DEVELOPMENT OF 3-PHASE TNVERTER WITH PID VOLTAGE CONTROL
MUHAMMAD FADDTL BIN AHMAD REBUDI
This project report is submitted in partial fulfilment o f the requirement for the award
of the Degree of Master of Electrical Engineering
Faculty o f Electrical and Electronic Engineering
Universiti Tun Hussein Onn Malaysia
JULY, 2014
ABSTRACT
The inverter converts electrical signal from DC to AC by using power electronic
circuits. Inverter output is square waves and a filter is needed to change the output to
a sinusoidal wave. A control method is implemented in this project to create stable
sinusoidal voltage amplitude. The benefit of using control method is the inverter can
be adjusted to adapt to a variety of loads and able to mitigate any small disturbance
in the system. PID controller is chosen for this project because the PID controller can
solve a very complex control for nonlinear element. The PID controller is deveioped
in Matlab Simulink simulation and coded into TMS320F28335 board. In
TMS320F28335 board, the desired voltages are compared with the output voltages to
get the error signal that will go into the PID controller. SPWM signal is created by
using the correction signal from PID controller and fed to the three-phase inverter
with filter to give a sinusoidal output. Based on the result obtained, the three-phase
inverter has successfully created a stable sinusoidal voltage output for any given
voltage references by utilizing the ADC module and the PID controller.
Penyongsang menukarkan isyarat elektrik dari DC ke AC dengan menggunakan litar
elektronik kuasa. Keluaran pengongsang adalah gelombang berbentuk segiempat dan
penapis diperlukan untuk menukar keluaran kepada gelombang berbentuk sinusoidal.
Satu kaedah kawalan perlu digunakan di dalam projek ini untuk mencipta voltan
sinusoidal. Manfaat menggunakan kaedah kawalan adalah penyongsang boleh
diselaraskan untuk menyesuaikan diri dengan pelbagai beban dan dapat
mengurangkan apa-apa gangguan kecil dalam sistem. PID telah dipilih untuk projek
ini kerana PID boleh menyelesaikan masalah yang kompleks. PID telah dibangunkan
menggunakan Matlab Sirnulink dan dimuat turun ke dalam papan TMS320F28335.
Voltan yang dikehendaki dibezakan dengan voltan k e l u m untuk mendapatkan
isyarat yang salah untuk dimasukkan ke dalam PID. Isyarat SPWM dicipta dengan
menggunakan isyarat pembetulan dari PID clan disambung kepada penyongsang tiga
fasa dengan penapis untuk memberi keluaran berbentuk sinusoidal. Berdasarkan
keputusan yang diperolehi, penyongsang tiga fasa telah berjaya mewujudkan output
voltan sinusoidal yang stabil bagi setiap voltan yang dikehendaki dengan
menggunakan modul ADC dan kaedah kawalan PJD.
i
TABLE OF CONTENTS
TITLE
PAGES
TITLE PAGE
i
DECLARATION
ii
ACKNOWLEDGEMENTS
iii
DEDICATION
iv
ACKNOWLEDGEMENT
v
ABSTRACT
vi
TABLE OF CONTENTS
viii
LIST OF TABLES
xii
LIST OF FIGURES
xiii
LIST OF SYMBOLS AND ABREVIATION
xv
LIST OF APPENDICES
xvi
ii
CHAPTER 1
CHAPTER 2
INTRODUCTION
1
1.1
Project Background
1
1.2
Problem Statement
2
1.3
Objective
3
1.4
Scope of Project
3
LITERATURE REVIEW
4
2.1
Inverter
4
2.1.1
Three Phase Voltage Source Inverter
4
2.1.2 PWM
6
2.1.3
SPWM
7
2.2
Distribution Generation
8
2.2.1
Introduction
8
2.2.2 Fuel cells (FCs)
9
2.2.3 Micro-turbines (MT)
10
2.2.4 Photovoltaic systems (PVs)
11
2.2.5 Wind energy conversion system
12
2.2.6 Small hydro-turbines
13
iii
CHAPTER 3
CHAPTER 4
2.2.7 Biomass
13
2.3
14
Controller Type
2.3.1 The Three Element of PID controller
15
2.4
20
IGBT and Mosfet
METHODOLOGY
23
3.1
Introduction
23
3.2
Block Diagram
23
3.3
Project Flowchart
24
3.4
TMS320F28335
25
3.5
Three Phase Inverter
26
3.6
Driver Circuit
26
3.7
ADC module TI board
29
3.8
Voltage sensor technique
30
3.9
PID Controller
32
3.10
Matlab Simulink
34
RESULT AND DISCUSSION
37
4.1
37
Simulation for Open Loop System
iv
4.2
Simulation Closed Loop
40
4.3
Hardware Open Loop Result
42
4.4
Hardware Closed Loop Result
45
4.4.1 20V Voltage reference
48
4.4.2 Comparison for Different Voltage
51
Reference
CHAPTER 5
CONCLUSION
53
5.1 Conclusion
53
5.2 Recommendation
54
REFERENCES
55
APPENDICES
58
v
LIST OF TABLES
3.1
Component for Inverter
27
3.2
Component for Driver Circuit
29
3.3
Component List for Voltage Sensor
31
4.1
PID Tuning Summation for Simulation
40
4.2
PID Tuning Table for Hardware
46
4.3
Hardware Result for ADC input and SPWM
51
4.4
Hardware Result for Inverter before and after Filtering
52
vi
LIST OF FIGURES
2.1
3 Phase Voltage Source Inverter
5
2.2
PWM waveform
6
2.3
SPWM waveform
7
2.4
Distributed Generation Technologies
8
2.5
Fuel Cell System
10
2.6
Micro Turbine System
11
2.7
Wind Energy Conversion System
12
2.8
Biomass Source
14
2.9
PI Controller in Automatic Reset Configuration
16
2.10
Block Diagram of a PID Controller in a Feedback Loop
19
3.1
Block Diagram of The Project
23
3.2
Flowchart of the Project
24
3.3
TMS320F28335 board
26
3.4
Three phase inverter
26
3.5
3 input 6 output driver circuits PCB design
28
3.6
