CONTROL OF POWER CONVERTER FOR GRID INTEGRATION
OF RENEWABLE ENERGY CONVERSION
AND STATCOM SYSTEMS
by
LING XU
A THESIS
Submitted in partial fulfillment of the requirements
for the degree of Master of Science in the
Department of Electrical and Computer
Engineering in the Graduate School of
The University of Alabama
TUSCALOOSA, ALABAMA
2009
Copyright Ling Xu 2009
ALL RIGHTS RESERVED
ABSTRACT
Investment in renewable energy is rapidly increasing worldwide. This is in response to a
number of global challenges and concerns, including climate change, increasing energy demand,
and energy security. The investment is widely spread over the leading renewable energy
technology sectors: wind, solar, biofuels, biomass, and fuel cells. Among those, wind, solar
photovoltaic, and fuel cells require power electronic converters for grid integration.
This thesis investigates advanced control technology for grid integration control of
renewable energy sources and STATCOM systems. First, the conventional control mechanism of
power converters applied in renewable energy conversion and STATCOM systems is studied.
Through both theoretical and simulation studies, a deficiency of the conventional control
mechanism is identified. It is found that malfunctions of traditional power converter control
techniques may occur when the controller output voltage exceeds the converter linear modulation
limit.
Then, the thesis proposes a novel control mechanism consisting of a current control loop
and a voltage control loop. The proposed control mechanism integrates PID, adaptive, and fuzzy
control techniques. An optimal control strategy is developed to ensure effective active power
delivery and to improve system stability. The behaviors of conventional and proposed control
techniques are compared and evaluated on both simulation and laboratory hardware testing
systems, which demonstrates that the proposed control mechanism is effective for grid
integration control over a wide range of system operating conditions while the conventional
ii
control mechanism may behave improperly, especially when the converter operates beyond its
linear modulation limit and under variable system conditions.
iii
LIST OF ABBREVIATIONS AND SYMBOLS
V
Volts: Unit of voltage.
A
Amperes: Unit of current.
kW
kilo Watts: Unit of active power.
kVar
kilo Vars: Unit of reactive power.
mH
milli Henry: Unit of inductance.
uF
micro Farad: Unit of capacitance.
Hz
Hertz: Unit of frequency.
s
Second: Unit of time.
°
Degree: Unit of angle.
DC
Direct current
AC
Alternative current
FACTS
Flexible AC transmission system
STATCOM
Static Synchronous Compensator
MOSFETs
MOS Field Effect Transistors
GTOs
Gate Turn Off Thyristors
IGBTs
Insulated Gate Bipolar Transistors
PID
Proportional-integral-derivative
DSP
Digital Signal Processing
iv
ADC
Analog to Digital Converter
>=
Great than or equal to
<=
Less than or equal to
=
Equal to
v
ACKNOWLEDGMENTS
I would like to express my grateful appreciation to my thesis committee chairperson and
my advisor, Dr. Shuhui Li, for his patient guidance and great help in my research and study life
throughout my study at The University of Alabama.
I would like to thank Dr. Timothy A. Haskew for his great help in the lab and his careful
guidance on the high power experimental equipments. I would also like to thank Dr. Keith A.
Williams for his patience in serving on my thesis committee member.
I would also like to thank the Department of Electrical and Computer Engineering for the
funding support in my research and providing equipments in the lab.
Finally, I would like to thank my parents, my fiancee and my friends for their countless
love, encouragement and help.
vi
CONTENTS
ABSTRACT...................................................................................................... ii
LIST OF ABBREVIATIONS AND SYMBOLS ............................................ iv
ACKNOWLEDGMENTS ............................................................................... vi
LIST OF TABLES............................................................................................ x
LIST OF FIGURES ......................................................................................... xi
1. INTRODUCTION ........................................................................................ 1
1.1 Grid integration of renewable energy conversion system........................... 1
1.2 Grid integration of energy storage system .................................................. 2
1.3 Grid integration of STATCOM system ...................................................... 2
1.4 Challenges in the grid integration of renewable energy and STATCOM
systems........................................................................................................ 3
1.5 Purpose of this thesis .................................................................................. 4
2. GENERAL STRUCTURE FOR GRID INTEGRATION OF RENEWABLE
ENERGY CONVERSION AND STATCOM SYSTEMS........................... 5
2.1 Structure of wind energy conversion system .............................................. 5
2.2 Structure of solar energy conversion system .............................................. 7
2.3 Structure of energy storage system ............................................................. 7
2.4 Structure of STATCOM system ................................................................. 8
2.5 Conclusions for grid integration of renewable energy and STATCOM
systems........................................................................................................ 9
vii
3. CONTROL OF POWER CONVERTER FOR GRID INTEGRATION.... 10
3.1 Introduction............................................................................................... 10
3.2 Mathematical model of the grid side converter system ............................ 16
3.3 Conventional control scheme of the grid side converter .......................... 21
3.4 Proposed control scheme of the grid side converter ................................. 25
3.5 Machine side converter controller ............................................................ 28
4. SIMULATION STUDY OF RENEWABLE ENERGY GRID
INTEGRATION CONTROL...................................................................... 31
4.1 Introduction............................................................................................... 31
4.2 Simulation models for grid integration of renewable energy conversion
system ....................................................................................................... 31
4.3 Simulation results and analysis................................................................. 43
5. SIMULATION STUDY FOR CONTROL OF PWM-BASED
STATCOM.................................................................................................. 53
5.1 Introduction............................................................................................... 53
5.2 STATCOM configuration and its control system..................................... 53
5.3 STATCOM simulation models ................................................................. 56
5.4 Simulation results and analysis................................................................. 58
6. LABORATORY HARDWARE EXPERIMENTAL STUDY AND
COMPARISON .......................................................................................... 73
6.1 Introduction............................................................................................... 73
6.2 Experimental setup.................................................................................... 73
6.3 Controller implementation ........................................................................ 74
6.4 Experiment results .................................................................................... 77
6.5 Conclusions............................................................................................... 96
viii
7. SUMMARY AND FUTURE WORK ........................................................ 97
8. REFERENCES ......................................................................................... 100
ix
LIST OF TABLES
4.1 System parameters of renewable energy conversion system model......... 43
5.1 System parameters of STATCOM model................................................. 59
6.1 Experiment parameters ............................................................................. 77
x
LIST OF FIGURES
2.1 Variable speed wind turbine with a PMSG................................................. 5
2.2 Variable speed wind turbine with a DFIG................................................... 6
2.3 Solar energy conversion system.................................................................. 7
2.4 Energy storage system ................................................................................ 8
2.5 A STATCOM system ................................................................................. 9
3.1 A typical AC/DC/AC converter.................................................................11
3.2 A typical DC/AC inverter ......................................................................... 12
3.3 Clarke transformation ............................................................................... 14
3.4 Park transformation................................................................................... 14
3.5 Grid side converter equivalent circuit in dq axes reference frame ........... 17
3.6 DC-link model .......................................................................................... 18
3.7 Grid side converter integrated with grid ................................................... 19
3.8 Conventional control scheme of the grid side converter .......................... 22
3.9 Current control loop.................................................................................. 23
3.10 DC-link voltage control loop .................................................................. 25
3.11 Proposed control scheme of the grid side converter ............................... 26
3.12 Proposed current control loop................................................................. 27
3.13 Structure of two types of AC/DC converter ........................................... 29
xi
3.14 Conventional control scheme of the machine side converter ................. 30
3.15 Proposed control scheme of the machine side converter ........................ 30
4.1 Simulation structure of AC/DC/AC converter system for grid integration
of renewable energy conversion systems.................................................. 32
4.2 abc to dq axis frame transformation ......................................................... 34
4.3 Core control system module using proposed control theory .................... 35
4.4 Vd1 and Vq1 signals generation blocks in proposed control system........... 36
4.5 Core control system module using conventional control theory .............. 36
4.6 Vd1 and Vq1 signals generation blocks in conventional control system..... 37
4.7 PWM pulse signals generation module in proposed control system ........ 38
4.8 Details of linear modulation limit in proposed control system................. 39
4.9 Reactive power optimal control block and algorithm............................... 40
4.10 Core control system module of machine side converter using proposed
control theory .......................................................................................... 41
4.11 Filter and power calculation block.......................................................... 42
4.12 Performance of renewable energy conversion system using conventional
control mechanism under case 1 ............................................................. 46
4.13 Performance of renewable energy conversion system using proposed
control mechanism under case 1 ............................................................. 47
4.14 Performance of renewable energy conversion system using conventional
control mechanism under case 2 ............................................................. 49
4.15 Performance of renewable energy conversion system using proposed
control mechanism under case 2 ............................................................. 51
5.1 Configuration of STATCOM.................................................................... 54
5.2 Equivalent circuit of grid integration of STATCOM ............................... 54
5.3 Conventional control system of STATCOM ............................................ 55
xii
5.4 Proposed control system of STATCOM................................................... 56
5.5 Simulation model of STATCOM for system voltage support control
application................................................................................................. 57
5.6 Core control system of STATCOM using conventional control
mechanism ................................................................................................ 58
5.7 Core control system of STATCOM using proposed control mechanism . 58
5.8 Performance of STATCOM using conventional control mechanism in
reactive power compensation mode under case 1..................................... 60
5.9 Performance of STATCOM using proposed control mechanism in reactive
power compensation mode under case 1 .................................................. 61
5.10 Performance of STATCOM using conventional control mechanism in
reactive power compensation mode under case 2................................... 63
5.11 Performance of STATCOM using proposed control mechanism in
reactive power compensation mode under case 2................................... 64
5.12 Performance of STATCOM using conventional control mechanism in bus
voltage support mode under case 1......................................................... 67
5.13 Performance of STATCOM using proposed control mechanism in bus
voltage support mode under case 1......................................................... 68
5.14 Performance of STATCOM using conventional control mechanism in bus
voltage support mode under case 2......................................................... 70
5.15 Performance of STATCOM using proposed control mechanism in bus
voltage support mode under case 2......................................................... 71
6.1 Controller of the AC/DC/AC converter system........................................ 75
6.2 dSPACE interface of real time application............................................... 75
6.3 Experiment platform and devices ............................................................. 78
6.4 AC/DC/AC experiment results using conventional control mechanism
under case 1............................................................................................... 79
6.5 AC/DC/AC experiment results using proposed control mechanism under
case 1......................................................................................................... 81
xiii
6.6 AC/DC/AC experiment results using conventional control mechanism
under case 2............................................................................................... 83
6.7 Simulation results of the AC/DC/AC converter system using conventional
control mechanism under case 2 ............................................................... 85
6.8 AC/DC/AC experiment results using proposed control mechanism under
case 2......................................................................................................... 86
6.9 STATCOM experiment results using conventional control mechanism
under case 1............................................................................................... 88
6.10 STATCOM experiment results using proposed control mechanism under
case 1....................................................................................................... 90
6.11 STATCOM experiment results using conventional control mechanism
under case 2............................................................................................. 91
6.12 Simulation results of the STATCOM system using conventional control
mechanism under case 2 ......................................................................... 93
6.13 STATCOM experiment results using proposed control mechanism under
case 2....................................................................................................... 95
xiv
CHAPTER 1
INTRODUCTION
1.1 Grid integration of renewable energy conversion system
Renewable energy is a kind of energy generated from natural resources. Sunlight, wind,
water, geothermal heat, and biomass can generate energy for human use. Renewable energy
supplied 18 percent of the energy consumption of the world in 2006 [1], and the investment in
renewable energy is increasing rapidly worldwide [2].
In a renewable energy conversion system, in wind, solar PV, and fuel cells, power
converters are necessary for grid integration [3]. For the wind energy conversion system, two
types of generators are normally used to produce electricity. One is the PMSG; the other is the
DFIG [4]. For both, their output has an AC voltage often at a frequency other than 60 Hz, the
electric utility grid frequency in the United States. As a result, power converters are needed at the
interface to the AC grid, which permits energy to flow from the wind turbine into the grid.
For solar energy and fuel cell energy conversion systems, there are some differences. The
output voltage of the solar panel and the fuel cell is DC. Again, since the grid is an AC power
system, a DC/AC power converter is necessary to integrate solar or fuel cell systems to the grid.
1
1.2 Grid integration of energy storage system
Integration of renewable energy in the power grid brings many challenges [5, 6]. The
power generation fluctuation, such as in a wind energy conversion system, may cause some
problems for the grid, especially in a weak grid. An energy storage system could be employed to
solve the potential problem. The energy storage device is usually a battery, which can provide
active power when the wind farm output is lower or store the excess active power generated by
the wind farm when its output is higher than usual. The output voltage of the energy storage
device is DC, thus a DC/AC power converter is necessary to integrate the energy storage device
to the grid.
1.3 Grid integration of STATCOM system
FACTS (Flexible AC transmission system) devices, widely used in today’s power system
[7], are critical for reactive power compensation and voltage support control in a renewable energy
conversion system [8]. Traditionally, reactive power compensation within the FACTS devices has
been handled with the thyristor-based static VAR compensator (SVC) [9].
Nevertheless, due to the developments of the power electronics technology, the
replacement of the SVC by a new breed of static compensators, STATCOM, based on the use of
voltage source PWM converter is looming [10-12]. The STATCOM system consists of a shunt
capacitor, a DC/AC power converter, and a grid filter. The grid integration of STATCOM is
based on the DC/AC power converter, which has a similar converter structure to that used in grid
integrated renewable energy conversion systems.
2
1.4 Challenges in the grid integration of renewable energy and STATCOM systems
Inherent characteristics of renewable energy resources cause technical issues not
encountered with conventional thermal, hydro, or nuclear power. These issues make operation of
the renewable energy resources and their integration with the grid system a technical challenge.
The rapid development of the renewable energy power industry, together with the rising
challenges, has drawn many of the world’s leading professional associations and organizations
into this fast growing field.
