Harmonics Analysis and Mitigation Using Passive Filters

Transcription

1 Harmonics Analysis and Mitigation Using Passive Filters By: Rooh Ul Amin Shaikh Abdul Basit Lashari Irfan Ansari 11EL01 11EL16 11EL37 Supervised By: Prof. Dr. ZUBAIR AHMED MEMON DEPARTMENT OF ELECTRICAL ENGINEERING, MEHRAN UNIVERSITY OF ENGINEERING & TECHNOLOGY, JAMSHORO Submitted in partial fulfillment of the requirement for the degree of Bachelors of Electrical Engineering January, 2015

2 In the name of Almighty Allah, The most gracious and the most merciful

3 CERTIFICATE This is to certify that the work presented in this thesis titled Harmonics Analysis and Mitigation Using Passive Filters is written by the following students as a partial fulfillment of the requirements for the degree of Bachelors of Electrical Engineering under the supervision of the Prof. Dr. Zubair Ahmed Memon Rooh Ul Amin Shaikh Abdul Basit Lashari Irfan Ansari 11EL01 11EL16 11EL37

4 ACKNOWLEGEMENTS We are grateful to our thesis mentor, Prof. Dr. Zubair Ahmed Memon for providing us the opportunity and guidance to complete this work, which is an integral part of the curriculum of Bachelors of Electrical Engineering at Mehran University of Engineering & Technology.

5 ABSTRACT Both electric utilities and end users of electric power are becoming increasingly concerned about the quality of electric power. It is an umbrella concept for a multitude of individual types of power system disturbances. One such major concern is the harmonics which is made the focus of study in this work. When electronic power converters first became commonplace in the late 1970s, many utility engineers became quite concerned about the ability of the power system to accommodate the harmonic distortion. Harmonics problems counter many of the conventional rules of power system design and operation that consider only the fundamental frequency. Therefore, the engineer is faced with unfamiliar phenomena that require unfamiliar tools to analyze and unfamiliar equipment to solve. This thesis is basically concerned with the Analysis and Mitigation of Harmonics generated by Power Electronic Converters. The investigation of harmonics has been carried out using Fast Fourier Transform (FFT) to evaluate the Total Harmonic Distortion (THD) of the converters with and without filters. And MATLAB/SIMULINK has been employed for presenting the simulation results because it is well established and recognized simulation software for the power system. Next, the designing of Passive Filter is carried out after a literature review and have been applied to the converters for harmonics mitigation. In the future, we would proceed to work on Active Filters for Harmonics Mitigation. i

6 CONTENTS CHAPTER 01 INTRODUCTION 1.1 ELECTRIC POWER QUALITY SOURCES OF ELECTRIC POWER QUALITY DETERIORATION IN A POWER SYSTEM NEED FOR ASSESSMENT OF ELECTRIC POWER QUALITY CLASSIFICATION OF POWER SYSTEM DISTURBANCES WHY ARE WE CONCERNED ABOUT POWER QUALITY? 06 CHAPTER 02 FUNDAMENTALS OF HARMONICS 2.1 INTRODUCTION TYPES OF HARMONICS Odd harmonics Even harmonics Inter harmonics Sub harmonics NON-SINUSOIDAL WAVEFORM Average value RMS value Form factor Harmonic power Total active power SOURCES OF HARMONICS Magnetization nonlinearities of transformer Rotating machines 11 ii

7 2.4.3 Arcing devices Power supplies with semiconductor devices Inverter Fed A.C drives Thyristor controlled reactors Phase controllers & AC regulators EFFECTS OF HARMONICS Resonance and effect on capacitor banks Poor damping Effects of harmonics on rotating machines Effects on transformer Effects on transmission lines Harmonic interference with power system protection Effects of harmonics on consumer equipment HARMONIC INDICES Total harmonic distortion Total demand distortion PRINCIPLES FOR HARMONICS CONTROL WHERE TO CONTROL HARMONICS on utility distribution Feeders In End-User Facilities USEFUL TOOLS FOR HARMONICS ASSESMENT 18 CHAPTER 03 HARMONIC FILTERS 3.1 INTRODUCTION PASSIVE FILTERS Passive shunt filters 20 iii

8 3.2.2 Passive series filter ACTIVE FILTERS HYBRID FILTERS DESIGN STEPS OF SERIES TUNED FILTERS DESIGN STEPS OF 2 ND ORDER HIGH PASS FILTER QUALITY FACTOR, BANDWIDTH AND SELECTIVITY 25 CHAPTER 04 STANDARD LIMITS OF HARMONIC DISTORTION 4.1 INTRODUCTION VOLTAGE HARMONIC DISTORTION LIMITS CURRENT HARMONICS DISTORTION LIMITS 28 CHAPTER 05 MATLAB SIMULATION & RESULTS 5.1 POWER CONVERTERS HARMONIC BEHAVIOUR HARMONIC ANALYSIS OF THREE PHASE 12 PULSE AC-DC CONVERTERS HARMONIC ANALYSIS OF THREE PHASE INVERTER 36 CHAPTER 06 CONCLUSIONS REFERENCES iv

