Literature Survey on Electric Power Quality

Transcription

1 Chapter 2 Literature Survey on Electric Power Quality Chapter at a Glance The chapter starts with an introduction. It categorizes different power quality problems followed by description of the sources of poor power quality and its effect on power system. Different IEC and IEEE standards and power quality related definitions are mentioned. It ends with a review on this issue during last few decades.

2 Literature Survey on Beeftic Power Qualify Chapter Introduction Development of technology in all its areas is progressing at faster rate. Power scenario has changed a lot. With the increase of size and capacity, power systems have become complex leading to reduce reliability. But, the development of electronics, electrical device and appliances has become more and more sophisticated and they demand uninterrupted and conditioned power. These have pushed the present complex electricity network and market in a strong competition resulting in the concept of deregulation. In this ever changing power scenario, quality assurance of electric power has also been affected. It demands a deep research and study on the subject Electric Power Quality. 2.2 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. EPQ has captured increasing attention in power engineering in recent years. Its main aspects [163] are: i. Fundamental concepts ii. iii. iv. Sources Effects. Modeling and Analysis v. Instrumentation vi. Solutions EPQ also describes the variation of the voltage, current and frequency in a power system. Most power system equipment has been able to operate successfully with relatively wide variations of these three parameters. However, within the last five to fifteen years, a large amount of equipment has been added to the power system, which is not so tolerant of these variations. The sophistication of electrical appliances with the development of electronics has added to the demand of quality power at the consumer premises. To ensure uninterrupted and quality power has thus become a point of competition for the power producers. Thus an open and competitive power market has paved its way. These situations have introduced the concept of deregulation in power sector. Like all other commodities, for electric power there should be quality issues at each physical location in all system especially in deregulated system. The sources of poor power quality can be categorized in two groups: (i) actual loads, equipment and components and (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 as mentioned in the International Electro-technical Commission (IEC)

3 Electric Power Quality in Power System, Ph, D. (Tech) Thesis, Department of Applied Physics, C U 2009 [17C] classification of power quality and relevant IEEE standard [58], [112], [113], [158]. The other 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. 2.3 Classification of Power System Disturbances The disturbances causing power quality degradation arising in a power system [70], [78], [82]. [97], [99], [102], [116], [133], [134], [139], [142], [152], [162], [163], [184], [186], [189], [195], [207]-[216] and their classification [29], [30], [187] mainly include: a. Voltage sag b. Voltage swell c. Flicker d. Voltage / Current unbalance 2. Ringing waves f. Outage g. Transients h. Harmonics Short definitions of the power system disturbances are summarized in Table 2.1 Table 2.1 Definition of power system disturbances SI No Disturbance Short definition a Voltage sag A reduction in RMS voltage over a range of 0.1 to 0.9 pu for a duration greater than 10 ms but less than 1 s [7], [191], b Voltage swell An increase in RMS voltage over a range of 1.1 to 1.8 pu for a duration greater than 10 ms but less than Is [49], [55]. c Flicker A visual effect of frequency variation of voltage in a system. d Voltage / Current unbalance e Ringing waves Deviation in magnitude of voltage / current of any one or two of the three phases. A transient condition which decays gradually f Outage Power interruption for not exceeding 60 s duration due to fault or maltrippirg of switchgear / system g Transients Sudden rise of sigial h Harmonics Non-sinusoidal wave forms I 14 V % J

4 Literature Survey on Electric Power Quality Chapter 2 The present thesis has mainly dealt with issues relating harmonics and voltage / current unbalance. Basic information and literature survey of power system harmonics are briefly discussed in sections 2.4 to 2.9 and those of unbalance in Researches on modeling of a polyphase system are reviewed in section Harmonic A pure polyphase system is considered to have pure sinusoidal alternating current and voltage waveforms. But, in reality, voltage and current waveforms are not pure sinusoidal. They are distorted. Normally they are called non sinusoidal waveforms. Non-sinusoidal waveform is formed with the combination of many sine waves of different frequencies. In these waves, there is fundamental wave as well as harmonic components. Fundamental wave The waveform in a non-sinusoidal wave having frequency at which the system has been designed and is expected to run and/or is being operated is called fundamental wave. The frequency is called fundamental frequency. Fundamental wave is a pure sine wave having fundamental frequency. In power network of India fundamental frequency is 50 Hz. Let f be the fundamental frequency in current waveforms, then fundamental component of current will be ii = /j smlirfi (21) where, I] is amplitude of fundamental component Harmonics The waveforms in a non-sinusoidal wave having frequencies other than fundamental frequency are called harmonics The frequencies of harmonic components are known as harmonic frequency. In most of the cases harmonic frequencies are integer multiple of fundamental frequency. The integer factor is known as the order of harmonic component Let n be the order, then n will be 1,2, 3,..., etc. Harmonic component of current of order n can be represented as i,, = /,, sin 2rrnft {2 2) where In is the amplitude of harmonic component of order n Integer harmonics are divided into two categories: odd harmonics and even harmonics ji

