POWER Prafulla 2015
POWER Prafulla 2015
POWER Prafulla 2015
Master of Technology
In
By
Rourkela
2015
POWER QUALITY IMPROVEMENT USING
MULTILEVEL INVERTER BASED SHUNT ACTIVE
FILTER
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
Master of Technology
In
By
Rourkela
2015
DECLARATION
I hereby declare that the work in the thesis entitled “Power Quality Improvement Using
Multilevel Inverter Based Shunt Active Filter” presented by me in partial fulfillment of the
requirements in partial fulfillment of the requirements for the award of Master of Technology
Degree in Electrical Engineering with specialization in “Power Control and Drives” at the
National Institute of Technology, Rourkela (Deemed University) is an authentic work carried out
by me. To the best of my knowledge, the matter embodied in the thesis has not been submitted to
any other University/ Institute for the award of any degree or diploma
CERTIFICATE
This is to certify that the thesis entitled, “Power Quality Improvement using
Multilevel Inverter based Shunt Active Filter” submitted by Mandava Divya Prafulla (Roll.
No. 710EE2008) in partial fulfillment of the requirements for the award of Master of
To the best of my knowledge, the matter embodied in the thesis has not been submitted to any
Rourkela –769008
ACKNOWLEDGEMENT
I would like to convey my appreciation and gratefulness to all those who have
contributed their efforts for me complete the project work .I specially look forward to thank my
thesis guide Prof. Anup Kumar Panda who always have been motivating me constantly and
always have been giving his best support throughout the thesis time in and out as well. His
readiness in providing us technical help and resources have been always invaluable. It was great
interference with the near, by communication lines, high power loss, low power factor and hence
poor usage of equipment. Shunt active filters are used to compensate these harmonics and draw
only fundamental component of current from the supply. The shunt active filters be designed
using multilevel inverters are more efficient when compared to the classical inverters due to their
In this work, multilevel inverter based shunt active filters are designed using different
mathematical model when fuzzy logic controllers are used for their design unlike the PI
controller. A three level H-bridge was used as the inverter for the filter design. The instantaneous
active(iLd), reactive(iLq) components of load current are used to produce the reference
compensation currents in this Id-Iq control method. Matlab simulation was used to verify the
results.
CONTENTS
CONTENTS .................................................................................................................................... 7
LIST OF FIGURES ........................................................................................................................ 9
LIST OF TABLES ........................................................................................................................ 11
Chapter 1 ......................................................................................................................................... 1
Introduction ................................................................................................................................. 1
1.1 Power Quality .................................................................................................................... 1
1.2 Literature Survey ............................................................................................................... 1
1.3 Motivation ......................................................................................................................... 3
1.4 Objective ............................................................................................................................ 4
1.5 Thesis Outline .................................................................................................................... 4
Chapter 2 ......................................................................................................................................... 6
Multilevel Inverters ..................................................................................................................... 6
2.1 Introduction .................................................................................................................. 6
2.2 Multilevel Inverters ........................................................................................................... 6
2.3 Comparison between different topologies ....................................................................... 11
2.4 Advantages ...................................................................................................................... 11
2.5 Summary.......................................................................................................................... 12
Chapter 3 ....................................................................................................................................... 13
Modulation Techniques in Multilevel Inverters .................................................................... 13
3.1 Introduction ..................................................................................................................... 13
3.2 Sinusoidal Pulse Width Modulation ................................................................................ 13
3.3 Multicarrier Pulse Width Modulation Techniques .......................................................... 14
3.3.1 Phase Shifted Modulation:............................................................................................ 15
3.3.2. Level Shifted Multicarrier Modulation: ...................................................................... 15
3.4 Simulation Results for Multilevel Inverters .................................................................... 17
3.4 Summary.......................................................................................................................... 22
7
Chapter 4 ....................................................................................................................................... 23