3 input 6 output driver circuit
28
3.7
Voltage Divider and Voltage Shifter Design
30
3.8
Voltage Divider and Voltage Shifter PCB Design
31
3.9
PID Control System Block Diagram
32
3.10
SPWM Generation for Three Phase Inverter
33
3.11
Target Preference Block and Setting
34
3.12
Digital Output Block and Setting
35
3.13
ADC Block and Setting
36
4.1
Inverter Circuit Simulations for Open Loop
38
4.2
SPWM Waveform for Three Phase Inverter
38
4.3
Simulation Output of Three Phase Inverter before Filter
39
vii
4.4
Simulation Output of Three Phase Inverter after Filter
39
4.5
Inverter Circuit Simulations for Closed Loop
40
4.6
Output Voltage for Close Loop
41
4.7
Open Loop Matlab Simulink Circuit for Hardware
42
Implementation
4.8
Three Phase Output SPWM
43
4.9
Three Phase Output SPWM after Driver Circuit
43
4.10
Three Phase Inverter Output before Filter
44
4.11
Three Phase Inverter Output after Filter
44
4.12
Closed Loop Matlab Simulink Circuit for Hardware
45
Implementation
4.13
Hardware for Closed Loop Circuit
45
4.14
Line To Line Voltage Output
46
4.15
Line to Neutral Output Value for Inverter after Filter
47
4.16
SPWM Output for 20V of Vref
48
4.17
Inverter Output for 20V of Vref
49
4.18
Filtered inverter output for 20V of Vref
49
ADC Input for 20V of Vref
50
viii
LIST OF SYMBOLS AND ABREVIATION
ADC
-
Analog to Digital Converter
DAC
-
Digital to Analog Converter
DG
-
Distributed Generation
DSP
-
Digital Signal Processing
IC
-
Integrated Circuit
PID
-
Proportional Integral Derivative
PWM
-
Pulse Width Modulation
SPWM
-
Sinusoidal Pulse Width Modulation
V
-
Voltage
Vdc
-
Direct Current Voltage
Vref
-
Voltage Reference
Vp-p
-
Voltage Peak to Peak
VSI
-
Voltage Source Inverter
ix
LIST OF APPENDICES
APPENDIX
TITLE
PAGE
A
GANTT CHART FOR MASTER PROJECT 1
58
B
GANTT CHART FOR MASTER PROJECT 2
59
C
ADC module
60
1
CHAPTER 1
INTRODUCTION
1.1
Project Background
Malaysia is actively involved with promoting renewable energy to feed the needs of
the country. Renewable energy is opportunities and challenges in unlocking the
potential of biomass, solar, wind and small hydro through a clear regulatory and
incentive driven framework. The government of Malaysia has launched a feed in
tariff scheme for renewable energy in December 2011 to encourage the development
of renewable energy [1]. The continuously increasing energy consumption may cause
overloads and creating problems such as outages, grid instability or deterioration of
power quality. To balance the energy demand and generation, renewable energy
resources such as Photovoltaic (PV), Wind, and Biomass is a good solution to these
problems. Since the generated renewable energy is in DC, an inverter is needed to
convert the DC supply to AC supply before connecting it to the equipment. There are
three basic types of DC to AC converters, depending on their output waveform
which are square wave, modified sine wave and pure sine wave. Considering power
wattage, efficiency and harmonic content, pure sine wave inverters has proved to
have the best quality among the three types because a pure sine wave inverter
produces power that is exactly like the power which is produced by the utility
company.
A number of different controllers are used in industry and in many other
fields. Generally, these controllers can be divided into two main groups that are
conventional controllers and unconventional controllers. The example of
conventional controllers is P, PI, PD, PID and Otto-Smith [2]. A mathematical model
2
for the process of conventional controller is needed in order to design these
controllers. While unconventional controllers utilize a new approaches to the
controller design in which knowledge of a mathematical model of a process
generally is not required. The examples of unconventional controller are fuzzy
controller and neuro-fuzzy controllers [2]. Majority of industrial processes are
nonlinear and thus complicate to describe mathematically. However, it is known that
many processes can be controlled using PID controllers providing that controller
parameters have been tuned well. This type of control has a lot of sense since it is
simple and based on 3 basic behavior types that is proportional (P), integration (I)
and derivation (D). Instead of using a small number of complex controllers, a larger
number of simple PID controllers are used to control simpler processes in an
industrial assembly in order to automate the certain more complex process. In spite
their simplicity, they can be used to solve even a very complex control problems.
While proportional and integrative modes are also used as single control modes, a
derivative mode is rarely used on its own in control systems. Combinations such as
PI and PD control are very often in practical systems. PID controller can be
integrated with inverter as one of its controlling techniques to ensure a stable
voltage[3].
1.2
PROBLEM STATEMENT
Electrical power system consists of several stages, which are power
