To my parentsMaria Helena and José Maria

List of Papers

This thesis is based on the following papers, which are referred to in the textby their Roman numerals.

I J. G. Oliveira, H. Bernhoff, “Battery recharging issue for a two-power-level flywheel system”. Journal of Electrical and Computer Engineer-ing, Vol. 2010, Article ID 470525, 5 pages, 2010.

II J. G. Oliveira, J. Lundin, J. Santiago, H. Bernhoff, “A double woundflywheel system under standard drive cycles: simulations and experi-ments”. International Journal of Emerging Electric Power Systems, Vol.11, Iss. 4, Article 6, 2010.

III J. G. Oliveira, J. Abrahamsson, H. Bernhoff, “Battery dischargingpower control in a double-wound flywheel system applied to electricvehicles”. International Journal of Emerging Electric Power Systems,Vol. 12, Iss. 1, Article 7, 2011.

IV J. G. Oliveira, J. Lundin, H. Bernhoff, “Power balance control in anAC/DC/AC converter for regenerative braking in a two-voltage-levelflywheel based driveline”. Accepted for publication in InternationalJournal of Vehicular Technology, August 2011.

V J. G. Oliveira, H. Schettino, V. Gama, R. Carvalho, H. Bernhoff,“A study on doubly fed flywheel machine based driveline withan AC/DC/AC converter”. Submitted to IET Electrical Systems inTransportation, 2011.

VI J. G. Oliveira, A. Larsson, H. Bernhoff, “Controlling a permanent mag-net motor using PWM converter in flywheel energy storage systems”.Proceedings of the 34th Annual Conference of the IEEE Industrial Elec-tronics Society, Orlando, USA, pp. 3364-3369, 2009.

VII J. G Oliveira, H. Bernhoff, “Power electronics and control oftwo-voltage-level flywheel based all-electric driveline”. Proceedings ofthe IEEE International Symposium on Industrial Electronics, Gdansk,Poland, pp. 1-7, 2011.

VIII J. Santiago, J. G. Oliveira, J. Lundin, J. Abrahamsson, A. Larsson,H. Bernhoff, “Design parameters calculation of a novel driveline forelectric vehicles”. World Electric Vehicle Journal, Vol. 3, ISSN 2032-6653-2009.

IX M. Hedlund, J. G. Oliveira, H. Bernhoff, “Sliding Mode 4-QuadrantDC/DC Converter for a Flywheel Application”. Submitted to ControlEngineering Practice, 2011.

X J. Abrahamsson, J. Santiago, J. G. Oliveira, J. Lundin, H. Bernhoff,“Prototype of electric driveline with magnetically levitated doublewound motor”. Proceedings of the International Conference onElectrical Machines, Rome, Italy, pp. 1-5, 2010.

XI H. Schettino, V. Gama, R. Carvalho, J. G. Oliveira, H. Bernhoff, “Im-plementation and control of an AC/DC/AC converter for double woundflywheel application”. Accepted for publication in Proceedings of theIEEE International Conference on Control and Automation, Santiago,Chile, 2011.

Reprints were made with permission from the publishers.

The author has contributed to the following papers which are not included in thethesis.

XII J. G. Oliveira, R. Carvalho, V. Gama, H. Schettino, H. Bernhoff, “Im-plementation of an AC/DC/AC converter for electric vehicle applica-tion”. Accepted for publication in Proceedings of the IV Brazilian Con-ference on Energy Efficiency, Juiz de Fora, Brazil, 2011.

XIII J. Santiago, J. G. Oliveira, J. Lundin, A. Larsson, H. Bernhoff, “Lossesin axial-flux permanent-magnet coreless flywheel energy storage sys-tems”. Proceedings of the 18th International Conference on ElectricalMachines, Vilamoura, Portugal, pp. 1-5, 2008.

XIV J. Santiago, J. G. Oliveira, J. Lundin, J. Abrahamsson, A. Larsson andH. Bernhoff, “Design parameters calculation of a novel driveline forelectric vehicles”. Proceedings of the EVS- 24th International Battery,Hybrid and Fuel Cell Electric Vehicle Symposium and Exhibition, Sta-vanger, Norway, 2009.

XV J. Lundin, J. G. Oliveira, C. Bostrom, K. Yuen, J. Kjeilin, M. Rahm,H. Bernhoff, M. Leijon, “Dynamic stability of an electricity genera-tion system based on renewable energy”. Proceedings of the Interna-tional Conference on Electricity Distribution, Frankfurt, Germany, Pa-per 0940, pp. 1-4, 2011.

XVI J. Santiago, J. G. Oliveira, H. Bernhoff, “Filter influence in rotor lossesin coreless axial flux permanent magnet machines”. Submitted to Jour-nal of Electrical Systems, 2011.

Book Chapter

XVII J. Santiago, J. G. Oliveira, Electric machines topologies in energystorage systems. Chapter 1 in Energy Storage, edited by Rafiqul IslamSheikh, Sciyo, 2010.

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.1 Flywheels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.2 Flywheel Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3 Vehicular Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.3.1 Energy Storage Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 161.3.2 Applications of Flywheels . . . . . . . . . . . . . . . . . . . . . . . . 171.3.3 Power Buffer Technologies . . . . . . . . . . . . . . . . . . . . . . . 181.3.4 Safety and Gyroscopic Forces . . . . . . . . . . . . . . . . . . . . . 20

1.4 Two-Power-Level Flywheel System . . . . . . . . . . . . . . . . . . . . . 201.4.1 Double Wound Flywheel Machine and Applications . . . . . 211.4.2 Application Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.1 Energy, Power and Torque in Flywheel Systems . . . . . . . . . . . . 252.2 Electric Machines Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.2.1 Two-Voltage-Level Machine Equations . . . . . . . . . . . . . . 262.2.2 Mathematical Model of a Permanent Magnet

Synchronous Machine Drive . . . . . . . . . . . . . . . . . . . . . . 282.3 Power Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.3.1 AC/DC Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.3.2 DC/AC Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.3.3 DC/DC Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.4 Control of AC Machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.4.1 Scalar V/F Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.4.2 Vector Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2.5 Switching Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.5.1 Pulse Width Modulation . . . . . . . . . . . . . . . . . . . . . . . . . 342.5.2 Space Vector Modulation . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.6 Semiconductor Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.6.1 Losses in Semiconductor Devices . . . . . . . . . . . . . . . . . . 36

2.7 Control Systems Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373 Components Description and Control Strategies . . . . . . . . . . . . . . . 39

3.1 System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.2 Battery Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.2.1 Unidirectional DC/DC Converter . . . . . . . . . . . . . . . . . . . 413.2.2 Bidirectional DC/DC Converter . . . . . . . . . . . . . . . . . . . . 413.2.3 ON/OFF Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.3 Flywheel Charge Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.4 Drive Mode Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.4.1 AC/DC Converter Control . . . . . . . . . . . . . . . . . . . . . . . . 463.4.2 DC/AC Converter Control . . . . . . . . . . . . . . . . . . . . . . . . 47

3.5 Braking Mode Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.1 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494.1.1 PID Controller Design . . . . . . . . . . . . . . . . . . . . . . . . . . . 494.1.2 Drive Cycles Investigation . . . . . . . . . . . . . . . . . . . . . . . . 51

4.2 Experimental Set-Ups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514.2.1 Flywheel Charging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524.2.2 Loaded Flywheel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 534.2.3 Flywheel Driveline with DC Wheel Machine . . . . . . . . . . 554.2.4 Complete Driveline Set-Up . . . . . . . . . . . . . . . . . . . . . . . 574.2.5 Measurement System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5 Summary of the Results and Discussions . . . . . . . . . . . . . . . . . . . . 615.1 Battery Charging System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

5.1.1 Unidirectional Converter . . . . . . . . . . . . . . . . . . . . . . . . . 615.1.2 Bidirectional Converter . . . . . . . . . . . . . . . . . . . . . . . . . . 63

5.2 Battery Discharging Control . . . . . . . . . . . . . . . . . . . . . . . . . . 655.2.1 Discharging Simulations . . . . . . . . . . . . . . . . . . . . . . . . . 655.2.2 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

5.3 Full System connected to Variable Resistive Load . . . . . . . . . . 695.4 Full System connected to AC Machine . . . . . . . . . . . . . . . . . . . 70

5.4.1 Traction Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 705.4.2 Braking Mode Simulations . . . . . . . . . . . . . . . . . . . . . . . . 73

5.5 Driveline Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 765.5.1 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 765.5.2 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 797 Suggestions for Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 818 Summary of Papers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 839 Svensk Sammanfattning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8910 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Nomenclature

B [Nms] Friction factorC [F] CapacitanceCw [-] Air drag coefficientD [-] Duty ratioeind [V] Induced voltageEn [J] Energy in flywheelEA, EH , EL [V] Internal voltagef [Hz] Electric frequencyfs [Hz] Switching frequencyid , iq [A] Direct and quadrature axis currentI, IH , IL [A] Phase currentI f [A] Field currentI0 [A] Current through a switchJ [kg ·m2] Moment of inertiakw [-] Constant representing construction of the machineKp [-] Proportional gainKi [-] Integral gainKd [-] Derivative gainL, LH , LL [H] Internal inductanceLd , Lq [H] Direct and quadrature axis inductanceM [H] Mutual InductanceN [-] Number of coils exposed to the same magnetic flux variationp [-] Number of pole pairsP [W] PowerPs [W] Switching losses

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Pon [W] On-state lossesR, RH , RL [Ω] Internal resistanceR0 [Ω] Resistive loadT , Te, Tl [Nm] Torquetc(on) [s] Turn-on timetc(o f f ) [s] Turn-off timeton [s] Conducting timeTs [s] Switching periodV , VH , VL [V] Phase voltagevd , vq [V] Direct and quadrature axis voltageVd [V] Voltage over a switchVon [V] On-state voltageδ [-] Hysteresis bandζ [-] Damping factorξ [-] Swift angle between winding setsσ [-] Error signalφ [Wb] Magnetic fluxω [rad/s] Rotational speedω0 [rad/s] Natural frequency

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Abbreviations

AC Alternating CurrentCPS Constant Pressure SystemCVT Continuously Variable TransmissionDC Direct CurrentDSC Digital Signal ControllerDSP Digital Signal ProcessorEMF Electro-Motive ForceESR Equivalent Series ResistanceEVs Electric VehiclesFESS Flywheel Energy Storage SystemHP High PowerICE Internal Combustion EngineIGBT Insulated-Gate Bipolar TransistorLP Low PowerMCU Micro Control UnitMMF Magneto-Motive ForceMOSFET Metal-Oxide-Semiconductor Field-Effect TransistorPCS Power Converter SystemPID Proportional-Integral-Derivative ControllerPMSM Permament Magnet Synchronous MotorPV PhotovoltaicPWM Pulse Width ModulationRPM Rotations Per MinuteSVM Space Vector ModulationTHD Total Harmonic DistortionTVLM Two-Voltage-Level Machine

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1. Introduction

Energy and power: words that are spoken more and more frequently nowa-days. There are several ways of generating usable energy, but once the energyis converted it is also important to be able to store it, allowing humans tobalance the supply and demand of energy.

Many countries have been changing their policies in order to develop tech-nologies aiming for more sustainable energy systems. Efficient and reliableElectric Vehicles (EVs) will contribute as a key technology in this transfor-mation. However, electric energy storage components are still limited due totheir low energy density and long recharge time when compared to internalcombustion engine vehicles [1].

The storage of energy in an efficient and secure way is a very importantissue and it dates from ancient times. Today, the commercial use of energystorage systems can be broadly categorized as mechanical, electrical, chemi-cal, biological, thermal and nuclear [2].

In this chapter, a basic introduction to flywheel systems is given, along witha description of the flywheel research project at Uppsala University. The ap-plication context of the work done within this thesis is also discussed.

1.1 FlywheelsFlywheel Energy Storage Systems (FESS) are classed in the group of me-chanical storage systems [3]. The principle of energy storage with a flywheelis not new. It is based on the rotating mass principle. A flywheel stores kineticenergy of rotation, where the stored energy depends on the moment of inertiaand the rotational speed of the flywheel.

The history of flywheels goes back thousands of years. The potter’s wheeland the spinning wheel are two examples where the flywheel, with its inertia,has converted a pulsating input power to a smooth output power. Early FESSwere purely mechanical, consisting of only a stone attached to an axle, asshown in Figure 1.1.

Early publications about the application of flywheels date from the begin-ning of the 20th century. A study of the inertia of the rotating parts of a train,specially the flywheel capacity of armatures with small diameter, was pre-sented by N. W. Storer, 1902 [4].

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A modification of the Ward-Leonard system of speed control [5], known asIlgner System, used a heavy flywheel on the motor-generator shaft to smoothout peak loads, which would otherwise be taken from the power supply. In1907, A. P. Wood [6] studied different ways of using Ilgner systems, so that a3-phase motor could be worked in case of failure of the flywheel system.

Figure 1.1: Example of an old application of FESS [7].

The application of flywheel as a load equalizer was described in 1909 byJ. S. Peck [8]. Peck claimed that flywheels have long been used as a loadequaliser, being cheaper, more efficient, and in general better suited than stor-age batteries.

Over time, the utilization of traditional flywheels decreased with the devel-opment of the electric grid. However, the technology came around again afterundergoing a round of improvements in materials, magnetic bearing control,and power electronics [9].

The energy stored in flywheels can be transferred in or out by using anelectric machine, which is mechanically connected. Hence, the flywheel canbe accelerated by the machine acting as a motor when it is supplied with elec-tric energy. Inversely, the machine acting as a generator can provide electricalenergy by slowing down the flywheel.

Progress in power electronics makes it possible to operate flywheels at highpower, with a power electronics unit comparable in size to the flywheel it-self or smaller. Composite materials enables high rotational velocity. Mag-netic bearings and vacuum operation offer very low friction during long-termstorage and longer life expectancy for high rotational speeds. High speed isdesirable since the energy stored is proportional to the square of the speed butonly linearly proportional to the mass [10].

A basic layout of the structure of a modern flywheel is shown in Figure 1.2.

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Figure 1.2: Basic layout of a modern flywheel energy storage system.

1.2 Flywheel TechnologiesSeveral hundreds of years ago, purely mechanical flywheels were used to keepmachines running smoothly from cycle to cycle. Later on, in the 1950’s anearly example of a power generating flywheel system is the "Gyrobus", pro-duced in Switzerland, powered by a 1500 kg flywheel [11]. However, the de-velopment of modern flywheels started in the 1970’s, when NASA sponsoredprograms proposing energy storage flywheels as possible primary sources forspace missions [12].

