development of a novel ultracapacitor electric
TRANSCRIPT
Development of a Novel Ultracapacitor ElectricVehicle and Methods to Cope with Voltage Variation
Kiyotaka KawashimaDepartment of Electrical Engineering
University of TokyoTokyo, Japan
Toshiyuki UchidaDepartment of Electrical Engineering
University of TokyoTokyo, Japan
Yoichi HoriInstitute of Industrial Science,
University of TokyoTokyo Japan
Abstract— In this paper, we introduce a novel DC-DC convert-erless ultracapacitor electric vehicle (EV) and propose methodsto cope with voltage variation of ultracapacitor. In generalcapacitor applications, a DC-DC converter is used to suppressultracapacitor’s voltage variation. In the case of the proposedEV, the ultracapacitor storage system is directly connected toan inverter for the purpose of weight, space utilization, andenergy efficiency. It is shown that the inverter, which is speciallydesigned to cope with voltage variation and operates withoutany problem between 50V - 200V. Dq axis vector control forthe interior permanent magnet synchronous motor is applied toachieve wide-speed operation and cope with voltage variation.
Keywords-Electric vehicle, ultracapacitor, motor control, dqaxis vector control
I. INTRODUCTION
A. Back Ground of the Research
The global climate has changed significantly owing toan increase in CO� emissions. Automotive industries areresponsible for a quarter of the total CO� emissions. Sincepure electric vehicles (EVs) are powered only by electricity,CO� emissions from them are lower than those from anyother vehicles such as internal combustion engine vehicles andhybrid electric vehicles.
Among technologies related to electric vehicles, remarkabletechnical progress has been made with respect to energystorage devices, such as Li-ion batteries, NiMH batteries,fuel cells(FCs), and ultracapacitors. However, serious safetyproblem with secondary batteries have yet to be resolved andFCs have associated cost and infrastructure problems.
Ultracapacitors have attracted considerable attention re-cently owing to their advantages. Fig. 1 shows power rateand life length comparison of energy storage devices [1].Ultracapacitors have low energy density, however, they havehigh power density and life length is much longer thanthose of secondary batteries. Furthermore, ultracapacitors areenvironmentally friendly because they do not use heavy metals[2]. Therefore, ultracapacitor applications in vehicle energymanagement and storage systems are becoming increasinglypopular [3]. In the next section, some ultracapacitor applica-tions for pure EVs are introduced.
Figure 1. Power rate and life length comparison of energy storage devices[1]
B. Ultracapacitor Applications for EVs
In Shanghai, electric buses with ultracapacitor energy stor-age systems have been operating since Aug. 2006 (Fig.2) [4].Because the system was converted from trolley bus system,it was already equipped with a DC - DC converter. Theultracapacitor energy storage system installed on the bus ischarged within a few minutes at bus stations by expanding apantograph installed on the roof. The energy stored in the busis sufficient only to cover the distance between stations (about500 m).
In Canada, Zenn Motor Company announced that theydeveloped a ceramic capacitor electric vehicle with a range of800km in 5 min. charging time (Fig. 3) [5] [6]. Developmentof a DC-DC converter that can accept such amount of powerfor 5 min. charging is a very technically challenging task.Nevertheless, a ceramic capacitor with such large energydensity is worthy of attention.
In Japan, Meisei University has developed an electric doublelayer capacitors (EDLCs) EV without a DC-DC converter [7].The capacitor voltage is maintained at a certain voltage levelby utilizing the bank switching of series and parallel capacitorconnections.
Ultracapacitors have also been adopted for automatic guide
978-1-4244-2601-0/09/$25.00 ©2009 IEEE 724
Figure 2. Ultracapacitor electric bus @ Shanghai
Figure 3. Ceramic capacitor EV @ Canada
vehicle (AGV) systems and forklifts which require instanta-neous power within a short period of time.
An ultracapacitor EV is a novel transportation system whichrepeatedly charges with a little energy storage device incomparatively limited region. The EVs that are currently in userequire large secondary batteries that are not only dangerous,but also inefficient in terms of weight, space and life length.Since energy grids are present throughout the country, thedevelopment of this new transportation system is significant.
The Shanghai bus system uses only 10% of the ultraca-pacitor’s voltage range. Since the energy stored in a capac-itor is proportional to the square of capacitor’s voltage, theSOC(state-of-charge) swing is only 20 % of the total energystored in the capacitor.
