Chapter 1. Introduction

Transcription

1 Chapter 1. Introduction The economical and ecological conversion of energy becomes one of the most acute problems of the present day. Power electronics - due to its capability to efficient energy processing plays a key role in solving this issue. Power electronic converters transform the nature and/or parameters of electrical energy using power electronic devices (tranzistos, tyristors, IGBTs) working in switch-mode, i.e., they change cyclical their blocked-state into conduction-state and vice versa, switching the energy circulated between supply and load. Due to this pulsed energy transfer, the voltages and currents become non-sinusoidal, characterized by presence of harmonics. The high switching speed of electric and magnetic quantities develops a large, conducted and radiated harmonic spectrum, which may affect the quality of power and perturb the operation of others electronic or electric devices. The use of power electronic converters is increasing, therefore, in order to avoid the increasing of pollution level, it is necessary to regulate and limit the generated perturbation under acceptable levels. This work contains the results of a research performed by author regarding to take obvious the presence of low- and medium-frequency perturbations created by electronic converters on high-, mediumand low-voltage-level grid, on point of connection of different type consumers, to find solutions for analysis of perturbations and their mitigation. The field of author s interest is a multidisciplinary one. The treated problems are at confluence of Power Electronics, Power Quality, Electromagnetic Compatibility, Power Systems, Computer Techniques, Signal Electronics. A short presentation of chapters closes this introduction. The thesis is structured in seven Chapters and two Appendix. Chapter 2. Electronic Energy Converters This chapter presents a review of the electrical energy conversion with the help of power electronic converters. Converters classification criteria are defined. Phenomena related to commutation process in converters are analyzed. DC to Dc converters (choppers) are discussed. Operation principle of step-down ( buck ) converter working in quadrant I IV and step-up ( boost ) converter in quadrant I are analyzed in continuous and discontinuous conduction mode. In order to plot external- and command-characteristics Matlab-Simulink structures were created. In the second part of this Chapter an implemented step-up converter is presented, which is integrated into a laboratory test rig represented in figure The test rig is composed by an uncontrolled rectifier feeding the stepup converter, a power-factor compensation converter (described in Chapter 5), the load of the step-up converter may be an active load (DC motor) or a passive (resistive) one. The control keys of the rig permit easy set up of different working configurations: the step-up converter operating alone, Fig Photo of laboratory test rig. step-up converter working with powerfactor correction etc. Chapter 3. Power Quality in Energy Conversion Chapter 3 deals with power quality parameters affected by power converters. There are reviewed the distortion factor, total harmonic distortion and the power-factor given by product of distortion factor and Displacement Power Factor. Furhermore, national and international standards and rules are discussed which regulate the perturbation of power line generated by consumers containing electronic converters. There are 1

2 mentioned conditions imposed by electromagnetic compatibility for voltage at consumer and limits of current harmonics. Chapter 4. Evidencing the Perturbations Chapter 4 contains results obtained by author in his research to furnish evidence of perturbations presence on high-, medium- and low-voltage-level power grid and on point of connection of different type of consumers. A synthesis is offered about measuring instruments used in study of power quality and perturbations generated by power electronic converters. There is given a description of a current- and voltage-transducerunit realized by the author (Fig. 4.2), followed by a short presentation of high performance industrial instruments and data loggers used by author in measurements. Were examined seven industrial data logger/data acquisition units: TRINET data logger, Handyscope HS3 data acquisition unit, Lab-PC-12/AI data acquisition card, MEMOBOX 3 Smart data logger, ABB ALPHA PowerPlus power-meter, NEXUS1252 data logger, TDS22 storage-scope. It is presented an implemented SQL application for harmonic analysis of captured data. The Application operates on data represented in CSV format, saved as independent files by the data logger unit. The program computes for measured current and voltage the power, the Fourier harmonics (frequency, amplitude and phase), sum of harmonics. According to the user settings the computation is performed only for selected harmonics or for all harmonics, up to the 3th order. Testing of the Application was made using data captured by Handyscope HS3 data acquisition unit connected to an uncontrolled rectifier unit. Fig shows the computed odd harmonics. The computed data is saved also in CSV format as files, and may be further processed by other software. Fig Transducer unit for current and voltage measurement (removed cover). Fig Presentation of odd harmonics contained by the input current of a rectifier. There are presented and discussed results of measurements carried out in different high-, medium- and low-voltage power stations located in Sibiu County. Long-term measurements were executed using ABB Alpha Power Plus intelligent power-meter installed in 18 medium- and low-voltage power stations located in Sibiu County: Sibiu Sud, Micăsasa TR CFR, Sadu V - Dumbrava, Sadu V - Lotru, CHE Avrig, Cârţişoara, Orlat, Mârşa, Compa Sibiu, Orlat - Gura Râului, and energy transport lines: Mediaş Târnăveni, Sibiu Nord - Copşa Mică, Micăsasa Tăuni, Micăsasa Târnăveni. The captured data was transferred on telephone-lines, wireless and GSM, and processed using Galaxy software. As example fig presents the recorded data in Sibiu Sud AT1 2 kv station. Representation of measured data may be done in different forms: line, bar-graph, columns, etc. Short-term measurements were performed in three power-stations in Sibiu County and also were investigated a low power rectifier unit 5 V / 7 A. The used equipment was the TRINET data logger, the captured data was processed by TRIFAZ 4. software. Fig shows the screen-capture of measurement executed in 11kV power-station CFR Micăsasa. Very short-term measurements were performed in order to capture the perturbations caused by isolated, non-repetitive events on the power lines, using NEXUS 1252 data logger installed in different power stations located in city Sibiu. Figure 4.7 presents a distorted voltage wave-form, and the corresponding harmonic spectrum. The measurement shows that also the transient phenomena contribute to the harmonic pollution. 2

