sindhureport.pdf

HYBRID FILTER FOR POWER QUALITY

IMPROVEMENT

A PROJECT REPORT

submitted by

SINDHU.S (CB207PE019)

in partial fulfillment for the award of the degree of

MASTER OF TECHNOLOGY

IN

POWER ELECTRONICS

AMRITA SCHOOL OF ENGINEERING, COIMBATORE

AMRITA VISHWA VIDYAPEETHAM

COIMBATORE 641 105

JULY 2009


AMRITA SCHOOL OF ENGINEERING, COIMBATORE, 641105

BONAFIDE CERTIFICATE

This is to certify that the project report entitled “HYBRID FILTER FOR POWER

QUALITY IMPROVEMENT” submitted by SINDHU.S (REGNO: CB207PE019)

in partial fulfillment of the requirements for the award of the Degree of Master of

Technology in POWER ELECTRONICS is a bonafide record of the work carried

out under my guidance and supervision at Amrita School of Engineering,

Coimbatore.

SIGNATURE SIGNATURE Mrs.M.R.Sindhu Dr.T.N.P.Nambiar Assistant Professor Chairperson Department of Electrical Department of Electrical and Electronics Engineering and Electronics Engineering This project report was evaluated by us on………….. INTERNAL EXAMINER EXTERNAL EXAMINER

Dedicated To ….

My Son Adithya


AMRITA SCHOOL OF ENGINEERING, COIMBATORE

DEPARTMENT OF ELECTRICAL AND ELECTRONICS

ENGINEERING (POWER ELECTRONICS)

DECLARATION

I, SINDHU.S (Reg. No: CB207PE019) hereby declare that this project report,

entitled “HYBRID FILTER FOR POWER QUALITY IMPROVEMENT", is a

record of the original work done by me under the guidance of Mrs. M.R.Sindhu,

Assistant Professor, Department of Electrical and Electronics Engineering, Amrita

School of Engineering, Coimbatore and that this work has not formed the basis for

the award of any degree/ diploma/ associateship/ fellowship or a similar award, to any

candidate in any University, to the best of my knowledge.

Signature of the student Place: Coimbatore Date:

COUNTERSIGNED

Mrs. M.R.Sindhu

Assistant Professor

Department of Electrical and Electronics Engineering,

Amrita School of Engineering, Coimbatore

ACKNOWLEDGEMENT

I express my sincere gratitude to my guide Mrs. M.R.Sindhu, Assistant

Professor, Electrical and Electronics Engineering Department, for her valuable help,

timely guidance and supervision throughout my project work.

I express my sincere thanks to Dr T. N. P. Nambiar, Chairperson, Electrical

and Electronics Engineering Department, for his guidance, supervision and support

throughout the course of this project.

I am thankful to Prof.A.T.Devarajan, Asst. Professor, Electrical and

Electronics Engineering Department, for his valuable suggestions during the entire

course of the project.

I express my thanks to the all the staffs of EEE department, Electrical

Machines Lab and Power Electronics Lab for their kind help and co-operation.

I express my sincere gratitude to my family, for their unending support, love

and encouragement especially, the sacrifices done by my dear son, Adithya. Finally I

express my bows to all of those who are remotely involved in this project.

Above all, I thank Almighty for giving me the strength, courage and blessings

to complete this project.

i

ABSTRACT

The effect of harmonics and power quality problems were introduced into the power

system with the usage of switching converters and other power electronic devices.

Since then, many works have been done to solve these power quality problems and

provide clean power to the customers. Passive tuned filters were the initial step taken

towards this. It consists of tuned LC filters for removing specific harmonic

component or blocking a band of harmonic frequencies generated by the nonlinear

loads. Low cost is a great benefit of these filters. But the installation of passive filters

may result in resonance between the impedance of passive filter and the system

resulting in amplifying the harmonics of source current. Resonance, Fixed

compensation and huge size in passive filters were overcome in active power filters.

Active power filter introduces equal and opposite harmonics and/or reactive

components in to the three phase system so that clean power at unity power factor is

drawn from supply mains. However, depending upon the amount and nature of

compensation demanded, the size and hence the cost of active filters in a practical

industry is too high.

Hybrid power filter topologies have been developed to solve the problems of

harmonic currents and reactive power effectively. Using low cost passive filters in the

hybrid active filter, the power rating of active converter is reduced compared with

that of pure active filters. Hybrid power filters retain the advantages of active filters

and do not have the drawbacks of passive and active filters. This topology lends itself

to retrofit applications with existing passive filters. The hybrid filters are cost-

effective and become more practical in industry applications. This thesis presents simulation studies and hardware testing of a hybrid power

filter using a combination of shunt active power filter and passive filter. The active

power filter employs Icosφ algorithm as control algorithm. The effectiveness of

hybrid power filter is evident from the simulation and hardware results presented in

thesis.

TABLE OF CONTENTS

CHAPTER TITLE PAGE NO

ABSTRACT i

LIST OF FIGURES v

LIST OF TABLES vii

LIST OF SYMBOLS viii

LIST OF ABBREVATIONS ix

1 INTRODUCTION 1

1.1 Power quality improvement 2

1.2 Scope of the work 2

1.3 Organization of the thesis 3

2 LITERATURE SURVEY 4

2.1 Introduction 4

2.2 Literature review 4

2.2.1 Passive filter 5

2.2.2 Active filter 5

2.2.3 Hybrid filter 6

2.3 Conclusion 9

3 POWER SYSTEM HARMONICS – SOURCES 10

AND EFFECTS

3.1 Introduction 10

3.2 Sources of harmonics 10

3.2.1 Transformer 11

3.2.2 Rotating machines 11

3.2.3 Arc furnaces 12

3.2.4 Fluorescent lights 12

3.2.5 Static var converters 12

3.2.6 Cycloconverters 12

3.3 Effects of harmonic distortion 12


3.3.1 Thermal losses in a harmonic 13

environment

3.3.2 Resonances 14

3.3.3 Effects of harmonics on rotating 14

Machines

3.4 Conclusion 15

4 POWER SYSTEM HARMONICS – 16

MITIGATION TECHNIQUES

4.1 Power quality improvement techniques 16

4.2 Passive filters 16

4.2.1 Single tuned filters 17

4.2.2 Double tuned filters 17

4.3.3 Automatically tuned filters 17

4.3.4 Damped filters 18

4.3 Active filters 19

4.3.1 Series active filter 20

4.3.2 Shunt active filter 20

4.3.3 Control algorithm 21

4.3.3.1 Instantaneous PQ theory 21

4.3.3.2 Synchronous detection algorithm 22

4.3.3.3 DC bus voltage algorithm 24

4.3.3.4 Icosφ algorithm 25

4.4 Hybrid filter 27

4.5 Conclusion 29

5 COMPARISON OF ACTIVE FILTERING 30

ALGORITHMS – SIMULATION RESULTS

5.1 Introduction 30

5.2 Simulation model requirements 30

5.3 Simulation of the full controlled converter feeding 32

an RL load without shunt active power filter


5.4 Simulation of Shunt Active Filter under 35

Balanced Source and Balanced Load

Conditions

5.4.1 Simulation of Instantaneous PQ 35

Theory

5.4.2 Simulation of the Synchronous 39

Detection Algorithm

5.4.3 Simulation of DC bus voltage 42

algorithm

5.4.4 Simulation of Icosφ algorithm 45

5.5 Simulation of shunt Active Filter using Icosφ 48

under Unbalanced Source and Balanced Load

conditions

5.6 Simulation of shunt Active Filter using Icosφ 51

algorithm under Balanced Source and unbalanced

load Conditions

5.7 Conclusion 54

6 SIMULATION OF HYBRID POWER FILTER 55 6.1 Introduction 55


an RL load with out filter


an RL load with passive filter


an RL load with shunt active power filter


an RL load with hybrid filter

6.6 Conclusion 71

7 HARDWARE SETUP – DESIGN AND 72

MODELLING OF HYBRID FILTER

7.1 Introduction 72


7.2 Hardware implementation – laboratory 73

prototype

7.3 Control circuit design and test results 75 7.3.1 Fundamental detection of load 75

current

7.3.1.1 Op-amp based circuit of 76

biquad filter

7.3.2 Zero crossing detection 76

7.3.3 Sample and hold circuit 77

7.3.4 The multiplication circuit 78

7.3.5 The subtractor circuit 78

7.3.6 The comparator circuit for 79

PWM generation

7.3.7 Isolation and amplification 79

circuit

7.4 Conclusion 80

8 HARDWARE IMPLEMENTATION OF HYBRID 81

POWER FILTER – UNDER DIFFERENT SOURCE

AND LOAD CONDITIONS

8.1 Introduction 81

8.2 Experimental results 81

8.1.1 Balanced source and balanced load 81

condition

8.1.2 Distorted source and a balanced load 85

condition

8.1.3 Balanced source and unbalanced load 88

condition

8.3 Conclusion 91

9 CONCLUSION AND SCOPE FOR FUTURE WORK 93

9.1Scope and future work 94


REFERENCES 95

APPENDICES 98

Appendix-1 Details of load, Passive filter and 98

inverter

Appendix-2 Datasheet 99

Appendix-3 Photograph of 113

experimental set up

ii

LIST OF FIGURES FIGURE TITLE PAGE NO:

2.1 Hybrid filters- Classification 7

4.1 Transformation from (a) two single 17

tuned filters to (b) double tuned filters

(c) the impedance versus frequency of

filter double tuned for 5th and 7th.

4.2 High pass damped filters: (a) first order 18

(b) second order (c) third order (d) C-type

4.3 (a) Voltage fed PWM inverter (b) Current 19

fed PWM inverter

4.4 (a) Series Active filter (b) Shunt Active filter 20

4.5 Block diagram for implementing Instantaneous 22

PQ theory

4.6 Block diagram for implementing Synchronous 23

Detection Algorithm

4.7 Block diagram for implementing DC Bus Voltage 24

Algorithm

4.8 Basic block diagram of Icosφ algorithm 26

4.9 Basic set up of three phase shunt hybrid filter 27

5.1 Basic block diagram for the implementation of 31

shunt active filter

5.2 Basic block diagram for full controlled converter 32

feeding an RL load

5.3 Source voltage, load current, source current in phase A 33

at α=0° without filter.

5.4 Source current spectrum at α=0° without filter 33

5.5 Source voltage, load current, source current in phase A 34

at α=30° without filter

5.6 Source current spectrum at α=30° without filter 34

iii

FIGURE TITLE PAGE NO:

5.7 Basic setup for shunt active filtering with 36

Controller

5.8 Source voltage, load current, source current 37

in phase A at α=0° with Active Power Filter

using IRPT control algorithm

5.9 Source current spectrum at α=0° with Instantaneous 37

Reactive PQ theory

5.10 Source voltage, load current, source current in 38

phase A at α=30° with Active Power Filter using

IRPT control algorithm

5.11 Source current spectrum at α=30° with Instantaneous 38

Reactive PQ theory



Synchronous Detection algorithm

5.13 Source current spectrum at α=0° with Synchronous 40

Detection algorithm



Synchronous Detection algorithm

5.15 Source current spectrum at α=30° with Synchronous 41

Detection algorithm


in phase A at α=0° with Active Power Filter using

DC Bus Voltage algorithm

5.17 Source current spectrum at α=0° with DC Bus 43

Voltage algorithm




iv


5.19 Source current spectrum at α=30° with 44




using with Icosφ algorithm

5.21 Source current spectrum at α=0° with Icosφ 46

algorithm



with Icosφ algorithm


algorithm



with IcosΦ algorithm


algorithm for unbalanced source and balanced

load condition



using with IcosΦ algorithm

5.27 Source current spectrum at α=30° for unbalanced 50

source and balanced load condition




5.29 Source current spectrum at α=0° with Icosφ algorithm 52

for balanced source and unbalanced load condition




v


5.31 Source current spectrum at α=30° with 53

Icosφ algorithm for balanced source

and unbalanced load condition

6.1 Basic simulation setup for full controlled 55

converter feeding an RL load


in phase A at α=0° without filter

6.3 Source current spectrum at α=0° without 57

filter


in phase A at α=30° without filter


filter

6.6 Matlab simulation of the full controlled converter 60

feeding RL load with passive filters


in phase A at α=0° with Passive filter

6.8 Source current spectrum at α=0° Passive filter 62

6.9 Source voltage, load current, source current

in phase A at α=30° with Passive filter


Passive Filter

6.11 Matlab simulation of the full controlled converter 63

feeding RL load with Active Power filter

6.12 Source voltage, load current and source current in 64

phase A at α =0° with Active Power Filter

6.13 Source current spectrum at α =0° with Active Power 65

Filter

6.14 Source voltage, load current and source current in 65

phase A at α =30° with Active Power Filter

vi


6.15 Source current spectrum at α =30° with 66

Active Power Filter

6.16 Matlab simulation of the full controlled 67

converter feeding RL load with Hybrid Filter

6.17 source voltage, load current and source current 68

in phase A at α =0° with Hybrid Filter

6.18 Source current spectrum at α =0° with Hybrid 69

Filter

6.19 source voltage, load current and source current 69

in phase A at α =30° with Hybrid Filter

6.20 Source current spectrum at α =30° with Hybrid 70

Filter

7.1 Basic Block diagram of three phase hardware setup 73

7.2 Basic setup for hardware using passive filters 74

7.3 The block diagram of the Icosφ analog controller 75

for phase A.

