Abstractâ Wind energy conversion systems (WECS) is currently a hot topic for electrical and electronic engineers because of the wide range of topologies that ...
Wind Energy Conversion System Course for Electrical Engineers. Part 1: Theoretical Background Mario J. Durán, Federico Barrero, Hugo Guzmán, Francisco Guzmán, Ana Pozo
Abstract— Wind energy conversion systems (WECS) is currently a hot topic for electrical and electronic engineers because of the wide range of topologies that both manufacturers and researchers are designing. Different types of electrical generators, power converters and configurations have been recently proposed and this serves as a centre of interest for engineering students. Wind energy is also an application that founds a great interest among students and a huge potential for employment of engineers. For these reasons, teaching power electronics and electrical machines using WECS as the main center of interest can be advantageous. This work describes a novel teaching experience using wind energy as the starting point for understanding power electronics and electrical machines. Part 1 of this work describes the theoretical background that is provided to the students, showing the vast range of concepts and competences involved in the course. Index Terms— Wind energy conversion systems, power electronics, electrical machines, simulation tools.
I. INTRODUCTION
E
lectric, hybrid and plug-in vehicles (EVs) in automotive industry, photovoltaic (PV) and wind energy conversion systems (WECS) in renewable energy industry or high voltage direct current (HVDC) and flexible AC transmission systems (FACTS) in utilities are changing the role and industrial use of power electronics and electrical machines. In these emerging areas the use of power electronics and electrical machines is growing at an enormous pace [1]. This trend is changing educational methodologies towards the instruction of proficient engineers with a multidisciplinary knowledge in these industrial technologies [2]-[3]. It is noticeable that the amount of electrical power which is processed through power converters is continuously growing but the degree curricula and the subjects’ programs are often focused on steady-state models for bulky ac power systems [4]. At the same time, conventional and bad connected bottomto-top power electronics and electrical machines courses
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poorly satisfy the industrial requirements. These forthcoming industrial challenges could be better faced following a different educational orientation and designing modern courses aiming to: Follow a top-to-bottom approach: starting the lectures with the application and descending to the underlying mathematical description and modeling. Include multidisciplinary knowledge: mixing electrical, electronic, automatic, mechanical and informatic engineering areas. Promote hands-on tasks and active role of students: including some kind of problem-based learning activities. Look for learners’ centers of interest: including hot topics such as renewable energies, smart grids and electrical vehicles. Use simulation and experimental platforms: to promote different skills and understand the scientific procedure. Highlight industry applications: to have an updated knowledge of the new developments in evolving areas. The aforementioned guidelines have served to design a course that selects WECS as the center of interest for the teaching of power electronics and electrical machines for engineering students at the University of Malaga (Spain). The fact that WECS are good candidates to be a centre of interest is justified considering that wind energy is the most mature and widely installed renewable energy in the world, representing around 17% of the total electrical generation in Spain. Furthermore, Spanish companies are also leading manufacturers of wind generators [5], this serving as an extra motivation for the students. The experience is designed to improve students’ competences from different approaches: From a scientific point of view, the study of the WECS seems to be an ideal candidate, covering not only power electronics but also electrical machines and power systems, and being an example of a multidisciplinary area [6]-[7], as schematically shown in Fig. 1. From a technological point of view, WECS show multiple topologies with different types of generators and converters,
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resulting of high interest for electric/electronic engineering students. In spite of being a mature technology, different manufacturers are also testing different strategies [8]-[9], leading to different generating voltages, gearbox design, etc. From the educational point of view the blended methodology that combines speech and hands-on work, theoretical explanation and problem-based learning, teacher-centered and student-centered strategies proves to obtain practical results in terms of motivation and students’ performance. The paper is structured so that a description of the course (part 1 and part 2) is included in section II. Then, the theoretical background, including a review of the power converters (PC) and electrical machines (EM) used in wind energy systems, is included in section III. The main conclusions are finally summarized in section IV. POWER ELECTRONICS
From the methodological point of view, the course follows a combined strategy that includes both direct lectures with hands-on working in a simulation-based environment. The pros and cons of using simulations vs. real labs or direct learning vs. problem/research based learning have been intensively discussed [2], [12], and each option is always trade-off between different features. The proposed pedagogical approach opts for a low-cost and flexible solution that aims to provide the student both with a state-of-the-art knowledge and scientific/technical skills to analyze a multidisciplinary problem. The course lasts for a period of 40 hours and consists of two parts: •Part 1: theoretical background of wind energy and description of the technological status of the WECS (power electronics, electrical generators and topologies). This part of the course is the one described in this first part of the paper. •Part 2: simulation of a full-power WECS with a permanent magnet synchronous generator, two-level IGBT-based voltage source converters, PI-based field oriented control (FOC) and carrier-based pulse width modulation (PWM). A full description of this second part can be found in the second part of the paper.