5 Electric Power Quality in Power System, Pit. D. (Tech) Thesis, Department of Applied Physics, C.U., 2009 Odd harmonics Integer harmonics having frequencies which are odd integer multiple of fundamental frequency are called odd harmonics. Odd harmonics can be expressed as L = L sin Innft (2.3) where, n = 3, 5, 7, etc. and I is the amplitude of harmonic component of order n. Even harmonics Integer harmonics having frequencies which are even integer multiple of fundamental frequency are called even harmonics. Even harmonics can be expressed as tn=!n sin 2'Ttnfi (2.4) where, n = 2, 4, 6,..., etc. and I is the amplitude of harmonic component of order n. 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, '= x L sin 2/T.nf =i»,; (2.5) A non sinusoidal wave is written as y,v = y sin. 2/Tji/I sin 2-izft -f h sin Anft -r l3 sin. 6nnft -r/4 sin 8 nnft ie sin 1 Qnnft (2 6) SiXllTlfJt - L sin 2ttf^t - iz sin 2nf-i j s sin 2nf4t Harmonic component of order n is ip = / sin 2-jrnft = /n sin 2irfnt where, A. = nf (2.7)

6 Literature Survey on Electric Power Qualify Chapter 2 Considering phase angle, the component can be expressed as *» = ln sin(2~/;;i - ) (2.8) where is the phase angle of nlh order current harmonic component. Considering phase angles of harmonic components, non-sinusoidal wave can be written as / '75 I -5 * sfe(2<t/rr - (2.9) Inter Harmonics Sometimes 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, in = In sin 2znft (2.10) where,n > 1 but not integer; e.g.t 1.2, i.s, etc Sub Harmonic Sometimes in non sinusoidal waveform there are harmonics having frequencies which are smaller than fundamental frequency. These are known as sub-harmonics. Mathematically, C = Ir< sir 2~nft (2 H) where,r, < 1; e.g.: 0.2,0.5, etc Average value Harmonic components having phase angle can be expressed as f = In sm(2~/ t - Average value is given by (2.12) 1 % 17

7 Electric Power Quality in Power System, Pk D, (Tech) Thesis, Department of Applied Physics, C U, 2009 Average value of current is given as (2.13) Rms value Rms value of the non sinusoidal current wave I is given by V overage of i~ (2.14) Form factor Form factor is the ratio of rms value to average value. In case of non sinusoidal wave it is given by RMS Value FormFactor = Average Value (2.15) Harmonic power Let. current waveform is i i *=! :, s.. sin {2rrfrt *«) (2.16) Voltage waveform is s.=t.2, (2.17) Power contributed by harmonic components of voltage and current waveforms can be expressed as Average power of harmonics of order n = Pn

8 Literature Survey on Electric Power Quality Chapter 2 irvnd(2nft) i rt sir.(27rf t - a,,)!'; sin(2rr/ t - /? ) d (Ixft) 17z -"o L K, = ^cosivff /? ) V2-V2 where, (2.18) V* = (««4) = p/i«5e cliff erence between harmonic component of voltage and current wavef arms 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 v» -p,.;n5 cos tp.,. COS Cp2 cos cp2 (2.19) Power factor Power factor of non sinusoidal waveform is written as Active Power 2., i,.,r.v. cos <p Power Factor = Apparant Power where,= y average of i- v. V average of v I i \ /- n (2 20) A 1,23..., 19 it 3