7.4 Op-amp circuit of the biquad filter 76

7.5 Negative zero crossing detection circuit 77

with monostable multivibrator

7.6 sample and hold circuit for getting Icosφ value 77

7.7 The multiplier circuit for producing desired 78

source current

7.8 The subtractor circuit for producing compensation 78

current.

7.9 The comparator circuit for PWM generation 79

7.10 The isolation and amplification circuit 80

8.1 Diode bridge rectifier load with active filter: 82

(a) Source voltage and load current (phase A)

(b) Load current and reference compensation

current (phase A)

vii


8.2 Diode bridge rectifier load: Reference 82

Compensation current and actual compensation

current in closed loop (phase A)

8.3 Diode bridge rectifier load with hybrid filter 83

(shunt active and shunt passive): (a) Load current

and reference compensation current (phase A)

(b)Reference compensation current and actual

compensation current in (phase A)

8.4 a) source voltages b) source current without 84

filtering c) source current with hybrid filter

for balanced source and load condition

8.5 a) a-phase voltage and current without filtering 84

b) a-phase voltage and current with hybrid

filtering for balanced source and load condition

8.6 THD % of Source current spectrum (a) without 85

filtering (b) with active filter for balanced source and

load condition

8.7 THD % of Source current spectrum (a) With passive 85

filter (b) with hybrid filter for balanced source and

load condition

8.8 a) source voltages b) source current without 86

filtering c) source current with hybrid filter

filtering for distorted source and balanced load

condition


b) a-phase voltage and current with hybrid

filtering for distorted source and balanced load

condition

viii



filtering (b) with active filter for distorted source

and balanced load condition


filter (b) with hybrid filter for distorted source and

balanced load condition

8.12 a) source voltages b) source current without filtering 88

c) source current with hybrid filter for balanced

source and unbalanced load condition


b) a-phase voltage and current with hybrid filtering

for balanced source and unbalanced load condition


filtering (b) with active filter for balanced source

and unbalanced load condition


filter (b) with hybrid filter for balanced source and

unbalanced load condition

ix

LIST OF TABLES

TABLES TITLE PAGE NO:

5.1 %THD in the mains currents before and after 48

shunt compensation for two firing angles α=0°

and α=30°

5.2 %THD in the mains currents before and after 53

shunt compensation under Balanced/ Unbalanced

Source and Balanced/unbalanced Load Conditions

using Icosφ algorithm

6.1 parameter values of passive filter for simulation 61

6.2 Simulation of the three phase system with 70

passive filter, active filter and hybrid filter

8.1 Comparison of source current spectrum with and 90

without hybrid filter under balanced source and

load condition (50V/phase)


without hybrid filter under distorted source and

load condition (50V/phase)


without hybrid filter under balanced source and

unbalanced load condition (50V/phase)

x

LIST OF SYMBOLS ea,eb,ec or va,vb,vc Three phase instantaneous mains voltages.

iLa,iLb,iLc Three phase instantaneous load currents

PL Instantaneous real power

qL Instantaneous reactive power

pL Ripple (AC) in real power

Ripple in reactive power

icα, icβ or icαβ Reference compensation currents in αβ plane

i∗ca, i∗cb, i∗cc or i∗cabc Three phase reference compensation currents

P Instantaneous three phase power

Pdc Average value of three phase power

Pa, Pb, Pc Average power in each phase

Ea, Eb, Ec Three phase mains voltage amplitudes

Etot Sum of Ea, Eb, Ec

ima, imb, imc Three phase desired main currents

Vm Phase voltage magnitude

ω Fundamental frequency in rad/sec

ILa1, ILb1, ILc1 Fundamental component of load current in each

phase

I.cosφ real component of fundamental component of load

current

Φ phase angles of fundamental load current

desired amplitude of source current

Ua,Ub,Uc Unit amplitude phase voltages in the three phases

isa(ref), isb(ref), isc(ref) Three phase desired (reference) mains currents

ia(comp), ib(comp), ic(comp) Three phase reference compensation currents

THD Total harmonic distortion

1

CHAPTER I

INTRODUCTION

The solid state power conversion is rapidly increasing due to wide

application of adjustable speed drives (ASDs), arc furnaces, bulk rectifiers, TV

sets, fax machines, computers, fluorescent tubes and microwave ovens, etc. Apart

from this a large number of solid state power converters such as diode bridge

rectifiers and thyristor rectifiers are used in the numerous industrial applications

and transmission/distribution networks. All these breeds of power converters are

nonlinear in nature and cause serious problems of current harmonics, poor power

factor, non sinusoidal supply voltage, reactive power burden and low system

efficiency. Hence, due to these serious issues there has been an increasing interest

in the subject of power quality and in the equipments which can suppress supply

harmonics, improve power factor and balance the input supply.

The various power quality problems are voltage sags and swells, voltage

transients, voltage flicker, harmonic distortions, voltage spikes etc. The overall

degradation in the electric power utility affects sensitive loads such as computers,

automated processing in industries and other microprocessor controlled medical

equipments risking the loss of valuable data. With growing awareness about these

problems, the power system utilities, industries and commercial establishments

started protecting themselves by investing in sophisticated protection equipment

for harmonic mitigation. The IEEE 519, IEC-6100, EN-50160 are some of the

standards defining power quality issues [4]. All these standards give an account of

harmonic generation and stipulated limits in the system in the presence of static

converters such as arc furnaces, static VAR compensator, inverters, switched

mode power supplies, pulse width modulated drives etc. The effects of harmonics

on various equipments are also described with guidelines for the measurement and

analysis of harmonics.

2

1.1 POWER QUALITY IMPROVEMENT

The quality of electric power received by the customer has always been a great

concern as reliability of electric supply. The term “Power Quality” is associated

with the fact that the consumer is bound to receive clean power, which provides

balanced, undistorted voltages of constant amplitude at a specified frequency i.e.;

the power quality is associated with the delivery of a sufficiently high grade of

electrical service [5]. It should have regulated voltage as well as steady current

waveforms without dips and surges and should maintain the desired load

frequency. The current and voltage waveforms are expected to be sinusoidal and

at unity power factor.

Conventionally harmonic distortion has been dealt-with by establishing banks

of tuned passive filters. Tuned passive filters have advantages of being simple to

design, cheap and reliable. However, they are limited by drawbacks such as fixed

compensation, resonance and huge size. Active filters were developed to mitigate

problems of passive filters. They are more effective in harmonic compensation

and improved performance. But pure active filters have high cost and require

comparatively high power converter ratings. Hybrid power filters, inheriting the

advantages of both passive filters and active filters provide improved performance

with a reduction in the overall cost of the power circuit.

1.2 SCOPE OF THE WORK

This work deals with power quality improvement by means of hybrid power

filter in which the shunt active power filter uses Icosφ control algorithm. Initially,

simulation and comparative study of existing shunt active filter algorithms in

Matlab simulation platform were carried out to verify the effectiveness of the

control algorithm under balanced source and balanced load conditions. Also, the

simulations of the control algorithm have been carried out for unbalanced source

and unbalanced non-linear load conditions. Finally simulation and hardware

testing of hybrid power filter under various source and load conditions was also

developed.

3

1.3 ORGANIZATION OF THE THESIS

The thesis is presented as 11 chapters. The chapter 1 brief introduction to the

thesis work ands scope of the work Chapter 2 covers the literature survey which is

a collection of research works done in areas of passive filter, active filter, hybrid

filter, control techniques etc. Chapter 3 describes sources of harmonics and

effects. Chapter 4 describes the mitigation techniques. In chapter 5, using

simulations a comparative study of the commonly used control algorithms is done.

Design of passive filter and simulation of hybrid power filter and results are

included in chapter 6. The hardware setup of hybrid filter is given in chapter 7. In

chapters 8, hardware implementation results of hybrid filter with diode bridge

rectifier load under different conditions including balanced/unbalanced source and

balanced/unbalanced load are presented. Chapter 9 gives the conclusion of the

work.

4

CHAPTER II

LITERATURE SURVEY

2.1 INTRODUCTION

Solid-state conversion of AC power using diodes and thyristors is widely adopted

to control a number of processes such as adjustable speed drives (ASD), furnaces,

chemical processes such as electroplating etc., power supplies, welding, heating

etc. These solid-state converters are also used in power industries such as

transmission systems, battery energy storage systems and interfacing renewable

energy electricity generating systems. Some of these solid-state controllers draw

harmonic currents and reactive power from the AC mains and behave as nonlinear

loads. Moreover, in three-phase AC mains, they also cause unbalance and poor

efficiency of the systems. Initially, lossless passive filters (LC) have been used to

reduce harmonics, and capacitors have been chosen for power-factor correction of

these nonlinear loads. But passive filters have the demerits of fixed compensation,

large size, detuning [15] and resonance with the supply system.

2.2 LITERATURE REVIEW

In recent years, the increasing application of power electronic equipments which

results in the generation of harmonic current components and causes higher

distortion levels throughout power systems. Harmonic resonance may cause

serious problems such as overvoltage in power systems. Because of such severity

of power quality problems, several standards have been developed [3] and are

being enforced on consumers, manufacturers and utilities. Moreover the power

community has become more conscious of these power quality problems and

numbers of technology options have been reported in the literature and research

publications. Detailed analysis of sources, effects and mitigation of harmonics

have been done and which is given in chapter 3 and 4.

5

2.2.1 PASSIVE FILTERS

Passive filters are widely used in power systems for harmonic mitigation. Passive

filters are classic methods for power quality improvement of distribution systems

consist of series LC tuned for removing a specific harmonic or blocking a

bandwidth of severe harmonics of nonlinear load current. These filters have low

impedances for the tuned frequencies such as 5th and 7th and for these

frequencies, the lower impedance of the filter in comparison with system

impedance, the better filtering characteristics of the passive filter. These devices

have advantages of being simple to design, cheap and reliable. However, shunt

passive filters have the following inherent problems which discourage their

application [21-23]:

Mistuning due to component tolerances of the inductors and capacitors,

variation of component values will strongly affect the filtering

characteristics.

The source impedance strongly influences filtering characteristics of the

shunt passive filter.

The shunt passive filter acts as a sink to the harmonic current flowing

from the source. In the worst case, the shunt passive filter falls in series

resonance with the source impedance.

At a specific frequency, a parallel resonance occurs between the source

impedance and the shunt passive filter, which is the so-called harmonic

amplification.

2.2.2 ACTIVE FILTER

Active power filters are one of the most important remedial measures to solve

power quality problems. Configurations of active filters can be classified based on

converter type, topology, and the number of phases. The converter type can be

either CSI or VSI bridge structure. The topology can be shunt, series, or a

combination of both. The third classification is based on the number of phases,

such as two-wire (single phase) and three- or four-wire three-phase systems [7,

20].

6

The control scheme used to generate the compensation signals forms the heart of

the active filtering unit. Instantaneous PQ theory [7], synchronous detection

algorithm [8], DC bus voltage algorithm [9] and synchronous reference frame

theory [10] are some of the widely used three-phase shunt active filtering

algorithms. Existing control schemes still require fine-tuning to make the

computations and circuit implementation as simple and rugged as possible. A

simple and effective control circuit enhances the speed of response and efficiency

of the filter. Icosφ algorithm is such a simple current compensation algorithm to

compensate for harmonics and reactive power in three-phase shunt active filters

[12, 13]. The active filter is expected to provide reactive power compensation

along with harmonic compensation in the case of non-linear reactive loads such as

thyristor converters, ac voltage regulators, etc. in the Icosφ algorithm, the

compensation currents are based on the active part of the load current [1].