The fast response time of flywheels make them suitable for different appli-cations in power systems. Flywheels have been used for harmonic compen-sation, being able to reduce them about 50% up to the 11th harmonic [13].Companies from Europe and USA have developed flywheels with the purposeof keeping the power quality; providing ride through for momentary poweroutages, reducing harmonic distortions, eliminating voltage sags, etc [14]. Forexample, Piller GmbH has installed flywheel energy storage system which canabsorb or supply 5 MW for 5 s in Dresden, Germany. Active Power (Austin,Texas) has produced 4.75 MW flywheels for power conditioning and protec-tion against power outages [15]. In Japan, a 200 MJ flywheel energy storagesystem has been used for eliminating fluctuations in the active power suppliedto the magnets in the High-Energy Accelerator Research Organization [16].Finally, in distribution network, a 10 MJ flywheel energy storage system, usedto maintain high quality electric power, managed to keep the voltage in the dis-

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tribution network between 98-102% and was capable of supplying 10 kW for15 min [17].

The storage of electricity generated by renewable sources is suitable for fly-wheels, since it can be used to match the fluctuating supply to the changingdemand of energy. A wind-diesel generator with flywheel energy storage hasbeen reported [18] with the goal of creating a unit where the regular windoscillations are compensated by the diesel generator and the flywheel. A mo-tor flywheel integrated with a photovoltaic system has been simulated and itmanaged to prolong the load supply from 9 a.m. to 3 p.m. to 8 a.m. beyond 6p.m. [19].

In space applications, the International Space Station [20,21] uses the sun asa primary power source. A Permanent Magnet (PM) motor/generator flywheelhas been simulated to keep the station functional during eclipses. In 1994,The NASA Glenn Research Center devoted new efforts to develop flywheelsystems on satellites, combining energy storage capability and attitude control[20, 22]. Nowadays, each NASA flywheel unit can store in excess of 15 MJand can deliver a peak power of 4.1 kW.

Other flywheel applications include aircraft launch systems [23] and pulsedpower systems [24].

Flywheels are able to absorb and deliver high power with high efficiency.These advantages have made them a very appealing choice for vehicular appli-cations, whose interest has increased over the years because of environmentalissues and projected shortage of oil. Since the vehicular application is the fo-cus of the present thesis, it will be discussed in more detail in the next section.

1.3 Vehicular TechnologyElectric vehicles were quite commonplace during the end of the 19th century.However, later on, they were completely abandoned in favour of internal com-bustion engine (ICE) vehicles. At that time, no one could envision the day theworld could run out of its fossil fuels reserves. But after many decades of highconsumption and increased concern over the environmental impact, the needfor new vehicular technologies became urgent.

Hybrid vehicles or pure electric vehicles are becoming more popular [25].The first, as the name indicates, are not 100% electric and still use a conven-tional ICE propulsion system combined with an electric propulsion system.

1.3.1 Energy Storage SystemsThe traditional disadvantages of EVs compared to internal combustion en-gine vehicles are limited driving range and relatively long time needed torecharge [26]. Even modern batteries have an energy density roughly up totwo orders of magnitude smaller than those of fossil fuels and a limited power

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density preventing rapid charging. To overcome these difficulties, differenttechnologies have been investigated including more powerful batteries, super-capacitors and flywheels [27].

Batteries

Flywheels

Ultracapacitors

Electrolyticcapacitors

Filmcapacitors

10 s3

10 s2

10 s1

10 s0

10s-1

10s-2

10s-3

101

102

103

104

105

106

107

10-2

10-1

100

101

102

103

Sp

ecif

icen

erg

y(W

h/k

g)

Specific power (W/kg)

Figure 1.3: Estimation of specific energy vs. specific power for different energy stor-age devices [28].

An estimation of the expected power/energy capabilities for different ve-hicular technologies in near future is shown in Figure 1.3. As illustrated, bat-teries have higher specific energy, but lower specific power, although the pic-ture can slightly change depending on the battery technology. Ultracapacitors(followed by electrolytic and film capacitors) have higher specific power butlower specific energy. Flywheels can combine reasonable specific power andspecific energy, as shown in Figure 1.3. However, none of the presented tech-nologies approaches the numbers for specific energy or power of fossil fuelspowered cars, which are around 104 Wh/kg and 105 W/kg respectively [29].

1.3.2 Applications of FlywheelsA FESS installed in a hybrid bus has been tested at the University of Texas atAustin. The unit accelerates a fully loaded bus to 100 km/h, stores about 7.2MJ and has a peak power capability of 150 kW, as well as a specific energyof more than 120 kJ/kg of rotating mass and a specific power of 2.5 kW/kg ofrotating mass [30].

A new conceptual hybrid EV equipped with flywheel and photovoltaic (PV)cell has been reported [31]. By employing the flywheel and PV cell as energyregeneration unit, the electric power consumption rate of the vehicle can be

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188 km/l in the community-driving schedule, and over 50 km/l in the longdriving schedules (the electric power consumption rate has been converted tothe fuel consumption rate of gasoline).

A novel flywheel-engine hybrid system employing Constant Pressure Sys-tem (CPS) to replace complex systems such as a planetary gear set or Contin-uously Variable Transmissions (CVTs) has been proposed [32].

The US Federal Railroad Administration has a program to develop FESSfor high speed rail applications. A CVT power train is used with a planetarygear set and compact steel flywheel [33]. The flywheel plays a part only intransient situations by compensating the engine inertia, making it possible tooptimize fuel economy in stationary situations without losing driveability intransients.

In South Africa, flywheel systems for the purpose of allowing trams to op-erate beyond wires have been supplied by Alstom APS Flywheel Systems[34, 35].

There is a particular interest in applying FESS in pulsed-power systemsincluded in all-electric/hybrid combat vehicles. In military affairs, the recentreleased modernization plans for both U.S Navy and U.S. Army indicate theirintention to depend more heavily on electricity for both ships and ground ve-hicles [3].

Recent progress in the area of vehicular technology has been driven by Fly-brid Systems [36]. The use a high-speed flywheel system for acceleration andenergy recovering during braking has been allowed since 2009 in FormulaOne World Championship, where the system manufactured by Flybrid Sys-tems has been used. The high-speed flywheel works completely mechanically(using CVT and fixed gears), being capable of storing 400 kJ.

1.3.3 Power Buffer TechnologiesThe combination of a primary energy source, e.g. batteries, and a power buffercan be used to meet the peak energy/power requirements of an electric vehicle.Electric vehicle traction systems that combine a supercapacitor or flywheelpeak power buffer with the battery energy source, also called dual powersources, have been evaluated [37].

A simple idealized power management scheme can be implemented withinthe model such that:

i) the buffer unit normally supplies or absorbs the peak power;ii) the battery supplies the average power;The power demand simulated at wheel shafts of an ordinary vehicle during

a standard FTP 75 (Federal Test Procedure) urban drive cycle is shown inFigure 1.4. The vehicle considered for this simulation has a mass of 1500kg, a dimensionless drag coefficient Cw of 1.35, and a frontal area of 1.73m2. The power demand varies from 34 kW (when accelerating) to -26 kW(when braking). However, the average electric power from the energy storage,

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needed to propel an ordinary vehicle according to a standard FTP 75 urbandrive cycle, is about 2.2 kW (not considering the internal losses of the system),i.e. less than one tenth of the maximum power needed during the drive cycle.A power buffer could handle all great variations in power to/from the wheelsinstead of transferring them to the battery.

Figure 1.4: The power-time graph of the FTP-75 drive cycle.

The battery-supercapacitor combination for vehicular applications has beenreported in the literature [38–41]. Results claim that supercapacitors offer highefficiency (around 90%) and can be charged and discharged a large number oftimes without performance deterioration. However, the supercapacitor kWhcost is estimated to be between 10000-20000$/kWh. Flywheels, on the otherhand, have an estimated kWh cost of 500-1000$/kWh [42, 43]. Furthermore,flywheels offer steady voltage and power level, independent of load, tempera-ture or state of charge; no chemistry included, thus no environmental pollutionassociated and efficiency and life cycles similar to the ones presented for su-percapacitors [44, 45].

FlywheelElectric Machine

Battery

Vehicle DriveSystem

DC Link

Peak Power Transfer

Figure 1.5: Example of a driveline incorporating a flywheel system.

Three systems with different specifications and based on using the bat-tery during normal driving condition and the flywheel during acceleration andbraking situations have been reported [46–48]. A diagram of such system isshown in Figure 1.5. The battery and flywheel are connected to the DC-link

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and the flywheel is responsible for absorbing the peak power transfer in thesystem.

1.3.4 Safety and Gyroscopic ForcesSafety is a natural concern with FESS when in close proximity to people[49]. An inertial containment system becomes necessary to minimize col-lateral damage in case of failure. Several safety projects have been fundedin the United States by the Defense Advanced Research Projects (DARPA),the Houston Metro Transit Authority and NASA [50]. Flywheels have beendesigned and operated with safe failure mode. Shock and vibration test ofan active magnetic bearing supported energy storage flywheel have been re-ported [51].

The gyroscopic forces are important for flywheels situated in vehicles,satellites or space stations. The energy content of a rotor increases as thesquare of the angular velocity, whereas the corresponding gyroscopicmoment increases linearly [52]. Gyroscopic forces would not present a riskfor a vehicle with a suspended flywheel at high rotational speeds, but theeffect should be considered when designing the system.

If necessary, one way to cope with the interaction of the forces in a vehicleis to place the flywheel in a gimbal system, keeping the relative position of theflywheel when the vehicle turns or lean, as shown in Figure 1.6.

1.4 Two-Power-Level Flywheel SystemMany flywheel systems are under development around the world. Differentapplications in EVs or hybrid vehicles have been investigated. However, exis-tent flywheel-battery systems (dual power sources systems that combine bat-tery and flywheel) have both sources placed on the same voltage level [46–48],as shown in Figure 1.5. This could decrease the efficiency of the system dueto battery voltage limitations.

The flywheel project at Uppsala University has its novelty in the drivelinetopology which is divided in two different power and voltage levels. The High-Power (HP) side is connected to wheel motor and the Low-Power (LP) sideis connected to the battery, see Figure 1.6. The key component of this systemis a Two-Voltage-Level-Machine (TVLM), with stator windings arranged todivide the system in two different voltage levels, similar to an electric trans-former. In this configuration, an efficient system which handles the powerdeveloped during fast dynamical processes is provided [53].

20

Figure 1.6: The EV propulsion system based upon a flywheel energy storage devicewith two power levels.

1.4.1 Double Wound Flywheel Machine and ApplicationsThe flywheel system under development at Uppsala University is based on adouble wound synchronous flywheel machine. The novelty lies in the config-uration of the stator, which has two sets of three-phase windings with a differ-ent number of turns. The two sets of windings are magnetically coupled andthe transformer characteristic has to be taken into account. Thus, the TVLMcan operate as a motor and a generator between two power buses at differentpower rating [54].

Other applications of similar machines can be found in the literature. In thelate 1920’s, six phase synchronous machines were used for power generation[55]. The extra phases were needed to overcome the limitation imposed byfault currents interrupting the capacity of circuit breakers.

In 1983, with increased demand for higher power drive systems, six phasestator machines helped to overcome the current limitation imposed by semi-conductor devices. These six phase drive systems improved torque and mag-netomotive force (MMF) characteristics over those of standard three phaseinverter drive systems [56].

Double wound synchronous machine systems were also used as DC to ACmotor/generators in the field of electric railways, in order to improve serviceto supply the air conditioners from the DC supply (pantograph) [57].

A new concept of rotating machines that enable direct connection of syn-chronous generators to the transmission network without any intervening step-up transformers was developed by Leijon et al. [58]. Such a synchronous ma-chine, called Powerformer, has the possibility of simultaneous direct connec-

21

tion to several different grid voltages through different stator windings. Thesecondary stator winding can also be used for power supply at the standardmedium and low-voltage levels to feed power plant auxiliaries.

The Optimal Flywheel Power Module [23], manufactured by Optimal En-ergy systems, is used to provide pulses of energy for charging high voltagecapacitors in a mobile military system. As in the present flywheel system, alow power side in the machine is used to receive energy from a DC bus. Highoutput power is provided on the secondary side.

1.4.2 Application ContextThe here investigated flywheel system is physically divided in two power lev-els through the flywheel machine stator windings. Each side connects the fly-wheel machine to another component of the system (e.g. battery or wheelmotor). The connection is made through electrical power converters, whichconvert input/output signal to the shape and frequency needed for couplingthe system. The power converters are controlled so the desired energy flow inthe battery-flywheel and flywheel-wheel machine link is obtained.

A diagram of the complete flywheel system is shown in Figure1.7. It contains three different bidirectional Power Converter Systems(PCS): an AC/DC/AC converter on the HP side, connecting the flywheelmotor/generator to the wheel motor and a DC/AC plus a DC/DC converter onthe LP side, connecting the flywheel motor/generator to the battery.

Wheelmotor

AC/DC/ACConverter

AC/DCConverter Battery

Flywheel

Motor/GeneratorC

C

C

Connected to the control unit

High Power Side (HP) Low Power Side (LP)

DC/DCConverter

C

Figure 1.7: Power electronics and control of the proposed EV propulsion system.

The present thesis focuses on the field of electrical engineering and, morespecifically, on the electric power conversion system and control. This thesistreats the design, simulation and construction of the power converter systemsand their control strategy. It addresses the complete assembled driveline.

22

The control and power electronics are very important for the present sys-tem. A correct control strategy is required to provide efficient and robust func-tionality of the driveline. The flywheel system has, in comparison to otherflywheel-based drivelines, a large number of power electronic converters andconsequently, control systems. The power converters should be designed sothe maximal possible efficiency in the driveline is achieved. Nonetheless, theconnection of two electrical machines with different and variable frequencyand amplitude of operation (i.e. the flywheel and the wheel motor) is particu-larly challenging.

Considering the importance of the electronic converters and their controlfor the system [59], this thesis aims to present and discuss the proposed powerelectronics and control strategy used in the two-power-level flywheel system.The results are presented based on the system functionality.

This thesis is organized as follows: Chapter 2, Theory, covers part of thetheoretical background of the thesis, describing the basic theory of electri-cal machines and the main approaches in control of AC machines. Chapter 3,Components Description and Control Strategies, describes the different powerconverters investigated and the suggested control strategies. Chapter 4, Meth-ods, treats the simulations tools and the experimental set-ups which were im-plemented. Chapter 5, Summary of the Results, discusses the most importantresults published in the papers attached to this thesis. Chapter 6, Conclusions,summarize the results and discussions. Finally, Chapter 7, Future Work, com-ments the plans to be continued after the presented work.

23

2. Theory

This chapter gives a theoretical background to different areas presented in thethesis. The first section presents general equations behind the flywheel energystorage functionality. The second presents a brief description of electric ma-chines theory. The third, fourth and fifth sections discuss the theory of powerelectronics and control systems which have been applied in this thesis.

