Key-Words: - Electrical Engineering education, simulation, power electronics, high voltages. with the classic methods of theory and experimental work.

Transcription

1 New ethods in experiental work along with siulation odeling and analysis in the laboratories of the Electrical Engineering Departent of the T.E.I of Larissa. ATHANASIOS MAGLARAS, JOHN ANDRITSOS, GEORGE SOULTIS, LEANDROS MAGLARAS, Electrical Engineering Departent T.E.I. of Larissa T.E.I. of Larissa, 4111 Larissa GREECE Abstract: - The goal of the Electrical Departent of the T.E.I of Larissa is to keep its New Course in high acadeic standards. In order to eet that goal the use of new technologies and especially coputer- based software is introduced in teaching procedures. Thus the teaching staff has adopted for their lectures the use of coputers, projectors, as well as siulation and presentation software, appearing very satisfying results. More efficient work is done in the laboratories, where the experiental work is successfully cobined with siulation work, as well as theoretical-atheatical analysis. The results taken fro the three different procedures are sorted into tables and copared, and students attept to explain and ake rearks concerning the differences of the values. The outcoe of such an approach is of great iportance for engineering education, and can assuredly be considered as the basis of Virtual Engineering. The students learn to trust the coputer applications in electro technical subjects, and understand how siulation can give faster and easier results copared to difficult experiental arrangeents or atheatical equations. Laboratory work becoes ore interesting and essential and the education provided is synchronous and effective. Key-Words: - Electrical Engineering education, siulation, power electronics, high voltages. 1 Introduction In the last few years, the Electrical Engineering departent of the T.E.I of Larissa is fulfilling European prograes for the reforation and iproveent of its course. Through these prograes a great effort has been ade to introduce new technologies and especially the use of special coputer software in teaching procedures. Thus the teaching staff has adopted for their lectures the use of coputers, projectors and presentation software, appearing very satisfying results. Especially in the laboratories, presentation software and siulation progras are used parallel with experiental arrangeents and easureents. Experiental work is successfully cobined with siulation odelling and analysis, as well as atheatical equation solutions. The results taken fro the three different procedures are sorted in tables and copared, and students attept to explain and ake rearks concerning the differences of the values. Thus in the ajority of the departents laboratories siulation and other coputer software have been adopted, since siulation has proven to be an excellent educational tool once cobined with the classic ethods of theory and experiental work. 2 The laboratories of the departent. The electrical engineering departent of the T.E.I. of Larissa has 1 ain laboratories in which students ake their practical exercises for ore than 15 odules. Most of the departents laboratories have introduced the new ethods giving efficient results for instance the laboratories of High Voltages, Power Electronics, Control Systes, electric achines, electronics, digital systes, CAD/CAE, etc. 2.1 High Voltage Laboratory. In the High voltage laboratory of the Electrical Engineering departent of the T.E.I. of Larissa, the dielectric properties (dielectric strength, insulating resistance, tanδ, etc) of air gaps and other insulated aterials are easured, under special test arrangeents. Typical voltage ultiplying and voltage easuring arrangeents are also built and used for easureents and tests.

2 2.1.1 The air gaps dielectric strength easureents. The dielectric strength of air gap arrangeents of different geoetries is easured. The analysis of the electric field distribution in the gap with Finite Eleent Method and special software is deonstrated. The connection between the axiu value of field strength and the breakdown voltage is investigated. (Fig. 1, 2, 3, 4) Ffield strength E/Eav ELECTRIC FIELD DISTRIBUTION ALONG THE AXIS IN AIR GAPS OF DIFFERENT GEOMETRIES Distance fro rod Fig. 1. Field strength distribution along the axis of the gap for three basic sall air gap arrangeents: sphere-sphere, rod-rod and rod-plate. FIELD FACTOR OF AIR GAPS OF DIFFERENT GEOMETRIES. spheresphere Spheresphere Rod - rod Rod - plate Breakdown voltage KV BREAKDOWN VOLTAGES OF AIR GAPS OF DIFFERENT GEOMETRIES Gap length plateplate rod - plate Πολυων Fig. 3. Values of the breakdown voltage for spheresphere, plate-plate, and rod plate sall air gaps. Breakdown voltage KV Rod - plate air gap 1 c long Rod diaeter Field strength on the rod KV/c Vbr, rod Vbr, plate Er, rod Er, plate FIELD FACTOR Eax/Eav GAP LENGHT ROD-ROD. ROD DIAMETER 1 ROD-PLATE GAP. ROD DIAMETER 1. PLATE GROUNDED ROD-PLATE GAP, ROD DIAMETER 1, ROD GROUNDED SPHERE - SPHERE GAP. SPHERE DIAMETER 45 Fig. 4. The breakdown voltage and the axiu Field Strength in sall rod plate air gaps, in function with the rod diaeter. Rod Vr Plate V= Fig. 2. Field factor (Eax/Eav) of air gaps of different geoetries. Fro fig. 1, 2, 3 and 4 it results that the breakdown voltage of an air gap arrangeent is lower, when the axiu field strength in the gap is higher. The breakdown voltage of a rod plate air gap is lower than of the plate-plate and sphere-sphere air gaps of the sae length. Fig. 5. Field strength distribution in a rod plate air gap odel It is obvious fro fig. 5 that the field distribution in a rod - plate air gap is inhoogeneous.