Active filters (AFs) [7, 20] in shunt and series configurations can be used to

compensate for different types of nonlinear loads. However, they have the

drawback that their rating is sometimes very close to load (up to 80%) in some

typical applications and thus it becomes a costly option for power quality

improvement in a number of situations. Moreover, a single active filter does not

provide a complete solution for compensation in many cases of nonlinear loads

due to presence of both voltage and current power quality problems. Because of

the higher rating of AFs and cost considerations, the acceptability of AFs by users

has been limited in practical situations. In response to these factors, a series of

hybrid filters has been evolved and extensively used in practice as a cost-effective

solution for the compensation of nonlinear loads. Moreover, these hybrid filters

(HFs) are found to be more effective in providing complete compensation of

various types of nonlinear loads.

2.2.3 HYBRID FILTERS

Hybrid filters are combination of more than one active filter or passive filter. It is

quite popular because the solid state devices used in the active part can be of

reduced size and cost (about 5% of load size).Here, generally, passive filters are

7

used to eliminate lower order harmonics. The higher order harmonic currents,

which are much less compared to lower order harmonics, are eliminated by active

filter. According to research publications on hybrid filters [3], hybrid filters can be

classified based on a number of elements in topology, supply system and types of

converters used in their circuits, as shown in Fig.13.

Fig.2.1: Hybrid filters- Classification

The number of elements in the topology can either be two, three or more, which

may either be active filters or passive filters. The supply system can be single

phase 2 wire, three phase 3 wire or three phase 4 wire to feed variety of nonlinear

loads. Voltage source inverters (VSI) or current source inverters (CSI) can be

used to realise the active filters as part of hybrid filters with appropriate control.

Main classification of hybrid filters is made on the basis of supply system, with

topology of filters as sub-classification.

Control scheme of hybrid filter require a control scheme, which has three major

stages. The first stage includes sensing of instantaneous current and voltage

signals such as AC voltage at the point of common coupling, injected voltages by

series active filter element, AC currents injected by shunt active filter element,

DC bus voltage or current depending on use of VSI or CSI in the implementation

of active filter by means of isolation amplifier or hall effect sensors. Second stage

is derivation of compensating signals. Control techniques based on time domains

8

are simple to implement and result in fast dynamic response due to instantaneous

derivation of compensation commands and nowadays they are only used in hybrid

filters. Many time domain control approaches such as instantaneous p – q theory,

synchronous reference frame theory, synchronous detection method etc. are used

to derive the compensating voltages in the case of series active filter or

compensating currents in the case of shunt active filters. The derived

compensating command signals are compared with sensed feedback signals and

error is processed in PWM controller to generate digital gating signals. These

digital (low/high) gating signals are buffered, isolated and amplified to feed the

gate of the solid state switching devices of active filter used in hybrid filters.

Selection of components of hybrid filters is important to attain high level

performance. There are a number of components in hybrid filters, such as passive

filter elements, active filter elements, control scheme employing sensors,

processor, isolation amplifier circuits, interfacing circuits, injection transformers

etc.

Passive filter consists of several AC capacitors, inductors and a small resistor to

be used in damped high pass filter. These inductors must have a quality factor as

high as possible to reduce the losses in the system and they must be designed in

such a way that they must not saturate in the whole current operating range. In

passive filters, capacitor is decided by the required reactive power in the system,

inductor is calculated by tuning it to particular harmonic frequency. Resistance of

the filter is calculated reducing the losses to optimum value of quality factor.

Another important component in the hybrid filter is the active filter element,

which is realised using VSI with ripple filter on AC side. Solid state switching

device used is a MOSFET for small ratings, an IGBT for medium power ratings

and a GTO for exceptionally high power ratings. Major component is a processor,

which receives the input signals, computes the algorithm and generates optimised

PWM signals. One of the major reasons for the advance of hybrid filter

technology consisting active filter elements is the development of fast self

commutating solid state devices such as MOSFET, IGBT etc. An improved low

cost sensor technology, compact isolation amplifiers and evolution of

9

microelectronics have made hybrid filters affordable. The development of low

cost, high accuracy and fast digital signal processors, microcontrollers and

application specific integrated circuits (ASICs) has made possible the

implementation of complex control algorithms for real time control at an

acceptable price

2.3 CONCLUSION

This chapter gives review of different types of filters used to mitigate distortion

and unbalance in power supply. The earliest type of filters were shunt /series

passive filters connected at PCC which provide a low/high impedance path to the

selected harmonics, preventing most of the selected harmonics from appearing at

the source. But they have drawbacks such as resonance, fixed compensation, high

no load losses, bulky size etc. As a better option of complete compensation of

distortions, active power filters have been researched and developed. Many

mature control algorithms are available in literature for the control of three-phase

active filters. Most of them use tedious computations and complex circuits and

hence highly expensive and slow in response. Hence as a better and economical

option, combination of passive and active filters, named hybrid filters, are

implemented. Hybrid filters allow designing active filters for only a fraction of

total load power, reducing costs and increasing overall system efficiency.

10

CHAPTER III

POWER SYSTEM HARMONICS - SOURCES AND EFFECTS

3.1 INTRODUCTION

Harmonics in power systems have been known since the adoption of alternating

current as means for electrical energy transmission. They have been magnified

with the increased use of non-linear devices. A non-linear device produces a non-

sinusoidal current when applied with a sinusoidal voltage and vive versa. The

harmonic sources can be broadly classified in to major and minor sources. Prior to

the appearance of power semiconductors, the main sources of waveform distortion

were electric arc furnaces, the accumulated effect of fluorescent lamps, and to a

lesser extent electrical machines and transformers. The increasing use of power

electronic devices for the control of power apparatus and systems has been the

reason for the greater concern about waveform distortion in recent times.

3.2 SOURCES OF HARMONICS

The sources of power quality issues can be broadly classified to two categories:

nonlinear loads, power system equipment and components, subsystems of

transmission and distribution systems [15]. In the former category, thyristor

converters, UPS, pulse modulated load, ASDs, arc furnaces, welding machines,

static var compensators, inverters, SMPS, fluorescent and other gas discharge

lighting, etc. In the latter category, grounding systems and resonance problems

can be included. Huge amounts of harmonic currents are generated by single

phase power electronic loads such as desktop computers, TVs, Fax Machines,

Copiers, Microwave ovens, heat pumps and electric vehicle battery chargers, etc.

The switching or commutation of power semiconductor devices generates voltage

or current transients that are characterized by a whole spectrum of frequencies.

During turn on or turn off, sudden collapse of electromagnetic field takes place

and EMI generated.

11

The most common power electronic aid is the single-phase rectifier, used to

power most modern office and domestic appliances. Although the individual

ratings are always small, their combined effect can be an important source of

waveform distortion [14]. The sources of harmonics can be classified as

(1) Traditional types

a. Transformers

b. Rotating machines

c. Arc furnaces

(2) Modern types

a. Fluroscent lamps

b. Thyristor-controlled devices which include

i. Rectifiers

ii. Inverters

iii. Static VAR compensators

iv. Cycloconverters

v. HVDC transmission

3.2.1 TRANSFORMER

Power transformers are sources of harmonics since they use magnetic materials

that are operated very close to and often in the non-linear region for economic

purposes. This result in the transformer magnetizing current being non-sinusoidal

and containing harmonics (mainly third harmonics) even if the applied voltage

were sinusoidal. In the three-phase transformers a delta or ungrounded wye

connection blocks the flow of zero sequence triplen harmonics currents.

3.2.2 ROTATING MACHINES

Rotating machines are considered as sources of harmonics because the windings

are embedded in slots which can never be exactly sinusoidal distributed so that the

mmf is distorted. Large generators are usually connected to power grid through

delta-connected transformers thus blocking the flow of third harmonic current.

Generally, harmonics produced by rotating machines are considered negligible

compared to those produced by other sources.

12

3.2.3 ARC FURNACES

The voltage-current characteristics of electric arcs are highly nonlinear. Following

arc ignition the voltage decreases due to the short-circuit current, the value of

which is only limited by the power system impedance. The main harmonic

sources in this category are the electric arc furnace, discharge type lighting with

magnetic ballasts, and to a lesser extent arc welders.

3.2.4 FLUORESCENT LIGHTS

In a fluorescent lamp, the voltage builds up in each half cycle till ignition occurs.

The lamp then appears as a negative resistance, the current limited by the non-

linear reactive ballast. The current is thus distorted.

3.2.5 STATIC VAR COMPENSATORS

Static VAR compensators are balanced three-phase devices that use SCR’s to

control the conduction time of shunt capacitors or inductors during each half cycle

in order to maintain a desired terminal voltage. Thus, non sinusoidal “chopped ”

currents are produced.

3.2.6 CYCLOCONVERTERS

A cycloconverter is a variable frequency a.c. motor drive composed of two three-

phase bridges supplying a single-phase output. It converts ac power at a higher

frequency to one at a lower frequency. With a line frequency of 50 Hz, the

cycloconverter output frequency can be varied from 0 to 10 Hz. The speed of

cycloconverter-driven large slow-speed synchronous motors can be continuously

varied between zero and 15 rpm.

3.3 EFFECTS OF HARMONIC DISTORTION

The main effects of voltage and current harmonics within the power system are:

• the failure of capacitor banks due to dielectric breakdown or reactive power

overload

13

• interference with ripple control and power line carrier systems

• causing mis-operation of systems which accomplish remote switching, load

control and metering

• dielectric breakdown of insulated cables resulting from harmonic over

voltages

• inductive interference with communications systems

• errors in meter reading

• signal interference and relay malfunction particularly in solid state and

microprocessor based control systems

• mechanical oscillations of induction and synchronous machines

• unstable operation of firing circuits based on zero crossing detecting or

latching

• excessive heating of transformers due to frequency dependent core

• change in TV picture size and brightness if harmonics affect the peak

voltage

• effects on computer and computerised automation production.

Among the possible external effects of harmonics are degradation in

communication systems performance, excessive audible noise and harmonic-

induced voltage and currents. Harmonics have the effect of increasing equipment

losses and thus the thermal stress.

3.3.1 THERMAL LOSSES IN A HARMONIC ENVIRONMENT

Harmonics have the effect of increasing equipment copper, iron and dielectric

losses and thus the thermal stress. The per unit increase in copper loss is

determined by the current distortion factor alternatively voltage distortion factor

both being equal for pure resistance. Iron Losses are those losses taking place in

an iron core which is magnetized by an applied excitation or by rotating in a

magnetic field. These losses consist of hysteresis loss and eddy current loss and

result in reducing the efficiency and increasing the core temperature thus limiting

the output. Hysteresis loss is due to reversal of magnetization of an iron core and

depends on the volume and quality of used magnetic material, maximum value of

flux density and frequency of electric current. Eddy current loss is associated with

14

flow of eddy current induced in the armature core of a rotating machine as a result

of its rotation in the magnetic field or in the core of a transformer as a result of ac

excitation. The dielectric loss in a capacitor or insulation loss in a cable is due to

the fact that it is not an ideal capacitor whose current leads the voltage by a factor

of 90o.

3.3.2 RESONANCES

The presence of capacitors, such as those used for power factor correction, can

result in local system resonances, which lead in turn to excessive currents and

possibly subsequent damage to the capacitors. Parallel resonance results in high impedance at the resonant frequency being

presented to the harmonic source. Parallel resonances can occur in a variety of

ways, the simplest perhaps being that where a capacitor is connected to the same

busbar as the harmonic source. A parallel resonance can then occur between the

system impedance and the capacitor.

The concern with series resonance is that high capacitor currents can flow for

relatively small harmonic voltages.

3.3.3 EFFECTS OF HARMONICS ON ROTATING MACHINES

Non-sinusoidal voltages applied to electrical machines may cause overheating.

Motors are not normally derated so long as the harmonic distortion remains within

the 5% normally recommended by the regulations. Above that limit they will

often experience excessive heating problems.

Harmonic voltages or currents give rise to additional losses in the stator windings,

rotor circuits, and stator and rotor laminations. The losses in the stator and rotor

conductors are greater than those associated with the d.c. resistances because of

eddy currents and skin effect.