9 LIST OF FIGURES Figure 1.1 Harmonics of laptop 04 Figure 1.2 Utility Capacitor Switching Transient 04 Figure 1.3 Voltage Sag 04 Figure 1.4 Voltage Swell 05 Figure 1.5 Flicker 05 Figure 2.1 Harmonic representation 07 Figure 2.2 Harmonic currents flow in a radial system 17 Figure 3.1 Passive shunt filters 20 Figure 3.2 Impedance vs Frequency curve of series tuned filter 21 Figure 3.3 Impedance vs Frequency curve of damped filter 21 Figure 3.4 Passive series filter 22 Figure 3.5 Quality factor effect on resonance curve 25 Figure 3.6 Quality factor effect on selectivity 26 Figure 5.1 Displacement of harmonics as a function of firing angle 30 Figure 5.2 FFT analysis of three phase inverter 31 Figure 5.3 Three phase 12 pulse AC-DC converter model 32 Figure 5.4 Source voltage without filters 32 Figure 5.5 Source current without filters 33 Figure 5.6 FFT analysis of distorted source voltage 33 Figure 5.7 FFT analysis of distorted source current 33 Figure 5.8 Impedance vs Frequency response of designed filter 34 Figure 5.9 Source voltage after filtration 35 v

10 Figure 5.10 Source current after filtration 35 Figure 5.11 FFT analysis of filtered source voltage 35 Figure 5.12 FFT analysis of filtered source voltage 36 Figure 5.13 Inverter circuit with MOSFETs 36 Figure 5.14 Three phase inverter model schematic 37 Figure 5.15 AC side voltage and FFT analysis without filters 37 Figure 5.16 AC side current and FFT analysis without filters 38 Figure 5.17 Three phase inverter model schematic with filter 38 Figure 5.18 AC voltage with filtration 39 Figure 5.19 FFT window of AC voltage with filtration 39 Figure 5.20 AC current with filtration 40 Figure 5.21 FFT window of AC current with filtration 40 vi

11 ABBREVATIONS EPQ THD TDD FFT RMS APFs PPFs Electric Power Quality Total Harmonic Distortion Total Demand Distortion Fast Fourier Transform Root Mean Square Active Power Filters Passive Power Filters vii

12 TABLES Table 4.1- ANSI/IEEE 519 voltage distortion limits Table 4.2-IEC voltage harmonic distortion limits in public low-voltage network Table 4.3- IEC voltage harmonic distortion limits in industrial plants Table 4.4-IEC class 3 Table 4.5-IEC maximum permissible harmonic currents for class D equipment Table 4.6- IEEE 519 current distortion limits. Table5.1- Circuit parameters of 12 pulse AC-DC converter Table 5.2-Designed Series tuned filter specifications Table 5.3- Circuit parameters of three phase inverter Table5.4-Designed Passive series filter specifications viii

13 CHAPTER # 1 INTRODUCTION 1.1 ELECTRIC POWER QUALITY Electric Power Quality (EPQ) is a term that refers to maintaining the near sinusoidal waveform of power distribution bus voltages and currents at rated magnitude and frequency Thus Electric Power Quality is often used to express voltage quality, current quality, reliability of service, quality of power supply, etc. Power Quality is ultimately a consumer driven issue defined as: Any power problem manifested in voltage, current or frequency deviation that results in failure or mal-operation of consumers equipment. In the study of (EPQ), different branches are being formed which deals with different issues related to electric power quality.these branches are divided into following stages. 1. Fundamental concepts 2. Sources 3. Effects 4. Modeling and Analysis 5. Instrumentation 6. Solutions (i) Fundamental concepts The fundamental concept of EPQ identifies the parameters and their degree of variation with respect to their rated magnitude which are the base reason for degradation of quality of electric power. (ii) Sources Sources are the regions or locations or events which causes the unwanted variation of those parameters. It s really a big challenge to the power engineers to find out the exact sources of power quality related disturbance in the ever increasing complex network. 1

14 (iii) Effects Effects of poor quality of power are the effects faced by the system and consumer equipment after the occurrence of different disturbances. (iv) Modeling and Analysis In modeling and analysis, attempts are taken to configure the disturbance, its occurrence, sources and effect; mainly based on the mathematical background. (v) Instrumentation For monitoring of EPQ, constant measurement and instrumentation of the electric parameters are necessary. (vi) Solutions Complete solution, i.e. delivery of pure power to the consumer side is practically impossible. Our target is to minimize the probability of occurrence of disturbances and to reduce the effects of EPQ problems. 1.2 SOURCES FOR ELECTRIC POWER QUALITY DETERIORATION IN A POWER SYSTEM The sources of poor power quality can be categorized in two groups: (i) Actual loads, Equipment and Components. (ii) Subsystems of transmission and distribution systems. Poor quality is normally caused by power line disturbances such as impulses, notches, voltage sag and swell, voltage and current unbalances, momentary interruption and harmonic distortions. The major contributors to poor power quality are harmonics and reactive power. Solid state control of ac power using high speed switches are the main source of harmonics whereas different non-linear loads contribute to excessive drawl of reactive power from supply. It leads to catastrophic consequences such as long production downtimes, malfunction of devices and shortened equipment life. 1.3 NEED FOR ASSESSMENT OF ELECTRIC POWER QUALITY It is common experience that electric power of poor quality has detrimental effects on health of different equipment and systems. Moreover, power system stability, continuity and reliability fall with the degradation of quality of electric power. 2

