control strategy for multiple input dc-dc power converters devoted to hev's.pdf

7/29/2019 Control Strategy for Multiple Input DC-DC Power Converters Devoted to HEV's.pdf

http://slidepdf.com/reader/full/control-strategy-for-multiple-input-dc-dc-power-converters-devoted-to-hevspdf 1/6

Control Strategy for M ultiple Input DC-DC Power Converters Devotedto Hybrid Vehicle Propulsion Systems

A . DI NAPOLI’, F.CRESCIMBINI1,Member. IEEE,F. GIUL II CAPPON12,Member, IEEE, and L . SOLERO’,Member. IEEE

‘Dept. of Mech. and Indus. Eng., Univ. of ROM A TRE , 00146 Roma, Italy, email: [email protected]

’Dept. of E lect. Eng., Univ. of Rome “La Sapienza”, 00184Roma, Italy, email: gi ul i i@lett ri ca. i ng.uni romal .it

Abstract - Customer demands for greater acceleration,

performance, and vehicle range in pure EVs plus mandatedrequir ements to fur ther r educe emissions in H EVs increase theappeal for combined on-board energy storage systems andgenerators. T his paper deals with the control system of an

original HEV pr opulsion system that includes fuel cel l

generator and storage energy system combining ultracapacitortank and battery. T he three on-board power sources supply thevehicle traction drive through a multi-input dc-to-dc powerconverter which provides the desired management of the powerflows. In particular, in the proposed arrangement theultr acapacitor tank is used for l eveling the battery load currentduring transients resulting from either acceleration or brakingoperation of the vehicle. The paper outli nes the features of thecontrol unit of the dc-to-dc power converter being used in theproposed propulsion system and describes the maincharacteristics of a 3SkW prototype developed to conductlaboratory experi ments and vali date the control strategy.

1. INTRODUCTION

To date on-road hybrid-electric vehicles (H EV ) make useof high-power density ac propulsion systems to provide

comparable performance with vehicles using internal

combustion engine (ICE) technology. Customer demands

for greater acceleration, performance, and vehicle range inpure EVs plus mandated requirements to further reduce

emissions in HEVs increase the appeal for combined on-board energy storage systems and generators. Electricmotor, inverter, and associated control technology has madesubstantial progress during the past decade and it is not the

limiting factor to either vehicle performance or the large-

scale production of EV s and HEVs. The search for acompact, lightweight. and efficient energy storage system(battery and/or combination of other emerging technologies,

including ultracapacitors, flywheel energy storage system,advanced batteries, and fuel cells) that is both affordable

and has acceptable cycle life remains the major roadblock to

large-scale production of EV s and HEVs.

Proton exchange membrane (PEM ) fuel-cell s are being

increasingly accepted as the most appropriate power sourcefor future generation vehicles. This acceptance isevident bythe formation of a new global alliance for the

commercialization of this technology and by the growingparticipation of major automotive manufacturers inproducing sophisticated demonstration vehicles.

With the rapid rate of technical advances in fuel-cells(FCs), and with major resources of the automotive industry

being directed to commercialization of FC propulsionsystems, it looks more promising than ever that FCs will

soon become a viable alternative to internal combustion

engine technology.

One issue that has been driving the development of FCsfor automotive applications is their potential to offer clean

and efficient energy without sacrificing performance or

driving range. In the case of a PEM FC powered vehiclerealizing this potential means ensuring that the complete FC:

system operates as eff iciently as possible over.the range ofdriving conditions that may be encountered.

