control of power converter for grid integration of renewable energy conversion and statcom

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

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].

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,

2 2cos( ) cos( ) cos( )2 3 3

2 23 sin( ) sin( ) sin( )3 3

ad

bq

c

it t tii

it t t i

ω ω π ω π

ω ω π ω π

⎡ ⎤ ⎡ ⎤− +⎢ ⎥⎡ ⎤ ⎢ ⎥= ⎢ ⎥⎢ ⎥ ⎢ ⎥⎢ ⎥⎣ ⎦ − − − − + ⎢ ⎥⎣ ⎦⎢ ⎥⎣ ⎦

(3.4)

The coefficient 23

is convenient in power calculation, which will be discussed later.

The voltage transformation matrix has the same form as (3.4).

2 2cos( ) cos( ) cos( )2 3 3

2 23 sin( ) sin( ) sin( )3 3

ad

bq

c

vt t tvv

vt t t v

ω ω π ω π

ω ω π ω π

⎡ ⎤ ⎡ ⎤− +⎢ ⎥⎡ ⎤ ⎢ ⎥= ⎢ ⎥⎢ ⎥ ⎢ ⎥⎢ ⎥⎣ ⎦ − − − − + ⎢ ⎥⎣ ⎦⎢ ⎥⎣ ⎦

(3.5)

The inverse transformation from d, q system to a, b, c system can be expressed as

following.

cos( ) sin( )2 2 2cos( ) sin( )3 3 3

2 2cos( ) sin( )3 3

ad

bq

c

t tii

i t ti

it t

ω ω

ω π ω π

ω π ω π

⎡ ⎤⎢ ⎥−

⎡ ⎤ ⎢ ⎥ ⎡ ⎤⎢ ⎥ ⎢= − − − ⎥ ⎢ ⎥⎢ ⎥ ⎢ ⎥ ⎣ ⎦⎢ ⎥ ⎢ ⎥⎣ ⎦⎢ ⎥+ − +⎣ ⎦

(3.6)

cos( ) sin( )2 2 2cos( ) sin( )3 3 3

2 2cos( ) sin( )3 3

ad

bq

c

t tvv

v t tv

vt t

ω ω

ω π ω π

ω π ω π

⎡ ⎤⎢ ⎥−

⎡ ⎤ ⎢ ⎥ ⎡ ⎤⎢ ⎥ ⎢= − − − ⎥ ⎢ ⎥⎢ ⎥ ⎢ ⎥ ⎣ ⎦⎢ ⎥ ⎢ ⎥⎣ ⎦⎢ ⎥+ − +⎣ ⎦

(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 , are the d

and q components of the converter output voltages in a dq rotating reference frame, and 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 and

respectively. Therefore, the amplitude modulation ratio of PWM generator is

1dV 1qV

dcV

_ 1 2 /d norm d dcv V= ⋅ V

V_ 1 2 /q norm q dcv V= ⋅

2 2_ _a d norm q normm v v= + , and the converter output phase peak voltage can be extracted.

2a dc

convm VV = (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 fR and the inductance is fL . 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).

1

1

1

a a a

b f b f b b

c c c

v i idv R i L i vdt

v i i

⎡ ⎤ ⎡ ⎤ ⎡ ⎤ ⎡ ⎤⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥= + +⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥⎣ ⎦ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦

a

c

v

v (3.9)

Appling d, q reference frame, (3.9) becomes (3.10),

1

1

d d d qf f s f

q q q d

v i i idR L Lv i i idt

ω−⎡ ⎤ ⎡ ⎤ ⎡ ⎤ ⎡ ⎤⎡ ⎤

= + + +⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥⎢ ⎥⎣ ⎦⎣ ⎦ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦

d

q

vv

(3.10)

where sω is the angular frequency of the grid voltage. With space vectors theory, (3.10) can be

written as (3.11),

1dq

dq dq dq dqf f s fd iv R i L j L i vdt

ω= ⋅ + + ⋅ + (3.11)

where , and are the instantaneous space vectors of grid voltage, line current and

converter output voltage respectively.

dqv dqi 1dqv

In steady-state condition, the derivative part can be removed, and (3.11) becomes (3.12),

1dq dqdq dqf s fV R I j L I Vω= ⋅ + ⋅ + (3.12)

17

where and are the steady-state grid side converter equivalent output voltage and the

gird equivalent voltage in dq reference frame, respectively. The current

1dqV dqV

dqI 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.

