Super Capacitor Energy Storage Based MMC for Energy Harvesting in Mine Hoist Application

Abstract

This paper proposes a super capacitor energy storage-based modular multilevel converter (SCES-MMC) for mine hoist application. Different from the conventional MMCs, the sub-modules employ distributed super capacitor banks, which are designed to absorb the regenerative energy of mine hoist and released in the traction condition, so as to improve energy utilization efficiency. The key control technologies are introduced in detail, followed by analysis of the configuration and operation principles. The feasibility of the proposed SCES-MMC topology and the control theory are also verified. Simulation results show that SCES-MMC can adapt to the variable frequency speed regulation of the motor drive, which shows good application prospects in the future for medium- and high-voltage mine hoist systems.

Full text

energies
Article
Super Capacitor Energy Storage Based MMC for
Energy Harvesting in Mine Hoist Application
Xiaofeng Yang 1,*ID , Piao Wen 1, Yao Xue 1, Trillion Q. Zheng 1and Youyun Wang 2
1School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044, China;
17121507@bjtu.edu.cn (P.W.); 16117392@bjtu.edu.cn (Y.X.); tqzheng@bjtu.edu.cn (T.Q.Z.)
2Tianshui Electric Drive Research Institute Co. LTD, Tianshui 741020, China; wyy@vip.163.com
*Correspondence: xfyang@bjtu.edu.cn; Tel.: +86-10-5168-7064
Received: 23 August 2017; Accepted: 13 September 2017; Published: 17 September 2017
Abstract:
This paper proposes a super capacitor energy storage-based modular multilevel converter
(SCES-MMC) for mine hoist application. Different from the conventional MMCs, the sub-modules
employ distributed super capacitor banks, which are designed to absorb the regenerative energy
of mine hoist and released in the traction condition, so as to improve energy utilization efficiency.
The key control technologies are introduced in detail, followed by analysis of the configuration and
operation principles. The feasibility of the proposed SCES-MMC topology and the control theory are
also verified. Simulation results show that SCES-MMC can adapt to the variable frequency speed
regulation of the motor drive, which shows good application prospects in the future for medium-
and high-voltage mine hoist systems.
Keywords:
super capacitor energy storage (SCES); modular multilevel converter (MMC); mine hoist;
state of charge; regenerative energy; energy harvesting
1. Introduction
The mine hoist conveyor is typically one of the largest consumers of electric power, besides of the
excavator systems, of all equipment in the mining industry. As the only inoue-to-downhole access,
the output performance of mine hoist electric transmission systems may not only play key roles in
economical mine production, but may also affect equipment and personal safety [
1
,
2
]. In view of the
large power rating requirements for mine hoist electric transmission systems, medium-voltage source
converters (VSC) have increasingly gained importance due to their high power density, excellent
efficiency, and high reliability [
3
–
5
]. For the last two decades, three-level neutral-point-clamped (NPC)
VSCs have been the standard solution in the medium-voltage range for industrial applications [
6
,
7
].
Since the regenerative energy in an electric mine hoister and excavator may be as high as 60 percent of
the motoring power [
8
,
9
], active front-end rectifiers (AFE) are usually adopted to feed the regenerative
energy back into the power distribution grid in order for it not to be wasted. However, the practical
constraints in the controller bandwidth may place restrictions on the system’s regenerative power
handling. Therefore, protective circuits, such as DC choppers and crowbars, are usually added to the
system for suppressing the DC bus over-voltage during regeneration [10].
Compared with NPC-VSC, modular multilevel converter (MMC) provides advantages such
as high modularity, lower switching efficiency, high efficiency, better output voltage performance,
etc. Thus, in addition to its successful commercial implementation in high voltage direct current
(HVDC) transmission projects [
11
–
13
], it also shows good application prospects in fields such as
power quality control [
14
,
15
]. However, AC drive-based MMCs are only suitable for industrial fans,
pumps and other applications of small speed range close to the rated value. In order to solve the
low-frequency torque ripple issues of conventional MMC motor drive systems, many scholars have
Energies 2017,10, 1428; doi:10.3390/en10091428 www.mdpi.com/journal/energies

