1-s2.0-S0306261921014161-main

Applied Energy 306 (2022) 118140
Contents lists available at ScienceDirect
Applied Energy
journal homepage: www.elsevier.com/locate/apenergy
Novel modular multilevel converter-based five-terminal MV/LV hybrid AC/
DC microgrids with improved operation capability under unbalanced
power distribution
Qian Xiao a, b, Yunfei Mu a, b, Hongjie Jia a, b, Yu Jin c, *, Xiaodan Yu a, b, *, Remus Teodorescu d,
Josep M. Guerrero d
a
Key Laboratory of Smart Grid of Ministry of Education, Tianjin University, Tianjin 300072, China
Key Laboratory of Smart Energy & Information Technology of Tianjin Municipality, Tianjin 300072, China
c
Department of Electrical Engineering and Automation, Harbin Institute of Technology, Harbin 150006, China
d
Department of Energy Technology, Aalborg University, Aalborg 9220, Denmark
b
H I G H L I G H T S
• The proposed topology has a more flexible interconnection with hybrid microgrids.
• The number of power switches is reduced compared with conventional MMC-based hybrid.
• Improved operation capability can be obtained under unbalanced power distribution in microgrids.
• A hierarchical energy control method is proposed with balanced capacitor voltages in MMC.
• The microgrids can operate normally at different operation modes.
A R T I C L E I N F O
A B S T R A C T
Keywords:
Multi-terminal microgrids
Hybrid AC/DC microgrids
Modular multilevel converter
Unbalanced power distribution
Energy control
Conventionally, the multilevel converter-based multi-terminal hybrid microgrids require a large number of
power switches and have a limited operation capability under unbalanced power distribution in medium and low
voltage (MV/LV) AC/DC microgrids. To solve this issue, this paper proposes the novel modular multilevel
converter (MMC)-based five-terminal MV/LV hybrid AC/DC microgrids. The proposed hybrid microgrids realize
the interconnection between the medium-voltage AC (MVAC), MVDC, low voltage AC (LVAC), and two LVDC
terminals. In addition, the MVAC grid is connected to the AC terminal of MMC, and the MVDC microgrid is
connected to the DC terminal of MMC through a dual active bridge (DAB) converter. Based on MMC, the compact
interlinking converters are established, providing three LVDC terminals, which are connected to two LVDC
microgrids and one LVAC microgrid through a DC/AC converter. Compared with the conventional MMC-based
hybrid microgrids, the proposed topology can significantly reduce the number of power switches. Moreover, to
overcome the control challenge of arm energy balancing in MMC and meet the requirement of different operation
modes in microgrids, a hierarchical energy control method is proposed, where the low circulating currents are
injected to balance the arm energy. Therefore, the system operation capability can be improved under unbal­
anced power distribution. Validation results in different conditions (power step, power reversal, and unbalanced
MVAC voltages) indicate that by the proposed method, the arm energy and capacitor voltages in MMC are well
balanced, and the proposed hybrid AC/DC microgrids can operate normally at different modes.
* Corresponding authors.
E-mail addresses: [email protected] (Q. Xiao), [email protected] (Y. Mu), [email protected] (H. Jia), [email protected] (Y. Jin), [email protected]
(X. Yu), [email protected] (R. Teodorescu), [email protected] (J.M. Guerrero).
https://doi.org/10.1016/j.apenergy.2021.118140
Received 29 July 2021; Received in revised form 8 October 2021; Accepted 24 October 2021
Available online 20 November 2021
0306-2619/© 2021 Elsevier Ltd. All rights reserved.
Q. Xiao et al.
Applied Energy 306 (2022) 118140
1. Introduction
(such as wind power and PV) and energy storage systems (ESSs) into DC
systems rather than AC systems [7]. On the other hand, a vast demand
for DC power is raised by modern loads like variable-speed drives for
elevators [8]. In addition, electric vehicles (PEVs) also appear as a
crucial modern load in future DC distribution systems, and extensive
novel electronic loads (such as high-quality and highly-efficient DC
lighting systems) have already been integrated into modern houses. The
increasing DC sources and DC loads provide a strong motivation to shift
the mainstream AC microgrids to DC or hybrid microgrids. However, the
widespread AC power system promotes the hybrid microgrids concept as
the preferable candidate, considering its compatibility [9].
In general, the hybrid microgrids are mainly designed for lowvoltage applications, where the AC terminal is connected to the lowvoltage AC (LVAC) microgrids, and the DC terminal is connected to
the low-voltage DC (LVDC) microgrids [10,11]. The AC terminal and DC
terminal are interconnected by bidirectional AC/DC power converters,
and the AC and DC DGs and loads can be connected to the corresponding
terminal. However, to connect the medium-voltage AC (MVAC) grid, the
bulky and volume-occupying line-frequency transformers are usually
necessary.