The effects of harmonics on various power system equipments are discussed

in [14]. The oscillatory transient can lead to transient over voltage and consequent

damage to power line insulators. Short duration voltage variations such as voltage

sags or voltage swells can cause loss of production in automated processes.

Voltage imbalance can cause temperature rise in motors and can even cause a

15

large motor to trip. Inter-harmonic voltages can upset the operation of fluorescent

lamps and television receivers. DC offsets can cause saturation in the power

transformer magnetic circuits. A notch can damage capacitive components

connected in shunt due to high rate of voltage rise at the notches. Voltage flickers

may adversely effect human health as the high frequency flickering of light bulbs,

fluorescent tubes or television screen can cause strain on the eyes resulting in

headache and migraine. The voltage flicker can also reduce the lifespan of

electronic equipment, lamps etc.

3.4 CONCLUSION

Harmonic sources in power system and their effects are studied.

16

CHAPTER IV

POWER SYSTEM HARMONICS - MITIGATION

TECHNIQUES

4.1 POWER QUALITY IMPROVEMENT TECHNIQUES

The power quality improvement techniques can be classified into precautionary

(preventive) solutions and corrective (remedial solutions).Phase cancellation or

harmonic control in power converters, usage of low distortion loads etc. are

preventive solutions. Usage of harmonic filters for elimination of harmonics and

unbalance are corrective solutions. Harmonic elimination is done commonly using

passive and active filter. The combination of active and passive filter is known as

hybrid filer.

4.2 PASSIVE FILTERS

The passive filter consists of capacitors and inductors which are connected in

shunt with power system. The passive filter is tuned to a desired frequency which

is to be filtered out from the power system. At resonance condition, tuned

frequency components will be bypassed. Conventionally tuned passive LC filters

have been used to compensate a portion of reactive power and harmonics in

power system. A number of configurations are suggested such as single tuned,

double tuned, triple tuned, quadruple tuned, damped, auto tuned etc. Series

passive filters are required to prevent a particular component from entering

selected plant components or parts of a power system which offers large

impedance to the relevant frequency component. Shunt passive filters offer very

low impedance to the harmonic frequencies and prevents from entering rest of the

system. The sharpness of tuning of filter is denoted by Q-factor. For tuned filters,

Q-factor is recommended in the range 30 – 60 and for damped filters it is 0.5 -

5.But variations in filter capacitance and inductance due to ageing and

temperature, may cause detuning from nominal tuned frequency. Passive filter

are of various configuration which explained below.

17

4.2.1 SINGLE TUNED FILTERS

A single tuned filter is a series RLC circuit in Figure 1 tuned to the frequency of

one harmonic (generally a lower characteristic harmonic). Its impedance is given

by Z 1= R+ j (ωL -1/ ωC) and which at the resonant frequency (fn) reduces to R.

There are two basic design parameters to be considered prior to the selection of R,

L and C. These are the quality factor (Q), and the relative frequency deviation (δ),

already defined.

In order to express the filter impedance in terms of Q and δ, the following

relationships apply:

ω= ωn(1 + δ) , where ωn=1/√ (LC). The reactance of inductor or capacitor in ohms

at the tuned frequency is X0= ωnL=1/(ωnC).

4.2.2 DOUBLE-TUNED FILTERS

Double tuned filter have the advantage of reducing the power loss at fundamental

frequency and recommended for high voltage applications, because of the

reduction in number of inductors subjected to full line impulse voltages and

shown in Fig.1.

Fig.4.1: Transformation from (a) two single tuned filters to (b) double tuned filters

(c) the impedance versus frequency of filter double tuned for 5th and 7th.

4.2.3 AUTOMATICALLY TUNED FILTERS

Automatically tuned filters use a control system to measure reactive power and

hence control the value of inductance and capacitance based on sign and

magnitude of reactive power. In tuned filter design it is advantageous to reduce

18

the maximum frequency deviation. This can be achieved by making the filters

tunable by either automatically switching the capacitance or by

varying the inductance. A range of ±5% is usually considered adequate. It has

advantages such as low inductor rating and small capacitor rating.

4.2.4 DAMPED FILTERS

For filtering a range of harmonic frequencies, damped filters are recommended,

which is shown in Fig4.2. The damped filter offers several advantages:

(1) Its performance and loading are less sensitive to temperature variation,

frequency deviation, component manufacturing tolerances, loss of capacitor

elements, etc.

(2) It provides low impedance for a wide spectrum of harmonics without the need

for subdivision of parallel branches, which increases switching and maintenance

problems.

(3) The use of tuned filters often results in parallel resonance between the filter

and system admittances at a harmonic order below the lower tuned filter

frequency, or in between tuned filter frequencies. In such cases the use of one or

more damped filters is a more acceptable alternative.

Fig 4.2: High pass damped filters: (a) first order (b) second order (c) third order

(d) C-type

The main disadvantages of the damped filter are as follows:

(4) To achieve a similar level of filtering performance the damped filter needs to

be designed for higher fundamental VA ratings, though in most cases a good

performance can be met within the limits required for power factor correction.

(5) The losses in the resistor and reactor are generally higher.

19

4.3 ACTIVE FILTER

Practically passive filters suffer from drawbacks as dependence of filtering

characteristics on source impedance, detuning, parallel/series resonance between

power system, high no load losses, bulky size and fixed compensation. It cannot

solve random variations in the load current waveform. To overcome the

difficulties explained with the passive filters, active filters have been developed,

which provide dynamic and adjustable solutions to power quality issues. Active

filters compensate harmonic components of load current by injecting equal but

opposite harmonics i.e. mains only need to supply the fundamental component of

load current. The design complexity and high cost of losses of the conventional

passive filters, as well as their restricted capability to eliminate inter-harmonics

and non-characteristic harmonics, has encouraged the development of harmonic

compensation by means of power electronic devices, and commonly referred to as

active filters.

These active filter circuitries have been developed employing modern fast

switching power devices with turn off capability like insulated gate bipolar

transistors (IGBTs). The converters commonly used in active filters are current

fed PWM modulation inverter and voltage fed PWM inverter, which is shown in

Figure 4.3. Voltage fed PWM inverter is more dominant, since it is lighter,

cheaper and expandable to multilevel and multistep versions, to enhance the

performance with lower switching frequencies.

Fig 4.3(a): Voltage fed PWM inverter Fig 4.3(b): Current fed PWM inverter

20

According to their connection to the network, active filters can be of the series

type, as shown in figure 4.4 (a), to prevent the transfer of harmonic current, or of

the shunt type, shown in Figure 4.4(b), to reduce the harmonic content in the

network.

Fig 4.4 (a) Series Active filter Fig 4.4 (b): Shunt Active filter

4.3.1 SERIES ACTIVE FILTER An active series filter is connected in series with the mains using a matching

transformer to eliminate voltage harmonics and to balance and regulate terminal

voltage of the load or line and shown in Fig 4.4(a). It also helps to damp out

harmonic propagation caused by resonance with line impedance and passive shunt

compensators. Series active filters are less common industrially than parallel

active filters. It is because of the main drawback of series circuits, that they have

to handle high load currents, which increases their current rating considerably.

This category of filters are mainly used to improve the quality of system voltage,

which is important for voltage sensitive devices such as superconducting magnetic

energy storage and power system protection devices.

4.3.2 SHUNT ACTIVE FILTER

Shunt active filters have the advantage of carrying only the compensation current

plus a small amount of active fundamental current supplied to compensate system

losses and shown in Fig 4.4(b). They can be made suitable for a wide range of

power ratings, by connecting several filters in parallel to supply higher currents.

21

Shunt active filters can be again subdivided into standard inverter, switched

capacitor, lattice structured and voltage regulator filters.

4.3.3 CONTROL ALGORITHM

The active filtering algorithm computes the reference compensation signals to be

generated by the active power filter (APF) to provide reactive power / harmonic /

unbalance compensation. The choice of the control algorithm therefore decides

the accuracy and the speed of response of the filter. The calculation steps involved

in the control technique have to be simple and minimal to make the control circuit

compact and reliable. Instantaneous PQ theory [8], synchronous detection

algorithm [9], DC bus voltage algorithm [10], synchronous reference frame theory

[11] etc. are some of the widely used three-phase active filtering algorithms.

4.3.3.1 INSTANTANEOUS PQ THEORY

In this algorithm proposed by Akagi [8], the three phase mains voltages and load

currents of the system are sensed and converted into the α − β (two) phase plane

using Park’s transformation.

Where ea, eb, ec are the three phase mains voltages.

where iLa, iLb, iLc are the three-phase load currents.

The instantaneous real power pL and the instantaneous imaginary power

consumed by load current are derived as,

where pL and qL contain a DC term and an AC term.

22

They can be represented as,

For harmonic elimination and reactive power compensation, the ripple in the real

power and the whole of reactive power are to be supplied by the active filter. The

reference compensation currents are therefore derived as,

Applying inverse Park’s transformation on the above signals gives the reference

compensation currents in the three-phases as,

Fig 4.5: Block diagram for implementing Instantaneous PQ theory

4.3.3.2 SYNCHRONOUS DETECTION ALGORITHM

The synchronous detection algorithm [9] computes the loading on individual

phases of the system, from the three phase mains voltages and load currents as:

23

The average value of ‘P’ then computed as Pdc, by passing ‘P’ through a low pass

filter. Pdc is divided among the three phases of the system as:

where Ea, Eb, Ec are the amplitude three phase main voltages and Etot is the sum

Ea, Eb, Ec.

With objective of achieving unity power factor at the source end, the desired

mains currents in the three phases are computed as:

The reference compensation signals are therefore the differences between the

desired mains currents and the actual load currents. i.e.

i *ca= ima−iLa; i*cb= imb−iLb ;

i *cc= imc−iLc

Fig.4.6: Block diagram for implementing Synchronous Detection Algorithm

24

4.3.3.3 DC BUS VOLTAGE ALGORITHM

The DC Bus Voltage Algorithm [10] is based on the idea that active power filter

forces the mains current to be sinusoidal and in phase with the main voltage, in

spite of the load characteristics. The magnitude of the mains current is related to

the power balance in the system. Any power unbalance affects the average voltage

of the DC link capacitor of the inverter. The change in the average voltage of the

DC link capacitor from its set value is fed to PI controller, whose output gives the

corresponding amplitude of the desired mains currents. The output of the PI

controller is multiplied with unit amplitude sinusoidal reference waves in phase

with the mains voltage, in order to get the desired mains currents. The actual

mains currents are detected and compared with desired values at a current mode

controller to generate the switching patterns for the APF.

Fig.4.7: Block diagram for implementing DC Bus Voltage Algorithm

25

4.3.3.4 Icosφ ALGORITHM

In the Icosφ algorithm [1], the desired mains current is assumed to be the product

of the magnitude Icosφ and a unit amplitude sinusoidal wave in phase with the

mains voltage. The source is required to supply only the active portion of the load

current as the shunt active power filter is expected to provide compensation for

the harmonic and reactive portion of the three-phase load current, and also for any

imbalance in the three-phase load currents. Hence, only balanced currents will be

drawn from the mains which will be purely sinusoidal and in phase with the mains

voltages. Fig 4.8 gives the block diagram for implementing the algorithm.

The reference compensation currents for the shunt active filter are thereby

deduced as the difference between the actual load current and the desired source

current in each phase. i.e.

where, the desired (reference) source currents in the three phases are given as,

Ua, Ub and Uc are the unit amplitude templates of the phase to ground source

voltages in the three phases respectively where

26

Fig 4.8: Basic block diagram of Icosφ algorithm

For any phase load current can be written as, imsin(wt-φ). Fundamental component

of load current is getting after low pass filtering operation. Low pass filtering is

done using biquad filter and its output is fundamental component of load current

which is 90° phase shifted and so fundamental form can be written as imsin(wt –

φ- 90o). At the time of the zero crossing of the input voltage ωt becomes 180° and

fundamental component of load current at that time becomes Icosφ. This value is

the magnitude of active component of load current.

However, the rating of active filters is very close to load (up to 80%) and

hence cost of shunt active filters is high and they are difficult to implement in

large scale. Additionally, the efficiency of active filter is determined by its control

algorithm. The above reasons led to different solutions to improve the practical

utilisation of active filters. One of them is to use a combined system of passive

filters and active filters as hybrid filters.