2.1 Energy, Power and Torque in Flywheel SystemsThe principle of energy storage with a flywheel is not new. It is based onthe same principle as the potter’s wheel: a rotating mass. The flywheel storeskinetic energy of rotation, where the stored energy En depends on the momentof inertia J and the rotational speed ω [60]:

En =12

Jω2 (2.1)

The flywheel can be used as a power handling device, in result from ad-vances in key enabling technologies. Power is the rate at which energy is con-verted, given by:

P =∆En

∆t=

12

Jω2

2 −ω21

∆t(2.2)

The torque T for a given (instantaneous) power output in electrical ma-chines can be calculated as:

T =Pω

(2.3)

Note that the instantaneous power injected by the torque depends only onthe instantaneous angular speed and not on whether the angular speed in-creases, decreases, or remains constant while the torque is being applied.

25

2.2 Electric Machines TheoryAn electric machine converts either mechanical energy to electrical energy(generator) or electrical energy to mechanical energy (motor). The principleof operation is based on interaction between the magnetic fields existent in theinterior [61]. Concerning the generator, the rotating magnetic field of the rotorinduces three-phase AC voltages into the stator armature windings, accordingto the Faraday’s law:

eind =−Ndφ

dt(2.4)

where N is the number of turns in the windings and φ is the magnetic fluxpassing through the windings. The RMS voltage in any phase of a three-phasestator is:

EA =√

2πkwNφ f (2.5)

where kw is a constant representing the construction of the machine and f isthe electrical frequency.

Conversely in motors, a three-phase set current in the stator armature wind-ings produces a rotating magnetic field which interacts with the rotor magneticfield, producing a torque.

The AC electrical machines are divided into synchronous andasynchronous. Synchronous machines are usually more efficient thanasynchronous machines and can more easily accommodate load power factorvariations. Permanent magnet synchronous machines do not require rotorfield excitation. These advantages of synchronous machines make themsuitable for electric vehicular applications [62].

A basic four pole synchronous machine is illustrated in Figure 2.1. Therotor consists of salient poles which are wound with coils. The stator is slottedto accommodate three sets of stator coils, displaced circumferentially at 120

intervals. A direct current I f is supplied to the rotor field winding.

2.2.1 Two-Voltage-Level Machine EquationsThe two-power-level system presented in this thesis is obtained by usinga double-fed synchronous machine. The equivalent circuit of the TVLMis shown in Figure 2.2. The windings’ neutral points are not necessarilyconnected. The rotor is magnetically linked to both sets of stator windings,but the stator windings are also magnetically linked to each other constitutinga transformer. The voltage is, in this case, the result of both the mutualmagnetic coupling and the electromotive force induced by the rotor.

The three-phase high power windings are represented in black whereas thelow power windings are represented in red in Figure 2.2. The current in dif-

26

Figure 2.1: Basic four pole, three-phase synchronous machine with rotor field excita-tion.

Figure 2.2: Schematics of the two sets of three-phase windings in a two-voltage-levelmachine.

ferent phases is represented by i and Va is the line to neutral voltage in phasea. Internal resistance and inductance are indicated by R and L, meantime Mrepresents the mutual inductance. ξ is the swift angle between winding sets.

The equation that governs the equivalent electric circuit of a permanentmagnet synchronous machine is:

V = RI +LdIdt

+EA (2.6)

where V is the machine output voltage, I is the stator current and the backElectro-Motive Force (EMF) EA can be calculated from Equation 2.5.

The TVLM can be evaluated as two separate synchronous machines withmagnetic coupling and common rotor speed. To represent the various opera-

27

tional models in the equivalent circuit, different loads are coupled to the con-nections of the machine. The most common operation mode of the TVLM iswhen the high power side is acting as a generator and the low power side as amotor; see Figure 2.3.

HE cos ( t+ )w x LE cos ( t)w

RH

RL

V

LO

AD

M

LH

LL

LV

HHigh Power

sideLow Power

side

Figure 2.3: Equivalent circuit of a TVLM. The low power side is acting as a motor,while the high power side is acting as a generator.

For the operational mode shown in Figure 2.3, Equation 2.6 can be re-written as:

VH =

(RH +LH

dIH

dt

)+

dMIL

dt+EH (2.7)

VL =

(RL +LL

dIL

dt

)+

dMIH

dt+EL (2.8)

where R, L and E represent the internal resistance, internal inductance andback EMF, respectively, both for the high- and low-power sides. V representsthe output voltage on the high power side (acting as a generator) and the ap-plied voltage on the low power side (acting as a motor). M represents themutual inductance.

The coupling condition for the voltage equations sets the same frequencyof both sides:

ω = ωH = ωL (2.9)

2.2.2 Mathematical Model of a Permanent Magnet SynchronousMachine DriveThe d-q transformation is a mathematical transformation used to reduce thethree-phase stationary coordinate system to the d-q rotating coordinate sys-tem [63]. In the case of permanent magnet synchronous motors (PMSM),which are described by a multivariable, coupled and nonlinear model, d-q

28

transformation is used to transform these nonlinear equations into a simpli-fied linear state model. The voltage equations of the PMSM in the rotatingreference frame are [64]:

vd = Rid +Lddiddt−ωLqiq (2.10)

vq = Riq +Lqdiqdt

+ωφ (2.11)

The electromagnetic torque Te can be written as

Te =32

p2[φ iq +(Ld−Lq) iqid ] (2.12)

where vq, vd , iq, id are the stator voltages and currents respectively. R is thestator resistance. Lq and Ld are the d-q axis stator inductances respectively. φ

is the rotor flux. p is the number of pole pairs and ω is the electrical speed ofthe motor.

The torque can be related to the d- and q-axes currents, to the rotor type, itsinductances Lq and Ld and to the magnets mounted on the rotor, as expressedin Equation 2.12. The electromechanical equation of a PMSM is given by:

p2(Te−Tl) = J

dω

dt+Bω (2.13)

where Tl , J and B represent the load torque, the inertia and the friction factorof the motor respectively.

2.3 Power ConvertersPower converters are an application of solid state electronics for the controland conversion of electric power. Power electronic converters can be foundwherever there is a need to modify a form of electrical energy (i.e. change itsvoltage, current or frequency).

Power conversion systems can be classified according to the type of theinput and output power: DC to AC (inverter), AC to DC (rectifier), DC to DCand AC to AC.

There are different types of AC to AC conversion [65]. The present workdeals with a back-to-back converter, which is composed of an indirect AC/ACconverter connected via DC-link capacitor. The AC/DC/AC converter is alsocalled rectifier-inverter pair and can be studied as separated AC/DC andDC/AC converters.

29

2.3.1 AC/DC ConvertersRectifiers are mainly divided into passive and active converters. Passive recti-fiers use diodes to perform the signal conversion whereas active rectifiers useswitching devices (e.g. thyristors or transistors). Forced-commutated rectifiersare built with semiconductors with gate-turn-off capability (transistors). Themain circuit of a force-commutated rectifier is shown in Figure 2.4. They arebidirectional converters and can also be used as inverters when reverse powerflow is obtained [66].

Figure 2.4: Main circuit of PWM rectifier, connected to a three-phase voltage source.

The voltage source rectifier operates by keeping the DC-link voltage at a de-sired reference value, using a feedback control loop. To function as a rectifier,the voltage at the DC-link must be larger than the peak DC voltage generatedby the rectifying diodes in passive mode (Vbridge), as shown in Figure 2.5.Otherwise, the diodes conduct and there is no full control of the rectifier.

The choice of the values of the inductors L and the capacitor C are criticalto the proper functionality of the rectifier.

Vdc

VBridge

Figure 2.5: DC link voltage and the diode rectification voltage.

30

When the synchronous rotating d-q reference frame is adopted, id and iqbecome DC currents and express the active and reactive currents. Therefore,active and reactive power can be decoupled and controlled independently. TheDC output voltage can be controlled by the voltage loop, and the current re-sponse can be controlled by the current loop, as shown in Figure 2.6.

-+ PI

Currentlimiter

0

dq

-ab

c

Cu

rren

t re

gu

lato

r

Qin

Pin

*

*V

V

*

iabc

Pulses

Figure 2.6: Control block diagram for the DC-link voltage regulation.

2.3.2 DC/AC ConvertersThe DC to AC power conversion is realized by inverters. The main circuit ofan inverter is equal to the forced-commutated rectifier circuit, shown in Figure2.4. When the converter shown in Figure 2.4 operates as an inverter, the powerflow changes the direction [67].

The control strategy of both inverter and rectifier is similar. The invertercontrol will be further discussed later, when the control of AC machines ispresented, in Section 2.4.

2.3.3 DC/DC ConvertersA buck converter is a step-down DC to DC converter [67], meaning that theoutput voltage is lower than the input voltage. The buck converter conversionratio is:

M (D) =ton

TS=

Vout

Vin= D (2.14)

where ton is the interval in which the switch is conducting and Ts is the switch-ing time period.

A buck converter circuit is shown in Figure 2.7.A boost converter regulates the output voltage to a higher level over the

input voltage, being often referred to as step-up converter. The conversionratio is:

31

+-

L

C+

-V

in

S1

R0

Figure 2.7: Step-down buck converter with a resistive load R0.

M (D) =Vout

Vin=

11−D

(2.15)

A basic circuit of the boost converter is shown in Figure 2.8.

+-

L

RC+

-V

in

S1

0

Figure 2.8: Step-up boost converter with a resistive load R0.

2.4 Control of AC MachinesMany three-phase loads need a supply of variable frequency, requiring fastand high-efficiency control by electronic means. In variable speed AC drives,inverters are used to control the rotor speed through the supplied frequencyand the machine flux through the supply voltage [68].

2.4.1 Scalar V/F ControlThe open loop scalar control is one way of controlling AC motors for variablespeed applications. It has the advantage of being relatively simple to imple-ment and sensorless [69].

Voltage/frequency control is a scalar control method based on static modelof the motor. Its goal is to keep the stator flux linkage constant by control-ling the V/f ratio, so that the maximum torque/current and the fastest torque

32

response of the motor can be obtained [70]. In order to keep the stator fluxlinkage constant, generally:

Vf=

Vrated

frated(2.16)

At low frequencies, the stator resistance cannot be ignored, being necessaryto maintain the voltage at a fixed value in this range of operation. A minimumfrequency is also used to improve the motor start-up. Through PWM (to bediscussed in Section 2.5.1), open-loop control acts on the motor, as shown inthe simplified block diagram of Figure 2.9.

Figure 2.9: Structure Block of V/f Control of PMSM.

2.4.2 Vector ControlVector control of PMSM allows, by using d-q components, separating closedloop of both flux and torque [71]. The electromagnetic torque can be ex-pressed in d-q components according to Equation 2.12. To achieve the maxi-mum torque/current ratio, which is a desired characteristic during accelerationand deceleration in EVs, the d-axis current is set to zero during the constanttorque control so that the torque is proportional only to the q-axis current.

PMSM speed can be controlled by closing a speed feedback loop as il-lustrated in Figure 2.10. The torque request, Te, is generated by the speedcontroller dependent on the speed error. By keeping the current id to zero,maximum torque can be achieved.

2.5 Switching TechniquesThe conversion of DC power to three-phase AC power can only be performedin the switched mode. Power semiconductor switches connect the two DCterminals and the three phases of the AC terminals at high repetition rates.The actual power flow in each motor phase is controlled by the duty cycle

33

-+ PI

Torquelimiter

0

dq-a

bc

Curr

ent

regula

tor

qr

Te*w

w

*

iabc

q qr e=

iT

idid

iq

qe

*

* Pulses

T i

Angle conversion

Figure 2.10: Typical permanent magnet synchronous machine control with currentand speed control loops.

of the respective switches. The desired sinusoidal waveform of the currentscan be achieved by varying the duty cycles sinusoidally with time, employingtechniques as Pulse Width Modulation (PWM) or Space Vector Modulation(SVM) [72].

2.5.1 Pulse Width ModulationPulse-width modulation is a way of delivering energy through a succession ofpulses rather than a continuously varying (analog) signal [73, 74]. The con-troller regulates energy flow to the motor shaft by increasing or decreasingpulse width. The motor’s own inductance acts like a filter, storing energy dur-ing the "on" cycle while releasing it at a rate corresponding to the input orreference signal.

A simple comparator with a sawtooth carrier can turn a sinusoidal commandinto a pulse-width modulated output, as shown in Figure 2.11. In general, thelarger the command signal, the wider the pulse.

+

-

Low

High

PMW signal

Comparator

Chopping

signal

Command

signal

Figure 2.11: Pulse Width Modulation Strategy.

2.5.2 Space Vector ModulationSpace vector modulation technique was originally developed as a vector ap-proach to PWM for three-phase inverters [75]. SVM is mostly used whenimplementing digital control, which is the case where PWM technique canbe difficult to implement. SVM is a more sophisticated technique for gener-

34

ating sine wave that provides a higher voltage to the motor with lower totalharmonic distortion (THD) [76].

The concept of space vector is derived from the rotating field of an ACmachine used for modulating the inverter output voltage. According to thespace vector theory, there are eight switch states, named as S0-S7 as shownin Figure 2.12a. The output voltage of the inverter is composed by these eightswitch states, represented as vectors with 60o rotation between each state. Aclassical sinusoidal modulation limits the phase duty cycle to the inner circleas shown in Figure 2.12b. The space vector modulation schemes extend thislimit to the hexagon by injecting third order harmonics in the signal. The resultis about 10% higher phase voltage at the inverter output.

0

1

abc

0

1

abc

0

1

abc

0

1

abc

0

1

abc

0

1

abc

0

1

abc

0

1

abc

S0=000 S1=100 S2=110 S3=010

S4=011 S5=001 S6=101 S7=111

b

aS0=000

S1=100

S2=110S3=010

S4=011

S5=001 S6=101

S7=111

I

II

III

IV

V

VI

(a)

(b)

Figure 2.12: (a) Eight switching states, (b) Eight voltage space vectors of a three-phase voltage source inverter.

2.6 Semiconductor DevicesMetal-oxide-semiconductor field-effect transistor (MOSFET) is used for am-plifying or switching electronic signals, which came along in the 1970’s.

35

Insulated gate bipolar transistor (IGBT) is a three-terminal power semicon-ductor device, noted for high efficiency and relatively fast switching. It camealong in the end of the 1980’s.

MOSFETs’ and IGBTs’ structures look very similar [77]. IGBTs are usedin medium- to high-power applications such as switched-mode power supply,traction motor control and induction heating. Large IGBT modules typicallyconsist of many devices in parallel and can have very high current handlingcapabilities in the order of hundreds of amperes with blocking voltages of6000 V, equating to hundreds of kilowatts.

MOSFETs, on the other hand, are preferred in high frequency applications(> 200 kHz) and low voltage applications (< 250 V), such as switch modepower supplies with hard switching and rated power below 1000 W.

Between 250 and 1000 V, choosing between IGBTs and MOSFETs is veryapplication-specific. Cost, size, speed and thermal requirements should beconsidered [78].

