To my parents
Maria Helena and José Maria
List of Papers
This thesis is based on the following papers, which are referred to in the text
by their Roman numerals.
I J. G. Oliveira, H. Bernhoff, “Battery recharging issue for a two-powerlevel flywheel system”. Journal of Electrical and Computer Engineering, Vol. 2010, Article ID 470525, 5 pages, 2010.
II J. G. Oliveira, J. Lundin, J. Santiago, H. Bernhoff, “A double wound
flywheel system under standard drive cycles: simulations and experiments”. International Journal of Emerging Electric Power Systems, Vol.
11, Iss. 4, Article 6, 2010.
III J. G. Oliveira, J. Abrahamsson, H. Bernhoff, “Battery discharging
power control in a double-wound flywheel system applied to electric
vehicles”. 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 an
AC/DC/AC converter for regenerative braking in a two-voltage-level
flywheel based driveline”. Accepted for publication in International
Journal 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 with
an AC/DC/AC converter”. Submitted to IET Electrical Systems in
Transportation, 2011.
VI J. G. Oliveira, A. Larsson, H. Bernhoff, “Controlling a permanent magnet motor using PWM converter in flywheel energy storage systems”.
Proceedings of the 34th Annual Conference of the IEEE Industrial Electronics Society, Orlando, USA, pp. 3364-3369, 2009.
VII J. G Oliveira, H. Bernhoff, “Power electronics and control of
two-voltage-level flywheel based all-electric driveline”. Proceedings of
the 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 for
electric vehicles”. World Electric Vehicle Journal, Vol. 3, ISSN 20326653-2009.
M. Hedlund, J. G. Oliveira, H. Bernhoff, “Sliding Mode 4-Quadrant
DC/DC Converter for a Flywheel Application”. Submitted to Control
Engineering Practice, 2011.
X J. Abrahamsson, J. Santiago, J. G. Oliveira, J. Lundin, H. Bernhoff,
“Prototype of electric driveline with magnetically levitated double
wound motor”. Proceedings of the International Conference on
Electrical Machines, Rome, Italy, pp. 1-5, 2010.
XI H. Schettino, V. Gama, R. Carvalho, J. G. Oliveira, H. Bernhoff, “Implementation and control of an AC/DC/AC converter for double wound
flywheel application”. Accepted for publication in Proceedings of the
IEEE International Conference on Control and Automation, Santiago,
Chile, 2011.
IX
Reprints were made with permission from the publishers.
The author has contributed to the following papers which are not included in the
thesis.
XII J. G. Oliveira, R. Carvalho, V. Gama, H. Schettino, H. Bernhoff, “Implementation of an AC/DC/AC converter for electric vehicle application”. Accepted for publication in Proceedings of the IV Brazilian Conference on Energy Efficiency, Juiz de Fora, Brazil, 2011.
XIII J. Santiago, J. G. Oliveira, J. Lundin, A. Larsson, H. Bernhoff, “Losses
in axial-flux permanent-magnet coreless flywheel energy storage systems”. Proceedings of the 18th International Conference on Electrical
Machines, Vilamoura, Portugal, pp. 1-5, 2008.
XIV J. Santiago, J. G. Oliveira, J. Lundin, J. Abrahamsson, A. Larsson and
H. Bernhoff, “Design parameters calculation of a novel driveline for
electric vehicles”. Proceedings of the EVS- 24th International Battery,
Hybrid and Fuel Cell Electric Vehicle Symposium and Exhibition, Stavanger, 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 generation system based on renewable energy”. Proceedings of the International Conference on Electricity Distribution, Frankfurt, Germany, Paper 0940, pp. 1-4, 2011.
XVI J. Santiago, J. G. Oliveira, H. Bernhoff, “Filter influence in rotor losses
in coreless axial flux permanent magnet machines”. Submitted to Journal of Electrical Systems, 2011.
Book Chapter
XVII
J. Santiago, J. G. Oliveira, Electric machines topologies in energy
storage systems. Chapter 1 in Energy Storage, edited by Rafiqul Islam
Sheikh, Sciyo, 2010.
Contents
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1 Flywheels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Flywheel Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Vehicular Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.1 Energy Storage Systems . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.2 Applications of Flywheels . . . . . . . . . . . . . . . . . . . . . . . .
1.3.3 Power Buffer Technologies . . . . . . . . . . . . . . . . . . . . . . .
1.3.4 Safety and Gyroscopic Forces . . . . . . . . . . . . . . . . . . . . .
1.4 Two-Power-Level Flywheel System . . . . . . . . . . . . . . . . . . . . .
1.4.1 Double Wound Flywheel Machine and Applications . . . . .
1.4.2 Application Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1 Energy, Power and Torque in Flywheel Systems . . . . . . . . . . . .
2.2 Electric Machines Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Two-Voltage-Level Machine Equations . . . . . . . . . . . . . .
2.2.2 Mathematical Model of a Permanent Magnet
Synchronous Machine Drive . . . . . . . . . . . . . . . . . . . . . .
2.3 Power Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1 AC/DC Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2 DC/AC Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.3 DC/DC Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Control of AC Machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.1 Scalar V/F Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.2 Vector Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 Switching Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.1 Pulse Width Modulation . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.2 Space Vector Modulation . . . . . . . . . . . . . . . . . . . . . . . . .
2.6 Semiconductor Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.1 Losses in Semiconductor Devices . . . . . . . . . . . . . . . . . .
2.7 Control Systems Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Components Description and Control Strategies . . . . . . . . . . . . . . .
3.1 System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Battery Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Unidirectional DC/DC Converter . . . . . . . . . . . . . . . . . . .
3.2.2 Bidirectional DC/DC Converter . . . . . . . . . . . . . . . . . . . .
3.2.3 ON/OFF Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
13
15
16
16
17
18
20
20
21
22
25
25
26
26
28
29
30
31
31
32
32
33
33
34
34
35
36
37
39
39
40
41
41
43
3.3 Flywheel Charge Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4 Drive Mode Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.1 AC/DC Converter Control . . . . . . . . . . . . . . . . . . . . . . . .
3.4.2 DC/AC Converter Control . . . . . . . . . . . . . . . . . . . . . . . .
3.5 Braking Mode Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.1 PID Controller Design . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.2 Drive Cycles Investigation . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Experimental Set-Ups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1 Flywheel Charging . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.2 Loaded Flywheel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.3 Flywheel Driveline with DC Wheel Machine . . . . . . . . . .
4.2.4 Complete Driveline Set-Up . . . . . . . . . . . . . . . . . . . . . . .
4.2.5 Measurement System . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 Summary of the Results and Discussions . . . . . . . . . . . . . . . . . . . .
5.1 Battery Charging System . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.1 Unidirectional Converter . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.2 Bidirectional Converter . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Battery Discharging Control . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 Discharging Simulations . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Full System connected to Variable Resistive Load . . . . . . . . . .
5.4 Full System connected to AC Machine . . . . . . . . . . . . . . . . . . .
5.4.1 Traction Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.2 Braking Mode Simulations . . . . . . . . . . . . . . . . . . . . . . . .
5.5 Driveline Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.1 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.2 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7 Suggestions for Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8 Summary of Papers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9 Svensk Sammanfattning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
46
46
47
47
49
49
49
51
51
52
53
55
57
58
61
61
61
63
65
65
67
69
70
70
73
76
76
78
79
81
83
89
91
93
Nomenclature
B
C
Cw
D
eind
En
EA , EH , EL
f
fs
id , iq
I, IH , IL
If
I0
J
kw
Kp
Ki
Kd
L, LH , LL
Ld , Lq
M
N
p
P
Ps
[Nms]
[F]
[-]
[-]
[V]
[J]
[V]
[Hz]
[Hz]
[A]
[A]
[A]
[A]
[kg · m2 ]
[-]
[-]
[-]
[-]
[H]
[H]
[H]
[-]
[-]
[W]
[W]
Friction factor
Capacitance
Air drag coefficient
Duty ratio
Induced voltage
Energy in flywheel
Internal voltage
Electric frequency
Switching frequency
Direct and quadrature axis current
Phase current
Field current
Current through a switch
Moment of inertia
Constant representing construction of the machine
Proportional gain
Integral gain
Derivative gain
Internal inductance
Direct and quadrature axis inductance
Mutual Inductance
Number of coils exposed to the same magnetic flux variation
Number of pole pairs
Power
Switching losses
9
Pon
R, RH , RL
R0
T , Te , Tl
tc(on)
tc(o f f )
ton
Ts
V , VH , VL
vd , vq
Vd
Von
δ
ζ
ξ
σ
φ
ω
ω0
10
[W]
[Ω]
[Ω]
[Nm]
[s]
[s]
[s]
[s]
[V]
[V]
[V]
[V]
[-]
[-]
[-]
[-]
[Wb]
[rad/s]
[rad/s]
On-state losses
Internal resistance
Resistive load
Torque
Turn-on time
Turn-off time
Conducting time
Switching period
Phase voltage
Direct and quadrature axis voltage
Voltage over a switch
On-state voltage
Hysteresis band
Damping factor
Swift angle between winding sets
Error signal
Magnetic flux
Rotational speed
Natural frequency
Abbreviations
AC
CPS
CVT
DC
DSC
DSP
EMF
ESR
EVs
FESS
HP
ICE
IGBT
LP
MCU
MMF
MOSFET
PCS
PID
PMSM
PV
PWM
RPM
SVM
THD
TVLM
Alternating Current
Constant Pressure System
Continuously Variable Transmission
Direct Current
Digital Signal Controller
Digital Signal Processor
Electro-Motive Force
Equivalent Series Resistance
Electric Vehicles
Flywheel Energy Storage System
High Power
Internal Combustion Engine
Insulated-Gate Bipolar Transistor
Low Power
Micro Control Unit
Magneto-Motive Force
Metal-Oxide-Semiconductor Field-Effect Transistor
Power Converter System
Proportional-Integral-Derivative Controller
Permament Magnet Synchronous Motor
Photovoltaic
Pulse Width Modulation
Rotations Per Minute
Space Vector Modulation
Total Harmonic Distortion
Two-Voltage-Level Machine
11
1. Introduction
Energy and power: words that are spoken more and more frequently nowadays. There are several ways of generating usable energy, but once the energy
is converted it is also important to be able to store it, allowing humans to
balance the supply and demand of energy.
Many countries have been changing their policies in order to develop technologies aiming for more sustainable energy systems. Efficient and reliable
Electric Vehicles (EVs) will contribute as a key technology in this transformation. However, electric energy storage components are still limited due to
their low energy density and long recharge time when compared to internal
combustion engine vehicles [1].
The storage of energy in an efficient and secure way is a very important
issue and it dates from ancient times. Today, the commercial use of energy
storage systems can be broadly categorized as mechanical, electrical, chemical, biological, thermal and nuclear [2].
In this chapter, a basic introduction to flywheel systems is given, along with
a description of the flywheel research project at Uppsala University. The application context of the work done within this thesis is also discussed.
1.1
Flywheels
Flywheel Energy Storage Systems (FESS) are classed in the group of mechanical storage systems [3]. The principle of energy storage with a flywheel
is not new. It is based on the rotating mass principle. A flywheel stores kinetic
energy of rotation, where the stored energy depends on the moment of inertia
and the rotational speed of the flywheel.
The history of flywheels goes back thousands of years. The potter’s wheel
and the spinning wheel are two examples where the flywheel, with its inertia,
has converted a pulsating input power to a smooth output power. Early FESS
were purely mechanical, consisting of only a stone attached to an axle, as
shown in Figure 1.1.
Early publications about the application of flywheels date from the beginning 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 presented by N. W. Storer, 1902 [4].
13
A modification of the Ward-Leonard system of speed control [5], known as
Ilgner System, used a heavy flywheel on the motor-generator shaft to smooth
out peak loads, which would otherwise be taken from the power supply. In
1907, A. P. Wood [6] studied different ways of using Ilgner systems, so that a
3-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 by
J. S. Peck [8]. Peck claimed that flywheels have long been used as a load
equaliser, being cheaper, more efficient, and in general better suited than storage batteries.
Over time, the utilization of traditional flywheels decreased with the development of the electric grid. However, the technology came around again after
undergoing 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 an
electric machine, which is mechanically connected. Hence, the flywheel can
be accelerated by the machine acting as a motor when it is supplied with electric energy. Inversely, the machine acting as a generator can provide electrical
energy by slowing down the flywheel.
Progress in power electronics makes it possible to operate flywheels at high
power, with a power electronics unit comparable in size to the flywheel itself or smaller. Composite materials enables high rotational velocity. Magnetic bearings and vacuum operation offer very low friction during long-term
storage and longer life expectancy for high rotational speeds. High speed is
desirable since the energy stored is proportional to the square of the speed but
only linearly proportional to the mass [10].
A basic layout of the structure of a modern flywheel is shown in Figure 1.2.
14
Figure 1.2: Basic layout of a modern flywheel energy storage system.