Energies 2017,10, 1428 2 of 11
carried out extensive research and proposed control strategies for common mode voltage/current
multi-component injection and pulse optimization [
16
–
19
]. However, the above method does not
fundamentally eliminate the limitations of MMC variable frequency-speed regulation control. On the
other hand, domestic mine accidents have been frequent in recent years. If the high-voltage converter
could continue to operate for a short time under outage conditions, the reliability of the mine hoist
equipment would be significantly improved. In addition, when the mine hoist is braking, the electric
transmission system will be in the generation condition, and the regenerating energy will charge the
DC bus capacitor. An energy storage-enabled MMC seems an ideal solution for the abovementioned
issues, since the super capacitors show better power density and a longer service life compared to
traditional batteries. Thus, the super capacitor energy storage based MMC (SCES-MMC) is more
suitable for mine hoist application. However, published technical papers focus mainly on battery
energy storage system (BESS)-based MMCs [
20
,
21
]. In view of the above issues, this paper adopts the
SCES-MMC for energy-harvesting applications. The super capacitor banks within the SCES-MMC
operates as a power source to ensure the mine hoist keeps working for a short time when power
failures occur unexpectedly. As a result, the safety of the mine hoist equipment can be significantly
improved to avoid mine accidents. In addition, when the mine hoist is braking, the regenerative energy
is quickly transferred to the SCES to avoid DC bus overvoltage.
This paper is organized as follows. Section 2presents the structural characteristics and operation
principles of SCES-MMC topology. In Section 3, the system control strategies including motor frequency
controls and state of charge (SOC) balancing controls are explored in detail. To validate the feasibilities
and effectiveness of the proposed topology and theory, extensive simulation results are demonstrated
in Section 4. Finally, Section 5reports the main conclusions.
2. Topology and Operation Principles
2.1. Topology Analysis
A typical topology for three-phase SCES-MMC is shown in Figure 1, where L
s
is the output
equivalent AC inductance, ois the neutral point of reference potential between the positive P and
negative N of the DC bus. The SCES-MMC is three-phase balanced, and its phase-leg consists of 2n
sub-modules SM
jk
(phase j=a, b, c, SMs number k= 1, 2,
. . .
, 2n) and leg inductance L
a
. Figure 1b
shows the basic building blocks-SCES-SM, which consists of two IGBTs (T
1
, T
2
), two anti-parallel
diodes (D1, D2), a capacitor CSM, a super capacitor bank interface circuit.
Energies 2017, 10, 1428 2 of 11
component injection and pulse optimization [16–19]. However, the above method does not
fundamentally eliminate the limitations of MMC variable frequency-speed regulation control. On the
other hand, domestic mine accidents have been frequent in recent years. If the high-voltage converter
could continue to operate for a short time under outage conditions, the reliability of the mine hoist
equipment would be significantly improved. In addition, when the mine hoist is braking, the electric
transmission system will be in the generation condition, and the regenerating energy will charge the
DC bus capacitor. An energy storage-enabled MMC seems an ideal solution for the abovementioned
issues, since the super capacitors show better power density and a longer service life compared to
traditional batteries. Thus, the super capacitor energy storage based MMC (SCES-MMC) is more
suitable for mine hoist application. However, published technical papers focus mainly on battery
energy storage system (BESS)-based MMCs [20,21]. In view of the above issues, this paper adopts the
SCES-MMC for energy-harvesting applications. The super capacitor banks within the SCES-MMC
operates as a power source to ensure the mine hoist keeps working for a short time when power
failures occur unexpectedly. As a result, the safety of the mine hoist equipment can be significantly
improved to avoid mine accidents. In addition, when the mine hoist is braking, the regenerative
energy is quickly transferred to the SCES to avoid DC bus overvoltage.
This paper is organized as follows. Section 2 presents the structural characteristics and operation
principles of SCES-MMC topology. In Section 3, the system control strategies including motor
frequency controls and state of charge (SOC) balancing controls are explored in detail. To validate
the feasibilities and effectiveness of the proposed topology and theory, extensive simulation results
are demonstrated in Section 4. Finally, Section 5 reports the main conclusions.
2. Topology and Operation Principles
2.1. Topology Analysis
A typical topology for three-phase SCES-MMC is shown in Figure 1, where L
s is the output
equivalent AC inductance, o is the neutral point of reference potential between the positive P and
negative N of the DC bus. The SCES-MMC is three-phase balanced, and its phase-leg consists of 2n
sub-modules SMjk (phase j =a, b, c, SMs number k = 1, 2, …, 2n) and leg inductance La. Figure 1b shows
the basic building blocks-SCES-SM, which consists of two IGBTs (T1, T2), two anti-parallel diodes (D1,
D2), a capacitor CSM, a super capacitor bank interface circuit.
abc
L
a
P
N
SM
a1
SM
b1
SM
c1
O
U
DC
2
U
DC
2
SM
a2
SM
an
SM
a(n+1)
SM
a(n+2)
SM
a(2n)
SM
b2
SM
bn
SM
b(n+1)
SM
b(n+2)
SM
b(2n)
SM
c2
SM
cn
SM
c(n+1)
SM
c(n+2)
SM
c(2n)
U
sc
U
sb
U
sa
L
s
L
s
L
s
i
sa
i
sc
i
sb
L
a
i
Pc
i
Nc
i
Pb
i
Nb
i
Pa
i
Na
I
DC
L
a
L
a
L
a
L
a
Phase-leg Upper
/
lower arm
MLoad :
motor
i
SM
Interface
C
SC
X
1
X
2
C
SM
T
1
T
2
D
1
D
2
U
SC
(a) (b)
Figure 1. Configuration of three-phase SCES-MMC and its sub-module: (a) Three-phase SCES-MMC
topology; (b) SCES-SM.
Figure 1.
Configuration of three-phase SCES-MMC and its sub-module: (
a
) Three-phase SCES-MMC
topology; (b) SCES-SM.

Energies 2017,10, 1428 3 of 11
Figure 2demonstrates three widely-applied topologies for SCES-SM interface circuits: direct
connection, bidirectional buck/boost converter and dual active bridge converter. The main difference
between the latter two is the isolating transformer. To simplify the analysis, Figure 2a is selected as
the SCES-SM interface circuit, but this does not affect the correctness of the theory. Additionally,
the distributed super capacitors C
SC
results in an SCES-MMC with a highly modular structure
and redundant capability, which further improves the reliability of the SCES-MMC for mine hoist
applications. What is more, the circulating energy of SCES-MMC is exchanged between phase-legs
through the common DC bus. Therefore, the SCES-SMs’ state of charge (SOC) may also be applied for
system controls.
Energies 2017, 10, 1428 3 of 11
Figure 2 demonstrates three widely-applied topologies for SCES-SM interface circuits: direct
connection, bidirectional buck/boost converter and dual active bridge converter. The main difference
between the latter two is the isolating transformer. To simplify the analysis, Figure 2a is selected as
the SCES-SM interface circuit, but this does not affect the correctness of the theory. Additionally, the
distributed super capacitors CSC results in an SCES-MMC with a highly modular structure and
redundant capability, which further improves the reliability of the SCES-MMC for mine hoist
applications. What is more, the circulating energy of SCES-MMC is exchanged between phase-legs
through the common DC bus. Therefore, the SCES-SMs’ state of charge (SOC) may also be applied
for system controls.
Figure 2. Topology of SCES-SM interface circuit: (a) Direct connection (b) Bidirectional buck/boost
converter (c) Dual active bridge converter.
2.2. Operation Principles of SCES-MMC
The switching states of the SCES-SM are shown in Table 1, and the output voltage of SCES-SM
is switched between zero and USC by controlling T1 and T2, where USC is the DC voltage of the SCES-
SM. At the same time, SOC control of the corresponding super capacitor banks (SCBs) may also be
implemented in this process.
Table 1. Switching states of SCES-SM.
Mode T1
/
D1 T2
/
D2iSM Output Voltage SOC
Charging 0/1 0/0 >0 USC increasing
Discharging 1/0 0/0 <0 USC decreasing
Bypass 0 1 >0 0 maintaining
Bypass 0 1 <0 0 maintaining
According to the reference direction shown in Figure 1, AC current of phase-j during normal
operation is expressed as
PDCsZ
11
32
jjj
iI ii=++
(1)
NDCsZ
11
32
jjj
iI ii=−+
(2)
sPN
j
jj
iii=− (3)
CDCZ
1
3
jj
iIi=+
(4)
U
SC
C
SC
U
SC
C
SC
L
SM
Interfac
e
C
SC
U
SC
U
SC
C
SC
(a)(b)
(c)
Figure 2.
Topology of SCES-SM interface circuit: (
a
) Direct connection (
b
) Bidirectional buck/boost
converter (c) Dual active bridge converter.
2.2. Operation Principles of SCES-MMC
The switching states of the SCES-SM are shown in Table 1, and the output voltage of SCES-SM
is switched between zero and U
SC
by controlling T
1
and T
2
, where U
SC
is the DC voltage of the
SCES-SM. At the same time, SOC control of the corresponding super capacitor banks (SCBs) may also
be implemented in this process.
Table 1. Switching states of SCES-SM.
Mode T1/D1T2/D2iSM Output Voltage SOC
Charging 0/1 0/0 >0 USC increasing
Discharging 1/0 0/0 <0 USC decreasing
Bypass 0 1 >0 0 maintaining
Bypass 0 1 <0 0 maintaining
According to the reference direction shown in Figure 1, AC current of phase-jduring normal
operation is expressed as
iPj=1
3IDC +1
2isj+iZj(1)
iNj=1
3IDC −1
2isj+iZj(2)
isj=iPj−iNj(3)
iCj=1
3IDC +iZj(4)