generation, power transmission and power distribution. Usually power generated
from the power plant is in DC but it will be converted to AC in order to be
transmitted. For power transmission, the AC voltage is connected through the
National Grid for Malaysia and then it is distributed to different kind of user such as
industrial and domestic user. System frequency is a continuously changing variable
that is determined and controlled by the real time balance between system demand
and total generation. Nowadays renewable energy is becoming popular in this
country because the government is encouraging the usage of the renewable energy to
promote green technology. The most common renewable energy that can be
generated in Malaysia is solar energy.
3
The problem with solar energy is the voltage generated in DC, while majority
of electrical appliance in Malaysia is using AC system. A solution to this problem is
using a three-phase inverter. A three-phase inverter can only change the output
waveform from VDC to VAC but the VAC amplitude is directly proportional to the
input of the VDC. Thus, a control method is needed to enable the VAC to be adjusted
without changing the VDC input of the inverter. The benefit of using a control method
is the three-phase inverter can adapt to a variety of loads and able to mitigate any
small disturbance in the system.
1.3
AIM AND OBJECTIVES
Creating a stable voltage output for 3 phase inverter is the aim of this project. To
achieve this aim, the objectives are shown as below:
a. To design a 3 phase inverter.
b. To use TI C2000 TMS320F28335 board as a signal processing device.
c. To implement PID control system in voltage control of the inverter.
1.4
SCOPE
This project is primarily concerned with the control method of the 3 phase inverter.
To achieve the objective, these are the scope that needed to be done:
a. To simulate the inverter in closed loop and open loop system by using Matlab
Simulink.
b. Voltage sensor techniques are needed in determining the appropriate SPWM
signal to be feed to the inverter.
c. Development of the Matlab Simulink model for hardware implementation
4
CHAPTER 2
LITERATURE REVIEW
2.1
Inverter
Inverter is one of static power converters. It produces an ac output waveform
from a dc power supply. These are the types of waveforms required in adjustable
speed drives (ASD), uninterruptible power supplies (UPS), static var compensators,
active filters, and voltage compensators, which are only a few applications. For
sinusoidal AC outputs, the magnitude, frequency, and phase should be controllable.
According to the type of ac output waveform, these topologies can be considered as
voltage source inverters (VSI), where the independently controlled ac output is a
voltage waveform. Static power converters, specifically inverters, are constructed
from power switches and the ac output waveforms are therefore made up of discrete
values. This leads to the generation of waveforms that feature fast transitions rather
than smooth ones [4],[5].
5
2.1.1
Three Phase Voltage Source Inverter
Figure 2.1: 3 Phase Voltage Source Inverter [6]
Bridge configuration is most commonly used for generating output because
the transformer required in this case is not as complicated as in the case of other
inverter circuits. For high power applications a fast switching thyristors are used.
While for low and medium power applications IGBT is used.
Figure 2.1 shows the circuit topology for a three-phase inverter. In this
circuit, the transistors are made to conduct in the order T1, T6 , T2, T4 ,T1, T5. The
recommended operation for this inverter is 120° inverter. Each leg is delayed by
120°. This mode of operation has the advantage of no possibility of a short circuit
across the dc input because the period of 60° elapses between the end of conduction
of one thyristor and the beginning of conduction of the other thyristor of the same
branch. In this particular mode of operation, three thyristors will be conducting at
any time. Triggering frequency of the thyristors will decide the output voltage wave
frequency. The output voltage amplitude can be change by changing the dc input
voltage [6].
6
2.1.2
PWM
Figure 2.2: PWM waveform [6]
Pulse Width Modulation is a signal which is generated by comparing the
amplitudes of the triangular wave (carrier) and sine wave (modulating). These are
done by using simple analogue comparator. Another method to create a PWM is
using digital sampling. Digital sampling is widely used in industry. The duty ratio D
is the fraction of time during which the switch is on. For control purposes the pulse
width can be adjusted to achieve a desired result by adjusting the Vin for the
comparator. Figure 2.2 shows the waveform of PWM [7].
7
2.1.3
SPWM
Figure 2.3: SPWM waveform [6]
The sinusoidal PWM (SPWM) method also known as the triangulation,
subharmonic, or suboscillation method, is very popular in industrial applications. For