With the development of remote area mine sites [12] and DC electric
ships [13], the medium-voltage DC (MVDC) microgrid is gaining more
attention in various applications [14]. Therefore, multiple intercon­
nection schemes have been proposed to realize power conversion be­
tween the MVDC and the LVDC system [15]. In addition, with the
increase of electrical power consumption, the efficiency of the power
interchange can be greatly improved by the integration of the MVAC
grid. Therefore, it is necessary to realize the direct power integration
between the MVAC grid and MVDC microgrids through multilevel
converter technology [16]. Considering the DC microgrids can better
consume the renewable DGs, and the AC appliance still accounts for the
major part of power loads, it is necessary to connect the MVDC terminal
to the MVAC terminal in the hybrid microgrids together with LVDC and
LVAC microgrids.
1.1. Backgrounds and motivation
For the past two decades, the proportion of electricity in global en­
ergy consumption increase from 17% in 2000 to 22% in 2018 [1]. With
the rapid growth of the economy and deterioration of the environment,
gas emission reduction and carbon neutrality are becoming important
global recognition [2]. The electricity, especially that generated by clean
energy, is gaining much more popularity, of which the electric hybrid
vehicles is a good example. It is estimated that by 2050, the growth rate
of electricity consumption will reach 80% to 90% [3], and it will rapidly
increase the demand for renewable clean energy [4].
The microgrid is an effective solution for the integration and con­
sumption of renewable energy. Conventionally, the AC microgrids are
applied in low-voltage applications, where all distributed generations
(DGs) and loads are connected to the common AC bus, such as smart
buildings, military areas, and rural sectors [5,6]. However, in recent
years, the DC microgrid has been drawing much attention. On the one
hand, it is more economical to integrate renewable energy resources
1.2. Existing hybrid microgrids
Conventionally, the grid frequency transformer is applied to realize
the interconnection between the medium-voltage and low-voltage sys­
tems [17]. These transformers are usually bulky and heavy, occupying a
large volume. In addition, the high-power grid-frequency transformer
can be expensive [18]. Therefore, to realize flexible power conversion
and meet the above interfacing requirements of the multi-terminal
hybrid microgrids, multiple mainstream structures have been pro­
posed in the past decades. One of the popular topologies is motivated by
the cascaded H-bridge (CHB) converter-based power electronic trans­
former (PET) [19]. This topology realizes power conversion between
MVAC and LVDC terminals [20], as shown in Fig. 1. It uses a CHB
converter as the main structure, where the AC output terminal of the
CHB converter is connected to the MVAC grid. The dual active bridge
(DAB) converters are connected to each full-bridge (FB) submodule (SM)
in the CHB converter, and the output terminals of DABs are connected in
parallel to increase the power rating of the LVDC microgrid. The hybrid
microgrids connect the MVAC grid with the LVDC microgrid without a
grid frequency transformer. Moreover, to increase the flexibility of this
topology, a DC/AC converter is usually included to provide an addi­
tional terminal for the LVAC microgrid. It is noted that the LVAC grids
can be integrated into the single-phase and three-phase AC grids.
Therefore, the single-phase and three-phase DC/AC converters are
applied individually for different AC grids. In this interconnection
scheme, the power difference between LVDC and LVAC microgrids will
not be influenced by the power rating of the CHB converter, and the two
microgrids controllers can be designed independently. However, it lacks
the interconnection flexibility for more microgrids of different voltage
levels. The improved three-terminal hybrid AC/DC microgrids are pro­
posed in Fig. 2 [21]. As shown in the figure, the DAB converters are
Fig. 1. The CHB-based hybrid AC/DC microgrids in [20].
Fig. 2. The CHB-based multi-terminal hybrid AC/DC microgrids in [21].
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Applied Energy 306 (2022) 118140
connected to each FB SM. The DABs are divided into two groups, the
output terminals of DABs in each group are connected together, forming
the LVDC terminal for LVDC microgrid.
The topology in Fig. 1 includes one LVDC microgrid and can be
connected to one LVAC microgrid through an additional power trans­
formation stage (DC/AC converter). The topology in Fig. 2 provides two
different LVDC microgrids, and it can also provide an additional LVAC
terminal through an additional DC/AC converter after LVDC microgrid1 or LVDC microgrid-2. In addition, the DC/AC converter can be con­
nected to an LVDC microgrid with a closer voltage level to improve the
efficiency of power conversion. However, the above CHB-based hybrid
microgrids cannot integrate the MVDC microgrid.
A modular multilevel converter (MMC)-based PET is analyzed in
[22], where each half-bridge (HB) SM is connected to a DAB converter,
and the output terminals of the DABs are paralleled connected to an
LVAC microgrid through an DC/AC converter, as shown in Fig. 3. It
realizes power conversion between the MVAC grid, MVDC microgrid,
and LVAC microgrid, which improves the interconnection adaptability
of the multi-terminal hybrid microgrids. Reference [23] analyzes the
improved MMC-based hybrid AC/DC microgrids, which is shown in
Fig. 4, where each SM is connected to a DAB converter. The output
terminals of DAB converters are connected to an LVDC microgrid or an
LVAC microgrid through DC/ AC converter. It integrates a number of
LVDC and LVAC terminals, together with an MVAC terminal and an
MVDC terminal. However, this topology has a great control challenge
under unbalanced power distribution in microgrids, which limits its
application.
