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IJCSIET--International Journal of Computer Science information and Engg., Technologies ISSN 2277-4408 || 01062015-019

IJCSIET-ISSUE5-VOLUME2-SERIES2

Integration of Multilevel inverters with energy storage systems for

hybrid vehicle applications

1Mr.Gattigunde Ramesh,2 Dr.J.NamrathaManohar 2Professor, EEE Department

1,2Lords Institute of Engineering and Technology, Hyderabad, India

Abstract

There are numbers of alternative energy resources

being studied for hybrid vehicles as preparation to

replace the exhausted supply of petroleum

worldwide. The use of fossil fuel in the vehicles is a

rising concern due to its harmful environmental

effects. Among other sources battery, fuel cell (FC),

super capacitors (SC) and photovoltaic cell i.e. solar

are studied for vehicle application. Combinations of

these sources of renewable energies can be applied

for hybrid electric vehicle (HEV) for next

generation of transportation. Various aspects and

techniques of HEV from energy management

system (EMS), power conditioning and propulsion

system are explored in this paper. Other related

fields of HEV such as DC machine and vehicle

system are also included. Various type models and

algorithms derived from simulation and experiment

are explained in details. The performances of the

various combination of HEV system are

summarized in the table along with relevant

references. This paper provides comprehensive

survey of hybrid electric vehicle on their source

combination, models, energy management system

(EMS) etc. developed by various researchers. From

the rigorous review, it is observed that the existing

technologies more or less can capable to perform

HEV well; however, the reliability and the

intelligent systems are still not up to the mark.

Accordingly, this review have been lighted many

factors, challenges and problems sustainable next

generation hybrid vehicle.

I. INTRODUCTION

AN energy storage system plays an important role in

electricvehicles (EV). Batteries, such as lead-acid or

lithiumbatteries, are the most popular units because

of their appropriateenergy density and cost. Since the

voltages of these kinds of batterycells are relatively

low, a large number of battery cells needto be

connected in series to meet the voltage requirement

of themotor drive [1], [2]. Because of the

manufacturing variab ility,cell arch itecture and

degradation with use, the characters such asvolume

and resistance will be different between these

cascadedbattery cells. In a trad itional method, all the

battery cells are directlyconnected in series and are

charged or discharged by thesame current, the

terminal voltage and state-of-charge (SOC)will be

different because of the electrochemical

characteristicdifferences between the battery cells.

The charge and

Discharges have to be stopped even though only one

of the cells reaches itscut-off voltage. Moreover,

when any cell is fatally damaged, thewhole battery

stack cannot be used anymore. So the battery cell

screening must be processed to reduce these

differences, andvoltage or SOC equalizat ion circuit is

often needed in practicalapplicat ions to protect the

battery cells from overcharging oroverdischarging

[3], [4].

Generally, there are two kinds of equalizat ion

circuits. Thefirst one consumes the redundant energy

on parallel resistanceto keep the terminal voltage of

all cells equal. For example,in charging course, if one

cell arrives at its cut-off voltage,the availab le energy

in other cells must be consumed in theirparallel-

connected resistances. So the energy utilizat ion ratio

isvery low. Another kind of equalizat ion circuit is

composed of agroup of inductances or transformers

and converters, which canrealize energy transfer

between battery cells. The energy in thecells with

higher terminal voltage or SOC can be transferred

toothers to realize the voltage and SOC equalization.

Since thevoltage balance is realized by energy

exchange between cells,

the energy utilizat ion ratio is improved. The

disadvantage is thata lot of inductances or isolated

multiwinding transformers arerequired in these

topologies, and the control of the convertersis also

complex [5]–[13]. Some studies have been

implementedto simplify the circuit and improve the

balance speed by multistageequalization [9]–[13].

Some zero voltage and zero currentswitching

techniques are also used to reduce the loss of

theequalization circuit [13].

