1. IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 7, JULY 2014 3537
A Hybrid Cascaded Multilevel Converter for Battery
Energy 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 large
number of battery cells are usually connected in series to enhance
the output voltage for motor driving. The difference in electro-
chemical characters will cause state-of-charge (SOC) and terminal
voltage imbalance between different cells. In this paper, a hybrid
cascaded multilevel converter which involves both battery energy
management and motor drives is proposed for EV. In the proposed
topology, each battery cell can be controlled to be connected into
the 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 bus
voltages to make up ac voltages. The outputs of the converter are
multilevel voltages with less harmonics and lower dv/dt, which is
helpful to improve the performance of the motor drives. By sepa-
rate control according to the SOC of each cell, the energy utilization
ratio of the batteries can be improved. The imbalance of terminal
voltage and SOC can also be avoided, fault-tolerant can be easily
realized by modular cascaded circuit, so the life of the battery stack
will be extended. Simulation and experiments are implemented to
verify the performance of the proposed converter.
Index Terms—Battery cell, charging and discharging, elec-
tric vehicle (EV), hybrid cascaded multilevel converter, voltage
balance.
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 appropriate
energy density and cost. Since the voltages of these kinds of bat-
tery cells are relatively low, a large number of battery cells need
to be connected in series to meet the voltage requirement of the
motor drive [1], [2]. Because of the manufacturing variability,
cell architecture and degradation with use, the characters such as
volume and resistance will be different between these cascaded
battery 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 China
under 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 of
Power System, Department of Electrical Engineering, Tsinghua University, Bei-
jing 100084, China (e-mail: zzd@tsinghua.edu.cn; wangkui@tsinghua.edu.cn;
xulie@tsinghua.edu.cn)
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: liyd@tsinghua.edu.cn).
Color versions of one or more of the figures in this paper are available online
at 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 characteristic
differences between the battery cells. The charge and discharge
have to be stopped even though only one of the cells reaches its
cut-off voltage. Moreover, when any cell is fatally damaged, the
whole battery stack cannot be used anymore. So the battery cell
screening must be processed to reduce these differences, and
voltage or SOC equalization circuit is often needed in practical
applications to protect the battery cells from overcharging or
overdischarging [3], [4].
Generally, there are two kinds of equalization circuits. The
first one consumes the redundant energy on parallel resistance
to 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 their
parallel-connected resistances. So the energy utilization ratio is
very low. Another kind of equalization circuit is composed of a
group of inductances or transformers and converters, which can
realize energy transfer between battery cells. The energy in the
cells with higher terminal voltage or SOC can be transferred to
others to realize the voltage and SOC equalization. Since the
voltage balance is realized by energy exchange between cells,
the energy utilization ratio is improved. The disadvantage is that
a lot of inductances or isolated multiwinding transformers are
required in these topologies, and the control of the converters
is also complex [5]–[13]. Some studies have been implemented
to simplify the circuit and improve the balance speed by multi-
stage equalization [9]–[13]. Some zero voltage and zero current
switching techniques are also used to reduce the loss of the
equalization circuit [13].
Multilevel converters are widely used in medium or high volt-
age motor drives [14]–[19]. If their flying capacitors or isolated
dc sources are replaced by the battery cells, the battery cells
can be cascaded in series combining with the converters instead
of connection in series directly. In [20]–[24], the cascaded H-
bridge converters are used for the voltage balance of the battery
cells. Each H-bridge cell is used to control one battery cell; then
the voltage balance can be realized by the separate control of
charging and discharging. The output voltage of the converter
is multilevel which is suitable for the motor drives. When used
for power grid, the filter inductance can be greatly reduced. The
cascaded topology has better fault-tolerant ability by its modular
design, and has no limitation on the number of cascaded cells,
so it is very suitable to produce a higher voltage output using
these low-voltage battery cells, especially for the application in
power grid. Similar to the voltage balance method in traditional
multilevel converters, especially to the STATCOM using flying
capacitors, the voltage balance control of the battery cells can
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2. 3538 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 7, JULY 2014
Fig. 1. Traditional power storage system with voltage equalization circuit and
inverter.
also be realized by the adjustment of the modulation ratio of
each H-bridge [25]. Compared to the traditional voltage bal-
ance circuit, the multilevel converters are very suitable for the
balance 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 of
the power density limitation of batteries, some ultracapacitors
are used to improve the power density. Some converters must be
used for the battery and ultracapacitor combination [26], [27].