Shunt Active Power Filter ......................................................................................................... 23
4.1 Introduction ..................................................................................................................... 23
4.2 Shunt Active Filters ......................................................................................................... 24
4.3 Basic principle ................................................................................................................. 24
4.4 Shunt Active Filter Design .............................................................................................. 25
4.5 Role of DC link capacitor ................................................................................................ 26
4.6 Design using PI controller ............................................................................................... 27
4.6.1 Introduction .................................................................................................................. 27
4.7 The fuzzy Logic control scheme ..................................................................................... 29
4.8 Fuzzy Algorithm .............................................................................................................. 30
4.9 Summary.......................................................................................................................... 34
Chapter 5 ....................................................................................................................................... 35
Simulation results and Discussion............................................................................................. 35
5.1 Simulation Results ........................................................................................................... 35
5.2 Summary.......................................................................................................................... 39
Chapter 6 ....................................................................................................................................... 40
Conclusion and Future Scope .................................................................................................... 40
6.1 Conclusion ....................................................................................................................... 40
6.2 Future Scope .................................................................................................................... 41
References ............................................................................................................................. 42
8
LIST OF FIGURES
Figure 2.1 Diode Clamped Multilevel Inverter…………………………………………………..8
Figure 2.2 Capacitor Clamped Multilevel Inverter………………………………………………9
Figure 2.3 Cascaded H Bridge Multilevel Inverter…………………………………………….10
Figure 3.1 Bipolar Modulation Scheme…………………………………………………………14
Figure 3.2 Unipolar Modulation Scheme…………………………………………………..……14
Figure 3.2 In Phase Disposition PWM Scheme…………………………………………………15
Figure 3.3 Phase Opposition Disposition…………………………………………………….….16
Figure 3. 4 Alternate Phase Opposition Disposition……………………………………….……16
Figure 3.5 Output for 3 Level Diode Clamped Inverters Using In phase Disposition PWM
Technique
……………………………………………………………………………………….…………..18
Figure 3.6 Output for 3 Level Diode Clamped Inverters Using Out Of Phase Disposition PWM
Technique ………………………………………………………………………………………..18
Figure 3.7 Output for 5 Level Diode Clamped Inverters Using In phase Disposition PWM
Technique……………………………………………………………………………..……...….19
Figure 3.8 Output for 5 Level Diode Clamped Inverters Using Alternate Phase Disposition PWM
Technique……………………………………………………………………….…………....….19
Figure 3.9 Output for 5 Level Diode Clamped Inverters Using Out Of Phase Disposition PWM
Technique………………………………………………………………….…………….…....…20
Figure 3.10 Output for 3 Level Capacitor Clamped Inverters Using In Phase Disposition PWM
Technique……………………………………………………………….……………………….20
Figure 3.11 Output for 3 Level Capacitor Clamped Inverters Using Alternate Phase Disposition
PWM Technique…………………………………………………………………………..……21
Figure 3.12 Output for 9 level H-Bridge…………..…...……………………...…..…………….22
Figure 4.1 Shunt Active Power filter Block Diagram ……………………………...……….…...25
Figure 4.2 PI Control Scheme Block Diagram ………………………………………...………..29
Figure 4.3 Block Diagram for System Using Fuzzy Logic Control …………………...….…….32
Figure 4.4 Input Error Membership Function…………………………………………..….…….35
Figure 4.5 Input Change in Error Membership Function……………………………….....…….35
9
Figure 4.6 Output Current Membership Function………………………………………….……36
Figure 5.1 Source Voltage Vs Time ………………………………….…………………………38
Figure 5.2 Source Current without using Filter Vs time………………….………….………….38
Figure 5.3 Source current after using PI Controller based filter Vs time…………………..…...39
Figure 5.4 Source current Harmonic Spectrum before compensation……………………….…..39
Figure 5.5 Source Current Harmonic Spectrum after Using PI Controller Based Filter……...…40
Figure 5.6 Source Current after using Fuzzy Controller based filter Vs time………..............….41
Figure 5.7 Source Current Harmonic Spectrum after using Fuzzy Logic controller……………41
10
LIST OF TABLES
Table 1 .......................................................................................................................................... 11
Table 2 .......................................................................................................................................... 34
11
Chapter 1
Introduction
1.1 Power Quality
Power quality is becoming a major issue in modern day power equipment because of the
high usage of power electronic devices. The pollution in the equipment is mainly due to the
nonlinear characteristics of the devices. They cause various problems like voltage distortion,
current distortion, low power factor, interference in the nearby communication networks etc. in
the form of harmonics. These harmonics are responsible for over-heating of equipment, noise or
vibrations and may even cause damage. Due to the various problems caused due to the power
quality issues, there is an urgent need for improving the power quality thereby reducing losses
[1].