Multilevel converters are widely used inmedium or

high voltagemotor drives [14]–[19]. If their fly ing



capacitors or isolateddc sources are replaced by the

battery cells, the battery cellscan be cascaded in

series combining with the converters insteadof

connection in series directly. In [20]–[24], the

cascaded Hbridgeconverters are used for the voltage

balance of the batterycells. Each H-bridge cell is used

to control one battery cell; thenthe voltage balance

can be realized by the separate control ofcharging

and discharging. The output voltage of the

converteris multilevel which is suitable for the motor

drives. When usedfor power grid, the filter

inductance can be greatly reduced. Thecascaded

topology has better fault-tolerant ability by its

modulardesign, and has no limitation on the number

of cascaded cells,so it is very suitable to produce a

higher voltage output usingthese low-voltage battery

cells, especially for the application inpower grid.

Similar to the voltage balance method in

traditionalmultilevel converters, especially to the

STATCOM using flyingcapacitors, the voltage

balance control of the battery cells can

Fig.1.Traditional power storage system with voltage

equalization circuit andinverter.

also be realized by the adjustment of the modulation

ratio ofeach H-bridge [25]. Compared to the

traditional voltage balancecircuit, the multilevel

converters are very suitable for thebalance of battery

cells. Besides the cascaded H-bridge circuit,some

other hybrid cascaded topologies are proposed in

[18], [19]which use fewer devices to realize the same

output. Because ofthe power density limitation of

batteries, some ultra-capacitorsare used to improve

the power density. Some converters must beused for

the battery and ultra-capacitor combination [26],

[27].Multilevel converters with battery cells are also

very convenientfor the combination of battery and

ultra-capacitors.

A hybrid cascaded multilevel converter is proposed

in thispaper which can realize the terminal voltage or

SOC balancebetween the battery cells. The converter

can also realize thecharge and discharge control of

the battery cells. A desired acvoltage can be output at

the H-bridge sides to drive the electricmotor, or to

connect to the power grid. So additional

batterychargers or motor drive inverters are not

necessary any moreunder this situation. The ac output

of the converter is multilevelvoltage, while the

number of voltage levels is proportional to thenumber

of cascaded battery cells. So in the applications of

EVor power grid with a larger number of battery

cells, the output acvoltage is approximately ideal sine

waves. The harmonics anddv/dt can be greatly

reduced than the traditional two-level converters.The

proposed converter with modular design can

realizethe fault redundancy and high reliab ility easily.

Simulation andexperimental results are proposed to

verify the performance ofthe proposed hybrid

cascaded mult ilevel converter in this paper.

II. TOPOLOGY OF THE HYBRID

CASCADEDMULTILEVEL

CONVERTER

One of the popular voltage balance circuits by energy

transferis shown in Fig. 1 [5], [28]. There is a half-

bridge arm andan inductance between every two

nearby battery cells. So thenumber of switching

devices in the balance circuit is 2∗n–2 andthe number

of inductance is n–1 where n is the number of

thebattery cells. In this circuit, an additional inverter

is needed forthe motor drive and a charger is usually

needed for the batteryrecharge [29]. In fact, if the

output of the inverter is connected

Fig.2.Hybrid cascaded mult ilevel converter.



With the three-phase ac source by some filter

inductances, thebattery recharge can also be realized

by an additional controlblock which is similar with

the PWM rectifier. The recharg ingcurrent and voltage

can be adjusted by the closed-loop voltageor power

control of the rectifier.

The hybrid-cascaded mult ilevel converter proposed

in thispaper is shown in Fig. 2, which includes two

parts, the cascadedhalf-bridges with battery cells

shown on the left and the Hbridgeinverters shown on

the right. The output of the cascadedhalf-bridges is

the dc bus which is also connected to the dcinput of

the H-bridge. Each half-bridge can make the

batterycell to be involved into the voltage producing

or to be bypassed.Therefore, by control of the

cascaded half-bridges, the numberof battery cells

connected in the circuit will be changed, thatleads to

a variable voltage to be produced at the dc bus.

TheH-bridge is just used to alternate the direction of

the dc voltageto produce ac waveforms. Hence, the

switching frequency ofdevices in the H-bridge equals

to the base frequency of thedesired ac voltage.