9 Electric Power Quality in Power System, Pit. D. (Tech) Thesis, Department of Applied Physics, C.U., Sources of Harmonics in Power System Conventional electromagnetic devices as well as semiconductor applications act as sources of harmonics. In electric power, system sources of harmonics [215], [216] are classified as 1 Transformer magnetization nonlinearities 2. Rotating machine harmonics 3. Distortion caused by arcing devices 4. Semiconductor based power supply system 5. Inverter fed A. C. drives 6. Thyristor controlled reactors Transformer Magnetization Nonlinearities Transformer magnetic material characteristic is non linear. This non linearity generates harmonics during excitation. Sources of harmonics in transformer are classified into four categories: 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: Transformers are designed to make good use of the magnetic properties of the core material. When such 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 If the fundamental component is ignored, and all triplen harmonics being absorbed by delta windings, then the harmonics generated are of order of 5, 7, 11, 13, 17, 19.,..i.e those of orders 6k ± 1, where k is an integer. c. Inrush Current Harmonics: When a transformer is switched off, some times there exists a residual flux density in the core. When the transformer is re-energized the flux density can reach peak levels of twice the maximum flux density or more It produces high ampere-turns in the core. This causes magnetizing currents to reach up to 5-10 per unit of the rated value, which is very high as compared to the normal values of a few percentage points. This is known as inrush current. This causes generation of enormous second harmonic component in the transformer current. d. D. C. Magnetization: Under magnetic imbalance, the shape of the magnetizing characteristics and the excitation currents are different from those under no load conditions. When the flux is unbalanced, the core contains an average value of flux if 20

10 Literature Survey on Bectric Power Quality Chapter 2 (<f>dc), which is equivalent to a direct component of excitation current of the transformer. Under such unbalance conditions, the transformer excitation current contains both odd and even harmonic components Rotating Machine Harmonics Rotating machines are considerable source of harmonics in power system. Harmonics produced in rotating electrical machines are classified into following categories: 1. M. m. f. Distortion of A. C. Windings 2. Slot Harmonics 3. Voltage Harmonics Produced by Synchronous Machine 4. Rotor Saliency Effects 5. Voltage Harmonics Produced by Induction Motors Distortion Caused by Arcing Devices Arcing devices are very important source of power system harmonics. The voltage versus current characteristics of an electric arc in an arcing device are highly non linear. Arc ignition is equivalent to a short circuit current with decrease in voltage. The voltage-current is controlled by the power system impedance. In respect of harmonic generation, arcing devices are divided into three main categories: 1. electric arc furnace 2. discharge type lighting 3. arc welders Power Supplies with Semiconductor Devices Semiconductor based power supply systems are the main sources of harmonics. Harmonics generated in power supply include integer harmonics, inter harmonics and sub harmonics. 255 Thyristor Controlled Reactors a. V4R Compensator VAR compensators used in power system network are also source of harmonics. SVC circuits are sources of harmonics in power system. Normally, SVC produces odd harmonics. Under perfectly symmetrical voltage conditions, triplen harmonics are kqpt out of the line by delta connection. h. Modulated Phase Controller: Modulated phase control method is used in cyclo- Qonverter. It performs static power conversion from one frequency to another frequency. Most of the cyclo-converter waveforms contain frequencies which are not integer multiples of the main output frequency.

11 Electric Power Quality in Power System, Ph B. (Tech) Thesis, Department of Applied Physics, C U Effects of Harmonic Distortion In electrical power system, harmonics are not desirable in most of the applications and operations. Harmonics have adverse effect on power system equipments as well as in operation. Effects of harmonics are classified in the following way: 1. Resonance 2. Poor Damping 3. Effects of Harmonics on Rotating Machines 4. Effects of Harmonics on Transformer 5. Effects of Harmonics on Transmission Lines 6. Effects of Harmonics on Measuring Instruments 7. Harmonic Interference with Power System Protection 8. Effects of Harmonics on Capacitor Banks 9. Effects of Harmonics on Consumer Equipment Resonance Capacitors used for pow'er factor correction cause system resonances due to harmonic frequencies. This results in excessive high current, which can produce damage to the capacitors. 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 prohlems in operation of power factor correction capacitors. There are three types of resonance occurred in power system a. Parallel Resonance, which offers high impedance at the resonance frequency and increases harmonic voltages and harmonic currents b. Series Resonance, which results in high capacitor current at relatively small harmonic voltages. Magnitude of this current depends upon the quality factor of the circuit c. Complementary and Composite Resonance: Normally, composite resonant frequency is of non-integer type frequencies generated from conversion from the fundamental frequency and d. c. components of the converter control circuit Poor Damping In presence of harmonics, variable speed drive motors or a switched mode power supply introduces small negative impedance or resistance. It deceases current with the rise, of voltage, which reduces the damping or broadband energy absorption capability of the system. 22