15 To avoid such effects, it is important to continuously assess the quality of power supplied to a consumer. 1.4 CLASSIFICATION OF POWER SYSTEM DISTURBANCES Power quality problems occur due to various types of electrical disturbances. Most of the EPQ disturbances depend on amplitude or frequency or on both frequency and amplitude. Based on the duration of existence of EPQ disturbances, events can divided into short, medium or long type. These disturbances are mainly classified as: (i) Interruption/under voltage/over voltage During power interruption, voltage level of a particular bus goes down to zero. The interruption may occur for short or medium or long period. Under voltage and over voltage are fall and rise of voltage levels of a particular bus with respect to standard bus voltage. Such disturbances increase the amount of reactive power drawn or deliver by a system, insulation problems and voltage stability. (ii) Voltage/Current unbalance Voltage and current unbalance may occur due to the unbalance in drop in the generating system or transmission system and unbalanced loading. During unbalance, negative sequence components appear. It hampers system performance and voltage stability. (iii) Harmonics Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate (termed the fundamental frequency; usually 50 or 60 Hz). Periodically distorted waveforms can be decomposed into a sum of the fundamental frequency and the harmonics. Harmonic distortion originates due to the nonlinear characteristics of devices and loads on the power system. Harmonics are classified as integer harmonics, sub harmonics and inter harmonics. Integer harmonics have frequencies which are integer multiple of fundamental frequency, sub harmonics have frequencies which are smaller than fundamental frequency and inter harmonics have frequencies which are greater than fundamental frequencies. Sometimes harmonics are classified as time harmonics and spatial (space) harmonics. Monitoring of harmonics with respect to fundamental is important consideration in power system application. 3

16 Figure 1.1 (iv) Transients Transients which are the sudden rise of signal may generate in the system itself or may come from the other system. Transients are classified into two categories: dc transient and ac transient. The figure 1.2 shows utility capacitor-switching transient. Figure 1.2 (v) Voltage Sag It is a short duration disturbance. During voltage sag, RMS voltage falls to a very low level for short period of time. It is actually a reduction in RMS voltage over a range of pu for a duration greater than 10 ms but less than 1 s Figure 1.3 4

17 (vi) Voltage Swell It is a short duration disturbance. During voltage sag, RMS voltage increases to a very high level for short period of time. It is an increase in RMS voltage over a range of pu for a duration greater than 10 ms but less than 1 s. Figure 1.4 (vii) Flicker It is undesired variation of system frequency. The voltage variations resulting from flicker are often within the normal service voltage range, but the changes are sufficiently rapid to be irritating to certain end users. Flicker can be separated into two types: cyclic and noncyclic. Cyclic flicker is a result of periodic voltage fluctuations on the system, while Non-cyclic is a result of occasional voltage fluctuations. The usual method for expressing flicker is similar to that of percent voltage modulation. It is usually expressed as a percent of the total change in voltage with respect to the average voltage ( V/V) over a certain period of time. The figure 1.5- shows a typical flicker waveform. Figure 1.5 5

18 (viii) Ringing waves Oscillatory disturbance of decaying magnitude for short period of time is known as ringing wave. It may be called a special type transient. (ix) Outage It is special type of interruption where power cut has occurred for not more than 60 s. 1.5 WHY ARE WE CONCERNED ABOUT POWER QUALITY? The ultimate reason that we are interested in power quality is economic value. There are economic impacts on utilities, their customers, and suppliers of load equipment. The quality of power can have a direct economic impact on many industrial consumers. There is big money associated with these disturbances. It is not uncommon for a single, commonplace, momentary utility breaker operation to result in a $10,000 loss to an average-sized industrial concern by shutting down a production line that requires 4 hours to restart. In the semiconductor manufacturing industry, the economic impacts associated with equipment sensitivity to momentary voltage sags resulted in the development of a whole new standard for equipment ridethrough. The electric utility is concerned about power quality issues as well. Meeting customer expectations and maintaining customer confidence are strong motivators. The loss of a disgruntled customer to a competing power supplier can have a very significant impact financially on a utility. 6

19 Chapter # 2 FUNDAMENTALS OF HARMONICS 2.1 INTRODUCTION A pure poly-phase system is expected to have pure sinusoidal alternating current and voltage waveforms of single frequency. But, the real situation deviates from this purity. Real voltage and current waveforms are distorted. Normally they are called non sinusoidal waveforms. Non sinusoidal waveform is formed with the combination of many sine waves of different frequencies. Thus actual power system signals have fundamental component as well as harmonic components. Harmonic distortion is caused by nonlinear devices in the power system. A nonlinear device is one in which the current is not proportional to the applied voltage. When a waveform is identical from one cycle to the next, it can be represented as a sum of pure sine waves in which the frequency of each sinusoid is an integer multiple of the fundamental frequency of the distorted wave. This multiple is called a harmonic of the fundamental, hence the name of this subject matter. The sum of sinusoids is referred to as a Fourier series, named after the great mathematician who discovered the concept. Harmonic component of current of order n can be represented as in = In sin 2πnft (2.1) Where In is the amplitude of harmonic component of order n. Figure 2.1 7

20 Figure 2.1 illustrates that any periodic, distorted waveform can be expressed as sum of sinusoids. 2.2 TYPES OF HARMONICS Integer harmonics are divided into two categories: odd harmonics and even harmonics. Other than integer harmonics there are sub and inter harmonics where n is fractional Odd Harmonics: Integer harmonics having frequencies which are odd integer multiple of fundamental frequency are known as odd harmonics. Odd harmonics may be expressed as i n = I n sin 2πnft (2.2) Where, n = 3, 5, 7,... etc. and In is the amplitude of harmonic component of order n Even Harmonics: Integer harmonics having frequencies which are even integer multiple of fundamental frequency are knows as even harmonics. Even harmonics may be expressed as i n = I n sin 2πnft (2.3) Where, n = 2, 4, 6,... etc. and In is the amplitude of harmonic component of order n Inter Harmonics: Often in non-sinusoidal waveform there are harmonics having frequencies which are greater than fundamental but not integer multiple of fundamental frequency. These are known as inter-harmonics. Mathematically, i n = I n sin 2πnft (2.4) Where, n > 1 but not integer; e.g.: 1.2, 1.5, etc Sub Harmonics: Often in non-sinusoidal waveform there are harmonics having frequencies which are smaller than fundamental frequency. These are known as subharmonics. Mathematically, i n = I n sin 2πnft (2.5) Where, n < 1; e.g.: 0.2, 0.5, etc 8