A battery storage unit can be employed combi ned with a

FC stack in order to achieve the operating voltage-current

point of maximum eff iciency for the FC system. In such aconventional arrangement, the battery is sized to deliver the

difference between the energy required by the traction drive

and the energy supplied by the FC system, as well as it hasto deal with power peaks being on demand during

acceleration or overtaking phases resulting from the driving

cycle on which the vehicle is expected to operate. Such peak

power transients result in a hard constraint for batteries, as

in the battery a higher peak than rated power implies an

increasing of losses and temperature and so a decreasing of

li fetime. Thereby, it is desirable to reduce these power peaks

by introducing an additional auxiliary power device, load

leveling of the FC-battery system by means of

ultracapacitors (UCs).UCs, which have high power density, obtain regeneration

energy at high efficiency during decelerations and supply

the stored energy during accelerations in order to reduce thepeak power requirements for the FC-battery unit. The

capacitors are sized to meet energy storage load levelingrequirements resulting from the driving cycle on which thevehicle isexpected to operate. Th e UC tank must supply all

the power required in excess of the FC-battery system ratedpower, provided that the UC state of charge (SOC) isgreater than a specified minimum. Whenever the power

required to operate the vehicle is lower than the FC-battery

rated power, the UCs can be charged with the power inexcess. Whenever regenerative braking operations occur,

energy is put into the UC tank provided this device is not

fully charged yet.L imitations to the use of U C tanks primaril y originate

from the characteristics of the UCs that to date are being

available in the market. In fact, at the to-date stage ofdevelopment, U Cs have a too low value of the cell voltage,as well as a cell leakage current that may change from one

cell to the others, which fact may result in significantvoltage unbalances in stacked unit. T he use of a dc-to-dcpower electronic converter in the dc-link of propulsion

drives substantially reduce the drawback resulting from

having a cell voltage lower than 2.5 V and allows a suitable

regulation of the energy flow coming in and out the UCtank.

0-7803-7369-3/02/$17.0002002 IEEE 1036



In this paper the control strategy for a multiple input dc-

dc power converter (M I-PEC) devoted to combine the

power flowing of multi-source on-board energy systems is

presented; the assumed energy storage arrangement, shown

in Fig. 1, includes fuel cell (FC) generator, ultracapacitor

(UC)tank, and battery system.

-

1

Fig. 1 . Proposed hybrid drive-train

The FC generator is the main energy source; however,FC generators have very poor efficiency at light load, thus

the battery storage system is required to supply the tractionpower at light load in order to save total efficiency [1, 21. A s

a result, the FC generator is sized in order to supply the

traction electric drive up to 415 of the cruising maximumpower, whereas the storage battery should feed at least 115

of the cruising maximum power for the time calculated on

the basis of selected driving cycles. FC systems can neither

recover energy nor provide high dynamic response. morethen ever when large auxiliary devices are used to feed

hydrogen and to accomplish the reforming process; then UCtank is used to satisfy acceleration and regenerative brakingrequirements accomplishing the system load transients andimproving the on-board battery cycle life [3].

The paper deals with the design and experimental testing

of the control board for a M I-PEC topology proposed for

hybrid vehicles' propulsion systems.

ILINK

A

VL IN K

11. M U L T I P L ENPUT CONVERTEROPOLOGY

In order to optimize the mode of operation of each on-

board power source, different topologies for the multi inputpower electronic converter (M I-PEC) were investigated.The configuration shown in Fig. 2 was chosen for its

feasibility in hardware arrangement and in control strategy

acconiplishment [4,51. As such a power converter interfaceis accomplished by means of connecting in parallel among

them the output circuits of three bi-directional step-up/step-down dc-to-dc converters, the joint output terminals of suchstep-uplstep-down converters are used to provide voltage

regulated dc power supply to the traction drive. On the other

hand, the three couples of M IPEC input terminals are

separately fed through FC stack, battery and UC tank,respectively. In each dc-to-dc converter the step-up mode of

operation is used to supply power to the dc input terminalsof the traction drive, whereas the step-down mode ofoperation is used whenever the power flow is required to

reverse because of regenerative braking operations beingcommanded in the traction drive. Theselected bi-directionaltopology is suitable to connect low voltage power sources,in which the voltage level can change as a function of the

I t I I 1

load power requirement and is affected by the state ofcharge (SOC), to traction electrical drives, which are usuallyfed at few hundreds Vol ts in order to reach high eff iciency.On the contrary, in F C generators, UC tanks, and batterysystems the number of elements connected in series is

limited to improve the system reliability.