1.2
Flywheel Technologies
Several hundreds of years ago, purely mechanical flywheels were used to keep
machines running smoothly from cycle to cycle. Later on, in the 1950’s an
early example of a power generating flywheel system is the "Gyrobus", produced in Switzerland, powered by a 1500 kg flywheel [11]. However, the development of modern flywheels started in the 1970’s, when NASA sponsored
programs proposing energy storage flywheels as possible primary sources for
space missions [12].
The fast response time of flywheels make them suitable for different applications in power systems. Flywheels have been used for harmonic compensation, being able to reduce them about 50% up to the 11th harmonic [13].
Companies from Europe and USA have developed flywheels with the purpose
of keeping the power quality; providing ride through for momentary power
outages, reducing harmonic distortions, eliminating voltage sags, etc [14]. For
example, Piller GmbH has installed flywheel energy storage system which can
absorb or supply 5 MW for 5 s in Dresden, Germany. Active Power (Austin,
Texas) has produced 4.75 MW flywheels for power conditioning and protection against power outages [15]. In Japan, a 200 MJ flywheel energy storage
system has been used for eliminating fluctuations in the active power supplied
to the magnets in the High-Energy Accelerator Research Organization [16].
Finally, in distribution network, a 10 MJ flywheel energy storage system, used
to maintain high quality electric power, managed to keep the voltage in the dis15
tribution network between 98-102% and was capable of supplying 10 kW for
15 min [17].
The storage of electricity generated by renewable sources is suitable for flywheels, since it can be used to match the fluctuating supply to the changing
demand of energy. A wind-diesel generator with flywheel energy storage has
been reported [18] with the goal of creating a unit where the regular wind
oscillations are compensated by the diesel generator and the flywheel. A motor flywheel integrated with a photovoltaic system has been simulated and it
managed to prolong the load supply from 9 a.m. to 3 p.m. to 8 a.m. beyond 6
p.m. [19].
In space applications, the International Space Station [20,21] uses the sun as
a primary power source. A Permanent Magnet (PM) motor/generator flywheel
has been simulated to keep the station functional during eclipses. In 1994,
The NASA Glenn Research Center devoted new efforts to develop flywheel
systems on satellites, combining energy storage capability and attitude control
[20, 22]. Nowadays, each NASA flywheel unit can store in excess of 15 MJ
and can deliver a peak power of 4.1 kW.
Other flywheel applications include aircraft launch systems [23] and pulsed
power 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 applications, whose interest has increased over the years because of environmental
issues and projected shortage of oil. Since the vehicular application is the focus of the present thesis, it will be discussed in more detail in the next section.
1.3
Vehicular Technology
Electric vehicles were quite commonplace during the end of the 19th century.
However, later on, they were completely abandoned in favour of internal combustion engine (ICE) vehicles. At that time, no one could envision the day the
world could run out of its fossil fuels reserves. But after many decades of high
consumption and increased concern over the environmental impact, the need
for 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 conventional ICE propulsion system combined with an electric propulsion system.
1.3.1
Energy Storage Systems
The traditional disadvantages of EVs compared to internal combustion engine vehicles are limited driving range and relatively long time needed to
recharge [26]. Even modern batteries have an energy density roughly up to
two orders of magnitude smaller than those of fossil fuels and a limited power
16
density preventing rapid charging. To overcome these difficulties, different
technologies have been investigated including more powerful batteries, supercapacitors and flywheels [27].
3
s
-1
s
10
0
s
1
10
2
10
s
3
10
-2
s
10
Batteries
2
10
-3
s
10
Flywheels
1
10
10
Specific energy (Wh/kg)
s
10
0
10
Ultracapacitors
-1
10
Electrolytic
Film
capacitors capacitors
-2
10
1
10
2
10
3
4
5
6
10
10
10
10
Specific power (W/kg)
7
10
Figure 1.3: Estimation of specific energy vs. specific power for different energy storage devices [28].
An estimation of the expected power/energy capabilities for different vehicular technologies in near future is shown in Figure 1.3. As illustrated, batteries have higher specific energy, but lower specific power, although the picture can slightly change depending on the battery technology. Ultracapacitors
(followed by electrolytic and film capacitors) have higher specific power but
lower specific energy. Flywheels can combine reasonable specific power and
specific energy, as shown in Figure 1.3. However, none of the presented technologies approaches the numbers for specific energy or power of fossil fuels
powered cars, which are around 104 Wh/kg and 105 W/kg respectively [29].
1.3.2
Applications of Flywheels
A FESS installed in a hybrid bus has been tested at the University of Texas at
Austin. The unit accelerates a fully loaded bus to 100 km/h, stores about 7.2
MJ and has a peak power capability of 150 kW, as well as a specific energy
of more than 120 kJ/kg of rotating mass and a specific power of 2.5 kW/kg of
rotating 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 energy
regeneration unit, the electric power consumption rate of the vehicle can be
17
188 km/l in the community-driving schedule, and over 50 km/l in the long
driving schedules (the electric power consumption rate has been converted to
the fuel consumption rate of gasoline).
A novel flywheel-engine hybrid system employing Constant Pressure System (CPS) to replace complex systems such as a planetary gear set or Continuously Variable Transmissions (CVTs) has been proposed [32].
The US Federal Railroad Administration has a program to develop FESS
for high speed rail applications. A CVT power train is used with a planetary
gear set and compact steel flywheel [33]. The flywheel plays a part only in
transient situations by compensating the engine inertia, making it possible to
optimize fuel economy in stationary situations without losing driveability in
transients.
In South Africa, flywheel systems for the purpose of allowing trams to operate beyond wires have been supplied by Alstom APS Flywheel Systems
[34, 35].
There is a particular interest in applying FESS in pulsed-power systems
included in all-electric/hybrid combat vehicles. In military affairs, the recent
released modernization plans for both U.S Navy and U.S. Army indicate their
intention to depend more heavily on electricity for both ships and ground vehicles [3].
Recent progress in the area of vehicular technology has been driven by Flybrid Systems [36]. The use a high-speed flywheel system for acceleration and
energy recovering during braking has been allowed since 2009 in Formula
One World Championship, where the system manufactured by Flybrid Systems 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 Technologies
The combination of a primary energy source, e.g. batteries, and a power buffer
can be used to meet the peak energy/power requirements of an electric vehicle.
Electric vehicle traction systems that combine a supercapacitor or flywheel
peak power buffer with the battery energy source, also called dual power
sources, have been evaluated [37].
A simple idealized power management scheme can be implemented within
the 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 in
Figure 1.4. The vehicle considered for this simulation has a mass of 1500
kg, a dimensionless drag coefficient Cw of 1.35, and a frontal area of 1.73
m2 . The power demand varies from 34 kW (when accelerating) to -26 kW
(when braking). However, the average electric power from the energy storage,
18
needed to propel an ordinary vehicle according to a standard FTP 75 urban
drive 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 wheels
instead 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 been
reported in the literature [38–41]. Results claim that supercapacitors offer high
efficiency (around 90%) and can be charged and discharged a large number of
times without performance deterioration. However, the supercapacitor kWh
cost is estimated to be between 10000-20000$/kWh. Flywheels, on the other
hand, have an estimated kWh cost of 500-1000$/kWh [42, 43]. Furthermore,
flywheels offer steady voltage and power level, independent of load, temperature or state of charge; no chemistry included, thus no environmental pollution
associated and efficiency and life cycles similar to the ones presented for supercapacitors [44, 45].
Peak Power Transfer
Flywheel
Electric Machine
DC Link
Vehicle Drive
System
Battery
Figure 1.5: Example of a driveline incorporating a flywheel system.
Three systems with different specifications and based on using the battery during normal driving condition and the flywheel during acceleration and
braking situations have been reported [46–48]. A diagram of such system is
shown in Figure 1.5. The battery and flywheel are connected to the DC-link
19
and the flywheel is responsible for absorbing the peak power transfer in the
system.
1.3.4
Safety and Gyroscopic Forces
Safety is a natural concern with FESS when in close proximity to people
[49]. An inertial containment system becomes necessary to minimize collateral damage in case of failure. Several safety projects have been funded
in the United States by the Defense Advanced Research Projects (DARPA),
the Houston Metro Transit Authority and NASA [50]. Flywheels have been
designed and operated with safe failure mode. Shock and vibration test of
an active magnetic bearing supported energy storage flywheel have been reported [51].
The gyroscopic forces are important for flywheels situated in vehicles,
satellites or space stations. The energy content of a rotor increases as the
square of the angular velocity, whereas the corresponding gyroscopic
moment increases linearly [52]. Gyroscopic forces would not present a risk
for a vehicle with a suspended flywheel at high rotational speeds, but the
effect should be considered when designing the system.
If necessary, one way to cope with the interaction of the forces in a vehicle
is to place the flywheel in a gimbal system, keeping the relative position of the
flywheel when the vehicle turns or lean, as shown in Figure 1.6.
1.4
Two-Power-Level Flywheel System
Many flywheel systems are under development around the world. Different
applications in EVs or hybrid vehicles have been investigated. However, existent flywheel-battery systems (dual power sources systems that combine battery 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 due
to battery voltage limitations.
The flywheel project at Uppsala University has its novelty in the driveline
topology which is divided in two different power and voltage levels. The HighPower (HP) side is connected to wheel motor and the Low-Power (LP) side
is connected to the battery, see Figure 1.6. The key component of this system
is a Two-Voltage-Level-Machine (TVLM), with stator windings arranged to
divide the system in two different voltage levels, similar to an electric transformer. In this configuration, an efficient system which handles the power
developed during fast dynamical processes is provided [53].
20
Figure 1.6: The EV propulsion system based upon a flywheel energy storage device
with two power levels.
1.4.1
Double Wound Flywheel Machine and Applications
The flywheel system under development at Uppsala University is based on a
double wound synchronous flywheel machine. The novelty lies in the configuration of the stator, which has two sets of three-phase windings with a different number of turns. The two sets of windings are magnetically coupled and
the transformer characteristic has to be taken into account. Thus, the TVLM
can operate as a motor and a generator between two power buses at different
power rating [54].
Other applications of similar machines can be found in the literature. In the
late 1920’s, six phase synchronous machines were used for power generation
[55]. The extra phases were needed to overcome the limitation imposed by
fault currents interrupting the capacity of circuit breakers.
In 1983, with increased demand for higher power drive systems, six phase
stator machines helped to overcome the current limitation imposed by semiconductor devices. These six phase drive systems improved torque and magnetomotive force (MMF) characteristics over those of standard three phase
inverter drive systems [56].
Double wound synchronous machine systems were also used as DC to AC
motor/generators in the field of electric railways, in order to improve service
to supply the air conditioners from the DC supply (pantograph) [57].
A new concept of rotating machines that enable direct connection of synchronous generators to the transmission network without any intervening stepup transformers was developed by Leijon et al. [58]. Such a synchronous machine, called Powerformer, has the possibility of simultaneous direct connec21
tion to several different grid voltages through different stator windings. The
secondary stator winding can also be used for power supply at the standard
medium and low-voltage levels to feed power plant auxiliaries.
The Optimal Flywheel Power Module [23], manufactured by Optimal Energy systems, is used to provide pulses of energy for charging high voltage
capacitors in a mobile military system. As in the present flywheel system, a
low power side in the machine is used to receive energy from a DC bus. High
output power is provided on the secondary side.
1.4.2
Application Context
The here investigated flywheel system is physically divided in two power levels through the flywheel machine stator windings. Each side connects the flywheel machine to another component of the system (e.g. battery or wheel
motor). The connection is made through electrical power converters, which
convert input/output signal to the shape and frequency needed for coupling
the system. The power converters are controlled so the desired energy flow in
the battery-flywheel and flywheel-wheel machine link is obtained.
A diagram of the complete flywheel system is shown in Figure
1.7. It contains three different bidirectional Power Converter Systems
(PCS): an AC/DC/AC converter on the HP side, connecting the flywheel
motor/generator to the wheel motor and a DC/AC plus a DC/DC converter on
the LP side, connecting the flywheel motor/generator to the battery.
Low Power Side (LP)
High Power Side (HP)
DC/DC
Converter
AC/DC/AC
Converter
C
AC/DC
Converter
Battery
C
C
C
Wheel
motor
Motor/Generator
Flywheel
Connected to the control unit
Figure 1.7: Power electronics and control of the proposed EV propulsion system.
The present thesis focuses on the field of electrical engineering and, more
specifically, on the electric power conversion system and control. This thesis
treats the design, simulation and construction of the power converter systems
and their control strategy. It addresses the complete assembled driveline.
22
The control and power electronics are very important for the present system. A correct control strategy is required to provide efficient and robust functionality of the driveline. The flywheel system has, in comparison to other
flywheel-based drivelines, a large number of power electronic converters and
consequently, control systems. The power converters should be designed so
the maximal possible efficiency in the driveline is achieved. Nonetheless, the
connection of two electrical machines with different and variable frequency
and amplitude of operation (i.e. the flywheel and the wheel motor) is particularly challenging.