1d d f q s fV I R I L dVω= − + + (3.13)

1q q f d sV I R I fLω= − − (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

Grid side converter

C

+

-

PgPm

dcvdcimi gi

Fig. 3.6. DC-link model

In figure 3.6, the DC-link voltage is , the capacitance is , the power flows from a

renewable source to the machine side converter is and the power flows from grid to the grid

side converter is

dcv C

mP

gP .

The energy stored in the capacitor is given by equation (3.15).

18

212dc dcW Cv= (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 gP and the power flow from machine

side . The relationship can be expressed as equation (3.16). mP

dcg m

dW P Pdt

= + (3.16)

Extracting the derivative part, then (3.16) becomes (3.17).

dcdc g m

dvCv P Pdt

= + (3.17)

From equation (3.17), it is clear that the constant DC-link voltage requires g mP P− = , 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 , and 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

1av 1bv 1cv

19

source , and . The grid filter consists of a resistor and an inductor, in which av bv cv fR and fL

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 is zero. The instantaneous active power transmitted from the grid to the grid side

converter can be calculated as equation (3.18).

qv

*Re( )dq dqa a b b c c d d q q d dp v i v i v i v i v i v i v i= + + = = + = (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).

*

Im( )dq dq q d d q d qq v i v i v i v= = − = i− (3.19)

In terms of the circuit shown in figure 3.7, since the d axis is aligned with grid voltage

position, the grid voltage can be written as 0dq dV V j= + . The output voltage of the grid side

converter can be written as , the current flowing from grid to converter is

described in equation (3.20).

1 1dq dV V jV= + 1q

1 1 1 1 12 2 2

( ) [( )dq dq d d f q s f d d s f q fdq

f s f f s f

V V R V L j V V L V RV VIR j L R L

]ω ωω ω

− − − − +−= =

+ + (3.20)

In terms of the steady state calculation, the active and reactive power can be expressed as

following.

* 1 12 2 2

[( ) ]Re( ) d d d f q s f

dqdq

f s f

V V V R V LP V I

R Lω

ω− −

= =+

(3.21)

20

* 1 12 2 2

[( ) ]Im( ) d d d s f q f

dqdq

f s f

V V V L V RQ V I

R Lωω

− += =

+ (3.22)

Since the grid filter resistance is much smaller than the inductance (i.e., f s fR Lω<< ),

the active power is mainly controllable by , and the reactive power is mainly controllable by

.

1qV

1dV

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

PI

, ,a b cv

, ,a b ci

,vα β

,iα β

eθ

dv+

1dv

1qv

1vα

1vβ

1, 1, 1a b cvdv′

qv′

_d refi

_q refi_dc refV dcV

diqi

fR

fL

CdcV−

+

+PI

−

−

PI +−

+−

+ −

eje θ−

eje θ−

eje θ−

s fLω

s fLω

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 fR and fL respectively. The currents in dq reference frame flowing through grid filter are

and 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).

di qi

1 ( )dd f d f s f q

div R i L L idt

ω= − + + + dv (3.23)

1 ( )qq f q f s

div R i L L

dtω= − + − f qi (3.24)

The items in bracket can be rewritten as dv′ and qv′ 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 dv′ should control the reactive

power and the q axis voltage 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),

qv′

1( )cf f

G sR L s

=+

(3.25)

the feedback current control loop is shown in figure 3.9,

1

f fR L s+pwmkPI+

−refi iv′

Fig. 3.9. Current control loop

where is the gain of the grid side converter. pwmk

The current loop controller is a typical PID controller, which can be expressed as

equation (3.26).