3 Ipulse voltage generation. i) An experiental arrangeent of a siple circuit for an ipulse voltage generation is built, with the use of a siple placket and soe contours, (fig. 6). The output ipulse voltage is taken with an oscilloscope, and the rise tie and the fall tie are easured for different values of the coponents R 1, R 2, C 1, C 2 (fig. 7). Fig. 6. The experiental arrangeent for ipulse voltage generation in the laboratory of High Voltages. Fig. 9. The rise (tr), and fall (tf) tie fro the analysis with the use of the ultisi software. iii) Fro atheatical analysis it results that the rise and the fall tie of an ipulse voltage generated by the circuit of fig. 8 can be calculated fro equations: Fig. 7. The oscilloscope output ipulse voltage of the arrangeent. ii) With the use of the Multisi software a odel of the circuit is drawn and analyzed. The rise tie and the fall tie of the ipulse voltage are easured for the sae above values of coponents R1, R 2, C 1, and C2. (fig. 8, 9). Figure 8. The circuit odel drawn with the use of the Multisi software. t r = K 1.R 2.C 2 (1) and t f = K 2.R 1.(C 1 +C 2 ), (2) where K 1 and K 2 are coplicated functions of R 1, R 2, C 1, C 2, and of the parasite capacitances and self inductances of the circuit. The students calculate the values of t r, and t f, for the sae above values of R1, R 2, C 1, C 2. iv) The three different results of the t r,, and t f, fro experient, siulation and atheatical analysis, are sorted in tables and copared, followed by discussion and special rearks concerning the differences of the resulted values. Special rearks are also ade for the values of the coefficients K 1 and K 2, in function with the values of the coponents R1, R 2, C 1, C 2, Cockroft arrangeent. i) The circuit of a Cockroft arrangeent is built in the laboratory, with the use of a special placket and soe contours. The input and the output voltage are

4 easured with the aid of a volteter and an oscilloscope. (fig 1 and 11). Fig. 13. The odels input and output fro the ultisi analysis. Fig. 1. The experiental Cockroft arrangeent in the laboratory Figure 11. The input and output voltage of the arrangeent on the oscilloscope in the laboratory. ii) With the use of the Multisi software a odel of the circuit is drawn and analyzed. The input and the output voltage is easured (figures 12,13) 2.2. Power Electronics Laboratory. In The Power Electronics laboratory of fig 14 of the electrical engineering departent, of the T.E.I. of Larissa the subjects that are studied and exained are: a) the way to fire a thyristor and its behaviour, b) single phase and three phase AC switches, c) dc choppers of one and four quadrant, d) power seiconductors, e) control and speed regulation in dc - ac otors and f) full controlled single-phase and three phase ac/dc converters. Fig. 12. The Cockroft circuit odel fro Multisi siulation. iii) Fro atheatical analysis it results that the relation between the input and the output voltage of the Cockroft arrangeent is: Vout =Vinax= 2*Vinrs (3) iii) The three different results fro experient, siulation and atheatical analysis, are listed and copared, and special rearks are ade. Students try to explain the reason for the differences of the 3 values Fig. 14. The laboratory of Power Electronics Full controlled single-phase bridge with thyristors. i) The circuit of a full controlled single-phase bridge with thyristors (fig. 18) is built in the laboratory (fig. 15). The bridge is supplied with 22V/5 Hz. The load is resistant with R=2 Oh and conductive with L=5 Η and the firing angle is φ=7. The thyristors T 1 and T 4 are fired siultaneously and so are thyristors T 2 and T 3 but with a 18 delay. The deviation of 18 in the drive of the thyristors is