There are two kinds of power electronics devices in

the proposed circuit. One is the low voltage devices

used in the cascaded half-bridges which work in

higher switching frequency to reduce harmonics,

such as MOSFETs with low on-resistance. The other

is the higher voltage devices used in the H-bridges

which worked just in base frequency. So the high

voltage large capacity devices such as GTO or IGCT

can be used in the H-bridges.

The three-phase converter topology is shown in Fig.

3. If thenumber of battery cells in each phase is n,

then the devicesused in one phase cascaded half-

bridges is 2∗n. Compared tothe traditional

equalization circuit shown in Fig. 1, the numberof

devices is not increased significantly but the

inductancesare eliminated to enhanced the system

power density and EMIissues.

Since all the half-bridges can be controlled

individually, astaircase shape half-sinusoidal-wave

voltage can be producedon the dc bus and then a

multilevel ac voltage can be formed atthe output side

of the H-bridge, the number of ac voltage levelsis

2∗n–1 where n is the number of cascaded half-

bridges in each

Fig.3. Three-phase hybrid cascaded multilevel

converter.

Fig.4.Output voltage and current of the battery cell.

Phase. On the other hand, the more of the cascaded

cells, themore voltage levels at the output side, and

the output voltage iscloser to the ideal sinusoidal. The

dv/dt and the harmonics arevery little . So it is a

suitable topology for the energy storagesystem in

electric vehicles and power grid.

III. CONTROL METHOD OF THE

CONVERTER

For the cascade half-bridge converter, define the

switchingstate as follows:

The modulation rat io mx of each half bridge is

defined as theaverage value of the switching state in a

PWM period. In therelative half-bridge converter

shown in Fig. 4, when Sx = 1, thebattery is connected

in the circuit and is discharged or chargedwhich is



determined by the direction of the external

current.When Sx = 0, the battery cell is bypassed

from the circuit, thebattery is neither discharged nor

charged. When 0 < mx < 1,the half-bridge works in a

switching state. The instantaneousdischarging power

from this cell is.

P = Sx·ux· i. (1)

Here ux is the battery cell voltage and i is the

charging currenton the dc bus.

In the proposed converter, the H-bridge is just used to

alternatethe direction of the dc bus voltage, so the

reference voltage ofthe dc bus is the absolute value of

the ac reference voltage, justlike a half-sinusoidal-

wave at a steady state. It means that notall the battery

cells are needed to supply the load at the same

time. As the output current is the same for all cells

connectedin the circuit, the charged or discharged

energy of each cell isdetermined by the period of this

cell connected into the circuit,which can be used for

the voltage or energy equalization. Thecell with

higher voltage or SOC can be discharged more or

tobe charged less in using, then the energy utilizat ion

ratio canbe improved while the overcharge and

overdischarged can beavoided.

For the cascaded multilevel converters, generally

there aretwo kinds modulation method: phase-shift

PWM and carriercascadedPWM. As the terminal

voltage or SOC balance controlmust be realized by

the PWM, so the carrier-cascaded PWM issuitable as

the modulation ratio difference between differentcells

can be used for the balance control [30]– [32].

In the carrier-cascaded PWM, only one half-bridge

converterin each phase is allowed to work in

switching state, the otherskeep their state

unchangeable with Sx= 1 or Sx= 0, so theswitching

loss can be reduced. When the converter is used

tofeed a load, or supply power to the power grid, the

batterywith higher terminal voltage or SOC is

preferentially used toform the dc bus voltage with

Sx= 1. The battery with lowerterminal voltage or

SOC will be controlled in switch state with0 < mx <1

or be bypassed with Sx= 0.

The control of the converter and voltage equalizat ion

canbe realized by a modified carrier-cascaded PWM

method asshown in Fig. 5. The position of the battery

cells in the carrierwave is determined by their

terminal voltages. In the dischargingprocess, the

battery cells with higher voltage are placed at

thebottom layer of the carrier wave while the cells

with lowervoltage at the top layer. Then, the cells at

the top layer will beused less and less energy is

consumed from these cells.