Among all the rising challenges, one important issue is how to integrate renewable energy
sources with the grid through power electronic converters as well as associated control system
designs. Although traditional approaches have been developed, mainly in Europe, for power
converter control of renewable energy systems during the last decade, there is a critical need to
develop new and improved power converter control technologies for many reasons. 1) The
existing power converter control technologies in grid integrated renewable energy generation
systems do not perform well in some cases. 2) Unbalance and high harmonic distortion have been
found in renewable energy conversion systems, which not only affect the grid system but also
affect the renewable energy sources. 3) The power quality is not an issue to be considered in the
existing controller design for the power converter in renewable energy conversions. However, the
power quality is a critical factor in power system, which has to be improved to ensure the quality of
service and security of the grid. 4) The existing power converter control mechanism has an
inherent deficiency, which can cause malfunctions of the system, such as abnormal DC capacitor
voltage, active and reactive power, or output currents. These malfunctions may make the gird
integration of the renewable energy sources unstable and may even cause power system trips
[13-15].
3
1.5 Purpose of this thesis
This thesis concentrates mainly on the control system study and development for DC/AC
converters used in the grid integration of renewable energy conversion and STATCOM systems.
The purpose of this thesis is to investigate and implement a novel control strategy for power
converters for enhanced and reliable grid integration of renewable energy conversion and
STATCOM systems. The conventional control mechanism for power converters is studied
theoretically and through computer simulation. Then, the thesis proposes a novel control
mechanism for power converters and analyzes the implementation details. Through both
computer simulation and real-world experiments, a deficiency of the conventional control
mechanism is identified. It is found that the malfunctions of the conventional control mechanism
may occur when the controller output voltage exceeds the linear modulation limit of the power
converters. The simulations and experiments also demonstrate that the proposed control
mechanism performs well even in extreme abnormal operating conditions, which verifies the
reliability and stability of the proposed control mechanism designed in this thesis.
4
CHAPTER 2
GENERAL STRUCTURE FOR GRID INTEGRATION OF RENEWABLE ENERGY
CONVERSION AND STATCOM SYSTEMS
2.1 Structure of wind energy conversion system
In a typical wind energy conversion system, a wind turbine captures the power from wind,
which rotates a generator in the huge wind turbine box. Wind turbines can operate with either fix
speed or variable speed. For a fix speed wind turbine, the generator is connected to the grid
directly. Since the speed is fixed, this kind of wind turbines cannot respond the turbulence of
wind speed effectively, which could result in the power swing transmitted to the grid and affects
the power quality [16]. For a variable speed wind turbine, the generator is connected to the grid
through power electronics equipments. The rotor speed has the possibility to be controlled by
those equipments. As a result, the power fluctuations caused by the wind speed variations can be
reduced, which improves the power quality comparing with the fix speed wind turbine system
[17].
Fig. 2.1. Variable speed wind turbine with a PMSG
5
Fig. 2.2. Variable speed wind turbine with a DFIG
Figure 2.1 shows the configuration of a PMSG wind turbine connected with the grid. The
power converters, between the generator and the grid, control the behaviors of the power flow of
the wind turbine to the grid. Figure 2.2 shows the configuration of a DFIG wind turbine
connected with grid. The main power flows through the upper lines between the generator and
the transformer. The path from the DFIG rotor to the transformer, through power converters, only
has to transfer 20%~30% of the total power, which reduces the losses in the power converters
comparing with the system shown in figure 2.1.
The power converters in both figure 2.1 and figure 2.2 perform as an AC/DC/AC
converter, which means that the AC power has to be converted to DC and then to be inverted
back to AC in order to be connected with the AC grid. The AC/DC/AC converter has to prevent
the potential damage transmitted to the grid, which might come from the power variation, wind
speed turbulence or current oscillation in the wind turbine side. In this thesis, the AC/DC
converter, which is the left hand part of the power converter in figure 2.1 and figure 2.2, is called
machine side converter. The DC/AC converter, which is the right hand part of the power
6
converter in figure 2.1 and figure 2.2, is called grid side converter. The AC/DC/AC interface
between the wind turbines and grid requires robust control scheme in order to provide the precise
and effective control signals to both the machine side converter and the grid side converter.
2.2 Structure of solar energy conversion system
Solar energy is one of the most important renewable energy resources. Sunlight can be
converted to electricity for the home and office uses. It is also clean and inexhaustible. In a
typical solar energy conversion system, photovoltaic (PV) devices are used to capture the energy
from the sunlight. A PV cell can convert light into direct current through the photoelectric effect.
However, direct current power cannot be directly connected with the AC grid. As a result, a
DC/AC converter is necessary to integrate the direct current power to the grid system [18-20].
Fig. 2.3. Solar energy conversion system
Figure 2.3 shows a typical structure of a solar energy conversion system. The power
converters connect a solar array with the grid and transmit the power captured from sunlight. The
left hand side of the power converter is a DC/DC converter, the right hand side of the power
converter is again a DC/AC converter.
2.3 Structure of energy storage system
The energy storage system can be used in a renewable energy conversion system for the
backup power supply. Due to the variation of the wind speed, the active power output of a wind
7
farm may vary from time to time, which is not good for the grid, especially in a weak grid. The
energy storage system can be one of the solutions for the challenges since it can provide power
when the output power of the wind power generator is lower than usual or it can store the excess
power when the output power of the wind power generator is higher than usual. Figure 2.4
depicts the configuration of an energy storage system. The energy source is a battery in this
application, the interface between the battery and the grid is a DC/AC power converter [21].
=
Grid
≈
Transformer
Power Converter
Fig. 2.4. Energy storage system
The controller of the power converter in figure 2.4 should control the converter to
generate active power to the grid when the output power of the wind farm is low, or store the
excess active power from the grid when the output power of the wind farm is high than the
desired value.
2.4 Structure of STATCOM system
The STATCOM system consists of a shunt connected capacitor, a DC/AC power
converter, and a grid filter [22]. Figure 2.5 shows the configuration of a typical STATCOM
system. The power converter in figure 2.5 is a DC/AC converter, which is similar to the power
converters shown in figure 2.1 to 2.4. The DC/AC converter is the interface connecting the shunt
capacitor with the grid. The controller of the DC/AC converter is the core part of the STATCOM
8
system. It should control the converter so as to generate reactive power to the grid if the grid
voltage is lower than the reference; or to absorb reactive power from the grid if the grid voltage
is higher than the reference.
Fig. 2.5. A STATCOM system
2.5 Conclusions for grid integration of renewable energy and STATCOM systems
Through the brief introduction of the general configurations of renewable energy and
STATCOM systems, it is clear that the grid integration of renewable energy and STATCOM
systems are similar in structure and function. All of the grid integrations require a DC/AC power
converter as the power exchange interface. Actually, the controller designs of the interface power
converters are similar to each other in the past. In the following chapters, the thesis first studies
the conventional control mechanism of the grid-side converter and analyzes a deficiency of the
conventional control mechanism both theoretically and through computer simulation. Then, the
thesis proposes a new control method. The behaviors of the conventional and proposed control
techniques are compared and evaluated in both simulation and laboratory real-time environments,
which demonstrates that the proposed control mechanism is effective for grid integration control
of renewable energies in a wide system operating conditions while the conventional control
mechanism may behave improperly especially when the converter operates beyond its linear
modulation limit and under variable system conditions.
9
CHAPTER 3
CONTROL OF POWER CONVERTER FOR GRID INTEGRATION
3.1 Introduction
3.1.1 AC/DC/AC converter
The AC/DC/AC converter discussed in this thesis is widely used in renewable energy
systems. For example, in a variable-speed wind energy conversion system, the general function
of the AC/DC/AC converter is to transmit the power generated from wind turbines to the grid.
The converter should provide good abilities to transmit power effectively, respond quickly and
accurately, and operate stably in potential extreme conditions.
Nowadays, some kinds of power electronics semiconductors are popular [23], including
Power MOS Field Effect Transistors (Power MOSFETs), Gate Turn Off Thyristors (GTOs), and
Insulated Gate Bipolar Transistors (IGBTs). The AC/DC/AC converter usually utilizes IGBT
devices in the power industry. The IGBT combines the advantages of the MOSFETs and the
advantages of the bipolar transistors by using an isolated gate FET as the control unit, and
utilizing a bipolar power transistor as the switch to transmit high currents. The IGBT is used in
medium to high power applications. The control unit in an IGBT is much simpler than a GTO,
and the switch frequency can be up to 40 kHz. High power IGBT modules may consist of many
devices in parallel and can have very high current handling capabilities.
10
+
ia2
ia1
Vdc
ib1
C
ib2
ic2
ic1
-
Fig. 3.1. A typical AC/DC/AC converter
Figure 3.1 shows a typical AC/DC/AC converter, which consists of 12 IGBTs. The left
hand side is an AC/DC converter (also called machine side converter), the right hand side is a
DC/AC inverter (also called grid side converter), and the middle part between the two converters
is a DC-link capacitor. The AC/DC converter converts AC power input into DC power output,
and the DC/AC converter inverts DC power input back into AC power output. This converter is
very important for transmitting power from wind turbines to the grid in practice. As a result, the
control scheme of the AC/DC/AC converter should be designed carefully and should control the
behaviors of the converters effectively.
3.1.2 Grid side converter
As shown in figure 3.1, the AC/DC/AC converter consists of an AC/DC converter and a
DC/AC inverter. Actually, these two types of converters are very similar to each other, the
fundamental control theories of these two types of converter are almost the same.
11
Fig. 3.2. A typical DC/AC inverter
Figure 3.2 shows a typical DC/AC inverter, there are 6 IGBTs in this inverter, which
inverts a DC power input into a controlled 3 phase AC power output based on the control signals
applied on the gate circuits of IGBTs.
The control signals used for the gate circuits of IGBT are usually generated through a
PWM signal generator. The simplest way to get a PWM signal requires a repetitive
switching-frequency sawtooth or triangular waveform and a comparator. In order to produce a
sinusoidal output voltage waveform, a sinusoidal control signal is compared with a triangular
waveform. The amplitude Vtri of the triangular waveform is always kept as constant value such
as 1 V. When the value of the sinusoidal control signal is greater than the triangular waveform
value, the PWM generator output is in high state, otherwise it is in low state. The frequency of
the triangular waveform creates the inverter switching frequency and the fundamental output
voltage waveform frequency is the same as the frequency of the sinusoidal control signal. Two
terms are defined in PWM algorithm, one is called amplitude modulation ratio, and the other is
called frequency modulation ratio. The amplitude modulation ratio ma is defined as
ma =
Vcontrol
Vtri
12
(3.1)
where Vtri is the amplitude of the triangular waveform, and Vcontrol is the amplitude of the
sinusoidal control signal. The frequency modulation ratio m f is defined as
mf =
ftri
f control
(3.2)
where f tri is the frequency of the triangular waveform (also called carrier frequency), and
f control is the frequency of the sinusoidal control signal [23].
3.1.3 Space vectors
The key point of space vectors is the transformation between a three-phase stationary
coordinate system and a two-phase rotating coordinate system [24]. The transformation can be
achieved through two steps.
a) Clarke transformation (abc system to αβ system).
b) Park transformation (αβ system to dq system).
Assuming ia , ib , ic are the three phase instantaneous currents, then, the complex
current is defined as
K
i s = ia + α ib + α 2ic
where α = e
2
j π
3
and α 2 = e
2
−j π
3
, represent the spatial operators.
13
(3.3)
β
b
iβ
is
iα
α
a
c
Fig. 3.3. Clarke transformation
Figure 3.3 shows the Clarke transformation, where α axis and a axis are in the same
K
direction. The complex current i s is projected on two orthogonal axes, which are α and β
axes. These two axes are also static as the three-phase stationary coordinate system.
β
q
iq
is
id
θ = ωt
d
α
Fig. 3.4. Park transformation
In Park transformation, the d axis is aligned with grid voltage position. Park
K
transformation is a projection, which projects i s onto dq rotating orthogonal axes. Figure 3.4
shows Park transformation, the dq coordinates system is a rotating system, where θ = ωt is the
grid voltage position.
14
As a result, these two transformations can be combined and written as a matrix form,
2
2 ⎤
⎡
cos(ωt ) cos(ωt − π ) cos(ωt + π ) ⎥ ⎡ia ⎤
⎡id ⎤
2⎢
3
3
⎢ ⎥
⎢
⎥ ⎢ib ⎥
⎢i ⎥ =
2
2
3⎢
⎣ q⎦
− sin(ωt ) − sin(ωt − π ) − sin(ωt + π ) ⎥ ⎢⎣ ic ⎥⎦
⎢⎣
3
3 ⎥⎦
The coefficient
(3.4)
2
is convenient in power calculation, which will be discussed later.
3
The voltage transformation matrix has the same form as (3.4).
2
2 ⎤
⎡
cos(ωt ) cos(ωt − π ) cos(ωt + π ) ⎥ ⎡ va ⎤
⎢
⎡ vd ⎤
2
3
3
⎢ ⎥
⎢
⎥ ⎢ vb ⎥
⎢v ⎥ =
2
2
3⎢
⎣ q⎦
− sin(ωt ) − sin(ωt − π ) − sin(ωt + π ) ⎥ ⎢⎣ vc ⎥⎦
⎢⎣
3
3 ⎥⎦
(3.5)
The inverse transformation from d, q system to a, b, c system can be expressed as
following.
⎡
⎤
⎢ cos(ωt )
− sin(ωt ) ⎥
⎡ia ⎤
⎢
⎥
⎢i ⎥ = 2 ⎢ cos(ωt − 2 π ) − sin(ωt − 2 π ) ⎥ ⎡id ⎤
⎢ ⎥
⎢ b⎥
3⎢
3
3 ⎥ ⎣ iq ⎦
⎢⎣ ic ⎥⎦
⎢
2
2 ⎥
⎢cos(ωt + π ) − sin(ωt + π ) ⎥
3
3 ⎦
⎣
(3.6)
⎡
⎤
⎢ cos(ωt )
− sin(ωt ) ⎥
⎡ va ⎤
⎢
⎥
⎢ v ⎥ = 2 ⎢ cos(ωt − 2 π ) − sin(ωt − 2 π ) ⎥ ⎡ vd ⎤
⎢ ⎥
⎢ b⎥
3⎢
3
3 ⎥ ⎣ vq ⎦
⎢⎣ vc ⎥⎦
⎢
2
2 ⎥
⎢cos(ωt + π ) − sin(ωt + π ) ⎥
3
3 ⎦
⎣
(3.7)
15
3.2 Mathematical model of the grid side converter system
3.2.1 Grid-side converter system model
The converter employs PWM and vector control approaches. Suppose Vd 1 , Vq1 are the d
and q components of the converter output voltages in a dq rotating reference frame, and Vdc is
the DC-link voltage. If the amplitude of triangular waveform in PWM generator is 1 V, the d and
q component signals required to generate gate signals are
vd _ norm = Vd 1 ⋅ 2 / Vdc
and
vq _ norm = Vq1 ⋅ 2 / Vdc respectively. Therefore, the amplitude modulation ratio of PWM generator is
ma = vd2 _ norm + vq2 _ norm , and the converter output phase peak voltage can be extracted.