creating a SPWM signal, a high-frequency triangular carrier wave is compared with a
sinusoidal reference of the desired frequency. The intersection of wave determines
the switching instants and commutation of the modulated. When sinusoidal wave has
magnitude higher than the triangular wave the comparator output is high, otherwise it
is low [6].
8
2.2
Distribution Generation
Distributed generation is electric power generators that produce electricity at a site
close to customers or that are tied to an electric distribution system and generally
refers to small-scale generation from 1 kW to 50 MW.
2.2.1
Introduction
Figure 2.4: Distributed Generation Technologies [8]
Figure 2.4 shows the distributed energy generation that are divided into two, which
are conventional generators and non-conventional generators. It is shown that DG
can be powered by reciprocating engine, gas turbine, electrochemical devices and
renewable energy devices. DG takes place on two levels that is the local level and the
end point level. Local level power generation plants often include renewable energy
technologies that are site specific, such as wind turbines, geothermal energy
production, solar systems, and some hydro-thermal plants. At the end-point level, the
individual energy consumer can apply many of these same technologies with similar
effects. One DG technology frequently employed by end point users is the modular
internal combustion engine [8].
9
2.2.2
Fuel cells (FCs)
Fuel cells have the same process with normal batteries as they both convert chemical
energy directly into electrical energy and heat. Fuel cells have two electrode
separated by an electrolyte. Fuel cells are generally characterized by the material of
electrolyte used. Presently five major types of fuel cells in different stages of
commercial availability exist. They include proton exchange membrane fuel cell
(PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), solid oxide
fuel cell (SOFC), and molten carbonate fuel cell (MCFC). AFC is not suitable for
DG application since they are nearly zero tolerance to CO2 and CO constituents in
the fuel. To obtain AC current from fuel cell technology, power conditioning
equipment is required to handle the inversion of DC current generated by fuel cell to
AC current that is required to be integrated into the distribution network. Physically a
fuel cell plant consists of three major parts as shown in Figure 2.5. A fuel processor
that removes fuel impurities and may increase concentration of hydrogen in the fuel,
a power section (fuel cell itself) which consists of a set of stacks containing catalytic
electrodes, generating the electricity and lastly a power conditioner that converts the
direct current produced in the power section into alternating current to be connected
to the grid. Resulting advantages of this technology are high efficiency, almost at
partial load, low emissions, and noiselessness as a result of non-existence of moving
parts, and free adjustable ratio from 50 kW to 3MW of electric and heat generation.
The energy savings result from the high conversion efficiency, is typically 40% or
higher, depending on the type of fuel cell. When utilized in a cogeneration
application by recovering the available thermal energy output, fuel cell’s overall
energy utilization efficiencies can be in the order of 85% or more [8].
Figure 2.5: Fuel Cell System [8]
10
2.2.3
Micro-turbines (MT)
Micro-turbines are becoming widespread for distributed power and combined heat
and power applications as they can start quickly. They are one of the most promising
technologies for powering hybrid electric vehicles. Generally micro-turbine systems
have range of power from 30 to 400 kW, while conventional gas turbines range from
500 kW to more than 300MW. Part of their success is due to advances in power
electronics, which enables unattended operation and interfacing with the commercial
power grid. Typical micro-turbine efficiencies are between 33% and 37%, especially
with 85% effective recuperator. It could achieve efficiencies above 80% in a
combined heat and power (CHP) application. Micro-turbines operate in a similar
manner as conventional gas turbines, based on the thermodynamic cycle known as
the Brayton cycle. Air is drawn into the compressor via the air inlet pipe as
illustrated in Figure 2.6. In the compressor, it is pressurized and forced into the cold
side of the recuperator, where it is preheated before it enters the combustion
chamber. The heated air and fuel are thoroughly mixed together and burnt. It is the
mixture, which expands through the turbine that is used to drive the turbine at a
speed of 96,000 rpm, since this has been coupled to the shaft of the generator. The
generator thus produces high frequency AC power that is converted to power
frequency by the use of power electronic devices. Micro-turbine systems have many
advantages over reciprocating engine generators, such as higher power density, with