1

I IFC LINK

Fig. 2. Multi input power electrmc converter layout

Each step-uplstep-down converter includes two powerswitches (IGB T) and two power diodes being arranged

among them to achieve a single three-terminal assembly, as

shown in Fig. 2. Such an I GBT-diode assembly is being

comniercially available at reasonable cost in the form of the

so-called "dual-IGBT power module", and the use of such a

quite inexpensive power semiconductor device justifies the

useless presence of one switch and one diode in the FC-fed

dc-to-dc converter which actually is not required to operate

as step-down converter during regenerative braking mode of

operation. In addition to a dual-I GB T power module, the

power circuit of each step-up/step-down converter includes

both an input inductoricapacitor filter and an output

capacitor filter. Both such power filters are required in order

to limit the current ripple in the output circuitof

each power

source, as well as to minimize the voltage ripple at the input

terminals of the traction drive. Due to the presence of such a

quite large capacitor fil ters in the M lP EC ci rcuit, capacitor

pre-charge circuits are included in the complete layout to

avoid dangerous over-currents at start-up.

In the MIPEC the IGBT duty cycles are controlled inorder to meet the power demand of the traction drive. Indoing that, the MIPE C control manager provides sharing

1037



open circuit voltage [VI

voltage@ inax power [VI

current (5)max power [ A ]

Batter S stein

rated voltage [V I

innx current A

111. PROPOSED CONTR OL SYSTEM

The control system of the MI-PEC should be designed in

order to satisfy the current and voltage requirements at the

input of the traction drive. In addition, the electrical power

sources should be operated on the basis of their peculiar

characteristics. In the M I-PEC the IGBT duty cycles are

controll ed in order to meet the power demand of the traction

drive. In doing that, the MI -PEC control manager provides

sharing among the three power sources of the power flow

being on demand. Such a control strategy is accomplished

by taking into account the state of charge (SOC) of UC and

battery, as well as both the maximum time derivative

allowed for the output current and the efficiency map of

each the power sources. T he FC generator can neither

recover energy nor provide high dynamic response, more

then ever when large auxiliary devices are used to feed

hydrogen and to accomplish the reforming process; as wellthe FC source usually shows very low efficiency when is

called to supply power in the 0-15% range of its rated

power. Thus, the FC generator is operated only when the

total power request (i.e. traction drive power demand plus

chargeable power sources additional request) is higher than

15% of i ts rated power. In addition, the FC source current is

controlled with the aim of complying with the dynamic

behavior of the whole FC system.

250

I25

160

For both the FC-fed and UC-fed dc-to-dc converters the

IGBT duty cycle is regulated in order to achieve the desired

control of the converter input current. As a result,

appropriate dynamic response during either acceleration or

braking operation of the vehicle is achieved, together withthe desired working point on FC generator V-lcharacteristic. On the other hand, the MI-PEC output

voltage is regulated at the desired value by controlling theduty cycle of the battery-fed dc-to-dc converter.

Due to the original M I-PEC structure, it is possible to

suitably control the instantaneous values of the output

currents from three power sources. In order to get the bestperformance, the FC is used to supply the average powerrequired by the vehicle driving cycle whereas the UC tank

provides load-leveling of the power peak demand and the

battery supplies the extra power - with respect to the FCmaximum output power - required to meet the actual power

demand resulting from vehicle operation. As during thedriving cycle the vehicle is continuously either accelerated

or braked the system has to either regenerate energy during

decelerations or supply the stored energy during

accelerations with as high as possible efficiency. At thesame time, it should be assured that both battery and UC

tank become not discharged during acceleration transient.Thereby. the system control algorithm has to include