In this paper, the development of a novel EV poweredsolely by an ultracapacitor is introduced. The EV utilizes morethan 90 % of the capacitor energy using a specially designedinverter (SOC swing = 90 %). In the next section, structure andadvantages of electric double layer capacitor (EDLC), whichis adopted to our EV, are introduced. In the third section,development of ultracapacitor EV is introduced. In the forthsection, methods for wide-speed operation are explained.
II. CHARACTERISTICS OF EDLC
A. Structure of EDLC
An electric double layer (EDL) was discovered byHelmholtz in 1879. The basic structure of Stern’s EDL modelis shown in Fig. 4 [8]. An EDL is composed of a thinmonomolecular layer and an outer diffusion zone. These twolayers are collectively called EDL.
An EDLC consists of plural capacitor cells, and the max-imum voltage is determined by the electrical dissociationvoltage of the electrolyte. The electrolyte used in our EDLCis an ionized liquid having high ionized conduction [9].
Since the EDLC module has many capacitor cells, repetitionof charging and discharging may result in voltage variationamong the cells and cause damage to the cells. To overcomethis problem, the module has a small electronic circuit thatequalizes voltages among the cells [10].
positive electrode
aluminumelectrode
electrolyte
separator activated carbon
negative electrode
ion
monomolecular layer
diffusion zone
{Figure 4. EDLC cell structure
B. Advantages of EDLC
The EDLC has the following advantages because it is notbased on chemical reactions.
� It can be charged and discharged very quickly withoutheat generation.
� It can endure repetitive charging and discharging.� The remaining energy level can be precisely known by
measuring the capacitor voltage.� It has a wide operating temperature range� It is environmentally friendly because it does not use
heavy metals.
C. Equivalent Circuit of EDLC
Several models for an ultracapacitor have been proposedpreviously. For example, a frequency domain model wasproposed and its validity was experimentally verified [11].
The simple equivalent circuit of an EDLC has a series ofmulti-stage RC circuits, as shown in Fig. 5. The followingphenomena are observed owing to the RC circuits.
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� Capacitance-voltage characteristic is nonlinear� Voltage drops remarkably after charging due to charging
of the latter-stage of the RC circuit� Voltage increases after discharging due to discharging of
the latter-stage of the RC circuit
Given these phenomena, it should be noted that the terminalvoltage does not necessarily always indicate precise SOC.Dynamic current and voltage data are required to determinethe SOC when the ultracapacitor system is not in use for longperiod of time.
R1 R2
C1
I1 I2 R3 I3
V1
C2
V2
C3
V3
V0 Rs
Figure 5. Equivalent circuit of EDLC
If the EDLC equivalent circuit is represented as a lumpedparameter circuit, the V-I equation can be written as a lineardifferential equation. Generally, a two or three-stage RC circuitis sufficient to match the experimental results. Fig.6 showsthe comparison between the voltages calculated by a three-stage RC model (�� = 0.059, �� = 5.0, �� = 100, �� =78.2, �� = 18.2, �� = 7.8, �� = 0) and a single-stage RCmodel (��=96.4,��=0.059,��=0) using three-parallel seven-series (3P7S) modules (TABLE II). The voltage error is largein the case of the single-stage RC model. On the other hand,voltage obtained using the three-stage RC model matchesthe experimental results. By assuming that the changes inthe electrical parameters represents a secular change, lifeestimation and fault diagnosis of EDLC are possible usingthe simulation model.
0 50 100 150 20050
60
70
80
90
100
Time(sec)
Vol
tage
(V)
Vccalculated by 3-s RC modelcalculated by 1-s RC model
Figure 6. Voltage calculations base on EDLC models
Capacitor COMS1
Capacitor 200F, 15V
Inverter 2kW(30-100V)
Steering sensor
Inertia/gyro sensors
Stroke sensorsElectrical control unit
Figure 7. Experimental vehicle for electric stability program
D. Charging and Discharging Efficiency
In the case of secondary batteries, charging efficiency de-creases due to internal resistance. On the other hand, in thecase of EDLC, the charging efficiency is very high becausethe internal resistance is very low. High charging efficiencydecreases charging time.
Charging and discharging efficiency are represented by thefollowing equations.
������� ��
� � �����(1)
��������� � �� ����� (2)
High-efficiency charging (nearly 100%) is possible if the RCproduct and charging time are properly selected. According toour experimental data, ������� is 98.4% at a charging currentof 25A and 94.8% at 150A [12].
