials in wind turbine power conversion

may improve the voltage rating, the power handling capacity and decrease the overall

system size. In addition to better performance, this may result in the decrease of overall

system costs. [9, p. 87]

With the enhancements in the generating technology, a single wind turbine could

produce substantially more power which will no doubt require the converter technology

to develop as well. In addition, if the power level grows higher, a single wind turbine

plays even more important role in the grid. Thus the grid connection of wind turbines

will no doubt stay an important research topic in the future.

35

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