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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 5, OCTOBER 2014
Evaluation of Emerging Modular Multilevel
Converters for BESS Applications
Theodore Soong, Student Member, IEEE, and Peter W. Lehn, Senior Member, IEEE
Abstract—The power conversion system for a battery-energy
storage system typically employs a conventional voltage-source
converter with battery strings directly connected to the dc bus.
This system configuration presents several issues, such as limited
efficiency of two-level converter systems and the limited reliability
associated with the use of long battery strings. This paper examines three viable multilevel converter solutions for integrating
battery energy storage that offer the potential for enhanced efficiency and reliability. These solutions are the modular multilevel
converter (MMLC) with battery energy storage distributed into
its submodules, the cascaded converter, and the MMLC with
battery energy storage centralized on its dc link. The three systems are compared in terms of efficiency, reliability, and module
redundancy. It is determined that the MMLC with distributed
battery energy storage must operate differently from conventional
MMLC systems. Its operation is therefore studied in detail and
validated through simulation to demonstrate its suitability for
distributed energy-storage integration. The analysis shows that
the MMLC with distributed battery energy storage requires the
largest number of semiconductor devices for a given power level;
however, it also provides the most efficient, reliable, and versatile
solution of energy-storage integration.
Index Terms—Battery storage plants, DC–AC power conversion, energy storage, power conversion, power electronics.
I. INTRODUCTION
D
UE TO higher penetration of renewable resources,
grid-connected energy storage is becoming a necessity
for future power grids. Energy storage helps to mitigate the
stochastic nature of renewable resources. In addition, energy
storage can quickly deliver active power to provide services,
such as spinning reserve, peak shaving, load levelling, and
load frequency control [1], [2]. These services increase the
reliability and stability of the grid [3], [4]. Available energy
storage technologies include hydro, battery, flywheel, superconducting magnetic energy storage, and supercapacitors [5].
Of these technologies, batteries are the dominant solution for
large-scale energy storage, and their use is cost effective when
designed to provide less than 5 h of service at rated output
power [6]. This allows the energy-storage system to operate
Manuscript received April 18, 2013; revised August 23, 2013 and March 06,
2014; accepted July 13, 2014. Date of publication August 12, 2014; date of
current version September 19, 2014. Paper no. TPWRD-00464-2013.
The authors are with the Department of Electrical and Computer Engineering, University of Toronto, Toronto, ON M5S 2J7, Canada (e-mail:
theodore.soong@mail.utoronto.ca; lehn@ecf.utoronto.ca).
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/TPWRD.2014.2341181
on time scales that complement generators rather than compete. This paper targets large battery energy-storage system
(BESS) installations for medium-voltage (MV) applications
in the megawatt/megawatt-hour range since conventional
two-level converters can be used for lower voltage and power
applications.
A BESS installation has two major hardware components: a
power conversion system (PCS) and a network of battery energy
storage units. The focus of this paper is the PCS component of a
three-phase BESS, and it is assumed that a mature battery technology is used, such as lead acid, sodium sulfur, or lithium-ion
batteries [7], [8]. When choosing a PCS topology, the most important features are the reliability as well as the efficiency of
the topology. Reliability is influenced by the configuration of
the battery energy-storage units. The conventional configuration
in existing BESSs places the batteries in long series strings to
create higher voltages. In this configuration, the battery lifespan
can be negatively affected by overcharging unless an equalization method is applied [9]. Furthermore, if one battery cell faults
or becomes dangerous to operate, an entire string must be disconnected for service.
In a BESS, energy must be transferred into and out of the battery. Thus, the overall efficiency of the BESS, or round trip efficiency, is the square of the converter efficiency. For example,
the modular multilevel converter (MMLC) is computed to have
an efficiency of approximately 99.3% compared to 98.0% of
a two-level VSC [10]. The round trip efficiency of these converters would be approximately 98.6% compared to 96.0%. This
serves to highlight the motivation for multilevel solutions.