Vconv =
maVdc
2
(3.8)
Thus, the grid side converter can be treated as a gain of control voltage outputs. The
coefficient of the gain is shown in equation (3.8).
The equivalent circuit of the grid side converter with grid filter in dq axes reference
frame is shown in figure 3.5. The grid filter consists of a resistor and an inductor in the
equivalent circuit, in which the resistance is R f and the inductance is L f . Applying
Kirchhoff’s voltage law, the relationship of the grid voltage and the converter output voltage in
terms of current and grid filter parameters in abc and dq axes reference frame has been described
in (3.9) and (3.10) respectively.
16
Fig. 3.5. Grid side converter equivalent circuit in dq axes reference frame
The voltage balance across the grid filter is described in equation (3.9).
⎡ va ⎤
⎢v ⎥ = R
f
⎢ b⎥
⎢⎣ vc ⎥⎦
⎡ia ⎤
⎡ia ⎤ ⎡ va1 ⎤
⎢i ⎥ + L d ⎢i ⎥ + ⎢ v ⎥
f
b
b1
⎢ b⎥
dt ⎢ ⎥ ⎢ ⎥
⎢⎣ ic ⎥⎦
⎢⎣ ic ⎥⎦ ⎢⎣ vc1 ⎥⎦
(3.9)
Appling d, q reference frame, (3.9) becomes (3.10),
⎡vd ⎤
⎢v ⎥ = R f
⎣ q⎦
⎡id ⎤
⎡ −iq ⎤ ⎡ vd 1 ⎤
d ⎡id ⎤
⎢ i ⎥ + L f ⎢ i ⎥ + ωs L f ⎢ ⎥ + ⎢ v ⎥
dt ⎣ q ⎦
⎣ id ⎦ ⎣ q1 ⎦
⎣ q⎦
(3.10)
where ωs is the angular frequency of the grid voltage. With space vectors theory, (3.10) can be
written as (3.11),
K
K
K
K
K
di dq
+ jωs L f ⋅ i dq + v dq1
v dq = R f ⋅ i dq + L f
dt
(3.11)
K
K
K
where v dq , i dq and v dq1 are the instantaneous space vectors of grid voltage, line current and
converter output voltage respectively.
In steady-state condition, the derivative part can be removed, and (3.11) becomes (3.12),
JK
K
K
JK
V dq = R f ⋅ I dq + jωs L f ⋅ I dq + V dq1
17
(3.12)
JK
JK
where V dq1 and V dq are the steady-state grid side converter equivalent output voltage and the
K
gird equivalent voltage in dq reference frame, respectively. The current I dq flows from the grid
to the converter.
The grid side converter dq output voltage can also be expressed in terms of currents and
grid voltage.
Vd 1 = − I d R f + I qωs L f + Vd
(3.13)
Vq1 = − I q R f − I d ωs L f
(3.14)
3.2.2 DC-link model
The DC-link capacitor connects the machine side converter and the grid side converter
[25-27], the equivalent circuit is shown in figure 3.6.
Machine side
converter
im
idc
ig
+
Grid side
converter
vdc
C
Pm
Pg
Fig. 3.6. DC-link model
In figure 3.6, the DC-link voltage is vdc , the capacitance is C , the power flows from a
renewable source to the machine side converter is Pm and the power flows from grid to the grid
side converter is Pg .
The energy stored in the capacitor is given by equation (3.15).
18
1
Wdc = Cvdc2
2
(3.15)
When the energy losses are small enough and can be neglected, the energy in the DC-link
capacitor depends on both the power flow from grid side Pg and the power flow from machine
side Pm . The relationship can be expressed as equation (3.16).
dWdc
= Pg + Pm
dt
(3.16)
Extracting the derivative part, then (3.16) becomes (3.17).
Cvdc
dvdc
= Pg + Pm
dt
(3.17)
From equation (3.17), it is clear that the constant DC-link voltage requires − Pg = Pm , which
means all the power from machine side has to be delivered to the grid side.
3.2.3 Active and reactive power calculation
Fig. 3.7. Grid side converter integrated with grid
Consider a practical grid side converter system shown in figure 3.7, the three-phase output
voltage of grid side converter is va1 , vb1 and vc1 respectively. The left-hand side part is the
DC-link capacitor C, the right-hand side is the grid, which is represented by a three-phase AC
19
source va , vb and vc . The grid filter consists of a resistor and an inductor, in which R f and L f
are the resistance and inductance, respectively.
In this thesis, the d axis of the reference frame is aligned with the grid voltage position so
that vq is zero. The instantaneous active power transmitted from the grid to the grid side
converter can be calculated as equation (3.18).
K K*
p = va ia + vbib + vcic = Re(v dq i dq ) = vd id + vq iq = vd id
(3.18)
Note that there is no other coefficient rather than 1 in (3.18). This is achieved by the effort of (3.4)
and (3.5). This modification benefits a lot in further calculation in three-phase system, which can
be treated as a single phase system and relieve complex coefficient calculations significantly.
Similarly, the instantaneous reactive power transmitted from grid to grid side converter can be
calculated as equation (3.19).
K K*
q = Im(v dq i dq ) = vq i d −vd iq = −vd iq
(3.19)
In terms of the circuit shown in figure 3.7, since the d axis is aligned with grid voltage
JK
position, the grid voltage can be written as V dq = Vd + j 0 . The output voltage of the grid side
JK
converter can be written as V dq1 = Vd 1 + jVq1 , the current flowing from grid to converter is
described in equation (3.20).
JK
JK
K
(V − Vd 1 ) R f − Vq1ωs L f − j[(Vd − Vd 1 )ωs L f + Vq1 R f ]
V dq − V dq1
I dq =
= d
R f + jωs L f
R 2 f + ω 2 s L2 f
(3.20)
In terms of the steady state calculation, the active and reactive power can be expressed as
following.
JK K *
V [(V − V ) R − V ω L ]
P = Re(V dq I dq ) = d d 2d 1 f 2 2q1 s f
R f + ωs L f
20
(3.21)
JK K *
V [(V − V )ω L + V R ]
Q = Im(V dq I dq ) = d d 2d 1 s 2 f 2 q1 f
R f + ωs L f
(3.22)
Since the grid filter resistance is much smaller than the inductance (i.e., R f << ωs L f ),
the active power is mainly controllable by Vq1 , and the reactive power is mainly controllable by
Vd 1 .
3.3 Conventional control scheme of the grid side converter
The conventional control scheme of the grid side converter utilizes PID control theory
[25]. The objective of the grid side converter is to keep the DC-link capacitor voltage constant
and regulate reactive power flowing between the grid and the grid side converter. There are two
control loops in the grid side converter control system, which is a current control loop and a
DC-link voltage control loop. If the machine is generating active power, the grid side converter
should transmit the active power from machine side converter to the grid. If the machine is
absorbing active power, the grid side converter should transmit the active power from grid to the
machine side converter. Otherwise, the voltage over DC-link capacitor may vary and cause
problems to the system. The DC-link voltage control loop tries to stabilize the voltage over the
capacitor. The current control loop tries to regulate dq currents to the d and q current references.
The overall control strategy is shown in figure 3.8, which employs vector control approaches.
The grid three phase voltage and currents are measured and transformed into dq reference frame
for control purpose. The output voltage signals of the controller are used to generate PWM
signals, which are used as gate commands for the IGBT modules in the grid side converter.
21
V dc
Vdc _ ref +
iq _ ref
−
PI
id _ ref
+
+
−
PI
−
PI
vd′
−
vq1
−
ωs Lf
vd
+ +
id
vα 1
va1,b1,c1
V dc
C
e− jθe vβ 1
+
−
vq′
vd 1
Lf
− jθe
vα ,β
va , b , c
− jθe
iα , β
ia ,b,c
e
ωs Lf
iq
Rf
θe
e
Fig. 3.8. Conventional control scheme of the grid side converter [25, 26]
3.3.1 Current loop controller design
The grid filter consists of a resistor and an inductor, where the resistance and inductance
are R f and L f respectively. The currents in dq reference frame flowing through grid filter are
id and iq respectively. The relationship of the currents, converter output voltage and grid
voltage are obtained by equation (3.10). Rewrite the equation (3.10) in dq axis reference frame
separately, which becomes equation (3.23) and (3.24).
vd 1 = −( R f id + L f
vq1 = −( R f iq + L f
did
) + ωs L f iq + vd
dt
diq
dt
) − ωs L f iq
(3.23)
(3.24)
The items in bracket can be rewritten as vd′ and vq′ respectively, which are the
controller output signals in figure 3.8. Actually, this is the deficiency of the conventional control
22
mechanism. As described in section 3.1.4, the d axis voltage vd′ should control the reactive
power and the q axis voltage vq′ should control the active power. However, the relationship in
the conventional control mechanism is opposite [25, 26, 36]. The plant for the current loops is
obtained from (3.23) and (3.24),
Gc ( s) =
1
Rf + Lf s
(3.25)
the feedback current control loop is shown in figure 3.9,
iref +
−
PI
v′ k
pwm
1
Rf + Lf s
i
Fig. 3.9. Current control loop
where k pwm is the gain of the grid side converter.
The current loop controller is a typical PID controller, which can be expressed as
equation (3.26).
G p (s) = K p +
Ki
s
(3.26)
The switching frequency of the PWM converter is f s , which is equal to f tri . The frequency
response design method is applied to design the controller. The crossover frequency is f c , which
is two order smaller than the switching frequency. The desired phase margin φ pm is 60°, which
is good enough to obtain system stability with the controller. Solving equations (3.27) and (3.28),
23
then get the two parameters K p and K i of the controller. After the controller output voltage
signals are generated, the actual drive voltage signals need to add some compensation items.
(K p +
K pwm
Ki
)
=1
s Rf + Lf s
⎡
K pwm ⎤
K
D
arg ⎢ ( K p + i )
⎥ = −180 + φ pm
s R f + L f s ⎦⎥
⎣⎢
(3.27)
(3.28)
The dq axis drive voltage signals are generated through equations (3.29) and (3.30)
respectively.
vd 1 = −vd' + ωs L f iq + vd
vq1 = −vq' − ωs L f iq
(3.29)
(3.30)
3.3.2 DC-link voltage loop controller design
The DC-link model is demonstrated in figure 3.6, the machine side current im flowing
into the DC-link is represented as a disturbance. Neglecting the losses in grid filter, the DC-link
model transfer function can be derived from equation (3.31).
vdc ig = vd id
vd =
ma 3
vdc
2 2
ig =
ma 3
id
2 2
C
dvdc
= ig + im
dt
Gd ( s ) =
vdc ( s ) ma 3
=
id ( s ) 2 2Cs
24
(3.31)
Once the DC-link model transfer function has been derived, the standard classical control
design method can be applied. The DC-link voltage control loop is shown in figure 3.10.
vdc _ ref +
−
id _ ref
PI
ma
2
3
vdc
2C s
Fig. 3.10. DC-link voltage control loop
The controller in figure 3.10 is a typical PID controller, whose transfer function form is
identical with (3.26). Similar to the current loop controller design procedure, the frequency
response design method is applied. The crossover frequency is three orders smaller than
switching frequency, and the phase margin is 60°. The controller parameters K p and K i can
be solved through equations (3.32) and (3.33).
K i ma 3
)
=1
s 2 2Cs
(3.32)
⎡
K m 3⎤
D
arg ⎢ ( K p + i ) a
⎥ = −180 + φ pm
s 2 2Cs ⎦
⎣
(3.33)
(K p +
3.4 Proposed control scheme of the grid side converter
The proposed control scheme of the grid side converter consists of a current control loop
and a DC-link voltage control loop. The three phase grid voltage and current are measured and
transformed into dq reference frame, which are then used for the control purpose. The current
control loop regulates dq axis currents to the d and q current references. The DC-link voltage
control loop regulates DC-link voltage over the capacitor and maintains the voltage at a desired
25
value. The main modifications of the proposed control scheme of the grid side converter are the
current control loop. The DC-link voltage control loop is similar with the conventional control
approach and the design steps follow the conventional and classical procedures.
Vdc _ ref
V dc
−
PI
+
id _ ref
+
−
id′
PI
Rf
+
−
+
+
ωs Lf
+
−
id
iq
PI
i q′
Rf
vq1
vα 1
e− jθe vβ 1
θe
ωs Lf
iq _ ref
vd 1
vd
−
−
V dc
va1,b1,c1
2/3
PWM
Rf
Voltage
angle
calculation
e− jθe
e− jθe
vα ,β
iα , β
C
Lf
3/2
3/2
v a ,b ,c
ia , b , c
Fig. 3.11. Proposed control scheme of the grid side converter
Figure 3.11 shows the proposed control scheme of the grid side converter, which depicts
the new designed control approach.
3.4.1 Proposed current loop controller
The proposed current loop controller is designed based on the equation (3.11). Instead of
generating dq axis voltage signals by the conventional control scheme, the proposed current loop
controller outputs dq axis current signals, which are id' and iq' respectively. The transformation
between d-q output current signals from the controller, id' and iq' , and the dq control voltages
driving the converter are calculated by equations (3.34) and (3.35).
vd 1 = − R f id' + ωs L f iq' + vd
vq1 = − R f id' − ωs L f iq'
26
(3.34)
(3.35)
Before the dq axis currents measured from grid lines are feed to the controller, a signal
processing unit has to be used, which is a typical low pass filter and a mean value calculation unit.