( ) ip p

KG s Ks

= + (3.26)

The switching frequency of the PWM converter is sf , which is equal to trif . The frequency

response design method is applied to design the controller. The crossover frequency is cf , which

is two order smaller than the switching frequency. The desired phase margin pmφ 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 pK and of the controller. After the controller output voltage

signals are generated, the actual drive voltage signals need to add some compensation items.

iK

( ) pwmip

f f

KKKs R L s

1+ =+

(3.27)

arg ( ) 180pwmip

f f

KKKs R L s pmφ

⎡ ⎤+ = −⎢ ⎥

+⎢ ⎥⎣ ⎦+

dv

(3.28)

The dq axis drive voltage signals are generated through equations (3.29) and (3.30)

respectively.

'1d d s f qv v L iω= − + + (3.29)

'1q q s fv v Lω= − − qi (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 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).

mi

dc g d dv i v i=

32 2

ad

mv = dcv

32 2

ag d

mi = i (3.31)

dcg m

dvC idt

i= +

( ) 3( )( ) 2 2

dc ad

d

v s mG si s Cs

= =

24

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.

32 2

amC sPI+

−

_dc refv dcv_d refi

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 pK and can

be solved through equations (3.32) and (3.33).

iK

3( )2 2

i ap

K mKs Cs

1+ = (3.32)

3arg ( ) 1802 2

i ap

K mKs Cs pmφ

⎡ ⎤+ = −⎢ ⎥

⎣ ⎦+ (3.33)

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.

3/2

3/2

2/3di′

diqi

, ,a b cv

, ,a b ci

fR

fL

CdcV

PWM

Voltageangle

calculation

,vα β

eθ

dv

,iα β

1, 1, 1a b cv1vα1vβ

1dv

1qvPI

PI

PI+−

+−

+

−

−−

−

+

+

+

dcV

qi ′

_dc refV

_q refi

_d refieje θ−

eje θ−

eje θ−

fR

s fLω

s fLω

fR

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 and respectively. The transformation

between d-q output current signals from the controller, and , and the dq control voltages

driving the converter are calculated by equations (3.34) and (3.35).

'di

'qi

'di

'qi

' '1d f d s f qv R i L iω dv= − + + (3.34)

(3.35) '1q f d sv R i Lω= − − '

f qi

26

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,

PI+−

re fi i'i 'v

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 and , 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

1dv 1qv

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: actual refQ Q−

Subject to:

_ _dc actual dc refV V=

2 2

3d q

rated

I II

+≤ ,

2 21 12( )

22 2 3 1

d q

conv

dc dc

V VVm

V V

+

= = ≤

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.

Ai

Bi

Ci

dcV dcV

Ai

Bi

Ci

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 and

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.

_d refi _q refi

29

PI

, ,a b cv

, ,a b ci

,vα β

,iα β

eθ

dv+

1dv

1qv

1vα

1vβ

1, 1, 1a b cvdv′

qv′

_d refi

_q refi

diqi

dcV−

+

+PI

−

−

+

−+−

eje θ−

eje θ−

eje θ−

mR

mLs mLω

s mLω

Fig. 3.14. Conventional control scheme of the machine side converter

di′

diqi

, ,a b cv

, ,a b ci

mR

mL

dcV

,vα β

eθ

dv

,iα β


1dv

1qvmRPI

PI

+

−

+

−

−−

−

+

+

+

qi ′

s mLω

_q refi

_d refieje θ−

eje θ−

eje θ−

s mLω

mR

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). signal generation in proposed control system 1dV

35

(b). signal generation in proposed control system 1qV

Fig. 4.4. and signals generation blocks in proposed control system 1dV 1qV

Figure 4.4 (a) and (b) show the and signals generated from the d and q axis

currents, and , using equations (3.34) and (3.35).

1dV 1qV

'di

'qi

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 and signals is different from that of the 1dV 1qV

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). signal generation in conventional control system 1dV

(b). signal generation in conventional control system 1qV

Fig. 4.6. and signals generation blocks in conventional control system 1dV 1qV

4.2.2.3 PWM signals generation module

After the voltage signals and 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 and have to be transformed

into PWM pulse signals and applied on the gate circuit of the IGBT converter module.