This paper identifies current PCS topologies that best address
the aforementioned issues. These topologies are the cascaded
converter, MMLC with centralized battery energy storage, and
the MMLC with distributed battery energy storage. As of yet,
proper operation of the MMLC with distributed battery energy
storage has not been demonstrated; therefore, simulation results
are presented. The three converters are then assessed based on
their efficiency, cost, module redundancy, and reliability. The
main contribution of this paper is the presentation of the proper
operation of an MMLC with distributed battery energy storage,
and the identification and comparison of PCS topologies which
are modular, efficient, and increase the reliability of a BESS.
II. POWER CONVERSION SYSTEMS
The PCS in modern BESS installations, within the megawatt/
megawatt-hour range, typically consists of a conventional twolevel or three-level insulated-gate bipolar transistor (IGBT) converters. Recent power converters used, or proposed for use, in
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SOONG AND LEHN: EVALUATION OF EMERGING MODULAR MULTILEVEL CONVERTERS FOR BESS APPLICATIONS
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Fig. 1. Generalized BESS structures: (a) single stage and (b) two stage.
a BESS can be placed into three general categories: 1) singlestage, 2) two-stage, and 3) multilevel systems [11]–[13].
1) Single-Stage Systems: The most common systems are
single-stage systems where the battery energy storage is directly connected to the dc bus of an IGBT-based inverter, as
illustrated in Fig. 1(a). Some examples of companies that use
this type of system are Parker SSD and ABB. Parker SSD employs two-level voltage-source converters [14], [15]. The ABB
product DynaPeaQ utilizes a neutral-point clamped converter
[16], [17].
Though single-stage systems are simple, integrating battery
energy storage directly on the dc link of an inverter can negatively impact system efficiency, cost, and reliability. Significant battery voltage variations exist over time as a result of the
storage system’s state of charge (SOC) [11], [18]. In single-stage
systems, this full range of voltage variation must be accommodated by the grid-tied inverter. To ensure proper inverter operation even under low SOC (low dc-link voltage), the converter is
nominally operated with an elevated dc bus voltage level. This
requires the use of reduced modulation indices, driving down
efficiency, and increasing ac output harmonics, thus increasing
ac harmonic filter costs. Furthermore, the difference in voltage
of the battery string when charged versus discharged forces the
grid-tied inverter to be over rated in terms of voltage, which further increases the costs.
As discussed in Section I, reliability is also a serious issue
in single-stage systems due to the large number of series-connected batteries. For example, more than 36 000 battery cells
distributed in two battery strings are needed in order to provide 5.36 MWh of capacity for the DynaPeaQ [19]. To improve
safety, and minimize maintenance time, it is common for these
large strings of batteries to be composed of modularized battery packs [16], [20]. Each module consists of a short string of
batteries, and a battery-management system that monitors and
balances the SOC of the batteries. Since the large series strings
of batteries are already modularized, it would be advantageous
if the power conversion system also becomes modular.
2) Two-Stage Systems: A two-stage PCS consists of a bidirectional dc–dc converter that feeds a grid-tied inverter stage.
The simplest type of two-stage BESS is shown in Fig. 1(b) from
[12]. It utilizes a bidirectional dc–dc converter to decouple the
batteries from the dc link of the three-phase inverter. By decoupling the batteries from the dc bus, the voltage variation of the
batteries is no longer an issue for the inverter. Any bidirectional
dc–dc converter can be used. If a boost-type converter is used at
an efficient operating point, the required number of series-connected batteries can be reduced.
One company that produces this type of two-stage system is
S&C Electric Company, which has several operational BESS
Fig. 2. Two-stage systems introduced in the literature, or built by companies.