The signal processing unit prevents the high order harmonics from getting into the controller,
which may cause malfunctions of the controller. The diagram of the current loop controller is
shown in figure 3.12,
ir e f +
−
PI
i'
v'
i
Fig. 3.12. Proposed current control loop
in which the PID controller can combine with adaptive and fuzzy control technologies [28, 29].
The controller operates on a direct target control principle, the controller parameters can be
adjusted by the adaptive and fuzzy parts based on the difference between the measurements and
reference values.
The saturation limit values are important in the control procedure, which prevent the
output signals of the controller exceeding the tolerable level of the device in the system. The
final output signals of the controller are dq axis voltage signals vd 1 and vq1 , which are then
used to generate PWM pulses for the grid side converter. In order to prevent the converter from
operating beyond various constraints, a nonlinear programming strategy is developed. The basic
principle of the nonlinear programming strategy is that, under the converter rated power and
linear modulation constraints, the system should operate with constant DC-link capacitor voltage
while minimize the difference between the reference and actual reactive power delivered to the
27
grid. The first goal of the strategy is to deliver the active power effectively, then, the second goal
is to generate the expected reactive power. The nonlinear programming strategy can be written as
following.
Minimize: Qactual − Qref
Subject to:
Vdc _ actual = Vdc _ ref
I +I
2
d
3
2
q
≤ I rated ,
m=
2 2Vconv
=
Vdc
2
2(Vd21 + Vq21 )
3
Vdc
≤1
3.4.2 Proposed DC-link voltage loop controller
The proposed DC-link voltage loop controller is designed based on the DC-link model
described in section 3.2.2. The controller design procedure is the same as the steps demonstrated
in section 3.3.2. It follows the conventional and classical design procedure. The controller
parameters are determined by equations (3.32) and (3.33).
3.5 Machine side converter controller
The purpose of the machine side converter is to deliver active power generated by a
renewable source to the grid side converter through DC-link capacitor. The DC-link capacitor
voltage should be stable while the system is operating. This goal is achieved by the DC-link
voltage loop controller of the grid side converter. As a result, the function of the machine side
converter controller is only to deliver the active power generated by the machine to the grid side
converter.
Two types of AC/DC converters can be used to form a machine side converter, one
28
utilizes diode and the other utilizes IGBT. Figure 3.13 demonstrates the basic structure of these
two types of AC/DC converter.
Vdc
Vdc
iA
iB
iA
iB
iC
iC
Fig. 3.13. Structure of two types of AC/DC converter
The output DC voltage of the AC/DC converter can be fully controlled if IGBT switches
are adopted. The machine side converter utilizes IGBT switches to form an AC/DC converter to
deliver the active power generated from machine to the grid side converter over the DC-link
capacitor in wind energy conversion system.
Figure 3.14 shows the conventional control scheme of machine side converter, and figure
3.15 shows the proposed control scheme of machine side converter. The voltage and current
sensors measure the voltages and currents of the connection point between a renewable energy
source and the machine side converter. The dq axis current reference id _ ref and iq _ ref
determines the desired active and reactive power generated by the renewable source, respectively.
The conventional and proposed control strategies of the machine side converter are similar to the
grid side converter control approach described in section 3.3 and 3.4. The difference is the q axis
current reference of the machine side converter controller is determined by an arbitrary value,
while the d axis current reference of the grid side converter controller is determined by the
variation of the DC-link capacitor voltage value.
29
id _ ref
iq _ ref
+
PI
−
+
PI
−
vd′
−
vα 1
vq1
−
V dc
vd
+ +
Rm
θe
ωs Lm
iq
Lm
− jθe
vα ,β
va ,b , c
− jθe
iα , β
ia ,b,c
e
ωs Lm
id
va1,b1,c1
e− jθe vβ 1
+
−
vq′
vd 1
e
Fig. 3.14. Conventional control scheme of the machine side converter
id _ ref
+
−
PI
id′
Rm
+
−
+
+
ωs Lm
vd
+
−
id
iq
PI
i q′
Rm
−
−
− jθe
e
− jθe
e
V dc
Rm
θe
ωs Lm
iq _ ref
vd 1 vα 1
e− jθe vβ 1
vq1
va1,b1,c1
Lm
vα ,β
v a ,b , c
iα , β
ia , b , c
Fig. 3.15. Proposed control scheme of the machine side converter
30
CHAPTER 4
SIMULATION STUDY OF RENEWABLE ENERGY GRID INTEGRATION CONTROL
4.1 Introduction
This chapter discusses the simulation models, approaches and results for grid integration
control of renewable energy conversion systems. The simulation models are built in
Matlab®/Simulink® development environment. All simulation models in this thesis are
implemented using the SimPowerSystem toolbox, a special toolbox in Simulink for power
systems and power electronics simulation. The AC/DC/AC converter control system is
implemented by utilizing conventional and proposed control methods respectively. Some normal
and extreme abnormal operating conditions for the energy conversion system are tested by
computer simulation, and the results are presented and analyzed.
4.2 Simulation models for grid integration of renewable energy conversion system
The grid integration control study of renewable energy conversion systems is investigated
in Matlab®/Simulink® environment using 1) conventional control theory described in section 3.3,
and 2) the proposed control theory described in section 3.4. The simulation models for the two
different control approaches have the same structure and most of the components are identical.
The only difference is the control system block. Figure 4.1 shows the top level simulation
structure of the AC/DC/AC converter system under feedback control. The upper part of the
simulation system consists of power generation, switch-mode converters, filter, and the grid. The
31
Fig. 4.1. Simulation structure of AC/DC/AC converter system for grid integration of
renewable energy conversion systems
32
details of the control system and data processing system are packaged into two subsystems. All
the details are hidden in the blocks to avoid complexity on the top level simulation system. The
simulation results are recorded to files and stored in hard disk, which makes it possible to
simulate a long time process.
4.2.1 High power path and related modules
The high power path consists of 1) a variable amplitude and variable frequency AC
voltage source representing a renewable energy source, 2) a three-phase machine side
switch-mode converter, 3) a DC-link capacitor, 4) a three-phase grid side switch-mode converter,
5) a three-phase grid filter, and 6) a three-phase AC voltage source representing the grid.
Four three-phase AC voltage and AC current measurements and a DC voltage
measurement are used to collect the information of the system. Measurements are set at the
renewable energy source, AC input terminal of the machine side converter, AC output terminal of
the grid side converter and point of the common coupling with the grid. The three-phase current
flowing between the renewable energy source and the machine side converter are measured, and
the three-phase current flowing between the grid and the grid side converter are also measured.
4.2.2 Grid side converter control system module
4.2.2.1 abc to dq axis frame transformation
The whole control system consists of two parts. One is grid side converter control system;
the other is machine side converter control system. The inputs of the control system are
three-phase voltages of machine side converter and grid side converter, three-phase currents
flowing into machine side converter and grid side converter, and the DC-link voltage. Since the
33
control system is designed based on the space vector theory, the three phase abc axis variables
have to be transformed into dq axis frame.
Fig. 4.2. abc to dq axis frame transformation
Figure 4.2 shows the transformation of abc to dq axis frame. The “theta” subsystem block
calculates the grid voltage phase position using Clarke transformation. The “abc_dq” subsystem
block transforms the three phase abc currents into d and q axis currents. Two “mean value”
blocks are added to process the d and q axis output current signals in proposed control system,
while the “mean value” blocks are not necessary in conventional control system.
4.2.2.2 Core control system module of grid side converter
1) Core control system module using proposed control method
Once the three phase grid currents have been transformed into dq axis frame, then they
are used as the inputs to the core control system module of the grid side converter.
34
Fig. 4.3. Core control system module using proposed control theory
Figure 4.3 demonstrates the core control system module using proposed control technique.
The d axis current reference is generated by comparing the difference of actual and desired
DC-link voltage. The q axis current reference is generated when running the initialization file,
which sets the desired reactive power transmitted to the grid at different simulation time. The
current-loop controllers update d and q axis current commands based on the error signals of the d
and q axis currents. The d and q axis current commands are then used to generate d and q axis
voltage commands.
(a). Vd 1 signal generation in proposed control system
35
(b). Vq1 signal generation in proposed control system
Fig. 4.4. Vd 1 and Vq1 signals generation blocks in proposed control system
Figure 4.4 (a) and (b) show the Vd 1 and Vq1 signals generated from the d and q axis
currents, id' and iq' , using equations (3.34) and (3.35).
2) Core control system module using conventional control theory
The structure of the core control system module using conventional control theory is
shown in figure 4.5. The procedure to generate d and q axis currents references is the same as the
structure shown in figure 4.3.
Fig. 4.5. Core control system module using conventional control theory
However, the procedure to generate Vd 1 and Vq1 signals is different from that of the
36
proposed control system. The calculation model is implemented by equations (3.29) and (3.30),
and figure 4.6 describes the calculation procedure.
(a). Vd 1 signal generation in conventional control system
(b). Vq1 signal generation in conventional control system
Fig. 4.6. Vd 1 and Vq1 signals generation blocks in conventional control system
4.2.2.3 PWM signals generation module
After the voltage signals Vd 1 and Vq1 are generated, the grid side converter output
voltage is determined. However, the grid side converter output voltage is actually determined by
the PWM pulse signals. As a result, the voltage signals Vd 1 and Vq1 have to be transformed
into PWM pulse signals and applied on the gate circuit of the IGBT converter module.
37
Fig. 4.7. PWM pulse signals generation module in proposed control system
Figure 4.7 shows the PWM pulse signals generation module in the proposed control
system. The inputs of this module are d and q axis voltage signals Vd 1 , Vq1 and DC-link voltage
Vdc . The d and q axis components of PWM pulse signals are generated using equations (4.1) and
(4.2).
vd _ norm =
vq _ norm =
2Vd 1
Vdc
2Vq1
Vdc
(4.1)
(4.2)
The PWM pulse signals generation requires the linear modulation limit not exceed 1,
otherwise, the PWM pulse signals may bring harms and huge distortions to the converter and its
output voltage.
With the proposed control system, the linear modulation limit block can limit the
modulation ratio not exceeding 1. Figure 4.8 shows the details of the linear modulation limit
block. A reactive power optimal control block is applied in the linear modulation limit block. The
algorithm has been described in section 3.4.1.
38
Fig. 4.8. Details of linear modulation limit in proposed control system
Figure 4.9 (a) and (b) describe the details of reactive power optimal control block and the
corresponding algorithm.
(a). Details of reactive power optimal control block
39
Start
Calculate
v d2 _ norm + v q2 _ norm
No
>=
3
2
Yes
No
vq _ norm >=
Output original
dq components
3
2
Yes
Calculate new dq
components
Calculate new
v
*
d _ norm
vd _ norm
= 1− v
2
q _ norm
Output updated
dq components
vd* _ norm =
vq* _ norm =
vd _ norm
vd2 _ norm + vq2 _ norm
vq _ norm
vd2 _ norm + vq2 _ norm
Output updated
dq components
END
(b). Algorithm of reactive power optimal control block
Fig. 4.9. Reactive power optimal control block and algorithm
40
With the calculation procedure of d and q axis values completed, the dq to abc frame
transformation equation (3.7) is used to generate three phase reference voltage and applied on the
PWM pulse signals generation block.
4.2.3 Machine side converter control system module
The machine side converter control system module is similar to the grid side converter
control system module. However, the procedure to generate the d axis current reference is
different from that used in the grid side converter control system module. To simulate the active
power flow variation of the renewable energy source, the active power generated by the machine
in the model may vary at different simulation time. Changing the d axis current reference
command can adjust the active power generated by the renewable energy source. The grid side
converter control system should deliver the active power from the renewable energy source to
the grid efficiently and avoid DC-link voltage variation.
Fig. 4.10. Core control system module of machine side converter using proposed control
theory
Figure 4.10 shows the core control system module of machine side converter using the
proposed control theory, the difference from figure 4.3 is the d axis current reference generation
as described above.
41
Besides the difference of core control system between machine side converter and grid
side converter, the abc to dq axis frame transformation block and the PWM signals generation
block are exactly the same as that used in the grid side converter control system module.
4.2.4 Data processing module
The data processing module collects the information of the system and process the data
collected. The voltages and currents information of the system are measured and transmitted to
the data processing module. Some filters and three phase active and reactive power calculation
blocks are used in the data processing module.
(a). Three phase voltage filter in data processing module
(b). Three phase active and reactive power calculation in date processing module
Fig. 4.11. Filter and power calculation block
Figure 4.11 (a) and (b) depict a filter block and a three phase active and reactive power
42
calculation block in the data processing module. The module consists of several filter blocks and
power calculation blocks as described above.
4.2.5 Results recording module
The simulation system utilizes “to file” block in Simulink® to record the simulation
results into file and store in hard disk rather than store in memory using scope. This feature
makes long time simulation possible, which may be needed for investigation of the overall
performance of the system in detail.
4.3 Simulation results and analysis
The performance of the conventional and the proposed control techniques are tested using
the simulation system developed under normal and extreme operating conditions. The simulation
results are recorded and compared in details, which demonstrates that the performance of
proposed control system is better than the conventional control system.
The system parameters are listed in table 4.1.
Table 4.1. System parameters of renewable energy conversion system model
Grid line voltage (V)
690
Grid filter resistor (Ω)
0.012
Grid filter inductor (mH)
2
DC-link capacitor (μF)
16000
DC-link voltage (V)
1200
Machine line voltage (V)
500
Machine side resistor (Ω)
0.012
43
Machine side inductor (mH)
3
System frequency (Hz)
60
Switching frequency (Hz)
1980
Sample time (s)
5e-6
Two cases are studied to evaluate the performance of renewable energy conversion
system using the conventional and the proposed control mechanisms, respectively. In the first
case, the operating condition is normal and the controller output voltage is always within the
converter linear modulation limit. In the second case, the operating condition is abnormal and the
controller output voltage command may exceed the linear modulation limit. Passive sign
convention is used, i.e., power absorbed toward the converter is positive.
