The 5th PSU-UNS International Conference on Engineering and Technology (ICET-2011), Phuket, May 2-3, 2011

Prince of Songkla University, Faculty of Engineering Hat Yai, Songkhla, Thailand 90112

Abstract: Teaching engineering courses always requires optimization between theoretical knowledge, mathematical modeling and problem solving from one side and experimental, laboratory and practical experience approach from the other side. The aim of the paper is to present past experiences of teaching in one of such courses - Power Electronics at the University of Novi Sad. Modern approach and application of new technologies will be presented. An analysis of student results confirms that students have successfully passed all of the course’s objectives. Key Words: Engineering Education / Power Electronics Course / Laboratory exercises

1. INTRODUCTION Teaching engineering courses always requires

optimization between theoretical knowledge, mathematical modeling and problem solving from one side and experimental, laboratory and practical experience approach from the other side. Modern computer technology provides tools for numerical, digital simulation of power electronics systems, enabling easier insight in the practical solutions and application of new technologies, adding another, virtual approach in teaching. Optimization is not easy task and teachers found them in complex situation to preset all aspects, but not to overload the students.

One of such courses at the University of Novi Sad, Faculty of Technical Sciences – Department of Power, Electronics and Telecommunication Engineering is Power Electronics (PEL), the course which is very often offered to students in similar departments at European, Asian or American universities. The authors are dealing with this course for more than 20 years, but over the last decade a modern approach has been introduced. Such concept consists of carefully picked balance between above mentioned three areas introducing students with the most advanced theoretical knowledge, giving them practical experience with power electronics system models in laboratory (lab) and introducing the skills of

using modern simulation tools and verifying the simulation results in laboratory.

The aim of the paper is to present past experiences of teaching such a course - Power Electronics and present application of new teaching methodology and technology at the University of Novi Sad.

2. POWER ELECTRONICS COURSE Power electronics course is organized for the students

of the third year (junior students) of power engineering major, as a compulsory course, and for students of the final year (senior students) of major Microelectronics–Applied Electronics as an elective course. These two groups of the students differ between themselves not only by age, but also by their background knowledge and course duration.

The first group is larger (between 50 to 80 students), and have PEL course spread over two semesters (PEL1 and PEL2), with 4 hours per week of teaching and lab. Their background is related to power engineering courses, with a few electronics and system modeling & simulation tools related courses.

The second group is smaller (10-15 students) and has one semester PEL course, but with 6 hours of teaching and lab per week. Their backgrounds are electronics courses, with a lot of digital, computer and electronics simulation subjects.

2.1. Power Electronics Teaching

The main PEL course outline is following the approach presented in [1,2]. The course begins with presentation of the complete electric power system (from generation to the consumption) with details of PEL’s role and place in it. Then a PEL system is described, with explanation of the main blocks and power processing tasks. After that, the course continues with presentations of components, converters and applications.

However, the two above mentioned groups have a little bit different approach. The main focus of the first group in PEL teaching is application of power electronics systems in industry and transportation, as the

TEACHING ENGINEERING COURSES AT THE UNIVERSITY OF NOVI SAD – CASE

OF POWER ELECTRONICS COURSE Vladimir Katić*, Stevan Grabić, Zoltan Čorba, Boris Dumnić, Dragan Milićević

University of Novi Sad, Faculty of Technical Sciences, Novi Sad, Serbia *katav@uns.ac.rs

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main power processing stage in an electric drive. The textbook [1] is recommended to the students. The main focus of the second group is application of power electronics for power supplies of electronics, mechatronics and computer devices, which is covered by the textbook [2]. The teaching of the both groups is followed by problem solving examples [3].

2.2. Power Electronics Lab

Very important part of the PEL course is laboratory practice [4,5,6]. The students are building very simple PEL systems and by changing their parameters learn how they operate and gain practical experience.

There are 10 different lab setups: 1. Fundamental Power electronics components and

circuits, 2. Single-phase AC/DC converters, 3. Three-phase AC/DC converters, 4. AC/AC converters – voltage controllers 5. DC/DC converters – Choppers, 6. Single-phase DC/AC converters, 7. Three-phase DC/AC converters, 8. DC/DC converters – Direct switch-mode power

supplies 9. DC/DC converters – Isolated switch/mode power

supplies 10. PEL system application – Wind generation systems

The complete description of the setups, with guidelines of application and tasks to be achieved are presented in [4]. The content is updated in 2005 and in 2011 in a form of scripts, but an official publication (like [4]) has not been published, yet.

An example of the lab setup for the second exercise is presented in Fig.1. The single-phase diode rectifier is tested without and with different DC side filter capacitances. Their effects are measured, regarding DC voltage ripple in cases of different R-type loads. They also find out some negative effects of PEL converters operation, like false reading of instruments in case of distorted current wave-shapes (Fig.1).

Fig.1. False reading of right multi-meter in case of

distorted line current measurement.