Multilevel converters with battery cells are also very convenient
for the combination of battery and ultracapacitors.
A hybrid cascaded multilevel converter is proposed in this
paper which can realize the terminal voltage or SOC balance
between the battery cells. The converter can also realize the
charge and discharge control of the battery cells. A desired ac
voltage can be output at the H-bridge sides to drive the electric
motor, or to connect to the power grid. So additional battery
chargers or motor drive inverters are not necessary any more
under this situation. The ac output of the converter is multilevel
voltage, while the number of voltage levels is proportional to the
number of cascaded battery cells. So in the applications of EV
or power grid with a larger number of battery cells, the output ac
voltage is approximately ideal sine waves. The harmonics and
dv/dt can be greatly reduced than the traditional two-level con-
verters. The proposed converter with modular design can realize
the fault redundancy and high reliability easily. Simulation and
experimental results are proposed to verify the performance of
the 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 transfer
is shown in Fig. 1 [5], [28]. There is a half-bridge arm and
an inductance between every two nearby battery cells. So the
number of switching devices in the balance circuit is 2∗n–2 and
the number of inductance is n–1 where n is the number of the
battery cells. In this circuit, an additional inverter is needed for
the motor drive and a charger is usually needed for the battery
recharge [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, the
battery recharge can also be realized by an additional control
block which is similar with the PWM rectifier. The recharging
current and voltage can be adjusted by the closed-loop voltage
or power control of the rectifier.
The hybrid-cascaded multilevel converter proposed in this
paper is shown in Fig. 2, which includes two parts, the cascaded
half-bridges with battery cells shown on the left and the H-
bridge inverters shown on the right. The output of the cascaded
half-bridges is the dc bus which is also connected to the dc
input of the H-bridge. Each half-bridge can make the battery
cell to be involved into the voltage producing or to be bypassed.
Therefore, by control of the cascaded half-bridges, the number
of battery cells connected in the circuit will be changed, that
leads to a variable voltage to be produced at the dc bus. The
H-bridge is just used to alternate the direction of the dc voltage
to produce ac waveforms. Hence, the switching frequency of
devices in the H-bridge equals to the base frequency of the
desired ac voltage.
There are two kinds of power electronics devices in the pro-
posed circuit. One is the low voltage devices used in the cascaded
half-bridges which work in higher switching frequency to re-
duce 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 the
number of battery cells in each phase is n, then the devices
used in one phase cascaded half-bridges is 2∗n. Compared to
the traditional equalization circuit shown in Fig. 1, the number
of devices is not increased significantly but the inductances
are eliminated to enhanced the system power density and EMI
issues.
Since all the half-bridges can be controlled individually, a
staircase shape half-sinusoidal-wave voltage can be produced
on the dc bus and then a multilevel ac voltage can be formed at
the output side of the H-bridge, the number of ac voltage levels
is 2∗n–1 where n is the number of cascaded half-bridges in each

3. 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, the
more voltage levels at the output side, and the output voltage is
closer to the ideal sinusoidal. The dv/dt and the harmonics are
very little. So it is a suitable topology for the energy storage
system in electric vehicles and power grid.
III. CONTROL METHOD OF THE CONVERTER
For the cascade half-bridge converter, define the switching
state 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 the
relative half-bridge converter shown in Fig. 4, when Sx = 1, the
battery is connected in the circuit and is discharged or charged
which is determined by the direction of the external current.
When Sx = 0, the battery cell is bypassed from the circuit, the
battery is neither discharged nor charged. When 0 < mx < 1,
the half-bridge works in a switching state. The instantaneous
discharging power from this cell is
P = Sx · ux · i. (1)
Here ux is the battery cell voltage and i is the charging current
on the dc bus.
