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
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Control of power converter for grid integration of renewable energy conversion and STATCOM
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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
1.1 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
4.2 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
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].
Transformer
Grid
=
≈
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
C
+
-
Vdc
ia2
ib2
ic2
ia1
ib1
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 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 is defined as
triV
am
controla
tri
VmV
= (3.1)
12
where is the amplitude of the triangular waveform, and is the amplitude of the
sinusoidal control signal. The frequency modulation ratio
triV controlV
fm is defined as
trif
control
fmf
= (3.2)
where trif is the frequency of the triangular waveform (also called carrier frequency), and
is the frequency of the sinusoidal control signal [23]. controlf
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 , , are the three phase instantaneous currents, then, the complex
current is defined as
ai bi ci
2s a bi i i icα α= + + (3.3)
where 23
je
πα = and
22 3
je
πα
−= , represent the spatial operators.
13
si iα
iβ
β
αa
b
c
Fig. 3.3. Clarke transformation
Figure 3.3 shows the Clarke transformation, where α axis and a axis are in the same
direction. The complex current si 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.
si
β
αd
q
diqitθ ω=
Fig. 3.4. Park transformation
In Park transformation, the d axis is aligned with grid voltage position. Park
transformation is a projection, which projects si onto dq rotating orthogonal axes. Figure 3.4
shows Park transformation, the dq coordinates system is a rotating system, where tθ ω= is the
grid voltage position.
14
As a result, these two transformations can be combined and written as a matrix form,
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.
2 2.5 3 3.5 4 4.5 5 5.5 61100
1150
1200
1250
1300
1350
Time (s)
DC
cap
acito
r vol
tage
(V)
(a) DC capacitor voltage waveform
66
2 2.5 3 3.5 4 4.5 5 5.5 6-200
-150
-100
-50
0
50
100
150
200
Time (s)
Grid
pow
er (k
W/k
Var
) Active power
Reactive power
(b) Active and reactive power waveform
2 2.5 3 3.5 4 4.5 5 5.5 60.7
0.8
0.9
1
1.1
1.2
Time (s)
Grid
vol
tage
(pu)
Bus voltage with STATCOM
Bus voltage without STATCOM
(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
2 2.5 3 3.5 4 4.5 5 5.5 61100
1150
1200
1250
1300
1350
Time (s)
DC
cap
acito
r vol
tage
(V)
(a) DC capacitor voltage waveform
2 2.5 3 3.5 4 4.5 5 5.5 6-200
-150
-100
-50
0
50
100
150
200
Time (s)
Grid
pow
er (k
W/k
Var
) Active power
Reactive power
(b) Active and reactive power waveform
2 2.5 3 3.5 4 4.5 5 5.5 60.7
0.8
0.9
1
1.1
1.2
Time (s)
Grid
vol
tage
(pu)
Bus voltage with STATCOM
Bus voltage without STATCOM
(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.
2 2.5 3 3.5 4 4.5 5 5.5 6400
600
800
1000
1200
1400
1600
1800
Time (s)
DC
cap
acito
r vol
tage
(V)
(a) DC capacitor voltage waveform
2 2.5 3 3.5 4 4.5 5 5.5 6-400
-300
-200
-100
0
100
200
300
400
Time (s)
Grid
pow
er (k
W/k
Var
) Active power
Reactive power
(b) Active and reactive power waveform
69
2 2.5 3 3.5 4 4.5 5 5.5 6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
Time (s)
Grid
vol
tage
(pu)
Bus voltage with STATCOM
Bus voltage without STATCOM
(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.
2 2.5 3 3.5 4 4.5 5 5.5 61100
1150
1200
1250
1300
1350
Time (s)
DC
cap
acito
r vol
tage
(V)
(a) DC capacitor voltage waveform
70
2 2.5 3 3.5 4 4.5 5 5.5 6-300
-200
-100
0
100
200
Time (s)
Grid
pow
er (k
W/k
Var
)Active power
Reactive power
(b) Active and reactive power waveform
2 2.5 3 3.5 4 4.5 5 5.5 60.6
0.7
0.8
0.9
1
1.1
Time (s)
Grid
vol
tage
(pu)
Bus voltage with STATCOM
Bus voltage without STATCOM
(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
Diodes module: CRYDOM EFG15F.
IGBT module: POWEREX PM300R060.
DC link capacitor: CORNELL DUBILIER DCMC902T450DG2B.
Power supply: Lab-Volt® 8821-20.
Inductor module: Lab-Volt® 8321-00 and Lab-Volt® 8325-10.
Voltage probe: Tektronix P5205 100MHz High Voltage Differential Probe.
Current probe: Tektronix A6303 current probe and Tektronix A6312 current probe.
Multimeter: Fluke 45 Dual Display Multimeter.
Oscilloscope: Tektronix TPS2024 Four Channel Digital Storage Oscilloscope.
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:
Table 6.1. Experiment parameters Source line voltage (V) 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
0 10 20 30 40 50 60 70 80 90 10040
45
50
55
60
65
Time (s)
DC
-link
vol
tage
(V)
(a) DC link voltage waveform
0 10 20 30 40 50 60 70 80 90 100-1
-0.5
0
0.5
1
Time (s)
Grid
d a
xis
curre
nt (A
)
(b) Grid d axis current waveform
0 10 20 30 40 50 60 70 80 90 100-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Time (s)
Grid
q a
xis
curre
nt (A
)
(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.