1) In case 1, the active power generated to grid is 100 kW. The reactive power reference
is 100 kVar absorbing from grid during the time period from 0s to 6s, while the reactive power
reference changes to -20 kVar during the time period from 6s to 12s. The reactive power
reference in this simulation is within the linear modulation limit, which is normal for the
operation of the system.
Figure 4.12 (a) to (d) show the DC-link voltage waveform, active and reactive power
waveforms, grid d and q axis current waveforms and grid three phase current waveform of the
renewable energy conversion system using the conventional control mechanism under case 1.
44
1800
1700
DC-link voltage (V)
1600
1500
1400
1300
1200
1100
1000
900
0
2
4
6
Time (s)
8
10
12
(a) DC-link voltage waveform
200
Grid power (kW/kVar)
150
100
Reactive power
50
0
Active power
-50
-100
-150
-200
0
2
4
6
Time (s)
8
10
12
10
12
(b) Active and reactive power waveform
100
50
Grid dq current (A)
0
q axis current
-50
-100
-150
-200
d axis current
-250
-300
0
2
4
6
Time (s)
8
(c) Grid dq axis current waveform
45
200
150
Grid current (A)
100
50
0
-50
-100
-150
-200
6
6.01
6.02
6.03
6.04
6.05
Time (s)
6.06
6.07
6.08
6.09
6.1
(d) Grid three phase current waveform
Figure 4.12. Performance of renewable energy conversion system using conventional
control mechanism under case 1
Figure 4.13 (a) to (d) show the DC-link voltage waveform, active and reactive power
waveforms, grid d and q axis current waveforms and grid three phase current waveform of the
renewable energy conversion system using the proposed control mechanism under case 1.
1700
DC-link voltage (V)
1600
1500
1400
1300
1200
1100
1000
0
2
4
6
Time (s)
8
(a) DC-link voltage waveform
46
10
12
500
400
Grid power (kW/kVar)
300
200
Reactive power
100
0
-100
-200
Active power
-300
-400
0
2
4
6
Time (s)
8
10
12
(b) Active and reactive power waveform
200
Grid dq current (A)
100
0
q axis current
-100
-200
d axis current
-300
0
2
4
6
Time (s)
8
10
12
(c) Grid dq axis current waveform
200
150
Grid current (A)
100
50
0
-50
-100
-150
-200
6
6.01
6.02
6.03
6.04
6.05
Time (s)
6.06
6.07
6.08
6.09
6.1
(d) Grid three phase current waveform
Figure 4.13. Performance of renewable energy conversion system using proposed control
mechanism under case 1
47
2) In case 2, the active power generated to grid is 100 kW before t=3s. At t=3s, the active
power reference changes to generate 200 kW. The reactive power reference is 100 kVar
absorbing from grid during the time period from 0s to 6s. The reactive power reference changes
to -50 kVar during the time period from 6s to 9s and changes back to 50 kVar during the time
period from 9s to 12s. The reactive power reference in this simulation exceeds the linear
modulation limit during the time period from 6s to 9s.
Figure 4.14 (a) to (d) show the DC-link voltage waveform, active and reactive power
waveforms, grid d and q axis current waveforms and grid three phase current waveform of the
renewable energy conversion system using the conventional control mechanism under case 2.
1800
1700
DC-link voltage (V)
1600
1500
1400
1300
1200
1100
1000
900
0
2
4
6
Time (s)
8
(a) DC-link voltage waveform
48
10
12
300
Reactive power
Grid power (kW/kVar)
200
100
0
-100
Active power
-200
-300
0
2
4
6
Time (s)
8
10
12
10
12
(b) Active and reactive power waveform
200
Grid dq current (A)
100
q axis current
0
-100
d axis current
-200
-300
-400
0
2
4
6
Time (s)
8
(c) Grid dq axis current waveform
400
300
Grid current (A)
200
100
0
-100
-200
-300
-400
6
6.01
6.02
6.03
6.04
6.05
Time (s)
6.06
6.07
6.08
6.09
6.1
(d) Grid three phase current waveform
Figure 4.14. Performance of renewable energy conversion system using conventional
control mechanism under case 2
49
Figure 4.15 (a) to (d) show the DC-link voltage waveform, active and reactive power
waveform, grid dq axis current waveform and grid three phase current waveform of the
renewable energy conversion system using the proposed control mechanism under case 2.
1700
DC-link voltage (V)
1600
1500
1400
1300
1200
1100
1000
0
2
4
6
Time (s)
8
10
12
10
12
(a) DC-link voltage waveform
200
Grid power (kW/kVar)
100
Reactive power
0
-100
Active power
-200
-300
0
2
4
6
Time (s)
8
(b) Active and reactive power waveform
50
200
q axis current
Grid dq current (A)
100
0
-100
-200
-300
-400
-500
d axis current
0
2
4
6
Time (s)
8
10
12
(c) Grid dq axis current waveform
300
Grid current (A)
200
100
0
-100
-200
-300
6
6.01
6.02
6.03
6.04
6.05
Time (s)
6.06
6.07
6.08
6.09
6.1
(d) Grid three phase current waveform
Figure 4.15. Performance of renewable energy conversion system using proposed control
mechanism under case 2
From figure 4.12 to figure 4.15, the following conclusions can be obtained:
(1) The AC/DC/AC converter works properly for both DC capacitor voltage and
reactive power controls if the controller output voltage does not exceed the linear
modulation or the saturation limit.
(2) Whenever the reactive power control demand makes the controller output voltage
go over the linear modulation or the saturation limit, then, the actual DC capacitor
voltage becomes uncontrollable using the conventional control method. The more
51
the controller output voltage exceeds the limit, the more the DC link voltage
deviates from the reference DC link voltage.
(3) After the controller output voltage exceeds the linear modulation or saturation
limit even just one time, the DC capacitor voltage using the conventional control
method becomes uncontrollable and floating with the reactive power demand after
that, showing the inherent deficiency of the conventional control mechanism.
(4) During the malfunction of the conventional control mechanism, there are more
oscillations in the DC capacitor voltage and the active and reactive powers
absorbed by the grid side converter, and the current taken by the grid side
converter from the grid becomes more unbalanced during each control transition.
(5) The AC/DC/AC converter works properly with the proposed control mechanism
whether the reactive power reference makes controller output voltage exceeds the
linear modulation limit or not.
(6) The current taken by the grid side converter from the grid changes smoothly
during each control transition when the proposed control mechanism is adopted.
However, the current oscillation is remarkable at each control transition when
conventional control mechanism is adopted.
52
CHAPTER 5
SIMULATION STUDY FOR CONTROL OF PWM-BASED STATCOM
5.1 Introduction
A STATCOM (Static Synchronous Compensator) is a device that can compensate reactive
power and provide voltage support to a bus. In a renewable energy conversion system,
STATCOM is used to improve the system stability [30, 32, 33, 34]. This chapter discusses the
control system for a PWM-based STATCOM and the simulation models for performance study of
the STATCOM system. The simulation models are built in Matlab®/Simulink® development
environment using SimPowerSystem toolbox. The STATCOM control system is implemented
utilizing the conventional control and proposed control techniques, respectively. Some normal
and extreme operation conditions for the STATCOM are tested by computer simulation, and the
results are presented and analyzed.
5.2 STATCOM configuration and its control system
Figure 5.1 depicts the basic configuration of a PWM-based STATCOM system connected
with the grid, where a capacitor is shunt connected with a voltage source PWM converter. A
transformer and a grid filter are connected between the converter and the grid [31, 32]. The grid
filter consists of a resistor R f and an inductor L f . The transformer can also be modeled as an
inductor plus a small resistor. Hence, the equivalent circuit between the converter and the grid can
be modeled as a resistor and an inductor in series for convenient analysis.
53
Fig. 5.1. Configuration of STATCOM
Figure 5.2 shows the equivalent circuit of the STATCOM system, where Vdc represents
the voltage over the capacitor C, the resistor R p represents the power loss in the converter and
the DC circuit. The voltages va1 , vb1 , and vc1 represent the three-phase output voltage of the
PWM converter, and the voltages va , vb , and vc represent the three- phase grid voltage at the
grid connection point. The transformer and grid filter in figure 5.1 are represented as a series
combination of a resistor R and an inductor L.
Fig. 5.2. Equivalent circuit of grid integration of STATCOM
Since the equivalent circuit in figure 5.2 is similar with the equivalent circuit of the
grid-side converter system shown in figure 3.7, the control system designed in chapter 3 can be
applied to control the STATCOM.
Figure 5.3 and figure 5.4 demonstrate the conventional and proposed control system of
54
STATCOM, respectively. Comparing with the control system designed in chapter 3, there are
some differences for the STATCOM system.
−
Vdc _ ref +
Vbus _ ref
PI
iq _ ref
+
−
Vbus
V dc
V dc
id _ ref
+
+
PI
−
PI
−
PI
iq _ ref
vd′
−
vα 1
vq1
−
Rp
va1,b1,c1
C
e− jθe vβ 1
+
−
vq′
vd 1
θe
ωL
vd
+
− jθe
+ e
R
L
vα ,β
va ,b , c
iα , β
ia ,b,c
ωL
id
− jθe
iq
e
Fig. 5.3. Conventional control system of STATCOM [26, 31]
In figure 5.3, the main structure of the control system is the same as that shown in figure
3.8. The q axis current reference could be determined in two ways: 1) a reactive power
compensation demand, or 2) a bus voltage support requirement [32]. For reactive power
compensation control, the q axis current reference is determined according to a reactive power
compensation demand. For system bus voltage support control, the q axis current reference is
determined based on the error signal between the actual and a desired system bus voltage, which is
equivalent to regulate the reactive power generation to the grid so as to adjust the actual bus
voltage to the desired value.
Figure 5.4 shows the proposed STATCOM control system. In figure 5.4, the main
structure is the same as that used in figure 3.11. However, the q axis current reference is
determined as shown in figure 5.3.
55
V dc
V dc
Vdc _ ref
−
PI
+
id _ ref
+
PI
−
id′
R
+
−
+
+
ωL
Vbus _ ref
+
−
Vbus
+
PI
−
PI
iq _ ref
id
iq
i q′
vq1
vd
ωL
iq _ ref
vd 1
−
R
−
Rp
vα 1 va1,b1,c1
PWM
e− jθe vβ 1 2/3
θe
e− jθe
e− jθe
R
Voltage
angle
calculation
vα ,β
iα , β
C
L
3/2
3/2
v a ,b ,c
ia , b , c
Bus Voltage
Magnitude
Calculation
Fig. 5.4. Proposed control system of STATCOM
5.3 STATCOM simulation models
The STATCOM simulation models are built in Simulink®, which consist of modules of
high power components, control modules and data processing modules. The STATCOM is
connected to the grid for either reactive power or the grid voltage support control.
Figure 5.5 depicts the top level of STATCOM simulation models built in Simulink®. A
fault switch is adopted to simulate a short circuit in a transmission line, which will cause a
voltage drop at the bus where the STATCOM is connected.
56
Fig. 5.5. Simulation model of STATCOM for system voltage support control application
57
Fig. 5.6. Core control system of STATCOM using conventional control mechanism
Fig. 5.7. Core control system of STATCOM using proposed control mechanism
Figure 5.6 shows the core control system module of the STATCOM using the
conventional control mechanism, and figure 5.7 shows the core control system module of the
STATCOM using the proposed control mechanism.
5.4 Simulation results and analysis
The performance of conventional and proposed STATCOM control systems is evaluated
under several different operating conditions. Since the STATCOM can operate at the reactive
power compensation mode or the bus voltage support mode, the simulation is conducted for each
58
of the two different modes. However, the system parameters for simulation of the two modes are
identical.
The system parameters are shown in Table 5.1.
Table 5.1. System parameters of STATCOM model
Grid line voltage (V)
570
Equivalent resistor (Ω)
0.0012
Equivalent inductor (mH)
1.2
Shunt capacitor (μF)
16000
Capacitor voltage (V)
1200
System frequency (Hz)
60
Switching frequency (Hz)
1980
5.4.1 Simulation Study of PWM STATCOM for reactive power compensation Control
Two cases are tested to evaluate the performance of STATCOM under the reactive power
compensation mode.
1) Passive sign convention is used, i.e., power absorbed toward the converter is positive. In
case 1, the STATCOM output reference is 1) 100 kVar from 0s to 2s; 2) -30 kVar from 2s to 5s; 3)
30 kVar from 5s to 8s; 4) -100 kVar from 8s to 10s. The controller output voltage is always
within the converter linear modulation limit.
Figure 5.8 (a) to (c) show the DC capacitor voltage waveform, output active and reactive
power waveforms and grid d and q current waveforms of the STATCOM system using the
conventional control mechanism.
59
1350
DC capacitor voltage (V)
1300
1250
1200
1150
1100
1050
1000
0
1
2
3
4
5
Time (s)
6
7
8
9
10
8
9
10
(a) DC capacitor voltage waveform
300
Grid power (kW/kVar)
200
Reactive power
100
0
Active power
-100
-200
-300
0
1
2
3
4
5
Time (s)
6
7
(b) Active and reactive power waveform
300
q axis current
Grid dq current (A)
200
100
0
-100
d axis current
-200
-300
0
1
2
3
4
5
Time (s)
6
7
8
9
10
(c) Grid dq axis current waveform
Fig. 5.8. Performance of STATCOM using conventional control mechanism in reactive
power compensation mode under case 1
60
1350
DC capacitor voltage (V)
1300
1250
1200
1150
1100
1050
1000
0
1
2
3
4
5
Time (s)
6
7
8
9
10
8
9
10
(a) DC capacitor voltage waveform
300
Grid power (kW/kVar)
200
Reactive power
100
0
Active power
-100
-200
-300
0
1
2
3
4
5
Time (s)
6
7
(b) Active and reactive power waveform
300
q axis current
Grid dq current (A)
200
100
0
-100
d axis current
-200
-300
0
1
2
3
4
5
Time (s)
6
7
8
9
10
(c) Grid dq axis current waveform
Fig. 5.9. Performance of STATCOM using proposed control mechanism in reactive
power compensation mode under case 1
61
Figure 5.9 (a) to (c) show the DC capacitor voltage waveform, output active and reactive
power waveforms and grid d and q current waveforms of the STATCOM system using the
proposed control mechanism.