Considering the importance of the electronic converters and their control
for the system [59], this thesis aims to present and discuss the proposed power
electronics 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 the
theoretical background of the thesis, describing the basic theory of electrical machines and the main approaches in control of AC machines. Chapter 3,
Components Description and Control Strategies, describes the different power
converters investigated and the suggested control strategies. Chapter 4, Methods, treats the simulations tools and the experimental set-ups which were implemented. Chapter 5, Summary of the Results, discusses the most important
results published in the papers attached to this thesis. Chapter 6, Conclusions,
summarize the results and discussions. Finally, Chapter 7, Future Work, comments the plans to be continued after the presented work.
23
2. Theory
This chapter gives a theoretical background to different areas presented in the
thesis. The first section presents general equations behind the flywheel energy
storage functionality. The second presents a brief description of electric machines theory. The third, fourth and fifth sections discuss the theory of power
electronics and control systems which have been applied in this thesis.
2.1
Energy, Power and Torque in Flywheel Systems
The principle of energy storage with a flywheel is not new. It is based on
the same principle as the potter’s wheel: a rotating mass. The flywheel stores
kinetic energy of rotation, where the stored energy En depends on the moment
of inertia J and the rotational speed ω [60]:
1
En = Jω 2
2
(2.1)
The flywheel can be used as a power handling device, in result from advances in key enabling technologies. Power is the rate at which energy is converted, given by:
P=
∆En 1 ω22 − ω12
= J
∆t
2
∆t
(2.2)
The torque T for a given (instantaneous) power output in electrical machines can be calculated as:
T=
P
ω
(2.3)
Note that the instantaneous power injected by the torque depends only on
the instantaneous angular speed and not on whether the angular speed increases, decreases, or remains constant while the torque is being applied.
25
2.2
Electric Machines Theory
An electric machine converts either mechanical energy to electrical energy
(generator) or electrical energy to mechanical energy (motor). The principle
of operation is based on interaction between the magnetic fields existent in the
interior [61]. Concerning the generator, the rotating magnetic field of the rotor
induces three-phase AC voltages into the stator armature windings, according
to the Faraday’s law:
eind = −N
dφ
dt
(2.4)
where N is the number of turns in the windings and φ is the magnetic flux
passing through the windings. The RMS voltage in any phase of a three-phase
stator is:
√
(2.5)
EA = 2πkw Nφ f
where kw is a constant representing the construction of the machine and f is
the electrical frequency.
Conversely in motors, a three-phase set current in the stator armature windings produces a rotating magnetic field which interacts with the rotor magnetic
field, producing a torque.
The AC electrical machines are divided into synchronous and
asynchronous. Synchronous machines are usually more efficient than
asynchronous machines and can more easily accommodate load power factor
variations. Permanent magnet synchronous machines do not require rotor
field excitation. These advantages of synchronous machines make them
suitable for electric vehicular applications [62].
A basic four pole synchronous machine is illustrated in Figure 2.1. The
rotor consists of salient poles which are wound with coils. The stator is slotted
to 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 Equations
The two-power-level system presented in this thesis is obtained by using
a double-fed synchronous machine. The equivalent circuit of the TVLM
is shown in Figure 2.2. The windings’ neutral points are not necessarily
connected. The rotor is magnetically linked to both sets of stator windings,
but the stator windings are also magnetically linked to each other constituting
a transformer. The voltage is, in this case, the result of both the mutual
magnetic coupling and the electromotive force induced by the rotor.
The three-phase high power windings are represented in black whereas the
low power windings are represented in red in Figure 2.2. The current in dif26
Figure 2.1: Basic four pole, three-phase synchronous machine with rotor field excitation.
Figure 2.2: Schematics of the two sets of three-phase windings in a two-voltage-level
machine.
ferent phases is represented by i and Va is the line to neutral voltage in phase
a. Internal resistance and inductance are indicated by R and L, meantime M
represents the mutual inductance. ξ is the swift angle between winding sets.
The equation that governs the equivalent electric circuit of a permanent
magnet synchronous machine is:
V = RI + L
dI
+ EA
dt
(2.6)
where V is the machine output voltage, I is the stator current and the back
Electro-Motive Force (EMF) EA can be calculated from Equation 2.5.
The TVLM can be evaluated as two separate synchronous machines with
magnetic coupling and common rotor speed. To represent the various opera27
tional models in the equivalent circuit, different loads are coupled to the connections of the machine. The most common operation mode of the TVLM is
when the high power side is acting as a generator and the low power side as a
motor; see Figure 2.3.
LH
LL
VH
LOAD
RH
M
High Power
side
RL
Low Power
side
EH cos (wt+x)
VL
EL cos (wt)
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 rewritten as:
dMIL
dIH
+
+ EH
(2.7)
VH = RH + LH
dt
dt
dIL
dMIH
VL = RL + LL
+
+ EL
dt
dt
(2.8)
where R, L and E represent the internal resistance, internal inductance and
back EMF, respectively, both for the high- and low-power sides. V represents
the output voltage on the high power side (acting as a generator) and the applied voltage on the low power side (acting as a motor). M represents the
mutual inductance.
The coupling condition for the voltage equations sets the same frequency
of both sides:
ω = ωH = ωL
(2.9)
2.2.2 Mathematical Model of a Permanent Magnet Synchronous
Machine Drive
The d-q transformation is a mathematical transformation used to reduce the
three-phase stationary coordinate system to the d-q rotating coordinate system [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 simplified linear state model. The voltage equations of the PMSM in the rotating
reference frame are [64]:
vd = Rid + Ld
did
− ωLq iq
dt
(2.10)
diq
+ ωφ
dt
(2.11)
vq = Riq + Lq
The electromagnetic torque Te can be written as
Te =
3p
[φ iq + (Ld − Lq ) iq id ]
22
(2.12)
where vq , vd , iq , id are the stator voltages and currents respectively. R is the
stator 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 of
the motor.
The torque can be related to the d- and q-axes currents, to the rotor type, its
inductances Lq and Ld and to the magnets mounted on the rotor, as expressed
in Equation 2.12. The electromechanical equation of a PMSM is given by:
dω
p
(Te − Tl ) = J
+ Bω
2
dt
(2.13)
where Tl , J and B represent the load torque, the inertia and the friction factor
of the motor respectively.
2.3
Power Converters
Power converters are an application of solid state electronics for the control
and conversion of electric power. Power electronic converters can be found
wherever there is a need to modify a form of electrical energy (i.e. change its
voltage, current or frequency).
Power conversion systems can be classified according to the type of the
input and output power: DC to AC (inverter), AC to DC (rectifier), DC to DC
and AC to AC.
There are different types of AC to AC conversion [65]. The present work
deals with a back-to-back converter, which is composed of an indirect AC/AC
converter connected via DC-link capacitor. The AC/DC/AC converter is also
called rectifier-inverter pair and can be studied as separated AC/DC and
DC/AC converters.
29
2.3.1
AC/DC Converters
Rectifiers are mainly divided into passive and active converters. Passive rectifiers use diodes to perform the signal conversion whereas active rectifiers use
switching devices (e.g. thyristors or transistors). Forced-commutated rectifiers
are built with semiconductors with gate-turn-off capability (transistors). The
main circuit of a force-commutated rectifier is shown in Figure 2.4. They are
bidirectional converters and can also be used as inverters when reverse power
flow 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 desired 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 generated
by 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 critical
to 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 iq
become DC currents and express the active and reactive currents. Therefore,
active and reactive power can be decoupled and controlled independently. The
DC output voltage can be controlled by the voltage loop, and the current response can be controlled by the current loop, as shown in Figure 2.6.
PI
-
V
Current
limiter
0
P*in
Q*in
iabc
Current regulator
+
dq-abc
V*
Pulses
Figure 2.6: Control block diagram for the DC-link voltage regulation.
2.3.2
DC/AC Converters
The DC to AC power conversion is realized by inverters. The main circuit of
an inverter is equal to the forced-commutated rectifier circuit, shown in Figure
2.4. When the converter shown in Figure 2.4 operates as an inverter, the power
flow changes the direction [67].
The control strategy of both inverter and rectifier is similar. The inverter
control will be further discussed later, when the control of AC machines is
presented, in Section 2.4.
2.3.3
DC/DC Converters
A buck converter is a step-down DC to DC converter [67], meaning that the
output voltage is lower than the input voltage. The buck converter conversion
ratio is:
M (D) =
ton Vout
=
=D
TS
Vin
(2.14)
where ton is the interval in which the switch is conducting and Ts is the switching 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 conversion
ratio is:
31
L
S1
Vin +-
C
+ R
- 0
Figure 2.7: Step-down buck converter with a resistive load R0 .
M (D) =
1
Vout
=
Vin
1−D
(2.15)
A basic circuit of the boost converter is shown in Figure 2.8.
L
S1
Vin +-
C
+
- R0
Figure 2.8: Step-up boost converter with a resistive load R0 .
2.4
Control of AC Machines
Many three-phase loads need a supply of variable frequency, requiring fast
and high-efficiency control by electronic means. In variable speed AC drives,
inverters are used to control the rotor speed through the supplied frequency
and the machine flux through the supply voltage [68].
2.4.1
Scalar V/F Control
The open loop scalar control is one way of controlling AC motors for variable
speed applications. It has the advantage of being relatively simple to implement and sensorless [69].
Voltage/frequency control is a scalar control method based on static model
of the motor. Its goal is to keep the stator flux linkage constant by controlling 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 flux
linkage constant, generally:
Vrated
V
=
f
frated
(2.16)
At low frequencies, the stator resistance cannot be ignored, being necessary
to maintain the voltage at a fixed value in this range of operation. A minimum
frequency is also used to improve the motor start-up. Through PWM (to be
discussed in Section 2.5.1), open-loop control acts on the motor, as shown in
the simplified block diagram of Figure 2.9.
Figure 2.9: Structure Block of V/f Control of PMSM.
2.4.2
Vector Control
Vector control of PMSM allows, by using d-q components, separating closed
loop of both flux and torque [71]. The electromagnetic torque can be expressed in d-q components according to Equation 2.12. To achieve the maximum torque/current ratio, which is a desired characteristic during acceleration
and deceleration in EVs, the d-axis current is set to zero during the constant
torque 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 illustrated in Figure 2.10. The torque request, Te , is generated by the speed
controller dependent on the speed error. By keeping the current id to zero,
maximum torque can be achieved.
2.5
Switching Techniques
The conversion of DC power to three-phase AC power can only be performed
in the switched mode. Power semiconductor switches connect the two DC
terminals 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
-
w
Angle conversion
qr
T
T
i
i
qr=qe
id
0
i*q
qe
id*
iabc
Current regulator
+
T*e
Torque
limiter
dq-abc
w*
Pulses
Figure 2.10: Typical permanent magnet synchronous machine control with current
and speed control loops.
of the respective switches. The desired sinusoidal waveform of the currents
can be achieved by varying the duty cycles sinusoidally with time, employing
techniques as Pulse Width Modulation (PWM) or Space Vector Modulation
(SVM) [72].
2.5.1
Pulse Width Modulation
Pulse-width modulation is a way of delivering energy through a succession of
pulses rather than a continuously varying (analog) signal [73, 74]. The controller regulates energy flow to the motor shaft by increasing or decreasing
pulse width. The motor’s own inductance acts like a filter, storing energy during the "on" cycle while releasing it at a rate corresponding to the input or
reference signal.
A simple comparator with a sawtooth carrier can turn a sinusoidal command
into a pulse-width modulated output, as shown in Figure 2.11. In general, the
larger the command signal, the wider the pulse.
High
Command
signal
+
Low
Comparator
Chopping
signal
PMW signal
-
Figure 2.11: Pulse Width Modulation Strategy.
2.5.2
Space Vector Modulation
Space vector modulation technique was originally developed as a vector approach to PWM for three-phase inverters [75]. SVM is mostly used when
implementing digital control, which is the case where PWM technique can
be difficult to implement. SVM is a more sophisticated technique for gener34
ating sine wave that provides a higher voltage to the motor with lower total
harmonic distortion (THD) [76].
The concept of space vector is derived from the rotating field of an AC
machine used for modulating the inverter output voltage. According to the
space vector theory, there are eight switch states, named as S0-S7 as shown
in Figure 2.12a. The output voltage of the inverter is composed by these eight
switch states, represented as vectors with 60o rotation between each state. A
classical sinusoidal modulation limits the phase duty cycle to the inner circle
as shown in Figure 2.12b. The space vector modulation schemes extend this
limit to the hexagon by injecting third order harmonics in the signal. The result
is about 10% higher phase voltage at the inverter output.
