DEVELOPMENT OF 3-PHASE TNVERTER WITH PID VOLTAGE CONTROL

MUHAMMAD FADDTL BIN AHMAD REBUDI

This project report is submitted in partial fulfilment of the requirement for the award

of the Degree of Master of Electrical Engineering

Faculty of 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 INTRODUCTION 1

1.1 Project Background 1

1.2 Problem Statement 2

1.3 Objective 3

1.4 Scope of Project 3

CHAPTER 2 LITERATURE REVIEW 4

2.1 Inverter

2.1.1 Three Phase Voltage Source Inverter

2.1.2 PWM

2.1.3 SPWM

2.2 Distribution Generation

2.2.1 Introduction

2.2.2 Fuel cells (FCs)

2.2.3 Micro-turbines (MT)

2.2.4 Photovoltaic systems (PVs)

2.2.5 Wind energy conversion system

2.2.6 Small hydro-turbines

4

4

6

7

8

8

9

10

11

12

13

iii

2.2.7 Biomass

2.3 Controller Type

2.3.1 The Three Element of PID controller

2.4 IGBT and Mosfet

13

14

15

20

CHAPTER 3 METHODOLOGY 23

3.1 Introduction

3.2 Block Diagram

3.3 Project Flowchart

3.4 TMS320F28335

3.5 Three Phase Inverter

3.6 Driver Circuit

3.7 ADC module TI board

3.8 Voltage sensor technique

3.9 PID Controller

3.10 Matlab Simulink

23

23

24

25

26

26

29

30

32

34

CHAPTER 4 RESULT AND DISCUSSION

37

4.1 Simulation for Open Loop System 37

iv

4.2 Simulation Closed Loop

4.3 Hardware Open Loop Result

4.4 Hardware Closed Loop Result

4.4.1 20V Voltage reference

4.4.2 Comparison for Different Voltage

Reference

40

42

45

48

51

CHAPTER 5 CONCLUSION

53

5.1 Conclusion

5.2 Recommendation

53

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

Implementation

42

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

Implementation

45

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:

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.

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

(2.5)

(2.6)

17

(2.7)

(2.8)

(2.9)

(2.10)

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:

Where is the derivative gain. The corresponding controller transfer

function is

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:

If a control law proportional to this expression is considered as:

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 pre-

act. 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 the

controller output, the final form of the PID algorithm is:

19

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 and on a

closed-loop system are summarized in the Table 2.1.

(2.11)

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 and [3].

2.4 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 Mosfet

Preferred Condition:-

Low duty cycle

Low frequency

Narrow or small line or load

variation

High voltage application

Operation at high junction

Temperature is allowed (>100

C)

>5 kW output power

Preferred Condition:-

High frequency application

Wide line or load variation

Long duty cycle

Low voltage application (<250 V)

Typical IGBT application includes:

Motor control : frequency <20

kHz

Uninterruptible power supply

Low power lighting

Typical Mosfet application includes:

Battery charging

Switch mode power supply

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

Figure 3.1: Block Diagram of the Project

Filter

3 Phase

Inverter

DC Supply

AC Voltage

TI board

(SPWM)

Voltage

Divider

24

3.3 Project Flowchart

Create driver circuit

and inverter circuit

Vp-p = VDC

Start

Troubleshoot the

circuit

Create voltage

divider circuit and

offset circuit

ADC input is

less than 3V

Troubleshoot the

voltage divider

circuit

Literature Review

Software research Hardware research

Generate SPWM

Simulate 3phase

inverter

Choose proper LC

filter

Create PID control

system

Vp-p=Vref Tune PID

Combine hardware

and software

Test run

End

yes

yes

yes

no

no

no

Figure 3.2: Flowchart of the Project

55

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