In the proposed converter, the H-bridge is just used to alternate
the direction of the dc bus voltage, so the reference voltage of
the dc bus is the absolute value of the ac reference voltage, just
like a half-sinusoidal-wave at a steady state. It means that not
all the battery cells are needed to supply the load at the same
time. As the output current is the same for all cells connected
in 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. The
cell with higher voltage or SOC can be discharged more or to
be charged less in using, then the energy utilization ratio can
be improved while the overcharge and overdischarged can be
avoided.
For the cascaded multilevel converters, generally there are
two kinds modulation method: phase-shift PWM and carrier-
cascaded PWM. As the terminal voltage or SOC balance control
must be realized by the PWM, so the carrier-cascaded PWM is
suitable as the modulation ratio difference between different
cells can be used for the balance control [30]–[32].
In the carrier-cascaded PWM, only one half-bridge converter
in each phase is allowed to work in switching state, the others
keep their state unchangeable with Sx = 1 or Sx = 0, so the
switching loss can be reduced. When the converter is used to
feed a load, or supply power to the power grid, the battery
with higher terminal voltage or SOC is preferentially used to
form the dc bus voltage with Sx = 1. The battery with lower
terminal voltage or SOC will be controlled in switch state with
0 < mx < 1 or be bypassed with Sx = 0.
The control of the converter and voltage equalization can
be realized by a modified carrier-cascaded PWM method as
shown in Fig. 4. The position of the battery cells in the carrier
wave is determined by their terminal voltages. In the discharging
process, the battery cells with higher voltage are placed at the
bottom layer of the carrier wave while the cells with lower
voltage at the top layer. Then, the cells at the top layer will be
used 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 the
carrier arranged by SOC can realize the SOC balance. Since the
SOC is difficult to be estimated in the batteries in practice, the
terminal voltage balance is usually used. Normally, the cut-off
voltage during charge and discharge will not change in spite of
the variation of manufacturing variability, cell architecture, and
degradation with use. So the overcharge and overdischarge can
also be eliminated even the terminal voltages are used instead
of the SOC for the carrier-wave arrangement.
To reduce the dv/dt and EMI, only one half-bridge is allowed
to change its switching state at the same time for the continuous
reference voltage. Therefore, the carrier wave is only rearranged
when the modulation wave is zero and the rearranged carrier
only becomes effective when the carrier wave is zero. So the
carrier wave is only rearranged at most twice during one refer-
ence ac voltage cycle as shown in Fig. 5. The battery’s terminal
voltage and SOC change very slowly during the normal use, so
the carrier wave updated by base frequency is enough for the
voltage and SOC balance.
If the number of the cascaded cells is large enough, all the
half-bridges can just work in switch-on or switch-off state to
form the staircase shape voltage. So the switching frequency
of all the half-bridges can only be base frequency as shown
in Fig. 6, where the output ac voltage is still very approach to
the ideal sinusoidal wave which is similar with the multilevel
converter in [32].

4. 3540 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 7, JULY 2014
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 of
the phase with bypassed cell will be reduced. For symmetry, the
three-phase reference voltage must be reduced to fit the output
voltage ability. To improve the output voltage, the neutral shift
three-phase PWM can be adopted. The bypass method and the
neutral shift PWM is very similar with the method explained
in [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 used
for the discharging control and the energy transfer circuit is used
for the voltage balance. In the proposed hybrid-cascaded circuit,
the cascaded half-bridges are used for voltage balance control
and also the discharging control associated with the H-bridge
converters. The switching loss and the conduction losses in these
two circuits are quite different. To do a clear comparison, the
switching and conduction loss is analyzed in this section.
In the hybrid-cascaded converter, the energy loss is composed
of several parts
JLoss = Js B + Js H +Jc B + Jc H . (2)
Here, Js B and Js H are the switching losses of the cascaded
half-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 is
composed by
JLoss = Js I +Js T + Jc I + Jc T (3)
where Js I and Js T are the switching losses of the three-phase
inverter and the energy transfer circuit for voltage balance. Jc I
and Jc T are the conduction losses. In the traditional circuit, the
energy transfer circuit only works when there is some imbalance
and only the parts between the unbalance cells need to work.