2.6.1 Losses in Semiconductor DevicesIn power electronics, IGBTs and MOSFETs, as well as diodes, are operatedmainly as switches, taking on various static and dynamic states in cycles. Inany of these states, power dissipation is generated, which heats the semicon-ductor and adds to the total dissipation of the switch.

Different single power dissipations are possible during switch operation[67]. Switching losses are usually a major contribution to the switch totalloss, mainly in MOSFETs, where the switching frequencies are higher. Atevery change of state, if the switch carrying current is opened, the voltagerises across the switch and the current through it falls, resulting in dissipationof a short pulse of power in the switch. Similarly, as the switch is closed, thevoltage will take some time to fall and the current will take some time to rise,producing a pulse of power dissipation.

The average switching power loss Ps in the switch due to these transitionscan be approximated as:

Ps =12

VdI0 fs(tc(on)+ tc(o f f )

)(2.17)

where Vd is the voltage across the switch, I0 is the current flowing through theswitch and fs is the switching frequency. tc(on) and tc(o f f ) are the turn-on andturn-off time of the switch, respectively.

Another major contribution to the power loss in the switch is the averagepower dissipated during the on-state Pon, which varies in proportion to theon-state voltage. The on-state losses, or conduction losses, are given by:

Pon =VonI0ton

Ts(2.18)

36

which shows that the on-state voltage, Von, in a switch should be as small aspossible. ton is the interval in which the switch is conducting and Ts is theswitching time period.

2.7 Control Systems TheoryA study of control involves developing a mathematical model for each compo-nent of a self-contained process under study so called control system [79,80].Transfer functions commonly describe control systems. The transfer functionsare defined as the ratio of the Laplace transform of the output Y(s), and the in-put U(s), given by:

G(s) =Y (s)U (s)

(2.19)

Many different parameters can be investigated by knowing the transferfunction of a control system, including its stability. Stability is defined as theability of a system to return to equilibrium once disturbed.

Two methods of investigating the stability of a control system are the re-sponse to singularity functions and the root-locus.

The response to singularity functions requires that the transient responseshould decay to zero after some time as for the linear system to be stable. Thesteady state response of a linear system is generally of the same shape as theapplied input. Examples of singularity functions are the step response and theimpulse response.

The location of the poles and zeros of a transfer function in the Real X Imag-inary plane is analyzed in the root-locus method (and the poles/zeros maps).The root-locus gives the trajectories of the closed loop poles as a function ofthe feedback gain (assuming negative). A system is stable if all of its poles arein the left-hand side of the s-plane (for continuous systems) or inside the unitcircle of the z-plane (for discrete systems) [79].

Once a control system is verified to be unstable (or in the case where thesystem output needs to be precisely known), a compensator can be insertedin the system. Additional controllers are used to place the poles/zeros of thesystem in a desirable/known position.

The PID controller is one of the most used in feedback control design. PIDis an abbreviation for Proportional-Integral-Derivative, referring to the threeterms operating on the error signal to produce a control signal [81]. The PIDcontroller transfer function is given by:

G(s) = Kp +Ki

s+Kds (2.20)

37

where Kp is the proportional gain, Ki is the integral gain and Kd is the deriva-tive gain. Control can be provided by tuning the three constants in the PIDalgorithm, designed for specific process requirements.

A high proportional gain results in a large change in the output for a givenchange in the error, meantime the integral term accelerates the movement ofthe process towards set-point. It eliminates the residual steady-state error thatoccurs with a pure proportional controller. The derivative term slows the rateof change of the controller output, and might not be required in some applica-tions. The Proportional-Integral (PI) controller is a special case of the commonPID controller in which the derivative (D) of the error is not used.

38

3. Components Description andControl Strategies

The present flywheel system has a large number of power converters and con-sequently, control systems. The converters and their control are very importantin the system. They regulate the functionality and safety of the different com-ponents, and are responsible for a part of the losses in the driveline, requiringcareful design. This chapter aims to present and describe the different powerconverter systems and the control strategies used.

3.1 System OverviewThe complete flywheel system is shown in Figure 3.1.

Figure 3.1: The EV propulsion system based upon a flywheel energy storage devicewith two power levels.

The flywheel based driveline contains three different Power ConverterSystems (PCS): A DC/AC plus a DC/DC converter on the Low Power(LP) side, connecting the flywheel motor/generator to the battery and anAC/DC/AC converter on the High Power (HP) side connecting the flywheelmotor/generator to the wheel machine.

The DC/DC converter is used to control the battery output power, to limitthe battery output current or to boost the battery voltage. It can also be used torecharge the battery with the energy stored in flywheel. The DC/DC convertermight not be required if the battery output power is controlled using the sameinverter (DC/AC converter) and a unidirectional converter is used (on or off-board converter) for battery recharging. However, the DC/DC converter candecouple the control for battery output power and flywheel machine speed.

39

Furthermore, the voltage boost feature can be used to reduce the battery packvoltage and extend the flywheel speed range. The bidirectional DC/DC con-verter shown in Figure 3.1 can be used during both acceleration mode andbattery charging.

The low power DC/AC converter is also bidirectional. It controls thespeed/torque of the flywheel when working as an inverter. In rectifier mode,which occurs when the energy stored in the flywheel is sent back to thebattery, the body diodes from the two-level inverter bridge are used toperform passive rectification.

Figure 3.2: The EV propulsion system based upon a flywheel energy storage devicewith two power levels.

The high power side of the driveline connects the flywheel to the wheelmachine. AC machines are preferable as wheel machines, due to their highefficiency and power density [71]. If an AC machine is used, a three-phasefour-quadrant AC/DC/AC converter is required, as shown in Figure 3.2.

During drive mode, power flows to the wheel machine (working as a mo-tor), and the flywheel-side converter operates as a rectifier, whereas the load-side converter operates as an inverter, as shown in Figure 3.2. During brakingmode, the roles are reversed, i.e., the wheel machine-side converter operatesas a rectifier, whereas the flywheel-side converter operates as an inverter. Thewheel machine works as a generator.

The following sections briefly describe the control strategies implementedfor the different power converters used in the driveline. The subsections willbe divided as in Figure 3.1: Battery Control (Control 1), Flywheel ChargeControl (Control 2) and Drive/Braking Mode Control (Control 3).

3.2 Battery ControlBattery Control is used with the DC/DC converter. Two different power con-verter systems have been investigated. The control strategies presented in sub-

40

section 3.2.1 and 3.2.2 focus on battery recharging operation. ON/OFF controlstrategy, suggested for battery output power control, is presented in subsection3.2.3.

3.2.1 Unidirectional DC/DC ConverterThe aim of the buck/boost converter is to control the current and voltage dur-ing battery recharging, using the energy stored in the flywheel. The storedenergy is sent back to the battery when the vehicle is parked. The main chal-lenge is the control of the power flow to the battery despite the decay of theflywheel machine voltage, which is dependent on its rotational speed.

The flywheel motor/generator is connected to a passive rectifier, as shownin Figure 3.3. The rectifier output is connected to the unidirectional DC/DCconverter. Although connected in series, buck and boost functionality are notused simultaneously in the present application [82]. The converters are con-nected in series to maintain the current direction, regardless of the operationmode (boost or buck).

The unidirectional DC/DC converter can operate over a wide range of inputvoltages. The converter can also maximize power transfer, since the booststage of the converter can provide power factor correction circuitry [83].

Constant current/voltage control can be accomplished with a ProportionalIntegral (PI) controller by using the buck and boost converter transfer func-tions [84] in a closed loop system. The proportional and the integrative gainare chosen to obtain the desired pole placement and, consequently, the desiredrise time and peak overshoot of the system response.

The application of the unidirectional DC/DC converter shown in Figure 3.3is novel as both converters can work independently. To operate the converter asa buck, S1 is open and S2 is chopping. To operate it as a boost, S1 is choppingand S2 is conducting.

3.2.2 Bidirectional DC/DC ConverterThe DC/DC converter shown in Figure 3.4 operates in all four quadrants [85],meaning that it is capable of transporting energy in both directions with boostor buck functionality. The same dynamics of the singular boost and buck con-verters can be applied.

The DC/DC converter should control the energy flow between the batteryand the LP side of the flywheel machine. Hence, the desired control variablesare both output current and voltage. Table 3.1 shows the possible operationmodes and the control variables required.

A transition between quadrants (e.g. changing from buck to boost modeduring operation) implies change of transfer function. If a PID controller isused, the integral memory (which sets the control value due to the model er-rors) will then be invalid and must be reset. Assuming large changes in system

41

Figure 3.3: Equivalent circuit diagram of the unidirectional DC/DC converter.

S1 S2

S4S3

I L

VB

I B

VDC

I DC

C1 C2

L

Figure 3.4: Equivalent circuit diagram of the bidirectional DC/DC converter.

dynamics, transitions will become difficult, and the system must be modelledwith adaptive techniques [86].

One way of solving this problem is by using a non-linear approach as slid-ing mode control, which assumes direct control of each switch state [87]. Thecontroller has been based on a set of decision laws, which can decide the op-eration mode and control the output current/voltage, as shown in Figure 3.5.The current through the inductor, iL,max is always controlled, being one of theparameters considered when deciding the switching state. The error from thecomparison of the variables is called σ and the hysteresis band is representedby δ . The switches Sa,up and Sb,down are referred as S1 and S4 in the illustrationof the DC/DC converter shown in Figure 3.4.

A general scheme of the control, where two target control variables areused, is shown in Figure 3.5a. SM stands for Sliding Mode, and this block pro-cesses the logic structures presented in (b), (c) and (d). The operation modeselection, where buck or boost mode is chosen depending on the system volt-

42

Table 3.1: Modes of operation of the DC/DC converter.

Energy Flow Target control variable Usage in driveline applicationBattery charging Voltage/current control Battery charging processBattery discharging Voltage control Boost of battery output voltageBattery discharging Current control Constant/maximum power control

Figure 3.5: Non-linear control strategy of the bidirectional DC/DC converter.

ages, is illustrated in Figure 3.5b. The control signals of the switches duringbuck and boost mode are presented in Figure 3.5c and Figure 3.5d, respec-tively.

The control of the lower switch (Sb,down) during buck mode is required whenleaving boost mode, so any excess of energy stored in the inductor could bedissipated. The hysteresis bands are slightly lifted according to the gain k, asto avoid the switches to interfere with each other.

A more detailed description of the converter modelling and the controllerdecisions can be found in Paper IX and [88].

3.2.3 ON/OFF ControlThe total energy transferred from a flywheel is given by Equation 2.2. In orderto transfer energy, the flywheel speed must be able to vary; therefore a fixedspeed control on the LP side of the flywheel is not indicated. However, thisvariation should be kept between well-defined limits. The limits are chosendepending on the machine mechanical parameters and safety issues.

43

A suggested logic for the control of the LP converter takes into account thevariation in the flywheel rotational speed and the LP PCS different modes ofoperation, as illustrated in a flow chart in Figure 3.6.

Figure 3.6: Flow chart which describes the ON/OFF control strategy during batterydischarging mode.

When the vehicle starts, the control strategy will measure the actual speed(S), and then compare this speed to a minimum speed (T) of the rotating fly-wheel. If the speed S<T, the flywheel machine is accelerated to its nominalspeed (N). Once the machine rotates at nominal speed, the LP converter isturned off and the flywheel is controlled by the HP side. The battery systemis activated and increases the speed back to the nominal value every time it isbelow the minimum speed (T). When the vehicle stops, the energy stored inthe flywheel is used to recharge the battery.

When the battery is reconnected to the system, its output power is limited,so that no power peak would occur on the LP side.

44

A constant power discharging mode is optimal from the battery perspec-tive. However, it can become difficult to predict the behaviour of the loadand the driveline performance can become inefficient or even insufficient.The ON/OFF case is suitable from the system perspective and simple to im-plement. With this control, the flywheel speed would vary, since the batterywould only be connected to the system when the flywheel speed reaches aminimum value.

3.3 Flywheel Charge ControlFlywheel Charge Control is used with the DC/AC converter on the LP sideof the driveline. The LP DC/AC converter is a two-level inverter. The powerconverter controls the speed of the flywheel accordingly to the block diagramshown in Figure 3.7. If ON/OFF strategy is used, the control of the flywheelspeed is made accordingly to the values of minimum (T) and nominal speed(N), discussed in Section 3.2.3.

-+

PIDError

ReferenceSpeed Amplitude Sine-Wave

GenerationPWM

3- PhaseInverter

Low passFilter

PMSM

PositionSensors

Rotor sectorCalculation

PhaseAdvance

Phase Period

SpeedCalculation

Rotor sector

Direction

Measured Speed

DutyCycles

Figure 3.7: Block diagram of the PMSM control.

Permanent magnet synchronous motors have sinusoidal distribution of themotor windings, what produces sinusoidal currents, reducing the torque rip-ple. Therefore, a solid state converter is used to supply the machine and theoutput voltage must be sinusoidal or sinusoidal PWM modulated [89].

Hall effect sensors detect the rotor position and also the measured speedis derived from one hall effect sensor. Speed control is achieved using a ref-erence speed and PID controller. Pulse signals for the three-phase motor aregenerated using space vector modulation.

45

3.4 Drive Mode ControlDrive Mode Control is used on the HP side of the driveline during accelera-tion mode. This section focuses on the control strategies used when the wheelmachine is an AC machine. The control of the AC/DC/AC converter can bedivided in AC/DC and DC/AC converter controllers.

A DC machine has also been used as wheel machine during experimentaltests. The control used is described together with the experimental results, inSection 4.2.3.

3.4.1 AC/DC Converter ControlA diagram of the DC-link control is shown in Figure 3.8. Due to the charac-teristic of closed loop, it is necessary to measure the stator currents and rotorangular position. These measurements are carried out by two current sensorsand hall effect sensors. The instantaneous values of stator currents ia and ibare mathematically transformed (Clarke and Park transformations [90]) andthen used as the feedback for iq and id control loops.

An outer loop of voltage is connected to a PID regulator. The output of thevoltage controller is the reference of the quadrature current iq. The referenceof the direct current id is set to zero, in order to obtain unity power factoroperation. The output of iq and id PID controllers are transformed into α andβ components (Inverse Park transformation). SVM is used, generating thepulses which are inserted into the three-phase bridge rectifier.

-+

-+

-+

Flywheel (TVLM)

Hallsensor

Speedcalculator

Angleestimator

Park tr. Clarke tr.

DC-link

-

+

ia

ib

a,b,c

a,bia

ib

a,b

d,qiq

id

wrwrqr

qr

id(ref)= 0

PID PID

PID

V(ref)

Vdc

Vdciq(ref)

id

iq

Vq

Vd

d,q

a,b

Inv. Park tr.Va

Vb

SVPMW

3-phaserectifier

Figure 3.8: Block diagram of the DC-link control.