respect to footprint and weight, extremely low missions and few or just one moving
part. Those designed with foil bearings and air-cooling operates without oil, coolants
or other hazardous materials. Micro-turbines also have the advantage of having the
majority of their waste heat contained in their relatively high-temperature exhaust,
whereas the waste heat of reciprocating engines is split between its exhaust and
cooling system. However, reciprocating engine generators are quicker to respond to
changes in output power requirement and slightly more efficient. Micro-turbines also
lose more efficiency at low power levels than reciprocating engines [8].
11
Figure 2.6: Micro Turbine System [8]
2.2.4
Photovoltaic systems (PVs)
Conversion of solar energy directly to electricity is possible by using photovoltaic
systems (PVs). These systems are commonly known as solar panels. PV solar panels
consist of discrete multiple cells, connected together either in series or parallel, that
convert light radiation into electricity. PV technology could be standalone or
connected to the grid. The output power of PV panels is directly proportional to the
surface area of the cells and footprint sizes. Therefore, footprint needs to be
relatively large (0.02 kW/m2). Even though the operating efficiency of this
technology may be relatively low (10–24%), nevertheless, it cannot be compared
with non-renewable systems. Since the output current of PVs is a function of solar
radiation and temperature, a maximum power point tracking (MPPT) stage is
required in the converter to always obtain the maximum power output. PV units are
integrated into the grid by using inverters [8].
12
2.2.5
Wind energy conversion system (WECS)
Windmills or wind turbines convert the kinetic energy of the streaming air to electric
power. The power from wind turbine is produced in the wind speed of 4–25 m/s
range. The size of the wind turbine has increased rapidly during the last two decades
with the largest units now being about 4 MWcompared to the 1970s in which unit
sizes were below 20 kW. Wind turbines above 1.0MW size are equipped with a
variable speed system incorporating power electronics to overcome mechanical
stresses. Single units of wind turbine can normally be integrated to the distribution
grid of 10–20 kV. In the present, the trend for wind turbine is that the wind power is
being located off shore in larger parks that are connected to high voltage levels and
through the transmission system. The power quality of the wind turbine depends on
the system design. Direct connection of synchronous generators may result in
increased flicker levels and relatively large active power variation. At present, wind
energy has been found to be the most competitive among all renewable energy
technologies. Figure 2.7 presents the schematic block diagram of WECS connection
to the power grid [8].
Figure 2.7: Wind Energy Conversion System [8]
13
2.2.6
Small hydro-turbines
The terms ‘‘small hydro’’ define installations for the production of hydroelectricity
at low power levels. Usually, the power from such installations can be in the range of
5 to 100 kW for ‘‘micro hydroelectric’’ power stations, and between 500 kW and
10MW for mini hydro-power stations. The heads of such plants can be in the range
of 1.5 to 400 m with flows ranging from several hundreds of liters per second to
several tens of cubic meters per second. In a small hydro-power station the technical
electromechanical solutions to be adopted should be robust and as simple as possible
to reduce costs and decrease maintenance, assuring profitability of the investments.
Various turbine manufacturers have developed standardized units for small
hydroelectric systems. Their designs are based on these three principles that is the
optimum use of the latest research into turbine machinery, the supply of the
electromechanical equipment in a compact ready-to-install and ready to operate form
and simple hydraulic design, using standard components to reduce costs and delivery
times. This approach is applicable to powers between 100 and 2000 kW [8].
2.2.7
Biomass
Biomass is considered one of the most important energy sources among the
renewable energies in near future. Biomass is organic material made from plants and
animals. It is considered as a renewable energy source because trees and crops can
always be grown, and waste from them would always exist. Modern biomass energy
recycles organic waste from forestry and agriculture such as corn stovers, rice husks,
wood waste and pressed sugar cane. Fast growing energy crops like willow and
switchgrass also can be used as fuel for the biosmass. Some examples of biomass
fuels are wood, crops, manure, and some garbage as portrayed in Figure 2.8. The
potential of biomass to help the world energy demand has been widely recognized.
When burnt, the chemical energy in biomass is released as heat, which is used to
produce steam that can be used to either drive a turbine for the production of