Toward the end of 19th century the advancement of alternating current (air conditioning)
transmission framework was in view of voltage which is sinusoidal of consistent frequency era.
These voltages of consistent frequency are responsible for the configuration of transformer,
transmission lines and machines [1]. In the event that the voltage will be non-sinusoidal then it
will make numerous changes in the configuration of transformer, machine and transmission
framework. Routine power hypothesis was taking into account active, reactive and apparent-
power definitions were adequate to investigation and out lining of all the power frameworks. By
and by, a few papers were distributed in the 1920s, demonstrating the ordinary idea of apparent
1
power and reactive and its value under non-sinusoidal conditions. Fryze (1932) characterized
power in time area while for frequency area was done by Budeanu (1927). Power quality has
been becoming a very important criterion due to the high usage of power electronic equipment.
Connection of nonlinear loads is mainly leading to this problem of voltage or current harmonics.
[2]
When there is a flow of harmonics through the electric transmission lines or distribution systems
there will be an additional distortions in voltage mainly because of the impedance present in the
system. Therefore while generating and utilizing power there is a distorted voltage or current
waveform [3].The currents in the grid system must operate at unity power factor to transfer
maximum amount of power [4] There was usage of passive filters in the olden days for
compensation, but these filters had many draw backs like heavy size, tuning problems, resonance
etc. and they had to be replaced with active filters.[5] The shunt active filters used VSI for their
operation.
The traditional two-level VSI systems used as APFs were the split capacitor three-leg or four-leg
type. These models were only useful for low or medium power utilities. These drawbacks led to
the invention of multilevel inverters which could be used for both high voltage as well as
medium power applications[5,6] In the recent times multilevel inverters had been under attention
mainly due to their usage in batteries, wind turbines etc. which could be connected through the
inverter to the grid and reducing harmonic problems[7,8]. They were also a means to give a
multilevel stepped voltage source inverter for connection to an AC high voltage, high power
system for various applications like the photo voltaic, fuel cells, utility interface systems [9, 10].
Multilevel converters, mainly the three-level inverters, were the best tradeoff solution between
cost and performance for high-power systems with high voltage[11,12,13] Points of interest of
2
PWM controlled VSCs, for example, diminished line harmonics, a superior power component,
generously littler filters, and a higher framework productivity empower an expense decrease of
the framework in numerous applications, for example, rolling mills, marine and mining
particularly with high voltage, conventional two-level VSIs couldn't dodge to utilize the
rating used and it might be exceptionally bulky and even tricky essentially because of the trouble
designed shunt active filters solves this problem by working as current source by supplying
negative harmonics . Harmonic currents which have opposite phases to those of the harmonic
currents drawn from the source by the nonlinear loads are produced by the filter[17] The
reference current extraction has to be done for design of filter and their compensation capabilities
are not the same in every control strategy Some of them do not give a proper solution when the
source voltage is not balanced.[18,19] Fuzzy logic controllers unlike the PI controllers does not
require the precise scientific mathematical model regarding procedure that is under control.[20]
1.3 Motivation
Due to the various drawbacks of passive filters, active filters were developed for
reducing the harmonics in the equipment having nonlinear loads. Uninterrupted Power
supplies, variable speed drives, and all other types of rectifiers etc. all come under the nonlinear
loads. When properly designed shunt active filters solves this problem by working as current
source by supplying negative harmonics. Harmonic currents which have opposite phases to
those of the harmonic currents drawn from the source by the nonlinear loads are produced by
the filter. When coupled in parallel to that of nonlinear load all there is a reduction of harmonic
3
content by such a shunt active filter. Fundamental component of current only is thus drawn from
the network.