3 THDu AT1 Sibiu Sud Fig a) THDu on phase T frequent is above imposed 3% limit : : : : : : :4 7 2 :2 3 Distorsiuni armonice pe tensiune t Distorsiuni armoice pe curent r THDu Distorsiuni armonice pe tensiune r THDi 5 THDi AT1 Sibiu Sud Distorsiuni armoice pe curent t Fig b) THDi on phase T meets the imposed limit. Fig Results of measurement performed in 2 kv power station from Sibiu Sud. Fig Results of measurement performed in power station CFR Micăsasa. Fig Measurement of transient phenomena. Left: perturbed voltage wave-form; Right: Harmonic spectrum of the perturbed wave. The last part of Chapter 4 presents the perturbation measurements performed on different consumers containing power electronic converters, working in low-voltage, industrial-, office- and housekeeping environment. The Handyscope HS3 data acquisition unit, MEMOBOX 3 Smart data logger and the TDS22 storage-scope was used in investigations. Were measured light sources (incandescent and economic bulbs, in different combinations), uncontrolled rectifier, half-controlled rectifier, power-supply equipped with power-factor compensation; step-up DC to DC converter. The figure 4.96 presents the log generated by Handyscope HS3 data acquisition unit, corresponding to the investigation of a 15 W economic bulb. The absorbed current is very disturbed, presents large harmonic content. 3

4 Fig b) Results of measurements on a 15W economic light source. Left top: Waveforms of absorbed current and main voltage; Left down: Harmonic spectrum; Current (blue), Voltage (brown); Right: Tabular presentation of first 1 harmonics of the main voltage and absorbed current. Chapter 5. Procedures for Mitigation of Perturbations Fig Photograph of realized power-factor compensator converter using the peak current control method. T e n s iu n e a U s s i c u r e n t u l Is a s u r s e i ( x 1 ) i n r e g im s t a t i o n a r 3 a lfa = 3 g r Us U s [ V ], Is x 1 [ A ] Is T im e [s ] T e n s iu n e a s u r s e i U s s i t e n s iu n e a p e s a r c in a U d 3 2 U s [ V ], U d [V ] Chapter 5 deals with the methods and solutions for mitigation or elimination of perturbations generated by electronic power converters fed from single-phase line. There are discussed active and passive procedures. There is presented and simulated using Matlab-Simulink environment, a native unity-power-factor single-phase converter in H topology, using symmetrical control of power switches as against of supply voltage-peak. Fig. 5.8 presents the input current, main voltage and output voltage wave-forms for a command angle of 3. In second part of this Chapter there are described two implemented converters for power-factor compensation. One of them uses the peak current control method (fig ) the other one, the borderline (discontinuous peak current) method (fig ). The first one was integrated in the laboratory test rig presented in fig It feeds the step-up converter described in Chapter 2. Fig 5.2 shows the oscilograms of the absorbed current without and with power-factor compensator. 1 Ud -1-2 Us -3 a lf a = 3 g r T im p [ s ] Fig Top: Supply voltage (blue) and absorbed current (green, scale x1); Down: Supply voltage (blue) and output voltage (green). Fig Oscilograms of the current absorbed by the step-up converter, without (top) and with (down) power-factor compensation. For testing, the second power-factor compensation converter fed a switch-mode power supply. 4