12 Literature Survey on Electric Power Qualify Chapter Effects of Harmonics on Rotating Machines a. Harmonic Losses: Harmonic voltages or currents increase losses in the stator windings, rotor circuit, and stator and rotor lamination. Normally the losses in the stator and rotor conductors are greater than those associate with the D. C. resistances due to the presence of eddy current and skin effect in ac circuit. This results in overheating reducing the efficiency of a motor. b. Harmonic Torque: Harmonic currents present in the stator of an a. c. 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 harmonic will develop shaft torques aiding shaft rotation whereas negative sequence harmonics will have the opposite effect. c. Other Effects: The presence of harmonics has effect on speed/ torque characteristic and causes cogging. Cogging is the failure of an induction motor to run up to normal speed due to a stable operating point occurring at a lower frequency. The presence of harmonics increases capacitive current through the stray capacitance in ASD-fed electric motors which is a source of their failure Effects of Harmonics on Transformers Harmonics has effect on transformer in various ways, e.g.: 1. Core Loss: Harmonic voltage increases the hysteresis and eddy current losses in the laminations. The amount of the core loss depends on harmonic present in supply voltage design parameter of core materials and magnetic circuit. a 2. Copper loss: Harmonic current increases copper loss. The loss (^/,^ ), mainly depend on the harmonics present in the load and effective ac resistance of the winding. Copper loss increase temperature and create hot spots in that transformer. The effect is prominent in the case of converter transformers these transformers do not benefit from the presence of filters as filter are normally connected on the a.c. system side. 3. Stress: Harmonic voltages increase stresses of the insulation, 4. Core vibration: harmonic current and voltage increase small core vibrations. 5. Saturation problem: Sometimes additional harmonic voltage causes core saturation. Guideline for transformer derating in respect of the harmonic current are given in the ANSI/IEEE standard C n=2 ho CO

13 Electric Power Quality in Power System, Ph D. (Tech) Thesis, Department of Applied Physics, C U., Effects of Harmonics on Transmission System Harmonics has effect on transmission line n various ways. a. Skin effect and Proximity effect: these effects depend on frequency. Harmonics increase these effects. As a result effective ac resistance increases in presence of harmonics b. Loss: Additional harmonic current increases copper loss of transmission system and a reduce power transmitting capacity. Copper loss is given by ^ I*Rn, where In is the nth harmonic current and Rn is tie system resistance at that harmonic frequency. c. Voltage drop: Harmonic current produces harmonic voltage drops across various circuit impedances. As a result a weak system of large impedance has low fault level and greater voltage disturbances where as a stiff system of low impedance has high fault level and lower voltage disturbance. d Dielectric stress: Harmonic voltage increases dielectric stress of cables used in transmission line. The dielectric stress is proportional to their crest voltages. This reduces dielectric strength. This results in shortening of the useful life of the cable and probability of the number of faults and hence, the cost of repairs. e. Corona: Corona starting and extinction levels depend on peak to peak voltage. The peak voltage depends on die phase relationship between the harmonics and the fundamental. Harmonic voltage increases peak to peak voltage. It may happen that the peak voltage is above the rating while the r.m.s. voltage is well within this limit. In this matter the IEEE 519 standard [190] provides typical capacity derating curves for cables feeding six pulse converters. n= Effects of Harmonics on Measuring Instruments Harmonics has effect on measuring instruments [207] - [215] in various ways a. Error: Measuring instruments are calibrated on purely sinusoidal alternating current but they are used on a distorted electricity supply. This introduces error in measurement. b. Sign of error: Sign of error depends on the magnitude and the direction of the harmonic power. c. Harmonic torque. Torque produced by harmonics greatly affects operation of instruments. d Any d.c. power supplied to or generated by the customer will cause an error proportional to the harmonic-fundamental power ratio, with the error sign related to the direction of power flow.