A simple circuit is used to demonstrate operation of a single-phase AC/AC converter for voltage control. Fig. 2 shows complete circuit where a triac is used to regulate a voltage over a resistive (lamp) load. This is very effective method, as student can have visual observation of the regulation effects. The control is achieved by a single integrated circuit and operated by potentiometer (adjustable resistance) R6 (Fig.2).

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Fig.2. AC/AC converter–voltage controller setup. The lab exercises also use some commercially

available equipment giving students sense of controlling of real PEL systems applied in industry. This is the content of the tenth lab exercise – PEL system application. The setup is developed to model a wind plant (Fig. 3). A model of a wind turbine is represented with a fully controlled induction motor. It is coupled with a permanent-magnet synchronous generator and connected to the public grid via back-to-back converter (active front-end rectifier-regenerative feedback unit). The students may control the induction motor according to the wind speed and direction data, obtained from a public wind-speed measurement station and measure generated power. They also have opportunity to go through a process of synchronization of the “wind” plant to the grid. Using PQ meters, they may observe the power quality of generated voltage and current, also.

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2.3. Power Electronics Simulation Tools

Modern simulation tools have been developed in two directions: 1. to support PEL teaching and 2. to enable computer simulation of the PEL circuits and systems.

For the first application, the well-known free interactive web based software, i.e. applets developed by the ETH Zurich are used [7]. The applets are giving the details of PEL circuit operation and control enabling students to understand real processes. Fig. 4 gives presentations of the two cases of applets application: Operation of a Single-phase PWM DC/AC converter (Fig.4.a) and control method and switches positions in a Space Vector Controlled PWM Rectifier (Fig.4.b) [7]. For better understanding the PEL theory and to be able to gain the most from the lab exercises, the students are advised to prepare themselves by testing different applets on-line.

a)

b) Fig.4. Some of PEL applets: a) Single-phase PWM con-verter, b) Space-Vector Control of a PWM Rectifier [7].

For the computer simulation of PEL circuits, there are number of different open-access or commercial software tools available. For teaching purposes, the most convenient are open-access software. The ETH offers a free-trial on-line Java PEL circuit simulator, available for the most common circuit analysis – Fig.5 [8]. The PEL circuits can be composed using models of the main switching components and different passive elements. However, the complexity of the circuit is limited.

Fig.5. Buck-boost converter modeling using

GrecoCIRCUITS simulation tools [8].

There are numerous of other PEL simulation software, which are more or less commercial, but always offering a free trial possibility (SABER, PLEXIM, SIMPLIS, SIMPLORER, ENTDC, RT LAB, SIMETRIX, SPICE, SIM POWER SYSTEM, CASPOC, SIMULINK, PLESC, POSIM, PESIM). The PESIM is rated by students as the most convenient for application, regarding the required learning efforts and quality of simulation [9]. The software consists of three parts: SIMCAD, PESIM and SIMVIEW. An example of the software application for PEL converter (Three-phase AC/DC converter with thyristors and voltage feedback control) is presented in Fig.6.

Fig.6. An example of PESIM application for PEL

converter simulation [9].

3. MODEL-BASED CONTROL DESIGN A new approach in teaching at the Faculty of

Technical Sciences - PEL laboratory is achieved by using the new Total Development Environment (TDE) concept. The TDE is recently introduced by dSPACE [10] that allows a full visual block-oriented programming of dynamic real-time systems using the Matlab/Simulink

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environment, Fig. 7. Thus, the need for hand-written code during exercise is eliminated.

Fig.7. Total Development Environment – TDE

The off-line Simulink simulations are now to be

implemented on a real-time frame by simply adding some specific blocks from Real Time Interface (RTI) Simulink library that connects the physical I/O of the system to the DSP core. All the code generation and downloading tasks are performed automatically.

There has been reported a large number of papers where Simulink is being intensively used in the teaching process of power electronics due to some important advantages like: visual block-oriented programming concept, true interactive environment and low cost. Our students use many off-line computer simulators including: Pspice, Simulink with Power Systems Blockset and they solve many power electronics and drives exercises using them. Yet they did not get the feeling of a real-time system. Now, by using TDE they have the chance to test “live” their control strategies in very short time.

The Real Time Interface (RTI) is use to connect the Simulink model to the physical world. The Real Time Workshop (RTW) automatically converts the model to C code. The C code is compile to the assembly language of the target processors, assemble, link-edit and download. An experimentation tool - ControlDesk is use to run, tune and monitor the running process of the lab exercise. Building virtual instrument panels and having complete control over Simulink and real-time simulations, or setting up complex experiments, manage their parameter sets, and even automate complete test series is enabled by using ControlDesk (Fig. 8). The ControlDesk in conjunction with the MATLAB/Simulink gives features that are needed for advanced function development.

Fig.8. Control Desk – Simulink interaction

After introducing this modern software and hardware

laboratory tools we could notice that student become more attracted to attend power electronics and electrical

drives related courses. The student enrollment in the electrical engineering courses is expected to increase in the coming years.