27

4.4 HYBRID FILTER

Hybrid filters are combination of more than one active filter or passive filter. It is

quite popular because the solid state devices used in the active part can be of

reduced size and cost (about 5% of load size).Here, generally, passive filters are

used to eliminate lower order harmonics. The higher order harmonic currents,

which are much less compared to lower order harmonics are eliminated by active

filter. Hybrid filters allow designing active filters for only a fraction of total load

power, reducing costs and increasing overall system efficiency. Figure 4.9 shows

the basic set up of hybrid filter.

Fig 4.9: Basic set up of three phase shunt hybrid filter.

The number of elements in the topology can either be two, three or more, which

may either be active filters or passive filters. The supply system can be single

phase 2 wire, three phase 3 wire or three phase 4 wire to feed variety of nonlinear

28

loads. Voltage source inverters (VSI) or current source inverters (CSI) can be

used to realise the active filters as part of hybrid filters with appropriate control.

Main classification of hybrid filters is made on the basis of supply system, with

topology of filters as sub-classification.

Each category of above hybrid configurations can be classified to

(i) hybrid of two passive elements

(ii) hybrid of three passive elements

(iii) hybrid of one active and one passive filter

(iv) hybrid of three elements- two passive with one active

(v) one passive with two active filter elements

(vi) hybrid of two active elements

(vii) hybrid of three active filter elements

The hybrid filters of more than three elements are rarely used because of cost and

complexity considerations

Control scheme of hybrid filter require a control scheme, which has three major

stages. The first stage includes sensing of instantaneous current and voltage

signals such as AC voltage at the point of common coupling, injected voltages by

series active filter element, AC currents injected by shunt active filter element,

DC bus voltage or current depending on use of VSI or CSI in the implementation

of active filter by means of isolation amplifier or hall effect sensors. Second stage

is derivation of compensating signals.

Control techniques based on time domains are simple to implement and result in

fast dynamic response due to instantaneous derivation of compensation

commands and nowadays they are only used in hybrid filters. Many time domain

control approaches such as instantaneous p – q theory, synchronous reference

frame theory, synchronous detection method etc. are used to derive the

compensating voltages in the case of series active filter or compensating currents

in the case of shunt active filters. The derived compensating command signals are

compared with sensed feedback signals and error is processed in PWM controller

to generate digital gating signals. These gating signals are buffered, isolated and

amplified to feed the gate of the solid state switching devices of active filter used

in hybrid filters. Selection of components of hybrid filters is important to attain

29

high level performance. There are a number of components in hybrid filters, such

as passive filter elements, active filter elements, control scheme employing

sensors, processor, isolation amplifier circuits, interfacing circuits, injection

transformers etc.

4.5 CONCLUSION

Harmonic mitigation techniques are studied. Active filters can be discussed as a

well-established means of mitigation techniques of power quality problems in

retrofit systems even though rating and cost is high. This drawback is usually

solved by using a hybrid filter which used conventional tuned passive filters in

conjunction with the active filters so that the dominant lower order harmonics are

eliminated by passive filters. The remaining harmonics are compensated by the

active filter.

30

CHAPTER V

COMPARISON OF ACTIVE FILTER ALGORITHMS –

SIMULATION RESULTS

5.1 INTRODUCTION

The active filtering algorithm computes the reference compensation signals to be

generated by the active power filter (APF) to provide reactive power / harmonic /

unbalance compensation. The choice of the control algorithm therefore decides

the accuracy and the speed of response of the filter. Instantaneous PQ theory [8],

synchronous detection algorithm [9], DC bus voltage algorithm [10], Icosφ

algorithm [1] etc. are some of the widely used three-phase active filtering

algorithms.

Simulation is a simple and inexpensive way of checking the validity of any design

or concept before actually building a hardware prototype. MATLAB from

mathworks Inc. [19] is the one of the mostly used simulation packages.

MATLAB/SIMULINK is the main simulation part of this multi domain software

package. Simulink has become the most widely used software package for

modeling and simulating dynamic systems. It includes block sets specific to

power systems, control systems, mechanical systems and mathematical systems.

SimPowerSystem is the toolbox used for simulation of power system. It includes a

vast library of all the elements, machines, voltage sources, measurements and

power electronic components used in any power system. It also includes a huge

collection of demos on the state-of-the-art techniques used in power systems.

5.2 SIMULATION MODEL REQUIREMENTS

The three-phase system to be modeled in simulation consists of the following

components/ subsystems.

1. A three phase, three wire symmetrical, balanced, ac voltage supply with

source impedance

2. A three-phase fully controller thyristor bridge rectifier feeding RL load

31

3. A three-phase voltage source inverter (VSI) of suitable rating to act as the

shunt active filter with suitable value of DC link capacitance based on the

design.

4. Three-phase reactance with appropriate design

5. Voltage sensors for sensing the phase voltage in each phase. Current sensors

for sensing the load current and source current in each phase.

6. Computation of reference compensation current.

7. Generation of firing pulses for the active filter devices

The figure 5.1 shows the basic set up to be implemented in simulation.

Fig. 5.1: Basic block diagram for the implementation of shunt active filter.

The three phase inductive source block is being used for a three phase source with

suitable source impedance. The universal bridge block can be programmed as a

controlled or uncontrolled converter is being used as a non linear load. The

32

synchronized six pulse generator is used as the firing circuit block for the non

linear load where firing angle α can be suitably selected for the required mode of

operation.

5.3 SIMULATION OF THE FULL CONTROLLED CONVERTER

FEEDING AN RL LOAD WITHOUT SHUNT ACTIVE POWER FILTER

A full controlled converter can be operated for different values of triggering angle

α. For simulation, 15 kW RL load is considered. Resistance value (R) is

calculated as 20Ω and inductance value (L) is 80mH. Source and line impedance

(Ls) become 0.5633 and 1.83mH respectively.

The figure 5.2 shows Matlab simulation model. Figure 5.3 to 5.6 shows

waveforms of source voltage, source current, load current and %THD in source

current for different firing angles of thyristor converter load.

Fig 5.2: Basic block diagram for full controlled converter feeding an RL load.

Case A: at α =0°

When triggering angle α is zero thyristor converter will act as a diode bridge

rectifier. The waveform obtained from simulation shows that source current is

contaminated with lots of harmonics. From source current spectrum it can be

found that %THD is 18.45.

33

Fig 5.3: Source voltage, load current, source current in phase A at α=0° without

filter

Fig 5.4: Source current spectrum at α=0° without filter.

34

Case B: at α=30°

When triggering angle increases to 30°, total harmonic distortion also increases to

25.91 %. This means that when α increase, current waveform will be

contaminated with more harmonic contents.


filter.


35

5.4 SIMULATION OF SHUNT ACTIVE FILTER UNDER BALANCED

SOURCE AND BALANCED LOAD CONDITIONS

A balanced source is one that provides balanced sinusoidal voltages in the three

phases that are equal in magnitude and displaced exactly by 1200 in phase from

each other. The supply currents

will be balanced but wave shape will be depending on the type of load ,the source

is connected to. The three phase load is said to be balanced when the three phase

currents drawn from the supply are equal. Figure 5.5 to 5.8 shows waveforms of

source voltage, source current, load current and %THD in source current for

different firing angles of thyristor converter load.

5.4.1 SIMULATION OF INSTANTANEOUS PQ THEORY

The system considered for simulation is a three phase source supplying a three

phase on-linear load, which under normal conditions draws harmonic-rich

balanced three phase currents. A three phase, VSI based shunt active power filter

is connected through a large inductance to the system for compensating for

harmonics and reactive power required by the load, so the source will be

supplying only sinusoidal currents.

A 230V, 50 Hz balanced three phase source with suitable source impedance and a

thyristor converter feeding an R-L load is chosen as the non-linear load. The firing

angle α can be varied as required.

The simulation is done for two different load conditions:

Three phase diode rectifier feeding a resistance.

Three phase thyristor controlled converter feeding a resistance(α=30o,

α=60o)

The simulation circuit for the diode rectifier load is given in Figure 5.7. The

thyristor converter load block with firing angle set to 0o acts as the diode rectifier.

The reference compensation signals are generated by Instantaneous PQ control

scheme. This controller uses 10 multipliers and 2 divider blocks in addition to the

Park’s transformation blocks for the generation of the compensation signals. The

36

three phase main voltages “eabc” and the load currents “iLabc” are sensed and

passed through Park’s transformation block to yield e(αβ) and IL(αβ) respectively.

From e(αβ) and IL(αβ), the instantaneous values of real power and imaginary power

are derived. These are then split in to their DC and AC terms. The ripple in real

power and whole of imaginary power are used to generate ic(αβ) signals to inverse

Park’s transformation block.

Figure 5.8 to 5.11 shows waveforms of source voltage, source current, load

current and %THD in source current for different firing angles of thyristor

converter load.

Fig 5.7: Basic setup for shunt active filtering with controller.

Case A: at α =0°.

Comparing the wave forms with and without controller at α =0°, it can be seen

that source current became almost pure sine wave i.e., harmonic currents needed

by converter is provided by shunt active filter. Now source has to supply only real

component of load current and remaining reactive components and harmonics

required by the load current is supplied by shunt active filter. The total harmonic

37

distortion without controller at α =0° is 18.45 % and with controller it is reduced

to 3.40%.i.e. THD value is within the allowable limit.

Fig 5.8: Source voltage, load current, source current in phase A at α=0° with

Active Power Filter using IRPT control algorithm.

Fig 5.9: Source current spectrum at α=0° with Instantaneous PQ theory.

38

Case B: at α =30°

Total harmonic distortion with controller at α=30° is 9.60% whereas without

controller it was 25.91 %. Very clearly, this value is not with in IEEE standards


Active Power Filter using IRPT control algorithm

Fig 5.11: Source current spectrum at α=30° with Instantaneous Reactive PQ

theory.

39

LIMITATIONS OF IRPT

This algorithm involves complex calculations and complicated circuitry. The

IRPT controller uses 10 multiplier and 2 divider blocks in addition to the park’s

transformation blocks for the generation of compensation currents. But this

approach is applicable only for balanced three phase system and source voltage

waveform is a pure sine wave.

5.4.2 SIMULATION OF THE SYNCHRONOUS DETECTION

ALGORITHM

The controller consists of 6 multipliers in addition to the low pass filter and power

distributor blocks. The real power drawn from the three phases is calculated from

Ea, Eb, Ec and ILa, ILb, ILc signals. The real power is sent to a low pass filter to

obtain its average value Pdc. The average power is divided equally among the

three phases. The desired main currents are derived from the average power. The

active filter reference currents are then calculated as difference between the main

currents and the actual load currents using comparator block. Figure 5.12 to 5.15

shows waveforms of source voltage, source current, load current and %THD in

source current for different firing angles of thyristor converter load.

Case A: at α =0°.


that source current became sine wave i.e., harmonic currents needed by converter

is provided by shunt active filter. Now source has to supply only real component

of load current and remaining reactive components and harmonics required by the

load current is supplied by shunt active filter. The total harmonic distortion

without controller at α =0° is 18.45 % and with controller it is reduced to

1.30%.i.e. THD value is within the allowable limit.

40


Active Power Filter using Synchronous Detection algorithm

Fig 5.13: Source current spectrum at α=0° with Synchronous Detection algorithm

41

Case B: at α =30°

Total harmonic distortion with controller at α=30° is only 1.49% whereas without

controller it was 25.91 % and source current became pure sine wave in phase with

source voltage.


Active Power Filter using Synchronous Detection algorithm

Fig 5.15: Source current spectrum at α=30° with Synchronous Detection

algorithm

42

LIMITATIONS OF SD ALGORITHM:

The control circuit consists of 6 multiplier blocks in addition to low pass filter and

power distribution blocks. But it suffers from the drawbacks such as (i)

effectiveness of the algorithm depends on harmonic contents in voltage signal (ii)

assumption is made that three phase currents are balanced.

.

5.4.3 SIMULATION OF THE DC BUS VOLTAGE ALGORITHM

The three phase source voltages and currents are sampled and the unit vectors

required for generating the reference currents is derived from the source voltage

signals after filtering and processing of those signals. The error between the reference

DC bus voltage and the actual DC bus voltage is found and it is passed through a PI

controller to generate the amplitude of the desired mains current. The unit vectors

obtained is then multiplied with the amplitude of the desired mains current to

generate the three phase reference mains currents. These reference mains currents act

as one of the inputs to the hysteresis controller and the other input to it is the actual

mains current. The switching gate pulses are generated on comparison of these two

inputs. . Figure 5.16 to 5.19 shows waveforms of source voltage, source current,

load current and %THD in source current for different firing angles of thyristor

converter load.