1
1
1
a
b
c
0
1
a
b
c
0
S0=000
a
b
c
0
0
1
1
a
b
c
0
1
a
b
c
0
S4=011
S3=010
S2=110
S1=100
1
a
b
c
a
b
c
0
a
b
c
0
S7=111
S6=101
S5=001
(a)
b
S3=010
S2=110
II
III
I
a
S0=000
S4=011
S1=100
S7=111
VI
IV
V
S5=001
S6=101
(b)
Figure 2.12: (a) Eight switching states, (b) Eight voltage space vectors of a threephase voltage source inverter.
2.6
Semiconductor Devices
Metal-oxide-semiconductor field-effect transistor (MOSFET) is used for amplifying or switching electronic signals, which came along in the 1970’s.
35
Insulated gate bipolar transistor (IGBT) is a three-terminal power semiconductor device, noted for high efficiency and relatively fast switching. It came
along in the end of the 1980’s.
MOSFETs’ and IGBTs’ structures look very similar [77]. IGBTs are used
in medium- to high-power applications such as switched-mode power supply,
traction motor control and induction heating. Large IGBT modules typically
consist of many devices in parallel and can have very high current handling
capabilities in the order of hundreds of amperes with blocking voltages of
6000 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 mode
power supplies with hard switching and rated power below 1000 W.
Between 250 and 1000 V, choosing between IGBTs and MOSFETs is very
application-specific. Cost, size, speed and thermal requirements should be
considered [78].
2.6.1
Losses in Semiconductor Devices
In power electronics, IGBTs and MOSFETs, as well as diodes, are operated
mainly as switches, taking on various static and dynamic states in cycles. In
any of these states, power dissipation is generated, which heats the semiconductor 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 total
loss, mainly in MOSFETs, where the switching frequencies are higher. At
every change of state, if the switch carrying current is opened, the voltage
rises across the switch and the current through it falls, resulting in dissipation
of a short pulse of power in the switch. Similarly, as the switch is closed, the
voltage 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 transitions
can be approximated as:
1
Ps = Vd I0 fs tc(on) + tc(o f f )
2
(2.17)
where Vd is the voltage across the switch, I0 is the current flowing through the
switch and fs is the switching frequency. tc(on) and tc(o f f ) are the turn-on and
turn-off time of the switch, respectively.
Another major contribution to the power loss in the switch is the average
power dissipated during the on-state Pon , which varies in proportion to the
on-state voltage. The on-state losses, or conduction losses, are given by:
Pon = Von I0
36
ton
Ts
(2.18)
which shows that the on-state voltage, Von , in a switch should be as small as
possible. ton is the interval in which the switch is conducting and Ts is the
switching time period.
2.7
Control Systems Theory
A study of control involves developing a mathematical model for each component of a self-contained process under study so called control system [79, 80].
Transfer functions commonly describe control systems. The transfer functions
are defined as the ratio of the Laplace transform of the output Y(s), and the input U(s), given by:
G (s) =
Y (s)
U (s)
(2.19)
Many different parameters can be investigated by knowing the transfer
function of a control system, including its stability. Stability is defined as the
ability of a system to return to equilibrium once disturbed.
Two methods of investigating the stability of a control system are the response to singularity functions and the root-locus.
The response to singularity functions requires that the transient response
should decay to zero after some time as for the linear system to be stable. The
steady state response of a linear system is generally of the same shape as the
applied input. Examples of singularity functions are the step response and the
impulse response.
The location of the poles and zeros of a transfer function in the Real X Imaginary 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 of
the feedback gain (assuming negative). A system is stable if all of its poles are
in the left-hand side of the s-plane (for continuous systems) or inside the unit
circle of the z-plane (for discrete systems) [79].
Once a control system is verified to be unstable (or in the case where the
system output needs to be precisely known), a compensator can be inserted
in the system. Additional controllers are used to place the poles/zeros of the
system in a desirable/known position.
The PID controller is one of the most used in feedback control design. PID
is an abbreviation for Proportional-Integral-Derivative, referring to the three
terms operating on the error signal to produce a control signal [81]. The PID
controller transfer function is given by:
G (s) = K p +
Ki
+ Kd s
s
(2.20)
37
where K p is the proportional gain, Ki is the integral gain and Kd is the derivative gain. Control can be provided by tuning the three constants in the PID
algorithm, designed for specific process requirements.
A high proportional gain results in a large change in the output for a given
change in the error, meantime the integral term accelerates the movement of
the process towards set-point. It eliminates the residual steady-state error that
occurs with a pure proportional controller. The derivative term slows the rate
of change of the controller output, and might not be required in some applications. The Proportional-Integral (PI) controller is a special case of the common
PID controller in which the derivative (D) of the error is not used.
38
3. Components Description and
Control Strategies
The present flywheel system has a large number of power converters and consequently, control systems. The converters and their control are very important
in the system. They regulate the functionality and safety of the different components, and are responsible for a part of the losses in the driveline, requiring
careful design. This chapter aims to present and describe the different power
converter systems and the control strategies used.
3.1
System Overview
The complete flywheel system is shown in Figure 3.1.
Figure 3.1: The EV propulsion system based upon a flywheel energy storage device
with two power levels.
The flywheel based driveline contains three different Power Converter
Systems (PCS): A DC/AC plus a DC/DC converter on the Low Power
(LP) side, connecting the flywheel motor/generator to the battery and an
AC/DC/AC converter on the High Power (HP) side connecting the flywheel
motor/generator to the wheel machine.
The DC/DC converter is used to control the battery output power, to limit
the battery output current or to boost the battery voltage. It can also be used to
recharge the battery with the energy stored in flywheel. The DC/DC converter
might not be required if the battery output power is controlled using the same
inverter (DC/AC converter) and a unidirectional converter is used (on or offboard converter) for battery recharging. However, the DC/DC converter can
decouple the control for battery output power and flywheel machine speed.
39
Furthermore, the voltage boost feature can be used to reduce the battery pack
voltage and extend the flywheel speed range. The bidirectional DC/DC converter shown in Figure 3.1 can be used during both acceleration mode and
battery charging.
The low power DC/AC converter is also bidirectional. It controls the
speed/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 the
battery, the body diodes from the two-level inverter bridge are used to
perform passive rectification.
Figure 3.2: The EV propulsion system based upon a flywheel energy storage device
with two power levels.
The high power side of the driveline connects the flywheel to the wheel
machine. AC machines are preferable as wheel machines, due to their high
efficiency and power density [71]. If an AC machine is used, a three-phase
four-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 motor), and the flywheel-side converter operates as a rectifier, whereas the loadside converter operates as an inverter, as shown in Figure 3.2. During braking
mode, the roles are reversed, i.e., the wheel machine-side converter operates
as a rectifier, whereas the flywheel-side converter operates as an inverter. The
wheel machine works as a generator.
The following sections briefly describe the control strategies implemented
for the different power converters used in the driveline. The subsections will
be divided as in Figure 3.1: Battery Control (Control 1), Flywheel Charge
Control (Control 2) and Drive/Braking Mode Control (Control 3).
3.2
Battery Control
Battery Control is used with the DC/DC converter. Two different power converter systems have been investigated. The control strategies presented in sub40
section 3.2.1 and 3.2.2 focus on battery recharging operation. ON/OFF control
strategy, suggested for battery output power control, is presented in subsection
3.2.3.
3.2.1
Unidirectional DC/DC Converter
The aim of the buck/boost converter is to control the current and voltage during battery recharging, using the energy stored in the flywheel. The stored
energy is sent back to the battery when the vehicle is parked. The main challenge is the control of the power flow to the battery despite the decay of the
flywheel machine voltage, which is dependent on its rotational speed.
The flywheel motor/generator is connected to a passive rectifier, as shown
in Figure 3.3. The rectifier output is connected to the unidirectional DC/DC
converter. Although connected in series, buck and boost functionality are not
used simultaneously in the present application [82]. The converters are connected in series to maintain the current direction, regardless of the operation
mode (boost or buck).
The unidirectional DC/DC converter can operate over a wide range of input
voltages. The converter can also maximize power transfer, since the boost
stage of the converter can provide power factor correction circuitry [83].
Constant current/voltage control can be accomplished with a Proportional
Integral (PI) controller by using the buck and boost converter transfer functions [84] in a closed loop system. The proportional and the integrative gain
are chosen to obtain the desired pole placement and, consequently, the desired
rise time and peak overshoot of the system response.
The application of the unidirectional DC/DC converter shown in Figure 3.3
is novel as both converters can work independently. To operate the converter as
a buck, S1 is open and S2 is chopping. To operate it as a boost, S1 is chopping
and S2 is conducting.
3.2.2
Bidirectional DC/DC Converter
The 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 boost
or buck functionality. The same dynamics of the singular boost and buck converters can be applied.
The DC/DC converter should control the energy flow between the battery
and the LP side of the flywheel machine. Hence, the desired control variables
are both output current and voltage. Table 3.1 shows the possible operation
modes and the control variables required.
A transition between quadrants (e.g. changing from buck to boost mode
during operation) implies change of transfer function. If a PID controller is
used, the integral memory (which sets the control value due to the model errors) 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.
IB
I DC
S1
VB
S2
L
C1
C2
VDC
IL
S3
S4
Figure 3.4: Equivalent circuit diagram of the bidirectional DC/DC converter.
dynamics, transitions will become difficult, and the system must be modelled
with adaptive techniques [86].
One way of solving this problem is by using a non-linear approach as sliding mode control, which assumes direct control of each switch state [87]. The
controller has been based on a set of decision laws, which can decide the operation 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 the
parameters considered when deciding the switching state. The error from the
comparison of the variables is called σ and the hysteresis band is represented
by δ . The switches Sa,up and Sb,down are referred as S1 and S4 in the illustration
of the DC/DC converter shown in Figure 3.4.
A general scheme of the control, where two target control variables are
used, is shown in Figure 3.5a. SM stands for Sliding Mode, and this block processes the logic structures presented in (b), (c) and (d). The operation mode
selection, where buck or boost mode is chosen depending on the system volt42
Table 3.1: Modes of operation of the DC/DC converter.
Energy Flow
Battery charging
Battery discharging
Battery discharging
Target control variable
Voltage/current control
Voltage control
Current control
Usage in driveline application
Battery charging process
Boost of battery output voltage
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 during
buck and boost mode are presented in Figure 3.5c and Figure 3.5d, respectively.
The control of the lower switch (Sb,down ) during buck mode is required when
leaving boost mode, so any excess of energy stored in the inductor could be
dissipated. The hysteresis bands are slightly lifted according to the gain k, as
to avoid the switches to interfere with each other.
A more detailed description of the converter modelling and the controller
decisions can be found in Paper IX and [88].
3.2.3
ON/OFF Control
The total energy transferred from a flywheel is given by Equation 2.2. In order
to transfer energy, the flywheel speed must be able to vary; therefore a fixed
speed control on the LP side of the flywheel is not indicated. However, this
variation should be kept between well-defined limits. The limits are chosen
depending on the machine mechanical parameters and safety issues.
43
A suggested logic for the control of the LP converter takes into account the
variation in the flywheel rotational speed and the LP PCS different modes of
operation, as illustrated in a flow chart in Figure 3.6.
Figure 3.6: Flow chart which describes the ON/OFF control strategy during battery
discharging 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 flywheel. If the speed S<T, the flywheel machine is accelerated to its nominal
speed (N). Once the machine rotates at nominal speed, the LP converter is
turned off and the flywheel is controlled by the HP side. The battery system
is activated and increases the speed back to the nominal value every time it is
below the minimum speed (T). When the vehicle stops, the energy stored in
the 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 perspective. However, it can become difficult to predict the behaviour of the load
and the driveline performance can become inefficient or even insufficient.
The ON/OFF case is suitable from the system perspective and simple to implement. With this control, the flywheel speed would vary, since the battery
would only be connected to the system when the flywheel speed reaches a
minimum value.
3.3
Flywheel Charge Control
Flywheel Charge Control is used with the DC/AC converter on the LP side
of the driveline. The LP DC/AC converter is a two-level inverter. The power
converter controls the speed of the flywheel accordingly to the block diagram
shown in Figure 3.7. If ON/OFF strategy is used, the control of the flywheel
speed is made accordingly to the values of minimum (T) and nominal speed
(N), discussed in Section 3.2.3.
Reference
Speed
-+
Duty
Cycles
Error
Amplitude Sine-Wave
Generation
PID
Phase
Measured Speed
PWM
3- Phase
Inverter
Period
Phase
Advance
Low pass
Filter
Rotor sector
Direction
Rotor sector
Calculation
Speed
Calculation
PMSM
Position
Sensors
Figure 3.7: Block diagram of the PMSM control.
Permanent magnet synchronous motors have sinusoidal distribution of the
motor windings, what produces sinusoidal currents, reducing the torque ripple. Therefore, a solid state converter is used to supply the machine and the
output voltage must be sinusoidal or sinusoidal PWM modulated [89].
Hall effect sensors detect the rotor position and also the measured speed
is derived from one hall effect sensor. Speed control is achieved using a reference speed and PID controller. Pulse signals for the three-phase motor are
generated using space vector modulation.