Energies 2017,10, 1428 4 of 11
where i
Pj
and i
Nj
are the upper and lower arm currents, respectively; I
DC
is the DC bus current. The arm
current flows through both the upper and lower legs consist of half of the AC output current i
sj
and the
common-mode current i
Cj
, which consists of the DC component I
DC
/3 and the circulating component
i
Zj
. The former refers to the active power for charging and discharging the SM capacitors, while the
latter indicates the reactive power causing the SM capacitor voltage ripples. Similarly, the resulting
AC and DC voltages are determined by
uPj=UDC
2−uj−La
diPj
dt−RaiPj(5)
uNj=UDC
2+uj−La
diNj
dt−RaiNj(6)
uj=uNj−uPj
2−La
2
dij
dt−Ra
2ij(7)
UDC =uPj+uNj+2La
diCj
dt+2RaiCj(8)
where u
j
is the AC output phase voltage, U
DC
is the rated DC bus voltage, R
a
is the equivalent series
arm resistor, and uPj and uNj denote the upper and lower arm voltages, respectively.
2.3. Power Flow Analysis
Figure 3illustrates the power flow modes, where P
SCES
refers to the power absorbed/released
by SCES, the load side power P
load
is represented by P
trac
when under traction conditions, or by P
reg
when under regeneration conditions. The fundamental power exchange relation is as follows
PLoad =Pdc +PSCES (9)
Energies 2017, 10, 1428 4 of 11
where iPj and iNj are the upper and lower arm currents, respectively; IDC is the DC bus current. The
arm current flows through both the upper and lower legs consist of half of the AC output current isj
and the common-mode current iCj, which consists of the DC component IDC/3 and the circulating
component iZj. The former refers to the active power for charging and discharging the SM capacitors,
while the latter indicates the reactive power causing the SM capacitor voltage ripples. Similarly, the
resulting AC and DC voltages are determined by
P
DC
PaaP
d
2d
j
j
jj
i
U
uuLRi
t
=−− −
(5)
N
DC
NaaN
d
2d
j
j
jj
i
U
uuLRi
t
=+− − (6)
NP aa
d
22d2
jj j
j
j
uu i
LR
ui
t
−
=−−
(7)
C
DC P N a a C
d
22
d
j
j
jj
i
Uuu L Ri
t
=+ + + (8)
where uj is the AC output phase voltage, UDC is the rated DC bus voltage, Ra is the equivalent series
arm resistor, and uPj and uNj denote the upper and lower arm voltages, respectively.
2.3. Power Flow Analysis
Figure 3 illustrates the power flow modes, where PSCES refers to the power absorbed/released by
SCES, the load side power Pload is represented by Ptrac when under traction conditions, or by Preg when
under regeneration conditions. The fundamental power exchange relation is as follows
Load dc SCES
PPP=+ (9)
Figure 3. Illustration of power flow: (a) Conventional VSC without SCES; (b) Traction (discharging)
condition; (c) Regeneration (charging) condition; (d) Fault operation mode.
With regard to bidirectional power flow characteristics, SCES-SM charging/discharging is
realized through different operation modes. The two basic operation modes in SCES-MMC are
defined as follows.
M
Load
P
dc
>0 P
trac
P
reg
Power
Consum-
ption
DC
Sour ce
VSC
SCES-
MMC
M
Load
P
dc
>0
P
trac
P
SCES
>0
Power
Consum-
ption
DC
Sour ce
SCES
SCES-
MMC
M
Load
P
SCES
>0
Power
Consum-
ption
DC
Sour ce
P
reg
SCES
P
trac
P
reg
P
SCES
>0
P
SCES
<0
P
dc
=0 SCES-
MMC
M
Load
Power
Consum-
ption
DC
Source
SCES
(a)(b)
(c) (d)
Figure 3.
Illustration of power flow: (
a
) Conventional VSC without SCES; (
b
) Traction (discharging)
condition; (c) Regeneration (charging) condition; (d) Fault operation mode.

Energies 2017,10, 1428 5 of 11
With regard to bidirectional power flow characteristics, SCES-SM charging/discharging is realized
through different operation modes. The two basic operation modes in SCES-MMC are defined
as follows.
1. Normal operation mode: |PLoad |>|Pdc|
Mode (a): PLoad =Pdc ,PSCES = 0;
Mode (b): Pdc > 0, PSCES > 0 (discharging), Ptrac > 0 (traction condition);
Mode (c): Pdc < 0, PSCES < 0 (charging), Preg> 0 (regeneration condition);
2. Fault operation mode: Pdc = 0
Mode (d): PLoad =PSCES .
In mode (a), SCES does not participate in the work. As a result, the operation principles of
SCES-MMC are similar to the conventional MMCs. Load side power is fed exclusively from the DC
bus of the SCES-MMC. Notably, when the mine hoist motor is braking, the regenerative energy maybe
consumed on the DC bus due to the uncontrolled rectifier, as shown in Figure 3a.
In mode (b), the load side motor is under traction conditions, and the SCES functions as the DC
source; thus, SCBs discharge to provide part of the AC load power, as shown in Figure 3b.
In mode (c), the load side motor is under regenerative conditions, and the SCES absorbs part of the
regenerative power from the mine hoist motor. However, extra regenerative power is still consumed
on the DC bus.
In contrast to conventional MMCs, when power outage faults occur, SCES will play the role of
sole DC source, and feed the AC side load under similar control strategies as those shown in Figure 3d;
thus, the mine hoist motor system is capable of continuing to work for a short time. This is a major
advantage of SCES-MMC in terms of enhancing system reliability.
3. System Control Strategies
The SCES-MMC operates differently from regular MMCs. Since the SCBs within each SCES-SM
are able to act as the DC source for supplying the AC load, the power will be delivered not only from
the DC bus but also the SCES. Therefore, the SCES-MMC control strategy, when applied to the mine
hoist motor drive system, includes two main parts, namely, motor variable frequency speed regulation
(VFSR) control and SOC control.
3.1. Variable Frequency Speed Control
The typical operating states of the mine hoist include acceleration, uniform speed, and
deceleration; thus, speed regulation is one of the main control targets. Based on the active flux observer
(AFO)-based variable frequency speed regulation control proposed in [
22
,
23
], the SCES-MMC VFSR
control strategy proposed in this paper is shown in Figure 4, where
ω*
and
ω
are the reference mine
hoist rotor speed and the observed rotor speed obtained by the active flux observer (AFO) proposed
in [
22
,
23
], respectively. The reference current i
*q
,i
*d
is calculated according to the mathematical model
of synchronous motors, as proposed in [
22
]. Proportional integral regulator PI refers to the current
loop controller. Consequently, the upper- and lower-arm voltages
u∗
Pj
and
u∗
Nj
are calculated based on
Equations (5) and (6).