14 Literature Survey on Electric Power Qualify Chapter 2 e. Harmonic voltages or currents not only produce torques, but also degrade the capability of a meter to measure fundamental frequency power. f. The kilowatt-hour meter, based on the Ferraris (eddy current) motor principle, show generally high readings to the extent of up to several percentage points with a consumer generating harmonics through thyristor-controlled variable speed equipment particularly where even harmonics and d.c. are involved. By this way, consumers that generate harmonics are automatically penalized by higher apparent electricity consumption. This may well offset the supply authority s additional losses. It is therefore in the consumer s own interest to reduce harmonic generation to the greatest possible extent. g. There is no proof or evidence that the reading of kva demand meters is affected by network harmonics. h KW demand meters operating on the time interval Ferraris motor principle show high reading in presence of harmonics. i. Harmonics create problems in measurement of VAR values in power networks as VAR is a quantity defined with respect to sinusoidal waveforms. j. Absolute average and peak responding meters which are calibrated in r.m.s. are net suitable in the presence of harmonic distortion Harmonic Interference with Power System Protection a. Harmonics degrade the operating characteristics of protective relays. Effect cf harmonics on relay depends on the design features and principles of operation. b. Some digital relays and algorithms operate on sample data and zero crossing moment. Harmonic distortion creates error on such operation. c. 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 d Current harmonic distortion affects the interruption capability of circuit breakers and fuses. e. The fuses are thermally activated and inherently r. m. s. over current devices. Fuse materials are also susceptible to extra skin effect of the harmonic frequencies. However, the changes in operating characteristics are small and do not present a problem. Most studies say that it is difficult to predict relay performance without testing. A lot of studies have been published on electro-mechanical and electronic relays. But there is little information on digital relay. Presence of harmonic current, particularly third harmonic, in a fault situation, results in considerable measurement errors relative to the fundamental based setting. Harmonic content increases the possibility of mal-operation, unless only the fundamental waveforms are captured

15 Electric Power Qualify in Power System, Pk D. (Tccki Thesis, Department of Applied Physics, C U., Effects of Harmonics on Capacitor Banks a. Dielectric loss: Harmonic voltage increases the dielectric loss in capacitors. b. Thermal Stress: Harmonic voltage increases the stress on capacitor bank and the system connected to the bank. d. Resonance: Harmonics cause series amd parallel resonances between the capacitors and the rest of the system. This results in over voltage and high currents, which increase the losses and overheating of capacitors, and often leading to their destruction. e. Reactive power: Change of harmonic contents sometimes increases reactive power over permissible manufacturer tolerances. f. Power factor correction capacitors: Presence of harmonics causes malfunction of the operation of power factor correction capacitors Effects of Harmonics on Consumer Equipment IEEE Task Force on the Effects of Harmonics on Equipment [33] has made a wide study on this matter. The result can be summarized as 1. Television Receivers: Harmonics changes in TV picture size and brightness. Interharmonics changes 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. 2. Fluorescent and mercury arc lighting: capacitors used in such light applications together with the inductance of the balast and circuit produce a resonant frequency. It results in excessive heating and faiure in operation. Audible noise is produced due to harmonic voltage distortioa 3. Computers [34]: harmonics create problems in monitor and c.p.u. 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 ± Summery of Effects of Harmonics Effects of harmonics in power system [5], [7], [12], [25], [31], [32], [50], [58], [117], [121], [123], [131], [165], [179], [180], [181], [184], [190], [191], [192], [194], [196], [197], [200], [203], [204], [206] - [215] can as summarized as given in Table

16 Literature Survey on Electric Power Quality Chapter 2 Name of component Generator Motor Transformer Relaying Switchgear Capacitor Cables Consumer equipment Communication circuits Table 2.2 Effects of harmonics on different electrical components Effects of Harmonics Production of pulsating or oscillating torques which involve torsional oscillations of rotor elements of TG set and rotor heating Stator and rotor copper losses increase due to harmonic current flow', leakage flux created by harmonic currents causes additional stator and rotor losses, core loss increases due to harmonic voltages and positive sequence harmonics develop shaft torques that aid shaft rotations whereas negative sequence opposes it. Stray losses increase due tom harmonic current flow, hysteresis losses increase, due to presence of high frequency harmonics resonance may occur between winding inductance and line capacitance. Maltripping may occur due to presence of harmonics which affects the time delay characteristics Due to predominance of skin and proximity effects at higher frequencies, busbars behave like cables and transient recovery voltage changes which affects the operation of blow-out coils. Due to presence of harmonics, reactive power increases, dielectric losses increase causing additional heating and resonance and overvoltage may occur, resulting in reduced life. Due to increased skin and proximity effects at higher frequencies, additional heating occurs, Rao increases and ac copper loss increases. Life and efficiency reduce drastically Noise creeps in transmitted signals 2.7 Definitions and Standards Different standards have been prepared from time to time by IEEE, ANSI, CBEMA [113], [141], [148], [158], [170], [186], [204] which provide guidelines for power quality usages and practices. Harmonic standards mainly include the following issues [9], [13], [38], [41], [47], [68], [113], [139], [154], [187]-