From figure 5.8 and figure 5.9, it is clear that both the conventional and proposed control
mechanism works well if the controller output voltage is within the converter linear modulation
limit.
2) In case 2, the STATCOM output reference is 1) 100 kVar from 0s to 2s; 2) -30 kVar
from 2s to 5s; 3) -400 kVar from 5s to 10s; 4) -80 kVar from 10s to 13s; 5) 30 kVar from 13s
to16s. The controller output voltage exceeds the linear modulation limit during the time period
from 5s to 10s. The controller output voltage drops below the linear modulation limit after 10s.
Figure 5.10 (a) to (c) show the DC capacitor voltage waveform, output active and
reactive power waveforms and grid d and q current waveforms of the STATCOM using the
conventional control mechanism.
1800
1700
DC capacitor voltage (V)
1600
1500
1400
1300
1200
1100
1000
900
800
0
2
4
6
8
Time (s)
10
(a) DC capacitor voltage waveform
62
12
14
16
600
Grid power (kW/kVar)
400
Active power
200
0
-200
-400
-600
Reactive power
0
2
4
6
8
Time (s)
10
12
14
16
(b) Active and reactive power waveform
600
400
Grid current (A)
200
0
-200
-400
-600
10
10.01
10.02
10.03
10.04
10.05
Time (s)
10.06
10.07
10.08
10.09
10.1
(c) Grid current waveform
Fig. 5.10. Performance of STATCOM using conventional control mechanism in reactive
power compensation mode under case 2
Figure 5.11 (a) to (c) show the DC capacitor voltage waveform, output active and reactive
power waveforms and grid d and q current waveforms of the STATCOM using the proposed
control mechanism.
63
1400
DC capacitor voltage (V)
1350
1300
1250
1200
1150
1100
0
2
4
6
8
Time (s)
10
12
14
16
(a) DC capacitor voltage waveform
200
Active power
Grid power (kW/kVar)
100
0
-100
-200
-300
-400
Reactive power
0
2
4
6
8
Time (s)
10
12
14
16
(b) Active and reactive power waveform
300
Grid current (A)
200
100
0
-100
-200
-300
10
10.01
10.02
10.03
10.04
10.05
Time (s)
10.06
10.07
10.08
10.09
10.1
(c) Grid current waveform
Figure 5.11. Performance of STATCOM using proposed control mechanism in reactive
power compensation mode under case 2
64
From figure 5.8 to figure 5.11, the following conclusions are obtained:
(1) If the controller output voltage does not exceed the linear modulation or the
saturation limit, the STATCOM works properly for DC capacitor voltage and
reactive power controls using both the conventional and the proposed control
approaches.
(2) Whenever the reactive power control demand makes the controller output voltage
go over the linear modulation or the saturation limit, then, the actual DC capacitor
voltage becomes uncontrollable using the conventional control technique [35].
The more the controller output voltage exceeds the limit, the more the DC voltage
deviates from the reference DC voltage.
(3) Using the conventional control mechanism, when the controller output voltage
exceeds the linear modulation or saturation limit even just one time, the DC
capacitor voltage becomes uncontrollable and floating with the reactive power
demand after that, showing the inherent deficiency of the conventional control
mechanism.
(4) During the malfunction of the conventional control mechanism, there are more
oscillations in the DC capacitor voltage and the active and reactive powers
absorbed by the STATCOM, and the current taken by the STATCOM from the
grid becomes more unbalanced during each control transition.
(5) The STATCOM works properly with the proposed control mechanism. Whenever
the reactive power reference makes controller output voltage exceeds the linear
modulation limit, the proposed control mechanism operates in an optimal control
mode by maintaining a constant DC-link voltage as the first priority while
65
fulfilling the reactive power control demand as much as possible. The system
stability is improved by the proposed control mechanism.
5.4.2 Simulation Study of PWM STATCOM for System voltage support Control
For the voltage support control mode, a short-circuit fault is set during the simulation,
which causes a bus voltage sag.
The STATCOM should generate appropriate reactive power to
the grid to support the bus voltage.
The performance of STATCOM under bus voltage support mode is evaluated for two
cases. In the first case, the bus voltage sag is 20% of the rated bus voltage; in the second case,
the bus voltage sag is 40% of the rated bus voltage, which requires more reactive power to
support the bus voltage.
1) In case 1, the short-circuit fault occurs during the time period between 3s and 4s.
Figure 5.12 (a) to (c) show the performance of the STATCOM using the conventional control
mechanism in bus voltage support application under a low voltage sag condition.
DC capacitor voltage (V)
1350
1300
1250
1200
1150
1100
2
2.5
3
3.5
4
Time (s)
4.5
(a) DC capacitor voltage waveform
66
5
5.5
6
200
150
Grid power (kW/kVar)
100
Active power
50
0
-50
-100
-150
Reactive power
-200
2
2.5
3
3.5
4
Time (s)
4.5
5
5.5
6
(b) Active and reactive power waveform
Grid voltage (pu)
1.2
Bus voltage with
STATCOM
1.1
1
0.9
Bus voltage without
STATCOM
0.8
0.7
2
2.5
3
3.5
4
Time (s)
4.5
5
5.5
6
(c) Bus voltage waveform
Fig. 5.12. Performance of STATCOM using conventional control mechanism in bus
voltage support mode under case 1
Figure 5.13 (a) to (c) show the performance of the STATCOM using the proposed control
mechanism in the same bus voltage support application under a low voltage sag condition.
67
1350
DC capacitor voltage (V)
1300
1250
1200
1150
1100
2
2.5
3
3.5
4
Time (s)
4.5
5
5.5
6
5.5
6
(a) DC capacitor voltage waveform
200
Grid power (kW/kVar)
150
100
Active power
50
0
-50
-100
-150
-200
Reactive power
2
2.5
3
3.5
4
Time (s)
4.5
5
(b) Active and reactive power waveform
Grid voltage (pu)
1.2
Bus voltage with
STATCOM
1.1
1
0.9
0.8
0.7
Bus voltage without
STATCOM
2
2.5
3
3.5
4
Time (s)
4.5
5
5.5
6
(c) Bus voltage waveform
Fig. 5.13. Performance of STATCOM using proposed control mechanism in bus voltage
support mode under case 1
68
2) In case 2, the short circuit fault occurs during the time period between 3s and 4s.
However, the bus voltage sag is higher than that in case 1. Figure 5.14 (a) to (c) show the
performance of the STATCOM using conventional control mechanism in bus voltage support
mode under a high voltage sag condition.
1800
DC capacitor voltage (V)
1600
1400
1200
1000
800
600
400
2
2.5
3
3.5
4
Time (s)
4.5
5
5.5
6
5
5.5
6
(a) DC capacitor voltage waveform
400
Grid power (kW/kVar)
300
200
Active power
100
0
-100
-200
-300
-400
Reactive power
2
2.5
3
3.5
4
Time (s)
4.5
(b) Active and reactive power waveform
69
1.5
1.4
Bus voltage with
STATCOM
Grid voltage (pu)
1.3
1.2
1.1
1
0.9
0.8
Bus voltage without
STATCOM
0.7
2
2.5
3
3.5
4
Time (s)
4.5
5
5.5
6
(c) Bus voltage waveform
Fig.5.14. Performance of STATCOM using conventional control mechanism in bus
voltage support mode under case 2
Figure 5.15 (a) to (c) show the performance of the STATCOM using the proposed control
mechanism in the bus voltage support mode under a high voltage sag condition.
1350
DC capacitor voltage (V)
1300
1250
1200
1150
1100
2
2.5
3
3.5
4
Time (s)
4.5
(a) DC capacitor voltage waveform
70
5
5.5
6
200
Grid power (kW/kVar)
100
Active power
0
-100
-200
Reactive power
-300
2
2.5
3
3.5
4
Time (s)
4.5
5
5.5
6
(b) Active and reactive power waveform
Bus voltage with
STATCOM
Grid voltage (pu)
1.1
1
0.9
0.8
0.7
Bus voltage without
STATCOM
0.6
2
2.5
3
3.5
4
Time (s)
4.5
5
5.5
6
(c) Bus voltage waveform
Fig.5.15. Performance of STATCOM using proposed control mechanism in bus voltage
support mode under case 2
From figure 5.12 to figure 5.15, the following conclusions are obtained:
(1) If the controller output voltage does not exceed the linear modulation or the
saturation limit under a low bus voltage sag condition, the STATCOM works
properly for both DC capacitor voltage and system voltage support controls using
both the conventional and the proposed control approaches.
(2) Whenever the bus voltage sag makes the controller output voltage go over the
linear modulation or the saturation limit, then, the conventional control method
71
would cause the actual DC capacitor voltage uncontrollable. The more the
controller output voltage exceeds the limit, the more the DC voltage deviates from
the reference DC voltage.
(3) Using the conventional control method, when the bus voltage sag makes
controller output voltage exceed the linear modulation or saturation limit even just
one time, it could trigger the conventional control approach getting into a
malfunction state and cannot return to its normal operation even after the high
voltage sag condition. Since then, the DC capacitor voltage becomes oscillating
continuously, showing the inherent deficiency of the conventional control
mechanism.
(4) During the malfunction of the conventional control mechanism, there are more
oscillations in the DC capacitor voltage and the active and reactive powers
absorbed by the STATCOM, and the current taken by the STATCOM from the
grid becomes more unbalanced during each short circuit fault occurrence.
(5) The STATCOM works properly with the proposed optimal control mechanism
whenever the bus voltage sag makes controller output voltage exceed the linear
modulation limit or not. The DC capacitor voltage is stable no matter how bad the
bus voltage sag is.
72
CHAPTER 6
LABORATORY HARDWARE EXPERIMENTAL STUDY AND COMPARISON
6.1 Introduction
This chapter describes the experimental investigation of the conventional and proposed
control methods for the grid-side converter control in renewable energy conversion and
STATCOM applications. The experiments results are recorded and analyzed, which proves that
the proposed control mechanism works well for the grid-side converter control in both
applications. The results point out that the system performance is better when the proposed
control mechanism is used.
6.2 Experimental setup
The control systems of the AC/DC/AC energy conversion and STATCOM systems are
developed by dSPACE and Matlab®/Simulink®. First, the control system models are built in
Matlab®/Simulink®. Second, the models are compiled into real-time code using Real-Time
Workshop®. ControlDesk® is an experimental software tool provided by dSPACE, which can
process the generated real-time code and run the program in the embedded DSP. The dSPACE
ADC module collects the voltage and current measurements. Then, the DSP processor runs the
designed program and the PWM generator sends the command signals to the external drive
circuits of the power converter.
The experimental setup consists of 9 parts:
73
z
Diodes module: CRYDOM EFG15F.
z
IGBT module: POWEREX PM300R060.
z
DC link capacitor: CORNELL DUBILIER DCMC902T450DG2B.
z
Power supply: Lab-Volt® 8821-20.
z
Inductor module: Lab-Volt® 8321-00 and Lab-Volt® 8325-10.
z
Voltage probe: Tektronix P5205 100MHz High Voltage Differential Probe.
z
Current probe: Tektronix A6303 current probe and Tektronix A6312 current probe.
z
Multimeter: Fluke 45 Dual Display Multimeter.
z
Oscilloscope: Tektronix TPS2024 Four Channel Digital Storage Oscilloscope.
z
Controller: dSPACE 1103.
6.3 Controller implementation
The controllers of the AC/DC/AC converter and STATCOM systems are implemented in
Matlab®/Simulink® with Real-Time Workshop. Figure 6.1 shows the controller model, which
consists of voltage and current measurements, control system, protection unit and PWM signals
generator.
The control systems are implemented using conventional and proposed control
mechanisms described in Chapters 3, 4 and 5, respectively. After the controller is implemented in
Matlab®/Simulink®, the model can be compiled into real time code by Real-Time Workshop.
Figure 6.2 shows the dSPACE interface in real time application, which consists of voltage,
current, power waveform monitors, reference command buttons and emergency stop button.
74
Fig. 6.1. Controller of the AC/DC/AC converter system
Fig. 6.2. dSPACE interface of real time application
75
The details of the control system have been described in chapter 3. The configurations of
the controller are the same as those shown in chapter 3. However, in real world, there are some
very important issues that need be considered carefully. The first issue is the voltage and current
measurements. Unlike the simulation models built in Chapter 4 and Chapter 5, the measurements
in real world are a little bit different. The voltage probe used in the experiments has a 50X
attenuation, and the dSPACE Analog to Digital Converter has a 10X attenuation. As a result, in
order to get the real voltage value, a 500X gain is necessary in the Simulink® model. Similarly, a
250X gain is added to get the real current value. The second issue is the pre-measured resistance
and inductance. In simulation models, the resistance and inductance are accurate as the defined
value. However, the actual resistance and inductance in experiments may be different with
pre-measured values obtained from the measuring equipments. These factors may affect the
design of controller parameters.
The maximum allowable current of the inductor is 3.6 A, which should be considered in
the controller design. A protection unit is added to limit the possible high current or voltage. If
the RMS current of any phase of the inductors exceeds 3 A, or the DC link voltage exceeds 150
V, the PWM signals generator will stop automatically, which cuts off the main power flow path
for the safety concerns. Also, there is a manual stop button if the operator wants to stop the
experiment manually.
76
6.4 Experiment results
6.4.1 Introduction
The experiments are conducted for the following two applications. One is the control
study of AC/DC/AC converter system normally used in renewable energy application, and the
other is the control study of STATCOM system for reactive power compensation application. In
the AC/DC/AC converter system experiment, a diodes bridge and a three-phase AC source are
used while, in the STATCOM system experiment, those components are not needed. The rest
parts of the experimental system are the same for both cases. The experiment parameters are
listed below:
Source line voltage (V)
Table 6.1. Experiment parameters
0~35
DC link capacitor (uF)
18000
DC link voltage (V)
50
Grid filter resistor (Ω)
1.4
Grid filter inductor (mH)
74
Grid line voltage (V)
20
Figure 6.3 shows a corner of the experimental system. A data cable connects the dSPACE
board with the drive circuit of the IGBT module. The cable delivers the PWM signals from the
dSPACE board to the drive circuit of the IGBT module.