(a) Simplified diagram of S&C’s PureWave storage management system [21],
(b) isolated two-stage system [24], and (c) two-stage system with series-connected converters [11].
sites [21]. A simplified diagram of the system is shown in
Fig. 2(a). In the system, each battery bank is connected to a
dc bus through an interleaved dc–dc converter, and the dc bus
feeds a grid-connected inverter [22], [23]. Storage capability is
increased by placing additional dc–dc converters with battery
banks in parallel on the dc bus. If galvanic isolation is required, as it is for some battery types, then the dc–dc converter
would be changed to an isolated bidirectional topology. Such a
topology is studied in [24] and is shown in Fig. 2(b).
Another two-stage topology modularizes the dc–dc converter
stage into series-connected modules, each with their own batteries [13], [11]. Fig. 2(c) depicts a possible implementation of
such a system, which was presented in [11].
3) Multilevel Systems: Multilevel converters are converters
that use more than two voltage levels to produce the desired
ac output waveform. There are four main multilevel topologies,
which are depicted in Fig. 3. The topologies are the MMLC,
cascaded converter, flying capacitor converter, and neutral-point
clamped (NPC) converter.
The MMLC is shown in Fig. 3(a). It is composed of threephase legs, and each leg has two phase arms. A phase arm consists of
submodules along with a small inductor to prevent
current spikes during switch transitions. In a standard MMLC,
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Fig. 3. Most prevalent multilevel topologies. (a) Modular multilevel converter,
(b) cascaded multilevel converter, (c) diode-clamped converter (5-Level), and
(d) flying capacitor converter (5-Level).
each submodule would consist of a half bridge and a capacitor; this is labelled as the “Type 1” submodule depicted in
Fig. 3(a). To utilize the MMLC as a PCS, centralized battery
energy storage can be connected to the dc link of the converter
[25], but this type of arrangement would still require long strings
of batteries. To utilize the modularity of the converter, the batteries can be connected to each submodule in two methods. The
battery can be directly connected across the capacitor, as shown
in the “Type 2” submodule of Fig. 3(a), or it can be interfaced
through a dc–dc converter, as shown in the “Type 3” submodule
in Fig. 3(a) [26], [27]. The main difference between Type 2 and
Type 3 submodules is that the battery would be directly exposed
IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 5, OCTOBER 2014
to second harmonic power in Type 2 submodules, and a large
submodule capacitor would be required to minimize the exposure. The dc–dc converter decouples the battery from the submodule capacitor, reducing the dc filter required for the battery,
increases the lifespan of the battery, and allows the submodule
capacitor to be decreased. The arrangement using either Type
2 or Type 3 submodule, is referred to as the MMLC with distributed battery energy storage.
The cascaded converter is composed of three-phase legs,
where each phase leg is composed of a string of H-bridge modules, as shown in Fig. 3(b). Phase
, and output terminals
are on one side of each phase leg, while all three-phase legs are
wye connected. Fig. 3(b) also shows the different methods of
integrating batteries into each submodule, by either connecting
batteries directly across the capacitor (“Type 1”), or interfaced
through a dc–dc converter (“Type 2”) [26]. The advantages
and disadvantages between Type 1 and Type 2 are similar to
those of the MMLC with distributed battery energy storage.
The most common battery integration method is to use “Type
1” submodules [28], [29].
An NPC converter, shown in Fig. 3(c), utilizes diodes to
create additional voltage levels, and batteries are integrated
into the dc bus of the converter [30], [11]. To create additional
voltage levels, the dc bus capacitor would consist of a string of
capacitors, and clamping diodes that are connected at regular
intervals. This allows the battery energy storage to be subdivided into smaller strings, which is depicted in Fig. 3(c), and
studied in [3].
Similar to the NPC, the flying capacitor converter, shown in
Fig. 3(d), utilizes capacitors, as opposed to diodes, to create additional voltage levels. Batteries are integrated into the dc bus
of the converter [30]. Unlike the NPC, the battery is not subdivided into shorter strings, and a single centralized battery energy
storage is connected to the dc bus.