Voltage source as well as current source converters can be used in the filter design. Voltage
source converters were found to be better over current source converters because they cost less
and had higher efficiency than the latter. They could easily be cascaded for higher rating. Gating
PWM was considered the most efficient technique for controlling the output of multilevel
inverters. There are many different kinds of PWM techniques available. The most simple and
commonly used technique is using a high frequency triangular carrier signal, and a reference
signal. Both of them were compared using a comparator and the output was high when the
reference signal value exceeded the carrier signal value and low otherwise. Many other
techniques were also developed like the delta modulation, Space vector modulation, delta sigma
modulation etc. Each of the methods have their own advantages over the others.
1.4 Objective
2. Modelling of shunt active filters for the reduction of the harmonics in the equipment using
The thesis work is organized into the following chapters besides the present chapter 1
Chapter 2 - Different multilevel inverter topologies were discussed and their applications were
presented.
4
Chapter 3 - PWM techniques were discussed for giving the gating signals to the inverters.
Chapter 4 - Simulation results for the different multilevel inverter topologies were presented.
Chapter 6 – Shunt active power filter was designed using the PI control.
Chapter 7 – Shunt power filter was designed using a fuzzy logic controller.
5
Chapter 2
Multilevel Inverters
2.1 Introduction
Many of the industrial and commercial application require medium and high power
equipment in the range of megawatt. For all such applications a single switch shouldn’t be
connected and so a family of switches have to be connected. This led to the introduction of
multilevel inverters which can be applied effectively in high as well as medium power
applications. These inverters are made up of an array of capacitors voltage sources, power
semiconductor from which they tend to generate stepped output voltage waveforms [3, 4].
In the recent times many different multilevel inverter topologies were proposed. There are
mainly three different basic topologies which remain the basis for many of the recent ones
Here for this inverter model the clamping purpose is served by using a diode as in fig.2.1 to
output the required staircase voltage waveform. Figure shows connecting series of capacitors in
the 3 level diode clamped inverter. Multiple voltages will be provided for the M level inverter,
6
each of the capacitor has a voltage of VDC/m-1. Maximum output voltage which can be obtained
is half of the input DC voltage. This has been the main drawback for this topology. But this
problem can be overcome by increasing the number of switches, capacitors, diodes. Three level
inverters of this type are extensively being used in industries these days.
Applications
o They are used for motor drives with variable speeds [8].
7
2.2.2 Flying Capacitor Multilevel Inverter
This structure has series of capacitor clamped switching devices as in fig. 2.2[5]. This
topology has many advantages over diode clamped structure. One of the main advantages is that
it does not require additional clamping diodes. Unlike that of diode clamped structure where
capacitors of same leg are charged to equal voltages. Here the capacitors are charged to different
voltages. This is the main disadvantages which lead to the limited use of this inverter. Fig shows
Applications
o Used in the control of Induction motor using Direct Torque Control (DTC) circuit[8]
8
Figure 2.2 Capacitor Clamped Multilevel Inverter
Multiple single phase full bridges are connected in series where each bridge can switch
between +Vdc ,0 and –Vdc as in fig.2.3[6]. The voltage supply to each bridge is provided
separately .The requirement of separate voltage supply devices is the main disadvantage of this
9
Figure 2.3 Cascaded H Bridge Multilevel Inverter
Applications
10
2.3 Comparison between different topologies
Table 1
S.NO Component Diode-clamped Capacitor Clamped H-bridge
clamping
2. Balancing 0 (M-1)*(M-2)/2 0
capacitors
the DC bus
switches
2.4 Advantages
The multilevel converter topology has many advantages. Some of them are
1. They produce common mode voltages thereby reducing the stress on the motor and thus
11
3. These operate at both switching frequencies which could be much greater than the
fundamental switching frequency or even very less than that of the fundamental switching
frequency. A lower switching frequency always leads to lower loss and high efficiency.
4. The distortion in the output can be maintained extremely low using the selective harmonic
elimination technique.
2.5 Summary
This chapter discussed about the main three types of multilevel inverter topologies available
and their applications. Their advantages and drawbacks were discussed along with their
12
Chapter 3
3.1 Introduction
PWM was considered the most efficient technique for controlling the output of multilevel
inverters. There are many different kinds of PWM techniques available. The most simple and
commonly used technique is using a high frequency triangular carrier signal, and a reference
signal. Both of them were compared using a comparator and the output was high when the
reference signal value exceeded the carrier signal value and low otherwise. Many other
techniques were also developed like the delta modulation, Space vector modulation, delta sigma
modulation etc. Each of the methods have their own advantages over the others.