5 The grid voltage and the absorbed current were captured using Lab-PC-12/AI data acquisition card and the captured data were processed under LabView environment. Fig shows the virtual instrument displaying the voltage, current and computed power wave-forms. Also the computed harmonics are shown. Fig Photograph of realized power factor compensator converter using the borderline (discontinuous peak current) control method. Fig Waveform of main voltage (pink) and the current (brown) absorbed by loaded switching-mode power supply fed by power-factor compensator converter. Chapter 6. Conclusions The thesis contains the theoretical and practical results of a research performed by author in order to take obvious the presence of low- and medium-frequency perturbations created by electronic converters on high-, medium- and low-voltage-level grid, on point of connection of different type consumers, and to find solutions for analysis of perturbations and to mitigate or eliminate them. The subject of thesis is of high complexity, a multidisciplinary one; it is situated at the confluence of Power Electronics, Power Quality, Electromagnetic Compatibility, Power Systems, Computer Techniques, Signal Electronics, Signal Processing, Modeling and Simulation, Ecology. This subject is an actual one; the current use of converters and the demand for a clean electric power require this study. In the same time, the subject is an opportune one; the recent advances of related scientific disciplines offer favorable circumstance to a successful solution of the problem. The objective is fulfiled in following steps: -evidencing on theoretical basis the perturbing character of electronic energy converters; - study of instrumentation used in perturbation measurements; - taking obvious experimentally the presence of perturbations on high-, medium- and low-voltage-level grid; - evidencing experimentally the presence of perturbations generated by different products containing electronic converters; - theoretical and practical study of the possibilities for elimination or mitigation of the perturbation. High frequency perturbations are not studied in this work. After the introduction contained in Chapter 1, the Chapter 2 presents a review about the electrical energy conversion with the help of power electronic converters, concluding with followings: - the most significant classification criteria for converters are the nature of input and output energy and the nature of commutation process; - the control law of the forced commutation step-down and step-up DC to DC converters in continuous conduction is a linear one, in discontinuous mode is non-linear. In order to compute the control characteristics of step-up converter a Matlab Simulink structure was conceived, realized and used. - measurements on a step-up converter conceived, realized and integrated by author into a laboratory test rig take in evidence the perturbations caused by rectifier - step-up converter group. Chapter 3 deals with problems related to standards and regulations for power quality. Are reviewed the power quality parameters affected by power converters, concluding with: - the most important parameter in characterization of energy conversion in electronic converters is the power factor. It is given by product of distortion factor (reflecting the harmonic content) and displacement factor (referring to the fundamental component and reflecting the effect of the load); 5

6 - the limit of harmonics generated by converters are defined by standards; - national standards are harmonized to European standards At the beginning of the Chapter 4, is presented a review of the requirements imposed by perturbations measurement and the demanded features for the measuring instruments used in study of power quality and perturbations generated by power electronic converters. There is given a description of a current- and voltage-transducer-unit realized by the author, followed by a presentation of high performance industrial instruments and data loggers used by author in measurements. The related conclusions are: - the realized transducer unit is an useful interface to data acquisition units. In conjunction with Handyscope HS3 data acquisition unit, Lab-PC-12/AI data acquisition card, TDS22 storage-scope it was used successful in laboratory investigations; - the analyzed TRINET data logger, MEMOBOX 3 Smart data logger, ABB ALPHA PowerPlus power-meter, NEXUS1252 data logger are highly recommended for field-measurements to accomplish longterm monitoring and analyze of power quality; - the NEXUS1252 data logger permits easy recording and analyze of transient phenomena in power systems; - there is presented in tabular form a comparison of seven instruments used by the author. In the next part of this Chapter is presented an implemented SQL Application for harmonic analysis of captured data represented in CSV format. Analysis may be performed for selected harmonics or for first 3 harmonics. The output of analysis is saved also in CSV format. The Application was tested using data captured by Handyscope HS3 data acquisition unit. In continuation there are presented and discussed results of measurements carried out in different medium- and low-voltage power stations located in Sibiu County. Long-term, short term measurements and transient data recording were performed. The control of instruments and transmission of the captured data were performed via telephone line, wireless and GSM. Observations on measurements are the followings: regarding to the presence of non-sinusoidal operation: - the THDu on phase T frequently, phase S sometime exceeds the 3% limit; - the THDi is acceptable, usual 7 12%, under the prescribed limit; - measurements on direction Mediaş Târnăveni show sporadic increased THDi values up to 6% on both monitored phase, however the computed average value for a week is under 2%; - the THDi on direction Micăsasa Tăuni frequently exceed 2%; - measurements on direction Micăsasa Târnăveni show permanent increased THDi, exceeding 2%; - measurements performed in power station on Micăsasa TR CFR (supplying the rail-way) show increased THDi exceeding 2%; it was observed that the peak value of THDi is correlated to passing of the locomotive through the connection point; - the identification of the source and reason of perturbations may be performed only by supplementary field measurements, based on analysis of the local topology of distribution network and the structure of the consumers; - very short-term measurements demonstrate the contribution of transient phenomena to the pollution of the grid. Accordingly, by the frequently switched consumers one has to take into account their effect in degrading of power quality; regarding to the used instrumentation: - for the above presented measurements were used: ABB PowerPlus power-meter, TRINET, and NEXUS1252 data loggers. All of them worked according to their specifications; - it is to note the versatility of the ABB ALPHA PowerPlus power-meter, with its advanced communication possibilities (optical, RS485, ISDN, GSM) and related large software support; - the software related to instruments permits various graphical presentation (line, bar-graph, columns, etc.) of captured data and their analysis; - the captured data may be saved as independent files. This makes possible the use also of other software for data evaluation; The last part of Chapter 4 presents perturbation measurements performed on different consumers containing power electronic converters, working in low-voltage, industrial-, office- and housekeeping environment. The Handyscope HS3 data acquisition unit, MEMOBOX 3 Smart data logger and the TDS22 storage-scope was used in investigations. The conclusions are the followings: regarding to the absorbed average current: 6