III. DEVELOPMENT OF ULTRACAPACITOR EV
A. Purpose of Development
We developed a micro EV ”Capacitor-COMS (C-COMS)”,that is powered by ultracapacitors (EDLCs). Our purpose ofthe development is for the research of EV’s motion controlexperiments. Our group has been researching EV’s motioncontrol utilizing electric motor’s advantages. However, theexisting experimental vehicles take approximately half a dayto charge their secondary batteries and have a short life lengthwhich poses usage and safety problems. Since the experimentswith the EV can be performed in a short period of time andthe programs used in the EV are changed frequently, the EVdoes not require a large amount of energy for one operation.
The C-COMS can run for 20 min with a charging time of 30sec. The specifications of the EV and ultracapacitor are listedin TABLE I and II, respectively. Fig.7 shows the componentsof the capacitor used in the EV.
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TABLE I
SPECIFICATION OF ”C-COMS”
Category 1 passenger EVDrive 2 in-wheel motors
Motor(Rated/Max power) IPMSM(0.3/2kW)Max torque / Velocity 100Nm / 50km/h
Inverter PWM vec.(MOSFET)
TABLE II
SPECIFICATION OF ENERGY STORAGE DEVICE
Company Nisshinbo Industries, Inc.Energy / Power density 3.3Wh/kg / 7.1kW/kg
Device EDLC(3P7S) EDLC(2P14S)Voltage / Capacitance 100V / 85.7F 200V / 28.6F
Energy 131Wh 175WhCharging time 30sec(150A,55V-) 42sec(75A,80V-)
Distance / Time 4-5km/20min. 5.3-6.7km/27min.Speed range 50km/h 65km/h
B. Test-drive Results of C-COMS
Fig. 8 shows the test-drive results of the C-COMS. Thecourse was 80 m per track, maximum speed on a straight pathwas 9 m/s and the vehicle decelerates before a curve to 2 m/s.Maximum torque was 160 Nm per motor. When the vehicledecelerates, the regeneration brake was used and energy wasaccumulated in the ultracapacitor system.
The capacitor voltage decreases during driving, so that wecan precisely estimate the remaining energy level.
From the test-drive results, it was found that inverter op-erates between 50-200 V without any problem and that theenergy utilization is above 93%.
0 100 200-100
0
100
200
Cur
rent
(A)
0 100 2000
50
100
150
200
Rig
ht M
otor
Tor
que
(Nm
)
0 100 2000
50
100
150
200
Time(sec)
Vol
tage
(V)
0 100 2000
5
10
Time(sec)
Spee
d(m
/s)
regeneration
current compensation
200-50V SOC swing 94%
Figure 8. Voltage and current transition of driving experiment
In the next section, we investigated and compare the meth-ods used to achieve wide-speed operation of the ultracapacitorEV.
IV. METHODS FOR WIDE-SPEED OPERATION
In this section, we discuss hardware and software ap-proaches for achieving wide-speed operation of ultracapacitorEV.
A. Hardware Approach
To cope with voltage variation and obtain a wide-speed op-eration, the following four hardware solutions were examined.TABLE III shows the comparison between the methods usedto achieve wide-speed operation.
TABLE III
COMPARISON OF METHODS FOR WIDE-SPEED OPERATION
efficiency weight/space speed-torque rangewith DC-DC converter fair poor excellent
mechanical relay good poor fairsemiconductor switch fair poor fair
mechanical gear good poor gooddirect connection excellent excellent poor
1) Use of DC-DC converter: Although this is the mostcommonly used and simplest way to suppress voltage vari-ation, cost, efficiency, weight and space utilization are com-promised.
2) Bank switching: Electrical power switches are usedfor changing series and parallel connection of ultracapacitormodules. By applying bank switching, it becomes possibleto maintain the capacitor voltage within a particular range[13]. There are two methods to carry out bank switching;mechanical relay and semiconductor switch. The former isefficient but has a chattering problem; the latter has nochattering problem but is inefficient due to the forward voltagedrop of semiconductor.
Energy utilization rate � is expressed by the followingequation, where � and � are voltage utilization ratio (� � �) and number of bank switches, respectively.
���� � ��� ��� � ��� ����� � � ��� ����������
� �� �� (3)
Practically, one bank switch is sufficient because energyutilization � is greater than 93 % when r is 0.5 and anincrease in the number of switches would increase the spacerequirements and cost.
3) Mechanical gear: A mechanical gear can be used toachieve a smooth and wide-speed operation. Continuouslyvariable transmission may result in smooth shift in both theentire speed and voltage range. However, problems similar tothose listed in 1) also arise in this case.
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4) Direct connection: Taking the above mentioned prob-lems (limited space, weight, efficiency and cost) into consider-ation, we adopted an ultracapacitor - inverter direct connectionmethod for C-COMS. It can also be applied for electricbuses, AGVs, etc. However, inverter should be designed takingvoltage variation of capacitor into consideration to achievewide - speed operation in whole voltage range.