The best among all the PWM methods is the Sinusoidal Pulse Width Modulation (SPWM)
which can be implemented in two level as well as multilevel inverters. In this technique a
sinusoidal reference signal and a carrier signal of very high frequency (triangular signal) are
under comparison to give low or high output. There are 2 types of SPWM techniques i.e.
unipolar and bipolar pulse width modulation. The bipolar technique requires a single modulating
wave and a single carrier wave whereas unipolar modulation requires two sinusoidal waves and a
single carrier wave. These techniques are mainly used for H bridge inverters.
13
Figure 3.1 Bipolar Modulation Scheme
Cascaded Multilevel inverters use PWM techniques which are mainly classified as 1. Phase
shifted modulation and 2.level shifted modulation [11]. For both the techniques, (M-1) triangular
14
carrier waves with that of same frequency and with same peak-peak amplitudes will be required
Here, there will be a phase shift of 𝜙𝑐𝑟 between all the adjacent carrier signals. There is a phase
360°
shift 𝜙𝑟 = .
(𝑚−1)
In this technique all the triangular waves are vertically displaced as shown in fig..
Depending upon the disposition of the carrier waves it is further classified into
(i) In Phase Disposition PWM (IPD – PWM): In IPD -PWM all of carrier triangular waves
15
(ii) Phase Opposition Disposition PWM (POD – PWM)- Here the carrier waveforms will
be placed above with those below the zero reference .There will be a 180 degrees phase
shift in the ones below and the ones above zero as in fig 3.3 .
(iii) Alternate Phase Opposition Disposition PWM (APOD –PWM)- Here the carrier waves
are to be 180 degrees displaced from each other alternately as in fig 3.3.
16
Figure 3. 1 Alternate Phase Opposition Disposition
All the 3 types of multilevel inverter configurations were modelled and simulated in
Fig.3.3 and fig.3.4 shows the output voltage waveform for a 3 level diode clamped inverter using
17
Figure 3.3 Output for 3 level diode clamped inverters using in phase disposition PWM
technique
Figure 3.4 Output for 3 level diode clamped inverters using out of phase disposition PWM
technique
Fig 3.5, 3.6, 3.7 show the output for 5 level diode clamped inverters using different PWM
techniques. It can be seen the waveform that the output approaches more a sine wave here when
18
Figure 3.5 Output for 5 level diode clamped inverters using in phase disposition PWM
technique
Figure 3.6 Output for 5 level diode clamped inverters using alternate phase disposition
PWM technique
19
Figure 3.7 Output for 5 level diode clamped inverters using out of phase disposition PWM
technique
Figure 3.8 Output for 3 level capacitor clamped inverters using in phase disposition PWM
technique
Fig 3.8 and 3.9 show the output for a 3 level capacitor clamped inverter using different
PWM techniques
20
Figure 3.9 Output for 3 level capacitor clamped inverters using alternate phase disposition
PWM technique
Fig 3.10 shows the output voltage waveform for a 9 level H bridge .It can be seen from figure
that it gave a better waveform which is better approaching a sine wave when compared to the 3
21
3.4 Summary
In this chapter the different PWM modulation techniques were presented for two level
as well as multilevel inverters. Different sinusoidal pulse width modulation techniques for two
level and multilevel inverters were discussed. Simulation results using MATLAB for 3 level and
5 level diode clamped inverter, 3 level diode clamped inverter, 9 level H-bridge are presented.
22
Chapter 4
Shunt Active Power Filter
4.1 Introduction
Power quality improvement is becoming a major issue in modern day power equipment
because of the high usage of power electronic devices. The pollution in the equipment is mainly
due to the nonlinear characteristics of the devices. They cause various problems like voltage
distortion, current distortion, low power factor, interference in the nearby communication
networks etc. in the form of harmonics. These harmonics are responsible for over-heating of
equipment, noise or vibrations and may even cause damage. Due to the various problems caused
due to the power quality issues, there is an urgent need for improving the power quality thereby
reducing losses.