7 - different behavior of consumer is observed: in office and home environment there are consumers (i.e. a laptop, computer network, etc.) absorbing a relatively constant average value with light deviation around this, others (i.e. microwave oven) absorb pulsed current, varying between zero and a maximal value. The investigated industrial consumer presents large and frequent current variation, depending on technological process. To estimate or predict the created perturbations is necessary to know the nature and operation of consumers. regarding to the THDi: - the investigated laptop behave perturbing, it presents high THDi value, about 125%; the microwave oven is also perturbing, its THDi fluctuate between 5 95%. In case of computer network different pollution were observed on different phases (2%...8%). The average harmonic pollution created by the computer network is favorable in comparison with pollution of the single computer due to statistical compensation of created perturbations. However the office- and household-equipments are relatively small power, due to their increasing number care must taken regarding the introduced perturbations; - the THDi of the industrial consumer was %, depending on technological process. regarding to the significant current harmonics: - the most significant harmonic of examined office- and household-equipments is the third harmonic, (highest third harmonic value presented the microwave oven, 2% of rated current); the even harmonics are insignificant; - in case of industrial consumer important variation was observed on fundamental wave (5% of rated current), and significant harmonics were present up to 13th order. regarding to the absorbed power S, P, Q and D: - the highest value power component absorbed by the computer was the distortion power, the reactive power presented negative value; - in case of computer network the highest value component were the active power, followed by the distortion power, the reactive power presented negative value; - the highest value power component absorbed by the micro-oven was the active one, followed by the distortion power, the reactive power presents small value. -in case of the industrial consumer the active power is the highest, the reactive- and distortion components are relatively reduced. Chapter 5 deals with methods and solutions for elimination/mitigation of perturbations generated by electronic power converters fed from single-phase line, concluding with followings: - high power factor yields efficient use of energy and reduced pollution of the grid; - the power factor may be enhanced using passive or active procedures; - simulation confirms that using symmetrical control of power switches as against of moment corresponding to the supply voltage peak-value, there is a possibility to build converters having native unity power factor, (displacement factor) i.e. the fundamental of absorbed current is in phase with supply voltage. - elimination/mitigation of generated perturbations may be performed using a power factor compensator converter between grid and classical converter; - were realized and experimented two different versions of power factor compensation converter. One of them uses the peak current control method, the other one, the borderline (discontinuous peak current) method. - experimental results are the expected, confirming the effectiveness of the procedure. The present work has a pronounced practical character. The experimental measurement is interwoven with design, implementation, modeling and simulation. The thesis use rich references: 32 books, 18 scientific papers presented in journals, 2 papers presented at conferences, 4 doctoral thesis, 2 patents, 14 application manuals, 1 standard and 6 catalogs. The author published 15papers in domain of this work. As final conclusions of 12 years long research in this field, the author proposes the followings: - it is imposed the elaboration of a method regarding the risk of energy transfer in distorted operation; - it is imposed to correlate the technical research in this field with an economical research; - it is necessary the continuation of this research in order to obtain technical, administrative and political decisions regarding this field; - it is imposed to elaborate procedures for local mitigation of effects caused by distorted operation of the grid (nearer to the consumer), and a global legislative treatment (nearer to reality); - it is necessary the elaboration of simultaneous applied, technical and economical coercive measures. 7