45
3.4
Drive Mode Control
Drive Mode Control is used on the HP side of the driveline during acceleration mode. This section focuses on the control strategies used when the wheel
machine is an AC machine. The control of the AC/DC/AC converter can be
divided in AC/DC and DC/AC converter controllers.
A DC machine has also been used as wheel machine during experimental
tests. The control used is described together with the experimental results, in
Section 4.2.3.
3.4.1
AC/DC Converter Control
A diagram of the DC-link control is shown in Figure 3.8. Due to the characteristic of closed loop, it is necessary to measure the stator currents and rotor
angular position. These measurements are carried out by two current sensors
and hall effect sensors. The instantaneous values of stator currents ia and ib
are mathematically transformed (Clarke and Park transformations [90]) and
then used as the feedback for iq and id control loops.
An outer loop of voltage is connected to a PID regulator. The output of the
voltage controller is the reference of the quadrature current iq . The reference
of the direct current id is set to zero, in order to obtain unity power factor
operation. The output of iq and id PID controllers are transformed into α and
β components (Inverse Park transformation). SVM is used, generating the
pulses which are inserted into the three-phase bridge rectifier.
Vdc
iq(ref)
V(ref)
+
PID
-
+
-
PID
iq
id(ref)= 0
+
Inv. Park tr.
Vq d,q Va
Vd
Vb
PID
-
ia
d,q
a,b
Park tr.
Angle
estimator
wr
a,b
a,b,c
Clarke tr.
Speed
calculator
Flywheel (TVLM)
Figure 3.8: Block diagram of the DC-link control.
ia
ib
ib
id
+
DC-link
qr
iq
46
3-phase
rectifier
a,b
id
qr
Vdc
SV
PMW
Hall
sensor
3.4.2
DC/AC Converter Control
The 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 instantaneous values of line currents. In this case, an encoder was used to measure
the rotor angular position. The inner currents loops are maintained in the same
structure 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 controller
is the reference value for the quadrature current iq , which regulates the torque
needed to reach the desired speed. The reference of the direct current id is set
to zero, in order to obtain maximum torque per ampere operation.
iq(ref)
w(ref)
+
wr
PID
-
+
-
PID
iq
id(ref)= 0
+
-
Inv. Park tr.
Vq d,q Va
Vd
PID
id
SV
PMW
Vb
3-phase
inverter
+
a,b
DC-link
qr
iq
ia
d,q
a,b
ib
id
a,b
ia
ib
a,b,c
Park tr.
Clarke tr.
Wheel motor
Rotor
speed
calculator
qr
Encoder
Figure 3.9: Block diagram of the PMSM vector control.
3.5
Braking Mode Control
Power is transferred from the wheels directly to the main energy storage device (e.g. batteries) during regenerative braking in EVs reported in the literature [91, 92]. Traditionally, the battery is directly connected to the wheel
motor. In the present flywheel system, the battery is not charged during braking [93] and, ideally, all the power converted during regenerative braking is
absorbed by the flywheel.
The presented regenerative braking control is obtained by controlling the
wheel machine output power, which can be calculated in d-q coordinates.
Differently from other conventional control strategies with DC-link regulator [94, 95], the proposed control strategy uses power estimation to balance
the power flow in the flywheel-wheel machine link. Therefore, only current
controllers are required, eliminating the need for the outer loops and volt47
age/speed controllers. Hysteresis current control is applied, providing fast dynamic response.
The control system block diagram of the HP converter during braking mode
is 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 from
a reference power Pin , estimated by a drive cycle simulation. The reactive
component of the AC line current is set to zero in order to operate at unity
power factor [96].
The inverter control attempts to send the same amount of power Pin from the
DC-link to the flywheel. Direct current (id ) is set to zero in order to maximize
the output torque, as shown in Equation 2.12. The power consumed by the
flywheel becomes linearly proportional to the quadrature current iq .
In an ideal case, no storage element in between is needed if the input and
output power are equal. Nevertheless, differences between the input and output power are inevitable in real systems (e.g. losses in the converter and in the
wheel machine) and a storage element is needed for the functionality of the
rectifier bridge.
48
4. Methods
Simulation methods are presented in this chapter, describing briefly the
methodology used to calculate the PID controller parameters and the drive
cycle simulations. The different experimental set-ups are also described,
together with the measurement systems used during the experimental tests.
4.1
Simulations
Different simulation tools have been used to design and estimate the system
behavior, before implementing it experimentally. Simulations have been
carried out using the following softwares: PSpice Orcad, Matlab, Matlab
Simulink and Dymola.
Complete system simulations have been implemented. Important steps for
system design are the tuning of the PID controllers and the drive cycle simulations. These two items will be briefly presented in the following subsections.
PID/PI controllers have been used in different control systems in the driveline: the flywheel machine control, the wheel machine control, the forcedcommutated 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 the
complete system.
The drive cycles have been investigated by Lundin et al. [97] and are presented in Section 4.1.2. The results from the drive cycle simulations have been
used as input for the complete system simulation and for analysing the power
converters’ stability.
4.1.1
PID Controller Design
A transfer function of the system can be obtained by using the energy plus the
machine parameters, using d-q equations presented in Section 2.2.2. For the
case of the flywheel machine, which is a surface mounted PMSM, the magnet
saliency is rather small, implying Lq =Ld [98]. It can be seen from Equation
2.12 that the maximum torque per ampere for the flywheel machine is obtained
by keeping id = 0.
The transfer function can be investigated and the system response with and
without 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 time
scales in the upper and lower graphs.
troller relies on its successful usage for a wide range of applications, including
process control and motor drives [99].
The PID compensation can be improved by correctly selecting the values of
K p , Kd and Ki that lead to the desired closed-loop response. This selection can
be made by choosing the values of the damping factor ς and natural frequency
ω0 of the system that would result in acceptable rise time and peak overshoot
of the closed loop response [80]. Using the second flywheel machine prototype parameters (detailed description in Section 4.2.2), the PID compensator
constants are obtained.
Step and impulse response of the system without the PID controller are
shown in the upper graphs of Figures 4.1a and 4.1b. The system takes a long
time before reaching steady-state with constant error without the PID controller. The unit step response of the closed loop system (second graph of
Figure 4.1a) converges to the steady-state after a short time and with low oscillation. 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 Figure
4.2. One of the poles is close to the right half of the s-plane, indicating that
the system is close to being unstable. The poles of the system with a PID
controller are shown in the lower plot of Figure 4.2, located now away from
the right part of the s-plane, which indicates a better performance after the
insertion of the PID controller.
50
Imaginary axis × 10
-4
4
Without PID controller
2
0
-2
-4
-1200
-800
-400
0
-400
0
4
Imaginary axis × 10
-2
With PID controller
2
0
-2
-4
-1200
-800
Real axis
Figure 4.2: Root locus analysis of the system, without and with the PID controller.
4.1.2
Drive Cycles Investigation
Important information on the torque demand of the flywheel can be found
when studying different drive cycles [100]. Considering a control system, the
drive cycles are the load variation in the machine or, for the system transfer
function, they can represent an external disturbance.
The new European drive cycle, divided into one urban and one extra urban
part, 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 acceleration or speed at every instant. The considered vehicle has a mass of 1500
kg, a dimensionless drag coefficient of 1.35 and a frontal area of 1.73 m2 [97].
4.2
Experimental Set-Ups
Four different scaled experimental set-ups were implemented. The experiments allow measurements of complete drive cycles, improving the understanding of the constituting components and optimization of the complete
system.
The experimental parameters of the electric machines used and the electronic components are presented in the next subsections.
51
Figure 4.3: New European Urban Drive
Cycle.
Figure 4.4: New Europen Extra Urban
Drive Cycle.
Figure 4.5: US Urban Drive Cycle.
Figure 4.6: US Highway Drive Cycle.
4.2.1
Flywheel Charging
The 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 prototype
The machine used during the Flywheel Charging experiments was a smallscaled three-phase axial flux PM machine [101]. The main characteristics of
the machine are presented in Table 4.1. A picture of the machine is shown in
Figure 4.7.
52
Table 4.1: Motor/generator parameters.
Pole pairs
Moment of Inertia
Internal resistance
Internal inductance
Remanent magnetic flux density
14
0.219 kg · m2
0.134 Ω
0.3 µH
1.3 T
Figure 4.7: Picture of the first scaled motor/generator flywheel prototype.
4.2.1.2 Power Electronics Hardware
A two-level three-phase DC/AC converter was used to control the machine
shown in Figure 4.7. A pre-programmed microcontroller MC3PHAC, from
Freescale Semiconductors, generated six PWM signals which were modulated
using V/F control (Section 2.4.1) in order to control the three-phase AC motor.
A Standard IGBT Module (Semikron SKM22GD123D) was chosen. The
IGBT has a typical collector-emitter resistance (Rce ) of around 100 mΩ and
is suitable for high switching frequencies. A three-phase bridge driver (International Rectifier IR 2130) which has three independent high and low side
reference output channels was also used to ensure the correct polarization of
the transistors. Switching frequency was 5.291 kHz. An AC filter with a cutoff frequency of 1.59 kHz was used.
The flywheel converter system with the low-pass filter experimental set-up
is shown in Figure 4.8.
4.2.2
Loaded Flywheel
The second experimental set-up was constructed based on the second flywheel
machine prototype and power electronics presented in the following sections.
The LP side of the flywheel machine was connected to an inverter, used to
control 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 between
open 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 prototype
The second flywheel machine prototype was a double-wound axial-flux PMS
motor/generator with two input/output sides, corresponding to the high-power
and low-power sides [102]. The machine nominal parameters are presented in
Table 4.2. A picture of the prototype is shown in Figure 4.10.
4.2.2.2 Power Electronics Hardware
A microcontroller dsPIC30F2010 from Microchip was used to implement the
control system described in Section 3.3. DsPICs are 16-bit Digital Signal Controllers (DSC) that integrate the control attributes of a microcontroller (MCU)
with the computation and throughput capabilities of a Digital Signal Processor (DSP). Hall effect sensors A1101 from Allegro were employed as position sensors. An IGBT module SK22GD123D from Semikron was used. A
three-phase bridge driver (IR 2130) ensured the right transistor polarization.
Switching frequency was 20 kHz.
54
Table 4.2: Motor/generator parameters.
Nominal Speed (rpm)
Moment of Inertia (kg · m2 )
Friction factor (Nms)
Number of poles
Internal phase resistance (Ω)
Internal phase inductance (mH)
Mutual inductance (mH)
Magnetic flux (Wb)
Low Power
2200
0.364
0.22
6
0.04
0.019
0.079
0.002
High Power
2200
0.364
0.22
6
0.12
0.19
0.076
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 batteries. A three-phase passive rectifier, 60MT120KB, from International Rectifier, connected the HP side of the flywheel to the resistive load.
4.2.3
Flywheel Driveline with DC Wheel Machine
The same flywheel machine (Section 4.2.2.1) and DC/AC converter (Section
4.2.2.2) were used in the third experimental set-up. Batteries and a DC/DC
converter were added to the LP side of the driveline. A DC machine was connected to the driveline instead of a variable resistor load, requiring new power
electronics 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 and
a picture of the implemented system is shown in Figure 4.12.
The results of this experiment are presented in Paper III.
BATTERY
DC/DC
CONVERTER
FLYWHEEL
CONTROLLER
(DC/AC)
FLYWHEEL
MACHINE
RECTIFIER
(AC/DC)
CURRENT
CONTROLLER
(DC/DC)
DC
MACHINE
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 Converter
Lead acid batteries were chosen for powering the driveline, as they are relatively cheap, safe and easy to charge. Four 12 V batteries were connected in
series, giving an output voltage of 48 V. This was enough to bring the flywheel
to 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 output
current, using IGBTs (SKM600GB066D) from Semikron. The inductor in the
current limiting circuit was 3.75 mH. The filtering capacitor was chosen to 4.7
mF. The microcontroller chosen was the dsPIC30F2010 made by Microchip.
The driver was an IR2110 made by International Rectifier. Hysteresis control
was implemented for the current controller. The current sensor was a HAL
50-s made by LEM.
56
4.2.3.2 High Power Side Converter
Unidirectional AC/DC and DC/DC converters were implemented on the HP
side 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 value
through a potentiometer (or Labview), using a DC/DC converter. The desired
value of the current was then compared to the measured value, and the output
error signal was used for producing the switching signals.
MOSFETs (IXFN140N20P) from IXYS, rated at 200 V and 140 A, were
chosen. The same driver, current sensor and microcontroller used with the
DC/DC converter on the LP side were utilized. Switching frequency was 14
kHz.
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 rated
1.9 kW. The electrical load used to brake the generator was a variable resistor
rated 0-630 Ω. Both the braking torque and the output power could be adjusted
by regulating the field current in the generator and the resistance of the load.