suitable regulation of the SOC values of battery and UC, sothat the actual SOC is always kept within preset upper andlower bounds. In addition to that, for each of the three

power sources the control algorithm must provide limitationof the output current as well as it has to assure that any ofthe MI-PEC input voltages do not come down under athreshold value. As the MI -PEC output voltage is being kept

constant, at any given operating condition of the tractiondrive the demanded power is proportional to the dc-linkcurrent. Thereby:

inns voltage( 0OC=) [V I

min. voltage (@ SOC=0.6) [VI

max current [A]

where Ire, Ibt and lsc, are the current contributions of the three

power sources to the M I-PEC output current. Through (1 )

for each power source it is possible to superimpose themaximum permissible power variation by means of fixing

the maximum permissible value for the derivatives. Then, in

order to impose the maximum power value it issufficient to

set a maximum admissible current value. A fter all theconstrains are being fixed it results:

I40

85200

In addition to the above constraints, it is necessary that

the SOC values of battery and UC remain within preset

upper and lower bounds. This can be achieved by regulatingthe currents through a proportional controller. In the voltageloop control, a hysteresis threshold around the desired dc-

link voltage level is used to calculate the reference signal;

thus repetitive switching between step-up and step-downmodes of operation of the battery system converter is

1038



avoided when the measured, dc-link voltage level is quiteclose to the reference. Therefore the control algorithm beingimplemented in the MI -PEC controller, tums out tobe:

Kuc-SOC

The reference control signal for the FC generator consistsof three terms: the first is the current required by the electric

drive, the second and third are the terms which take in

account respectively the SOC values of battery and UC. Asan example, whenever either the battery system or the UCtank needs to be recharged, the FC generator change its

working point in order to both supply the electric dri ve andrecharge the on-board energy storage devices.

The reference control signal for the UC tank consists oftwo temis: the first one takes in account thc UC SOC level

and then either the required charging current or dischargingcurrent, whereas the second term is proportional to the

difference between the electric drive current request and thecurrent supplied by the FC generator. The reference control

signal for the battery system is kept constant in order to

follow the used electric drive specifications. However, a delink voltage level variable with the rotational speed of the

electric motor could be adopted in order to improve the

efficiency of the traction drive [7]. According to the above-described management of the power flows, the three powersources concur to supply the required output power. with the

UC tank being acting to achieve slow current (power) timevariations for both battery and FC.

In order to achieve optimal sizing of the power sources as

a function of either maximum total efficiency or vehicle

total mass, the proposed control strategy was investigated

concerning a number of vehicle classes and driving cycles.

To this aim, a suitable Simuli nfi model was implemented

on the computer to run simulations of the vehicle and

traction drive being operated over selected driving cycles.

The output variables of the Simulink'z! model are the dc link

current, voltage and power required by the propulsion

system in order to meet the driving cycle mission. A

PSpice'c model of the proposed MI-PEC topology was

realized in order to investigate the power flow control

strategy. On the basis of the dc link current and voltage,

obtained by means of the Si ndi nkg model, the power flow

sharing among the three power sources is investigated, and

valuation of the control loop response for each single

converter unit is studied.

As an example of simulation results achieved for a 35kWrating power propulsion system. Fig. 3 shows the dc current

contribution given by each of the on-board power sources to

the MI-PEC output current resulting from drivi ng a city-car

vehicle over a given driving cycle corresponding to the

urban mission for the vehicle. It can be observed in Fig. 3

that the proposed control strategy actually allows suitable

leveli ng of the battery l oad, with the UC tank being handling

current transients due to acceleration and braking operation

.3/2

of the vehicle. Table I I resumes the chosen values for each

parameter used in the reference control signals calculation:

by changing the parameter values it is possible to modify the

current contribution given by each of the on-board power

sources. The simulations give the opportunity to the

designer to valuate the control algorithm parameters forgiven power sources characteristics, as well to decide the

best rating of battery system and UC tank on the basis of the

FC generator characteristic.