1dV 1qV

1dV 1qV

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 , and DC-link voltage

. The d and q axis components of PWM pulse signals are generated using equations (4.1) and

(4.2).

1dV 1qV

dcV

1_

2 dd norm

dc

VvV

= (4.1)

1_

2 qq norm

dc

Vv

V= (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

Output original dq components

No Yes

No Yes

Output updated dq components

Output updated dq components

END

* 2_ _1d norm q normv v= −

_d normvCalculate new

>= 32

Calculate

2 2_ _d norm q normv v+

_32q normv >=

_*_ 2 2

_ _

d normd norm

d norm q norm

vv

v v=

+_*

_ 2 2_ _

q normq norm

d norm q norm

vv

v v=

+

Calculate new dq components

(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

0 2 4 6 8 10 12900

1000

1100

1200

1300

1400

1500

1600

1700

1800

Time (s)

DC

-link

vol

tage

(V)

(a) DC-link voltage waveform

0 2 4 6 8 10 12-200

-150

-100

-50

0

50

100

150

200

Time (s)

Grid

pow

er (k

W/k

Var

) Reactive power

Active power

(b) Active and reactive power waveform

0 2 4 6 8 10 12-300

-250

-200

-150

-100

-50

0

50

100

Time (s)

Grid

dq

curre

nt (A

) q axis current

d axis current

(c) Grid dq axis current waveform

45

6 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.1-200

-150

-100

-50

0

50

100

150

200

Time (s)

Grid

cur

rent

(A)

(d) Grid three phase current waveform

Figure 4.12. Performance of renewable energy conversion system using conventional control mechanism under case 1



renewable energy conversion system using the proposed control mechanism under case 1.

0 2 4 6 8 10 121000

1100

1200

1300

1400

1500

1600

1700

Time (s)

DC

-link

vol

tage

(V)


46

0 2 4 6 8 10 12-400

-300

-200

-100

0

100

200

300

400

500

Time (s)

Grid

pow

er (k

W/k

Var

)

Reactive power

Active power


0 2 4 6 8 10 12-300

-200

-100

0

100

200

Time (s)

Grid

dq

curre

nt (A

)

q axis current

d axis current


6 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.1-200

-150

-100

-50

0

50

100

150

200

Time (s)

Grid

cur

rent

(A)

(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.



renewable energy conversion system using the conventional control mechanism under case 2.

0 2 4 6 8 10 12900

1000

1100

1200

1300

1400

1500

1600

1700

1800

Time (s)

DC

-link

vol

tage

(V)


48

0 2 4 6 8 10 12-300

-200

-100

0

100

200

300

Time (s)

Grid

pow

er (k

W/k

Var

)


Reactive power

Active power

0 2 4 6 8 10 12-400

-300

-200

-100

0

100

200

Time (s)

Grid

dq

curre

nt (A

) q axis current

d axis current


6 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.1-400

-300

-200

-100

0

100

200

300

400

Time (s)

Grid

cur

rent

(A)


Figure 4.14. Performance of renewable energy conversion system using conventional control mechanism under case 2

49


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.

0 2 4 6 8 10 121000

1100

1200

1300

1400

1500

1600

1700

Time (s)

DC

-link

vol

tage

(V)


0 2 4 6 8 10 12-300

-200

-100

0

100

200

Time (s)

Grid

pow

er (k

W/k

Var

)

Reactive power

Active power


50

0 2 4 6 8 10 12-500

-400

-300

-200

-100

0

100

200

Time (s)

Grid

dq

curre

nt (A

)

q axis current

d axis current


6 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.1

-300

-200

-100

0

100

200

300

Time (s)

Grid

cur

rent

(A)


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 fR and an inductor fL . 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 represents

the voltage over the capacitor C, the resistor

dcV

pR represents the power loss in the converter and

the DC circuit. The voltages , , and represent the three-phase output voltage of the

PWM converter, and the voltages , , and 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.