5 taken fro the synchronisation voltage when reversing the coparators ends. ii) The wavefors of the voltage, the loads current, the current of each thyristors, the inputs current and the thyristors T 1 voltage are taken with the use of an oscilloscope. (fig. 16) iii) The average and RMS values of the loads voltages, the average and RMS values of the loads currents and of each thyristor, the RMS values of the input currents are calculated and the values are listed in tables. Fig. 17. Model of a one full controlled single-phase bridge drawn with the use of PSIM software. Fig. 18 Scheatic diagra of a full controlled single-phase bridge Fig. 15. Arrangeent of a full controlled singlephase bridge in the lab Fig. 16. Wavefors taken fro the oscilloscope iv) With the use of the PSIM software a odel of the circuit is drawn (fig. 17) and the wavefors fro siulation analysis appear. (fig. 19) Fig. 19. Wavefors taken fro with the process of siulation analysis. v) The atheatical equations used for the calculation of the above values are: R V (ωt-φ) i(ωt) = [ηµ(ωt θ) - ηµ(φ - θ)e Lω ] (4) Z

6 Ι rs = Ι rs(p) φ [i(ωt)] dωt = π φ = 2I rs(th) = V 2Z V 2Z α ηµασυν(2φ+ α + θ) π πσυνθ α ηµασυν(2φ+ α + θ) π πσυνθ V V 2V α α Ι dc dc = = (συνφ- συνφ ) = ηµ ηµ(φ+ ) R πr 1 πr 2 2 φ 1 1 2V α α V dc = V ηµωtdωt = ηµ ηµ(φ + ), π φ π 2 2 where α=φ 1 -φ is the conduction angle, φ 1 is the extinction angle, θ=tan -1 (Lω/R) and Z=sqrt(R 2 +(Lω) 2 ) vi) The results fro the above procedures are copared and lead to the conclusion that experiental work theoretical approach and siulation don t appear rearkable differences. 3 Results and Discussion The use of new technologies and especially of coputers and siulation odeling and analysis is of great iportance for the odern higher educational systes. Students learn to trust the coputer applications in electro subjects, and especially the siulation odeling and analysis. They get failiarized with plenty siulation software, and realize how to approach circuit analysis. They understand how siulation can give faster and easier results copared to difficult experiental arrangeents or atheatical equations. Laboratory work becoes ore interesting and essential and the education provided is very satisfactory as well as synchronous and effective. The knowledge received is ore coplete, since the cobination of siulation, theory and experient lead to a better coprehension of the probles. Finally the outcoe of such an approach is of great iportance for engineering education in general, and can assuredly be considered as the basis of Virtual Engineering. Acknowledgeent: Special thanks to assistant professor Mrs Konstantina Giannakopoulou who offered helpful suggestions during the preparation of this paper. This work has been funded by the Greek Operational Prograe for Education and Initial Training (O.P. Education) under the action: Reforation of Undergraduate Studies Progras References: [1] Sylvester P. P., Ferrari R. L. Finite eleents for electrical engineers, Cabridge University Press [2] M. S. Naidu, V. Kaaraju, High Voltage Engineering, McGraw Hill [3] Ε. Kuffel, W.S. Zaengl, J. Kuffel. High Voltage Engineering. Fundaentals. Newnes Oxford. 2 [4] Khalifa, M. High-Voltage Engineering, Theory and Practice. Marcel Dekker inc., New York. [5] I. Stathopoulos, High Voltages, SYMEON PUBLISHING Co. 2 [6] A. Maglaras. High voltage technology. T.E.I. of Larissa, 24 [7] I. Andritsos. Power electronics. T.E.I. of Larissa 24 [8] I. Gonos, V. Kontargiri, I. Stathopoulos, A.M. Michaelidis, C.P. Riley. Field solution of electroagnetic probles using the Finite Eleent Analysis. Congress of CINGRE, Athens. 23. [9] A. Maglaras. Nuerical analysis of electric field in air gaps, related to the Barrier Effect. 1 st IC-SCCE Athens, 24. [1] A. Maglaras, L. Maglaras. Modeling and analysis of electric field distribution in air gaps, stressed by breakdown voltages. WSEAS MMACTEE Athens 24 [11] Mohan, Undeland, Robbins. Power Electronics. Tziolas publishing Co, 1996 [12] Muhaad H. Rashid. Power Electronics (circuits, devices, and applications). Prentice Hall, 1998 [13] PC Sen, Power Electronics. McGraw Hill 1987 [14] B. M. Bird, K.G.King, D.A.G Pedder, An introduction to Power Electronics. John Wiley and Sons, Ltd, 1987 [15] Robert W. Erickson, Dragan Maksiovic, Fundaentals of Power Electronics. Kluwer Acadeic Publisher, 24