8 Chapter 7. Contributions There are presented the contributions of the author grouped according to the chapters: Chapter 2 - review of the DC/DC step-down and step-up converters; - conceiving and realization of Matlab-Simulink structures in order to compute and represent the control characteristics of a step-up converter in continuous and discontinuous operation; - conceiving and practical realization of a step-up power converter; - conceiving and practical realization of an experimental rig composed by following units: -step-up converter, - uncontrolled power supply (feeds the converter), - power-factor compensation converter, - active load (motor), - resistive load, - switching and measuring components; - measurements on step-up converter in different operation modes (with or without power-factor compensation, active or passive load) in order to evidence the perturbation in line current caused by converter; Chapter 3. - comparative study of national and international regulations and standards, regarding the harmonic content of absorbed current, and regarding to the framing of different products of different power range into standards. Chapter 4. - analysis of techniques and requirements for evidencing the perturbations generated on grid; - conceiving and practical realization of a transducer unit for voltage and current measurements in lowvoltage power lines in order to interface different data loggers. The unit accomplishes the signal conditioning and galvanic isolation; - comparative synthesis on different measurement equipments used by the author in investigation of perturbations: the realized transducer unit, TRINET data logger, Handyscope HS3 data acquisition unit, LabPC-12/AI data acquisition card, MEMOBOX 3 Smart data logger, ABB ALPHA PowerPlus powermeter, NEXUS1252 data logger, TDS22 storage-scope; - performing field and laboratory measurements and experiments with the above mentioned instruments using optical-line, telephone-line and GSM communication for instrument control and data transmission; - conceiving and implementation of a SQL Application for computing of harmonics of a captured signal represented in CSV format; testing of the Application using data captured by Handyscope HS3 unit. - measurements in order to take obvious the perturbations on medium- and low- voltage lines of the grid and at the connection points of industrial-, office-equipments and home appliances: long-term measurements carried out in different medium-voltage power stations, using 18 ABB Alpha PowerPlus power-meters installed in power-distribution stations located in Sibiu County: Sibiu Sud, Micăsasa TR CFR, Sadu V - Dumbrava, Sadu V - Lotru, CHE Avrig, Cârţişoara, Orlat, Mârşa, Compa Sibiu, Orlat - Gura Râului, and energy transport lines: Mediaş Târnăveni, Sibiu Nord - Copşa Mică, Micăsasa Tăuni, Micăsasa Târnăveni; short-term measurements using TRINET data logger, carried out in high- and medium-voltage power transformer stations from CFR Micăsasa, Dumbrăveni, PA3 rectifier-station at TOURSIB. Low voltage measurements were performed on a 38Vca 5VDC, 7A industrial rectifier. very short-term measurements were performed in order to capture the perturbations caused by isolated, non-repetitive events on the power lines using NEXUS 1252 data logger installed in power stations located in city Sibiu: PT 221 Siretului str.- 4 KVA; PT 233 Lomonosov str., 63 kva; PT 24 Turnătoriei Str., 1 kva; PT Fundătura Lânii str., 2*1 kva; measurements at the connection points of industrial-, office-equipments and home appliances containing electronic power converters, using Handyscope HS3 data acquisition unit, Memobox 3 Smart data logger, and the TDS224 storage-scope. Were tested following products: light sources, uncontrolled rectifier, half-controlled rectifier, power supply equipped with power factor correction, step-up converter, step-down converter, frequency converter, ZCS power supply, switching power supply. Chapter 5. - conceiving and realization of a Matlab-Simulink structures in order to simulate an unity power factor converter, using symmetrical control of power switches as against of moment of the supply voltage peak-value; - conceiving and practical realization of a power factor compensation converter using peak current control method. The converter is part of the experimental rig described in Chapter 2. Measurements confirm the efficiency of the compensation; - conceiving and practical realization of a power factor compensation converter using the borderline (discontinuous peak current) control method. The converter is part of the experimental rig described in Chapter 2. Efficiency of the converter is studied using the Lab-PC-12/AI data acquisition card and appropriate LabView environment. 8