4.2.4
Complete Driveline Set-Up
One of the goals with the flywheel project has been to implement a bidirectional driveline, based on a flywheel power buffer and an AC wheel machine.
Here, an AC synchronous machine was used as wheel machine. New electronics and control were required on HP side of the driveline. A block diagram of
the fourth experimental set-up is shown in Figure 4.13 and a picture of the
implemented system is shown in Figure 4.14.
The results obtained from this experimental set-up are presented in Paper
V, Paper XI and Paper XII.
Figure 4.13: Scheme of the complete driveline set-up.
4.2.4.1 High Power Side Converter
The HP side of the flywheel was connected to a 600 µH inductor on each
phase, required for the functionality of the forced-commutated rectifier. A 20
mF/350 V capacitor was connected to the DC-link. Two three-phase MOSFET bridges were built with discrete DSEI 2130. A snubber circuit was implemented 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 Instruments), based on the control strategies described in Section 3.4.
The driver used was IR2130, from International Rectifier. The driver was
electrically insulated from the MCU by digital optocouplers (A4800), with the
purpose of avoiding damage in the low power system. The system required a
large number of sensors: four current sensors (HAL50-S), one voltage sensor
(resistive divider) and three hall sensors (A1101). The current sensors were
used 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 voltage capture) were required for the rectifier control strategy. The encoder was
used to estimate the wheel motor rotor angle for the inverter board. Switching
frequency was 15 kHz.
A synchronous motor with permanent magnets from Leroy Somer, rated at
3.5 kW/380 V/1500 rpm, was used as wheel machine. It uses an incremental
rotary encoder with mounted stator coupling (ERN423).
4.2.5
Measurement System
Measurements are very challenging and a critical step when running an experimental test. A wrong acquisition rate can spoil the shape of the signal and
even hide noise and harmonic content. The higher the voltage level of the experiment, the more difficult and expensive it becomes to make measurements.
Different experimental set-ups were implemented and different measurement
systems were used.
A digital oscilloscope, TDS2004C, from Tektronix was used with the Flywheel Charging set-up. It allows USB memory connection, granting image
58
and data point storage. Measurements of current and voltage were made, with
an acquisition rate of 1 kHz. The accuracy of the measurements was ±3% on
all models.
Measurement systems from National Instruments (NI) were employed in
the Flywheel Loaded and the Flywheel with DC Wheel Machine experimental
set-ups. A DAQ card, NI USB-6259 BNC, 16-Bit, 1.25 MS/s, with coaxial
probes, was used for current measurements and analog outputs. A real time
controller, NI CRIO 9022 combined with measurement slots (NI 9225 and NI
9229) which can measure up to 300 V, including differential measurements,
was also used. The maximum voltage range accuracy of the measurements
was 0.034 V. Data acquisition rate was 1 kHz for voltage and 100 Hz for
current measurement. Speed measurements were performed with the same
hall effect sensors (A1101) used for the control of the flywheel machine.
A Labview interface is required for storing the data in a compatible format
when using NI measurement system. A small program was implemented in
Labview which filters, stores and displays the data. It was also possible to
generate analog output signals and this feature was implemented with the third
experimental set-up in order to control the speed of the DC wheel motor.
PicoScopes are computer based oscilloscopes, which offer the functionality of a standard digital storage oscilloscope, in a portable and easy-to-use
package. Two different PicoScopes were used for measurements in the Complete Driveline set-up: the PicoScope 2200 and the PicoScope 3425 Differential Oscilloscope. The first one can only measure up to 20 V, but the latter
can measure up to 300 V, including differential measurements. Both offer an
accuracy of ±1% of the measured voltage. The data acquisition rate was 3
kHz for both voltage and current measurements. The biggest advantage of
this measurement system was its simplicity, since it can be directly connected
to a computer, with no need for programming or data conversion.
59
5. Summary of the Results and
Discussions
The most relevant results obtained from the present work are discussed in this
chapter. The results are divided by subject and the papers where those results
were published are mentioned in the beginning of each section. The control
strategy and experimental set-up used will be referred to those discussed in
Chapters 3 and 4.
5.1
Battery Charging System
Two different power converter systems, aimed for controlling the battery
charging process, were investigated. Simulation results of the unidirectional
DC/DC converter are presented in Section 5.1.1. Simulation and experimental
results of the bidirectional DC/DC converter are presented in Section 5.1.2.
5.1.1
Unidirectional Converter
The output voltage of the flywheel machine decreases when it is used to
recharge the battery, due to the decreasing rotational speed. To accommodate the voltage difference, a buck/boost converter can be used, together with
passive rectification performed by the body diodes in the inverter bridge. The
two-stage unidirectional converter, described in Section 3.2.1, was modelled
and simulated.
The results of this investigation are presented in Paper I.
5.1.1.1 Converter Simulations
Simulations were based on the power converter circuit presented in Figure
3.3. Values of L and C were chosen to 100 µH and 200 µF. The switching
frequency was set to 10 kHz. The battery internal resistance RB was assumed
to be 1 Ω. Simulation conditions are given in Table 5.1.
The battery charging simulation results are shown in Figure 5.1. The battery
is typically selected as the main energy storage device in the system. There are
different methods for charging batteries, but constant current method requires
simple and inexpensive control equipment [104]. The charging is arranged
in two periods: constant current (the battery voltage progressively rises) and
61
Table 5.1: Motor/generator parameters.
Pole pairs
Flywheel machine initial speed
Flywheel machine initial output phase voltage
Battery type
Battery Voltage Range
Charging current
14
6000 rpm
42 V
Lithium-Ion
25 - 42 V
1.9 A ± 0.1 A
constant voltage charging (applied as soon as the battery voltage reaches the
trickle 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 flywheel
output voltage was higher than the battery voltage, followed by the boost operation mode. At 1.2 s the flywheel machine output voltage became equal to
the battery voltage. After 3.4 s, the battery reached its nominal voltage of 42
V. Input voltage was then kept constant (at 42 V) and charge current started
to drop as full charge was approached. Full charge was reached when battery
current was less than 3% of the rated current (1.9 A). The machine stopped
rotating after 4 s when the battery reached 100% of its total charge.
Flywheel machine output voltage, flywheel rotational speed, battery current, battery voltage and current error after the first comparator and the error
after the PI controller (Error 1 and 2) are shown in Figure 5.1. The lowest
panel presents the PWM pulses. Results are shown for the first second of buck
operation, followed by the corresponding data of boost operation and constant
voltage period.
Error 1 voltage varied around zero, since the actual current was kept equal to
the reference current, as shown in Figure 5.1. Error 2 increased as a function of
the PI controller proportional gain and changes the PWM duty cycle, allowing
the switch to be opened for longer periods of times. Longer pulses could be
seen at the end of each converter operation interval.
Simulations included losses in the passive components and in the battery
model, given that the present DC/DC converter has a two-stage filter, increasing the number of passive components in the system. The inductors’ and capacitors’ internal series resistance (ESR) were set to 5 mΩ. Ideal switches
were used during the simulation. An efficiency of 92% was obtained when
dividing the output power over the battery by the input power in the capacitor
C.
62
Figure 5.1: Battery current, voltage, control errors and PWM pulses during buck and
boost operation modes.
5.1.2
Bidirectional Converter
A four-quadrant DC/DC converter was simulated, and a small prototype was
implemented and tested. The focus of this investigation was the battery charging process, but a four-quadrant DC/DC converter offers the possibility of
controlling the battery output power using the same hardware.
The functionality of the DC/DC converter and the control strategy implemented are described in Section 3.2.2 and explained in detail in [88].
The results of this investigation are presented in Paper IX.
63
5.1.2.1 Converter Simulations
A complete simulation of the four-quadrant DC/DC converter described in
Section 3.2.2 was implemented and run in Simulink. Simulation aimed for a
transition between buck and boost mode, during a short time-frame, in order
to 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 results
are shown from Figure 5.2 to Figure 5.5.
Figure 5.2: Flywheel machine output voltage, ua and load voltage, ub .
Figure 5.3: Duty ratio for the active
switches.
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, before
the input voltage is lower than the output voltage, due to the control strategy
illustrated in Figure 3.5. The output current and voltage started to decrease
when Sb,down reached unity duty cycle.
64
The inductor current, shown in Figure 5.4, jumped from 15 A to 60 A when
the control jumped from buck to boost mode, but it was kept under the maximum 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 became
low and could not be boosted any longer.
5.1.2.2 Experimental Results
A scaled prototype of the described four-quadrant DC/DC converter was implemented, with the same parameters used during the simulations. The prototype was tested with the scaled flywheel machine (Section 4.2.2.1), while
running at a low rotational speed. The flywheel was accelerated by its high
power side, and the low power side was connected to the DC/DC converter,
after a passive rectifier. A resistor was used as load impedance. Transition
modes were investigated, similarly to simulations presented previously.
Both voltage and current were controlled during the transition between buck
and boost, as shown in Figures 5.6 and 5.7. The flywheel voltage, uA , was
falling in Figure 5.6, meantime the load voltage, uB , was kept constant until the
flywheel 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 from
boost to buck was tested. Results are shown in Figure 5.8 and 5.9. The input
voltage, uA , was increased from 0 V to 30 V. The output voltage, uB , was kept
around 15 V, with a small oscillation during the transition, as shown in Figure
5.8. The same oscillation is shown in the plot of the inductor current, iL in
Figure 5.9, together with the output current, iB , which was kept around 2 A.
5.2
Battery Discharging Control
A simulation model of the LP side of the system was implemented, based on
ON/OFF control. Experimental results were also obtained, in which a drive
cycle (similar to the one used in the simulations) was applied to the system
load.
A more detailed description of the control strategy can be found in Section
3.2.3. The results of this investigation are presented in Paper III.
5.2.1
Discharging Simulations
A simulation model of the LP side of the driveline was implemented in Dymola Software. A simple drive cycle consisting of two pulses of load torque
was applied to the HP side of the flywheel machine and the system was simulated. The power consumed by the load in consequence of the applied torque
is shown in Figure 5.10.
65
Figure 5.6: Flywheel machine output voltage, uA and load voltage, uB .
Figure 5.7: Inductor current, iL and load
current, iB .
Figure 5.8: Flywheel machine output voltage, uA and load voltage, uB .
Figure 5.9: Inductor current, iL and load
current, iB .
The nominal value of the motor speed was set to 2500 rpm, and the lower
limit to 2300 rpm. The flywheel used in the present simulations has a considerably high inertia, being capable of storing 15 kJ when rotating at 2500
rpm.
The ON/OFF control signal is shown in Figure 5.11. At the start of the
simulation, the motor was running at 2480 rpm, as seen in Figure 5.12. The
PID system forced the motor to reach its nominal speed in 1 s and the battery
was disconnected from the system. The speed remained almost constant for
another 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, reaching a final value of 10 Nm, with the peak lasting for 2 s. Subsequently, the
motor slowed down, reaching a speed of 2370 rpm. Another torque with the
same magnitude was applied at t = 12 s into the simulation, forcing the motor
66
speed below 2300 rpm. The battery was reconnected and caused the motor to
accelerate.
The power from the battery, shown in Figure 5.13, was zero between t = 2 s
and t = 15 s - a period when the battery was disconnected from the system. The
battery current tends to rise when reconnected, in order to bring the system
back to nominal speed. The average power from the battery was of around
700 W, meantime the peak power consumed by the load was around 2.6 kW.
Figure 5.10: The power consumed by the
load 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 Results
The Flywheel Driveline with DC Wheel Machine, described in Section 4.2.3,
was used to perform experimental tests using ON/OFF control. Interesting
observations about the system performance and dynamics could be made by
varying the generator load in different ways.
67
Figure 5.14: The power consumed by the
load 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 applied
to the load during the experiments, similarly to the simulations. The power
consumed by the load in consequence of the applied torque is shown in Figure
5.14.
The battery was disconnected when the flywheel reached its maximum
speed, here set to 1900 rpm. The speed of the flywheel machine decreased due
to the losses in the system and to the power consumed by the load. The battery
was reconnected when the flywheel reached the minimum speed of 1200 rpm.
The ON/OFF control signal used to connect/disconnect the battery from the
system 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 the
losses of the electrical machine, which have mostly been neglected during the
simulations. Even when no torque was being applied (between the time when
the 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
68
control was able to take the system back to its nominal speed once the battery
was reconnected to the system, at t = 30 s, as predicted by simulations.
The power from the battery was initially decreasing, since the speed of the
flywheel was approaching its nominal value, as shown in Figure 5.17. When
reconnected, battery output power was limited by the DC/DC converter, working as a buck converter. The average power from the battery was approximately 150 W, almost 3 times lower than the peak power consumed by the
load (370 W). The experimental set-up was scaled down and therefore the
applied torque represented a small amount of power (370 W peak). The flywheel prototype has a relatively small inertia, so the small applied torque was
enough to reduce the speed of the flywheel.