21 2.3 NON-SINUSOIDAL WAVEFORM Non-sinusoidal wave is constituted by the combination of odd-even harmonic components as well as fundamental component. Thus, mathematically, it can be expressed as: i= (2.6) A non-sinusoidal wave may be expressed in terms of harmonics as i= = = I1 sin 2πft + I2 sin 4πft + I3 sin 6πnft + I4 sin 8πnft + I5 sin 10πnft+ = I1 sin 2πf1ft + I2 sin 2πf2t + I3 sin 2πf3t + I4 sin 2πf4t + I5 sin 2πf5t.... (2.7) Average Value Harmonic components having phase angle (α n ) can be expressed as: in = In sin (2πf n t α n ) (2.8) Average value is given by: i nav = ( ) ( ) (2.9) RMS Value RMS value of the non-sinusoidal current wave is given by: i RMS = = (2.10) Form Factor Form factor is the ratio of RMS value to average value. In case of non-sinusoidal wave it is given by: Form Factor = (2.11) Harmonic Power The current and voltage are respectively given as: i = (2.12) 9

22 v = (2.13) Power contributed by harmonic components of voltage and current waveforms can be expressed as: Average power of harmonics of order n, Pn = = i n RMS v n RMS cos (2.14) Where = = phase difference between harmonic component of voltage and current waveforms of order n Total Active Power Total active power is contributed by fundamental as well as harmonic components of voltage and current waveform. Thus total power is written as: p = = = i 1 rms v 1 rms cos ϕ 1 + i 2 rms v 2 rms cos ϕ 2 + i 3 rms v 3 rms cos ϕ 3 (2.15) 2.4 SOURCES OF HARMONICS The main sources of harmonics in electric power systems can be categorized as: a) Magnetization nonlinearities of transformer b) Rotating machines c) Arcing devices d) Semi-conductor based power supply e) Inverter fed A.C drives f) Thyristor controlled reactors g) Phase controllers h) A.C regulators 2.4.1Magnetization nonlinearities of transformer Transformers magnetic material characteristic is non-linear. This non linearity is the main reason for harmonics during excitation. Sources of harmonics in transformer may be classified into four categories as follows: 10

23 a.normal Excitation: Normal excitation current of a transformer is non-sinusoidal. The distortion is mainly caused by zero sequence triplen harmonics and particularly the third present in the excitation current. b.symmetrical Over Excitation: When transformers are subjected to a rise in voltage, the cores face a considerable rise in magnetic flux density, which often causes considerable saturation. This saturation with symmetrical magnetizing current generates all the odd harmonics. c.inrush Current Harmonics: A switched-off transformer with residual flux in the core is re-energized, the flux density rises to peak levels of twice the maximum flux density or more, thus producing high-ampere turns causing magnetizing currents to reach up to 5-10 per unit of the rated value, typically known as inrush current. This causes generation of enormous second order harmonic component in the transformer current d.d.c Magnetization: Under magnetic unbalance, the core contains an average value of flux (φ dc ), which is equivalent to a direct component of excitation current of the transformer which contains both odd and even harmonic components Rotating machines Rotating machines also contribute in generation of harmonics, this maybe further classified as: a) Magnetic Nonlinearities of the core material causes harmonic generation b) Non-uniform flux distribution in the air gap leads to harmonic production c) Slots and Teeth presence changes the reluctance of magnetic flux and this variation results in harmonics d) Rotor Saliency also brings the variation of reluctance in the magnetic path and reactance in electric path which contribute to harmonics generation e) Crawling is a common problem faced by induction motors. During this fault, odd harmonics like 5th and 7th orders appear. f) Cogging, a problem when motor fails to start produces harmonics different from those in the normal condition g) Some other causes are Rotor misalignment, Mass unbalance, Fractal error and Unsymmetrical faults. 11

24 2.4.3 Arcing devices Electric Arc Furnace, Discharge type lighting, Arc welders have highly non-linear voltage vs current characteristics. Therefore, arc ignition is equivalent to a shortcircuit with a decrease in voltage. Hence they are a major source of power system harmonics Power supplies with semiconductor devices Harmonics generated by such supplies include integer, inter and sub harmonics whose magnitudes and frequencies depend upon the type of semiconductor devices used, operating point, nature of load variation, etc Inverter Fed A.C drives The use of switching devices like GTO, IGBT, etc. along with PWM technique has become popular in AC Drives applications which are sources of integer as well as fractional harmonics Thyristor controlled reactors Different types of thyristor controlled reactors used in power system like series controller, shunt controller, static VAR compensator (SVC), fixed capacitor thyristor controlled reactor (FCTCR), thyristor switched capacitor thyristor controlled reactor (TSCTCR) are sources of harmonics in power system Phase controllers & AC regulators Phase Controller for the supply of balanced electric power and AC voltage regulators when applied both online and offline for voltage regulation will result in harmonic generation. 2.5 EFFECTS OF HARMONICS Harmonics are not desirable in most applications and operations of electrical power system; therefore it has wide adverse effects on the system. The effects of harmonics may be classified as: 12