17 Electric Power Quality in Power System, Ph. D. (Tech) Thesis, Department of Applied Physics, C.U., 2009 (1) description and characterization of the phenomenon (2) major sources of harmonic problems (3) impact on other equipment and on the power system (4) indices and statistical analysis to provide a quantitative assessment and its significance (5) measurement techniques and guidelines (6) emission limits of quality degradation for different types and classes of equipment (7) immunity or tolerance level of different types of equipment (8) testing methods and procedures for compliance with limits (9) mitigation guidelines The IEC Standard [170] Geneva based International Electro-technical Commission or Commission Electro-technique Internationale (IEC) is the widely recognized organization as the curator of electric power quality standards. It has defined a series of standards, called Electro-Magnetic Compatibility (EMC) standards, to deal with power quality issues. The IEC series includes harmonics and inter-harmonics as one of the conducted low-frequency electro-magnetic phenomena. This series includes a concise description of the documents of the IEC. series. It provides internationally accepted information for the control of power system harmonic (and inter-harmonic) distortion. IEC : It provides the 'ationale for limiting power frequency conducted harmonic and inter-harmonic current emissions from equipment in the frequency range up to 9 khz. IEC : It outlines the major sources of harmonics in three categories of equipment, power system equipment, industrial loads and residual loads. The increasing use of the HVDC converters and FACTS devices has become the main source of harmonic distortion originating in the transmission system. Static power converters and electric arc furnaces are the main contributors in the industrial category, and appliances powered by rectifiers with smoothing capacitors (mostly PCs and TV receivers) the main distorting components in the residential category. IEC : it contains a section on the compatibility levels of the harmonic and inter-harmonic voltage distortion in public low-voltage power industry systems. IEC : It provides harmonic and inter-harmonic compatibility levels for industrial plant. It also describes the main effects of inter-harmonics, a subject discussed in chapter 3 28

18 Literature Survey on Electric Power Quality Chapter 2 IEC : Similarly to , this document deals with compatibility levels for low-frequency conducted disturbances, in this case relating to medium voltage power supply systems. It also covers the subject of injected signals such as those used in ripple control. IEC and 3-4: These contain limits for harmonic current emissions by equipment with input currents of 16A and below per phase. It also specifies the measurement circuit, supply source and testing conditions as well as the requirements for the instrumentation. IEC : First, indicates the capability levels for harmonic voltages in low and medium voltage networks as well as planning levels for MV, HV and EHV power systems. It then makes assessment of emission limits for distorting loads in MV and HV power systems. IEC : it provides limits for the harmonic currents produced by equipment connected to low voltage systems with input currents equal to and below 75 A per phase and subject to restricted connection. IEC : This is perhaps the most important document of the series, covering the subject of testing and measurement techniques. It is a general guide on harmonic and inter-harmonic measurements and instrumentation for pow'er systems and equipment connected thereto. IEC : this is also a document on testing and measurement techniques with reference to harmonics and inter-harmonics, including mains signaling at a. c. power ports as well as low-frequency immunity tests IEEE [158] IEEE document is a widespread alternative to the IEC series. It identifies the major sources of harmonic in power systems. The harmonic sources described in this standard include power converters, arc furnaces, static VAR compensators, inverters of dispersed generation, electric phase control of power, cyclo-converters, switch mode power supplies and pulse width modulated (PWM) drives. Some typical distorted wave shapes with the harmonic order and level of each harmonic component in the distortion caused by these devices have been illustrated in this section. It also describes the system response in presence of harmonics. It comprises parallel resonance, series resonance and the effect of system loading on the magnitude of these resonances. This document discusses the general response of these systems to harmonic distortion based on typical characteristics of lowvoltage distribution systems, industrial systems and transmission systems. The standard includes the effects of harmonic distortion on the operation of various devices or loads which comprise motors and generators, transformers, power cables, 29 J V