For compensation of the non-linear loads generated current harmonics due to the,
Conventionally LC passive power filters have been used, mainly because they were very cheap
as well efficient[12] . But there were many drawbacks because of series and parallel
resonances, their compensation largely depended on system impedance [12]. Source current
harmonics are eliminated the impedance of filter must be smaller than that of the source. They
are not suited for loads that vary with time, because the variation in load impedances have the
possibility of detuning the filter [12]. Classically passive filters were used to improve the power
quality. But these filters had many drawbacks like heavy size, resonance problems, fixed
compensation etc. So active filters were developed to overcome these drawbacks. Many
different active filter topologies were developed in the recent times. The active filter topology
was found to be very effective even when there was high non-linearity in the load.
23
4.2 Shunt Active Filters
Uninterrupted Power supplies, variable speed drives, and all other types of
rectifiers etc. all come under the nonlinear loads. They act as harmonic current sources because
they draw non-sinusoidal currents from the source [12]. When properly designed shunt active
filters solves this problem by working as current source by supplying negative harmonics.
Harmonic currents which have opposite phases to those of the harmonic currents drawn from
the source by the nonlinear loads are produced by the filter [12]. When coupled in parallel to
that of nonlinear load all its harmonic currents will be compensated by such a shunt active filter.
Fundamental component of current only is thus drawn from the network [12].
Conventionally the shunt APF’s are controlled such that harmonic or reactive
compensation currents are injected based on the reference currents calculated using different
methods [13]. These currents which are injected have to cancel reactive currents and also the
harmonics that are caused by non-linear loads. The desired reference currents that should be
The filter is designed as in fig 4.1 below such that the system draws only fundamental
component of current from the source and the harmonic currents are supplied by the filter.
24
Figure 4.1 Shunt Active Power filter Block Diagram [12]
To achieve good compensation the filter must be able to extract and inject the harmonics
maintain the DC capacitor link voltage constant iLc. There are many different types of control
strategies used to extract the compensation currents. Here we use the “Instantaneous active
current and the reactive current “Id-Iq” control strategy [2] which has many advantages over other
methods. The instantaneous active (iLd), reactive (iLq) components of load current are used to
produce the reference compensation currents ica*, icb* and icc* in this Id-Iq control method [7].
The load currents are tracked and changed to stationary α-β reference and then into d-q using the
25
𝑖𝐿𝑑 𝑐𝑜𝑠𝜃 𝑠𝑖𝑛𝜃 𝑖𝐿𝛼
[𝑖 ] = [ ]*[ ] …………………………… 2
𝐿𝑞 −𝑠𝑖𝑛𝜃 𝑐𝑜𝑠𝜃 𝑖𝐿𝛽
The angle theta can be obtained from the source voltage vectors for ideal source voltage
conditions. [12]
The fundamental component of d-axis and q-axis load current are filtered out and the harmonics
along with the power loss component id1h are converted to reference currents.
The i*cd and i*cq are changed back to the a-b-c coordinates. Transformation M matrix of the
𝑠𝑖𝑛𝜃 𝑐𝑜𝑠𝜃 1
M = [sin(𝜃 − 2𝜋/3) cos(𝜃 − 2𝜋/3) 1] ………………….. 7
2𝜋
sin(𝜃 + 2𝜋/3) cos(𝜃 + ) 1
3
During dynamic load conditions the real power difference which must be supplied to the
load is supplied by the capacitor by charging or discharging the capacitor. The DC link capacitor
voltage must be necessarily maintained constant to achieve good working of the active power
filter [13]. For this purpose a controller should be used. When DC voltage of capacitor is equal to
26
that of constant reference value, then all the real power that is been drawn from source will be
same as that of the power load the load consumes. When DC voltage of capacitor is less than that
of the reference voltage, it implies that the active power demanded by the load and source
current drawn by the equipment from the source must be increased because the source’s output
real power was not exactly enough to satisfy completely and a bigger value of DC capacitor
voltage means that there must be a decrease in the reference source current.
for all industrial controls. The PI control system evaluates the error value (e) as the difference of
the measured system variable with a reference variable. The controller tries to minimize
the errors by use of manipulating variable thereby adjusting the process as per the requirement.
the integral values are denoted as kp and ki. The kp value is based on the present error, and Ki is
based on the past errors. The system process is controlled by taking the sum of the two through a
control element.