In the proposed PWM method, the carrier arranged

by terminalvoltage can realize the terminal voltage

balance, while thecarrier arranged by SOC can realize

the SOC balance. Since theSOC is difficu lt to be

estimated in the batteries in practice, theterminal

voltage balance is usually used. Normally, the cut-off

voltage during charge and discharge will not change

in spite ofthe variation of manufacturing variability,

cell architecture, anddegradation with use. So the

overcharge and overdischarge canalso be eliminated

even the terminal voltages are used insteadof the

SOC for the carrier-wave arrangement.

To reduce the dv/dtand EMI, only one half-bridge is

allowedto change its switching state at the same time

for the continuousreference voltage. Therefore, the

carrier wave is only rearrangedwhen the modulation

wave is zero and the rearranged carrieronly becomes

effective when the carrier wave is zero. So thecarrier

wave is only rearranged at most twice during one

referenceac voltage cycle as shown in Fig. 6. The

battery’s terminalvoltage and SOC change very

slowly during the normal use, sothe carrier wave

updated by base frequency is enough for thevoltage

and SOC balance.

If the number of the cascaded cells is large enough,

all thehalf-bridges can just work in switch-on or

switch-off state toform the staircase shape voltage.

So the switching frequencyof all the half-bridges can

only be base frequency as shownin Fig. 6, where the

output ac voltage is still very approach tothe ideal

sinusoidal wave which is similar with the

multilevelconverter in [32].

Fig.5. Carrier wave during discharging.



Fig.6.Base frequency modulation.

When one cell is damaged, the half-bridge can be

bypassed,and there is no influence on the other cells.

The output voltage ofthe phase with bypassed cell

will be reduced. For symmetry, thethree-phase

reference voltage must be reduced to fit the

outputvoltage ability. To improve the output voltage,

the neutral shiftthree-phase PWM can be adopted.

The bypass method and theneutral shift PWM is very

similar with the method explainedin [20], [21].

IV. LOSS ANALYSIS AND

COMPARISON

Compared to the traditional circuit in Fig. 1, the

circuit topologyand voltage balance process is quite

different. In the traditionalcircuit in Fig. 1, the three-

phase two-level inverter is usedfor the discharging

control and the energy transfer circuit is usedfor the

voltage balance. In the proposed hybrid-cascaded

circuit,the cascaded half-bridges are used for voltage

balance controland also the discharging control

associated with the H-bridgeconverters. The

switching loss and the conduction losses in thesetwo

circuits are quite different. To do a clear comparison,

theswitching and conduction loss is analyzed in this

section.

In the hybrid-cascaded converter, the energy loss is

composedof several parts

JLoss = Js B + JsH+Jc B + JcH . (2)

Here, Js_ B and Js _H are the switching losses of the

cascadedhalf-bridges and the H-bridge converters,

while Jc_ H and Jc_ Bare the conduction losses.

Fig.7. DC bus voltage output by the cascaded half-

bridgesFFT Analysis of Existing system..

In the traditional circuit as shown in Fig. 1, the

energy loss iscomposed by

JLoss = Js I+Js T + Jc I + Jc T (3)

Where Js_ I and Js_ T are the switching losses of the

three-phaseinverter and the energy transfer circuit for

voltage balance.Jc_ Iand Jc_ T are the conduction

losses. In the traditional circuit, theenergy transfer

circuit onlyworks when there is some imbalanceand

only the parts between the unbalance cells need to

work.So the switching and conduction losses will be

very small if thebattery cells are symmetrical.

First, the switching loss is analyzed and compared

under therequirement of same switching times in the

output ac voltage.That means the equivalent

switching frequency of the cascadedhalf-bridges in

hybrid-cascaded converter is the same as

thetraditional inverter. The switching loss is

determined by thevoltage and current stress on the

semiconductor devices, andalso the switching time

(4)

In the proposed hybrid-cascaded converter, the H-

bridge converteris only used to alternate the direction

of the output voltageto produce the desired ac voltage

as shown in Fig. 7, the devicesin the H-bridge



converter always switch when the dc bus voltageis

zero. So the switching loss of the H-bridge is almost

zero

(5)

The equivalent switching frequency of the half-

bridges is thesame as the traditional converter, but

only one half-bridge isactive at the any instantaneous

in each phase. The voltage stepof each half-bridge is

only the battery cell voltage which is muchlower than

the whole dc bus voltage in Fig. 1. So in a single

Switching course, the switching loss is only

approximate 1/n ofthe one in traditional converter if

the same device is adopted inboth converters.