The converter topology that has garnered the most interest for
BESS applications is the cascaded multilevel converter. It has
been studied by Akagi et al. [28], and Doncker et al. [31] for
direct use as the PCS of a BESS or for applications to renewable resources, and electric drives by Tolbert et al. [29]. One
company that appears to be adopting such a topology for use
in a BESS is Altair Nanotechnologies, which published a white
paper demonstrating an experimental cascaded multilevel converter for use as a BESS [20]. The MMLC is another topology
of interest, and its suitability as a BESS or for the purpose of
integrating distributed energy resources to the grid has been investigated in [25]–[27].
III. CANDIDATE POWER CONVERTER SYSTEMS
As discussed in Sections I and II, an ideal PCS should be efficient, and utilize shorter strings of batteries in comparison to
current systems to enhance reliability. Of the PCSs presented in
Section II, the best candidates are the cascaded converter and
MMLC. Both of these topologies can integrate short strings of
batteries into their modular structure and have a wide application range because they can be scaled to higher voltage ratings
with the simple addition of modules.
On the system level, MMLC and the cascaded converter
allow for reduced harmonics, and higher uptime due to module
SOONG AND LEHN: EVALUATION OF EMERGING MODULAR MULTILEVEL CONVERTERS FOR BESS APPLICATIONS
redundancy. The MMLC with distributed battery energy storage
also has a fixed dc link, which can be used to interconnect it
with an MV dc network if desired. Thus, it can act as a dc/ac
interconnect and as a storage unit in a single system.
The two topologies that are under consideration—the
MMLC with distributed battery energy storage and cascaded
converter—are shown in Fig. 3(a), and (b), respectively. Since
battery costs are significant in a BESS, submodules chosen
for the cascaded and MMLC comparison utilize converters
to interface the battery to the submodule capacitors. These
converters, referred to as a battery interface converter (BIC),
were chosen primarily for their benefits in decreasing second
harmonic power, which increases the lifespan of the batteries.
Thus, “Type 2” and “Type 3” submodule were chosen for the
cascaded converter and MMLC, respectively. In this section,
the cascaded converter and MMLC with distributed battery
energy storage will be compared. The MMLC with centralized
battery energy storage does not meet the criterion of modularized battery energy storage, but is included for the purpose of
comparison.
A. MMLC With Centralized Battery Energy Storage
The MMLC with centralized battery energy storage operates
as a regular MMLC and uses “Type 1” submodules of Fig. 3(a).
Compared to a regular MMLC, the dc-link voltage of this
topology is not fixed since it is composed of series-connected
batteries and the dc-link voltage depends on their SOC. As
previously discussed, this causes the voltage rating of the
converter to increase, thus requiring additional modules in
each arm of the converter. This topology does not modularize
the battery energy storage nor does it offer a fixed dc-link
voltage for integration of other dc loads or sources. It offers no
additional benefits when compared to existing technology.
For the following discussion, the operation of the MMLC is
briefly reviewed. Fig. 4 shows a single-phase representation of
the MMLC.
and
represent the voltage created by the
upper and lower arm submodules, respectively. Each arm can
be represented as an ac voltage source approximately equal to
with a dc offset equal to
.
and
are the
upper and lower arm currents, which sum
at the output
node to equal the ac output current
. The difference of the
two arm currents equates to
; this is a current that conducts
through the phase leg and does not interact with the output. For
a three-phase MMLC,
conducts into the dc link, or the other
phase legs.
When uncontrolled, the difference current of the MMLC
would be primarily composed of a dc and second harmonic
current. The presence of the second harmonic current would
increase conduction losses, but can be used to decrease the
submodule capacitors’ voltage ripple, thus minimizing capacitor-size requirements [32], [33]. In this comparison, the
difference current is controlled such that the second harmonic
current is removed to maximize efficiency for a fair comparison. The difference current is therefore composed of a dc
current, henceforth referred to as the dc difference current, and
each arm of the MMLC conducts half the ac output current
along with the dc difference current. The ac output current
transfers power from the arm to the output terminal, while the
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Fig. 4. Single-phase MMLC.
dc current transfers power from the dc link to the arm. More
details on the converter’s current waveforms can be found in
[10], [34], and [35].