27
Figure 4.2 PI Control Scheme Block Diagram [12, 13]
The out value of that of the PI controller will be active current required to make up for
the power loss inside the active filter is obtained by sending the error into the PI controller and
28
Using a comparator all the reference compensation currents thus which are obtained are
compared with those of actual filter currents. Errors obtained are subjected to PWM using
triangular carrier signals [20] and hence the obtained gate pulses are given to the VSI to obtain
4.7.1 Introduction
Fuzzy logic is a type of numerous valued logic that can handle with surmised, as
opposed to settled and accurate thinking. Contrasted with conventional parallel logic (where
variables may tackle genuine or false values), the truth esteem of the fuzzy logic variables might
have that lies in value somewhere around 0 and 1. It has been stretched out to handle the idea of
incomplete truth, where reality quality may lie between totally genuine and totally false. Further
when the linguistic variables are utilized, the degrees might be calculated by particular functions.
4.7.2 Design
Fig 7.1 demonstrates the procedure utilized for fuzzy logic control plan for the shunt
active filter. The controlling model system diagram is as shown in the fig. For the
implementation of the shunt filter control calculation, the voltage of the DC capacitor voltage
ought to be measured and after that it is to be compared with that of the reference voltage [14].
The inputs for fuzzy system are the error(e) which is the difference of the values V DCref
and VDC at nth sampling along with that of the change in error that is equal to ce(n)=e(n)-e(n-1)
29
will be given as. Output of the fuzzy controller will be the active current needed to make up for
all the power loss inside the filter and subsequently will keep the DC capacitor voltage constant.
The reference currents for compensation acquired from the controller are compared to that of the
original currents by utilizing a comparator. These errors are then subjected to PWM by using
triangular carrier signals [14] and the obtained gate pulses are thus given to the VSI to obtain the
The control activity of that of a fuzzy logic controller is primarily gotten from assessment of
a set which contains straightforward simple rules. For the advancement of every one of these
standards we require good understanding of the procedure which should be controlled however
does not require the precise scientific mathematical model regarding procedure that is under
control. The inward structure of the controller and the essential calculation utilized for the fuzzy
The fuzzy rationale framework basically comprises of the detailing of mapping of a given data
set which is the input to the required output data set using fuzzy logic. The mapping procedure is
the premise from which the conclusion will be made. The fuzzy procedure follows the following
All the fresh inputs are firstly changed over to semantic variables in the fuzzification
30
characterizes how the estimations of the fuzzy variable in the specific area are mapped to
another MF μ which lies somewhere around 0 and 1. A Membership function may have
distinctive shape figure. The most basic utilized state of the MF is the triangular-sort which
could be symmetrical or asymmetrical. The trapezoidal MF has the shape of that of a truncated
triangle. Firstly, two MFs are based on a Gaussian distribution curve: the essential Gaussian
cure with a two-sided composite of two distinctive Gaussian distribution curves. The bell MF
with a level top is to some degree not the same as a Gaussian. The Gaussian and also the bell
MFs are both smooth and non-zero for all points which are non-zero [3].
The essential Boolean logic properties are likewise true for Fuzzy logic. We will know
the extent to which every of the antecedent part of the standard will be fulfilled after all the
inputs have been fuzzified. In light of this guideline, operations, for example, OR or AND are
The consequent part of the rule is evaluated with the help of the Implication part of the
process. There are myriad implication methods proposed in the literature. Among all the methods
Results of the fuzzy output which are thus obtained from that of the implication and aggregation
steps. The output will be union of that of all the outputs that is of that of all the individual rules.
defuzzification process is the conversion from fuzzy output into that of a crisp output. For this
process there have been many methods used .Of all the defuzzification methods Center of Area
For this scheme, the error (e) along with change of error (ce) are inputs to the real system
from. For the conversion of the input variables into the output variables, the seven fuzzy levels
31
are thus chosen as: NB,NM,NS,Z,PS,PM,PB (negative-big, negative-medium ,negative-small ,
The elements for the rule table are obtained from good analyzation of the filter behavior as
shown in table 2.