Energies 2017,10, 1428 6 of 11
Energies 2017, 10, 1428 5 of 11
1. Normal operation mode: |PLoad| > |Pdc|
Mode (a): PLoad = Pdc, PSCES = 0;
Mode (b): Pdc > 0, PSCES > 0 (discharging), Ptrac > 0 (traction condition);
Mode (c): Pdc < 0, PSCES < 0 (charging), Preg> 0 (regeneration condition);
2. Fault operation mode: Pdc = 0
Mode (d): PLoad = PSCES.
In mode (a), SCES does not participate in the work. As a result, the operation principles of SCES-
MMC are similar to the conventional MMCs. Load side power is fed exclusively from the DC bus of
the SCES-MMC. Notably, when the mine hoist motor is braking, the regenerative energy maybe
consumed on the DC bus due to the uncontrolled rectifier, as shown in Figure 3a.
In mode (b), the load side motor is under traction conditions, and the SCES functions as the DC
source; thus, SCBs discharge to provide part of the AC load power, as shown in Figure 3b.
In mode (c), the load side motor is under regenerative conditions, and the SCES absorbs part of
the regenerative power from the mine hoist motor. However, extra regenerative power is still
consumed on the DC bus.
In contrast to conventional MMCs, when power outage faults occur, SCES will play the role of
sole DC source, and feed the AC side load under similar control strategies as those shown in Figure 3d;
thus, the mine hoist motor system is capable of continuing to work for a short time. This is a major
advantage of SCES-MMC in terms of enhancing system reliability.
3. System Control Strategies
The SCES-MMC operates differently from regular MMCs. Since the SCBs within each SCES-SM
are able to act as the DC source for supplying the AC load, the power will be delivered not only from
the DC bus but also the SCES. Therefore, the SCES-MMC control strategy, when applied to the mine
hoist motor drive system, includes two main parts, namely, motor variable frequency speed
regulation (VFSR) control and SOC control.
3.1. Variable Frequency Speed Control
The typical operating states of the mine hoist include acceleration, uniform speed, and
deceleration; thus, speed regulation is one of the main control targets. Based on the active flux
observer (AFO)-based variable frequency speed regulation control proposed in [22,23], the SCES-
MMC VFSR control strategy proposed in this paper is shown in Figure 4, where ω* and ω are the
reference mine hoist rotor speed and the observed rotor speed obtained by the active flux observer
(AFO) proposed in [22,23], respectively. The reference current i*q, i*d is calculated according to the
mathematical model of synchronous motors, as proposed in [22]. Proportional integral regulator PI
refers to the current loop controller. Consequently, the upper- and lower-arm voltages *
P
j
u and *
Nj
u
are calculated based on Equations (5) and (6).
Figure 4. Block diagram of variable frequency speed control.
i
*q
_
+_
PI
PI
ω
*
∆
ω
ω
u
*j
Setting point
of the speed i*
q
calculation
according [22]
i
sa
i
sb
i
sc
i
d
i
q
abc
 dq
ω
+
u
*q
u
*d
i
*d
ω
u
*Pj
u
*Nj
i*
d
calculation
according [22]
Equation 5
Equation 6
dq

abc
Figure 4. Block diagram of variable frequency speed control.
3.2. SOC Control
SOC control is one of the main differences from conventional MMCs. It is essential to control the
circulating current of SCES-MMC to maximize the efficiency of the SOC controls. According to the
configuration features, The SOC control structure of SCES-MMC generally includes SOC control of
upper/lower arm sub-modules and SOC balancing control of phase-legs, as shown in Figure 5, where
K2to K5refer to closed-loop controllers such as PI controllers.
Energies 2017, 10, 1428 6 of 11
3.2. SOC Control
SOC control is one of the main differences from conventional MMCs. It is essential to control the
circulating current of SCES-MMC to maximize the efficiency of the SOC controls. According to the
configuration features, The SOC control structure of SCES-MMC generally includes SOC control of
upper/lower arm sub-modules and SOC balancing control of phase-legs, as shown in Figure 5, where
K2 to K5 refer to closed-loop controllers such as PI controllers.
Figure 5. Block diagram of SOC control: (a) Individual SOC control (upper arm); (b) Individual SOC
control (lower arm); (c) SOC balancing control.
Figure 5a,b shows the block diagram of individual SOC balancing controls, so as to make the
SCES-SM SCBs SOC on the same leg equal to its corresponding average arm SOC (
P
j
SOC , Nj
SOC ).
sign() denotes the signum function, and the average arm SOC Pj
SOC , Nj
SOC are expressed as
follows:
1
1n
Pj jk
k
SOC SOC
n=
= (10)
2
1
1n
Nj jk
kn
SOC SOC
n=+
= (11)
The arm SOC balancing control aims to eliminate the deviation between the average arm SOCs.
Similarly, the SOC balancing control of the phase-legs aims to eliminate the deviation between the
average SOC (
,jave
SOC ) of each phase and the three-phase average SOC ( ave
SOC ). Therefore,
,avej
SOC and ave
SOC are given by:
2
,
1
1
2
n
jave jk
k
SOC SOC
n=
= (12)
,
1
3
c
ave j ave
ja
SOC SOC
=
= (13)
SOC
ave
SOC
j,ave
K
3
SOC
Pj
SOC
Nj
K
4
+i
*
zj1
i
*
zj2
+
+
-
i
*
zj
+
-
+
-
+
i
Pj
i
Nj
+
i
Zj
K
5
U
*j,cir
0.5
Individu al SOC balancing
control of phase-leg s
SOC balanc ing control of
upper and lower arms Circulating current control
SOC
Nj
SOC
jk
K
2
+
-i
Nj
sign()
U
*Njk,ind
(k=n+1,…,2n)
U
*Pjk,ind
SOC
Pj
SOC
jk
K
2
+
-i
Pj
sign()
(k=1,…,n)
(a)(b)
(c)
Figure 5.
Block diagram of SOC control: (
a
) Individual SOC control (upper arm); (
b
) Individual SOC
control (lower arm); (c) SOC balancing control.
Figure 5a,b shows the block diagram of individual SOC balancing controls, so as to make the
SCES-SM SCBs SOC on the same leg equal to its corresponding average arm SOC (
SOCP j
,
SOCNj
).
sign() denotes the signum function, and the average arm SOC
SOCPj
,
SOCNj
are expressed as follows:
SOCP j =1
n
n
∑
k=1
SOCjk (10)