B. Cascaded Converter With Battery Energy Storage
The cascaded converter can only provide active power when
each module is connected to a dc source [36]–[38]. In this case,
battery energy storage acts as the dc source, and is distributed
into shorter series strings of batteries connected to each module.
In case of battery faults, extra modules can be placed in the
phase leg, and a single module would be serviced instead of an
MV string of batteries connected to the dc link. For the cascaded
converter, each phase leg, consequently each module, must conduct the full ac output current [28].
C. MMLC With Distributed Battery Energy Storage
The MMLC with distributed battery energy storage has batteries distributed into each module, and extra modules can be
placed in each arm of the converter for use in the case of battery
faults. An MMLC of this structure can transfer power from one
phase leg to another with the use of the dc link. This can be used
to balance the SOC of the batteries between phase legs, or compensate a phase leg if a battery in the submodule undergoes a
fault. This is an advantage compared to the cascaded converter
since the difference current is internal to the MMLC, and would
not affect the output of the converter.
The MMLC with distributed battery energy storage operates
differently than a regular MMLC. As previously described, each
arm must conduct half the ac output, in addition to a dc current.
Since each submodule includes battery energy storage, power
does not need to be delivered from the dc link, via the dc difference current. Therefore, each arm only conducts half the ac
output current, reducing conduction loss in the converter. Due to
the nonstandard operation of this topology, Section IV provides
simulated waveforms of the MMLC with distributed battery energy storage in operation.
IV. SIMULATION OF THE MMLC WITH DISTRIBUTED
BATTERY ENERGY STORAGE
A PSCAD simulation of the MMLC with distributed battery
energy storage was created to verify the expected current in
each arm of the converter. Simulation results for three scenarios
are shown—the first depicts the MMLC with distributed battery
energy storage operating without the dc difference current, the
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 5, OCTOBER 2014
Fig. 5. Diagram of the simulated system.
TABLE I
SIMULATION PARAMETERS
Fig. 6. MMLC control structure.
is the average of all submodule cais the number of submodules with energy storage in
pacitor voltages, and
operation for the phase arm.
second scenario demonstrates that energy storage in one phase
leg can supplement the other phase legs, and the third scenario
shows that MMLC operation is unaffected when a BIC is shut
down. In all scenarios, the MMLC provided rated real power to
the grid. Fig. 5 depicts the simulated system, and Table I summarizes all simulation quantities. Due to the low number of modules in the simulated MMLC, a switching frequency of 2.75 kHz
was used, but only one module in each arm is modulated at any
time, and the remainder are either on or off. The battery energy
storage is modelled as a constant voltage source due to short duration of the simulation.
The control system used in this simulation is depicted in
Fig. 6. The MMLC was controlled with proportional resonant
and resonant controllers, whose references for these controllers
were created in the
frame, and synchronized to the grid. For
each BIC, a PI controller was used to control the current of the
battery energy storage. When the MMLC operates without a dc
link, a PI controller is used to control the total output power of
the energy storage to maintain the total submodule capacitor
voltage of all arms.
A. Operation of an MMLC With Distributed Battery Energy
Storage
In Section III, it was stated that the arms of an MMLC with
distributed battery energy storage would only conduct half the
fundamental current. The operation of this topology was not
previously validated by simulation results. Fig. 7 shows current
Fig. 7. MMLC is shown transitioning from operation with the dc-link source
to operation with the distributed battery energy storage only. After the transition
at 0.05 s, the MMLC no longer requires a dc difference current (i.e., power from
the dc link) and the upper and lower arm currents align.
waveforms of phase . The MMLC in the simulation is initially
operated such that power is provided only by a dc-link source.
At 0.05 s, the dc-link source is disconnected, and power from the
battery energy storage is injected into each submodule to compensate. After the initial transient, the upper and lower arm currents
are aligned. Thus, the MMLC with distributed
battery energy sources only conducts half the fundamental current in each arm as opposed to half the fundamental current plus
the dc difference current.