0 5 10 15 20 2540
45
50
55
60
65
Time (s)
DC
-link
vol
tage
(V)
(a) DC link voltage waveform
0 5 10 15 20 25-1
-0.5
0
0.5
1
Time (s)
Grid
d a
xis
curre
nt (A
)
(b) Grid d axis current waveform
80
0 5 10 15 20 25-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Time (s)
Grid
q a
xis
curre
nt (A
)
(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
0 20 40 60 80 100 120 140 160 180 2000
20
40
60
80
100
Time (s)
DC
-link
vol
tage
(V)
(a) DC link voltage waveform
0 20 40 60 80 100 120 140 160 180 200-4
-2
0
2
4
Time (s)
Grid
d a
xis
curre
nt (A
)
(b) Grid d axis current waveform
0 20 40 60 80 100 120 140 160 180 200-4
-3
-2
-1
0
1
2
3
Time (s)
Grid
q a
xis
curre
nt (A
)
(c) Grid q axis current waveform
82
0 20 40 60 80 100 120 140 160 180 20016
18
20
22
24
Time (s)
Grid
d a
xis
volta
ge (V
)
(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.
0 10 20 30 40 50 6048
49
50
51
52
53
Time (s)
DC
-link
vol
tage
(V)
(a) DC link voltage waveform
0 10 20 30 40 50 60-1.5
-1
-0.5
0
0.5
1
1.5
Time (s)
Grid
d a
xis
curr
ent (
A)
(b) Grid d axis current waveform
84
0 10 20 30 40 50 60-1.5
-1
-0.5
0
0.5
1
1.5
Time (s)
Grid
q a
xis
curr
ent (
A)
(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
0 20 40 60 80 100 120 140 160 180 20040
45
50
55
60
Time (s)
DC
-link
vol
tage
(V)
(a) DC link voltage waveform
0 20 40 60 80 100 120 140 160 180 200-1
-0.5
0
0.5
1
Time (s)
Grid
d a
xis
curre
nt (A
) d axis current reference
Actual d axis current
(b) Grid d axis current waveform
0 20 40 60 80 100 120 140 160 180 200-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
Time (s)
Grid
q a
xis
curre
nt (A
)
q axis current reference
Actual q axis current
(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
0 20 40 60 80 100 120 140 160 180 20040
50
60
70
80
90
Time (s)
DC
-link
vol
tage
(V)
(a) DC link voltage waveform
0 20 40 60 80 100 120 140 160 180 200-1.5
-1
-0.5
0
0.5
1
1.5
2
Time (s)
Grid
d a
xis
curre
nt (A
)
(b) Grid d axis current waveform
0 20 40 60 80 100 120 140 160 180 200-1.5
-1
-0.5
0
0.5
1
1.5
Time (s)
Grid
q a
xis
curre
nt (A
)
(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.
0 20 40 60 80 100 120 140 160 180 20040
45
50
55
60
Time (s)
DC
-link
vol
tage
(V)
(a) DC link voltage waveform
0 20 40 60 80 100 120 140 160 180 200-1
-0.5
0
0.5
1
Time (s)
Grid
d a
xis
curre
nt (A
)
(b) Grid d axis current waveform
89
0 20 40 60 80 100 120 140 160 180 200-0.5
0
0.5
1
Time (s)
Grid
q a
xis
curre
nt (A
)
(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.
0 20 40 60 80 100 120 140 160 180 20020
30
40
50
60
70
80
90
Time (s)
DC
-link
vol
tage
(V)
(a) DC link voltage waveform
90
0 20 40 60 80 100 120 140 160 180 200-2
-1
0
1
2
3
Time (s)
Grid
d a
xis
curre
nt (A
)
(b) Grid d axis current waveform
0 20 40 60 80 100 120 140 160 180 200-3
-2
-1
0
1
2
3
Time (s)
Grid
q a
xis
curre
nt (A
)
(c) Grid q axis current waveform
0 20 40 60 80 100 120 140 160 180 20016
18
20
22
24
Time (s)
Grid
d a
xis
volta
ge (V
)
(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.
0 5 10 15 20 25 30 3546
48
50
52
54
56
Time (s)
DC
-link
vol
tage
(V)
(a) DC link voltage waveform
0 5 10 15 20 25 30 35-0.3
-0.2
-0.1
0
0.1
0.2
Time (s)
Grid
d a
xis
curre
nt (A
)
(b) Grid d axis current waveform
92
0 5 10 15 20 25 30 350
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Time (s)
Grid
q a
xis
curre
nt (A
)
(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.
0 20 40 60 80 100 120 140 160 180 20040
45
50
55
60
Time (s)
DC
-link
vol
tage
(V)
(a) DC link voltage waveform
0 20 40 60 80 100 120 140 160 180 200-1
-0.5
0
0.5
1
Time (s)
Grid
d a
xis
curre
nt (A
)
d axis current reference
Actual q axis current
(b) Grid d axis current waveform
94
0 20 40 60 80 100 120 140 160 180 200-1
-0.5
0
0.5
1
Time (s)
Grid
q a
xis
curre
nt (A
)
q axis current reference
Actual q axis current
(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.
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.
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
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