5.3
Full System connected to Variable Resistive Load
The scaled flywheel prototype was connected to a resistive load, as described
in Section 4.2.2. The functionality of the proposed driveline could be tested
experimentally, using the variable load connected to the flywheel high power
side, after a rectifier. The load current and flywheel output power change when
changing the resistive load, causing a torque to be applied on the flywheel. The
different voltage levels of the machine, the power buffer functionality and the
inverter 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 sequence, first to an open circuit and then to a low resistance, down to a minimum of 15 Ω.
AC voltages from the low (red line) and high power side (black line) of the
system are shown in Figure 5.18. Both voltages show a very low harmonic
content. The gain in the amplitude of the HP windings is a consequence of
the double wound machine, in which a higher voltage was obtained in the HP
side, 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 is
shown in Figure 5.20. Between 0 and 5 s, the speed increased since the extracted power was lower than the input power. After 5 s, the extracted power
increased and the flywheel speed decreased in order to supply the load. The
control system responded, taking around 4 s to return to 2080 rpm after the
maximum torque was applied.
The flywheel allowed a steady power delivery from the inverter, as shown
in Figure 5.21. The power fluctuations on the low power side remained lower
than 15 W for a fluctuation of 350 W on the load side.
69
Figure 5.18: Flywheel machine low- and
high-power side line voltages.
Figure 5.19: Line current on the low power
side.
Figure 5.20: Rotational speed of the flywheel machine.
Figure 5.21: Power delivered by the inverter and power consumed in the load.
5.4
Full System connected to AC Machine
The double wound flywheel machine was connected to a three-phase AC PM
synchronous machine, as described in Section 4.2.4. The AC wheel machine
in the driveline required a bidirectional AC/DC/AC converter to link the HP
side of the system.
Simulations and experimental results were obtained for the traction (or acceleration) mode of operation. The braking mode (regenerative braking) was
designed and simulated, and its implementation is currently under development.
The results of this investigation are presented in Papers IV, V, XI and XII.
5.4.1
Traction Mode
Energy flows from the battery to the wheel machine (working as a motor)
during traction mode and the flywheel-side converter operates as rectifier,
70
whereas the load-side converter operates as an inverter. DC-link voltage control is performed by the rectifier. The rotational speed of the wheel machine is
controlled by the inverter. The reference speed (or the signal used for setting
the speed and acceleration of the vehicle) is directly given, and continuously
updated, from a drive cycle simulation or from a sensor.
5.4.1.1 Traction Mode Simulations
Traction mode was simulated in a full system model implemented in Simulink.
Battery voltage was 120 V and DC-link reference voltage was 300 V. The
flywheel initial and reference speed were 5000 rpm and the wheel motor initial
and 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 machines.
Figure 5.25: Line current and voltage on
the flywheel HP side.
A constant load torque of 5 Nm was applied to the wheel motor at t = 1
s. The load increased the power consumed by the wheel machine, in order
to keep the same rotational speed. The increased power was taken from the
71
energy stored in the DC-link capacitor, which was recharged from the HP side
of the flywheel machine, since the control of the rectifier aimed at keeping the
same DC-link voltage (300 V). The voltage slightly varied when the torque is
applied, as shown in Figure 5.22.
The power from the battery on the LP side (Pin ) and the power consumed
by the wheel motor (Pout ) are shown in Figure 5.23. During the complete
simulation (3 s), the average power consumed by the wheel motor was 152
W, meantime the average input power was 173 W. This indicates an average
efficiency of 88% according to the simulations.
The rotational speed of the flywheel and the wheel motor are presented in
Figure 5.24. The speed of the flywheel varied less when compared to wheel
speed, since the flywheel was spinning with a relatively high speed. Shortly
after the torque being discontinued, both speeds returned to their reference
values.
Line current and voltage on the HP side of the flywheel are shown in Figure
5.25. Due to the implemented control of the force-commutated rectifier, unity
power factor operation was reached.
5.4.1.2 Experimental Results
A test similar to the one presented in the simulations was experimentally performed, during 50 s. Battery voltage was 25 V and DC-link voltage was equal
to 80 V. Initial rotational speed of the flywheel was around 2000 rpm, while
the 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 machine. The voltage over the DC-link sank slightly when the torque was applied, as shown in Figure 5.26.
The power from the battery on the LP side (Pin ) and the power consumed by
the 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
72
Figure 5.28: Rotational speed of the machines.
Figure 5.29: Line current and voltage on
the 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 the
simulations.
The rotational speed of the flywheel and wheel motor are shown in Figure
5.28. The rotational speed of the flywheel sank in order to supply the power
required to keep the DC-link.
Line current and voltage on the HP side of the flywheel are shown in Figure
5.29. Unity power factor operation was reached.
Additional experimental results are shown in Figure 5.30. The flywheel was
disconnected from the battery and the load was kept the same, so the output
current of the DC-link would not vary. The speed of the flywheel decreased
as shown in Figure 5.30a. The voltage in the DC-link was kept constant (80
V) 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. It
increased as the speed of the flywheel decreased, to compensate for the falling
output voltage.
5.4.2
Braking Mode Simulations
In regenerative braking mode, the wheel machine-side operates as a rectifier, whereas the flywheel-side converter operates as an inverter. The wheel
machine works as a generator. The here presented control was obtained by
monitoring the wheel machine output power, which now flows in the reverse
direction and attempting to send the same amount of power from the DC-link
to the flywheel. A description of the suggested control strategy can be found
in Section 3.5.
Two simulation models of the system were run according to the same drive
cycle. Model 1 represented the Simulink model, while a Matlab equivalent
model of the system was implemented in Model 2. The simulation model implemented in Simulink considered the detailed components and control of the
73
Figure 5.30: a) Speed of the flywheel, b) voltage over the DC link, c) flywheel output
current. (Unpublished results)
system described in Chapter 3. The Matlab simulation was a general model
of the flywheel during braking mode, based also on the drive cycles investigation. Both models used parameters from the experimental set up described in
4.2.4. The power Pin , which was the input to the simulation, was derived from
a vehicle weighing 70 kg and with a front area of 1.1 m2 . The drag coefficient
Cw was assumed to be 0.33.
The power and the actual speed of the car, calculated from the drive cycle
simulations are shown in Figure 5.31 and 5.32. The wheel machine, acting
as a generator, had an initial speed of 570 rpm and the flywheel had an initial
speed of 4780 rpm. The DC-link voltage was set to 300 V. The power extracted
from the wheels was the reference value in the present simulation. The same
reference power was used in both rectifier and inverter control, as described
in Section 3.5.
The DC-link voltage, which varies under braking mode, is shown in Figure
5.33. The capacitor discharged to supply the system losses as the power given
to 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, indicating
a variation of around 250 J of the energy stored in capacitor, in 5 s. Thus the
average power out of the capacitor was 50 W. The efficiency when transmitted
74
Figure 5.31: Regenerated power used as
reference power in the proposed control.
Figure 5.32: Speed of the car, obtained
from the drive cycle simulations.
Figure 5.33: DC-link voltage.
from the wheels to the flywheel storage was around 92%, considering that the
average power produced during braking was 608 W.
The speed of the flywheel, shown in Figure 5.34 for Models 1 and 2, increased due to the regenerated power. The speed of the flywheel increased
less when compared to the variation in the speed of the wheel machine, since
the flywheel had a higher initial speed and, consequently, a larger amount of
energy 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 were
regenerated.
75
Figure 5.34: Speed of the flywheel machine during braking.
5.5
Figure 5.35: Speed of the wheel machine
during braking.
Driveline Losses
The investigated driveline has a large number of components, and different
losses can be associated. The main losses to be considered in the system
are: battery internal losses, power electronics losses (IGBTs, MOSFETs and
diodes), flywheel machine losses, passive components losses (inductors, capacitors and resistors) and wheel machine losses.
Simulations were carried in Matlab in order to estimate the system total
losses. Experimental tests were also performed and compared to the results
obtained from the calculations.
5.5.1
Simulation Results
A theoretical calculation was implemented in Matlab, which used the parameters of the components in the driveline. Switching losses were calculated
considering turn-on and turn-off times obtained from measurements. On-state
losses were calculated according to the information provided in the data sheets
of the components. The losses in the electric machines were calculated based
on theoretical approaches described by Santiago [102].
The voltage levels and the speed of the machines were set equal to the
ones used in the simulations of the complete driveline. Instantaneous values
of 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 simulations and experimental results presented in Section 5.4.1. The losses were
divided 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 multiplied in order to calculate an approximated efficiency of the driveline. A
total efficiency of 86% was obtained. The lower efficiency obtained from the
calculations (in comparison with the simulations/experimental results) can be
76
3%
10%
21%
3%
3%
Battery
IGBTs
Flywheel machine
MOSFETs
Passive comp.
Wheel machine
60%
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.(Unpublished
results)
explained by the battery model, which was not considered in the simulations
or experiments. The flywheel machine was responsible for a major part of the
77
losses, and this could be attributed to the mechanical losses. The mechanical
losses are to be reduced in the close future, with the insertion of a vacuum
chamber and magnetic bearings to the system. Losses in the wheel machine
were also high, which could be explained since the machine was operated in
a power/speed level which is lower than its nominal conditions.
The IGBT total losses were higher than the MOSFET total losses, even
though there are two MOSFET boards and one IGBT board. This result can
be explained due to the nominal values of the IGBT board, which are higher
than those used during the experiment.
5.5.2
Experimental Results
The complete driveline set-up was tested, and measurements of voltage and
current were taken from different points in the driveline. The rotational speed
of the flywheel machine and wheel motor were approximately 2000 rpm and
180 rpm, respectively. A load torque, lasting around 10 s, was applied to the
wheel machine. The power consumed at different points in the driveline was
calculated, with and without the applied load. The results of this experiment
are shown in Figure 5.37.
During steady-state, all the power delivered by the battery was consumed
by losses of the system. When a load torque was applied, there was an increase
in the current levels and the ohmic losses were also increased.
According to the experimental results shown in Figure 5.37, the flywheel
machine consumed a major part of the power losses, as in the simulations.
Losses in the low power converter system were around 8 W (difference between battery power and LP side power), meantime the losses in the high
power converter system were around 12 W (difference between HP side power
and WM power). The LP side losses were mainly the losses in the IGBT based
converter, whilst the losses on the HP side were consumed by the MOSFET
converters and the passive components. Considering the number of components, the losses in the MOSFETs were relatively lower than the losses in the
IGBTs, showing agreement with the results obtained from the simulations.
78
6. Conclusions
A novel all electric driveline based on a double wound flywheel machine has
been demonstrated. Simulations and experimental results of the system have
been presented. The design and assembly of the power electronics and their
control scheme have been successfully implemented.
Reduced power variations on the battery side have been obtained under
heavy load conditions, proving the system’s functionality, which has several
advantages when compared to other all-electric drivelines.
A low power converter system, connecting the flywheel machine to the battery has been designed, constructed and tested. The present power converter
system controls the speed of the flywheel machine and the dynamics of the low
power side. Simulations and experimental results have shown that ON/OFF
control is operational with high energy density flywheel, since the speed variation is very small for high torque applications on the load side. The number
of discharging cycles over the battery can be reduced by letting the flywheel
vary within safe and controlled limits. Attention should be given to the chemical aspects of the battery and how a discontinuous discharging mode affects
its lifetime. However, the here suggested disconnection would have a reduced
impact on the battery, since it is still connected to the DC/DC converter filter
capacitor.
Two different DC/DC converters have been investigated. A unidirectional
DC/DC converter, which works as a battery charger converter system has been
presented. The designed converter stability has been evaluated through simulations under closed loop control. The proposed PI controller has kept a constant current or voltage during battery charging with a low ripple during buck
and boost operation. The simulation has indicated that the control is robust
enough to allow battery recharging despite the decrease in the flywheel output
voltage when slowing down. The battery recharging within the present flywheel system can also be combined to a charging process from the grid by
using 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 dynamics
change depending on the flywheel speed, and the requirements of power stability into and out of the flywheel are important. The DC/DC converter can
be used to limit the current into the flywheel during a start-up situation, but
also boost the voltage when the back-EMF is high. In the same setup, when
79
transferring power from the flywheel to the battery, the charging current can
be kept constant regardless of the declining input voltage.
Although the operation of both DC/DC converters has been focused on battery charging application, for a flywheel acting only as a power handling device, no significant amount of energy can be recharged in the battery. In practice the only moments to efficiently recharge the battery are at the end of a
drive 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 driveline has been simulated and built. Simulations and experimental results during
acceleration mode have shown good agreement. The designed controllers have
managed to keep the different controlled signals almost equal to their reference values. Unity power factor and low distortion have been achieved (both
in the simulations and experimental results) on the high power side terminals.