tib-soc

Fig. -3. Multiple i nput dc-dc converter cii ixnts for ECE 15drive cyclc(dc ink V. FC generator 0. JC tank A, battery system0)

.?I2

TABLE 11. PARAMETERSSE D I NCONTROLIGNALS \PRESSIONS

Fuel Cell Generator Current Retereiice

K fi- I 312

cost [V I 250

Kuc

K'fC 213

Iv. CONTROL ALGORITHM ~MPL ERIENTATI ON

The control algori thm main block scheme is shown in

Fig. 4. The required measurement data for the control

system are obtained by using Hall effect sensors for both

current and voltage signals, and then they are acquired by

means of 8 bits AID converters using the "put and take

technique". In case the measured values are not acceptable

for either the power sources or the components rating, one

or more alarm leds are turned on in order to identify the

malfunction; then the MI-PEC isoperated in order to assure

the dc-link power supplying even with exclusion of the

presumed damaged devices. The propulsion system is driven

with a minimum power level ignoring the cycle mission and

the MI -PEC is controlled fol lowing the optimum sharing of

the power flow among the still available on-board sources.As the measured values are checked, UC tank and battery

system SOC levels are investigated and either step-up or

step-down mode of operation is selected for each power

source converter. Following (3) the reference control signals

are generated, by comparing the reference control signalswith the measured values the error signals are found; a PI

controller for each power source converter is then

responsible for the switch duty-cycle command.

1039



order to avoid undesired reset of the microprocessor

because of the radiated interference.

Power Seiniconductors

Rated Current [ A ]

Rated V oltage [V I

r

PM 300DSA 060 (IPM 2-pack mod.)

300

600Ma functioningPower Source Inductors

Inductance [pH]

Rated Current [ A ]

Max Current Ripple [A ]

I &

L,< L, LR180 80 2.50

160 200 8032 80 16

Ma functioningPower Source

Exclusion.

Capacitors C,, ,T

Capacitance [mF] I S

Rated Vol tage [V I 385

I95ax RM S Current Ripple

I A l- CFCIN CliCIN c,,,1 2 I

.385 385 385

I 3 26 13

SOC Values

Step-UpiStep-Down Modeo f

GenerationIeasurement Data

Acquisition

4

Cycle Commands

MI-PECPower System

Fig. 4. Control algorithm inaiii block scheme

v. MI-PEC PROTOTYPEA ND EXPER IMENTALESULTS

A M I-PEC prototype was arranged to supply a 3SkW -

250V electric drive for a small-class hybrid vehicle; the

electric drive is composed of a IGBT voltage source inverter

and induction motor. The prototypal version of theinvestigated M I-PEC is shown in Fig. 5; ndependent phase-

legs were used for each power source in order to have high

flexibil ity in layout connections and air-forced heat sink waschosen to make easier the experimental rig setting up;however a six-pack module and water-cooled heat-sink can

be used to achieve sufficient compactness of the propulsionsystem. Two ferrite-core inductors were arranged in parallel

to achieve the required 1SopH, 160A equivalent inductancefor FC side converter.

The leading characteristics of each component used in

the MI -PE C prototype are shown in Table 111. Thecomponents were sized on the basis of the values in Table 1and assuming a switching frequency of 1SkHz. T he adopted

microprocessor-based control board is shown in Fig. 6; the

control board is placed in an appropriate shielded box in

Fig. 5. M I-PE C prototype

Fig. 6. MI-PEC microprocessor-based control board

In the arranged experimental test rig, the battery system

was composed of 12 battery connected in series and rated 3.5Ah - 12 V each; the U C tank was arranged by means of 3

UC subsystems rated 67 F - 42 V each and connected inseries; a dc generator with separate field winding was used

to simulate experimentally the electrical characteristic of the

FC generator. A s an example of the experimental tests

concerning the control system, duty-cycles of the FCs andUCs step-up switches are shown in Figs. 7 and 8

respectively at the beginning and at the end of the vehicle

simulated acceleration.