19 Electric Power Quality in Power System, Ph. D. (Te:li) Thesis, Department of Applied Physics, C. IJ., 2009 capacitors, electronic equipment, metering equipment, switchgear, relays and static power converters. It deals with the interference to the telephone networks as a result of harmonic distortion in the power systems with reference to the C-message weighting system created jointly by Bell Telephone Systems and Edison Electric Institute. The standard recommends several possible methods of reducing the amount of telephone interference caused by harmonic distortion in the power system. Some analysis methods and measurement requirements foi assessing the levels of harmonic distortion in the power system have been discussed in the standard It then illu strates the methods for the calculation of larmonic currents, system frequency responses and modeling of various power system components for the analysis of harmonic propagation. The section on measurements includes currently available harmonic monitoring techniques with the accuracy and selectivity ( the ability to distinguish one harmonic component from others) requiremaits on these monitors. The standard highlights the averaging or snap-shot techniques that can be used to smooth-out the rapidly fluctuating harmonic components and thus reduce the overall data bandwidth and storage requirements. Different design aspects have been discussed in the standard. It includes methods for designing reactive compensation for systems with harmonic distortion, various types of reactive compensation schemes, indicating that some of the equipment, such as TCR and TSC, are themselves sources of harmonic distortion. It also highlights the various techniques for reducing the amount of harmonic current penetrating into the a. c. systems. Some recommendation and useful suggestion for both individual consumers and utilities for controlling the harmonic distortion to tolerable levels have been made in this section. This standard ends with recommendations for evaluating new harmonic sources by measurements and detailed modeling simulation studies, providing several examples to illustrate how these recommendations can be implemented effectively in practical systems. The standards also include notching, the distortion caused on the line voltage waveform by the commutation process between valves in some power electronic devices, along with analyses the converter commutation phenomenon and describes the notch depth and duration with respect to the system impedance and load current Limits are expressed m terms of the notch depth, THD of supply voltage and notch area for different supply systems. 2.8 General Harmonic Indices Different useful definitions can be summarized as given in Table

20 Literature Survey on Bectric Power Qualify Chapter2 Table 2.3 Different power quality parameters and their definitions EPQ Parameter Total Voltage Harmonic Distortion (THDV) Total Current Harmonic Distortion (THDi) Definition P. >Vx / >lx Active Power Distortion Factor PDF= jlx 100% Px Reactive Power Distortion Factor QDF=Otx 100 ^ Apparent Power Distortion Factor A DF=^-x 100% A Flicker factor AVIV Crest factor PQ Index V peak 1V RUS ] 2*J ioo% ^(0-100% J 2.9 Review on Harmonic Assessment Different techniques for measurement and monitoring of harmonic related power quality parameters have been developed. It may be categorized as a. Fourier Transform Based Assessment: One of old techniques used in analysis of non -sinusoidal signals is Fourier transform [55], [216]. Fourier analysis has been used for power quality assessment [201] for a long period. It permits mapping of signals from time domain to frequency domain by decomposing the signals into several frequency components. Application of Discrete FT and Fast FT are very useful to overcome some of the disadvantages of the earlier one. b. Wavelet Transform Based Assessment: Fourier transform foils in the analysis of transients owing to the non-stationary property of its signals in both time and frequency domains. Wavelet transform (WT) [56], [57] helps us in such cases. Wavelet analysis has been suggested as a new tool for measurement and monitoring power quality problems [38], [39], [40], [42], [49], [68], [73], [89], [90], [96], [122], [125], [140] both in absence and presence of transients. Multi-resolution signal decomposition [63], [114] has been used to localize different power quality problems and assess them. 31

