EDUCATION STRATEGY REGARDING THE ELECTROMAGNETIC COMPATIBILITY AT LOW-FREQUENCY

Transcription

1 Rev. Roum. Sci. Techn. Électrotechn. et Énerg. Vol. 61, 1, pp , Bucarest, 2016 EDUCATION STRATEGY REGARDING THE ELECTROMAGNETIC COMPATIBILITY AT LOW-FREQUENCY VACLAV KUS 1, PAVEL DRABEK, TEREZA JOSEFOVA Key words: Electromagnetic compatibility, Power electronics converters, Power engineering education, Power system harmonics, Power quality. The problems of electromagnetic compatibility (EMC) have been taken into account in almost all faculties of electrical engineering in the world. However, EMC phenomena are only partially mentioned in the small chapters of individual electrical engineering courses and the education is largely focused on one specific EMC area. This paper describes the educational approach to teaching the subject of the EMC in the low-frequency interference. In accordance with the complexity of EMC issue courses and lessons have to be divided into several sections. Main attention is paid to the interference of power electronic converters with the electricity power net and to the negative effect of the converters on loads. Experiences from 15 years of teaching are presented. 1. INTRODUCTION General EMC issues are often presented in literature; the most recognized publication that discuss EMC issues is [1]. The EMC of power systems is most described in [2]. Papers on high-frequency interference appear in conferences and journals more frequently in comparison with papers on low-frequency interference. The power quality issues are described for example in [3, 4]. Measurements of harmonic currents and voltages and their analysis are shown in [5]. On the other hand, only few publications are focused on EMC education. Most publications about education in the EMC relate to high frequency or they discuss the general issues of the EMC (for example [6]). One of the basic articles which demonstrates the necessity of teaching EMC is [7]. The next article [8] presents the related education in individual schools, mainly focusing on the specialization of the school. Learning styles for engineering students are described in [9]. Most of these articles again discuss on high-frequency interference and they are oriented to the characteristics of these schools and to the descriptions of the laboratories. Valuable information is offered in the publication [10] about student activities within the education and student activities committee (ESAC). Only few publications are aimed at EMC education in the low frequencies. The first paper about the low-frequency EMC is [11]. The publication [12] deals with the calculation of harmonics (characteristic and non-characteristic harmonics). From the literature overview it is obvious that teaching EMC in the whole range is difficult. The new conception of the EMC education strategy began more than 15 years ago. 2. EDUCATION STRATEGY OF EMC The teaching of EMC is complicated and this paper demonstrates this complex issue. It requires sufficient knowledge of other subjects. Before EMC, it is necessary to know the basics of many fields, including mathematics (in the range of Fourier series), theoretical electrical engineering and electro-magnetic field, electrical power engineering, electrical machines, electronics and power electronics. Therefore spe-cialized EMC courses have to be placed in the Master s study programs. Bachelor study program Masters study program PhD study program MATHEMATICS (Fourier Series) THEORETICAL ELECTRIC ENGINEERING LOW FREQUENCY INTERFERENCE SPECIAL PARTS of EMC ELECTRICAL ENGINEERING ELECTRONICS, ELECTRONICS ELECTRICAL MACHINES ELECTROMAGNETIC S COMPATIBILITY DESIGN ELECTROMAGNETIC S COMPATIBILITY TESING R&D in EMC Basics of many Specialized EMC education Industry oriented Fig. 1 Basic structure of EMC education. 1 University of West Bohemia, Department of Electromechanics and Power Electronics, Univerzitni 24, Plzen, Czech Republic, kus@kev.zcu.cz.