25 2.5.1 Resonance and effect on capacitor banks: Resonance occurs when the frequency at which the capacitive and inductive reactance of the circuit impedance are equal. At the resonant frequency, a parallel resonance has high impedance and series resonance low impedance. Harmonic resonances create problems in operation of power factor correction capacitors. Capacitors used for power factor correction cause system resonances due to harmonic frequencies. This results in excessive high current, which can produce damage to the capacitors. Change of harmonic contents sometimes increases reactive power over permissible manufacturer tolerances Poor damping: In the presence of harmonics, undesirable variation of the degree of damping changes the operating performance of different measuring and controlling instruments Effects of harmonics on rotating machines: Harmonic voltages and currents increase losses in the stator windings, rotor circuit, and stator and rotor lamination; resulting in overheating and efficiency reduction. Harmonic currents present in the stator of an AC machine produce induction motoring action (i.e. positive harmonic slips Sn), which gives rise to shaft torques in the same direction as the harmonic field velocities in such a way that all positive sequence harmonics will develop shaft torques aiding shaft rotation whereas negative sequence harmonics will have the opposite effect, thus affecting the speed/torque characteristic considerably Effects on transformer: Harmonic Voltage increases the core losses in laminations and stresses the insulation, while harmonic current increase copper losses. They also cause increase in core vibrations Effects on transmission lines: Harmonics tend to increase Skin and Proximity Effects since both are frequency dependent. Harmonic currents reduce the power transmitting capacity by increasing copper losses and also produce harmonic voltage drops across various circuit impedances. As a result, a weak system of large impedance has low fault level and 13

26 greater voltage disturbances where as a stiff system of low impedance has high fault level and lower voltage disturbance. Harmonic voltages reduce dielectric strength of cables by causing an increase in dielectric losses Harmonic interference with power system protection: Harmonics degrade the operating characteristics of protective relays. Some digital relays and algorithms operate on sample data and zero crossing moment. Harmonic distortion creates error on such operation. Harmonics make higher di/dt at zero crossings and the current sensing ability of the thermal magnetic breakers and change trip point due to extra heating in the solenoid. Current harmonic distortion affects the interruption capability of circuit breakers and fuses Effects of harmonics on consumer equipment: IEEE Task Force on the Effects of Harmonics on Equipment has made a wide study on this matter. The result can be summarized as follows: a) Television Receivers: Harmonics changes in TV picture size and brightness. Interharmonics change amplitude modulation of the fundamental frequency. For example, even a 0.5% inter harmonic level can produce periodic enlargement and reduction of the image of the cathode ray tube. b) Fluorescent and mercury arc lighting: Capacitors used in such lighting applications together with the inductance of the ballast and circuit produce a resonant frequency. It results in excessive heating and failure in operation. Audible noise is produced due to harmonic voltage distortion. c) Computers: Harmonics create problems in monitor and CPU operation. Harmonic rate (geometric) measured in vacuum must be less than 3% (Honeywell, DEC) or 5% (IBM). CDC specifies that the ratio of peak to effective value of the supply voltage must equal to 1.41± HARMONIC INDICES The two most commonly used indices for measuring the harmonic content of a waveform are the Total Harmonic Distortion (THD) and the Total Demand Distortion. 14

27 2.6.1 Total harmonic distortion The THD is a measure of the effective value of the harmonic components of a distorted waveform. That is, it is the potential heating value of the harmonics relative to the fundamental. This index can be calculated for either voltage or current: THD= (2.16) Where Mh is the RMS value of harmonic component h of the quantity M. The RMS value of a distorted waveform is the square root of the sum of the squares as shown in Equations (2.16) and (2.17). The THD is related to the rms value of the waveform as follows: RMS= = M1 (2.17) The THD is a very useful quantity for many applications, but its limitations must be realized. It can provide a good idea of how much extra heat will be realized when a distorted voltage is applied across a resistive load. Likewise, it can give an indication of the additional losses caused by the current flowing through a conductor. However, it is not a good indicator of the voltage stress within a capacitor because that is related to the peak value of the voltage waveform, not its heating value. The THD index is most often used to describe voltage harmonic distortion. Harmonic voltages are almost always referenced to the fundamental value of the waveform at the time of the sample. Because fundamental voltage varies by only a few percent, the voltage THD is nearly always a meaningful number Total demand distortion Current distortion levels can be characterized by a THD value but this can often be misleading. A small current may have a high THD but not be a significant threat to the system. For example, many adjustable-speed drives will exhibit high THD values for the input current when they are operating at very light loads. This is not necessarily a 15

28 significant concern because the magnitude of harmonic current is low, even though its relative current distortion is high. Some analysts have attempted to avoid this difficulty by referring THD to the fundamental of the peak demand load current rather than the fundamental of the present sample. This is called total demand distortion and serves as the basis for the guidelines in IEEE Standard , Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems. It is defined as follows: TDD= (2.18) I L is the peak, or maximum, demand load current at the fundamental frequency component measured at the point of common coupling (PCC). 2.7 PRINCIPLES FOR HARMONICS CONTROL Harmonic distortion is present to some degree on all power systems. Fundamentally, one needs to control harmonics only when they become a problem. There are three common causes of harmonic problems: 1. The source of harmonic currents is too great. 2. The path in which the currents flow is too long (electrically), resulting in either high voltage distortion or telephone interference. 3. The response of the system magnifies one or more harmonics to a greater degree than can be tolerated. When a problem occurs, the basic options for controlling harmonics are: 1. Reduce the harmonic currents produced by the load. 2. Add filters to siphon the harmonic currents off the system, block the currents from entering the system, or supply the harmonic currents locally. 3. Modify the frequency response of the system by filters, inductors, or capacitors. 2.8 WHERE TO CONTROL HARMONICS? The strategies for mitigating harmonic distortion problems differ somewhat by location. The following techniques are ways for controlling harmonic distortion on both the utility distribution feeder and end user power system. 16