Case A: at α =0°.








43


Active Power Filter using DC Bus Voltage algorithm

Fig 5.17: Source current spectrum at α=0° with DC Bus Voltage algorithm

44

Case B: at α =30°



source voltage.


Active Power Filter using DC Bus Voltage algorithm

Fig 5.19: Source current spectrum at α=30° with DC Bus Voltage algorithm

45

5.4.4 SIMULATION OF Icosφ ALGORITHM

In MATLAB/SIMULINK model, the three phase V-I measurement block is used

to measure the three phase voltage and load current. Low pass filter gives

fundamental component of load current which is 90° phase shifted. Unit

amplitude sine wave is produced in phase with respective phase voltages. At the

time of zero crossing of the phase voltages, using sample and hold circuit, Icosφ

value is extracted. The desired source current is computed, using product block,

after multiplication of unit amplitude sine wave with Icosφ value. Reference

compensation currents are computed as the difference between the actual load

currents and desired source currents using subtractor blocks. The actual filter

output currents and reference compensation currents are fed to a hysteresis

controller block with a suitable hysteresis band to generate the firing signals for

the active power filter devices. Figure 5.20 to 5.23 shows waveforms of source

voltage, source current, load current and %THD in source current for different

firing angles of thyristor converter load.

Case A: at α =0°.








46


Active Power Filter using with Icosφ algorithm

Fig 5.21: Source current spectrum at α=0° with Icosφ algorithm

47

Case B: at α =30°



source voltage. For unbalanced source, the total THD is reduced from 25.5% to

1.42%.



Fig 5.23: Source current spectrum at α=30° with Icosφ algorithm

48

Table 5.1: %THD in the mains currents before and after shunt compensation

for two firing angles α=0° and α=30°

α=0° α=30°

Without APF

18.45% 25.91%

With

APF

IRPT 3.40% 9.60%

Synchronous

Detection 1.30% 1.49%

DC Bus Voltage

Algorithm 1.80% 2.79%

Icosφ Algorithm 0.91% 1.29%

5.5 SIMULATION OF SHUNT ACTIVE FILTER USING Icosφ

ALGORITHM UNDER BALANCED/UNBALANCED SOURCE AND

BALANCED/UNBALANCED LOAD CONDITIONS

An unbalanced source used is one in which a 30% amplitude unbalance is

introduced in phases a, b and c, respectively. The unbalance is reflected on the

load currents in the three phases too. However, the active filter is expected to

balance the three phase source currents effectively.



converter load under unbalanced source and balanced load conditions.

49

Case A: at α =0°.


Active Power Filter using with IcosΦ algorithm

Fig 5.25: Source current spectrum at α=0° with Icosφ algorithm for unbalanced

source and balanced load condition

50

Case B: at α =30°

For unbalanced source and balanced load condition, the total THD is reduced

from 25.5% to 1.34%.


Active Power Filter using with IcosΦ algorithm

Fig 5.27: Source current spectrum at α=30° for unbalanced source and balanced

load condition

51

5.6 SIMULATION OF SHUNT ACTIVE FILTER USING Icosφ

ALGORITHM UNDER BALANCED SOURCE AND UNBALANCED

LOAD CONDITIONS

An unbalance in load is introduced by connecting three different resistances of

100 Ω, 150 Ω and 50 Ω values in series with a, b and c phase voltage sources on

the AC side of the non linear load.



converter load under balanced source and unbalanced load conditions. Table 1.2

compares the %THD values of the main current for both types of load conditions

when shunt compensation is provided by Icosφ control schemes.

Case A: at α =0°.

In the IcosΦ algorithm, the unbalance in load is taken care of in the determination

of the reference compensation currents itself. The magnitudes of the active

portions of the fundamental load current in the three phases are detected and

average of these values is taken as the amplitude of the desired mains current.

This makes sure that the magnitudes of the desird source currents in all the three

phases remain the same even in case of load unbalance. the total THD is reduced

from 25.5% to 1.27%.



52

Fig 5.29: Source current spectrum at α=0° with Icosφ algorithm for balanced


Case B: at α =30°

For balanced source and unbalanced load codition, the total THD is reduced from

25.5% to 1.88%.



53

Fig 5.31: Source current spectrum at α=30° with Icosφ algorithm for balanced


Table 5.2: %THD in the mains currents before and after shunt compensation

under Balanced/ Unbalanced Source and Balanced/unbalanced Load

Conditions using Icosφ algorithm

Conditions α =0° α =30°

Without APF(THD in

source current)

With

APF

(THD

in

source

current)

Without APF(THD in

source current)

With

APF(THD

in source

current)

Phase

a

Phase

b

Phase

c

Phase

a

Phase

b`

Phase

c

Balanced

Source

27.81% 27.87 27.77% 0.91% 28.37% 28.50% 28.53% 1.29%

Unbalanced

source

24.81% 23.9% 33.17% 0.92% 27.08% 24.37% 36.58% 1.34%

Unbalanced

load

16.13% 20.63% 16.84% 1.27% 33.2% 29.85% 27.06% 1.86%

54

5.7 CONCLUSION

From the simulation results, it can be seen that Icosφ algorithm has been more

effective than other three schemes for shunt active value filtering. It has been

found to yield comparable or rather marginally better results as compared to the

existing algorithms in addition to being simpler than other algorithms.

Icosφ algorithm has been simulated under conditions generally prevailing in three

phase system such as a balanced three- phase voltage supply feeding a balanced

non-linear load, an unbalanced supply feeding a balanced non-linear load and a

balanced three-phase supply feeding an unbalanced load. Here also, Icosφ

algorithm made the source currents to be balanced, sinusoidal and unity power

factor currents. The source current THD% has been brought down to best value in

the case of Icosφ algorithm, which is very much with in the levels set by IEEE

standards.

The source current THD% has been brought down to a value ≤3% with the

individual harmonics less than 2%, in the case of Icosφ algorithm, which is very

much with in the levels set by IEEE standards. Hence Icosφ algorithm is the

control algorithm for the active filter used in implementing the hybrid filter.

55

CHAPTER VI

SIMULATION OF HYBRID POWER FILTER

6.1 INTRODUCTION

Hybrid active filters inherit the efficiency of passive filters and the improved

performance of active filters, and thus constitute a viable improved approach for

harmonic compensation. Active filters were developed to mitigate problems of

passive filters. They are more effective in harmonic compensation and improved

performance. But pure active filters are highly expensive and require

comparatively high power converter ratings. Hybrid filters, combination of both

passive filters and active filters, provide improved performance and cost-effective

solutions. Both the active filter and passive filters are connected in parallel with

the non-linear load, together contributes for harmonic and reactive compensation.

This topology lends itself to retrofit applications with existing passive filters.


FEEDING AN RL LOAD WITHOUT FILTER

A fully controlled thyristor converter can be operated for different values of

triggering angle α. For simulation, a full controlled converter feeding 15 kW RL

load is considered. Resistance value (R) is calculated as 20Ω and inductance value

(L) is 80mH. Source and line impedance can be assumed as 0.08 p u and thus

source resistance (Rs) and source impedance (Ls) become 0.5633 Ω and 1.83mH

respectively.

Fig 6.1: Basic simulation setup for full controlled converter feeding an RL load.

56

The figure 6.1 is the basic set up for simulation. Figure 6.2 shows waveforms of

source voltage, source current, load current and figure 6.3 shows %THD in source

current for firing angle α =0° of thyristor converter load.

Case A: at α = 0°

When triggering angle α is zero thyristor converter will act as a diode bridge

rectifier. The waveform obtained from simulation shows that source current is

contaminated with lots of harmonics. From source current spectrum it can be

found that %THD is 18.45.


filter.

57


Case B: at α=30° When triggering angle increases to 30°, total harmonic distortion also increases to

25.91 %. This means that when α increase, current waveform will be

contaminated with more harmonic contents. Figure 6.5 shows the current

spectrum analysis.

Fig6.4: Source voltage, load current, source current in phase A at α=30° without

filter.

58



FEEDING AN RL LOAD WITH PASSIVE FILTER

The passive filter is designed as follows: The reactive power requirement of the

load is calculated. The 5th and 7th order of harmonic shunt filters are designed to

sink in respective harmonic currents. The capacitors for the passive filter are

designed to supply the specified percentage of the reactive power requirement of

the load. The Icosφ algorithm thereby generates the reference compensation

currents for the active power filter each time, with the remaining harmonics and

the reactive contents in the source current after the insertion of passive tuned

filters.

The total VAR requirement is calculated under the worst possible condition of a

given load. The passive filter design is done based on the % VAR to be supplied

by the passive filter. The passive filter element values (R, L and C) are calculated

per phase as follows.

VAR supplied by passive filters per phase, VARp(ph) =(VARp)/3

This can be equated to V2/Xc where V is per phase (rated) voltage across the

passive filter and XC = capacitive reactance per phase

VARp(ph)=V2/Xc=V2. ω.C which gives,

C = VARp(ph)/V2.ω. (6.1)

59

Where C is the capacitance

VARp(ph) is the reactive power per phase

and ω=2.π.f

where f= frequency(50Hz).

Only the 5th and 7th order harmonic passive filters have been chosen for the

simulation study. These filters sink in the 5th and 7th order harmonics from the

three-phase system in addition to supplying the %VAR as specified. VARp(ph) is

shared between one 5th order and one 7th order passive filter in each phase. The

inductances (Lh) of these filters are calculated as follows.

Lh = 1/( ωh2.C) (6.2)

where ωh is the specific harmonic frequency to be absorbed by the filter. (ω5 for

5th and ω7 for 7th).

For a given Q factor , where Q=XLh/Rh

Resistance Rh= ω.Lh/Q. (6.3)

For each VAR supply, the value of C varies. For each value of C, Lh for each ‘h’

varies. For each Lh value, Rh is calculated based on the Q factor value. Thus C, Lh

and Rh design values of each harmonic order passive filter, are mutually

dependent on each other and also on the chosen values of the parameters such as

VAR and Q factor.

The figure 6.6 is the basic set up for simulation of three-phase system with

thyristor controlled converter compensated by passive filter. Figure 6.7 to 6.10

shows waveforms of source voltage, source current, load current and %THD in

source current for different firing angles of thyristor converter load.

60

Fig 6.6: Matlab simulation of the full controlled converter feeding RL load with

passive filters

61

Table 6.1 parameter values of passive filter for simulation

Harmonic Resistor Inductor Capacitor

5th 0.0314Ω 10mH 40µF

7th 0.0160Ω 5.1mH 40µF

Case A: at α =0°. Comparing the wave forms with and without controller at α =0°, it can be seen

that source current became sine wave i.e., the passive filter sinks the 5th and 7th

harmonic currents by providing a low impedance path. The total harmonic

distortion without controller at α =0° is 18.47 % and with controller it is reduced

to 1.62%.


Passive filter

62

Fig 6.8: Source current spectrum at α=0° with Passive Filter.

Case B: at α =30°


controller it was 25.91 %


Passive filter

63

Fig 6.10: Source current spectrum at α=30° with Passive Filter.


FEEDING AN RL LOAD WITH SHUNT ACTIVE POWER FILTER

The main purpose of using Icosφ control algorithm is to reduce the current

harmonic distortion. This section discusses the effects of shunt active filter with

Icosφ controller. The figure 6.10 shows the basic Icosφ for shunt active

filtering. Figure 6.11 to 6.14 shows waveforms of source voltage, source current,

load current and %THD in source current for different firing angles of thyristor

converter load.

64


Active Power filter .Case A: at α =0°.








65

Fig 6.12: source voltage, load current and source current in phase A at α =0° with

Active Power Filter.

Fig 6.13: Source current spectrum at α =0° with Active Power Filter.

66

Case B: at α =30°



source voltage.


Active Power Filter.

Fig 6.15: Source current spectrum at α =30° with Active Power Filter.

67


FEEDING AN RL LOAD WITH HYBRID FILTER

Hybrid system uses a passive filter connected in shunt to remove the lower order

harmonics and a shunt active filter is connected to remove the remaining

harmonics. The shunt active filer uses the Icosφ control algorithm. The figure

6.15 is the basic set up for simulation.