Furthermore, if the lower conduction voltagedrop and

faster turn-off device such as MOSFET is used in

theproposed converter, the switching loss of the half-

bridge will bemuch s maller

Js B < Js I /n. (6)

In the traditional circuit, the voltage balance circuit

will stillcause some switching loss determined by the

voltage imbalance.So in the proposed new topology,

the switching loss is muchsmaller compared to the

traditional two-level inverter

TABLE I

COMPARISON OF THE SWITCHING AND

CONDUCTION LOSS OF THE PROPOSED

CIRCUIT AND THE TRADITIONAL ONE.

The conduction loss is determined by the on-

resistance of theswitching devices and the current

value. Whatever the switchingstate, one switch

device in each half-bridge and two devices inH-

bridge are connected in the circuit of each phase, so

theconduction loss power can be calculated by

Pc B = I2Rc B · n (7)

Pc H = I2Rc H · 2. (8)

Here, I is the rms value of the output current, Rc_ B

is theon-resistance of the MOSFET in the half-

bridge, Rc_ H is thedevice on-resistance used in the

H-bridge, and n is the numberof the cascaded cells.

In the traditional three-phase inverter, only one

device is connectedin the circuit of each phase, the

conduction loss powerin each phase is just

Pc I = I2Rc I (9)

whereRc I is the on-resistance of the devices used in

the inverter.Normally, the same semiconductor

devices can be usedin the H-bridges and the

traditional three-phase inverters, sothe on-resistance

of the inverters is almost the same as the Hbridges.

The on-loss of the H-bridges cannot be reduced,

while theon-loss on cascaded half-bridges can be

reduced furthermoreby reducing the number o f the

cascaded cells. In practical applications,the battery

module of 12 and 24 V can be used for thecascaded

cells instead of the basic battery cell with only 2–3 V.

Also the semiconductor devices with low on-

resistance are usedin the half-bridges.

From the above analysis, the switching loss of the

proposedconverter is much less than the traditional

converter, althoughthe on-loss is larger than the

traditional converter. The comparedresults are shown

in Table I.

V. CHARGING METHOD

In the system using the proposed circuit in this paper,

a dcvoltage source is needed for the battery charging.

The chargingcurrent and voltage can be controlled by

the proposed converteritself according to the

necessity of the battery cells.. A circu it breaker is

used to switch the

Fig.8. Carrier waves during charging.



Fig.9.Charging circuit of battery with dc source.

Fig.10. Charging circuit of battery with ac source.

dc bus from the H-bridge to the dc voltage source.

Furthermore,a filter inductor is connected in series

with the dc source torealize the current control. The

dc voltage can also be realizedby the H-bridge and a

capacitor. The H-bridgeworked as a rectifier by the

diodes and a steady dc voltage isproduced with the

help of the capacitors.

In the charging course of the battery, the charging

currentshould be controlled. The current state

equation is as follows:

(10)

Here, udc is the dc bus voltage output by the

cascaded halfbridges,ucharge is the voltage of the dc

source, and Rf , Lfare the resistance and inductance

of the inductive filter betweenthe cascaded half-

bridges and the dc source. By this chargingmethod,

the voltage of the dc source must be smaller than

thepossible maximum value of the dc bus

ucharge ≤ udc ≤ n · u0 . (11)

Fig.10. Current control scheme for the battery

charging.

Fig.11.Output waveform for pulse generation circuit

Here, n is the number of the cascade half-bridges in

each phaseand u0 is the discharging cut-off voltage

of the battery cell.

During the charging cycle, the voltages of the battery

cells andthe dc voltage source might be variable, so

the switching states ofthe cascaded half-bridges will

be switched to make the chargingcurrent constant.

The charging current control scheme is shownin Fig.