Energies 2017,10, 1428 7 of 11
SOCNj =1
n
2n
∑
k=n+1
SOCjk (11)
The arm SOC balancing control aims to eliminate the deviation between the average arm SOCs.
Similarly, the SOC balancing control of the phase-legs aims to eliminate the deviation between the
average SOC (
SOCj,ave
) of each phase and the three-phase average SOC (
SOCave
). Therefore,
SOCj,ave
and SOCave are given by:
SOCj,ave =1
2n
2n
∑
k=1
SOCjk (12)
SOCave =1
3
c
∑
j=a
SOCj,ave (13)
The above SOC control is implemented by the control of the circulating current, and the reference
circulating current
i∗
Zj
is calculated from the outputs of the above two SOC balancing control
i∗
Zj1
and
i∗
Zj2, as follows
i∗
Zj =i∗
Zj1+i∗
Zj2(14)
The controller of the circulating current inner loop aims to make the actual circulating current i
zj
equal to the reference current
i∗
Zj
, and then to realize the SOC balancing control, with the reference
voltage U∗
j,cir being the output of the controller.
In summary, system control structures for the SCES-MMC are shown as Figure 6, where the
reference voltage of the phase arm is calculated as follows:
U∗
Pjk =
u∗
Pj
n+U∗
Pjk,ind +U∗
j,cir +U∗
DC
n(15)
U∗
Njk =
u∗
Nj
n+U∗
Njk,ind +U∗
j,cir +U∗
DC
n(16)
Energies 2017, 10, 1428 7 of 11
The above SOC control is implemented by the control of the circulating current, and the
reference circulating current *
Z
j
i is calculated from the outputs of the above two SOC balancing
control *
1
Z
j
i and *
2
Z
j
i, as follows
** *
12
Z
jZj Zj
iii=+ (14)
The controller of the circulating current inner loop aims to make the actual circulating current izj
equal to the reference current *
Z
j
i, and then to realize the SOC balancing control, with the reference
voltage *
,
j
cir
U being the output of the controller.
In summary, system control structures for the SCES-MMC are shown as Figure 6, where the
reference voltage of the phase arm is calculated as follows:
**
***
,,
Pj
D
C
Pjk Pjk ind j cir
uU
UUU
nn
=+ + + (15)
**
***
,,
Nj
D
C
Njk Njk ind j cir
uU
UUU
nn
=+ + + (16)
Figure 6. Overview of system control structures for SCES-MMC.
4. Verifications of SCES-MMC
To verify the feasibility of the proposed SCES-MMC mine hoist system and control strategies, a
simulation model of a nine-level SCES-MMC was created, as shown in Figure 7. The multi-pulse
rectifier and chopper cell are employed to supply the constant DC bus voltage UDC. The simulation
model parameters of SCES-MMC are listed in Table 2, and the initial SOC value of the SCBs is 100%.
The super capacitor parameter design and selection method are similar to [24], and the modulation
method adopted in this simulation is carrier phase-shifted sinusoidal pulse-width-modulation,
combined with the third harmonic injection method of [25]. In addition, assuming that the
synchronous motor reaches the rated speed at 0.6 s and starts decelerating at 0.9 s, the SCES-MMC
output performance is shown in Figure 8.
Figure 8a,b shows that the frequency of the voltage and the current will gradually rise to its rated
value under variable frequency speed regulation control. The SCES-MMC comes to its steady state
at 0.6 s, and then gradually decays when the system enters the deceleration condition at 0.9 s. From
the speed characteristics of the synchronous motor shown in Figure 8c, it can be seen that, before 0.6
s, the SCBs begin to release energy when the motor speed is rising and the system SOC is decreasing,
as shown in Figure 8d. Then, the motor speed starts to drop when the synchronous motor brakes at
0.9 s, and the regenera tive powe r in this process will also help charge the SCB of each SCES-SM, hence
its SOC will rise.
SM SOC
control
Figu re 5a
Phase-leg
SOC
bala ncing
Figu re 5b
Upper/lower leg
SOC
balancing
Figu re 5b
Cir culating
current
suppressor
Figu re 5b
Variable
freq uency
speed
regula tion
Figu re 4 PWM
gener ator
SCES-
MMC
SOC
jk
U
*Pjk,ind
U
*Njk,ind
u
*Pj
u
*Nj
U
*j,cir
U
*Pjk
U
*Njk
i
j*
I
*Zj2
SOC
Pj
SOC
Nj
SOC
Pj
SOC
Nj
SOC
j,ave
SOC
ave
I
*Zj1
ω
*
SOC balancing control VFSR contr ol
SOC
jk
SOC
average
calculation
Eq.(9)-(12)
Reference
voltage
allocation
Equation 15
and
Equation 16
Figure 6. Overview of system control structures for SCES-MMC.
4. Verifications of SCES-MMC
To verify the feasibility of the proposed SCES-MMC mine hoist system and control strategies,
a simulation model of a nine-level SCES-MMC was created, as shown in Figure 7. The multi-pulse
rectifier and chopper cell are employed to supply the constant DC bus voltage U
DC
. The simulation
model parameters of SCES-MMC are listed in Table 2, and the initial SOC value of the SCBs is 100%.
The super capacitor parameter design and selection method are similar to [
24
], and the modulation
method adopted in this simulation is carrier phase-shifted sinusoidal pulse-width-modulation,