So the switching and conduction losses will be very small if the
battery cells are symmetrical.
First, the switching loss is analyzed and compared under the
requirement of same switching times in the output ac voltage.
That means the equivalent switching frequency of the cascaded
half-bridges in hybrid-cascaded converter is the same as the
traditional inverter. The switching loss is determined by the
voltage and current stress on the semiconductor devices, and
also the switching time
Js=
Ts w it ch
0
u · idt. (4)
In the proposed hybrid-cascaded converter, the H-bridge con-
verter is only used to alternate the direction of the output voltage
to produce the desired ac voltage as shown in Fig. 7, the devices
in the H-bridge converter always switch when the dc bus voltage
is 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 the
same as the traditional converter, but only one half-bridge is
active at the any instantaneous in each phase. The voltage step
of each half-bridge is only the battery cell voltage which is much
lower than the whole dc bus voltage in Fig. 1. So in a single
switching course, the switching loss is only approximate 1/n of
the one in traditional converter if the same device is adopted in
both converters. Furthermore, if the lower conduction voltage
drop and faster turn-off device such as MOSFET is used in the
proposed converter, the switching loss of the half-bridge will be
much smaller
Js B < Js I /n. (6)
In the traditional circuit, the voltage balance circuit will still
cause some switching loss determined by the voltage imbalance.
So in the proposed new topology, the switching loss is much
smaller compared to the traditional two-level inverter.

5. ZHENG et al.: HYBRID CASCADED MULTILEVEL CONVERTER FOR BATTERY ENERGY MANAGEMENT APPLIED IN ELECTRIC VEHICLES 3541
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 the
switching devices and the current value. Whatever the switching
state, one switch device in each half-bridge and two devices in
H-bridge are connected in the circuit of each phase, so the
conduction loss power can be calculated by
Pc B = I2
Rc B · n (7)
Pc H = I2
Rc H · 2. (8)
Here, I is the rms value of the output current, Rc B is the
on-resistance of the MOSFET in the half-bridge, Rc H is the
device on-resistance used in the H-bridge, and n is the number
of the cascaded cells.
In the traditional three-phase inverter, only one device is con-
nected in the circuit of each phase, the conduction loss power
in each phase is just
Pc I = I2
Rc I (9)
where Rc I is the on-resistance of the devices used in the in-
verter. Normally, the same semiconductor devices can be used
in the H-bridges and the traditional three-phase inverters, so
the on-resistance of the inverters is almost the same as the H-
bridges.
The on-loss of the H-bridges cannot be reduced, while the
on-loss on cascaded half-bridges can be reduced furthermore
by reducing the number of the cascaded cells. In practical appli-
cations, the battery module of 12 and 24 V can be used for the
cascaded cells instead of the basic battery cell with only 2–3 V.
Also the semiconductor devices with low on-resistance are used
in the half-bridges.
From the above analysis, the switching loss of the proposed
converter is much less than the traditional converter, although
the on-loss is larger than the traditional converter. The compared
results are shown in Table I.
V. CHARGING METHOD
In the system using the proposed circuit in this paper, a dc
voltage source is needed for the battery charging. The charging
current and voltage can be controlled by the proposed converter
itself according to the necessity of the battery cells. The charging
circuit 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 to
realize the current control. The dc voltage can also be realized
by the H-bridge and a capacitor as shown in Fig. 9. The H-bridge
worked as a rectifier by the diodes and a steady dc voltage is
produced with the help of the capacitors.
In the charging course of the battery, the charging current
should be controlled. The current state equation is as follows:
Rf i + Lf
di
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 between
the cascaded half-bridges and the dc source. By this charging
method, the voltage of the dc source must be smaller than the
possible maximum value of the dc bus
ucharge ≤ udc ≤ n · u0. (11)

6. 3542 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 7, JULY 2014
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 phase
and u0 is the discharging cut-off voltage of the battery cell.