B. Software Approach
In this section, we discuss the software approach to copewith voltage variation of capacitor utilizing dq axis vectorcontrol for the interior permanent magnet synchronous motor(IPMSM).
1) Basic equation of IPMSM: A high efficiency drive forthe entire voltage range can be achieved using the d-axiscurrent vector control. By converting from the coordinatesystem at rest to the rotating coordinate system, the voltageequation for the IPMSM can be expressed as
�����
��
��� � �� ����
��� �� � ��
� �����
���
����
� (4)
Torque equation is
� � ������ � ��� � �������
�� (5)
� ���������� �
�
���� � ����
������
� (6)
where, �� and �� : dq axis armature voltages, ��: armature
resistance, �� and �� : dq axis armature inductances, �:electrical angular velocity, �� and ��: dq axis armature currents,�� �
����, ��: effective armature flux of permanent magnet,
� : pole pair, ��: effective current and �: phase delay angle.The current and voltage limit equations are
��� � ��� ����� (7)
������� � ����� � ���
� ���� ������
�
�� (8)
where, ��� and ��� are the maximum armature current and
maximum inverter output voltage, respectively.
2) Maximum output power (MOP) control: MOP control,–a combination of maximum torque control at low speed andfield weakening control at high speed–, is applied for motorcontrol [14].
In general motor control, since the DC link voltage ���is constant, the velocity �� at which torque reduction occursis also constant. However, in the case of the ultracapacitor- inverter direct connection method, �� varies depending on���. From (8) �� can be expressed by following primary linearequation of ���.
Figure 9. Switch map of MT control and FW control
�� ���
������� � ����� � �������
�������
������� � ����� � ���� (9)
Fig. 9 shows the switching map of MOP control based on(9). In the case of a DC-DC converterless EV, �� should bechanged depending on both the velocity and the voltage.
-Maximum torque (MT) controlDifferentiating (6) with respect to �, � �� , which max-
imizes torque with given ���, is derived. ��� and ��� whichmaximize torque can be expressed by
��� ���
��� � ����
����
���� � �����
�
������ (10)
��� ������ � ���� (11)
-Field weakening (FW) controlIf � is larger than ��, MT control is not applicable due
to voltage limit. At this speed range, a wide-speed operationcan be achieved by weakening the magnetic field using d axiscurrent.
Dq axis currents are given in (7) and (8).
��� ������
������������
� � ������������
�������������
���
��� � ��
�
�
(12)
��� ������ � ���� (13)
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0 10 20 30 40 500
50
100
150
200
Speed(km/h)
Torq
ue(N
m)
OMCMOPCMTC
0 1 2 3 4 5 6 7 80
10
20
30
40
Time(sec)
Spee
d(km
/h)
OMCMOPCMTC
MT FWOMMOP
Figure 10. Over modulation control for high voltage utilization rate
3) Over modulation (OM) control: For HEV motor appli-cations, OM control is applied to increase voltage utilization[15]. Fig. 10 shows a comparison between OM control withMOP control, MOP control and MT control. It shows that theoperation speed range expands approximately 17 % due to theOM control. However, dq axis vector control is not achievedaccurately due to a non-sinusoidal voltage wave, and torqueaccuracy is not guaranteed.
4) Maximum efficiency (ME) control: Although MOP con-trol optimizes �� to achieve maximum torque and a wide-speed operation, it does not take iron loss into consideration.ME control minimizes the summation of both iron and copperlosses [16]. Iron loss is the summation of the hysteresis loss,which is proportional to the product of the square of themagnetic field and motor speed, and eddy current loss whichis proportional to the product of the square of the magneticfield and the square of the motor speed.
Depending on the target speed range, it may be assumed thatsuch a wide-speed operation is not required at high voltage.Therefore by applying ME control at high voltage at theexpense of torque and speed range, high efficiency operationat whole voltage range will be possible. However, inverter andmotor should be redesigned based on target speed range andvoltage range.
V. CONCLUSION
In this paper, we proposed a novel ultracapacitor EV andmethods to cope with voltage variation. Basic and electricalcharacteristic of EDLC is introduced. Using a specially de-signed inverter, it was shown that no problem arose to driveIPMSM between 50V - 200V with dq axis vector control.Hardware and software approaches for wide-speed operationare investigated and the validity of dq current vector controlsis verified by experimental results.
ACKNOWLEDGMENT
The author and the work are supported by Japan Societyfor the Promotion of Science and global center of excellence(G-COE) program of the University of Tokyo.
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