Energies 2017,10, 1428 8 of 11
combined with the third harmonic injection method of [
25
]. In addition, assuming that the synchronous
motor reaches the rated speed at 0.6 s and starts decelerating at 0.9 s, the SCES-MMC output
performance is shown in Figure 8.
Figure 8a,b shows that the frequency of the voltage and the current will gradually rise to its rated
value under variable frequency speed regulation control. The SCES-MMC comes to its steady state at
0.6 s, and then gradually decays when the system enters the deceleration condition at 0.9 s. From the
speed characteristics of the synchronous motor shown in Figure 8c, it can be seen that, before 0.6 s,
the SCBs begin to release energy when the motor speed is rising and the system SOC is decreasing, as
shown in Figure 8d. Then, the motor speed starts to drop when the synchronous motor brakes at 0.9 s,
and the regenerative power in this process will also help charge the SCB of each SCES-SM, hence its
SOC will rise.
Table 2. Simulation parameters of SCES-MMC.
System Parameters Value
AC line-to-line voltage 1.45 kV
DC link voltage 3.6 kV
Rated power 5.3 MW
Leg inductance 5.0 mH
SM capacitor voltage 0.45 kV
Super capacitor capacitance 20 F
Rated speed 52 rpm
Energies 2017, 10, 1428 8 of 11
Table 2. Simulation parameters of SCES-MMC.
System Parameters Value
AC line-to-line voltage 1.45 kV
DC link voltage 3.6 kV
Rated power 5.3 MW
Leg inductance 5.0 mH
SM capacitor voltage 0.45 kV
Super capacitor capacitance 20 F
Rated speed 52 rpm
Figure 7. Configuration of SCES-MMC simulation model.
Figure 8. Output performance of SCES-MMC: (a) AC output voltage; (b) AC output current; (c) Motor
speed; (d) Average SOC of SCBs; (e) Voltage of the SCBs—phase a; (f) Voltage of the SCBs—phase b.
Three-phase SC ES-MMC
abc
L
a
P
N
SM
a1
SM
b1
SM
c1
O
U
DC
2
U
DC
2
SM
a2
SM
an
SM
a(n+1)
SM
a(n+2)
SM
a(2n)
SM
b2
SM
bn
SM
b(n+1)
SM
b(n+2)
SM
b(2n)
SM
c2
SM
cn
SM
c(n+1)
SM
c(n+2)
SM
c(2n)
U
sc
U
sb
U
sa
L
s
L
s
L
s
i
sa
i
sc
i
sb
L
a
i
Pc
i
Nc
i
Pb
i
Nb
i
Pa
i
Na
I
DC
L
a
L
a
L
a
L
a
Rectifire 1
Rectifire m
AC
grid
3-phase
transformer
M
multi-pulse
rectifire
Synchronous
motor
Chopper
(a) (b)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
(d)
99.6
99.7
99.8
99.9
100
SOC (%)
-10
0
10
20
30
40
50
60
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
(c)
Speed (rpm)
time (s) time (s)
(a) (b)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
(d)
99.6
99.7
99.8
99.9
100
SOC (%)
-10
0
10
20
30
40
50
60
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
(c)
Speed (rpm)
time (s) time (s)
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
-2.0
-1.5
-1.0
-0.5
0
0.5
1.0
1.5
2.0
AC voltage (kV)
time (s)
u
sa
u
sc
u
sb
-6.0
-4.0
-2.0
0
2.0
4.0
6.0
AC current (kA)
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
time (s)
i
sa
i
sc
i
sb
(e) (f)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0.2
0.3
0.4
0.5
U
cA,SM
(kV)
time (s) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0.2
0.3
0.4
0.5
U
cB,SM
(kV)
time (s)
Figure 7. Configuration of SCES-MMC simulation model.
Due to the high capacitance of the SCBs, even if a large amount of energy is released/absorbed,
there is no significant fluctuation in the sub-module capacitor voltage USM, as shown in Figure 8e,f.
Figure 9illustrates the power distribution laws of the proposed SCES-MMC. From the
above-mentioned analysis, it can be concluded that the SCBs in the SCES-MMC will be discharged to
provide about 69% of the AC load power requirement. Thus, less DC power is required compared
to those without super capacitor banks. Since SCES is involved in absorbing part of the regenerative
power from the mine hoist motor, under regeneration conditions, the power dissipated in the
SCES-MMC is also less than for conventional MMCs. This makes it possible to minimize the capacity
requirements of the AC grid and energy loss of the mine hoist system.