21 Electric Power Quality in Power System, Pk D. (Tech' Thesis, Department of Applied Physics, C U., 2009 c. Neuro-Fuzzy Based Assessment: An ANN-fuzzy logic combined system for classifying power system disturbances las been introduced to identify the event based quality issues [174], Fuzzy-Based Adaptive Digital Metering system and Genetic Algorithm [106], [108] have been imroduced to avoid effects of power quality problems. Different measurement and mcnitoring techniques have been discussed in [14], [15], [17], [18], [50], [60], [76], [81], [126], [132], [149], [167], [172], [178], [179], 2.10 Unbalance Unbalance in power system is defined as deviation in magnitude of voltage / current of any one or two of the three phases. Unbalance may generate from source, load, improper grounding, etc. Unbalance in power system s associated with stability problem, imbalance causes excessive drawl of reactive power, maloperation of equipment, maloperation of measuring instruments and shortening of life span of different appliances. Some useful definitions of electric quantities related to imbalance have been newly established in IEEE standard [21], [58], Unbalance in power system has been characterized with the help of symmetrical components [44], [70], [82], Line voltage drop due to unbalance has been formulated for some events in power system [45], A lot of research work has been done to assess the unbalance [10], [36], [180], Some ofthem have not measured phase angles [10], True imbalance factor UF is defined as V+ and V. represent the root mean square (RMS) voltages of the positive and negative sequence components, respectively. Some new definition regarding unbalance related problems have been introduced in [21], [35], Voltage unbalance has been calculated in presence of harmonic and inter-harmonic components using Park approach [8]. Voltage sag originated from unbalance has been characterized and measured in [44], [45]. IEEE Standard [58] aims to provide structure in the definitions and concepts. The definition of apparent power and of power factor has been discussed for sinusoidal three phase unbalanced and for non sinusoidal periodic situations. Two possible concepts, the one relating to the literature on the single phase sinusoidal case, and the other relating to the new IEEE Standard, are rigorously analyzed and the relationships as well as the differences between them are thoroughly discussed [9], Method for calculating the positive and negative sequence components cf a three phase, sinusoidal and unbalanced

22 Literature Survey on Electric Power Quality Chapter 2 voltage system without the application of Fortescue transformations in the complex plane, while the root mean square (RMS) line-to-line voltages are required, and without having to measure the phase relationships between these voltages has been discussed in [10], It leads to assess the voltage unbalance. Discrete or event type power quality (PQ) disturbances mainly include voltage sags, swells and the transients which are directly contributing unbalance. An extensive literature survey has been done which suggests that there is no generally accepted method for characterization of these disturbances and suitable limits are not yet found in any international standard One of the reasons for the lack of characterization methods is the difficulty of defining suitable site indices for each discrete disturbance type. The existing characterization method has been reviewed and discussed in [4], which suggests a new generalized approach is then given to show a better way of characterizing voltage sags, swells and transients Voltage tolerance curves, also known as power acceptability curves are plots of equipment maximum acceptable voltage deviation versus time duration for acceptable operation. Power acceptability curve defined by Computer Business Equipment Manufacturing Association (CBEMA) and issues of three-phase and rotating loads are discussed in [24], 2.11 Modeling of a Polyphase System Different models have also been developed for analysis of polyphase power system e.g. simplified d-q transformation followed by semiconductor modeling [161]. Use of TLM (Transmission Line Modeling) Technique [115] has increased. Some new analytical tools were proposed by Sudhoff [160] Performance of poly phase power system under unknown disturbance was studied in [211]. Contingency-based zonal reserve modeling [1] has been done for a co-optimized energy and reserve market. A new dynamic model considering effects of temperature, pressure and internal resistance has been proposed [2] for PEM fuel cell power modules. Dynamic Modeling has been done [6] for combined PEM fuel cell and ultra-capacitor system for stand-alone residential applications. Dynamic modeling has been done for drive application in a polyphase power system [3] and [4]. Passivity based method for induction motor control has been proposed in [79], [93], [95], [210]. The term passivity based control (PBC) was introduced to define a controller design methodology, the idea has been successful to control physical system in particular these described by Euler-Lagrange equation of motion which is detailed in [20]. Dynamic phasor method was introduced in [23], [66], [67]. Besides these some other techniques were also introduced in [37], [65] But none of these approaches included the effects of harmonics in modeling the power system and hence performance analysis does not include harmonic effect. In chapter 11, a passivity-

23 Electric Power Quality in Power System, Pit. D. (Tech) Thesis, Department of Applied Physics, C.U., 2009 based model of a poly phase power system in presence of harmonics has been developed. Limitation of this model has been overcome by introducing an activity-based model which caries generation of harmonics. A cased study on practical induction has been done based on this model Objective of the Thesis Literature survey shows that a lot of research works have already been done and are going on to assess electric power quality indices in respect of harmonics and unbalance. In this thesis, attempts have been made to presents some novel techniques for the assessment of the same having specific advantages. Consequently, modeling of a polyphase power system has been done in presence of harmonics followed by its implementation in power system components. Those assessment and modeling have been presented and discussed in the following chapters. 34