During the charging cycle, the voltages of the battery cells and
the dc voltage source might be variable, so the switching states of
the cascaded half-bridges will be switched to make the charging
current constant. The charging current control scheme is shown
in Fig. 10. A PI regulator is used to make the current constant
by changing the output dc voltages of the cascaded cells. The
voltage of the dc source is used as a feed-forward compensation
at the output of the PI controller. In practical applications, the
value of the dc source’s voltage is almost constant, so the feed-
forward compensation can also be removed and the variation of
the dc source voltage can be compensated by the feedback of
the PI controller.
In charging state, the arrangement of the carrier waves will
be opposite with discharge state. The battery cell with lower
voltage will be placed in the bottom then these cells will absorb
more energy from the dc source. The cells with higher voltages
will be arranged at the top levels to make these cells absorb
less energy. By the similar analysis as the discharging state, the
positions 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, so
the modulation will also be changed to the charging mode as
shown 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 the
charging current controlled by the scheme shown in Fig. 9.
When the reference dc bus voltage is variable, such as the
half-sinusoidal-wave, all the battery cells will be charged in-
termittently because of the PWM as shown in Fig. 12. That
is 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 proposed
converter can be larger than the ordinary charging method. When
the battery stack is charged by a steady dc source, the reference
dc bus voltage is nearly constant. To realize the fast-charge of
the battery cell, the carrier-wave must be forced to exchange the
position which is not arranged by the SOC or terminal voltages
any more. Then, the switching state of the half-bridges will be
changed to produce intermittently charging current.
VI. EXPERIMENTAL RESULTS
To verify the performance of the proposed topology and
the control method, a three-phase four-cell cascaded circuit is
erected at lab. The platform is shown in Fig. 13. The lead-acid
battery modules of 5 Ah and 12 V are used. The battery mod-
ules are monitored by the LEM Sentinel which can measure
the voltage, temperature, and resistance of the battery modules.
The measured information is transferred to the DSP controller by
RS-232 communication. The voltages and currents are measured
and recorded by YOKOGAWA ScopeCorder DL750. Since the
SOC of the battery cells are difficult to be estimated, the termi-
nal voltages are used for the PWM carrier-wave arrangement;

7. ZHENG et al.: HYBRID CASCADED MULTILEVEL CONVERTER FOR BATTERY ENERGY MANAGEMENT APPLIED IN ELECTRIC VEHICLES 3543
Fig. 14. Triphase output voltage of multilevel.
then the terminal voltage balance is realized instead of the SOC
balance which can still verify the effects of the converter.
The cascaded half-bridges are designed with MOSFET
IPP045N10N3 (Infineon) whose on-resistance is only 4.2 mΩ
and the rated current is 100 A. Even if the discharge or charge
current is 50 A, the conduction loss on the MOSFET of each
half-bridge is only
P = I2
R = 10.5W.
Then, the efficiency drop caused by the conduction loss of the
MOSFET in the whole phase is
Δη =
I2
R · n
U · I · n
= 1.75%.
So the conduction loss added by the equalization circuit is very
small compared to the output power of the whole converter.
Another device of MOSFET IPB22N03S4L-15 (Infineon) of
30 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% when
the output current is 11 A.
The three-phase output voltage is shown in Fig. 14. There
are nine levels at each phase. So it approaches more to the
ideal sinusoidal wave than the traditional two-level inverters. In
a steady state, the FFT analysis result of the output ac phase
voltage is shown in Fig. 15. It shows that the harmonics is little
compared 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, and
dc bus current are shown in Figs. 16 and 17, which indicate the
whole process from start-up to the stable state of the relative IM.
From Fig. 16, it is obvious that the output voltage levels keep
increasing according to the acceleration of the motor speed. The
stator current of the motor shown in Fig. 17 are of improved
sinusoidal shape which can reflect the control performance of
the motor. The dc bus current is also shown in Fig. 17. As the lag
between the voltage and current phase, when the phase voltage
change direction, the H-bridge will change its switching state
too. But the phase ac current will change its direction after a
Fig. 15. FFT analysis of the output ac voltage.