29 2.8.1 On Utility Distribution Feeders Harmonic problems on distribution feeders often exist only at light load. The voltage rises, causing the distribution transformers to produce more harmonic currents and there is less load to damp out resonance. Switching the capacitors off at this time frequently solves the problem. Should harmonic currents from widely dispersed sources require filtering on distribution feeders, the general idea is to distribute a few filters toward the ends of the feeder. Figure 2.2 shows one example of a filter installed on an overhead distribution feeder. This shortens the average path for the harmonic currents, reducing the opportunity for telephone interference and reducing the harmonic voltage drop in the lines. The filters appear as nearly a short circuit to at least one harmonic component. This keeps the voltage distortion on the feeder to a minimum. Figure In End-User Facilities When harmonic problems arise in an end-user facility, the first step is to determine if the main cause is resonance with power factor capacitors in the facility. If this is the case, a simple solution would be to use a different capacitor size. Installation of filters on end-user low-voltage systems is generally more practical and economical than on 17

30 utility distribution systems. The criteria for filter installation are more easily met, and filtering equipment is more readily available on the market. 2.9 USEFUL TOOLS FOR HARMONICS ASSESSMENT The basis of all harmonic assessment still depends on the measurement of amplitude and phase angle of the harmonics components. Different mathematical tools have come out for this purpose. Some of them are capable of measuring both integer and non-integer order of harmonics. Some of them are capable of measuring only integer type of harmonics. Also, in many cases, signals could not be captured in continuous form.to overcome these limitations modified mathematical tools have been developed to handle discrete signals. Mathematical tools for harmonics analysis may be in time domain, or frequency domain or both time-frequency domains. One of old techniques used in analysis of non-sinusoidal signals is Fourier transform. Fourier analysis has been used for power quality assessment for a long period. It permits mapping of signals from time domain to frequency domain by decomposing the signals into several frequency components. The transform method suffers from limitation to handle discrete or discontinuous, multi-valued and undefined signals which are often faced by electrical applications. In this thesis, we have employed Fast Fourier Transform (FFT) built-in tool available in MATLAB which can be directly applicable or after some alteration on captured discrete signals. 18

31 Chapter # 3 HARMONIC FILTERS 3.1 INTRODUCTION Any combination of passive (R, L, C) and/or active (transistors, op-amps) elements designed to select or reject a band of frequencies is called a filter. Filters are used to filter out any unwanted frequencies due to nonlinear characteristics of some electronic devices or signals picked up by the surrounding medium. Filters are one of those corrective (remedial) solutions aimed at overcoming harmonic problems and to keep them within safe limits. They provide a low impedance path or trap to a harmonic to which a filter is tuned, hence are called tuned (resonant) circuits. The process of tuning aims at setting the circuit to f r (resonant frequency) where the response is or at maximum. The circuit is then said to be in state of resonance. There are three types of filters. 1. Passive filters 2. Active filters 3. Hybrid filters We will be dealing with only Passive Filters as these are the main concern of this thesis work. A short description about active and hybrid filters is also provided in what follows: 3.2 PASSIVE FILTERS Passive filters are basically topologies or arrangements of R, L and C elements connected in different combinations to gain desired suppression of harmonics. They are employed either to shunt the harmonic currents off the line or to block their flow between parts of the system by tuning the elements to create a resonance at a selected frequency. The also provide the reactive power compensation to the system and hence improve the power quality. However, they have the disadvantage of potentially interacting adversely with the power system and the performance of passive filter depends mainly on the system source impedance. On the other hand they can be used 19

32 for elimination of a particular harmonic frequency, so number of passive filters increase with increase in number of harmonics on the system. They can be classified into: 1. Passive shunt filter 2. Passive series filter Passive shunt filters are the main focus of study in this thesis and are discussed here in detail while a little thought is presented on series filters Passive Shunt Filters They are classified as shown in the figure 3.1 below: Figure 3.1 Single Tuned Filter: The most common type of passive filter is the single-tuned notch filter. This is the most economical type and is frequently sufficient for the application. The notch filter is series-tuned to present low impedance to a particular harmonic current and is connected in shunt with the power system. Thus, harmonic currents are diverted from their normal flow path on the line through the filter Notch filters can provide power factor correction in addition to harmonic suppression. In fact, power factor correction capacitors may be used to make single-tuned filters. They are tuned at low harmonic frequencies. At the tuned harmonic, capacitor and reactor have equal reactance and the filter has purely resistive impedance. A general schematic diagram of series tuned filter is shown in the figure 3.1. The impedance vs frequency curve of this filter is shown in Figure

33 Figure 3.2 Double Band Pass Filters: A double Band Pass Filter is a series combination of a main capacitor, a main reactor and a tuning device which consists of a tuning capacitor and a tuning reactor connected in parallel. The impedance of such a filter is low at two tuned frequencies. Damped Filters: They can be 1 st, 2 nd, or 3 rd order type. The most commonly used is the 2 nd order. A 2 nd order damped filter consists of a capacitor in series with a parallel combination of a reactor and a resistor. It provides low impedance for a moderately wide range of frequencies. When used to eliminate high order harmonics ( 17 th and above), a damped filter is referred to as High Pass Filter, providing a low impedance for high frequencies but stopping low ones. Damped filters are usually tuned to hn<hr, that is 10.7, 16.5 and so on. The Impedance Vs Frequency curve of Second-Order High Pass Filter is shown in figure 3.3 Figure