10. A PI regulator is used to make the current

constantby changing the output dc voltages of the

cascaded cells. Thevoltage of the dc source is used as

a feed-forward compensationat the output of the PI

controller. In practical applications, thevalue of the

dc source’s voltage is almost constant, so the

feedforwardcompensation can also be removed and

the variation ofthe dc source voltage can be

compensated by the feedback ofthe PI controller.

In charging state, the arrangement of the carrier

waves willbe opposite with discharge state. The

battery cell with lowervoltage will be placed in the



bottom then these cells will absorbmore energy from

the dc source. The cells with higher voltageswill be

arranged at the top levels to make these cells

absorbless energy. By the similar analysis as the

discharging state, thepositions of the carrier waves in

charge state are shown in Fig. 8.During the

regeneration mode of the motor drive, for

example,when the EV is braking, the battery cells are

also charged, sothe modulation will also be changed

to the charging mode asshown in Fig. 11.

The energy charged into the battery cell is the same

as (1)

(12)

whereSxis the switching state of the bridge arms and i

is thecharging current controlled by the scheme

shown in Fig. 9.

When the reference dc bus voltage is variable, such

as thehalf-sinusoidal-wave, all the battery cells will

be charged intermittently. Thatis similar to the fast-

charge method of the battery to improve the charging

speed [33]. So the charging current in the

proposedconverter can be larger than the ordinary

charging method. Whenthe battery stack is charged

by a steady dc source, the referencedc bus voltage is

nearly constant. To realize the fast-charge ofthe

battery cell, the carrier-wave must be forced to

exchange theposition which is not arranged by the

SOC or terminal voltagesany more. Then, the

switching state of the half-bridges will bechanged to

produce intermittently charging current.

VI. EXPERIMENTAL RESULTS

To verify the performance of the proposed topology

andthe control method, a three-phase four-cell

cascaded circuit iserected at lab. The platform is

shown in Fig. 13. The lead-acidbattery modules of 5

Ah and 12 V are used. The battery modulesare

monitored by the LEM Sentinel which can

measurethe voltage, temperature, and resistance of

the battery modules.Themeasured information is

transferred to theDSP controller byRS-232

communicat ion. The voltages and currents are

measuredand recorded by YOKOGAWA Scope order

DL750. Since theSOC of the battery cells are d ifficult

to be estimated, the terminalvoltages are used for the

PWM carrier-wave arrangement;

Fig.12. Triphase output voltage of mult ilevel.

Then the terminal vo ltage balance is realized instead

of the SOCbalance which can still verify the effects

of the converter.

The cascaded half-bridges are designed with

MOSFETIPP045N10N3 (Infineon) whose on-

resistance is only 4.2 mΩand the rated current is 100

A. Even if the discharge or chargecurrent is 50 A, the

conduction loss on the MOSFET of eachhalf-bridge

is only

P = I2R = 10.5W.

Then, the efficiency drop caused by the conduction

loss of theMOSFET in the whole phase is

So the conduction loss added by the equalizat ion

circuit is verysmall compared to the output power of

the whole converter.Another device of MOSFET

IPB22N03S4L-15 (Infineon) of30 V, 22 A, and 14.6

mΩ will be used in our platform. The efficiencydrop

caused by the conduction loss Δη = 1.38% whenthe

output current is 11 A.

The three-phase output voltage is shown in Fig. 12.

Thereare nine levels at each phase. So it approaches

more to theideal sinusoidal wave than the traditional

two-level inverters. Ina steady state, the FFT analysis

result of the output ac phasevoltage is shown in Fig.

14. It shows that the harmonics is littlecompared to

the base-frequency component.

An induction motor (IM) was driven with the

variablevoltage–variable-frequency (VVVF) control

method. The parametersof the motor are shown in

Table II.



The dc bus voltage, ac output voltage, output current,

anddc bus current are shown in and 14, which

indicate thewhole process from start-up to the stable

state of the relative IM As the lagbetween the voltage

and current phase, when the phase voltagechange

direction, the H-bridge will change its switching

statetoo. But the phase ac current will change its

direction after a

Fig.13. FFT analysis of the output ac voltage.