77
Fig. 6.3. Experiment platform and devices
6.4.2 Experimental results for control of AC/DC/AC converter system
The performance of the AC/DC/AC converter system for two cases using the
conventional and the proposed control techniques. The results demonstrate that the proposed
control mechanism is effective in a wide system operating conditions while the conventional
control mechanism may behave improperly under some operating conditions.
In case 1, the reactive power reference is -5 Var, the DC link voltage reference is 50 V.
The case 1 demonstrates a situation that the AC/DC/AC converter system works well under both
the conventional and the proposed control methods.
78
65
DC-link voltage (V)
60
55
50
45
40
0
10
20
30
40
50
Time (s)
60
70
80
90
100
80
90
100
80
90
100
(a) DC link voltage waveform
Grid d axis current (A)
1
0.5
0
-0.5
-1
0
10
20
30
40
50
Time (s)
60
70
(b) Grid d axis current waveform
1
Grid q axis current (A)
0.8
0.6
0.4
0.2
0
-0.2
-0.4
0
10
20
30
40
50
Time (s)
60
70
(c) Grid q axis current waveform
Fig. 6.4. AC/DC/AC experiment results using conventional control mechanism under
case 1
79
Figure 6.4 shows the experiment result under case 1 using conventional control
mechanism. The DC link voltage is stable at 50 V as expected during case 1, and the grid d-q
axis currents are also stable at the expected value.
Figure 6.5 shows the experiment results of the AC/DC/AC converter system using the
proposed control mechanism for the same conditions of case 1.
65
DC-link voltage (V)
60
55
50
45
40
0
5
10
15
20
25
20
25
Time (s)
(a) DC link voltage waveform
Grid d axis current (A)
1
0.5
0
-0.5
-1
0
5
10
15
Time (s)
(b) Grid d axis current waveform
80
1
Grid q axis current (A)
0.8
0.6
0.4
0.2
0
-0.2
-0.4
0
5
10
15
20
25
Time (s)
(c) Grid q axis current waveform
Fig. 6.5. AC/DC/AC experiment results using proposed control mechanism under case 1
Comparing to the waveforms shown in figure 6.4, the waveforms shown by figure 6.5
demonstrate that the AC/DC/AC converter system performs better when the proposed control
mechanism is used. The oscillations of the DC link voltage and the d-q axis currents are much
smaller than those shown in the figure 6.4.
In case 2, there are some reactive power reference changes and source voltage change.
The purpose of the case 2 is to test the dynamic performance of the AC/DC/AC converter system
under control and to examine whether the controller can response quickly and correctly to those
changes.
Figure 6.6 shows the experiment results of the system using conventional control
mechanism under case 2.
81
100
DC-link voltage (V)
80
60
40
20
0
0
20
40
60
80
100
Time (s)
120
140
160
180
200
160
180
200
160
180
200
(a) DC link voltage waveform
Grid d axis current (A)
4
2
0
-2
-4
0
20
40
60
80
100
Time (s)
120
140
(b) Grid d axis current waveform
3
Grid q axis current (A)
2
1
0
-1
-2
-3
-4
0
20
40
60
80
100
Time (s)
120
140
(c) Grid q axis current waveform
82
Grid d axis voltage (V)
24
22
20
18
16
0
20
40
60
80
100
Time (s)
120
140
160
180
200
(d) Grid d axis voltage waveform
Fig. 6.6. AC/DC/AC experiment results using conventional control mechanism under
case 2
In case 2, the initial source voltage is 35 V, and the initial reactive power generated to the
grid is 0 Var. At t=50s, the source voltage drops to 0 V. At t=100s, the reactive power reference
changes from 0Var to -5Var, i.e., a generating reactive power to the grid. At t=150s, the source
voltage changes back to 35 V. As shown in figure 6.6, the dynamic response of the AC/DC/AC
converter system is not good when the conventional control mechanism is used. Around 20
second, there is a disturbance in the system, which made the DC link voltage and the grid current
oscillate away from the reference greatly. At each reference transition or source voltage change
point, the oscillation always occurs in both DC link voltage and grid current, and it takes long
time for the voltage and current to be stable at the expected level again.
Figure 6.7 shows the simulation results of the AC/DC/AC converter system using
conventional control mechanism under the same experimental condition used in case 2. . The
simulation time step for the controller part is the same as the sample time used in dSPACE digital
control system. The reactive power reference is 0 Var initially, and then changes to -5Var at t=35s.
The AC source voltage is 35 V initially, but changes to 0 V at t=15s, and changes back to 35 V at
83
t=50s. The only difference between the experiment and the simulation is the time scale. It is clear
that the DC link voltage and the grid d-q currents are stable and can be adjusted to the reference
value precisely in the simulation, demonstrating that the controller design for the AC/DC/AC
converter system is correct.
53
DC-link voltage (V)
52
51
50
49
48
0
10
20
30
Time (s)
40
50
60
50
60
(a) DC link voltage waveform
1.5
Grid d axis current (A)
1
0.5
0
-0.5
-1
-1.5
0
10
20
30
Time (s)
40
(b) Grid d axis current waveform
84
1.5
Grid q axis current (A)
1
0.5
0
-0.5
-1
-1.5
0
10
20
30
Time (s)
40
50
60
(c) Grid q axis current waveform
Fig. 6.7. Simulation results of the AC/DC/AC converter system using conventional
control mechanism under case 2
However, the controller using conventional control mechanism performs improperly
during some periods in the experiment under case 2. In the simulation, the grid voltage is ideal,
whose d axis component is always 20 V. However, the grid voltage in the experiment is
simulated by a Lab-Volt® power supply module. It is not as strong as in the simulation. The
actual d axis component of the grid voltage is oscillating and has a big deviation from 20 V
during the period of t=60s to t=120s in figure 6.6 (d). It may cause the significant oscillations in
DC-link voltage and actual dq axes currents. The grid voltage deviation may be caused by the
deficiency of the conventional control mechanism under huge reference change condition. Also,
the grid voltage variations may affect the function of the controller. The controller and the grid
voltage could influence each other. There are some more factors could affect the performance of
the actual controller, including inaccurate pre-measured resistance and inductance, unbalanced
three-phase grid filter or any other system condition change. Also, it is more challenging for the
conventional control mechanism to perform well due to the low ratings of various components of
the laboratory testing system.
85
DC-link voltage (V)
60
55
50
45
40
0
20
40
60
80
100
Time (s)
120
140
160
180
200
180
200
(a) DC link voltage waveform
Grid d axis current (A)
1
d axis current
reference
0.5
0
Actual d axis
current
-0.5
-1
0
20
40
60
80
100
Time (s)
120
140
160
(b) Grid d axis current waveform
0.5
Grid q axis current (A)
0.4
q axis current
reference
0.3
0.2
0.1
0
Actual q axis
current
-0.1
-0.2
0
20
40
60
80
100
Time (s)
120
140
160
180
200
(c) Grid q axis current waveform
Fig. 6.8. AC/DC/AC experiment results using proposed control mechanism under case 2
86
Figure 6.8 shows the simulation results of the AC/DC/AC converter system using the
proposed control mechanism under the same experimental condition used in case 2. Comparing
to figure 6.6, the performance of the AC/DC/AC converter system using the proposed control
mechanism is much better. The DC link voltage is stable around 50 V in the experiment. At each
transition time, the oscillation of the system is very small. The d-q axis currents can also track
the respective references precisely and quickly. At t=150s, the source voltage increases from 0 V
to 35 V, which means more active power should be delivered to the grid. The reactive power
reference remains unchanged. However, the actual q axis current decreases automatically, which
means the proposed control mechanism switching into the optimal control mode by ensuring that
the active power generated by the source can be delivered to the grid, but minimizing the
difference between the desired and actual reactive power as much as possible.
6.4.3 Experimental results for STATCOM system control
The laboratory setup of the STATCOM system is similar to that of the AC/DC/AC
converter system except no voltage source and the diode bridges are needed. Two cases are used
to evaluate the performance of the STATCOM system using the conventional and proposed
control mechanism, respectively.
The first case is to verify the STATCOM system works well under the normal operating
conditions. In case 1, the DC link voltage reference is 50 V, and the reactive power reference is
-5 Var, i.e., a generating reactive power to the grid. The DC link voltage and the grid current
oscillate a lot around 70 second due to a disturbance in the system.
87
90
DC-link voltage (V)
80
70
60
50
40
0
20
40
60
80
100
Time (s)
120
140
160
180
200
160
180
200
160
180
200
(a) DC link voltage waveform
2
Grid d axis current (A)
1.5
1
0.5
0
-0.5
-1
-1.5
0
20
40
60
80
100
Time (s)
120
140
(b) Grid d axis current waveform
1.5
Grid q axis current (A)
1
0.5
0
-0.5
-1
-1.5
0
20
40
60
80
100
Time (s)
120
140
(c) Grid q axis current waveform
Fig. 6.9. STATCOM experiment results using conventional control mechanism under case 1
88
Figure 6.9 shows the experiment results of the STATCOM system using the conventional
control mechanism under case 1 while Figure 6.10 shows the STATCOM system experiment
results using the proposed control mechanism under case 1.
DC-link voltage (V)
60
55
50
45
40
0
20
40
60
80
100
Time (s)
120
140
160
180
200
160
180
200
(a) DC link voltage waveform
Grid d axis current (A)
1
0.5
0
-0.5
-1
0
20
40
60
80
100
Time (s)
120
140
(b) Grid d axis current waveform
89
Grid q axis current (A)
1
0.5
0
-0.5
0
20
40
60
80
100
Time (s)
120
140
160
180
200
(c) Grid q axis current waveform
Fig. 6.10. STATCOM experiment results using proposed control mechanism under case 1
Comparing to the waveforms shown in figure 6.9, the DC link voltage and grid current
are always stable at the expected values in figure 6.10, demonstrating that the proposed control
mechanism works perfectly under normal operating conditions.
Similarly to Section 6.4.2, the case 2 is used to test the dynamic response of the
conventional and proposed control system under variable operating conditions.
Figure 6.11 shows the STATCOM experiment results using the conventional control
mechanism under case 2.
90
DC-link voltage (V)
80
70
60
50
40
30
20
0
20
40
60
80
100
Time (s)
120
140
(a) DC link voltage waveform
90
160
180
200
Grid d axis current (A)
3
2
1
0
-1
-2
0
20
40
60
80
100
Time (s)
120
140
160
180
200
160
180
200
160
180
200
(b) Grid d axis current waveform
3
Grid q axis current (A)
2
1
0
-1
-2
-3
0
20
40
60
80
100
Time (s)
120
140
(c) Grid q axis current waveform
Grid d axis voltage (V)
24
22
20
18
16
0
20
40
60
80
100
Time (s)
120
140
(d) Grid d axis voltage waveform
Fig. 6.11. STATCOM experiment results using conventional control mechanism under case 2
91
In case 2, the initial reactive power reference is -5 Var. The reactive power reference
changes to -2 Var around 60 second. The STATCOM system can work before the change of the
reactive power reference using the conventional control mechanism. After the change of the
reactive power reference around 60s, the DC link voltage and the grid currents start to oscillate
constantly and can not track with the expected references.
56
DC-link voltage (V)
54
52
50
48
46
0
5
10
15
20
25
30
35
25
30
35
Time (s)
(a) DC link voltage waveform
Grid d axis current (A)
0.2
0.1
0
-0.1
-0.2
-0.3
0
5
10
15
20
Time (s)
(b) Grid d axis current waveform
92
Grid q axis current (A)
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0
5
10
15
20
25
30
35
Time (s)
(c) Grid q axis current waveform
Fig. 6.12. Simulation results of the STATCOM system using conventional control
mechanism under case 2
Figure 6.12 shows the simulation results of the STATCOM system using the conventional
control mechanism for the same conditions used in case 2 of the laboratory experiment. The
reactive power reference is -5 Var, initially and changes to -2 Var at t=20s. The only difference
between the experiment and simulation is time scale. As it can be seen from figure 6.12, the
simulation results are different from the experimental results. It is clear that the controller using
the conventional control mechanism works properly in the simulation but not in the experimental
environment. In the simulation, the grid voltage is ideal, whose d axis component is always 20 V.
However, similar to the case 2 in AC/DC/AC converter experiment, the grid voltage in the
experiment is simulated by a Lab-Volt® power supply module. It is not as strong as in the
simulation. The actual grid voltage oscillates periodically from t=60s in figure 6.11 (d). It may
cause the significant oscillations in DC-link voltage and actual dq axes currents. The grid voltage
deviation may be caused by the deficiency of the conventional control mechanism under huge
reference change condition. Also, the grid voltage variations may affect the function of the
controller. The controller and the grid voltage could influence each other. There are some more
93
factors could affect the performance of the actual controller, including inaccurate pre-measured
resistance and inductance, unbalanced three-phase grid filter or any other system condition
change. Similarly, it is more challenging for the conventional control mechanism to perform well
due to the low ratings of various components of the laboratory testing system.
Figure 6.13 shows the experimental results of the STATCOM system using the proposed
control mechanism under case 2.
DC-link voltage (V)
60
55
50
45
40
0
20
40
60
80
100
Time (s)
120
140
160
180
200
(a) DC link voltage waveform
Grid d axis current (A)
1
d axis current
reference
0.5
0
Actual q axis
current
-0.5
-1
0
20
40
60
80
100
Time (s)
120
140
(b) Grid d axis current waveform
94
160
180
200
Grid q axis current (A)
1
q axis current
reference
0.5
0
Actual q axis
current
-0.5
-1
0
20
40
60
80
100
Time (s)
120
140
160
180
200
(c) Grid q axis current waveform
Fig. 6.13. STATCOM experiment results using proposed control mechanism under case 2
Comparing to the case 2 in figure 6.11, there are some differences in the test results when
the proposed control mechanism is used. The initial reactive power reference is -5 Var, i.e., a
generating reactive power to the grid. The reactive power reference changes to -2 Var around 60
second,, and changes to -9 Var around 100 second (a condition that the converter operates
beyond the linear modulation limit). Around 150 second, the reference changes to 5 Var, i.e., an
absorbing reactive power from the grid. As it is demonstrated in figure 6.13, the DC link voltage
is always stable at 50 V no matter how the reactive power reference changes. The grid d axis
current has the same performance as the DC link voltage. The grid q axis current can track the
reference change precisely and quickly at each transition time. If the reactive power reference
exceeds the linear modulation limit of the power converter, the controller turns into the optimal
control mode by limiting the reactive power output to the maximum capability of the STATCOM
system.