46

3.4.2 DC/AC Converter ControlThe vector control block diagram is similar to the DC-link control diagram,as shown in Figure 3.9. Current sensors are necessary to capture the instan-taneous values of line currents. In this case, an encoder was used to measurethe rotor angular position. The inner currents loops are maintained in the samestructure as for the DC-link control. The main difference is in the outer loop,where a speed loop control is implemented. The output of the speed controlleris the reference value for the quadrature current iq, which regulates the torqueneeded to reach the desired speed. The reference of the direct current id is setto zero, in order to obtain maximum torque per ampere operation.

-+

-+

-+

Encoder

Rotorspeed

calculator

Park tr. Clarke tr.

DC-link

-

+

ia

ib

a,b,c

a,bia

ib

a,b

d,qiq

id

wrwr

qr

qr

id(ref)= 0

PID PID

PID

iq(ref)

id

iq

Vq

Vd

d,q

a,b

Inv. Park tr.Va

Vb

SVPMW

3-phaseinverter

w(ref)

Wheel motor

Figure 3.9: Block diagram of the PMSM vector control.

3.5 Braking Mode ControlPower is transferred from the wheels directly to the main energy storage de-vice (e.g. batteries) during regenerative braking in EVs reported in the lit-erature [91, 92]. Traditionally, the battery is directly connected to the wheelmotor. In the present flywheel system, the battery is not charged during brak-ing [93] and, ideally, all the power converted during regenerative braking isabsorbed by the flywheel.

The presented regenerative braking control is obtained by controlling thewheel machine output power, which can be calculated in d-q coordinates.Differently from other conventional control strategies with DC-link regula-tor [94, 95], the proposed control strategy uses power estimation to balancethe power flow in the flywheel-wheel machine link. Therefore, only currentcontrollers are required, eliminating the need for the outer loops and volt-

47

age/speed controllers. Hysteresis current control is applied, providing fast dy-namic response.

The control system block diagram of the HP converter during braking modeis shown in Figure 3.10.

Figure 3.10: Control system block diagram of the HV converter during braking mode.

The direct (active) component of the AC line current id is calculated froma reference power Pin, estimated by a drive cycle simulation. The reactivecomponent of the AC line current is set to zero in order to operate at unitypower factor [96].

The inverter control attempts to send the same amount of power Pin from theDC-link to the flywheel. Direct current (id) is set to zero in order to maximizethe output torque, as shown in Equation 2.12. The power consumed by theflywheel becomes linearly proportional to the quadrature current iq.

In an ideal case, no storage element in between is needed if the input andoutput power are equal. Nevertheless, differences between the input and out-put power are inevitable in real systems (e.g. losses in the converter and in thewheel machine) and a storage element is needed for the functionality of therectifier bridge.

48

4. Methods

Simulation methods are presented in this chapter, describing briefly themethodology used to calculate the PID controller parameters and the drivecycle simulations. The different experimental set-ups are also described,together with the measurement systems used during the experimental tests.

4.1 SimulationsDifferent simulation tools have been used to design and estimate the systembehavior, before implementing it experimentally. Simulations have beencarried out using the following softwares: PSpice Orcad, Matlab, MatlabSimulink and Dymola.

Complete system simulations have been implemented. Important steps forsystem design are the tuning of the PID controllers and the drive cycle simula-tions. These two items will be briefly presented in the following subsections.

PID/PI controllers have been used in different control systems in the driv-eline: the flywheel machine control, the wheel machine control, the forced-commutated rectifier control and the unidirectional DC/DC converter.

The PID controller used with the LP inverter is discussed in Section 4.1.1.Similar steps were taken to design the other PID controllers present in thecomplete system.

The drive cycles have been investigated by Lundin et al. [97] and are pre-sented in Section 4.1.2. The results from the drive cycle simulations have beenused as input for the complete system simulation and for analysing the powerconverters’ stability.

4.1.1 PID Controller DesignA transfer function of the system can be obtained by using the energy plus themachine parameters, using d-q equations presented in Section 2.2.2. For thecase of the flywheel machine, which is a surface mounted PMSM, the magnetsaliency is rather small, implying Lq=Ld [98]. It can be seen from Equation2.12 that the maximum torque per ampere for the flywheel machine is obtainedby keeping id = 0.

The transfer function can be investigated and the system response with andwithout a PID controller can be studied. The motivation of using a PID con-

49

(a) Step response. (b) Impulse response.

Figure 4.1: The system response to various input signals. Note the different timescales in the upper and lower graphs.

troller relies on its successful usage for a wide range of applications, includingprocess control and motor drives [99].

The PID compensation can be improved by correctly selecting the values ofKp, Kd and Ki that lead to the desired closed-loop response. This selection canbe made by choosing the values of the damping factor ς and natural frequencyω0 of the system that would result in acceptable rise time and peak overshootof the closed loop response [80]. Using the second flywheel machine proto-type parameters (detailed description in Section 4.2.2), the PID compensatorconstants are obtained.

Step and impulse response of the system without the PID controller areshown in the upper graphs of Figures 4.1a and 4.1b. The system takes a longtime before reaching steady-state with constant error without the PID con-troller. The unit step response of the closed loop system (second graph ofFigure 4.1a) converges to the steady-state after a short time and with low os-cillation. The same behavior is shown in Figure 4.1b for the impulse response.

The system without the PID controller has two poles, as shown in Figure4.2. One of the poles is close to the right half of the s-plane, indicating thatthe system is close to being unstable. The poles of the system with a PIDcontroller are shown in the lower plot of Figure 4.2, located now away fromthe right part of the s-plane, which indicates a better performance after theinsertion of the PID controller.

50

-1200 -800 -400 0-4

-2

0

2

4Im

ag

inary

ax

is×

10

-2

Real axis

With PID controller

-1200 -800 -400 0-4

-2

0

2

4

Imag

inary

ax

is×

10

-4

Without PID controller

Figure 4.2: Root locus analysis of the system, without and with the PID controller.

4.1.2 Drive Cycles InvestigationImportant information on the torque demand of the flywheel can be foundwhen studying different drive cycles [100]. Considering a control system, thedrive cycles are the load variation in the machine or, for the system transferfunction, they can represent an external disturbance.

The new European drive cycle, divided into one urban and one extra urbanpart, is shown in Figures 4.3 and 4.4, respectively. The American drive cycles,one urban and one extra urban, are shown in Figures 4.5 and 4.6, respectively.The figures show the power needed to drive the vehicle at the requested accel-eration or speed at every instant. The considered vehicle has a mass of 1500kg, a dimensionless drag coefficient of 1.35 and a frontal area of 1.73 m2 [97].

4.2 Experimental Set-UpsFour different scaled experimental set-ups were implemented. The experi-ments allow measurements of complete drive cycles, improving the under-standing of the constituting components and optimization of the completesystem.

The experimental parameters of the electric machines used and the elec-tronic components are presented in the next subsections.

51

Figure 4.3: New European Urban DriveCycle.

Figure 4.4: New Europen Extra UrbanDrive Cycle.

Figure 4.5: US Urban Drive Cycle. Figure 4.6: US Highway Drive Cycle.

4.2.1 Flywheel ChargingThe flywheel charging experimental set-up consisted of a flywheel prototype,a DC/AC converter and DC voltage source.

The results of this experiment are presented in Paper VI and Paper XIII.

4.2.1.1 Flywheel prototypeThe machine used during the Flywheel Charging experiments was a small-scaled three-phase axial flux PM machine [101]. The main characteristics ofthe machine are presented in Table 4.1. A picture of the machine is shown inFigure 4.7.

52

Table 4.1: Motor/generator parameters.

Pole pairs 14Moment of Inertia 0.219 kg ·m2

Internal resistance 0.134 Ω

Internal inductance 0.3 µHRemanent magnetic flux density 1.3 T

Figure 4.7: Picture of the first scaled motor/generator flywheel prototype.

4.2.1.2 Power Electronics HardwareA two-level three-phase DC/AC converter was used to control the machineshown in Figure 4.7. A pre-programmed microcontroller MC3PHAC, fromFreescale Semiconductors, generated six PWM signals which were modulatedusing V/F control (Section 2.4.1) in order to control the three-phase AC motor.

A Standard IGBT Module (Semikron SKM22GD123D) was chosen. TheIGBT has a typical collector-emitter resistance (Rce) of around 100 mΩ andis suitable for high switching frequencies. A three-phase bridge driver (Inter-national Rectifier IR 2130) which has three independent high and low sidereference output channels was also used to ensure the correct polarization ofthe transistors. Switching frequency was 5.291 kHz. An AC filter with a cut-off frequency of 1.59 kHz was used.

The flywheel converter system with the low-pass filter experimental set-upis shown in Figure 4.8.

4.2.2 Loaded FlywheelThe second experimental set-up was constructed based on the second flywheelmachine prototype and power electronics presented in the following sections.The LP side of the flywheel machine was connected to an inverter, used tocontrol the speed of the machine. The HP side was connected to a variable

53

Figure 4.8: Picture of the power converter system in EMI protecting boxes.

resistor, as shown in Figure 4.9. The resistive load could be varied betweenopen circuit (infinite resistance) and 15 Ω.

The results of this experiment are presented in Paper II and Paper VIII.

Figure 4.9: Scheme of the Loaded Flywheel experimental set-up.

4.2.2.1 Scaled flywheel prototypeThe second flywheel machine prototype was a double-wound axial-flux PMSmotor/generator with two input/output sides, corresponding to the high-powerand low-power sides [102]. The machine nominal parameters are presented inTable 4.2. A picture of the prototype is shown in Figure 4.10.

4.2.2.2 Power Electronics HardwareA microcontroller dsPIC30F2010 from Microchip was used to implement thecontrol system described in Section 3.3. DsPICs are 16-bit Digital Signal Con-trollers (DSC) that integrate the control attributes of a microcontroller (MCU)with the computation and throughput capabilities of a Digital Signal Proces-sor (DSP). Hall effect sensors A1101 from Allegro were employed as posi-tion sensors. An IGBT module SK22GD123D from Semikron was used. Athree-phase bridge driver (IR 2130) ensured the right transistor polarization.Switching frequency was 20 kHz.

54

Table 4.2: Motor/generator parameters.

Low Power High PowerNominal Speed (rpm) 2200 2200Moment of Inertia (kg ·m2) 0.364 0.364Friction factor (Nms) 0.22 0.22Number of poles 6 6Internal phase resistance (Ω) 0.04 0.12Internal phase inductance (mH) 0.019 0.19Mutual inductance (mH) 0.079 0.076Magnetic flux (Wb) 0.002 0.002

Figure 4.10: Picture of the second scaled motor/generator prototype.

A voltage source, QPX1200 60 V/50 A from TTi, was used instead of bat-teries. A three-phase passive rectifier, 60MT120KB, from International Rec-tifier, connected the HP side of the flywheel to the resistive load.

4.2.3 Flywheel Driveline with DC Wheel MachineThe same flywheel machine (Section 4.2.2.1) and DC/AC converter (Section4.2.2.2) were used in the third experimental set-up. Batteries and a DC/DCconverter were added to the LP side of the driveline. A DC machine was con-nected to the driveline instead of a variable resistor load, requiring new powerelectronics to connect the HP side of the system [103]. A block diagram of

55

the flywheel driveline with a DC wheel machine is shown in Figure 4.11 anda picture of the implemented system is shown in Figure 4.12.

The results of this experiment are presented in Paper III.

FLYWHEEL

MACHINE

DC/DC

CONVERTERBATTERY

RECTIFIER

(AC/DC)

FLYWHEEL

CONTROLLER

(DC/AC)

DC

MACHINE

CURRENT

CONTROLLER

(DC/DC)

LOAD

Figure 4.11: Scheme of the flywheel driveline with DC wheel machine.

Figure 4.12: Picture of the flywheel driveline with DC wheel machine.

4.2.3.1 Low Power Side ConverterLead acid batteries were chosen for powering the driveline, as they are rela-tively cheap, safe and easy to charge. Four 12 V batteries were connected inseries, giving an output voltage of 48 V. This was enough to bring the flywheelto a speed which gives a voltage output of 80 V on the HP side.

A DC/DC buck converter was designed and built to limit the battery outputcurrent, using IGBTs (SKM600GB066D) from Semikron. The inductor in thecurrent limiting circuit was 3.75 mH. The filtering capacitor was chosen to 4.7mF. The microcontroller chosen was the dsPIC30F2010 made by Microchip.The driver was an IR2110 made by International Rectifier. Hysteresis controlwas implemented for the current controller. The current sensor was a HAL50-s made by LEM.

56

4.2.3.2 High Power Side ConverterUnidirectional AC/DC and DC/DC converters were implemented on the HPside of the system, connecting the flywheel to the DC wheel motor.

The AC/DC converter was a three-phase passive rectifier (60MT120KB)from International Rectifier.

The control of the DC motor was made by setting the desired current valuethrough a potentiometer (or Labview), using a DC/DC converter. The desiredvalue of the current was then compared to the measured value, and the outputerror signal was used for producing the switching signals.

MOSFETs (IXFN140N20P) from IXYS, rated at 200 V and 140 A, werechosen. The same driver, current sensor and microcontroller used with theDC/DC converter on the LP side were utilized. Switching frequency was 14kHz.

The wheel motor used was a compound DC-motor rated 10 kW at 60 V.The mechanical load required to brake the motor was a DC-generator rated1.9 kW. The electrical load used to brake the generator was a variable resistorrated 0-630 Ω. Both the braking torque and the output power could be adjustedby regulating the field current in the generator and the resistance of the load.

4.2.4 Complete Driveline Set-UpOne of the goals with the flywheel project has been to implement a bidirec-tional driveline, based on a flywheel power buffer and an AC wheel machine.Here, an AC synchronous machine was used as wheel machine. New electron-ics and control were required on HP side of the driveline. A block diagram ofthe fourth experimental set-up is shown in Figure 4.13 and a picture of theimplemented system is shown in Figure 4.14.

The results obtained from this experimental set-up are presented in PaperV, Paper XI and Paper XII.

Figure 4.13: Scheme of the complete driveline set-up.

4.2.4.1 High Power Side ConverterThe HP side of the flywheel was connected to a 600 µH inductor on eachphase, required for the functionality of the forced-commutated rectifier. A 20mF/350 V capacitor was connected to the DC-link. Two three-phase MOS-FET bridges were built with discrete DSEI 2130. A snubber circuit was im-plemented in order to eliminate the ringings presented in the MOSFET output,based on a parallel connection of a filter capacitor and a damping resistor.

57

Figure 4.14: Picture of the complete driveline set-up.

The control of the AC/DC/AC converter was performed using two MCUs(Microcontrollers) Piccolo TMDSCNCD28035 (manufactured by Texas In-struments), based on the control strategies described in Section 3.4.