Operation without dc difference current is achieved without
affecting the system dynamics of the MMLC. This is shown in
Fig. 8 where the MMLC operates in all four P and Q quadrants
while active power is supplied by battery energy storage only
with no dc-link source.
SOONG AND LEHN: EVALUATION OF EMERGING MODULAR MULTILEVEL CONVERTERS FOR BESS APPLICATIONS
Fig. 8. MMLC is operating at several P and Q operating points, measured at
the PCC, with all power provided by battery energy storage units located in each
submodule. The MMLC is operating without a dc-link source in this scenario.
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Fig. 10. MMLC is operating without a dc-link source. The battery energy
storage units in all submodules are outputting the same amount of power. At
0.05 s, the output power of one BIC drops to 0
. To compensate, the
have increased their output power
other batteries in the same arm as
while batteries in all other phase arms maintain their output power
.
power required from the dc-link sources, as demonstrated in
Section IV-A. In this case, the energy storage in phase leg
provided enough power for phase to operate without a power
source external to the phase leg, and delivered power to the
other two phases through the dc link. Thus, the average of
the phase difference current
is two-thirds that of the
dc-link source current before it was disconnected.
This simulation showed that the difference current of each
phase leg is independent of the other phases. In addition, batteries within a submodule can support active power transfer to
any phase, providing additional redundancy that is not offered
by the cascaded converter.
C. MMLC Redundancy Within a Phase Arm
Fig. 9. MMLC is shown transitioning from operation with the dc-link source to
0.05 s, the dc-link source
operation with energy storage in phase only. At
was disconnected and the battery energy storage in phase provides power to
all phases. After the initial transients, the phase difference current is seen,
transferring power to the other phase legs, while all other currents remain the
same.
B. MMLC Redundancy Between Phases
As mentioned in Section III, the MMLC can transfer power
between phase legs by utilizing the dc link. The simulation results presented in this section demonstrate this operation by
using only the energy storage in phase leg to provide power
to phases , and . The simulation results are shown in Fig. 9.
At the beginning of the simulation, only the dc-link source provides power to the MMLC. At 0.05 s, the dc-link source was
disconnected, and the battery energy storage in phase leg was
used to provide power to the MMLC.
Standard MMLC operation transfers power from the dc-link
source to the phase legs with the dc difference current. With
the addition of distributed battery energy storage, the power
delivered by the battery energy storage would reduce the
In this scenario, the MMLC is shown to operate with a disconnected BIC in a single-phase arm. When one of the BICs
in the phase arm is disconnected, the other BICs in the same
phase arm increase their output power to compensate for the lost
module. The simulation results are shown in Fig. 10. To display
the change in the output power of the BICs, three representative waveforms are shown. The current
is the current of the
battery that is disconnected,
represents the current of other
batteries in the same phase arm, and
represents the current
of all batteries in all other phase arms.
The simulation begins with the MMLC operating with battery
energy storage in all submodules and with no dc link. All three
battery currents are equal at this point. At 0.05 s, one battery
energy storage is shut down; thus,
drops to zero. The power
from the other batteries in the same phase arm is increased to
compensate for the loss of
, as shown by
, and the output
power of all other batteries in the MMLC remains the same,
as indicated by
. This simulation shows that an MMLC can
operate even if the energy storage is not present in all submodules without any affect on the output or difference currents of
all three phases.
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 5, OCTOBER 2014
TABLE II
SYSTEM PARAMETERS FOR EFFICIENCY COMPARISON
TABLE IV
CONVERTER PARAMETERS
BIC conduction and switching losses were calculated by
TABLE III
SWITCH LOSS PARAMETERS
(3)
(4)
is the average current conducted by a BIC, and
is the rms current conducted by a BIC.
is the
switching frequency of the BIC, and was assumed to operate
at a frequency of 1 kHz.