ELECTRICAL MACHINES CONTROL THEORY
POWER SYSTEMS
SIMULATION COMUTING
methodologies do not provide a proper multidisciplinary context to the learners and they are not structured to allow the student build their own simulations.
WECS
ECONOMY
SIGNAL PROCESSING
ENVIRONMENTAL REGULATIONS FLUID MECHANICS
MECHANICAL ENGINEERING
Fig 1. Multidisciplinary nature of Wind Energy Systems.
II. DESCRIPTION OF THE COURSE This course has been developed within the framework of a specialization course at the University of Malaga (Spain). These specialization courses provide undergraduate and graduated engineering students with an opportunity to approach to theory and applications of novel industrial developments, updating their professional skills. The course has been structured following a top-down approach [1], beginning with an overview of the wind energy, following with a description of the technological status, and ending with the simulation of a wind energy system. Opposite to other pedagogical approaches that provide the student with ready-touse simulations [10]-[11], the experience promotes hands-on learning by asking the students to build their own simulators step-by-step following an inclusive strategy. Although the use of simulations in power electronics [10] and electrical machines [11] is common in University teaching, most of the
In the first part, the lectures start with the general concepts of wind energy and end with the current trends at the wind energy industry (see table I). During the lectures the technical content is gradually increased and the focus is placed on the topologies, power electronics and electrical machines selected by different manufacturers. The students have the opportunity to review the gearless direct-drive multi-pole choice of Enercon, the medium speed permanent magnet generators with back-to-back 2L-VSCs used by Gamesa, the 3-level Neutral Point Clamped (NPC) VSIs used by Multibrid (now Areva), the diode-rectifier with boost stage used by Vensys or the doubly-fed induction generator for off-shore applications developed by Repower to name a few. Notice that lectures’ content is close to market trends in nowadays wind generation systems. Since the choice of each manufacturer is always a trade-off between different features, the course reviews electrical machines and power electronics characteristics within the specific context of WECS. Fortunately, the variety is so wide that the course can cover many aspects of interest for electric/electronic engineering students or professionals (more details in section III).
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In the second part of the course a specific topology is selected and the students are challenged to build in Matlab/Simulink and PSIM a wind energy conversion system (see table I). The student has to build the whole simulation starting from the scratch, with no pre-built black or blue boxes [13], aiming to promote the development of simulation skills and hands-on work. The simulation follows an inclusive strategy where every new element in the simulation is connected to the previous ones until the whole WECS is simulated. For the sake of generality, the course avoids specific toolboxes and builds every element from its mathematical model. The system selected for simulation includes a permanent magnet synchronous generator (used by Gamesa in its late developments) because of the simplicity of the equations and its industrial interest, a 2-level voltage source inverter and standard FOC control with PWM (more details in section IV).
Use of plot options and review of gear and gearless WECS Simulink implementation of differential equations Practical notions of PI tuning, 3D representation, harmonic calculation Practical notions of modulation, DC link role and control Promote student creativeness and joy
Competence Basic knowledge of WECS Simulation skills Active role and Autonomous learning Environmental concern Connection of technical knowledge Creative thinking Cooperative attitude Awareness of industrial concerns Critical judgment Initiative and problem-solving skills Long-life learning
TABLE I COURSE CONTENT AND SCHEDULE Objective Get to know the wind energy situation worldwide Obtain a general knowledge of non-electrical aspects of wind energy Review the basics about EM and PE for specific needs in the WE industry Acquire consciousness of the wide range of technological solutions Understand the new requirements of the TSO (REE in Spain) Evaluate doubly-fed induction generators (DFIG) in WE systems Understand full-power advantages in off-shore and on-shore WE systems Review the late industry development and show the foreseeable trend in WE Objective Review the bidirectional connection of Matlab and Simulink Use of functions, cascade simulations and knowledge of WE power curves
FAQS about WECS
Lec. hour 2 4
Electrical machines and converters in WECS WECS topologies
4
WECS regulation in Spain
1
Partial-power WECS
2
2
3 Full-power WECS
1
Mechanical system Simulation PMSG model and control
2
VSI model and PWM design
4
Free time for simulation
2
5
TABLE II COMPETENCES TO BE DEVELOPED DURING THE COURSE
During lecture and simulation parts, the course aims to promote and develop many different competences, some of them are briefly summarized in table II.
