2. i
ABSTRACT
Markus Ovaskainen: Grid Connection for Variable Speed Wind Turbines
Bachelor of Science Thesis, 36 pages
April 2015
Bachelorβs 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.
3. ii
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
4. iii
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
5. iv
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
Ο (Air) Density
Οm Rotational Speed of the Turbine
ΞΈ Angle Between the d- and the q- axes
Ξ»r Rotor Flux
Ξ»r Rotor Flux Vector
6. 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
Ξ±Ξ²-frame Two-phase stationary reference frame
dq- frame Rotational reference frame
7. 1
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 gridβs standpoint. In Chapter 6, the themes discussed are summarized and
concluded by elaborating on future trends and possibilities.
8. 2
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
9. 3
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.
10. 4
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
11. 5
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 Bernoulliβs 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]
12. 6
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]:
π =
1
2
ππ΄π£3
πΆ π, (1)
where π 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 turbineβs Tip
13. 7
Speed Ratio (TSR) which denotes the ratio between the tangential speed of the tip of the
blade and the actual velocity of the wind. Betzβs 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]
14. 8
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:
ππ = π π π π, (2)
where Pm is the mechanical power, Tm is the torque generated by the turbine shaft and
π π 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]
15. 9
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.
16. 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]
17. 11
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.
18. 12
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]
19. 13
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
20. 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]
21. 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.
22. 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 converterβs 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.
23. 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]
24. 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
25. 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.
26. 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 (Ξ±Ξ²-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]
27. 21
Utilizing trigonometrics, the abc/dq reference frame transformation can be expressed in
matrix form as follows [16, p. 52]:
[
π₯ π
π₯ π
] =
2
3
[
cos π cos (π β
2π
3
) cos (π β
4π
3
)
βπ πππ β sin (π β
2π
3
) β sin (π β
4π
3
)
] [
π₯ π
π₯ π
π₯ π
],
(3)
where xa, xb and xc are the abc-frame variables, xd and xq are the dq-frame variables, and
π 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 Ξ»r 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 Ξ»r and a torque-producing component
producing the electromagnetic torque Te. In rotor flux orientation, the rotor flux vector
Ξ»r 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 π 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
28. 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]
29. 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]
30. 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 10β90 % 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
31. 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 49.9β50.1 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 49.5β50.1 Hz. However, the
limits set by the British grid operator are between 47.5β52.0 Hz which is a relatively
broad operation range.
32. 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 1β30 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 1β15 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]
33. 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
34. 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]
35. 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
36. 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 doesnβt increase dramatically. The
power flow of the wind turbine can be described by the following equation [3, pp. 623-
624]:
ππ€ β ππ = ππ = π π π½
ππ π
ππ‘
, (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, π π 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
37. 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.
38. 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]
39. 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]
40. 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.
41. 35
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