The driver used was IR2130, from International Rectifier. The driver waselectrically insulated from the MCU by digital optocouplers (A4800), with thepurpose of avoiding damage in the low power system. The system required alarge number of sensors: four current sensors (HAL50-S), one voltage sensor(resistive divider) and three hall sensors (A1101). The current sensors wereused in both boards to capture the instantaneous values of the line AC currents.The hall sensor (flywheel speed capture) and the voltage sensor (DC-link volt-age capture) were required for the rectifier control strategy. The encoder wasused to estimate the wheel motor rotor angle for the inverter board. Switchingfrequency was 15 kHz.

A synchronous motor with permanent magnets from Leroy Somer, rated at3.5 kW/380 V/1500 rpm, was used as wheel machine. It uses an incrementalrotary encoder with mounted stator coupling (ERN423).

4.2.5 Measurement SystemMeasurements are very challenging and a critical step when running an ex-perimental test. A wrong acquisition rate can spoil the shape of the signal andeven hide noise and harmonic content. The higher the voltage level of the ex-periment, the more difficult and expensive it becomes to make measurements.Different experimental set-ups were implemented and different measurementsystems were used.

A digital oscilloscope, TDS2004C, from Tektronix was used with the Fly-wheel Charging set-up. It allows USB memory connection, granting image

58

and data point storage. Measurements of current and voltage were made, withan acquisition rate of 1 kHz. The accuracy of the measurements was ±3% onall models.

Measurement systems from National Instruments (NI) were employed inthe Flywheel Loaded and the Flywheel with DC Wheel Machine experimentalset-ups. A DAQ card, NI USB-6259 BNC, 16-Bit, 1.25 MS/s, with coaxialprobes, was used for current measurements and analog outputs. A real timecontroller, NI CRIO 9022 combined with measurement slots (NI 9225 and NI9229) which can measure up to 300 V, including differential measurements,was also used. The maximum voltage range accuracy of the measurementswas 0.034 V. Data acquisition rate was 1 kHz for voltage and 100 Hz forcurrent measurement. Speed measurements were performed with the samehall effect sensors (A1101) used for the control of the flywheel machine.

A Labview interface is required for storing the data in a compatible formatwhen using NI measurement system. A small program was implemented inLabview which filters, stores and displays the data. It was also possible togenerate analog output signals and this feature was implemented with the thirdexperimental set-up in order to control the speed of the DC wheel motor.

PicoScopes are computer based oscilloscopes, which offer the functional-ity of a standard digital storage oscilloscope, in a portable and easy-to-usepackage. Two different PicoScopes were used for measurements in the Com-plete Driveline set-up: the PicoScope 2200 and the PicoScope 3425 Differ-ential Oscilloscope. The first one can only measure up to 20 V, but the lattercan measure up to 300 V, including differential measurements. Both offer anaccuracy of ±1% of the measured voltage. The data acquisition rate was 3kHz for both voltage and current measurements. The biggest advantage ofthis measurement system was its simplicity, since it can be directly connectedto a computer, with no need for programming or data conversion.

59

5. Summary of the Results andDiscussions

The most relevant results obtained from the present work are discussed in thischapter. The results are divided by subject and the papers where those resultswere published are mentioned in the beginning of each section. The controlstrategy and experimental set-up used will be referred to those discussed inChapters 3 and 4.

5.1 Battery Charging SystemTwo different power converter systems, aimed for controlling the batterycharging process, were investigated. Simulation results of the unidirectionalDC/DC converter are presented in Section 5.1.1. Simulation and experimentalresults of the bidirectional DC/DC converter are presented in Section 5.1.2.

5.1.1 Unidirectional ConverterThe output voltage of the flywheel machine decreases when it is used torecharge the battery, due to the decreasing rotational speed. To accommo-date the voltage difference, a buck/boost converter can be used, together withpassive rectification performed by the body diodes in the inverter bridge. Thetwo-stage unidirectional converter, described in Section 3.2.1, was modelledand simulated.

The results of this investigation are presented in Paper I.

5.1.1.1 Converter SimulationsSimulations were based on the power converter circuit presented in Figure3.3. Values of L and C were chosen to 100 µH and 200 µF. The switchingfrequency was set to 10 kHz. The battery internal resistance RB was assumedto be 1 Ω. Simulation conditions are given in Table 5.1.

The battery charging simulation results are shown in Figure 5.1. The batteryis typically selected as the main energy storage device in the system. There aredifferent methods for charging batteries, but constant current method requiressimple and inexpensive control equipment [104]. The charging is arrangedin two periods: constant current (the battery voltage progressively rises) and

61

Table 5.1: Motor/generator parameters.

Pole pairs 14Flywheel machine initial speed 6000 rpmFlywheel machine initial output phase voltage 42 VBattery type Lithium-IonBattery Voltage Range 25 - 42 VCharging current 1.9 A ± 0.1 A

constant voltage charging (applied as soon as the battery voltage reaches thetrickle level).

The simulation started with the battery close to a discharged state (25 V).Initially, the control operated the system in buck mode, since the flywheeloutput voltage was higher than the battery voltage, followed by the boost op-eration mode. At 1.2 s the flywheel machine output voltage became equal tothe battery voltage. After 3.4 s, the battery reached its nominal voltage of 42V. Input voltage was then kept constant (at 42 V) and charge current startedto drop as full charge was approached. Full charge was reached when batterycurrent was less than 3% of the rated current (1.9 A). The machine stoppedrotating after 4 s when the battery reached 100% of its total charge.

Flywheel machine output voltage, flywheel rotational speed, battery cur-rent, battery voltage and current error after the first comparator and the errorafter the PI controller (Error 1 and 2) are shown in Figure 5.1. The lowestpanel presents the PWM pulses. Results are shown for the first second of buckoperation, followed by the corresponding data of boost operation and constantvoltage period.

Error 1 voltage varied around zero, since the actual current was kept equal tothe reference current, as shown in Figure 5.1. Error 2 increased as a function ofthe PI controller proportional gain and changes the PWM duty cycle, allowingthe switch to be opened for longer periods of times. Longer pulses could beseen at the end of each converter operation interval.

Simulations included losses in the passive components and in the batterymodel, given that the present DC/DC converter has a two-stage filter, increas-ing the number of passive components in the system. The inductors’ and ca-pacitors’ internal series resistance (ESR) were set to 5 mΩ. Ideal switcheswere used during the simulation. An efficiency of 92% was obtained whendividing the output power over the battery by the input power in the capacitorC.

62

Figure 5.1: Battery current, voltage, control errors and PWM pulses during buck andboost operation modes.

5.1.2 Bidirectional ConverterA four-quadrant DC/DC converter was simulated, and a small prototype wasimplemented and tested. The focus of this investigation was the battery charg-ing process, but a four-quadrant DC/DC converter offers the possibility ofcontrolling the battery output power using the same hardware.

The functionality of the DC/DC converter and the control strategy imple-mented are described in Section 3.2.2 and explained in detail in [88].

The results of this investigation are presented in Paper IX.

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5.1.2.1 Converter SimulationsA complete simulation of the four-quadrant DC/DC converter described inSection 3.2.2 was implemented and run in Simulink. Simulation aimed for atransition between buck and boost mode, during a short time-frame, in orderto test the controller response (t = 10 s). The capacitor values (C1 and C2)were 20 mF and 15 mF. The inductor L value was 320 µH. Simulation resultsare shown from Figure 5.2 to Figure 5.5.

Figure 5.2: Flywheel machine output volt-age, ua and load voltage, ub.

Figure 5.3: Duty ratio for the activeswitches.

Figure 5.4: Inductor current. Figure 5.5: Load current.

Input voltage ua was falling from 100 V to 0 V, as shown in Figure 5.2.Load voltage should be kept at 60 V and load current should be kept at 15 A.

The equivalent duty ratio for the active switches is shown in Figure 5.3.The jump between buck and boost was performed at around t = 1.4 s, beforethe input voltage is lower than the output voltage, due to the control strategyillustrated in Figure 3.5. The output current and voltage started to decreasewhen Sb,down reached unity duty cycle.

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The inductor current, shown in Figure 5.4, jumped from 15 A to 60 A whenthe control jumped from buck to boost mode, but it was kept under the maxi-mum value set by the control system. The load current, shown in Figure 5.5,was kept at 15 A until around t = 8 s. At this time, the input voltage ua becamelow and could not be boosted any longer.

5.1.2.2 Experimental ResultsA scaled prototype of the described four-quadrant DC/DC converter was im-plemented, with the same parameters used during the simulations. The pro-totype was tested with the scaled flywheel machine (Section 4.2.2.1), whilerunning at a low rotational speed. The flywheel was accelerated by its highpower side, and the low power side was connected to the DC/DC converter,after a passive rectifier. A resistor was used as load impedance. Transitionmodes were investigated, similarly to simulations presented previously.

Both voltage and current were controlled during the transition between buckand boost, as shown in Figures 5.6 and 5.7. The flywheel voltage, uA, wasfalling in Figure 5.6, meantime the load voltage, uB, was kept constant until theflywheel voltage reached a low value. Load current, iB, and inductor current,iL, are shown in Figure 5.7.

A second test was made, also with focus on the transitions between modes.A DC voltage source was used as the input voltage, and the transition fromboost to buck was tested. Results are shown in Figure 5.8 and 5.9. The inputvoltage, uA, was increased from 0 V to 30 V. The output voltage, uB, was keptaround 15 V, with a small oscillation during the transition, as shown in Figure5.8. The same oscillation is shown in the plot of the inductor current, iL inFigure 5.9, together with the output current, iB, which was kept around 2 A.

5.2 Battery Discharging ControlA simulation model of the LP side of the system was implemented, based onON/OFF control. Experimental results were also obtained, in which a drivecycle (similar to the one used in the simulations) was applied to the systemload.

A more detailed description of the control strategy can be found in Section3.2.3. The results of this investigation are presented in Paper III.

5.2.1 Discharging SimulationsA simulation model of the LP side of the driveline was implemented in Dy-mola Software. A simple drive cycle consisting of two pulses of load torquewas applied to the HP side of the flywheel machine and the system was simu-lated. The power consumed by the load in consequence of the applied torqueis shown in Figure 5.10.

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Figure 5.6: Flywheel machine output volt-age, uA and load voltage, uB.

Figure 5.7: Inductor current, iL and loadcurrent, iB.

Figure 5.8: Flywheel machine output volt-age, uA and load voltage, uB.

Figure 5.9: Inductor current, iL and loadcurrent, iB.

The nominal value of the motor speed was set to 2500 rpm, and the lowerlimit to 2300 rpm. The flywheel used in the present simulations has a con-siderably high inertia, being capable of storing 15 kJ when rotating at 2500rpm.

The ON/OFF control signal is shown in Figure 5.11. At the start of thesimulation, the motor was running at 2480 rpm, as seen in Figure 5.12. ThePID system forced the motor to reach its nominal speed in 1 s and the batterywas disconnected from the system. The speed remained almost constant foranother 3 s, as the losses in the modeled system were small.

The first load torque was applied to the motor 4 s into the simulation, reach-ing a final value of 10 Nm, with the peak lasting for 2 s. Subsequently, themotor slowed down, reaching a speed of 2370 rpm. Another torque with thesame magnitude was applied at t = 12 s into the simulation, forcing the motor

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speed below 2300 rpm. The battery was reconnected and caused the motor toaccelerate.

The power from the battery, shown in Figure 5.13, was zero between t = 2 sand t = 15 s - a period when the battery was disconnected from the system. Thebattery current tends to rise when reconnected, in order to bring the systemback to nominal speed. The average power from the battery was of around700 W, meantime the peak power consumed by the load was around 2.6 kW.

Figure 5.10: The power consumed by theload in consequence of the applied torque. Figure 5.11: The ON/OFF control signal.

Figure 5.12: Flywheel rotational speed. Figure 5.13: The power from the battery.

5.2.2 Experimental ResultsThe Flywheel Driveline with DC Wheel Machine, described in Section 4.2.3,was used to perform experimental tests using ON/OFF control. Interestingobservations about the system performance and dynamics could be made byvarying the generator load in different ways.

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Figure 5.14: The power consumed by theload in consequence of the applied torque. Figure 5.15: The ON/OFF control signal.

Figure 5.16: Flywheel rotational speed. Figure 5.17: The power from the battery.

A simple drive cycle consisting of two pulses of load torque was appliedto the load during the experiments, similarly to the simulations. The powerconsumed by the load in consequence of the applied torque is shown in Figure5.14.

The battery was disconnected when the flywheel reached its maximumspeed, here set to 1900 rpm. The speed of the flywheel machine decreased dueto the losses in the system and to the power consumed by the load. The batterywas reconnected when the flywheel reached the minimum speed of 1200 rpm.The ON/OFF control signal used to connect/disconnect the battery from thesystem is shown in Figure 5.15. The rotational speed, shown in Figure 5.16,varied in good agreement with the simulations. The difference relates to thelosses of the electrical machine, which have mostly been neglected during thesimulations. Even when no torque was being applied (between the time whenthe battery was disconnected and the first torque pulse was applied, around t =10 s), the speed of the flywheel decreased, due to internal losses. The designed

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control was able to take the system back to its nominal speed once the batterywas reconnected to the system, at t = 30 s, as predicted by simulations.

The power from the battery was initially decreasing, since the speed of theflywheel was approaching its nominal value, as shown in Figure 5.17. Whenreconnected, battery output power was limited by the DC/DC converter, work-ing as a buck converter. The average power from the battery was approxi-mately 150 W, almost 3 times lower than the peak power consumed by theload (370 W). The experimental set-up was scaled down and therefore theapplied torque represented a small amount of power (370 W peak). The fly-wheel prototype has a relatively small inertia, so the small applied torque wasenough to reduce the speed of the flywheel.

5.3 Full System connected to Variable Resistive LoadThe scaled flywheel prototype was connected to a resistive load, as describedin Section 4.2.2. The functionality of the proposed driveline could be testedexperimentally, using the variable load connected to the flywheel high powerside, after a rectifier. The load current and flywheel output power change whenchanging the resistive load, causing a torque to be applied on the flywheel. Thedifferent voltage levels of the machine, the power buffer functionality and theinverter time response were measured.

The results of this investigation are presented in Paper II and Paper VIII.The experiment initiated with a load of 80 Ω, which was changed in se-

quence, first to an open circuit and then to a low resistance, down to a mini-mum of 15 Ω.

AC voltages from the low (red line) and high power side (black line) of thesystem are shown in Figure 5.18. Both voltages show a very low harmoniccontent. The gain in the amplitude of the HP windings is a consequence ofthe double wound machine, in which a higher voltage was obtained in the HPside, for a lower voltage applied on the LP side. The inverter output current(LP side) is shown in Figure 5.19.