1040



. r j

Fig. 7. MI-PEC switch duty-cycles at the beginning of vehicle acceleratioii(hottom trace: FC side. upper trace: UC side)

(hottom trace: FC side. upper trace: UC side)

When a sudden increase of current is requested by theelectric dri ve, the UC tank immediately feeds the current

request, whereas the FC generator takes some seconds tosatisfy the electric drive request because of its low dynamictransient (if compared to UCs transients). As a result, at thebeginning of the vehicle simulated acceleration the duty-cycle value of the UC side switch rises and after someseconds decreases, whereas the contrary happens to theduty-cycle value ofthe FC side switch.

The experimental campaign also concerned tests devotedto evaluate the performance of the MIPEC prototype in

terms of current ripple in the MlPEC input and outputcircuits. Fig. 9 shows typical waveform recorded for theinput current flowing in the MIPEC ferrite-core inductorsand Fig. 10 shows the typical wavefomi of the currentflowing in the MIPEC output capacitors.

VI , CONCLUSIONS

In this paper a multiple input dc-dc power converterdevoted to combine the power flowing of multi-source on-board energy systems has been presented; the proposed

energy storage arrangement includes fuel-cell generator,ultracapacitor tank, and battery system. The paper has beenmainly focused on the control system of the adoptedmultiple-input power electronic converter. Future work will

be addressed to further optimization criterion for the control

Fig. 9. Wavefomof MI-PEC prototype electrical quantities: ferrite-coreinductor current (SOA/div)

the output capacitors (20A /di v)

strategy and size reduction of the MI-PEC volume andweight.

V I I . RE FE RE N CE S

V. Raman. "The Hydrogen Fuel Option for Furl Cell VehicleFleets." F iiel Cell Poiiwfbr.Troris/iortcitioii.SAE SP-1425, 1999.

ti. Dircks, "Recent Advances in Fuel Cells foi- Tsansportation

Applications." Firrl Cell Pouw for Tinrisport.. SAE SP-1425. 1999.F. Caricchi, F. Crescimhini, F. Cii~ii Capponi, L. Solero.

"Ultracapacitors Employment i n Supply System for EV Motor

Drives: Theoretical Study and Experimental Results." Proc. of tlir

14"' ltiter~i~tioiicillectr.ic- Ve/iic,leSi'rriposi~mi.997. cd-rom.

.4.Di Napoli. F. Giulii Capponi, L. Solero. "Power ConverterArrangements with Ultracapacitor T ank for Battery Load Leveling inEV Motor Drives." Pi.oc. of ' th e 8" Eirropeaii Coiif 017 PowerElectrotiics otid ilpplicntioric., 1909, cd-rom.

R.D. King. J . Schwartz. M. Cardinal. L . Salasoo, "Development and

System Test of a High Efficiency UltracapacitoriBattery ElectronicInterface," Pror. of' the I f " Iiiteniotioiicrl Electik VehicleSi.iiipo.siiriii. 1998. cd-rom.A . Di Napoli. F. Crrscimhini, L . Solero. G. Pede. G. L o Bianco, M.Pasquali. "U ltracapacitor and Battei-y Storage System Supporting

Fuel-Cell Powered Vehicle." Proc. of th e 18'" I tit~~rti otiot~ollectrk

VehicleSwiposiiirri, 200 ,cd-roin.

F. Caricchi, F . Crescimbini, G. Noia. D. Pirolo. "Experimental Studyof a Bi-Directional DC-DC: Converter for the DC Link VoltageControl and the Regenerative Bi-aking n PM Motor Drives Devotedto Electrical Vehicles," Proceedings of'tlieEEE Y t l i Applied Power.Electrotiics Cor!/:orid E .vpo.~tiori.994. pp. 38 1-386.

1041

control strategy for multiple input dc-dc power converters devoted to hev's.pdf

Documents