a hybrid cascaded multilevel converter for battery 2

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 7, JULY 2014 3537

A Hybrid Cascaded Multilevel Converter for BatteryEnergy Management Applied in Electric Vehicles

Zedong Zheng, Member, IEEE, Kui Wang, Member, IEEE, Lie Xu, Member, IEEE, and Yongdong Li, Member, IEEE

Abstract—In electric vehicle (EV) energy storage systems, a largenumber of battery cells are usually connected in series to enhancethe output voltage for motor driving. The difference in electro-chemical characters will cause state-of-charge (SOC) and terminalvoltage imbalance between different cells. In this paper, a hybridcascaded multilevel converter which involves both battery energymanagement and motor drives is proposed for EV. In the proposedtopology, each battery cell can be controlled to be connected intothe circuit or to be bypassed by a half-bridge converter. All half-bridges are cascaded to output a staircase shape dc voltage. Then,an H-bridge converter is used to change the direction of the dc busvoltages to make up ac voltages. The outputs of the converter aremultilevel voltages with less harmonics and lower dv/dt, which ishelpful to improve the performance of the motor drives. By sepa-rate control according to the SOC of each cell, the energy utilizationratio of the batteries can be improved. The imbalance of terminalvoltage and SOC can also be avoided, fault-tolerant can be easilyrealized by modular cascaded circuit, so the life of the battery stackwill be extended. Simulation and experiments are implemented toverify the performance of the proposed converter.

Index Terms—Battery cell, charging and discharging, elec-tric vehicle (EV), hybrid cascaded multilevel converter, voltagebalance.

I. INTRODUCTION

AN energy storage system plays an important role in elec-tric vehicles (EV). Batteries, such as lead-acid or lithium

batteries, are the most popular units because of their appropriateenergy density and cost. Since the voltages of these kinds of bat-tery cells 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 variability,cell architecture and degradation with use, the characters such asvolume and resistance will be different between these cascadedbattery cells. In a traditional method, all the battery cells are di-rectly connected in series and are charged or discharged by the

Manuscript received March 20, 2013; revised May 22, 2013 and June 30,2013; accepted August 15, 2013. Date of current version February 18, 2014.This work was supported by the National Natural Science Foundation of Chinaunder Grant NSFC 51107066. Recommended for publication by Associate Ed-itor P. C.-K. Luk.

Z. Zheng, K. Wang and L. Xu are with the State Key Laboratory ofPower System, Department of Electrical Engineering, Tsinghua University, Bei-jing 100084, China (e-mail: [email protected]; [email protected];[email protected])

Y. Li was with the School of Electrical Engineering, Xinjiang University,Urumqi, China. He is now with the Department of Electrical Engineering, Ts-inghua University, Beijing 100084, China (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPEL.2013.2279185

same current, the terminal voltage and state-of-charge (SOC)will be different because of the electrochemical characteristicdifferences between the battery cells. The charge and dischargehave 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 cellscreening must be processed to reduce these differences, andvoltage or SOC equalization circuit is often needed in practicalapplications to protect the battery cells from overcharging oroverdischarging [3], [4].

Generally, there are two kinds of equalization 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 available energy in other cells must be consumed in theirparallel-connected resistances. So the energy utilization ratio isvery low. Another kind of equalization 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 utilization 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 multi-stage equalization [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 in medium or high volt-age motor drives [14]–[19]. If their flying 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 H-bridge converters 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

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3538 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 7, JULY 2014

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 bal-ance circuit, 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 ultracapacitorsare used to improve the power density. Some converters must beused for the battery and ultracapacitor combination [26], [27].Multilevel converters with battery cells are also very convenientfor the combination of battery and ultracapacitors.

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 con-verters. The proposed converter with modular design can realizethe fault redundancy and high reliability easily. Simulation andexperimental results are proposed to verify the performance ofthe proposed hybrid cascaded multilevel converter in this paper.

II. TOPOLOGY OF THE HYBRID CASCADED

MULTILEVEL 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 multilevel 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 rechargingcurrent and voltage can be adjusted by the closed-loop voltageor power control of the rectifier.