The flywheel rotational speed variation between 2050 and 2150 rpm isshown in Figure 5.20. Between 0 and 5 s, the speed increased since the ex-tracted power was lower than the input power. After 5 s, the extracted powerincreased and the flywheel speed decreased in order to supply the load. Thecontrol system responded, taking around 4 s to return to 2080 rpm after themaximum torque was applied.

The flywheel allowed a steady power delivery from the inverter, as shownin Figure 5.21. The power fluctuations on the low power side remained lowerthan 15 W for a fluctuation of 350 W on the load side.

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Figure 5.18: Flywheel machine low- andhigh-power side line voltages.

Figure 5.19: Line current on the low powerside.

Figure 5.20: Rotational speed of the fly-wheel machine.

Figure 5.21: Power delivered by the in-verter and power consumed in the load.

5.4 Full System connected to AC MachineThe double wound flywheel machine was connected to a three-phase AC PMsynchronous machine, as described in Section 4.2.4. The AC wheel machinein the driveline required a bidirectional AC/DC/AC converter to link the HPside of the system.

Simulations and experimental results were obtained for the traction (or ac-celeration) mode of operation. The braking mode (regenerative braking) wasdesigned and simulated, and its implementation is currently under develop-ment.

The results of this investigation are presented in Papers IV, V, XI and XII.

5.4.1 Traction ModeEnergy flows from the battery to the wheel machine (working as a motor)during traction mode and the flywheel-side converter operates as rectifier,

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whereas the load-side converter operates as an inverter. DC-link voltage con-trol is performed by the rectifier. The rotational speed of the wheel machine iscontrolled by the inverter. The reference speed (or the signal used for settingthe speed and acceleration of the vehicle) is directly given, and continuouslyupdated, from a drive cycle simulation or from a sensor.

5.4.1.1 Traction Mode SimulationsTraction mode was simulated in a full system model implemented in Simulink.Battery voltage was 120 V and DC-link reference voltage was 300 V. Theflywheel initial and reference speed were 5000 rpm and the wheel motor initialand reference speed were 500 rpm.

Figure 5.22: System input/output power. Figure 5.23: Voltage over the DC link.

Figure 5.24: Rotational speed of the ma-chines.

Figure 5.25: Line current and voltage onthe flywheel HP side.

A constant load torque of 5 Nm was applied to the wheel motor at t = 1s. The load increased the power consumed by the wheel machine, in orderto keep the same rotational speed. The increased power was taken from the

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energy stored in the DC-link capacitor, which was recharged from the HP sideof the flywheel machine, since the control of the rectifier aimed at keeping thesame DC-link voltage (300 V). The voltage slightly varied when the torque isapplied, as shown in Figure 5.22.

The power from the battery on the LP side (Pin) and the power consumedby the wheel motor (Pout) are shown in Figure 5.23. During the completesimulation (3 s), the average power consumed by the wheel motor was 152W, meantime the average input power was 173 W. This indicates an averageefficiency of 88% according to the simulations.

The rotational speed of the flywheel and the wheel motor are presented inFigure 5.24. The speed of the flywheel varied less when compared to wheelspeed, since the flywheel was spinning with a relatively high speed. Shortlyafter the torque being discontinued, both speeds returned to their referencevalues.

Line current and voltage on the HP side of the flywheel are shown in Figure5.25. Due to the implemented control of the force-commutated rectifier, unitypower factor operation was reached.

5.4.1.2 Experimental ResultsA test similar to the one presented in the simulations was experimentally per-formed, during 50 s. Battery voltage was 25 V and DC-link voltage was equalto 80 V. Initial rotational speed of the flywheel was around 2000 rpm, whilethe initial speed of the wheel motor was around 300 rpm.

Figure 5.26: Voltage over the DC link. Figure 5.27: System input/output power.

A varying torque, with and average value of 5 Nm, was applied to the ma-chine. The voltage over the DC-link sank slightly when the torque was ap-plied, as shown in Figure 5.26.

The power from the battery on the LP side (Pin) and the power consumed bythe wheel motor (Pout) are shown in Figure 5.27. During the experimental test(50 s), the average power consumed by the wheel motor was 70 W, meantime

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Figure 5.28: Rotational speed of the ma-chines.

Figure 5.29: Line current and voltage onthe flywheel HP side.

the input power was 80 W. This indicates an average efficiency of around 87%according to the experimental results, similar to the results obtained from thesimulations.

The rotational speed of the flywheel and wheel motor are shown in Figure5.28. The rotational speed of the flywheel sank in order to supply the powerrequired to keep the DC-link.

Line current and voltage on the HP side of the flywheel are shown in Figure5.29. Unity power factor operation was reached.

Additional experimental results are shown in Figure 5.30. The flywheel wasdisconnected from the battery and the load was kept the same, so the outputcurrent of the DC-link would not vary. The speed of the flywheel decreasedas shown in Figure 5.30a. The voltage in the DC-link was kept constant (80V) until the speed the flywheel was around 20 rpm, as shown in Figure 5.30b.The output current on the flywheel high power side is shown in Figure 5.30c. Itincreased as the speed of the flywheel decreased, to compensate for the fallingoutput voltage.

5.4.2 Braking Mode SimulationsIn regenerative braking mode, the wheel machine-side operates as a recti-fier, whereas the flywheel-side converter operates as an inverter. The wheelmachine works as a generator. The here presented control was obtained bymonitoring the wheel machine output power, which now flows in the reversedirection and attempting to send the same amount of power from the DC-linkto the flywheel. A description of the suggested control strategy can be foundin Section 3.5.

Two simulation models of the system were run according to the same drivecycle. Model 1 represented the Simulink model, while a Matlab equivalentmodel of the system was implemented in Model 2. The simulation model im-plemented in Simulink considered the detailed components and control of the

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Figure 5.30: a) Speed of the flywheel, b) voltage over the DC link, c) flywheel outputcurrent. (Unpublished results)

system described in Chapter 3. The Matlab simulation was a general modelof the flywheel during braking mode, based also on the drive cycles investiga-tion. Both models used parameters from the experimental set up described in4.2.4. The power Pin, which was the input to the simulation, was derived froma vehicle weighing 70 kg and with a front area of 1.1 m2. The drag coefficientCw was assumed to be 0.33.

The power and the actual speed of the car, calculated from the drive cyclesimulations are shown in Figure 5.31 and 5.32. The wheel machine, actingas a generator, had an initial speed of 570 rpm and the flywheel had an initialspeed of 4780 rpm. The DC-link voltage was set to 300 V. The power extractedfrom the wheels was the reference value in the present simulation. The samereference power was used in both rectifier and inverter control, as describedin Section 3.5.

The DC-link voltage, which varies under braking mode, is shown in Figure5.33. The capacitor discharged to supply the system losses as the power givento the capacitor and the power consumed by the flywheel were the same.

The final voltage over the DC-link was 260 V for both models, indicatinga variation of around 250 J of the energy stored in capacitor, in 5 s. Thus theaverage power out of the capacitor was 50 W. The efficiency when transmitted

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Figure 5.31: Regenerated power used asreference power in the proposed control.

Figure 5.32: Speed of the car, obtainedfrom the drive cycle simulations.

Figure 5.33: DC-link voltage.

from the wheels to the flywheel storage was around 92%, considering that theaverage power produced during braking was 608 W.

The speed of the flywheel, shown in Figure 5.34 for Models 1 and 2, in-creased due to the regenerated power. The speed of the flywheel increasedless when compared to the variation in the speed of the wheel machine, sincethe flywheel had a higher initial speed and, consequently, a larger amount ofenergy stored.

The speed of the wheel machine during braking is shown in Figure 5.35.Different falling rates corresponded to different amounts of power which wereregenerated.

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Figure 5.34: Speed of the flywheel ma-chine during braking.

Figure 5.35: Speed of the wheel machineduring braking.

5.5 Driveline LossesThe investigated driveline has a large number of components, and differentlosses can be associated. The main losses to be considered in the systemare: battery internal losses, power electronics losses (IGBTs, MOSFETs anddiodes), flywheel machine losses, passive components losses (inductors, ca-pacitors and resistors) and wheel machine losses.

Simulations were carried in Matlab in order to estimate the system totallosses. Experimental tests were also performed and compared to the resultsobtained from the calculations.

5.5.1 Simulation ResultsA theoretical calculation was implemented in Matlab, which used the param-eters of the components in the driveline. Switching losses were calculatedconsidering turn-on and turn-off times obtained from measurements. On-statelosses were calculated according to the information provided in the data sheetsof the components. The losses in the electric machines were calculated basedon theoretical approaches described by Santiago [102].

The voltage levels and the speed of the machines were set equal to theones used in the simulations of the complete driveline. Instantaneous valuesof power were chosen: 300 W for the input power and 1kW for the load power.

The results obtained from the calculation were in agreement with the sim-ulations and experimental results presented in Section 5.4.1. The losses weredivided in the different components of the system as shown in Figure 5.36.

The efficiency of the LP and HP sides were computed separately, and mul-tiplied in order to calculate an approximated efficiency of the driveline. Atotal efficiency of 86% was obtained. The lower efficiency obtained from thecalculations (in comparison with the simulations/experimental results) can be

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3%

10%

60%

3%

3%

21%

Battery

IGBTs

Flywheel machine

MOSFETs

Passive comp.

Wheel machine

Figure 5.36: Losses in the complete driveline according to each different component.

Figure 5.37: Experimental result of the losses in the complete driveline.(Unpublishedresults)

explained by the battery model, which was not considered in the simulationsor experiments. The flywheel machine was responsible for a major part of the

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losses, and this could be attributed to the mechanical losses. The mechanicallosses are to be reduced in the close future, with the insertion of a vacuumchamber and magnetic bearings to the system. Losses in the wheel machinewere also high, which could be explained since the machine was operated ina power/speed level which is lower than its nominal conditions.

The IGBT total losses were higher than the MOSFET total losses, eventhough there are two MOSFET boards and one IGBT board. This result canbe explained due to the nominal values of the IGBT board, which are higherthan those used during the experiment.

5.5.2 Experimental ResultsThe complete driveline set-up was tested, and measurements of voltage andcurrent were taken from different points in the driveline. The rotational speedof the flywheel machine and wheel motor were approximately 2000 rpm and180 rpm, respectively. A load torque, lasting around 10 s, was applied to thewheel machine. The power consumed at different points in the driveline wascalculated, with and without the applied load. The results of this experimentare shown in Figure 5.37.

During steady-state, all the power delivered by the battery was consumedby losses of the system. When a load torque was applied, there was an increasein the current levels and the ohmic losses were also increased.

According to the experimental results shown in Figure 5.37, the flywheelmachine consumed a major part of the power losses, as in the simulations.Losses in the low power converter system were around 8 W (difference be-tween battery power and LP side power), meantime the losses in the highpower converter system were around 12 W (difference between HP side powerand WM power). The LP side losses were mainly the losses in the IGBT basedconverter, whilst the losses on the HP side were consumed by the MOSFETconverters and the passive components. Considering the number of compo-nents, the losses in the MOSFETs were relatively lower than the losses in theIGBTs, showing agreement with the results obtained from the simulations.

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6. Conclusions

A novel all electric driveline based on a double wound flywheel machine hasbeen demonstrated. Simulations and experimental results of the system havebeen presented. The design and assembly of the power electronics and theircontrol scheme have been successfully implemented.

Reduced power variations on the battery side have been obtained underheavy load conditions, proving the system’s functionality, which has severaladvantages when compared to other all-electric drivelines.

A low power converter system, connecting the flywheel machine to the bat-tery has been designed, constructed and tested. The present power convertersystem controls the speed of the flywheel machine and the dynamics of the lowpower side. Simulations and experimental results have shown that ON/OFFcontrol is operational with high energy density flywheel, since the speed vari-ation is very small for high torque applications on the load side. The numberof discharging cycles over the battery can be reduced by letting the flywheelvary within safe and controlled limits. Attention should be given to the chem-ical aspects of the battery and how a discontinuous discharging mode affectsits lifetime. However, the here suggested disconnection would have a reducedimpact on the battery, since it is still connected to the DC/DC converter filtercapacitor.

Two different DC/DC converters have been investigated. A unidirectionalDC/DC converter, which works as a battery charger converter system has beenpresented. The designed converter stability has been evaluated through simu-lations under closed loop control. The proposed PI controller has kept a con-stant current or voltage during battery charging with a low ripple during buckand boost operation. The simulation has indicated that the control is robustenough to allow battery recharging despite the decrease in the flywheel outputvoltage when slowing down. The battery recharging within the present fly-wheel system can also be combined to a charging process from the grid byusing the same DC/DC converter.

A bidirectional DC/DC converter has also been simulated and implemented.The control system has been useful in flywheel applications where dynamicschange depending on the flywheel speed, and the requirements of power sta-bility into and out of the flywheel are important. The DC/DC converter canbe used to limit the current into the flywheel during a start-up situation, butalso boost the voltage when the back-EMF is high. In the same setup, when

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transferring power from the flywheel to the battery, the charging current canbe kept constant regardless of the declining input voltage.

Although the operation of both DC/DC converters has been focused on bat-tery charging application, for a flywheel acting only as a power handling de-vice, no significant amount of energy can be recharged in the battery. In prac-tice the only moments to efficiently recharge the battery are at the end of adrive cycle (when the vehicle stops) and during long periods of braking, i.e.long downhill slopes.

A bidirectional AC/DC/AC converter for the high power side of the drive-line has been simulated and built. Simulations and experimental results duringacceleration mode have shown good agreement. The designed controllers havemanaged to keep the different controlled signals almost equal to their refer-ence values. Unity power factor and low distortion have been achieved (bothin the simulations and experimental results) on the high power side terminals.

A power balance control for the AC/DC/AC converter during braking modehas been proposed. The control allows robust response, where the speed ofwheel machine has been varied according to the power calculated from a drivecycle. The results obtained from this control have been compared to a Matlabmodel of the system, with satisfactory agreement.

A theoretical estimation of the losses in the driveline has been implemented,and an experimental test was performed in order to verify the results. Resultshave shown how the losses are divided in the different components of thesystem, improving the understanding of the constituting components and opti-mization of the complete system. A major part of the losses has been attributedto the flywheel machine, due to the mechanical losses. The mechanical lossesare to be reduced in the close future, with the insertion of a vacuum chamberand magnetic bearings to the system.

The average efficiency of the driveline has been estimated during acceler-ation to be around 87% (battery-wheels). A regenerative braking strategy hasbeen simulated and an efficiency wheel-to-wheel of around 80% is expected.The system efficiency can be improved to over 90% by reducing the losses inthe flywheel machine.