2 2 Education of electromagnetic compatibily at the low-frequency 49 Students first come in contact with the pheno-menon of EMC in the first year of their studies in the course Industrial Electronics and Mechatronics. In the Bachelor s studies the students of Power Engineering, Electronics and Industrial Electronics programs have to take the course Power Electronics. This course contains basic knowledge about EMC, especially in low-frequency disturbance. The specialized EMC education is divided into three main subjects on the Master s level. The course Electromagnetic Compatibility Testing is mandatory for students of Power Engineering. Lectures are especially focused on the measurement of high/-fre-quency emissions and immunity of power equipment. Students of Electronics attend the course Electro-magnetic Compatibility Design. The lectures are aimed at the design of electronic equipment with regard to EMC. Students of Industrial Electronics take the courses Low Frequency Interference. This course focuses on the basic influence of power converters on the power grid and fed devices and ways how these unfavorable effects can be minimized. Figure 1 presents the above mentioned structure of EMC education. All three courses are lectured for the whole semester and they contain theoretical sessions and practical measurements, as well. They are offered to students of all branches at the Faculty of Electrical Engineering (also to the entire university as an elective). Master s thesis and consequently PhD thesis have been written in the EMC fields introduced above ELECTROMAGNETIC COMPATIBILITY-TESTING The main part of the lectures is aimed at the European standards EN which deal with testing of electrical equipment. Laboratory exercises are focused on measurements, namely on emission and immunity testing of power 2.2. ELECTROMAGNETIC COMPATIBILITY-DESIGN This special course deals with the design of power converters including drivers. The object of the lectures is to design converters in order to meet the requirements of EMC testing LOW-FREQUENCY INTERFERENCE Nowadays, great attention is paid to the negative effects of semiconductor devices on the distribution network from EMC point of view. As the power electronics devices find their wide application in power systems, power quality is becoming more important issue to consider. Operation of indirect frequency converters with IGTB brings a lot of advantages, but these advantages are often accompanied by some unfavourable effects. The converters adversely influence the power distribution network due to a non-sinusoidal drawn current, the fed motor by a transient motor overvoltage and also the converter control circuits. Therefore the basic of the lectures is focused on power The lectures are divided into these parts (Fig. 2): Calculation of current harmonics (of power electronics converters) and their negative effect on the power electricity network [2 4, 12, 13]. Design of features to eliminate harmonics [2]. Impact of converters on fed devices especially on electric motors [14]. Design of protective elements to eliminate the influences of converters on a load [14]. Failure effect of a power supply on converters [15]. Measuring in the low-frequency interference field [5]. Standards for EMC in the low-frequency field etc. 3. EDUCATION OF EMC IN THE LOW-FREQUENCY NTERFERENCE Figure 1 shows that students must have know-ledge of many courses to understand lectures con-cerning EMC. Mathematical knowledge is one of the basic conditions for completion of the course, especially for understanding of Fourier series. Knowledge of classical electronics circuits and power electronics circuits is important for understanding of harmonics creation principles. Power engineering knowledge is necessary for understanding of grid impedance calculations and harmonics spreading. Theory of electric machines is the basis for under-standing of converters negative effects on electric motors. IMPACT OF CONVERTERS ON THE NET IMPACT OF CONVERTERS ON THE FED DEVICES DESIGN OF THE FEATURES TO ELIMINATE OF HARMONICS DESIGN OF THE FEATURES TO ELIMINATE OF VOLTAGE DROPS ELECTRONICS CONVERTER ELEMENTS TO ELIMINATE OF IMPACT ON THE FED DEVICES STANDARDS FOR EMC IN THE LOW FREQUENCY FIELD VOLTAGE DROPS AND IMPACT ON CONVERTERS MEASURING IN THE LOW FREQUENCY INTERFERENCE Fig. 2 Basic areas teaching low frequency interference.