Hybrid Filter

68

Case A: at α =0°.


that source current became sine wave i.e., 5th and 7th are removed by passive filter

connected in shunt and remaining harmonic currents needed by converter is

provided by shunt active filter. The total harmonic distortion without controller at

α =0° is 18.45 % and with controller it is reduced to 0.47%.i.e. THD value is

within the allowable limit. Figure 6.17 to 6.20 shows waveforms of source

voltage, source current, load current and %THD in source current for different

firing angles of thyristor converter load using hybrid filter.


Hybrid Filter

69

Fig 6.18: Source current spectrum at α =0° with Hybrid Filter.

Case B: at α =30°



source voltage.

Fig 6.19: source voltage, load current and source current in phase A at α =30°

with Hybrid Filter

70

Fig 6.20: Source current spectrum at α =30° with Hybrid Filter.

Table 6.2 Simulation of the three phase system with passive filter, active filter

and hybrid filter

Parameters Without filter With Passive

filter

With Active

filter

With Hybrid

filter

α = 0o α = 30o α = 0o α = 30o α = 0o α = 30o α = 0o α =30o

Fundamental

source

current in

rms (A)

16.87 14.5 21.47 17.04 18.04 14.96 18.55 15.92

THD in

Source

Current (%)

18.45 25.91 1.62 2.44 0.91 1.29 0.47 0.64

Time Delay

(in ms)

5 5 20 20 20 20

71

6.6 CONCLUSION

Using the proposed hybrid filter structure consisting of combination of shunt

active power filter and 5th and 7th shunt passive filters has the best performance

in harmonic reduction. The active filter is then required to compensate only for

the remaining harmonics which the passive filter has not removed. Thus the rating

and cost of the active power filter has been reduced. Thus the use of active power

filters for power quality improvement in retrofit systems is thus justified. The

results prove the efficiency of the hybrid filter in reducing the current harmonics.

72

CHAPTER VII

HARDWARE SETUP – DESIGN AND MODELLING OF

HYBRID FILTER

7.1 INTRODUCTION

The earliest type of filters were shunt /series passive filters connected at

PCC which provide a low/high impedance path to the selected harmonics,

preventing most of the selected harmonics from appearing at the source. But they

have drawbacks such as resonance, fixed compensation, high no load losses,

bulky size etc. As a better option of complete compensation of distortions, active

power filters have been researched and developed. Many mature control

algorithms are available in literature for the control of three-phase active filters.

Most of them use tedious computations and complex circuits and hence highly

expensive and slow in response. Hence as a better and economical option,

combination of passive and active filters, named hybrid filters, are implemented.

It is quite popular because the solid state devices used in the active part can be of

reduced size and cost. Here, generally, passive filters are used to eliminate lower

order harmonics. The higher order harmonic currents, which are much less in

magnitude compared to lower order harmonics, are eliminated by active filter.

Hybrid filters allow designing active filters for only a fraction of total load

power, reducing costs and increasing overall system efficiency. Based on reactive

power requirement of the system under the present / rated load condition, passive

LC filters lower order harmonic frequency can be designed and inserted. The

active filter is controlled by using Icosφ algorithm, which can be easily

implemented using analog circuit. The main advantages of using analog circuits

are easy availability, cost effectiveness of analog circuits for implementing any

mathematical function and very quick response with simple implementation

aspects. Based on reactive power requirement of the system under the present /

rated load condition, passive LC filters lower order harmonic frequency can be

designed and inserted.

73

Fig 7.1: Basic Block diagram of three phase hardware setup.

7.2 HARDWARE IMPLEMENTATION – LABORATORY

PROTOTYPE A voltage source inverter assembly, which consists of a

three phase IGBT based inverter along with large DC link capacitor, is

being used as the shunt active filter. DC link capacitor of 1650mF / 800V

is used for maintains steady voltage required by the inverter. Appendix 1

gives the details of the SEMIKRON make inverter. Different

combinations of inductor blocks are used as coupling inductor according

to the requirement of the non linear load. LEM Hall effect voltage sensors,

namely, LV 25-P and current sensor LA 25-NP have been used for

sensing phase voltages, load currents and active filter output currents.

Appendix 2 gives the data sheets of the sensors.

74

The passive filter design is done based on reactive power requirement. Since

most of the power system equipments are bilateral, even order harmonics will

not be present in power system. Therefore, to reduce the predominant

harmonics of 5th and 7th order harmonics to a great extent, 6th order harmonic

shunt passive filter is designed. The capacitor value, inductance value and

resistance value of the 6th harmonic order passive filter, are mutually depend

on each other and also on the chosen values of the parameters such as VAR

and Q factor. For this loading condition, the combination of 5mH-80µF is

selected as the most optimised filter. The passive filter is designed for 100%

VAR compensation and it is tuned to the sixth harmonic. A three phase diode

bridge rectifier, of current rating 16 A, feeding single phase, 230V ,3kW

resistive load is used as a non linear load for testing the control circuit. The

figure 7.2 shows the basic hardware setup.

7.2: Basic setup for hardware using passive filters

75

7.3 CONTROL CIRCUIT DESIGN AND TEST RESULTS

The modules of analog circuit controller used with shunt active power filter are

shown in figure 7.3.

Fig 7.3: The block diagram of the Icosφ analog controller for phase A. 7.3.1 FUNDAMENTAL DETECTION OF LOAD CURRENT

Low pass filtering for extracting fundamental component of load current is the

main operation to be done by the controller. Low pass filtering is done by the

biquad filter. The advantages of using biquad filter, rather than other low pass

filters, are it is easy to design, it gives unity gain and it also gives exact 90° phase

shift. Following session gives the design of biquad filter.

76

7.3.1.1. OP-AMP BASED CIRCUIT OF BIQUAD FILTER

To obtain op-amp circuit implementation a miller integrator circuit having time

constant CR=1/ω0 and replace summer block with an op-amp summing circuit.

The resulting circuit, known as Kervin-Huelsman-Newcomb or KHN biquad after

its inventors is shown figure 7.4.

Fig 7.4: Op-amp circuit of the biquad filter

For given values of K, Q and ω0 the design of circuit are straight forward. Expressing the output of the summer in terms of its inputs, Vhp can be written as,

----------------- (4)

------ (5)

Equating last terms of right hand side of the equation (4) and (5), Rf/R1=1. Again

equating second to last term of the right hand side of the equations (4) and

(5),R3/R2=2Q-1 .Finally equating coefficients of Vi, K=(2-1)/Q

7.3.2 ZERO CROSSING DETECTION

This circuit includes the comparator circuit with monostable multivibrator

74LS123 used for getting sharp output pulses at negative zero crossing of the

77

phase voltage. Figure 7.5, shows the simulation and outputs. Truth table of

74LS123 is selected in such a way that when input pin get transition from high to

low. Pulse width of the output of multivibrator is decided by the values of Rext and

Cext.

Fig 7.5: Negative zero crossing detection circuit with monostable multivibrator.

7.3.3 SAMPLE AND HOLD CIRCUIT

The sample and hold circuit (LF 398) gives the Icosφ value. This has got two

inputs, one is from the biquad filter and other is from monostable multivibrator.

The output is Icosφ which is getting at instant of negative of zero crossing voltage

and output is given to multiplier circuit. The circuit is shown in figure 7.6

78

Fig 7.6: The sample and hold circuit for getting Icosφ value.

7.3.4 THE MULTIPLICATION CIRCUIT

AD 633 JN is used as multiplier for multiplying IcosØ value and unit amplitude

sine wave. The output of the multiplier is the desired source current. The

multiplier circuit connection is shown in figure 7.7.

Fig 7.7: The multiplier circuit for producing desired source current.

7.3.5 THE SUBTRACTOR CIRCUIT

The subtracting circuit is formed as shown in figure 7.8 with two IC 741op-amps.

The load current and reference source current were given as input and verified the

output.

79

Fig 7.8: The subtractor circuit for producing compensation current.

7.3.6 THE COMPARATOR CIRCUIT FOR PWM GNERATION

The comparator circuit is used for producing PWM pulses for inverter. This

circuit compares reference compensation current and actual filter current. When

reference filter current is more than actual filter current, output of the comparator

is high and vice versa. The comparator is realized using op-amp 741 and IC

4049B. The 4049B is used for inverting comparator low output.

Fig 7.9: The comparator circuit for PWM generation

80

7.3.7 ISOLATION AND AMPLIFICATION CIRCUIT

The isolation circuit is provided for the isolation between power circuit and

controller circuit. The optocoupler 6N136 is used for isolation. The Semikron

make Inverter needs +15V in the gate of the IGBT. Normal PWM is less than 15V

and it is not sufficient to drive the gate of the Semikron make Inverter. The

transistor amplifier BC 547 is using for amplifying PWM pulses. The circuit

diagram is shown in figure 7.10

Fig 7.10: The isolation and amplification circuit.

7.4 CONCLUSION

This chapter describes the hardware setup of the shunt active filter along with

three phase non-linear load. The tuned passive filter is designed and inserted for

partial compensation. The analog circuit for Icos φ control algorithm has been

designed. The hardware testing results prove that the active filter and passive filter

are working satisfactorily, which are included in Chapter VIII.

81

CHAPTER VIII

HARDWARE IMPLEMENTATION OF HYBRID POWER

FILTER – UNDER DIFFERENT SOURCE AND LOAD

CONDITIONS

8.1 INTRODUCTION

The hybrid power filter has been designed and implemented for balanced

/unbalanced/distorted three phase source conditions with balanced/unbalanced

three-phase load. Also, the three-phase hybrid power filter results are compared

with that of passive filter and active filter results under various source/load

conditions.

A balanced source is one that provides balanced sinusoidal voltages in the three

phases that are equal in magnitude and displaced exactly by120o in phase from

each other. Unbalanced load currents in a three-phase system, is a common

phenomenon and should be considered as an operating condition that may vary

frequently. The supply current will be balanced or unbalanced depending on the

operating conditions, but the wave shape will depend on the type of load. Here,

the three phase diode bridge rectifier is considered as a non-linear load and this

introduces higher level of current harmonics. In this chapter, effectiveness of

hybrid power filter for compensating current harmonics is covered

8.2 EXPERIMENTAL RESULTS

8.2.1. BALANCED SOURCE AND BALANCED LOAD CONDITION

The experimental results on a scaled down balanced three phase system

connected to diode bridge rectifier feeding a resistive load (230V, 3kW) are

presented in this section. The Icosφ controller senses load current, supply voltage

and generate PWM pulses to IGBT inverter. The shunt active power filter is

connected to three phase supply at point of common coupling through 10mH,

coupling reactance.

82

The passive filter is designed for 100% VAR compensation and it is tuned to the

sixth harmonic so as to avoid resonance condition and to sink both 5th and 7th

harmonic currents to certain extent. This will reduce the size of the passive filter

and hence loading on the source also.

A scaled down three phase supply of 50V/phase is connected to a diode bridge

rectifier load and the reduction in source current harmonics are obtained with the

help of active filter and hybrid filter. The results obtained in DSO are shown in

Fig.8.1 to Fig.8.3.

Fig 8.1: Diode bridge rectifier load with active filter: (a) Source voltage and load

current (phase A) (b) Load current and reference compensation current (phase A).

.

Fig 8.2: Diode bridge rectifier load: Reference compensation current and actual

compensation current in closed loop (phase A).

83

Fig 8.3: Diode bridge rectifier load with hybrid filter (shunt active and shunt

passive): (a) Load current and reference compensation current (phase A)

(b) Reference compensation current and actual compensation

current in (phase A)

The tests results are also analyzed using FLUKE make power quality analyzer.