4. STUDENT RESPONSES Very important part of each course is student results

at the end, which is quantified by the final grade. The grade is consisting of students overall performance during the semester (pre-exam duties: tests, reports, seminar papers, home works, etc) and at the final exam. In the case of PEL course, a contribution of the pre-exam duties on the final grade is 50%, while the rest is earned at the exam.

The grading system in Serbia is based on grades between 10 (excellent-exceptional, the best grade) and 6 (satisfactory, the lowest passing grade). If a grade is 5, the student has failed the exam. The results are evaluated with points – from 0-100. There is relation between the awarded points and the final grade in the manner presented in the Table 1.

In some countries in Europe a statistical grading system is in use. Such system is recommended by European credit transfer system (ECTS) and proposed for use in student mobility networks, like Erasmus, Campus Europae, Erasmus Mundus and others. It should be noted that in Serbia method of statistical grading is not in use, although ECTS credits are applied. However, there is clear correspondence between the two systems, which is shown in the Table 1. Table 1. Relation between points and grades at UNS

Points Existing grades

Descriptive grades

Statist-ical grades

Percent. of

success 95-100 10 Excellent –

exceptional A 10%

85-94 9 Excellent B 25% 75-84 8 Very good C 30% 65-74 7 Good D 25% 55-64 6 Satisfactory E 10% 0-54 5 Unsatisfactory FX -

Unsatisfactory F -

In order to present exam results of the students of both majors, an analysis is presented in Figs. 9 and 10 (left columns). It can be seen that most of the students have grades above 8 and that no student has the lowest passing grade 6. Average grade for both groups is presented in Table 2. It can be seen that both groups have similar average grade, so that overall success is between very good and excellent.

Table 2. Characteristics of statistical grading system

Statistical grading system

Students of Microelectronics

major

Students of Power Engineering

major

x - grade average value

8,32 (EGS), 8,00 (SGS)

8,35(EGS), 8,00 (SGS)

σ - standard deviation 0,88 0,98

xσ - Variance [%] 10,60 11,74

EGS – Existing grading system; SGS – Statistical Grading System

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A comparison with statistical grading system is also very important and it is shown in Figs. 9 and 10, as well as in Table 2. It can be seen that overall average grade for the both groups is lower if statistical system is applied. Also, total number of the best grades is lower.

Fig.9. Grading comparison for the students of the Power

Engineering major

Fig.10. Grading comparison for the students of the

Microelectronics major.

5. CONCLUSION The paper presents the teaching methodology of an

engineering course at the University of Novi Sad-Faculty of Technical Sciences, which has theoretical, laboratory and computer simulation segments. A case of the Power Electronics course for two different groups of students has been analyzed. The final student results show that they well accepted this complex approach, especially the laboratory part. Their response is positive. Comparing existing grading system and statistical one, results show

that there are still a lot of differences and that the existing system is awarding students with better grades.

6. REFERENCES [1] N. Mohan, T. Undeland, W. Robbins, “Power

Electronics – Converters, Applications and Design”, 3rd Ed., John Wiley and Sons, USA, 2003.

[2] B. Dokic, “Power Electronics – Converters and Regulators”, Akademska misao, Belgrade, 2008 (In Serbian).

[3] V.A. Katic, “Power Electronics – Worked Problems", University of Novi Sad, University Textbooks Series No.66, Novi Sad, 1998 (In Serbian).

[4] V.A. Katić, D. Marčetić: "The Laboratory for Power Electronics at University of Novi Sad", Int. Conf. on Electrical Drives and Power Electronics: EDPE'94, Stara Lesna - High Tatras (Slovakia), Oct. 18-20, 1994, pp.436-440.

[5] V.A. Katic, D. Marcetic, D. Graovac, “Power Electronics – Laboratory Practice”, University of Novi Sad, University Textbooks Series No.124, Novi Sad, 2000 (In Serbian).

[6] D.Reljić, D.Milićević, E.Adžić, B.Dumnić, S. Grabić, V.Porobić, M.Vekić, Z.Ivanović, V.A.Katić, V.Vasić, D.Marčetić, Đ.Oros, Z.Čorba, “Modern Laboratory Tools for Experimental Research in the Field of Electric Drives”, 15th Inter. Symp. on Power Electronics – Ee 2009, Novi Sad (Serbia), 28-30th Oct. 2009, Paper No.T4-2.11, CD-ROM.

[7] U. Drofenic, J. Kolar, Interactive Power Electronics Seminar (iPES), Available On-line, 2011, http://www.ipes.ethz.ch

[8] ***, GeckoCIRCUITS Beginners Tutorial, Gecko Research, Available On-line, 2011, http://www.gecko-research.com

[9] The Power Electronics Simulation Software – PESIM, Lab-Volt Systems, Available On-line, 2011, http://www.labvolt.com/products/electric-powercontrols/power-electronics/power-electronics-simulation-software-pesim-8971

[10] dSpace, “Solutions for Control, DS1104 R&D Controller Board”, Paderborn, 2004.

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