3 50 Vaclav Kus, Pavel Drabek, Tereza Josefova CALCULATION OF CURRENT HARMONICS Calculations of current harmonics values are difficult to obtain from a real current waveform which is drawn from the grid by power converters, especially if an analogy with one over h rule is required. Therefore the simplification of the current wave-form drawn from the grid by singlephase and three-phase frequency converter is performed this is shown in Fig. 3. Harmonic analysis of these waveforms is easier and the modified one over h rule (3) can be derived. Equations (1) (3) present absolute values calculations of the first current harmonic and higher current harmonics. 4 I1 = I m sin d = 1, 273I sin d m, π 2 2 (1) (for single-phase bridge) 8 I m sin d π I1 = cos = 2, 205I sin d m, π (for three-phase bridge) hd 1 sin 1 Ih = I 2 I k h 1. = d h 1 d sin 2 modified one over h rule. (2) (3) 2π 2π i S ϕ d ϕ d d i S I s I sm π ωt ϕ d+ π ω t Fig. 3 Simplified waveforms of input current from single and three-phase frequency converter. Fig. 5 Possible current waveforms taken from the network. Fig. 4 Modeling scheme for harmonic current and voltage calculation in the system with the frequency converter with the 3-phase diode rectifier and voltage source inverter. In accordance with the model in Fig. 4, the influences of converters parameters on the grid, nature of the current waveform and harmonics values can be determined. Figure 5 shows possible current waveforms taken from the network DESIGN OF FEATURES TO ELIMINATE HARMONICS This part of the course deals with grid impedance calculations. The lectures and tutorials describe the calculations of industrial factories grid impedance so that installing of several semiconductor converters is shown. Electrical equivalent circuit of an industrial factory is

4 4 Education of electromagnetic compatibily at the low-frequency 51 used for the grid impedance calculation. From this equivalent circuit, the model scheme is derived. The converter is demonstrated as a source of harmonic currents (especially characteristics harmonics). The next task is calculation of grid frequency characteristics and harmonic voltages values that are propagated by Thereafter these results are checked according to relevant standards. Attention is also paid to the investigation of the resonant frequency. Design of power filters is lectured in the second part of the course too. It is presented with regard to filter and compensatory effects; the resonant frequency have to be taken into account. Figure 6 shows the frequency characte-ristics of a standalone filter tuned under the 238 Hz and the frequency characteristics of the filter connected to the grid. Fig 6 Frequency characteristics of a filter. With respect to the object of the course, the main part is focused on the filters. Resonant converters are described in detail in the course dealing with power electronics. The design of electric motors with reinforced insulation is necessary to discuss with specialists in material engineering VOLTAGE DROPS IN THE SYSTEM AND THEIR IMPACT ON OPERATION OF SEMICONDUCTOR CONVERTERS Nowadays, the problems with frequency converters caused by network voltage fault have to be solved. After the voltage drop in the ac network, the voltage in the dc part of a frequency converter decreases quickly and usually the voltage protections switch-off the converter. After voltage recovering, drives cannot generally operate MEASURING IN THE LOW-FREQUENCY INTERFERENCE FIELD A measurement in the faculty laboratory is the last part of the course. The measurement is carried out in accordance with the relevant standards IEC and IEC It is necessary that students understand the 3-second interval. Purchased analysers which meet international standards are used for testing. Results shown in the graphs and the possibility to process these data in PC are the advantages of these analysers. Students cannot understand some phenomena, this is the disadvantage of the measurement, because testing with analysers is not clear as measurement with classical voltmeters and ammeters STANDARDS FOR EMC IN THE LOW-FREQUENCY INTERFERENCE FIELD The last part of the course includes information about EMC standards. These lectures are focused on the standards in the low frequency field. The students have to orient in the systems of standards, their evolution and using in the practice. Fig. 7 Laboratory model of the compensation unit. Figure 7 presents the laboratory exercises where students learn to set and control compensators IMPACT OF CONVERTERS ON THE FED DEVICES The third part of the course is focused on modelling of inverter-cable-motor systems. This section of the lectures describes influences of system parameters (switching frequency of converters, dv/dt, cable parameters and electric motor parameters) on changing size and frequency of the over-voltage in electric motors ELEMENTS DESIGN FOR ELIMINATING THE CONVERTERS INFLUENCE ON THE LOAD This section of the course deals with the below issues: reducing the dv/dt using resonant converters, reducing the dv/dt using filters, using sine filter, design of special electric motors with reinforced insulation. 