For all three converters, it was assumed that the battery would
drop in voltage by 30% from its full charge voltage, and was
based on Saft’s VL45E lithium-ion cell [39]. For the MMLC
with centralized battery energy storage, the voltage drop would
increase the number of required switches and affect the dc difference current. For the MMLC with distributed battery energy
storage and cascaded converter, the voltage drop increases the
conduction current of the BIC. Efficiency was calculated at the
nominal battery voltage, which is assumed to be 85% of the battery’s full charge voltage.
In Table IV, the number of modules required for each converter was calculated with (5)–(7) for the MMLC with centralized battery energy storage, MMLC with distributed battery energy storage, and the cascaded converter, respectively
where
V. CONVERTER COMPARISON
The three candidate converters have been introduced. The
current waveforms have been described for the MMLC with
centralized battery energy storage and a cascaded converter.
Simulation of the MMLC with distributed energy storage
demonstrated its unique characteristics and its resulting current
waveforms. The objective of this section is to compare these
converters in terms of their efficiency and redundancy.
A. Efficiency Comparison
An efficiency study was performed to compare:1) the MMLC
with centralized battery energy storage; 2) the MMLC with distributed battery energy storage; and 3) the cascaded converter.
The system and converter parameters for this study are listed
in Tables II and IV, respectively. The efficiency comparison includes conduction and switching losses, and assumes that these
are the primary sources of loss. The parameters used in the loss
mechanics of these switch types were found from their respective data sheets and are given in Table III. Conduction loss was
modelled as a fixed voltage source
, in series with a
resistor
, and the energy loss at each turn ON and OFF of
the switch was assumed to have a fixed
and variable
component
. The loss calculations for a submodule
were separated into losses incurred by the standard submodule
of the converters, and those of the BIC. The standard submodule
conduction and switching losses were calculated by using
(1)
(2)
where
is the average current conducted by a submodule,
is the rms current conducted by a submodule,
is
the number of switches conducting at a given time,
is the
number of switches in the standard submodule, and
is the
frequency where a submodule switch is turned ON and OFF. Each
module was assumed to switch ON and OFF, once per period of
the grid.
%
(5)
(6)
(7)
where
is the ratio between the dc link and the peak ac voltage,
is
the drop in battery voltage from full charge to min%
imum charge in percent,
is the rated submodule capacitor voltage, and
is the rated system voltage. The constant
is the overhead voltage reserved for control and accounts for
impedance drops between the converter and the grid, and is set
to 1.17 in this comparison.
To compare the three converters on an equal basis,
was chosen such that a rated average current of 0.7 kA conducts through the submodule switch involved in the dc/ac power
conversion. The rated module current is derated from the actual switch current rating due to fault, overload, and cooling
considerations. For the MMLC with centralized battery energy
storage, the rated average current was calculated at the minimum dc-link voltage (i.e., when the batteries are discharged).
SOONG AND LEHN: EVALUATION OF EMERGING MODULAR MULTILEVEL CONVERTERS FOR BESS APPLICATIONS
Fig. 11. Comparison of efficiency between the MMLC with distributed battery energy storage, MMLC with centralized battery energy storage, and the
cascaded converter.
The efficiency curves of the cascaded converter, the MMLC
with centralized battery energy storage, and the MMLC with
distributed battery energy storage are shown in Fig. 11. The efficiency curves show that the MMLC with distributed battery
energy storage is the most efficient, followed by the cascaded
converter, and then the MMLC with centralized battery energy
storage. The difference in efficiency between the MMLC with
centralized battery energy storage, and the MMLC with distributed battery energy storage can be attributed to the current
that each module must conduct. As discussed in Section IV, the
MMLC with distributed energy storage does not need to transfer
power from the dc link to the converter. Therefore, the converter
arms do not need to conduct dc current, whereas the MMLC
with centralized battery energy storage does.