Energies 2017,10, 1428 9 of 11
Energies 2017, 10, 1428 8 of 11
Table 2. Simulation parameters of SCES-MMC.
System Parameters Value
AC line-to-line voltage 1.45 kV
DC link voltage 3.6 kV
Rated power 5.3 MW
Leg inductance 5.0 mH
SM capacitor voltage 0.45 kV
Super capacitor capacitance 20 F
Rated speed 52 rpm
Figure 7. Configuration of SCES-MMC simulation model.
Figure 8. Output performance of SCES-MMC: (a) AC output voltage; (b) AC output current; (c) Motor
speed; (d) Average SOC of SCBs; (e) Voltage of the SCBs—phase a; (f) Voltage of the SCBs—phase b.
Three-phase SC ES-MMC
abc
L
a
P
N
SM
a1
SM
b1
SM
c1
O
U
DC
2
U
DC
2
SM
a2
SM
an
SM
a(n+1)
SM
a(n+2)
SM
a(2n)
SM
b2
SM
bn
SM
b(n+1)
SM
b(n+2)
SM
b(2n)
SM
c2
SM
cn
SM
c(n+1)
SM
c(n+2)
SM
c(2n)
U
sc
U
sb
U
sa
L
s
L
s
L
s
i
sa
i
sc
i
sb
L
a
i
Pc
i
Nc
i
Pb
i
Nb
i
Pa
i
Na
I
DC
L
a
L
a
L
a
L
a
Rectifire 1
Rectifire m
AC
grid
3-phase
transformer
M
multi-pulse
rectifire
Synchronous
motor
Chopper
(a) (b)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
(d)
99.6
99.7
99.8
99.9
100
SOC (%)
-10
0
10
20
30
40
50
60
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
(c)
Speed (rpm)
time (s) time (s)
(a) (b)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
(d)
99.6
99.7
99.8
99.9
100
SOC (%)
-10
0
10
20
30
40
50
60
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
(c)
Speed (rpm)
time (s) time (s)
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
-2.0
-1.5
-1.0
-0.5
0
0.5
1.0
1.5
2.0
AC voltage (kV)
time (s)
u
sa
u
sc
u
sb
-6.0
-4.0
-2.0
0
2.0
4.0
6.0
AC current (kA)
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
time (s)
i
sa
i
sc
i
sb
(e) (f)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0.2
0.3
0.4
0.5
U
cA,SM
(kV)
time (s) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0.2
0.3
0.4
0.5
U
cB,SM
(kV)
time (s)
Figure 8.
Output performance of SCES-MMC: (
a
) AC output voltage; (
b
) AC output current; (
c
) Motor
speed; (d) Average SOC of SCBs; (e) Voltage of the SCBs—phase a; (f) Voltage of the SCBs—phase b.
Energies 2017, 10, 1428 9 of 11
Due to the high capacitance of the SCBs, even if a large amount of energy is released/absorbed,
there is no significant fluctuation in the sub-module capacitor voltage USM, as shown in Figure 8e,f.
Figure 9 illustrates the power distribution laws of the proposed SCES-MMC. From the above-
mentioned analysis, it can be concluded that the SCBs in the SCES-MMC will be discharged to
provide about 69% of the AC load power requirement. Thus, less DC power is required compared to
those without super capacitor banks. Since SCES is involved in absorbing part of the regenerative
power from the mine hoist motor, under regeneration conditions, the power dissipated in the SCES-
MMC is also less than for conventional MMCs. This makes it possible to minimize the capacity
requirements of the AC grid and energy loss of the mine hoist system.
Figure 9. Power distributions.
5. Conclusions
A super capacitor energy storage-based modular multilevel converter (SCES-MMC) topology
for mine hoist applications has been investigated in this paper. In contrast to conventional MMCs,
the sub-modules employ distributed SCBs, which are designed to absorb the regenerative energy of
the mine hoist, and release it under traction conditions. Due to the high power density of the super
capacitor, the sub-modules’ capacitor voltage will not significantly fluctuate. The configuration and
operation principles, together with control technologies were studied in detail. Moreover, the
feasibility of the proposed SCES-MMC topology and the control theory were also verified. Simulation
results show that SCES-MMC makes reasonable use of the energy of the system through the
distributed SCBs, improving the energy utilization efficiency, and shows good application prospects
in future medium-/high-voltage mine hoist systems.
Acknowledgments: The authors gratefully acknowledge technical support from the National Key Research and
Development Program of China (Award Number: 2016YFE0131700) and the State Key Laboratory of Large
Electric Drive System and Equipment Technology (Award Number: SKLLDJ042016005).
Author Contributions: Xiaofeng Yang contributed to the concept, undertook significant theory analysis, and
wrote the paper. Piao Wen and Yao Xue helped perform the simulation with constructive discussions, Trillion
Q. Zheng supplied guidance, and Youyun Wang provided the field testing parameters and edited the
manuscript. All authors read and approved the manuscript.
Conflicts of Interest: The authors declare no conflicts of interest.
-2.5
-2.0
-1.5
-1.0
-0.5
0
0.5
Power/MW
1.0
1.5
1.0 time (s)
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
Without SC
SCES-MMC
SCBs
SCBs discharging
SCBs charging
power dissipation
DC power supply
Figure 9. Power distributions.
5. Conclusions
A super capacitor energy storage-based modular multilevel converter (SCES-MMC) topology
for mine hoist applications has been investigated in this paper. In contrast to conventional MMCs,
the sub-modules employ distributed SCBs, which are designed to absorb the regenerative energy
of the mine hoist, and release it under traction conditions. Due to the high power density of the
super capacitor, the sub-modules’ capacitor voltage will not significantly fluctuate. The configuration
and operation principles, together with control technologies were studied in detail. Moreover, the

Energies 2017,10, 1428 10 of 11
feasibility of the proposed SCES-MMC topology and the control theory were also verified. Simulation
results show that SCES-MMC makes reasonable use of the energy of the system through the distributed
SCBs, improving the energy utilization efficiency, and shows good application prospects in future
medium-/high-voltage mine hoist systems.
Acknowledgments:
The authors gratefully acknowledge technical support from the National Key Research and
Development Program of China (Award Number: 2016YFE0131700) and the State Key Laboratory of Large Electric
Drive System and Equipment Technology (Award Number: SKLLDJ042016005).
Author Contributions:
Xiaofeng Yang contributed to the concept, undertook significant theory analysis, and wrote
the paper. Piao Wen and Yao Xue helped perform the simulation with constructive discussions,
Trillion Q. Zheng
supplied guidance, and YouyunWang provided the field testing parameters and edited the manuscript. All authors
read and approved the manuscript.
Conflicts of Interest: The authors declare no conflicts of interest.
Abbreviations
The following abbreviations are used in this manuscript:
AC alternating current
DC direct current
HVDC high voltage direct current
VSC voltage source converter
NPC neutral-point-clamped
AFE active front end
SCES super capacitor energy storage
MMC modular multilevel converter
SM sub-module
SCB super capacitor bank
SOC state of charge
IGBT insulated gate bipolar transistor
VFSR variable frequency speed regulation
References
1.
Chen, H. Research and Application Based on Full Digital DC Speed Adjusting Device in Mine Hoist
Automation System. In Proceedings of the International Symposium on Intelligence Information Processing
and Trusted Computing, Huanggang, China, 28–29 October 2010; pp. 524–527.
2.
Ferreira, V.N.; Mendonça, G.A.; Rocha, A.V.; Resende, R.S.; Cardoso Filho, B.J. Medium voltage IGBT-based
converters in mine hoist systems. In Proceedings of the IEEE Industry Applications Society Annual Meeting,
Portland, OR, USA, 2–6 October 2016; pp. 1–8.
3.
Abu-Rub, H.; Holtz, J.; Rodriguez, J.; Ge, B. Medium-Voltage Multilevel Converters—State of the Art,
Challenges, and Requirements in Industrial Applications. IEEE Trans. Ind. Electron.
2010
,57, 2581–2596.
[CrossRef]
4.
Kouro, S.; Malinowski, M.; Gopakumar, K.; Pou, J.; Franquelo, L.G.; Bin, W.; Rodriguez, J.; Perez, M.A.;
Leon, J.I. Recent Advances and Industrial Applications of Multilevel Converters. IEEE Trans. Ind. Electron.
2010,57, 2553–2580. [CrossRef]
5. Yang, X.; Li, J.; Wang, X.; Fan, W.; Zheng, T.Q. Circulating Current Model of Modular Multilevel Converter.
In Proceedings of the Asia-Pacific Power and Energy Engineering Conference (APPEEC), Wuhan, China,
25–28 March 2011; pp. 1–6.
6.
Krug, D.; Malinowski, M.; Bernet, S. Design and comparison of medium voltage multi-level converters for
industry applications. In Proceedings of the 39th IEEE Industry Applications Conference (IAS), Seattle, WA,
USA, 3–7 October 2004; pp. 781–790.
7.
Asea Brown Boveri. ACS 6000 Medium Voltage AC Drives—Commissioning Manual; ABB Switzerland Ltd.:
Zurich, Switzerland, 2004.