The hybrid-cascaded multilevel 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 H-bridge inverters 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 pro-posed circuit. One is the low voltage devices used in the cascadedhalf-bridges which work in higher switching frequency to re-duce harmonics, such as MOSFETs with low on-resistance. Theother is the higher voltage devices used in the H-bridges whichworked just in base frequency. So the high voltage large capacitydevices 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

ZHENG et al.: HYBRID CASCADED MULTILEVEL CONVERTER FOR BATTERY ENERGY MANAGEMENT APPLIED IN ELECTRIC VEHICLES 3539

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:

Sx=

{1

0

upper switch is conducted, lower switch is OFF

lower switch is conducted, upper switch is OFF.The modulation ratio mx of each half bridge is defined as the

average 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 sametime. As the output current is the same for all cells connectedin the circuit, the charged or discharged energy of each cell is

determined 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 utilization 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 carrier-cascaded PWM. 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 equalization canbe realized by a modified carrier-cascaded PWM method asshown in Fig. 4. 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 termi-nal voltage can realize the terminal voltage balance, while thecarrier arranged by SOC can realize the SOC balance. Since theSOC is difficult to be estimated in the batteries in practice, theterminal voltage balance is usually used. Normally, the cut-offvoltage 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/dt and 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 refer-ence ac voltage cycle as shown in Fig. 5. 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 topol-ogy and voltage balance process is quite different. In the tradi-tional circuit 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 + Js H +Jc B + Jc H . (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 B

are the conduction losses.

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

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 I

and Jc T are the conduction losses. In the traditional circuit, theenergy transfer circuit only works 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

Js=∫ T sw i t ch

0u · idt. (4)

In the proposed hybrid-cascaded converter, the H-bridge con-verter is 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

Js H ≈ 0. (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 singleswitching 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 smaller

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 ICOMPARISON 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 con-nected in the circuit of each phase, the conduction loss powerin each phase is just

Pc I = I2Rc I (9)

where Rc I is the on-resistance of the devices used in the in-verter. 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 H-bridges.

The on-loss of the H-bridges cannot be reduced, while theon-loss on cascaded half-bridges can be reduced furthermoreby reducing the number of the cascaded cells. In practical appli-cations, 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. The chargingcircuit is shown in Fig. 8. A circuit breaker is used to switch the

Fig. 8. Charging circuit of battery with dc source.

Fig. 9. 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 as shown in Fig. 9. 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:

Rf i + Lfdi

dt= udc−ucharge . (10)

Here, udc is the dc bus voltage output by the cascaded half-bridges, ucharge is the voltage of the dc source, and Rf , Lf

are 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. Carrier waves during charging.

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 feed-forward compensation 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. 9.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)

Pcharge = uxi = Sx · ux · i (12)

where Sx is 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 in-termittently because of the PWM as shown in Fig. 12. Thatis similar to the fast-charge method of the battery to improve

Fig. 12. Intermittent charging current of battery cells.

Fig. 13. Platform of the experiment.

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 mod-ules are monitored by the LEM Sentinel which can measurethe voltage, temperature, and resistance of the battery modules.The measured information is transferred to the DSP controller byRS-232 communication. The voltages and currents are measuredand recorded by YOKOGAWA ScopeCorder DL750. Since theSOC of the battery cells are difficult to be estimated, the termi-nal voltages are used for the PWM carrier-wave arrangement;


Fig. 14. Triphase output voltage of multilevel.

then the terminal voltage 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

Δη =I2R · nU · I · n = 1.75%.

So the conduction loss added by the equalization 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 effi-ciency drop caused by the conduction loss Δη = 1.38% whenthe output current is 11 A.

The three-phase output voltage is shown in Fig. 14. 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. 15. It shows that the harmonics is littlecompared to the base-frequency component.

An induction motor (IM) was driven with the variable-voltage–variable-frequency (VVVF) control method. The pa-rameters of the motor are shown in Table II.

The dc bus voltage, ac output voltage, output current, anddc bus current are shown in Figs. 16 and 17, which indicate thewhole process from start-up to the stable state of the relative IM.From Fig. 16, it is obvious that the output voltage levels keepincreasing according to the acceleration of the motor speed. Thestator current of the motor shown in Fig. 17 are of improvedsinusoidal shape which can reflect the control performance ofthe motor. The dc bus current is also shown in Fig. 17. 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. 15. FFT analysis of the output ac voltage.

TABLE IIPARAMETERS OF THE INDUCTION MOTOR

Fig. 16. DC and ac output voltage during motor acceleration.

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. 17. Output ac and dc current during motor acceleration.

Fig. 18. DC bus current with some loads.

the average dc bus voltage is positive which stands for the activepower flow from the battery to the electric motor.

When there is some load on the motor, the dc bus current isshown in Fig. 18. The reversed current is greatly reduced whichmeans the average power is flowed from the converter to themotor.

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 two dynamic courses are zoomed and shownseparately in Figs. 19 and 20.

The dc bus voltage and the charging current when connectingwith the dc source are shown in Fig. 19. 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. 18, there are two half-bridgesworking with Sx = 1 and the third working in switching state.