Part 1: lecture content Wind Energy Situation
Gearbox Simulation
Lectures X
Simulation
X
X X
X X
X X X
X X
X X
X
X
It must be highlighted that the blended methodology that combines theoretical explanation with simulation-based activities can successfully promote different competences. While some of them are overlapped, others are specifically promoted by a different part of the proposed course. III. POWER ELECTRONICS AND ELECTRICAL MACHINES IN WIND ENERGY APPLICATIONS. THEORETICAL BACKGROUND. This first part of the course includes the main theoretical content and it is lectured alternating class speech, technical videos (from manufacturers and institutions) and tests/questions. It is industry oriented and focused on issues of interests for electric/electronic engineers, including:
Future Trends in WECS
2
Part 2: simulation content Matlab/Simulink introduction
Sim. hour 3
1. Wind energy situation: this part provides a context of the application, sequentially describing the penetration of wind energy in the World, Europe, Spain and Andalusia.
Wind Simulation
3
2. FAQS about WECS: this section of the course is devoted to all non-electrical/electronic aspects of the wind energy and it is structured in the form of frequently asked questions
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(FAQS). It covers topics related to history, economy, renewable energy policies, environmental issues, WECS manufacturers, on-shore/off-shore solutions, DC/AC transmission, number of turbine blades, standard speed and power ranges and grid connection issues among others. All these questions aim to provide the student with a general knowledge of the application context, which is usually scarce in technical courses. 3. EM and PE in WECS: although students are familiar with electrical machines and power electronics at a basic level, this section reviews the specific content related to wind energy systems. Notice that some particular electrical machines (e.g. DFIG) and power converters (e.g. multilevel VSIs) not included in the degree curriculum are explained in detail and special attention is paid to them due to their industrial interest. Since the variety of both electrical machines and converters is very wide in wind energy systems, the course includes details of the manufacturer and model of the WECS to increase the motivation and industrial knowledge of the course students. Tables III and IV summarize some market solutions using different EM like squirrel cage (SC), wound rotor (WR) and doubly-fed (DF) induction generators (IG) and permanent magnet synchronous generators (PMSG), and PC in a particular two level (2L) and three-level (3L) voltage source converters (VSC) and boost converters (Boost), good examples of a back-to-back (B2B) configuration and in diode rectifier, boost stage and grid-side VSC configuration (D+VSC). Power range is considered low below 1.8MW and high above 3MW, voltage range is considered low below 1kV and medium above 1kV, and the number of poles is considered low below 8, and high above 50. The tables also show that fixed-speed topologies have no maximum power point tracking (MPPT) or low voltage ride through (LVRT) capability in contrast to full-power topologies.
topologies are more expensive than the fixed-speed counterparts. 4. WECS topologies: one the most interesting features of modern wind energy systems is the current technical competition to develop more accurate, inexpensive and efficient designs. This bunch of technical solutions is used by the lecturer to encourage students to contribute in a challenging field, and concepts related to medium voltage operation, reactive power control, low voltage ride through (LVRT) capability, efficiency, cost, robustness and maintenance are discussed for each considered topology [8][9]. 5. WECS regulation in Spain: as the wind power penetration increases (in March 2011 wind energy was the leading source in Spain with almost 20% of the mix), requirements of transmission system operators (TSO) to wind farms have being tighten in the country. The REE (Spanish TSO) and international conditions related to wind farms disconnection and LVRT are reviewed [14]-[15]. 6. Partial Power WECS: because of the industrial importance of DFIGs for the wind energy industry (around 5070% of the installed wind farms are based on this topology [8][9]), this section is devoted to explain the operation of this type of generators. Since the standard electrical machines course in a degree includes the squirrel cage and the wound rotor induction machine but not the DFIG, the sub and super synchronous modes of operation are detailed, pointing out why this machine suits so well the WECS requirements. LVRT limitations are also analyzed and some new developments towards full-power topologies are studied. TABLE IV POWER ELECTRONICS IN WECS Model
TABLE III ELECTRICAL MACHINES IN WECS Manuf.