95
6.5 Conclusions
The experiments of the AC/DC/AC converter and STATCOM systems show the real-life
performance of the conventional and the proposed control techniques and provide a chance to
compare the simulation results with hardware experimental results.
Through the real-time hardware experiments, it is clear that the conventional control
mechanism performs well under certain operating conditions. However, the conventional control
mechanism may not work properly in a real-time laboratory environment under some specific
conditions although it may perform pretty well in Matlab®/Simulink® simulation environment
under the same conditions. It means that the conventional control mechanism is not reliable and
the performance depends on the laboratory system conditions.
For the proposed control mechanism, the experimental results demonstrate that it can
work properly both in AC/DC/AC converter and STATCOM applications no matter how the
external conditions vary. The experiment results match the computer simulation results perfectly,
which is not achieved while using conventional control mechanism. The DC link voltage can be
stable at the expected value even for extreme conditions. The reactive power output of the
systems can be limited when more active power is delivered to the grid. The perfect performance
match between simulation and experiments for the controller using the proposed control
mechanism proves that the proposed control mechanism is not sensitive for the change of
pre-measured resistance and inductance of the grid filter. The proposed controller has a better
dynamic response with any system conditions change. The stability of the whole system is
improved due to the contribution of the proposed control mechanism.
96
CHAPTER 7
SUMMARY AND FUTURE WORK
Renewable energy, a clean energy source, is rapidly growing worldwide today. To
combat global climate change, there is an urgent need to take strong and early action to tackle
climate change in order to stabilize greenhouse gas concentrations at a level that would prevent
dangerous anthropogenic interference with the climate system. Generating electricity from
renewable energy recourses can make a considerable contribution to CO2 cuts.
However, due to the intermittent nature of renewable energy sources and incompatibility
of renewable electric energy generation systems with traditional electric utility systems,
generation, delivery and management of the renewable electric energy is a great challenge to the
energy industry, which usually requires the power converters for grid integration of renewable
energy source so as to assure the delivery of the energy generated from renewable sources
efficiently.
FACTS (Flexible AC transmission system) devices have been widely used in today’s
power system. STATCOM (Static Synchronous Compensator) is one kind of FACTS devices. To
increase the power system voltage stability under variable renewable energy generation conditions,
the STATCOM is important to provide reactive power support and compensate to the grid. It
becomes more and more popular and is usually equipped with a renewable energy conversion
system nowadays.
The control technology of power converters used in renewable energy conversion and
97
STATCOM systems was developed several decades ago. Although the power converters can
work properly in most normal operating conditions with the conventional control mechanism, the
malfunction may occur during some extremely operating conditions. The malfunction of the
conventional control mechanism may cause some severe harm to the power system and devices.
Throughout the simulation and experimental analysis, this thesis obtains some important
conclusions.
ƒ
Conventional control method
1) Power converters work properly for both DC capacitor voltage and reactive power
controls if the controller output voltage does not exceed the linear modulation or the saturation
limit.
2) Whenever the reactive power control demand makes the controller output voltage go
over the linear modulation or the saturation limit, then, the actual DC capacitor voltage becomes
uncontrollable. The more the controller output voltage exceeds the limit, the more the DC voltage
deviates from the reference DC voltage.
3) After the controller output voltage exceeds the linear modulation or saturation limit even
just one time, the DC capacitor voltage becomes uncontrollable and floating with the reactive
power demand after that, showing the inherent deficiency of the conventional control mechanism.
Even when the abnormal operating condition disappears after over modulation condition, the DC
capacitor voltage is still uncontrollable and more oscillation of active and reactive power absorbed
by the grid side converter may occur. To protect the power system and devices, the whole system
may need to be shut down and reset the initial value after abnormal operating condition occurred.
4) During the malfunction of the conventional control mechanism, there are more
98
oscillations in the DC capacitor voltage and the active and reactive powers absorbed by the grid
side converter, and the current taken by the grid side converter from the grid becomes more
unbalanced during each control transition.
ƒ
Proposed control technology
1) The power converter works properly with the proposed control mechanism all the time
no matter whether the reactive power reference makes controller output voltage exceeds the linear
modulation limit or not.
2) The current taken by the grid side converter from the grid changes smoothly during each
control transition when proposed control mechanism is adopted. However, the current oscillation
is remarkably at each control transition when conventional control mechanism is adopted.
In summary, the proposed control mechanism designed in this thesis can handle normal and
abnormal operating conditions for control of grid-side converter in renewable energy conversion
and STATCOM applications. Using the proposed control approach, the DC capacitor voltage is
stable and the oscillation of current taken by the grid side converter from grid is much less than that
using the conventional control mechanism. The benefits of utilizing the proposed control
mechanism include improving system stability, improving power quality, and protecting system
devices.
For the future work, some more intelligent control approaches need to be developed to
improve the performance of the control system for the grid integration control of three-phase
DC/AC power converters. The research can also be extended to the field of machine control
utilizing the proposed control mechanism.
99
REFERENCES
1) Renewables
2007
Global
Status
http://www.ren21.net/pdf/RE2007_Global_Status_Report.pdf
Report.
Available:
2) Global Trends in Sustainable Energy Investment 2007: Analysis of Trends and Issues in the
Financing of Renewable Energy and Energy Efficiency in OECD and Developing countries.
Available:
http://sefi.unep.org/fileadmin/media/sefi/docs/publications/SEFI_Investment_Report_2007.p
df
3) Arai, J., Iba, K., Funabashi, T., Nakanishi, Y., Koyanagi, K. and Yokoyama, R., “Power
electronics and its applications to renewable energy in Japan,” Circuits and Systems
Magazine, IEEE, vol. 8, issue 3, pp. 52-66, third quarter, 2008.
4) Tan, K. and Islam, S., “Optimum control strategies in energy conversion of PMSG wind
turbine system without mechanical sensors,” IEEE Trans. on Energy Conversion, vol. 19,
issue 2, pp. 392-399, Jun. 2004.
5) Wei Li and Joos, G., “Comparison of Energy Storage System Technologies and
Configurations in a Wind Farm,” Power Electronics Specialists Conference, 2007. PESC
2007. IEEE, pp.1280-1285, 17-21, June 2007.
6) Mohd, A., Ortjohann, E., Schmelter, A., Hamsic, N. and Morton, D., “Challenges in
integrating distributed Energy storage systems into future smart grid,” IEEE International
Symposium on Industrial Electronics, 2008. ISIE 2008, pp. 1627-1632, June 30-July 2, 2008.
7) N.G. Hingorani, “Flexible AC Transmission Systems”, IEEE Spectrum, vol. 30, no. 4, pp.
41-48, 1993.
8) A. R. Bergen and V. Vittal, Power System Analysis, 2nd Ed. Upper Saddle River, NJ:
Prentice Hall, 2000.
9) L. Gyugyi, “Power electronics in electric utilities: Static VAR compensators,” Proceedings
of the IEEE, vol. 76, no. 4, pp. 483-493, Apr. 1988.
10) E. Acha, C.R. Fuerte-Esquivel, H. Ambriz-Perez, and C. Angeles-Camacho, “FACTS –
Modeling and Simulation in Power Networks,” Chichester, England: John Wiley & Sons
Inc., 2004.
100
11) K. Fujii, K. Kunomura, K. Yoshida, A. Suzuki, S. Konishi, M. Daiguji, and K. Bada,
“STATCOM applying flat-packaged IGBTs connected in series,” IEEE Trans. on Power
Electronics, vol. 20, no. 5, pp. 1125-1132, Sep. 2005
12) C. Schauder, M. Gernhardt, E. Stacey, T. Lemak, L. Gyugyi, T. W. Cease, and A. Edris,
“Operation of +100 MVAr TVA STATCON,” IEEE Trans. on Power Delivery, vol. 12, no. 4,
pp. 1805-1811, Oct. 1997.
13) Shuhui Li and Ling Xu, “PWM converter control for grid integration of wind turbines with
enhanced power quality,” Industrial Electronics, 2008. IECON 2008. 34th Annual
Conference of IEEE, Orlando, FL, pp. 2218-2224, 10-13 Nov. 2008.
14) I. Codd, “Windfarm Power Quality Monitoring and Output Comparison with EN50160,”
Proc. of the 4th Intern. Workshop on Large-scale Integration of Wind Power and
Transmission Networks for Offshore Wind Farm, 20-21 Oct. 2003, Sweden.
15) L. Xu and Y. Wang, “Dynamic Modeling and Control of DFIG Based Wind Turbines under
Unbalanced Network Conditions,” IEEE Trans. on Power Systems, vol. 22, issue 1, pp.
314-323, Feb. 2007.
16) M. P. Papadopoulos, S. A. Papathanassiou, N. G. Boulaxis, and S. T. Tentzerakis, “Voltage
quality change by grid-connected wind turbines,” in European Wind Energy Conference, pp.
783-785, Nice, France, 1999.
17) Muljadi and C. P. Butterfield, “Pitch-Controlled Variable-Speed Wind Turbine Generation,”
IEEE Trans. on Industry Applications, vol. 37, no. 1, pp. 240-246, January/February 2001.
18) Casaro, M.M., and Martins, D.C, “Behavior matching technique applied to a three-phase
grid-connected PV system,” IEEE International Conference on Sustainable Energy
Technologies, 2008. ICSET 2008, pp. 12-17, 24-27, Nov. 2008.
19) Femia, N., Petrone, G., Spagnuolo, G. and Vitelli, M., “Optimization of perturb and observe
maximum power point tracking method,” IEEE Trans. on Power Electronics, vol. 20, issue 4,
pp. 963-973, Jul. 2005.
20) Shengyi Liu and Dougal, R.A., “Dynamic multiphysics model for solar array,” IEEE Trans.
on Energy Conversion, vol. 17, issue 2, pp. 285-294, Jun. 2002.
21) Maharjan, L., and Inoue, S.; Akagi, H., “A Transformerless Energy Storage System Based on
a Cascade Multilevel PWM Converter With Star Configuration,” IEEE Trans. on Industry
Applications, vol. 44, issue 5, pp. 1621-1630, Sept. - Oct. 2008.
22) Giroux, P., Sybille, G. and Le-Huy, H., “Modeling and simulation of a distribution STATCOM
using Simulink'sPower System Blockset,” IEEE Conference on Industrial Electronics Society,
2001, IECON’01, vol. 2, pp. 990-994, Nov. 29 - Dec. 2, 2001.
101
23) Eduard Ned Mohan, Tore M. Undeland, and William P. Robbins, Power Electronics:
Converters, Applications, and Design, 3rd Edition. Hoboken, NJ: John Wiley & Sons Inc.,
2003.
24) Field
oriented
control
of
http://focus.ti.com/lit/an/bpra073/bpra073.pdf
3-phase
AC-Motors.
Available:
25) R. Pena, J.C. Clare, and G.M. Asher, “Double fed induction generator using back-to-back
PWM converters and its application to variable speed wind-energy generation,” IEE
Proc.-Electr. Power Appl., vol. 143, no. 3, pp. 231-241, May 1996.
26) C. Schauder and H. Mehta, “Vector analysis and control of advanced static VAR
compensators,” IEE Proceedings-C, vol. 140, no. 4, pp. 299-306, Jul. 1993.
27) Andreas Petersson, “Analysis, Modeling and Control of Doubly-Fed Induction Generators for
Wind Turbines,” Ph.D. dissertation, Department of Energy and Environment, Chalmers
University of Technology, Goteborg, Sweden, 2005
28) K. M.,Passino, and S. Yurkovich, Fuzzy Control, Prentice Hall, 1998.
29) P. A. Ioannou, and J. Sun, Robust Adaptive Control, Prentice Hall, Inc, 1996.
30) M.S. El-Moursi and Adel M. Sharaf, “Novel STATCOM controllers for voltage stabilisation
of wind energy scheme,” International Journal of Global Energy Issues, vol. 26, no.3/4, pp.
382-400, 2006.
31) Pablo García-González and Aurelio García-Cerrada, “Control system for a PWM-based
STATCOM,” IEEE Trans. on Power Delivery, vol. 15, no. 4, pp. 1252-1257, Oct. 2000.
32) Pranesh Rao, M. L. Crow, and Zhiping Yang, “STATCOM control for power system voltage
control applications,” IEEE Trans. on Power Delivery, vol. 15, no. 4, pp. 1311-1317, Oct.
2000.
33) P. Maibach and T. Thurnherr, “PCS 6000 STATCOM System overview and operational
experience”, Proceedings of 2008 European Wind Energy Conference and Exhibition,
Belgium, March 31 - April 3, 2008.
34) M. Tavakoli Bina, M.D. Eskandari and M. Panahlou, “Design and installation of a ±250 kVAr
D-STATCOM for a distribution substation”, Electric Power Systems Research, vol. 73, issue 3,
pp. 383-391, March 2005.
35) Z. Xi and S. Bhattacharya, “STATCOM Control and Operation with Series Connected
Transformer Based 48-pulse VSC,” Proceedings of 33rd Annual Conference of the IEEE
Industrial Electronics Society, Taipei, Taiwan, pp. 1714-1719, Nov. 5-8, 2007.
102
36) S. Li and T.A. Haskew, “Analysis of Decoupled d-q Vector Control in DFIG Back-to-Back
PWM Converter,” Proceedings of IEEE 2007 Power & Energy Society General Meeting,
Tampa FL, June 24-28, 2007.
103