1av 1bv 1cv

av bv cv

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.

PI

, ,a b cv

, ,a b ci

,vα β

,iα β

eθ

dv+

1dv

1qv

1vα1vβ

1, 1, 1a b cvdv′

qv′

_d refi

_q refi_dc refV dcV

diqi

R

L

pRC

dcV

Lω

−

+

+PI

−

−

PI +−

+−

+ −

Lω

eje θ−

eje θ−

eje θ−

PI_bus refV +−

busV _q refi

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

3/2

3/2

2/3di′

diqi

, ,a b cv

, ,a b ci

R

L

pRC

dcV

PWM

Voltageangle

calculation

,vα β

eθ

dv

,iα β


1dv

1qvR

R

PI

PI

PI+−

+−

+

−

−−

−

+

+

+

dcV

qi ′

Lω

Lω

_dc refV

_q refi

_d refieje θ−

eje θ−

eje θ−PI

_bus refV +−

busV _q refi

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

0 1 2 3 4 5 6 7 8 9 101000

1050

1100

1150

1200

1250

1300

1350

Time (s)

DC

cap

acito

r vol

tage

(V)

(a) DC capacitor voltage waveform

0 1 2 3 4 5 6 7 8 9 10-300

-200

-100

0

100

200

300

Time (s)

Grid

pow

er (k

W/k

Var

) Reactive power

Active power


0 1 2 3 4 5 6 7 8 9 10-300

-200

-100

0

100

200

300

Time (s)

Grid

dq

curre

nt (A

)

q axis current

d axis current


Fig. 5.8. Performance of STATCOM using conventional control mechanism in reactive power compensation mode under case 1

60

0 1 2 3 4 5 6 7 8 9 101000

1050

1100

1150

1200

1250

1300

1350

Time (s)

DC

cap

acito

r vol

tage

(V)


0 1 2 3 4 5 6 7 8 9 10-300

-200

-100

0

100

200

300

Time (s)

Grid

pow

er (k

W/k

Var

) Reactive power

Active power


0 1 2 3 4 5 6 7 8 9 10-300

-200

-100

0

100

200

300

Time (s)

Grid

dq

curre

nt (A

)

q axis current

d axis current


Fig. 5.9. Performance of STATCOM using proposed control mechanism in reactive power compensation mode under case 1

61


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.

0 2 4 6 8 10 12 14 16800

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

Time (s)

DC c

apac

itor v

olta

ge (V

)


62

0 2 4 6 8 10 12 14 16-600

-400

-200

0

200

400

600

Time (s)

Grid

pow

er (k

W/k

Var

)Active power

Reactive power


10 10.01 10.02 10.03 10.04 10.05 10.06 10.07 10.08 10.09 10.1-600

-400

-200

0

200

400

600

Time (s)

Grid

cur

rent

(A)

(c) Grid current waveform

Fig. 5.10. Performance of STATCOM using conventional control mechanism in reactive power compensation mode under case 2


power waveforms and grid d and q current waveforms of the STATCOM using the proposed

control mechanism.

63

0 2 4 6 8 10 12 14 161100

1150

1200

1250

1300

1350

1400

Time (s)

DC c

apac

itor v

olta

ge (V

)


0 2 4 6 8 10 12 14 16-400

-300

-200

-100

0

100

200

Time (s)

Grid

pow

er (k

W/k

Var

)

Active power

Reactive power


10 10.01 10.02 10.03 10.04 10.05 10.06 10.07 10.08 10.09 10.1-300

-200

-100

0

100

200

300

Time (s)

Grid

cur

rent

(A)

(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.



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)


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


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)


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


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)




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)


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


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)




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)


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


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)




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.



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)


0 5 10 15 20 25-1

-0.5

0

0.5

1

Time (s)

Grid

d a

xis

curre

nt (A

)


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

)


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)


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

)


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

)


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)


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)


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)


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)


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


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)


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

)


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

)


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)


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

)


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

)


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)


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

)


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

)


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)


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

)


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

)


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)


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



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



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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