32
Figure 4.5 Input Change in Error Membership Function [16]
33
Table 2 [17]
4.9 Summary
The importance of the shunt active filter and its advantages over classical passive filters
were discussed in this chapter. The designing and calculation of compensation currents using the
“Id-Iq” control strategy, role of DC link capacitor in maintaining good power quality was
considered. Shunt active filter design utilizing the PI control and a fuzzy logic control were
presented. Fuzzy logic was a better scheme than the PI control as it did not require a precise
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Chapter 5
Simulation results and Discussion
A 3 phase system was taken and the active filter was modelled accordingly in
MATLAB/SIMULINK software using both the controllers. A diode bridge rectifier was taken as
the nonlinear load. The filter was designed using a 3 phase 3 level H bridge multilevel inverter.
Fig 5.1 shows the balanced source voltage waveform. Fig. 5.2 shows the load current waveform
for the non-linear load without using a filter. It can be observed from the figures below that the
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Figure 5.2 Source Current without using filter Vs time
Fig 5.3 shows the load current waveform after using a PI controller based shunt active filter
Figure 5.3 Source current after using PI controller based filter Vs time
Fig 5.4 and fig 5.5 show the THD analysis of the source current without using a filter and after
using a shunt active filter. It can be observed from THD values that the harmonics were
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Figure 5.4 Source current harmonic spectrum before compensation
Figure 5.5 Source Current Harmonic spectrum after Using PI Controller Based Filter
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Fig 5.6 shows the source current waveform after using a fuzzy logic controller based shunt
active filter. It can be seen from figure that it does not contain harmonics. Fig.5.7 shows the
THD analysis of the current waveform and it can be seen that the THD was reduced to 1.36%
which was much less than that of the PI controller based filter.
Figure 5.6 Source current after using fuzzy controller based filter Vs time
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Figure 5.7 source current harmonic spectrum after using fuzzy logic controller
5.2 Summary
Simulation results using MATLAB/SIMULINK software for the design of shunt active
filter using PI and fuzzy logic controllers were presented here. The harmonic distortion was also
presented for each of the configuration. The fuzzy controller gave a better result when compared
39
Chapter 6
Conclusion and Future Scope
6.1 Conclusion
In this work the multilevel inverter based shunt active filter was designed and verified
using MATLAB software. Initially different multilevel configurations were modelled using
MATLAB and the output was obtained. Level shifted PWM techniques were used to give the
gating signals. It was found that as the number of number of levels increased, the output was
found to have less harmonics and its shape was more resembling a perfect sine wave. The Id-Iq
control strategy was used to produce the reference currents for compensation. It was found to be
a much better technique as it does not require a phase locked loop to obtain the compensation
currents like other methods. Three level H-bridge was used in the design of the filter.
Maintaining the DC link capacitor voltage constant was the main task in designing the filter and
getting a harmonic free source current. Different controllers like the PI and fuzzy controllers
were employed for this purpose. Using a comparator all the reference compensation currents
thus obtained were compared with those of actual filter currents. Errors obtained are subjected to
PWM using triangular carrier signal. The filters were designed using both PI and Fuzzy logic
controllers. The THD was measured for both the models. The fuzzy controller gave a
compensation of 1.36% and the PI controller gave a compensation of 3.1%. It was found that the
fuzzy control scheme gave better compensation compared to the PI control scheme. Harmonics
were drastically reduced with the use of filters and almost sinusoidal source current waveform
was obtained.
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6.2 Future Scope
1. Many different control strategies like the p-q strategy etc. can be used for the filter design.
2 .Controllers can be designed and verified for distorted source voltage conditions like distorted
amplitude or distorted phase, dynamic load conditions. Practically since the loads are never
3. Many advanced switching techniques are being developed which can be used to give the
4. Use of many switching components which lead to a high cost of equipment in designing the
multilevel inverters is the major challenge in the multilevel inverter based shunt active filter
model. New topologies with less switching can be tries to be developed to overcome this
problem.
41
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