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7. Suggestions for Future Work

Even if there exists a complete and functional flywheel-based all-electric driv-eline, there are many improvements to be considered. Regarding the electricalsystem of the driveline, some suggestions can be pointed out for future work:

• Implementation of the regenerative braking and comparison to the simula-tion results: this work is currently under development. With the implemen-tation of the regenerative braking, the complete assembling of the drivelineis reached.• Battery constant output power: a full-scale four-quadrant DC/DC converter

can be implemented in order to control constantly the battery output power.• Voltage levels in the system: the voltage levels on the low power (LP)

and high power (HP) side of the driveline have not yet been defined. Thechoice of these voltage levels is an optimization between the reduction ofthe losses and the cost of the electrical/electronic components.• Exploring different battery technologies: depending on which battery tech-

nology to be used in the driveline, some changes in the control strategiesmight be required.• Grounding: a detailed study of the system grounding can be made.• Improvement of the electronic drivers: Gate drivers are the interface be-

tween control systems and high power electronic. The larger currents thedriver can handle, the faster the gate charge can be injected or removed andthe more efficient the power circuit will be.• Simulations: improvement of the complete simulation of the driveline, with

the development of a special model for the double wound flywheel ma-chine.• Power electronics: other types of power electronics technologies can be

tested in order to improve the system functionality and efficiency. Multi-level inverters can be an option if a high voltage level is chosen for the HPside, but the high swtching frequency required might be a drawback.• The driveline requires a large number of sensors for its functionality, what

can compromise the system’s cost and robustness. Future work may alsoconsist in advanced sensorless control techniques which might allow fullsystem operation with a reduced number of components.

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8. Summary of Papers

Paper IBattery Recharging Issue for a Two-Power-Level Flywheel System

The paper investigates the control of the power flow to the battery whenthe vehicle is parked, despite the decay of the flywheel machine voltage. Thedesign and simulation of an unidirectional DC/DC buck/boost converter fora variable rotational speed flywheel are presented. Conventional power elec-tronic converters are used in a new application, which can maintain a constantcurrent or voltage on the battery side. Successful PI current control has beenimplemented and simulated, together with the complete closed loop system.

The author has written the paper, performed the modelling and simulationsof the proposed DC/DC converter.

Published in Journal of Electrical and Computer Engineering, Vol. 2010,Article ID 470525, 5 pages, 2010.

Paper IIA Double Wound Flywheel System under Standard Drive Cycles: Simu-lations and Experiments

In this paper the functionality of the system is investigated by means ofsimulations and experiments. Different standard drive cycles are applied onthe high power side to assess the effect of load variations in the system as awhole and particularly in the speed control. The response of the speed controlsystem is investigated with computer simulations and experimental verifica-tion. The energy storage in the flywheel allows a steady power supply fromthe battery via the inverter, proving the functionality of the system.

The author has performed most of the writing, the Simulink simulations andthe experimental tests.

Published in International Journal of Emerging Electric Power Systems,Vol. 11, Iss. 4, Article 6, 2010.

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Paper IIIBattery Discharging Power Control in a Double-Wound Flywheel SystemApplied to Electric Vehicles

The paper focuses on the converter system and the control logic for regulat-ing the battery discharging process and the flywheel rotational speed. Empha-sis are given to the overall power/energy management of the system. Simula-tions and experimental results show that an ON/OFF battery control allows aefficient system, requiring a robust speed control and high energy density forthe flywheel machine.

The author has performed most of the writing, the Matlab simulations andthe experimental tests.

Published in International Journal of Emerging Electric Power Systems,Vol. 12, Iss. 1, Article 7, 2011.

Paper IVPower balance control in an AC/DC/AC converter for regenerative brak-ing in a two-voltage-level flywheel based driveline

A power matching control applied to an AC/DC/AC converter for regener-ative braking application is discussed in Paper IV. The AC/DC/AC converterregenerates the electric power converted during braking to the flywheel ma-chine, here used as power handling device. By controlling the power balance,the same hardware can be used for acceleration and braking providing reduc-tion of harmonics and robust response. A simulation of the complete systemduring braking mode is performed both in Matlab and Simulink and results arecompared. The functionality of the proposed control is shown and discussed,with full regeneration achieved.

The author has performed most of the writing, the modelling of theAC/DC/AC converter and the Simulink simulations (Model 1).

Accepted for publication in International Journal of Vehicular Technology,August 2011.

Paper VA study on doubly fed flywheel machine based driveline with anAC/DC/AC converter

The paper presents simulations and experimental results of the two-power-level driveline, where the control and electronics used are presented and thesystem efficiency is discussed. The control strategy of the AC/DC/AC con-

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verter used on the high power side of the driveline is discussed. Simulationsof the complete system are carried in Simulink and compared to the experi-mental results, obtained from the scaled experimental test set-up. Simulationsand experimental results show good agreement. The average efficiency of thedriveline during a simple drive cycle is obtained. A theoretical calculationbased on the real parameters of the system is implemented.

The author has performed most of the writing, the simulations and con-tributed to the experimental tests.

Submitted to IET Electrical Systems in Transportation, June 2011.

Paper VIControlling a Permanent Magnet Motor using PWM converter in Fly-wheel Energy Storage Systems

The paper presents a power DC/AC converter to govern an AC flywheelmachine. Different load connections are investigated. An output RLC filter isdesigned and built to minimize the harmonics due to the switching operationsof the Pulse Width Modulated (PWM) converter that drives the motor. Simu-lations are compared to the corresponding laboratory experiments. It is foundthat the harmonics are considerably reduced when a RLC output filter is in-cluded in the system. The simulation results are verified with the experimentalresults as they show a good agreement.

The author has written the paper, performed the simulations and the exper-imental tests.

Published in Proceedings of the 34th Annual Conference of the IEEE In-dustrial Electronics Society, Orlando, USA, pp. 3364-3369, 2009. (Presentedorally by the author.)

Paper VIIPower Electronics and Control of two-voltage-level flywheel based all-electric driveline

The paper presents the complete design and simulation of the proposed fly-wheel system when connected to an AC wheel machine. Vector control basedspeed regulators are designed and successfully simulated. DC link voltagecontrol is achieved by using synchronous rectification. Power estimation isused during regenerative braking in order to charge the flywheel with thepower generated from the vehicle speed reduction. Simulations verify thefunctionality of the proposed system.

The author has written the paper and performed the simulations.

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Published in Proceedings of the IEEE International Symposium on Indus-trial Electronics, Gdansk, Poland, pp. 1-7, 2011. (Presented orally by the au-thor.)

Paper VIIIDesign parameters calculation of a novel driveline for electric vehicles

The paper investigates the dynamic behaviour of a vehicle operating ac-cording to a standard drive cycle. Parameters of the flywheel based driveline(such as power rates and size of the flywheel) are obtained by optimization. Adescription of the performance of a Two-Voltage-Level Machine is presentedthrough its equivalent circuit and the control of the machine. Special attentionis given to the system losses. A scale prototype is constructed and tested undera drive cycle, demonstrating the system performance of the system.

The author has contributed to the written material, the motor control simu-lations and performing the experiment.

Published in World Electric Vehicle Journal, Vol. 3, ISSN 2032-6653-2009.

Paper IXSliding Mode 4-Quadrant DC/DC Converter for a Flywheel Application

The paper focuses on the design and construction of a four-quadrant DC/DCconverter. The target application is the flywheel based all-electric driveline,with focus on the battery recharging process. The control decisions are basedentirely on the latest available measurements, implying that no memory needsreinitializing when changing quadrant (such as for PI methods). The boostcontrol is based on a topology specific current source approximation. Thecontrol is found to be parameter invariant, regardless of high input/output dy-namics variance.

The author has contributed to the written material and the experimentalresults. (Diploma work thesis under the supervision of the author.)

Submitted to Control Engineering Practice, July 2011.

Paper XPrototype of electric driveline with magnetically levitated double woundmotor

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This paper describes a bench test set-up under construction to investigatethe properties of the flywheel system in details. The proposed set-up is ex-pected to achieve a level of power and energy close to that of a full scale sys-tem. This will allow measurements of complete drive cycles to be performed,improving the understanding of the constituting components and optimizationof the complete system.

The author has contributed to the written material.Published in Proceedings of the International Conference on Electrical Ma-

chines, Rome, Italy, pp. 1-5, 2010.

Paper XIImplementation and Control of an AC/DC/AC converter for doublewound flywheel application

This paper presents the implementation and control of the AC/DC/AC con-verter, used to connect the flywheel high voltage side to the wheel motor.Converter general operation and the control strategy adopted are discussed.The implementation of the AC/DC/AC converter is described from a practi-cal perspective. Results from experimental tests performed in the full systemprototype are presented. The prototype system is running with satisfactorystability during acceleration mode.

The author has contributed to the written material and the experimentaltests.

Accepted for publication in the Proceedings of the IEEE International Con-ference on Control and Automation, Santiago, Chile, 2011.

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9. Svensk Sammanfattning

Många länder har förändrat sin politik för att gynna utvecklingen av teknikersom strävar mot mer långsiktigt hållbara energisystem. Effektiva och tillförl-itliga elfordon kommer att bidra till denna utveckling.

Optimeringen av det elektriska drivsystemet är en av de viktigasteutmaningarna för att göra elbilar konkurrenskraftiga med traditionella bensin-och dieseldrivna bilar. Att ersätta förbränningsmotorn med en elmotor, elleratt använda en kombinerad el- och förbränningsmotor, är fördelaktigt. Dettap.g.a. den höga verkningsgraden som är högre än 90% för elfordon och igenomsnitt 40% för fordon med förbränningsmotorer.

För att kunna hantera den stora kraften vid acceleration och vid regenerativbromsning är svänghjulssystem attraktiva att använda i elfordon. Kombina-tionen av ett svänghjul och ett batteri har flera fördelar, såsom högre topp-effekt, högre energitäthet och en minskning av antalet partiella laddnings-/urladdningscykler i batteriet.

I det här projektet studeras en helt elektrisk drivlina baserad på ettsvänghjul. Svänghjulet är unikt då det har två olika spännings-/effektnivåeroch därför samtidigt kan fungera både som motor och generator. Systemetkan därför hantera effekten som utvecklats under dynamiska processer(bromsning/acceleration) på ett effektivt sätt.

Den kompletta drivlinan består av tre huvudkomponenter: den huvudsak-liga energikällan (t.ex. batteri), svänghjulet och drivmotorn. Högeffektsidan(HP) av svänghjulet förbinder svänghjulet till drivmotorn och lågeffektsidan(LP) förbinder svänghjulet till batteriet.

Det elektriska framdrivningssystemet är elbilens hjärta. Den överför elkraftmed en hög verkningsgrad och kopplar samman de mekaniska rörliga delarna.Den elektriska delen av ett elfordon består av en elektrisk maskin, kraftelek-tronik och kontrollsystem.

De olika huvudkomponenterna i systemet kan kopplas samman med hjälpav frekvensomriktare och DC/DC-omvandlare som omvandlar spänningen tillden frekvens och amplitud som krävs för sammankoppling. Styrningen avfrekvensomriktarna och DC/DC-omvandlarna är viktiga för funktionalitetenhos hela drivlinan, och är därför ett utmanande område inom detta projekt.

Denna avhandling fokuserar på frekvensomriktarna och DC/DC-omvandlarna och de kontrollstrategier som används för att styra dessa.Modellering, simulering och konstruktion av omvandlarsystemet,

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tillsammans med monteringen av hela drivlinan, har varit målet för den härdoktorsavhandlingen.

Tre olika omvandlartopologier har undersökts:• DC/DC-omvandlare. Två olika topologier har simulerats och testats; en

enkelriktad DC/DC-omvandlare, som fungerar som en batteriladdare, ochen dubbelriktad DC/DC-omvandlare.• DC/AC-omvandlare på LP-sidan, som kontrollera hastigheten på

svänghjulet.• AC/DC/AC-omvandlare på HP-sidan, som ansluter svänghjulet till drivmo-

torn.Fyra olika experimentuppställningar av systemet har satts upp. I experi-

menten har mätningar av drivsystemets olika cykler genomförts. Resultatenfrån experimenten har lett till en ökad förståelse av de ingående komponen-terna och förslag på hur det kompletta systemet kan optimeras.

Olika kontrollstrategier har föreslagits och undersökts och resultaten harvisat att kontrollstrategierna i drivlinan kan ge en jämn uteffekt från bat-terierna medan svänghjulet hanterar effektvariationerna på den drivande sidan.

En genomsnittlig verkningsgrad på cirka 87% (från batteri till hjul) harberäknats och bekräftas via simuleringar och mätningar. Omvandlarsystemethar visat sig vara effektivt och robust och kan hantera effektflödet i systemet.En regenerativ bromsningssekvens har simulerats med en förväntad verkn-ingsgrad hjul till hjul på cirka 80%.

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10. Acknowledgements

To my supervisor, Prof. Hans Bernhoff, for the opportunity and the confidencein my work. Also to my co-supervisor, Prof. Mats Leijon, for making thisdivision a great place to work.

To Ånpanneföreningens Forskningstiftelse and the Swedish Energy Agency(STEM), for funding this research project.

To Gunnel Ivarsson, Christina Wolf, Elin Tögenmark, Ingrid Ringård,Thomas Götschl and Ulf Ring for their help and kindness.

To Dr. Anders Larsson, Dr. Nelson Theethayi, Prof. Ladislav Bardos andProf. Hana Barankova, for the encouragement and help during the first yearsof my studies in Sweden.

To my colleagues Juan de Santiago, Johan Lundin, Johan Abrahamsson andMagnus Hedlund. Words can not describe the admiration and affection I feelfor you! Thank you for all the help, patience and everlasting discussions.

To my other colleagues at the Division for Electricity, for making this divi-sion the best place to work at. Special thanks to my colleagues on the secondfloor: Saman Majdi, Valeria Castellucci, Jose Perez and Kiran Kumar Kovi.

To Cecilia Boström, Johan Abrahamsson, Johan Lundin, Magnus Hedlund,Milena Moreira, Kiran Kumar Kovi and Katarina Yuen, for taking their timeto read this thesis and for the valuable comments. Thank you, Emilia Lalander,for the help with Latex.

To Nils Finnstedt, Henrique Schettino, Vinicius Gama and Renato Car-valho, for their contributions to this work.

To Prof. Francisco José Gomes, from the Federal University of Juiz de Fora,Brazil. Your work ethic is something I am still learning from.

To Carlos Martins and Arlei Lucas, for always answering to my calling forhelp.

To Alexandre Cury, for "being there", independently of which country ortime zone.

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To the new friends I gained after these years of adventure in Sweden, forenriching my life in so many different ways and lighting up the dark days.

To my friends in Brazil, for the support and for making this life a path worthto be continued.

To my parents, my mother Maria Helena and father José Maria, for havingtaught me everything, for the shared faith, for giving me your endless lovethroughout.

To my family in Brazil, my haven! My extended family in Costa Rica, thankyou for the sincere encouragement.

To my husband, Esteban, words can not describe my gratitude. You are abright, wonderful and tireless person, and I would never be writing these linestoday were it not for you. Obrigada!

"Stones in the road? I save every single one, one day I will build a castle"Fernando Pessoa

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