electricity or to provide heat to industries and homes. Combustion of biomass
involves burning the biomass in air at a flow rate of 4 to 5 kg of air per kg of
biomass. This process on small scale is always used for thermal applications, while a
14
large-scale combustion plant with steam cycle is essential for power generation.
Biomass can as well produce fuel for cars that is cleaner than oil [8].
Figure 2.8: Biomass Source [8]
2.3
Controller Type
Basically controller can be divided into two categories which is the adaptive and
passive controller. The example of adaptive controller is PID, Fuzzy and Neural
Network. Adaptive Control covers a set of techniques which provide a systematic
approach for automatic adjustment of controllers in real time, in order to achieve or
to maintain a desired level of control system performance when the parameters of the
plant dynamic model are unknown or change in time.
15
2.3.1
The Three Element of PID controller
The proportional control action is proportional to the current control error according
to the expression
(2.1)
Where
is the proportional gain. It implements the typical operation of
increasing the control variable when the control error is large. The transfer function
of a proportional controller can be derived as:
(2.2)
With respect to the On–Off controller, a proportional controller has the
advantage of providing a small control variable when the control error is small and
therefore to avoid excessive control efforts. The main drawback of using a pure
proportional controller is that it produces a steady-state error. It is worth noting that
this occurs even if the process presents an integrating dynamics, in case a constant
load disturbance occurs. This motivates the addition of a bias term
, namely,
(2.3)
The value of
can be fixed at a constant level or can be adjusted manually
until the steady-state error is reduced to zero. It is worth noting that in commercial
products the proportional gain is often replaced by the proportional band PB, which
is the range of error that causes a full range change of the control variable.
The integral action is proportional to the integral of the control error. The
equation is
(2.4)
16
Ki is the integral gain. It appears that the integral action is related to the past
values of the control error. The corresponding transfer function is:
(2.5)
The presence of a pole at the origin of the complex plane allows the reduction
to zero of the steady-state error when a step reference signal is applied or a step load
disturbance occurs. In other words, the integral action is able to set automatically the
correct value of
in (2.3) so that the steady-state error is zero. This fact is better
explained in Figure 2.9, where the resulting transfer function is a PI controller result.
(2.6)
For this reason the integral action is also often called automatic reset. Thus,
the use of a proportional action in conjunction to an integral action of a PI controller,
solves the main problems of the oscillatory response associated to an On–Off
controller and of the steady-state error associated to a pure proportional controller. It
has to be stressed that when integral action is present, the so-called integrator windup
phenomenon might occur in the presence of saturation of the control variable.
Figure 2.9: PI Controller in Automatic Reset Configuration
17
While the proportional action is based on the current value of the control
error and the integral action is based on the past values of the control error, the
derivative action is based on the predicted future values of the control error. An ideal
derivative control law can be expressed as:
(2.7)
Where
is the derivative gain. The corresponding controller transfer
function is
(2.8)
In order to understand better the meaning of the derivative action, it is worth
considering the first two terms of the Taylor series expansion of the control error at
time
ahead:
(2.9)
If a control law proportional to this expression is considered as:
(2.10)
This naturally results in a PD controller. The control variable at time t is
therefore based on the predicted value of the control error at time t + Td. For this
reason the derivative action is also called anticipatory control, or rate action, or preact. It appears that the derivative action has a great potentiality in improving the
control performance as it can anticipate an incorrect trend of the control error and
counteract for it.
18
A proportional-integral-derivative controller (PID controller) is a generic
control loop feedback mechanism widely used in industrial control systems. A PID
controller calculates an "error" value as the difference between a measured process
variable and a desired set point. The controller attempts to minimize the error by
adjusting the process control inputs. Many processes can be controlled using PID
controllers providing that controller parameters have been tuned well. Instead of