A power balance control for the AC/DC/AC converter during braking mode
has been proposed. The control allows robust response, where the speed of
wheel machine has been varied according to the power calculated from a drive
cycle. The results obtained from this control have been compared to a Matlab
model 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. Results
have shown how the losses are divided in the different components of the
system, improving the understanding of the constituting components and optimization of the complete system. A major part of the losses has been attributed
to the flywheel machine, due to the mechanical losses. The mechanical losses
are to be reduced in the close future, with the insertion of a vacuum chamber
and magnetic bearings to the system.
The average efficiency of the driveline has been estimated during acceleration to be around 87% (battery-wheels). A regenerative braking strategy has
been 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 in
the flywheel machine.
80
7. Suggestions for Future Work
Even if there exists a complete and functional flywheel-based all-electric driveline, there are many improvements to be considered. Regarding the electrical
system of the driveline, some suggestions can be pointed out for future work:
• Implementation of the regenerative braking and comparison to the simulation results: this work is currently under development. With the implementation of the regenerative braking, the complete assembling of the driveline
is 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. The
choice of these voltage levels is an optimization between the reduction of
the losses and the cost of the electrical/electronic components.
• Exploring different battery technologies: depending on which battery technology to be used in the driveline, some changes in the control strategies
might be required.
• Grounding: a detailed study of the system grounding can be made.
• Improvement of the electronic drivers: Gate drivers are the interface between control systems and high power electronic. The larger currents the
driver can handle, the faster the gate charge can be injected or removed and
the 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 machine.
• Power electronics: other types of power electronics technologies can be
tested in order to improve the system functionality and efficiency. Multilevel inverters can be an option if a high voltage level is chosen for the HP
side, 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 also
consist in advanced sensorless control techniques which might allow full
system operation with a reduced number of components.
81
8. Summary of Papers
Paper I
Battery Recharging Issue for a Two-Power-Level Flywheel System
The paper investigates the control of the power flow to the battery when
the vehicle is parked, despite the decay of the flywheel machine voltage. The
design and simulation of an unidirectional DC/DC buck/boost converter for
a variable rotational speed flywheel are presented. Conventional power electronic converters are used in a new application, which can maintain a constant
current or voltage on the battery side. Successful PI current control has been
implemented and simulated, together with the complete closed loop system.
The author has written the paper, performed the modelling and simulations
of the proposed DC/DC converter.
Published in Journal of Electrical and Computer Engineering, Vol. 2010,
Article ID 470525, 5 pages, 2010.
Paper II
A Double Wound Flywheel System under Standard Drive Cycles: Simulations and Experiments
In this paper the functionality of the system is investigated by means of
simulations and experiments. Different standard drive cycles are applied on
the high power side to assess the effect of load variations in the system as a
whole and particularly in the speed control. The response of the speed control
system is investigated with computer simulations and experimental verification. The energy storage in the flywheel allows a steady power supply from
the battery via the inverter, proving the functionality of the system.
The author has performed most of the writing, the Simulink simulations and
the experimental tests.
Published in International Journal of Emerging Electric Power Systems,
Vol. 11, Iss. 4, Article 6, 2010.
83
Paper III
Battery Discharging Power Control in a Double-Wound Flywheel System
Applied to Electric Vehicles
The paper focuses on the converter system and the control logic for regulating the battery discharging process and the flywheel rotational speed. Emphasis are given to the overall power/energy management of the system. Simulations and experimental results show that an ON/OFF battery control allows a
efficient system, requiring a robust speed control and high energy density for
the flywheel machine.
The author has performed most of the writing, the Matlab simulations and
the experimental tests.
Published in International Journal of Emerging Electric Power Systems,
Vol. 12, Iss. 1, Article 7, 2011.
Paper IV
Power balance control in an AC/DC/AC converter for regenerative braking in a two-voltage-level flywheel based driveline
A power matching control applied to an AC/DC/AC converter for regenerative braking application is discussed in Paper IV. The AC/DC/AC converter
regenerates the electric power converted during braking to the flywheel machine, here used as power handling device. By controlling the power balance,
the same hardware can be used for acceleration and braking providing reduction of harmonics and robust response. A simulation of the complete system
during braking mode is performed both in Matlab and Simulink and results are
compared. 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 the
AC/DC/AC converter and the Simulink simulations (Model 1).
Accepted for publication in International Journal of Vehicular Technology,
August 2011.
Paper V
A study on doubly fed flywheel machine based driveline with an
AC/DC/AC converter
The paper presents simulations and experimental results of the two-powerlevel driveline, where the control and electronics used are presented and the
system efficiency is discussed. The control strategy of the AC/DC/AC con84
verter used on the high power side of the driveline is discussed. Simulations
of the complete system are carried in Simulink and compared to the experimental results, obtained from the scaled experimental test set-up. Simulations
and experimental results show good agreement. The average efficiency of the
driveline during a simple drive cycle is obtained. A theoretical calculation
based on the real parameters of the system is implemented.
The author has performed most of the writing, the simulations and contributed to the experimental tests.
Submitted to IET Electrical Systems in Transportation, June 2011.
Paper VI
Controlling a Permanent Magnet Motor using PWM converter in Flywheel Energy Storage Systems
The paper presents a power DC/AC converter to govern an AC flywheel
machine. Different load connections are investigated. An output RLC filter is
designed and built to minimize the harmonics due to the switching operations
of the Pulse Width Modulated (PWM) converter that drives the motor. Simulations are compared to the corresponding laboratory experiments. It is found
that the harmonics are considerably reduced when a RLC output filter is included in the system. The simulation results are verified with the experimental
results as they show a good agreement.
The author has written the paper, performed the simulations and the experimental tests.
Published in Proceedings of the 34th Annual Conference of the IEEE Industrial Electronics Society, Orlando, USA, pp. 3364-3369, 2009. (Presented
orally by the author.)
Paper VII
Power Electronics and Control of two-voltage-level flywheel based allelectric driveline
The paper presents the complete design and simulation of the proposed flywheel system when connected to an AC wheel machine. Vector control based
speed regulators are designed and successfully simulated. DC link voltage
control is achieved by using synchronous rectification. Power estimation is
used during regenerative braking in order to charge the flywheel with the
power generated from the vehicle speed reduction. Simulations verify the
functionality of the proposed system.
The author has written the paper and performed the simulations.
85
Published in Proceedings of the IEEE International Symposium on Industrial Electronics, Gdansk, Poland, pp. 1-7, 2011. (Presented orally by the author.)
Paper VIII
Design parameters calculation of a novel driveline for electric vehicles
The paper investigates the dynamic behaviour of a vehicle operating according to a standard drive cycle. Parameters of the flywheel based driveline
(such as power rates and size of the flywheel) are obtained by optimization. A
description of the performance of a Two-Voltage-Level Machine is presented
through its equivalent circuit and the control of the machine. Special attention
is given to the system losses. A scale prototype is constructed and tested under
a drive cycle, demonstrating the system performance of the system.
The author has contributed to the written material, the motor control simulations and performing the experiment.
Published in World Electric Vehicle Journal, Vol. 3, ISSN 2032-6653-2009.
Paper IX
Sliding Mode 4-Quadrant DC/DC Converter for a Flywheel Application
The paper focuses on the design and construction of a four-quadrant DC/DC
converter. The target application is the flywheel based all-electric driveline,
with focus on the battery recharging process. The control decisions are based
entirely on the latest available measurements, implying that no memory needs
reinitializing when changing quadrant (such as for PI methods). The boost
control is based on a topology specific current source approximation. The
control is found to be parameter invariant, regardless of high input/output dynamics variance.
The author has contributed to the written material and the experimental
results. (Diploma work thesis under the supervision of the author.)
Submitted to Control Engineering Practice, July 2011.
Paper X
Prototype of electric driveline with magnetically levitated double wound
motor
86
This paper describes a bench test set-up under construction to investigate
the properties of the flywheel system in details. The proposed set-up is expected to achieve a level of power and energy close to that of a full scale system. This will allow measurements of complete drive cycles to be performed,
improving the understanding of the constituting components and optimization
of the complete system.
The author has contributed to the written material.
Published in Proceedings of the International Conference on Electrical Machines, Rome, Italy, pp. 1-5, 2010.
Paper XI
Implementation and Control of an AC/DC/AC converter for double
wound flywheel application
This paper presents the implementation and control of the AC/DC/AC converter, 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 practical perspective. Results from experimental tests performed in the full system
prototype are presented. The prototype system is running with satisfactory
stability during acceleration mode.
The author has contributed to the written material and the experimental
tests.
Accepted for publication in the Proceedings of the IEEE International Conference on Control and Automation, Santiago, Chile, 2011.
87
9. Svensk Sammanfattning
Många länder har förändrat sin politik för att gynna utvecklingen av tekniker
som strävar mot mer långsiktigt hållbara energisystem. Effektiva och tillförlitliga elfordon kommer att bidra till denna utveckling.
Optimeringen av det elektriska drivsystemet är en av de viktigaste
utmaningarna för att göra elbilar konkurrenskraftiga med traditionella bensinoch dieseldrivna bilar. Att ersätta förbränningsmotorn med en elmotor, eller
att använda en kombinerad el- och förbränningsmotor, är fördelaktigt. Detta
p.g.a. den höga verkningsgraden som är högre än 90% för elfordon och i
genomsnitt 40% för fordon med förbränningsmotorer.
För att kunna hantera den stora kraften vid acceleration och vid regenerativ
bromsning är svänghjulssystem attraktiva att använda i elfordon. Kombinationen av ett svänghjul och ett batteri har flera fördelar, såsom högre toppeffekt, högre energitäthet och en minskning av antalet partiella laddnings/urladdningscykler i batteriet.
I det här projektet studeras en helt elektrisk drivlina baserad på ett
svänghjul. Svänghjulet är unikt då det har två olika spännings-/effektnivåer
och därför samtidigt kan fungera både som motor och generator. Systemet
kan därför hantera effekten som utvecklats under dynamiska processer
(bromsning/acceleration) på ett effektivt sätt.
Den kompletta drivlinan består av tre huvudkomponenter: den huvudsakliga 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 elkraft
med en hög verkningsgrad och kopplar samman de mekaniska rörliga delarna.
Den elektriska delen av ett elfordon består av en elektrisk maskin, kraftelektronik och kontrollsystem.
De olika huvudkomponenterna i systemet kan kopplas samman med hjälp
av frekvensomriktare och DC/DC-omvandlare som omvandlar spänningen till
den frekvens och amplitud som krävs för sammankoppling. Styrningen av
frekvensomriktarna och DC/DC-omvandlarna är viktiga för funktionaliteten
hos hela drivlinan, och är därför ett utmanande område inom detta projekt.
Denna avhandling fokuserar på frekvensomriktarna och DC/DComvandlarna och de kontrollstrategier som används för att styra dessa.
Modellering, simulering och konstruktion av omvandlarsystemet,
89
tillsammans med monteringen av hela drivlinan, har varit målet för den här
doktorsavhandlingen.
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, och
en 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 drivmotorn.
Fyra olika experimentuppställningar av systemet har satts upp. I experimenten har mätningar av drivsystemets olika cykler genomförts. Resultaten
från experimenten har lett till en ökad förståelse av de ingående komponenterna och förslag på hur det kompletta systemet kan optimeras.
Olika kontrollstrategier har föreslagits och undersökts och resultaten har
visat att kontrollstrategierna i drivlinan kan ge en jämn uteffekt från batterierna medan svänghjulet hanterar effektvariationerna på den drivande sidan.
En genomsnittlig verkningsgrad på cirka 87% (från batteri till hjul) har
beräknats och bekräftas via simuleringar och mätningar. Omvandlarsystemet
har visat sig vara effektivt och robust och kan hantera effektflödet i systemet.
En regenerativ bromsningssekvens har simulerats med en förväntad verkningsgrad hjul till hjul på cirka 80%.
90
10. Acknowledgements
To my supervisor, Prof. Hans Bernhoff, for the opportunity and the confidence
in my work. Also to my co-supervisor, Prof. Mats Leijon, for making this
division 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 and
Prof. Hana Barankova, for the encouragement and help during the first years
of my studies in Sweden.
To my colleagues Juan de Santiago, Johan Lundin, Johan Abrahamsson and
Magnus Hedlund. Words can not describe the admiration and affection I feel
for you! Thank you for all the help, patience and everlasting discussions.
To my other colleagues at the Division for Electricity, for making this division the best place to work at. Special thanks to my colleagues on the second
floor: 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 time
to 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 Carvalho, 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 for
help.
To Alexandre Cury, for "being there", independently of which country or
time zone.
91
To the new friends I gained after these years of adventure in Sweden, for
enriching 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 worth
to be continued.
To my parents, my mother Maria Helena and father José Maria, for having
taught me everything, for the shared faith, for giving me your endless love
throughout.
To my family in Brazil, my haven! My extended family in Costa Rica, thank
you for the sincere encouragement.
To my husband, Esteban, words can not describe my gratitude. You are a
bright, wonderful and tireless person, and I would never be writing these lines
today were it not for you. Obrigada!
"Stones in the road? I save every single one, one day I will build a castle"
Fernando Pessoa
92
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