Fig. 19. DC bus voltage and current when connecting with the DC source.

Fig. 20. DC bus voltage and charging current when a charging current refer-ence was given.

When the charging current reference is given, the dc busvoltage will reduce first to establish the charging current asshown in Fig. 20. 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 noncharging state.

During the battery charging, if the voltage of the dc source ischanged, the charging current control result is shown in Fig. 21.It can be seen even there are changes on dc source voltages,the charging current can be controlled well by the close-loopcontrol of the dc current. As feed-forward compensation by thedc source voltage is not used in our system, there are someripples in the dc current during dynamic course.

From Figs. 19 and 21, it is clear that the proposed topology canbe charged under external input dc voltages and the feasibilityof the proposed charging method for practical EV application isproved.

When the initial values of the terminal voltages are differ-ent, the voltage can be balanced during discharging with the


Fig. 21. DC bus voltage and charging current during dc source voltage change.

Fig. 22. Terminal voltage balance result during discharging.

proposed PWM. The curve of the three cells terminal voltagesis shown in Fig. 22. The cell voltages are measured in every10 min. We can find that the three terminal voltages will fi-nally become equal voltage levels under the proposed voltagebalance algorithm. It must be mentioned that the balanced ter-minal voltages are the operating voltages of the battery cells.The open-circuit voltage 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

The hybrid-cascaded multilevel converter proposed in thispaper can actualize the charging and discharging of the batterycells while the terminal voltage or SOC balance control can berealized at the same time. The proposed converter with mod-ular structure can reach any number of cascaded levels and issuitable for the energy storage system control with low-voltagebattery cells or battery modules. The fault module can be by-passed without affecting the running of the other ones, so theconverter has a good fault-tolerant character which can signif-icantly improve the system reliability. The PWM method withlow switching loss for both discharging and charging control is

proposed considering the balance control at the same time. Theoutput of the circuit is multilevel ac voltages where the numberof levels is proportional to the number of battery cells. So theoutput ac voltage is nearly the ideal sinusoidal wave which canimprove the control performance of the motor control in EVs. Adc bus current control method for battery charging with externaldc or ac source is also studied where the constant-current con-trol can be realized and the additional charger is not needed anymore. Experiments are implemented and the proposed circuitand control method are verified.

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Zedong Zheng (M’09) was born in Shandong, China,in 1980. He received the B.S. and Ph.D. degreesin electrical engineering from the Department ofElectrical Engineering, Tsinghua University, Beijing,China, in 2003 and 2008, respectively.

He is currently a Faculty Member in the Depart-ment of Electrical Engineering, Tsinghua University.His research interests include power electronics con-verters and high-performance motor control systems.

Kui Wang (M’11) was born in Hubei, China, in 1984.He received the B.S. and Ph.D. degrees in electricalengineering from the Department of Electrical Engi-neering, Tsinghua University, Beijing, China, in 2006and 2011, respectively.

He is currently a Postdoctoral Researcher withTsinghua University. His research interests includemultilevel converters, control of power converters,and adjustable-speed drives.

Lie Xu (M’11) was born in Beijing, China, in 1980.He received the B.S. degree in electrical and elec-tronic engineering from the Beijing University ofAeronautics and Astronautics, Beijing, China, in2003, and the M.S. and Ph.D. degrees from The Uni-versity of Nottingham, Nottingham, U.K., in 2004and 2008, respectively.

From 2008 to 2010, he was a Research Fellow withthe Department of Electrical and Electronic Engineer-ing, The University of Nottingham. He is currentlya Faculty Member with the Department of Electrical

Engineering, Tsinghua University, Beijing, China. His research interests includemultilevel techniques, multilevel converters, direct ac–ac power conversion, andmultilevel matrix converter.

Yongdong Li (M’08) was born in Hebei, China, in1962. He received the B.S. degree from Harbin In-stitute of Technology, China, in 1982, and the M.S.and Ph.D. degrees from the Department of Electri-cal Engineering, Institut National Polytechnique deToulouse, Toulouse, France, in 1984 and 1987, re-spectively.

Since 1996, he has been a Professor in the Depart-ment of Electrical Engineering, Tsinghua University,Beijing, China. He was also an Invited Professor withthe Institut National Polytechnique de Toulouse and

the Dean of the School of Electrical Engineering, Xinjiang University, Urumqi,China. His research interests include power electronics, machine control, andwind power generation.

a hybrid cascaded multilevel converter for battery 2

Engineering

proposed converter

largenumber of battery

different cells

halfbridge converter

battery energymanagement

cascadedbattery cells

battery stackwill

hbridge converter