Energies 2017,10, 1428 11 of 11
8.
Abdel-baqi, O.; Nasiri, A.; Miller, P. Dynamic Performance Improvement and Peak Power Limiting Using
Ultracapacitor Storage System for Hydraulic Mining Shovels. IEEE Trans. Ind. Electron.
2015
,62, 3173–3181.
[CrossRef]
9.
Parkhideh, B.; Mirzaee, H.; Bhattacharya, S. Supplementary Energy Storage and Hybrid Front-End
Converters for High-Power Mobile Mining Equipment. IEEE Trans. Ind.Appl.
2013
,49, 1863–1872. [CrossRef]
10.
Parkhideh, B.; Bhattacharya, S.; Mazumdar, J.; Koellner, W. Utilization of Supplementary Energy Storage
Systems in High Power Mining Converters. In Proceedings of the 2008 IEEE Industry Applications Society
Annual Meeting, Edmonton, AB, Canada, 5–9 October 2008; pp. 1–7.
11.
Dorn, J.; Huang, H.; Retzmann, D. Novel Voltage-Sourced Converters for HVDC and FACTS Applications.
In Proceedings of the CIGRE Symposium, Osaka, Japan, 1–4 November 2007; pp. 314–321.
12.
Yang, X.; Xue, Y.; Chen, B.; Lin, Z.; Mu, Y.; Zheng, T.Q.; Igarshi, S. Reverse blocking sub-module based
modular multilevel converter with DC fault ride-through capability. In Proceedings of the IEEE Energy
Conversion Congress and Exposition (ECCE), Milwaukee, WI, USA, 18–22 September 2016; pp. 1–7.
13.
Yang, X.; Xue, Y.; Chen, B.; Lin, Z.; Zheng, T.Q.; Li, Y. Novel modular multilevel converter against DC faults
for HVDC applications. CSEE J. Power Energy Syst. 2017,3, 140–149.
14.
Ma, F.; Xu, Q.; He, Z.; Tu, C.; Shuai, Z.; Luo, A.; Li, Y. A Railway Traction Power Conditioner Using Modular
Multilevel Converter and Its Control Strategy for High-Speed Railway System. IEEE Trans. Transp. Electrif.
2016,2, 96–109. [CrossRef]
15.
Yang, X.; Zheng, T.Q.; Lin, Z.; Tao, X.; You, X. Power Quality Controller Based on Hybrid Modular Multilevel
Converter. In Proceedings of the the 21st IEEE International Symposium on Industrial Electronics (ISIE),
Hangzhou, China, 28–30 May 2012; pp. 1997–2002.
16. Hagiwara, M.; Akagi, H.; Nishimura, K. A Medium-Voltage Motor Drive with a Modular Multilevel PWM
Inverter. IEEE Trans. Ind. Electron. 2010,25, 1786–1799. [CrossRef]
17.
Li, B.; Zhou, S.; Xu, D.; Yang, R.; Xu, D.; Buccella, C.; Cecati, C. An Improved Circulating Current Injection
Method for Modular Multilevel Converters in Variable-Speed Drives. IEEE Trans. Ind. Electron.
2016
,63,
7215–7225. [CrossRef]
18.
Antonopoulos, A.; Angquist, L.; Harnefors, L.; Nee, H. Optimal Selection of the Average Capacitor Voltage
for Variable-Speed Drives With Modular Multilevel Converters. IEEE Trans. Ind. Electron.
2015
,30, 227–234.
[CrossRef]
19.
Kolb, J.; Kammerer, F.; Gommeringer, M.; Braun, M. Cascaded Control System of the Modular Multilevel
Converter for Feeding Variable-Speed Drives. IEEE Trans. Power Electron. 2015,30, 349–357. [CrossRef]
20.
Vasiladiotis, M.; Rufer, A. A Modular Multiport Power Electronic Transformer with Integrated Split Battery
Energy Storage for Versatile Ultrafast EV Charging Stations. IEEE Trans. Ind. Electron. 2015,62, 3213–3222.
21.
Quraan, M.; Yeo, T.; Tricoli, P. Design and Control of Modular Multilevel Converters for Battery Electric
Vehicles. IEEE Trans. Power Electron. 2016,31, 507–517. [CrossRef]
22.
Agarlita, S.; Coman, C.; Andreescu, G.; Boldea, I. Stable V/f control system with controlled power factor
angle for permanent magnet synchronous motor drives. IET Electron. Power Appl.
2013
,7, 278–286. [CrossRef]
23.
Boldea, I.; Paicu, M.C.; Andreescu, G. Active Flux Concept for Motion-Sensorless Unified AC Drives.
IEEE Trans. Power Electron. 2008,23, 2612–2618. [CrossRef]
24.
Sau, S.; Fernandes, B.G. Analysis and reduction of capacitor ripple current in modular multilevel converter
for variable speed drives. In Proceedings of the European Conference on Power Electronics and Applications
(EPE’16 ECCE Europe), Karlsruhe, Germany, 5–9 September 2016; pp. 1–10.
25.
Li, R.; Fletcher, J.E.; Williams, B.W. Influence of third harmonic injection on modular multilevel converter
-based high-voltage direct current transmission systems. IET Gener. Transm. Distrib.
2016
,10, 2764–2770.
[CrossRef]
©
2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).