34 3.2.2 Passive Series Filter Unlike a notch filter which is connected in shunt with the power system, a series passive filter is connected in series with the load. The inductance and capacitance are connected in parallel and are tuned to provide high impedance at a selected harmonic frequency. The high impedance then blocks the flow of harmonic currents at the tuned frequency only. At fundamental frequency, the filter would be designed to yield low impedance, thereby allowing the fundamental current to flow with only minor additional impedance and losses. Figure 3.4 shows a typical series filter arrangement. Series filters are used to block a single harmonic current (such as the third harmonic) and are especially useful in a single-phase circuit where it is not possible to take advantage of zero-sequence characteristics. The use of the series filters is limited in blocking multiple harmonic currents. Each harmonic current requires a series filter tuned to that harmonic. This arrangement can create significant losses at the fundamental frequency. Figure ACTIVE FILTERS Active filters are relatively new types of devices for eliminating harmonics. They are based on sophisticated power electronics and are much more expensive than passive filters. However, they have the distinct advantage that they do not resonate with the system. They can work independently of the system impedance characteristics. Thus, they can be used in very difficult circumstances where passive filters cannot operate successfully because of parallel resonance problems. They can also address more than one harmonic at a time and combat other power quality problems such as flicker. They are particularly useful for large, distorting loads fed from relatively weak points on the power system 22

35 3.4 HYBRID FILTERS Since the APFs (Active Power Filters) topologies are not cost-effective for the application of high power because of their high rating and very high switching frequency of PMW (Pulse Width Modulator) converters. Thus LC PPFs (Passive Power Filters) are used for harmonic filtration of such large nonlinear loads. However, Passive filters suffer from some shortcomings for example, the performance of these filters is affected due to the varying impedance of the system and with the utility system the series and parallel resonances may be created, which cause current harmonics increase in the supply. Therefore, another solution of harmonic mitigation, called HAPF (Hybrid Active Power Filter), has been introduced. HAPF provides the combined advantages of APF and PPF and eliminate their disadvantages. These topologies are cost effective solutions of the high-power power quality problems with well filtering performance. 3.5 DESIGN STEPS OF SERIES TUNED FILTERS In design of the filter, the proper selection of the capacitor size is very essential from power factor point of view. A series-tuned filters is a capacitor designed to trap a certain harmonic by adding a reactor such that X L =X c at the frequency f n. To design series-tuned following step are followed: Determine the capacitor size Q c in MVAR, say the reactive power requirement of the source. The capacitor reactance is (3.1) Capacitance for filters is calculated by (3.2) Where n=number of filters to be designed The resonance condition will occur when capacitive reactance is equal to inductive reactance as: X L =X C (3.3) 23

36 To trap the harmonics of order h, the reactance should be of size (3.4) The resistance of filter depends on the quality factor (Q) by which sharpness of the tuning is measured. (3.5) Where Q is the quality factor and for series tuned is 30<Q DESIGN STEPS OF 2 ND ORDER HIGH PASS FILTER To design 2 nd order high pass filter, following step are followed: Determine the capacitor size Q c in MVAR, say the reactive power requirement of the source. The capacitor reactance is (3.6) Capacitance for filters is calculated by (3.7) Where n=number of filters to be designed The resonance condition will occur when capacitive reactance is equal to inductive reactance as: X L =X C (3.8) To trap the harmonics of order h, the reactance should be of size (3.9) The resistance of filter depends on the quality factor (Q) by which sharpness of the tuning is measured. R = * Q (3.10) Where Q, the quality factor is 0.5 <Q< 5. 24

37 3.7 QUALITY FACTOR, BANDWIDTH AND SELECTIVITY The quality factor of a resonant circuit is defined as the ratio of reactive power of either the inductor or capacitor to the average power of resistor at resonance. Q For a Series Resonant Circuit, Qs= = (3.11) Where X L is the inductive reactance, R is the resistance, Qs is the quality factor of series resonant circuit and Then, f S = is the angular frequency that account for resonance. is called the resonant frequency of the series resonant circuit. The quality factor is a measure of the sharpness of the tuning frequency. It is determined by the resistance value. The effect of Q factor on the response curve is as shown in the figure 3.5 Figure 3.5 In terms of Qs, if R is large for the same X L, then Qs is less. A small Qs, therefore, is associated with a resonance curve having a large bandwidth and a small selectivity while a large Qs indicates otherwise. The selectivity indicates that one must be selective in choosing the frequency to ensure that it is in the bandwidth. The smaller the bandwidth, the higher is its selectivity as shown in the figure

38 Figure 3.6 The larger value of the quality factor gives the best reduction in harmonic reduction. However, it is necessary to take care of the harmonic frequencies because these harmonic current frequencies will also follow the least impedance path. These currents cause the increased power loss. Therefore it is necessary to perform the computer based harmonic simulation for analyzing the performance of the filters. The Bandwidth is related o the Quality Factor as: BW= or (3.12) BW= f 2 -f 1 = (3.13) Where f 1 and f 2 are the cutoff frequencies or half power frequencies which specify the range of Band Width 26

39 Chapter # 4 STANDARD LIMITS OF HARMONIC DISTORTION 4.1 INTRODUCTION The limits of allowable voltage and current harmonics distortion set by IEEE and IEC have been presented in this chapter which are just based on the personal experiences and involvement of Power Quality analysts in harmonic analysis research. These standards provide guidelines for power quality usages and practices. 4.2 VOLTAGE HARMONIC DISTORTION LIMITS Bus voltage at PCC Individual V h, % Voltage THD, % V<69KV V<161KV V 161KV Table 4.1- ANSI/IEEE 519 voltage distortion limits Odd harmonics Even harmonics Triplen harmonics H % V h H % V h H % V h Table 4.2-IEC voltage harmonic distortion limits in public low-voltage network 27