using a small number of complex controllers, a larger number of simple PID
controllers are used to control simpler processes in an industrial assembly in order to
automate the certain more complex process. In spite their simplicity, they can be
used to solve even a very complex control problems. While proportional and
integrative modes are also used as single control modes, a derivative mode is rarely
used on its own in control systems. Combinations such as PI and PD control are very
often in practical systems [2][3].
The PID controller involves three separate constant parameters that are
proportional (P), integration (I) and derivation (D) as in Figure 2.10. These values
can be interpreted in terms of time where P depends on the present error, I on the
accumulation of past errors, and D is a prediction of future errors, based on current
rate of change [2].
The PID control scheme is named after its three correcting terms, whose sum
constitutes the manipulated variable (MV). The proportional, integral, and derivative
terms are summed to calculate the output of the PID controller. Defining as
controller output, the final form of the PID algorithm is:
the
19
(2.11)
Where,
: Proportional gain, a tuning parameter
: Integral gain, a tuning parameter
: Derivative gain, a tuning parameter
e
: Error
t
: Time or instantaneous time (the present)
: Variable of integration, takes on values from time 0 to the present t.
Figure 2.10: Block Diagram of a PID Controller in a Feedback Loop
A proportional controller
will have the effect of reducing the rise time
but never eliminate the steady-state error. An integral control
will have the
effect of eliminating the steady-state error for a constant or step input, but it may
make the transient response slower. A derivative control
will have the effect of
increasing the stability of the system, reducing the overshoot, and improving the
transient response. The effects of each of controller parameters
closed-loop system are summarized in the Table 2.1.
and
on a
20
Table 2.1: Effects of each of controller parameters
CL Response
Rise Time
Overshoot
Settling Time
S-S Error
Kp
Decrease
Increase
Small Change
Decrease
Ki
Decrease
Increase
Increase
Eliminate
Kd
Small Change
Decrease
Decrease
No Change
These correlation may not be exactly accurate, because
and
are dependent
on each other. In fact, changing one of these variables can change the effect of the
two other. The Table should only be used as a reference when determining the values
for
2.4
and
[3].
IGBT and Mosfet
Insulated Gate Bipolar Transistor (IGBT) combined the best of conventional bipolar
transistors and FETs they only require a voltage across the base in order to conduct.
However, like conventional bipolar, they are efficient conductors of current through
their collector or emitters.
The bipolar transistor requires a high base current to turn on, has relatively
slow turn off characteristic and is liable for thermal runaway due to a negative
temperature coefficient. In addition, the lowest attainable on state voltage or
conduction loss is governed by the collector emitter saturation voltage VCE (SAT).
The Mosfet however is a device that is voltage and not current controlled.
Mosfet have a positive temperature coefficient, stopping thermal runaway. The on
state resistance has no theoretical limit hence on state losses can be far lower. The
Mosfet also has a body drain diode which is particularly useful in dealing with
limited freewheeling currents.
The IGBT has the output switching and conduction characteristics of a
bipolar transistor but is voltage controlled like a Mosfet. In general, this means it has
the advantages of high current handling capability of a bipolar with the ease of
21
control of a Mosfet. However, the IGBT still has the disadvantages of a
comparatively large current tail and nobody drain diode.
Another potential problem with some IGBT types is the negative temperature
coefficient, which could lead to thermal runaway and makes the paralleling of
devices hard to effectively achieve. This problem is now being addressed in the latest
generations of IGBT that they are based on non punch through technology. This
technology has the same basic IGBT structure but is based on bulk diffused silicon,
rather than the epitaxial material that both IGBT and Mosfets has historically used.
Comparing both of Mosfet and IGBT structures look very similar. The basic
difference is the addition of a P substrate beneath the N substrate. The IGBT
technology is certainly the device of choice for breakdown voltage above 1000 V,
while the Mosfet is certainly the device of choice for device breakdown voltage
below 250 V.
Choosing between IGBTs and Mosfets is very application specific and cost,
size, speed and thermal requirement should all be considered. Table 2.4 shows
differential between IGBT and Mosfet [9].
22
Table 2.2: Differential between IGBT and Mosfet
IGBT
Preferred Condition:-
Mosfet
Preferred Condition:-