The hybrid power filter, combination of shunt active and shunt passive filters,

reduces total harmonic distortion in source current. Passive filter reduces the

lower order (5th and 7th) harmonics and a shunt active power filter injects the

remaining harmonics in source current .Relevant results are shown in figures 8.4

to 8.7

84

Fig.8.4: a) source voltages b) source current without filtering c) source current

with hybrid filter

Fig.8.5: a) a-phase voltage and current without filtering b) a-phase voltage and

current with hybrid filtering

85

Fig 8.6: THD % of Source current spectrum (a) without filtering (b) with active

filter for balanced source and balanced load

Fig 8.7: THD % of Source current spectrum (a) With passive filter (b) with hybrid

filter for balanced source and balanced load

8.2.2. DISTORTED SOURCE AND A BALANCED LOAD CONDITION

The distorted source voltages are applied across a diode bridge rectifier feeding a

resistive load. When there is distortion in the supply voltages, the fundamental

components are first derived using suitably tuned second order low pass filters to

86

make the voltages balanced and sinusoidal. The unit sine wave of these balanced

voltages are used as templates as required by the Icosφ algorithm to generate

compensation currents. The hardware set up for this experiment has already been

described in section 8.0 which is used for balanced source and load conditions. he

results are shown from figures 8.8 and 8.11.


with hybrid filter



87


filter for distorted source and balanced load

Fig 8.11: THD % of Source current spectrum (a) With passive filter (b) with

hybrid filter for distorted source and balanced load

88

8.2.3. BALANCED SOURCE AND UNBALANCED LOAD CONDITION:

The balanced three phase supply is connected to the diode rectifier with a

resistance of 25Ω connected in series with a-phase supply line and 52Ω connected

in series with c-phase supply line on the AC side for introducing unbalance in the

load currents in the three phases. In the IcosΦ algorithm, the unbalance in load is

taken care of in the determination of reference currents itself as explained in

section 3.1 in chapter 3. The shunt active filter using the Icosφ algorithm makes

sure that the source currents in all the same phases remain balanced even in case

of load unbalance.

The active filter along with the passive filter reduces the THD and the results are

shown in fig 8.12 to 8.15.


with hybrid filter

89




filter for balanced source and unbalanced load

90

Fig 8.15: THD % of Source current spectrum (a) With passive filter (b) with

hybrid filter for balanced source and unbalanced load

Table 8.1: Comparison of source current spectrum with and without hybrid filter

under balanced source and load condition (50V/phase)

UNDER BALANCED SOURCE AND BALANCED LOAD CONDITION

Supply Voltage=50V/phase

THD(%

)

H5 (%) H7 (%) H11 (%) H13 (%)

WITHOUT

HYBRID

FILTER

R 29.8 24.8 9.0 9.1 5.1

Y 29.7 24.4 9.4 9.0 5.3

B 28.7 23.3 9.8 8.2 6.0

WITH

HYBRID

FILTER

R 3.6 2.7 2.9 0.9 0.2

Y 3.8 2.7 2.3 0.8 0.2

B 3.9 3.0 2.3 0.5 0.2

91


under distorted source and balanced load condition (50V/phase)

UNDER DISTORTED SUPPLY AND BALANCED LOAD CONDITION


THD(%

)

H5 (%) H7 (%) H11

(%)

H13

(%)

WITHOUT

HYBRID

FILTER

R 29.4 22.8 11.5 9.1 6.1

Y 29.2 22.4 11.7 9.2 6.0

B 29.1 22.5 11.5 9.1 6.0

WITH

HYBRID

FILTER

R 3.4 1.9 1.7 0.6 0.1

Y 3.3 2.0 1.2 0.3 0.2

B 3.8 2.5 2.0 0.7 0.2


under balanced source and unbalanced load condition (50V/phase)

UNDER BALANCED SOURCE AND UNBALANCED LOAD

CONDITION


THD(%

)

H5 (%) H7 (%) H11

(%)

H13

(%)

WITHOUT

HYBRID

FILTER

R 19.5 11.8 5.6 2.1 1.7

Y 16.1 8.1 8.2 2.1 1.9

B 24.8 19.1 5.0 1.3 1.0

WITH

HYBRID

FILTER

R 4.0 3.2 1.5 0.5 0.2

Y 3.3 2.8 1.2 0.5 0.2

B 3.7 3.3 1.5 0.5 0.1

92

8.3 CONCLUSION

Three phase diode bridge rectifier load produces lot of current harmonics and it is

injected in to supply mains. Total input current THD, has been above the

allowable limit and total input power factor was also reduced. After hybrid

filtering, the %THD has been reduced to allowable limit.

The three-phase hybrid filtering system works quite efficiently under balanced

source and balanced load conditions. The Total Harmonic Distortion (THD) in the

source current is with in 4% which conforms to IEEE power quality standards.

Unbalance in the three phase load currents drawn by non-linear loads is tested

with hybrid filtering techniques. This filtering technique has been found to be

very effective in this operating condition with the source current THD remaining

within 5% in this case as well.

93

CHAPTER IX

CONCLUSION AND SCOPE OF FUTURE WORK

9.1 CONCLUSION

Power quality problems such as harmonics injection, poor power factor,

unbalance and reactive power burden lead to low system efficiency. The power

quality problems are caused invariably by power electronic converters and other

non-linearities present in the power system. Passive, active and hybrid filters are

the three main solutions to mitigate power quality problems in retrofit systems.

Passive filter, filter out desired harmonic frequency component, but it may cause

resonance, detuning, fixed compensation etc. A number of research works are

carried out in active filters, but their rating and cost will be too high. Hence hybrid

filters, a combination of active and passive filter, are suggested for harmonic

compensation as more effective, economical and resonant free solution.

Standard active filter control algorithms such as instantaneous PQ theory,

synchronous detection algorithm, and DC bus voltage algorithm are

studied. Using MATLAB/SIMULINK software, models for the same have

been created and simulated.

Icosφ control algorithm has been studied and shunt active power filter

using this algorithm was simulated in Matlab with 3-phase thyristor

converter feeding RL load. It has been found to yield comparable or rather

marginally better results as compared to existing algorithms under various

source and load conditions, in addition to being simpler than the existing

power filtering algorithms.

Tuned passive filters were designed and simulated in Matlab along with

shunt active filter. The passive filter was designed to filter out the lower

order harmonics, mainly the fifth and the seventh harmonics thereby

reducing the rating of the active power filter.

The prototype of three-phase hybrid filter connected to a three phase

system with diode bridge rectifier feeding a resistive load has been

developed in the laboratory. It uses a shunt connected passive filter in each

94

phase to filter out the tuned frequency harmonics and the remaining

harmonics were filtered by the shunt connected active filter which used the

Icosφ control algorithm. The prototype has been tested for different system

and load conditions such as balanced supply and balanced load, distorted

source voltage and balanced load and also on balanced supply and

unbalanced load condition.

The three-phase hybrid filtering system works quite efficiently under

balanced source and balanced load conditions. The Total Harmonic

Distortion (THD) in the source current is with in 4% which conforms to

IEEE power quality standards.

Unbalance in the three phase load currents drawn by non-linear loads is

tested with hybrid filtering techniques. This filtering technique has been

found to be very effective in this operating condition with the source

current THD remaining within 5% in this case as well.

The three-phase system is tested under distorted source voltages. Here

also, the THD is found to be less than 5% with individual harmonics less

than 3%

The implementation results show that effective current harmonics is

reduced using hybrid power filter.

9.2 SCOPE OF FUTURE WORK

The traditional passive filters will be permanently connected to system and

draws large amount of source current even at light load conditions. By

using auto-tuned filters, the passive filter components can be controlled

according to load variations. The selection of filter parameters,

capacitance and inductance, can be done with the help of a properly tuned

knowledge base, using Artificial Neural Networks, to provide fast

compensation using digital controller.

Fuzzy techniques can be used for more fast and effective compensation.

The Icosφ algorithm can be implemented digitally so that problems of

analog circuits such as ageing, tolerance etc can be avoided.

95

REFERENCES

[1] Manjula G.Nair and G. Bhuvaneswari, “Design, Simulation and Analog

Circuit Implementation of a Three-phase Shunt Active Filter using the Icos Ǿ

Algorithm” IEEE PEDS 2005.

[2] Bor-Ren Lin .et.al. "Analysis and operation of hybrid active filter for harmonic

elimination", Electric Power Systems Research 2002, Vol.62, pp.191-200

.

[3] B. Singh, V. Verma, A. Chandra and K. Al-Haddad, "Hybrid filters for power

quality improvement"IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 3, May

2005

[4] IEEE Guide for harmonic control and reactive compensation of Static Power

Converters, IEEE Standard 519-1992.

[5] Power quality files available (on-line) at: http://www.e-shikshalaya.com

[6] H.L.jou, J.C.Wu and H.Y.chu, “New single phase active power filter” IEE

Proc.-Electr. Power Appl., Vol. 141, No. 3, May 1994.

[7] Bhim Singh, Kamal Al-Haddad and Ambrish Chandra, “A Review of Active

Filters for Power Quality Improvement” IEEE Transactions On Industrial

Electronics, Vol. 46, No. 5, October 1999.

[8] H. Akagi, Y. Kanazawa & A. Nabae, "Instantaneous reactive power

compensators comprising switching devices without energy storage components”,

IEEE Trans. Industry Applications, Vol. 20(3), pp. 625-630, 1984.

[9] C.L. Chen, C.E. Lin & C.L. Huang, "Reactive and harmonic current

compensation for unbalanced three-phase systems using the synchronous

detection method”, Electric Power systems Res., Vol. 26, pp.l63-170, 1993.

96

[10] H. L. Jou, "Performance comparison of the three-phase-active-power filter

Algorithms", in Proc. IEE Conf. on Generation, Transmission, Distribution, pp.

646-652, 1995.

[11] S. Bhattacharya & D. Divan, "Synchronous frame based controller

implementation for a hybrid series active power filter system”, in Proc. 13th IAS

Annual meeting, pp.2531-2540, 1995.

[12] G. Bhuvaneswari & M.G.Nair, "A novel current compensation technique for

shunt active power filters", in Proc. IASTED Conf On Power and Energy

systems, pp. 109-113, 2003.

[13] G. Bhuvaneswari, Manjula G.Nair, “Comparison of Synchronous Detection

and Icosφ Shunt Active Filtering Algorithms”, IEEE 2006.

.

[14] Jos Arrillaga and Neville R. Watson, “power system harmonics”, John Wiley

& Sons Ltd, 2nd edition, 2003.

[15] George. J. Wakileh, “Power System Harmonics-Fundamentals, Analysis and

Filter Design”, Springer International Edition, 2001.

[16] Akagi. H., Navea. A. and Atoh.S, “Control strategy of active power filters

using multiple voltage-source PWM converters”, Trans. IEEE Industry

Applications, IA22 (3), 460–5,1996.

[17] Hayafune.K, Ueshiba.T, Masada.E and Ogiwara.Y, “Microcomputer

controlled active power filter”, International Conference on Industrial Electronics,

Control and Instrumentation”, Tokyo, pp. 1221–26,1984.

[18] Detjen.O, Jacobs.J, De Doncker.R.W and Mall.H.G, “A new hybrid filter to

dampen resonances and compensate harmonic currents in industrial power

97

systems with power factor correction equipment”, Trans. IEEE Power

Electronics, 16(6), 821–7, 2001.

[19] MATLAB/SIMULINK User’s Guide, The MathWork Inc., USA.

[20] M.EI-Habrouk, M.K.Darwish and P.Mehta, “Active power filters: a review”,

IEEE Proc.-Electr. Power Appl., Vol. 147, No. 5, May 2000.

[21] Z.P.Fang, H.Akagi, A.Nabae, "A New Approach to Harmonic Compensation

in Power Systems-a Combined System of Shunt Passive and Series Active

Filters", IEEE Trans, Industry applications. Vo1.26, No.6, Nov/Dec 1990, pp983-

990. [22] F.Z.Peng, H.Akagi and A. Navae, "Compensation Characteristics of the

Combined System of Shunt Passive and Series Active Filters", IEEE-T Ind.

Applications, vol. IA-29, no.1, JadFeb 1993, pp141-151.

[23] H.Fujita, H.Akagi, "A Practical Approach to Harmonic Compensation in

Power Systems-Series Connection of Passive and Active Filters", IEEE-T

industry Applications 1991, pp1020-1025.

98

APPENDIX –1

Three phase IGBT based inverter, 20 KVA, 10A, 20 KHZ PWM –Semikron make

Voltage Source Inverter (VSI)

DC link capacitor of 1650 µfd / 800V.

Three phase diode bridge rectifier having current rating of 16A.

Non-linear loads

400V, 5A, 10mH each in three phase phases.

Coupling Inductance

R =50ohms, 5A; L=5mH,5A; C=80µfd / 440V for each of the three phases

Passive Filter components

99

APPENDIX – 2

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

APPENDIX-3

Photograph of Experimental Setup

Active filter with controller -Hardware

Passive Filter – Hardware circuit

Documents

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