In Table IV, the cost of the converter was measured with
the “Installed Switch MVA per MW output” ratio. The ratio is
computed by summing the switch VA of the BICs and dc/ac
converter switches, and dividing it by
. While the MMLC
with distributed battery energy storage is the most efficient
topology, it has a higher cost when compared to the cascaded
converter. In addition, the MMLC with centralized battery
energy storage has the highest cost and lowest efficiency, and
is the least preferred option. Comparing the two MMLC variants, contrary to intuition, the addition of the BICs enhances
efficiency and ultimately reduces the cost, for the same
.
The presence of the dc current in the MMLC with centralized
batteries decreased efficiency, and derated the output power of
the topology. Evidently, dc current has a higher impact on cost
than the addition of the BICs.
B. Redundancy
As previously discussed in Section I, reliability is a major
issue for battery energy storage. This discussion focuses on reliability of the different converters in terms of redundancy. The
MMLC with centralized battery energy storage does not have
increased redundancy compared to existing systems, but this is
not the case for the MMLC with distributed energy storage, nor
for the cascaded converter, which both subdivide the battery energy storage into shorter strings.
2093
A major advantage of the MMLC with distributed energy
storage over the cascaded converter is its flexible module redundancy, which further increases reliability. For the cascaded
converter utilized as part of a BESS, it has been shown by [28]
that power can be independently delivered to each submodule.
Reference [40] also demonstrated that a cascaded converter is
able to deliver limited active power when a single submodule of
each phase leg is connected to a dc source, but as of the writing
of this paper, operation of the cascaded converter with an arbitrary number of integrated sources is still an open research
question. Thus, redundant modules may be required to address
battery failures.
In contrast, the MMLC with distributed energy resources
offers more flexibility and redundancy than the cascaded
converter. Work presented in [27] and simulation results from
Section IV-C showed that submodules can operate with some
modules that do not provide real power flow without impacting
MMLC operation. The simulation results in Section IV-B also
demonstrated that power can be readily transferred between
phase legs through the dc link. Moreover, this topology has a
fixed dc link, which can allow the BESS to be integrated into
a dc network without any additional complexity. Thus, the
MMLC can offer redundancy and versatility beyond that of the
cascaded converter due to independent power delivery from
any submodule, and its capability of dc-link interconnection.
VI. CONCLUSION
This paper reviewed current converter topologies for BESSs,
and selected three topologies for detailed comparison studies.
These topologies are the MMLC with centralized battery energy
storage, the cascaded converter, and the MMLC with distributed
battery energy storage. The ideal power conversion system is
shown to be the MMLC with distributed battery energy storage.
This topology was chosen for three reasons. First, the converter,
similar to the cascaded converter, subdivides the battery string,
which increases reliability, but the MMLC with distributed battery energy storage also allows for more redundancy when compared to the cascaded converter. Second, an efficiency study was
conducted, and the MMLC with distributed energy storage was
found to be the most efficient with comparable cost compared
to the cascaded converter, which has the lowest cost. Finally,
the voltage rating of the MMLC with distributed energy storage
does not need to be overrated to account for the voltage variation of the batteries. This allows for the possibility of interconnecting the MMLC to a dc network while operating as a BESS;
a feature that the other two converters do not possess. In terms
of efficiency, reliability, and future applications beyond that of a
stand-alone BESS, the MMLC with distributed energy storage
offers the most versatile topology for future development.
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Theodore Soong (S’11) received the B.A.Sc. degree in engineering science and
the M.A.Sc. degree in electrical engineering from the University of Toronto,
Toronto, ON, Canada, in 2009 and 2012, respectively, where he is currently
pursuing the Ph.D. degree in electrical engineering.
Peter W. Lehn (SM’05) received the B.Sc. and M.Sc. degrees in electrical engineering from the University of Manitoba, Winnipeg, MB, Canada, in 1990
and 1992, respectively, and the Ph.D. degree in electrical engineering from the
University of Toronto, Toronto, ON, Canada, in 1999.
From 1992 to 1994, he was with the Network Planning Group of Siemens AG,
Erlangen, Germany. Currently, he is a Professor at the University of Toronto.