4. EDUCATION IN THE PHD STUDYING PROGRAM PhD students of the Faculty of Electrical Engineering can enrol the course Low-Frequency Interference (LFI). In this course, they deal with the specific part of EMC which is important for their PhD thesis. The second option for students who are interested in LFI is to choose the PhD thesis which directly related to this topic. Topics of PhD thesis discuss both theoretical knowledge and requirements of industry. For example the topics of PhD thesis are: Investigation of non-characteristics harmonics currents of semiconductors Investigation of inter-harmonics currents of semiconductors Influence of frequency converters on fed devices. Investigation of input harmonics currents of active rectifiers. Interaction of compensation equipment and mass remote control signals of electrical appliance. Influence of voltage drops of the grid on semiconductors

5 52 Vaclav Kus, Pavel Drabek, Tereza Josefova 5 5. CONCLUSIONS The paper describes the educational strategy of the course Low-Frequency Interference which is not as usually taught at universities on an international scale and it also presents experiences of teaching. The course Low-Frequency Interference is one of the most demanding, because knowledge of the mathematics, electrical engineering, electronics, etc. is necessary for understanding of the lectures. Therefore students of fulltime study are generally more successful than students of combined form of study. Exceptions are students of combined form of study who have practical experience related to EMC; these students are an asset for lecturers. Experiences of teaching and cooperation with graduates who deal with EMC show that the educational strategy of the course is appropriately chosen. Therefore, in the lessons, it is necessary to include theoretical knowledge and practical exercises in the laboratory. ACKNOWLEDGMENTS This work was supported by the European Regional Development Fund and the Ministry of Education, Youth and Sports of the Czech Republic under project No. ED / : Regional Innovation Centre for Electrical Engineering (RICE). Received on July 23, 2015 REFERENCES 1. P. Chatterton, M. Houlden, EMC Electromagnetic Theory to Practical Design, John Wiley & Sons. 2. J. Arrilaga, D. A. Bradley, P.S. Bodger, Power System Harmonics,Wiley Interscience Publication. 3. L. E. Petrean, M. Horgos, N. Pavel, L. Petrean, The effect of power quality disturbances on the electromagnetics compatibility, Rev. Roum. Sci. Techn. Électrotechn. et Énerg., 53 (Suppl.), pp , Bucarest, G. L. Wakileh, Power systems harmonics: fundamentals, analysis and filter design, Berlin, Springer, F.I. Hantila, M. Maricaru, R. M. Ciuceanu, L. Corlan, Harmonics analysis of circuits with nonlinear resisstive elements. Rev. Roum. Sci. Techn. Électrotechn. et Énerg., 57, 4, pp , C. Medrano, F. Arcega, A. Lopez, I. Plaza, T. Pollan, Electromagnetic Compatibility: Learning from experience by means of practical cases. Technologies Applied to Electronics Teaching (TAEE), 2012, pp G. K. Deb, Importance of EMC education. Proceedings of the International Conference on Electromagnetic Interference and Compatibility, 1999, pp V. Ungvichian, A Positive Step Toward Education in Electromagnetic Compatibility, IEEE Transactions on Education, 33, 4, A. D. Ionita, A. Olteanu, Domain specific models, knowledge and tools to support multiple learning styles for engineering students, Rev. Roum. Sci. Techn. Électrotechn. et Énerg., 59, 4, pp , M. Steffka, Education and student activities committee (ESAC). IEEE Electromagnetic Compatibility Magazine, 2, 1, pp , Z. Yang, Y.S. Kye, A practical approach to EMC education at the undergraduate level. IEEE Transactions on Education, 47, 4, pp , V. Kus, Z. Peroutka, P. Drabek, Non-characteristic harmonics and interharmonics of power electronic converters, International Conference and Exhibition on Electricity Distribution, 2005, CIRED, Turin, Italy. 13. F.I. Hantila, F. Constantinescu, A. G. Gheorghe, M. Nitescu, M. Maricaru, A new algorithm for frequency domain analysis of nonlinear circuits, Rev. Roum. Sci. Techn. Electrotechn. et Energ., 54, 1, pp , V. Kus, Z. Peroutka, Adverse Effects in Voltage Source Inverter- FedDrive Systems, Applied Power Electronics Conference and Exposition, 3/2002, Dallas, USA, pp T. Benslimane, Open switch faults detection and localization in three phases shunt active power filter, Rev. Roum. Sci. Techn. Électrotechn. et Énerg., 52, 3, pp , 2007.