TABLE II

PARAMETERS OF THE INDUCTION MOTOR

.period of time, so the direction of the dc bus current

is reversedwhen the directions of phase voltage and

current are different.The reversed dc bus current also

reflects the reactive power ofthe load. As the

induction motor is of nearly no load, the phasecurrent

is nearly π/2 lag with the phase voltage and the

averagedc current is almost zero in steady state. But

in starting course,

Fig.14 .Output voltage and current

Fig.15. DC bus current with some loads.

the average dc bus voltage is positive which stands

for the active power flow from the battery to the

electric motor

When there is some load on the motor, the dc bus

current is shown in Fig. 15. The reversed current is

greatly reduced which means the average power is

flowed from the converter to the motor.

When the charging method introduced in above

section is usedwith a steady dc source, the cascaded

half-bridges are connectedwith the dc source first,

then a charging current reference isgiven, the output

dc bus voltage and the dc current of the wholecourse

are recorded as a single figure by the

YOKOGAWAScopeCorder.



The dc bus voltage and the charging current when

connectingwith the dc source are shown in Fig. 16. It

shows that when thedc source was connected, the

cascaded half-bridges changedtheir switching state

quickly to keep the charging current sameas the

reference value. In Fig. 16, there are two half-bridges

Working with Sx= 1 and the third working in

switching state.

Fig.16. DC bus voltage and current when connecting

with the DC source.

When the charging current reference is given, the dc

busvoltage will reduce first to establish the charging

current asshown in Fig. 17. As the resistance in the

filter inductance isvery little, the voltage drop on the

filtered inductance by dccurrent is also near zero, the

dc bus voltage will become almostthe same as the no

charging state.

Fig.17. DC bus voltage and charging current during

dc source voltage change.

proposed PWM. The cell voltages are measured in

every10 min. We can find that the three terminal

voltages will finallybecome equal voltage levels

under the proposed voltagebalance algorithm. It must

be mentioned that the balanced terminalvoltages are

the operating voltages of the battery cells.The open-

circuit vo ltage which can reflect the SOC cannot

bebalanced in real time. Due to the cell difference,

the balancedterminal voltage will become different

when the battery cellsare out of use for a period of

time.

VII. CONCLUSION

A hybrid cascaded multilevel converter topology based on a small and cheap H-Bridge converter has

been proposed, simulated and verified. The proposed converter with modular structure can reach any number of cascaded levels and is suitable for the energy storage system control with low-

voltage battery cells or battery modules. The output of the circuit is multilevel ac voltages where the number of levels is proportional to the number of

battery cells. So the output ac voltage is nearly the ideal sinusoidal wave which can improve the control performance of the motor control in EVs. A fifteen level 3phase proto type using Multicarrier Pulse

Width Modulation Control was implemented to demonstrate some of its advantages: excellent voltage waveforms, THD reduces and, hence, almost perfect current waveforms. The proposed solution is

Multilevel Inverters become a real solution for EV and traction applications.

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[2] H. M. Zhang and S. P. Ding, “Application of

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Mr.Gattigunde Ramesh. He is

pursuing M.Tech (Power Electronics) at Lords

Institute of Engineering and

Technology,Hyderabad.Completed his B.Tech(EEE)

from LakireddyBalireddy College of Engineerig

,Mylavaram,Andhra Pradesh state,India.

Dr.J.NamrathaManohar is a

Doctorate in Electrical Engineering from JNTUH. She

has acquired her Bachelor of Engineering(BE) Degree

from Osmania University in the year 1982, MCA from IGNOU in the year 2004; M.Tech in the year 2006.She

is presently Professor in the Department of Electrical

and Electronics Engineering and Dean Academics at

Lords Institute of Engineering and Technology,

Hyderabad, India She has a total of 32years of experience. She has Served as Manager at M/S.

Hindustan Cables Limited for 17 years in various

departments as Quality Control, Engineering and

Production Planning and Control. She has 15years of

experience as a Professor. Her research areas include Power System Performance Optimization, FACTS

Devices and Neural Networks. She has published about

11 papers and prepared several Manuals.

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