Model
Gen. Type
Pole Num
Power Range
Vestas Vestas Gamesa Repow. Gamesa Multibr. Enercon Vensys
V82 V80 G90 5M G10 M500 E112 62
SCIG WRIG DFIG DFIG PMSG PMSG WRIG PMSG
Low Low Low Low Med. Med. High High
Low Med. Med. High High High High Low
Volt Ran ge Low Low Low Low Low Med Low Low
Cont. Astall Pitch Pitch Pitch Pitch Pitch Pitch Pitch
Finally, issues related to price and power quality are included just as indicator to show the student that, for example, D+VSC configuration provides higher total harmonic distortion (THD) than B2B configurations or that full-power
VESV82 VESV80 GAMG90 REP5M GAMG10 M500 0
Conv. Cap. -
Conv. type -
Conv. Conf. -
Power Qual. High
-
-
-
2LVSC 2LVSC 2LVSC 3LVSC 2LVSC 2LBoost 2LVSC 3LBoost
Partial Partial Full Full Full
ENEE112 Full Vensy s-62
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Low
MPPT LVRT No
High
Low
No
B2B
Med.
Med.
Partial
B2B
Med.
Med.
Partial
B2B
Med.
High
Yes
B2B
High
High
Yes
D+ VSC
Low.
Med.
Yes
D+ VSC
Low
Med.
Yes
Price
Fig. 2. WECS with multilevel grid-side converter and six-phase generator.
7. Full-power WECS: this section covers in detail the fullpower topologies with PMSG and IG and with back-to-back and D+VSC configurations. Control schemes are reviewed in depth as they will be implemented in part 2 of the course by the students. Off-shore implementations with HVDC transmission (promoted in the Marco Program of the European Union) are also compared with traditional on-shore wind farms with AC connection. 8. Future trends in WECS: the theoretical part is finally completed showing the industry trends and future developments (to the best knowledge of the lecturer) in the wind energy field. Possibilities including distributed power converters, multiple stator windings, distributed drive trains [8], high temperature superconductor generator (e.g. WindTec 10 MW Sea Titan) [16], multilevel grid-side converters and multiphase generators [17] are presented as promising candidates to replace existing topologies. Some new developments [16] and research proposal [17] including modern types of generators and converters are shown to challenge the student and promote curiosity. Figure 2 shows a novel energy conversion system based on a multiphase system [18]-[20] using a PMSG and a medium-voltage multilevel converter proposed in [17]. To summarize, some of the main features of part 1 are: The theoretical explanation provides a context that includes non-electric/electronic multidisciplinary aspects. Industrial aspects are emphasized showing wind energy manufacturers’ catalogues and videos where some of the hottest technical aspects in the field can be identified. It is shown the wide variety of technical solutions, including the Vestas V112 generator model and all the technical solutions adopted by Vestas and other manufacturers.
Teacher-centered methodology is broken including more than 100 test questions and short problems within the lectures and building a competition among groups of students. IV. CONCLUSIONS Although the topics related to electrical machines and power electronics can be taught out of context, designing a top-tobottom approach that starts in the application and descends to the mathematical models of its components can be advantageous from a pedagogical point of view. The motivation is increased when the knowledge that is taught is immediately used to solve a real industrial problem. With this idea in mind, this part 1 of the work has shown that wind energy conversion systems (WECS) provide such a wide range of technical solutions that makes this application a good candidate to teach electrical engineers. The second part of this work will show the assessment of both parts showing not only a good acceptance among the students but also a good performance in terms of knowledge and acquired competences. ACKNOWLEDGEMENT The authors gratefully acknowledge the Spanish Government and the European Union (FEDER funds) for the economical support provided within the National Research, Development and Innovation Plan, DPI2011-25396. REFERENCES [1] [2] [3]
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