TABLE II
PARAMETERS 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 reversed
when the directions of phase voltage and current are different.
The reversed dc bus current also reflects the reactive power of
the load. As the induction motor is of nearly no load, the phase
current is nearly π/2 lag with the phase voltage and the average
dc current is almost zero in steady state. But in starting course,

8. 3544 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 7, JULY 2014
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 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. 18. 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 used
with a steady dc source, the cascaded half-bridges are connected
with the dc source first, then a charging current reference is
given, the output dc bus voltage and the dc current of the whole
course are recorded as a single figure by the YOKOGAWA
ScopeCorder. The two dynamic courses are zoomed and shown
separately in Figs. 19 and 20.
The dc bus voltage and the charging current when connecting
with the dc source are shown in Fig. 19. It shows that when the
dc source was connected, the cascaded half-bridges changed
their switching state quickly to keep the charging current same
as the reference value. In Fig. 18, there are two half-bridges
working 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 bus
voltage will reduce first to establish the charging current as
shown in Fig. 20. As the resistance in the filter inductance is
very little, the voltage drop on the filtered inductance by dc
current is also near zero, the dc bus voltage will become almost
the same as the noncharging state.
During the battery charging, if the voltage of the dc source is
changed, 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-loop
control of the dc current. As feed-forward compensation by the
dc source voltage is not used in our system, there are some
ripples in the dc current during dynamic course.
From Figs. 19 and 21, it is clear that the proposed topology can
be charged under external input dc voltages and the feasibility
of the proposed charging method for practical EV application is
proved.
When the initial values of the terminal voltages are differ-
ent, the voltage can be balanced during discharging with the

9. ZHENG et al.: HYBRID CASCADED MULTILEVEL CONVERTER FOR BATTERY ENERGY MANAGEMENT APPLIED IN ELECTRIC VEHICLES 3545
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 voltages
is shown in Fig. 22. The cell voltages are measured in every
10 min. We can find that the three terminal voltages will fi-
nally become equal voltage levels under the proposed voltage
balance 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 be
balanced in real time. Due to the cell difference, the balanced
terminal voltage will become different when the battery cells
are out of use for a period of time.
VII. CONCLUSION
The hybrid-cascaded multilevel converter proposed in this
paper can actualize the charging and discharging of the battery
cells while the terminal voltage or SOC balance control can be
realized at the same time. The proposed converter with mod-
ular 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 fault module can be by-
passed without affecting the running of the other ones, so the
converter has a good fault-tolerant character which can signif-
icantly improve the system reliability. The PWM method with
low switching loss for both discharging and charging control is
proposed considering the balance control at the same time. 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
dc bus current control method for battery charging with external
dc or ac source is also studied where the constant-current con-
trol can be realized and the additional charger is not needed any
more. Experiments are implemented and the proposed circuit
and 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. degrees
in electrical engineering from the Department of
Electrical 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 electrical
engineering from the Department of Electrical Engi-
neering, Tsinghua University, Beijing, China, in 2006
and 2011, respectively.
He is currently a Postdoctoral Researcher with
Tsinghua University. His research interests include
multilevel 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 of
Aeronautics and Astronautics, Beijing, China, in
2003, and the M.S. and Ph.D. degrees from The Uni-
versity of Nottingham, Nottingham, U.K., in 2004
and 2008, respectively.
From 2008 to 2010, he was a Research Fellow with
the Department of Electrical and Electronic Engineer-
ing, The University of Nottingham. He is currently
a Faculty Member with the Department of Electrical
Engineering, Tsinghua University, Beijing, China. His research interests include
multilevel techniques, multilevel converters, direct ac–ac power conversion, and
multilevel matrix converter.
Yongdong Li (M’08) was born in Hebei, China, in
1962. 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 de
Toulouse, 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 with
the 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, and
wind power generation.