Low duty cycle

High frequency application

Low frequency

Wide line or load variation

Narrow or small line or load

Long duty cycle
variation

Low voltage application (<250 V)

High voltage application

Operation at high junction
Temperature is allowed (>100
C)

>5 kW output power
Typical IGBT application includes:

Typical Mosfet application includes:
Motor control : frequency <20

Battery charging
kHz

Switch mode power supply

Uninterruptible power supply

Low power lighting
23
CHAPTER 3
METHODOLOGY
3.1
Introduction
This chapter will describe the method for this subject in order to achieve the desired
objective. Figure 3.1 consist of block diagram of the project and Figure 3.2 shows the
flowchart of the project.
3.2
Block Diagram
TMS320F28335 board can create SPWM pulse by using Matlab Simulink and Code
Composer Studio V3 (CCSV3). Six pulses that have been created are fed into tree
phase inverter. DC voltage from the DC supply is converted to an AC voltage using a
3 phase inverter. The voltage output from the three-phase inverter is connected to RL
filter to get sine wave. The voltage of the inverter is then step down by using voltage
divider and the voltage level is increase to be in range of 0V to 3V as
TMS320F28335 ADC module can only read from 0V to 3V
DC Supply
3 Phase
Inverter
AC Voltage
Filter
TI board
(SPWM)
Figure 3.1: Block Diagram of the Project
Voltage
Divider
24
3.3
Project Flowchart
Start
Literature Review
Software research
Hardware research
Generate SPWM
Create driver circuit
and inverter circuit
Simulate 3phase
inverter
Vp-p = VDC
no
Troubleshoot the
circuit
yes
Choose proper LC
filter
Create voltage
divider circuit and
offset circuit
Create PID control
system
ADC input is
less than 3V
yes
Combine hardware
and software
Test run
no
Vp-p=Vref
Tune PID
yes
End
Figure 3.2: Flowchart of the Project
no
Troubleshoot the
voltage divider
circuit
55
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