40 Even harmonics Triplen harmonics H % v h H % V h H % V h x Table 4.3- IEC voltage harmonic distortion limits in industrial plants Odd harmonics Even harmonics Triplen harmonics H %, V h H %, V h H %, V h Table 4.4-IEC class CURRENT HARMONIC DISTORTION LIMITS H Max I h h Equipment input current 16 A per phase Table 4.5-IEC maximum permissible harmonic currents for class D equipment 28

41 I SC /I L I h / I L, % ---- general distribution systems (120V-69KV) TDD h<11 11 h<17 17 h<23 23h h<35 h 35 < > I SC /I L I h / I L, % ---- general sub transmission systems (69-161KV) TDD h<11 11 h<17 17 h<23 23 h<35 h 35 I SC /I L I h / I L, % ---- general transmission systems (>161KV) TDD h<11 11 h<17 17 h<23 23 h<35 h 35 % < Table 4.6- IEEE 519 current distortion limits. 29

42 Chapter # 5 MATLAB SIMULATION & RESULTS 5.1 POWER CONVERTERS HARMONIC BEHAVIOUR In almost all electronic equipment, the devices directly connected with the power network are converters; their characteristics determine the harmonic behavior of the complete system and the impact on power supply depends on the rectifier topology and the type of power devices employed. The characteristic harmonic components of the current pulses supplying converters have harmonic orders n, such as n = k p±1, where k = 1, 2, 3, 4 and p is the number of converter arms (pulse number). Thyristor rectifiers have the advantage of a relatively simple control system over uncontrolled rectifiers and can be found in D.C drives or other many applications. The harmonics generated by a phase-controlled rectifier on the A.C side may be calculated as for uncontrolled rectifiers, depending on pulse number. For example, for six-pulse rectifiers, the main harmonic components are the fifth and the seventh. However, in this case, new even and odd harmonics, referred to as noncharacteristic harmonics, of low amplitudes, are produced; on the other hand, the amplitudes of the characteristic harmonics are modified by several factors including asymmetry, inaccuracy in thyristor firing times, switching times, imperfect filtering. A displacement of the harmonics as a function of the thyristor phase angle may also be observed as shown in the figure 5.1 below: Figure

43 Three-phase electronic power converters differ from single-phase converters mainly because they do not generate third-harmonic currents as shown in the figure 5.2 for a three phase inverter circuit. This is a great advantage because the third-harmonic current is the largest component of harmonics. However, they can still be significant sources of harmonics at their characteristic frequencies. Figure HARMOINIC ANALYSIS OF THREE PHASE 12 PULSE AC-DC CONVERTER In this work, three-phase ac to dc converter has been simulated with and without passive shunt filters using MATLAB/SIMULINK environment. This system is analyzed without and with passive filters using Total Harmonic Distortion as an index.the circuit parameters used in simulation are presented in table below: Supply Voltage Source Inductance Load (Resistive) Transformer (three winding) 220 V RMS 1.06mH 100 ohms Yg/y/d1, 1200 VA, 220V/100V/100V Table 5.1 The system considered for harmonic analysis here consists of a three phase converter consisting of two 6 pulse bridges modeled to work as an uncontrolled rectifier. The schematic of the system is shown in the figure

44 Figure 5.3 The circuit response without filters is demonstrated in what follows. The figure 5.4 and 5.5 shows three phase supply voltage and current respectively. The FFT analysis windows of VS and Is are given in figure 5.6 and 5.7 which shows the percentage harmonics in the spectrum before the filters are incorporated. Since the waveforms are distorted, it implies the presence of harmonics. Figure

45 Figure 5.5 Figure 5.6 Figure 5.7 Without passive filters the total harmonic distortion of the current is above the range specified by the power quality standards. To follow the recommended IEEE 519 power harmonic standards the total harmonic distortion must be less than 5%. This 33

46 can be obtained by connecting the passive filters to the system. For reducing the THD below 5% passive filters have been designed. There are three filters used, two of which are single tuned at 11 th and 13 th harmonic and the other is high pass filter for high order harmonics. The filters specifications for each type of filter are shown in the below: Harmonic Order Capacitance (µf) Inductance(mH) Resistance 11 th th Higher order Table 5.2 The Q Factor for the filters is chosen to be 40. The Impedance VS Frequency curve for the desired response is as shown in the figure 5.8 Figure 5.8 After connecting the filters the three-phase supply currents become near to sinusoidal and harmonics are decreased below 5%. The simulation results are presented in Figure 5.9 and 5.10 for the voltage and current on the AC side. The FFT analysis windows of the source voltage and current are given in figure 5.11 and 5.12 respectively which shows the percentage harmonics in the spectrum with the filters incorporated. 34

47 Figure 5.9 Figure 5.10 Figure

48 Figure HARMONIC ANALYSIS OF THREE PHASE INVERTER In this analysis, a three phase inverter supplying a three phase resistive load has been investigated with and without passive series filters and the effects on parameters on AC side has been analyzed with THD as an index. The inverter model along with circuit parameters are presented in the table below: V dc Three Phase Load (Resistive) Table V 53kW Figure

49 Figure 5.14 The circuit waveforms without filters for the voltage and current on AC side along with their FFT analysis are shown respectively in the figures below: Figure

50 Figure 5.16 When LC series filter is employed, it is observed that THD becomes less than 5% and the waveforms become near to sinusoidal. The model is as shown in the figure. Its waveforms along with FFT analysis are also shown. Figure 5.17 The values of filter s inductance and capacitance are : Capacitance (µf) Inductance (mh)

51 Figure 5.18 Figure

52 Figure 5.20 Figure