To improve the system operation capability under unbalanced power
distribution, the MMC-based hybrid microgrids are proposed in [24], as
shown in Fig. 5. The DAB converters are connected to each SM, and the
output terminals of DABs in the upper arm are connected together to an
LVDC microgrid. The output terminals of DABs in the lower arm are
connected to another LVDC microgrid. This topology realizes the
interconnection between the MVAC grid, MVDC microgrid, and two
different LVDC microgrids. However, the system operation capability is
still limited under unbalanced power distribution among LVDC micro­
grids. In addition, the control methods in the above papers only discuss
the conditions where the microgrids operate at voltage source mode.
The grid-connected operation of microgrids needs to be discussed.
To further enhance the interconnection flexibility and improve the
system operation capability under unbalanced power distribution in
LVAC and LVDC microgrids, this paper proposes the novel MMC-based
five-terminal MV/LV hybrid AC/DC microgrids and a novel energy
control method. The main contributions and innovations of this paper
can be listed as follows.
Fig. 3. The MMC-based hybrid AC/DC microgrids in [22].
Fig. 4. The MMC-based multi-terminal hybrid AC/DC microgrids in [23].
(1) The proposed hybrid microgrids can improve the interconnection
flexibility of the power system. It can realize flexible intercon­
nection and power support between the MVAC grid, MVDC
microgrid, LVAC microgrid, and two LVDC microgrids. These, in
return, will strengthen the mutual energy support between the
different microgrids and increase the consumption of clean
energy.
(2) Compared with the conventional MMC-based hybrid microgrids,
the proposed hybrid microgrids can significantly reduce the
number of required power switches and lower the system cost for
energy integration.
(3) With the proposed hybrid microgrids topology and hierarchical
energy control method, the required circulating current injection
for arm energy balancing is lower. Therefore, the operation
capability under unbalanced power distribution can be greatly
improved.
(4) With the proposed hierarchical energy control method, the
MVDC microgrid, LVAC microgrid, and LVDC microgrids can
work at different operation modes, whether the grid-connected
operation mode or the voltage-source operation mode.
Fig. 5. The MMC-based four-terminal hybrid AC/DC microgrids in [24].
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Applied Energy 306 (2022) 118140
Fig. 6. The proposed MMC-based five-terminal MV/LV hybrid AC/DC microgrids.
Table 1
Advantages and disadvantages of the existing multilevel converter-based hybrid microgrids.
Items
Topologies
Advantages
Fig. 1 [20]
CHB-PET-based
Fig. 2 [21]
CHB-based
1. No control challenge under unbalanced power distribution.
Fig. 3 [22]
MMC-PET-based
Fig. 4 [23]
MMC-based
Fig. 5 [24]
MMC-based
1.
2.
1.
2.
1.
2.
Disadvantages
1. Realize interconnection between two different LVDC microgrids.
No control challenge under unbalanced power distribution;
Provide the MVDC terminal.
A number of microgrids can be integrated;
Provide the MVDC terminal.
improved operation capability under unbalanced power distribution;
MVDC microgrid is integrated
The rest of the paper is organized as follows. The topology and hi­
erarchical control structure of the proposed hybrid microgrids are pre­
sented in Section 2. The proposed energy control method is introduced
in detail in Section 3. Section 4 analyzes the operation capability of
proposed hybrid microgrids and provides the design principle of MMC.
In Section 5, the simulation model of the proposed five-terminal hybrid
microgrids is established, and simulations under various conditions are
conducted to validate the feasibility and advantages of the proposed
operation scheme. The conclusions are listed in Section 6.
1.
2.
1.
2.
3.
1.
Only two microgrids (one LVDC, one LVAC microgrid) are integrated;
No MVDC terminal.
Limited operation capability under unbalanced power distribution;
No MVDC terminal;
Limited to voltage source operation mode.
Only one LVAC microgrid is integrated.
1. Highly limited operation capability under unbalanced power distribution.
1. High circulating current injection under unbalanced power distribution;
2. Limited to MVDC and LVDC voltage source operation mode.
j = a, b, c; k = 1, 2, 3, 4). Four SMs and an arm inductor Larm are included
in each arm. There are two arms in each phase, defined as the upper arm
and the lower arm according to their position. One side of the upper and
lower arms are connected together as the AC output terminal of MMC,
and the other sides of the upper and lower arms are connected to the
positive and the negative poles of the DC bus in MMC. The AC terminal
of MMC, T1, can be directly connected to the MVAC grid through a filter
inductor L, without grid-frequency transformers. The DC terminal of
MMC is connected to a DAB converter, providing an MVDC terminal T2
and flexible MVDC voltage for the MVDC microgrid.
To further connect the LVDC and LVAC microgrids, three groups of
compact interlinking converters are established based on MMC, and they
are described as follows. In the compact interlinking converters, the
capacitor of each HB-SM is connected with the full-bridge high-fre­
quency transformer (FB-HFT). In the FB-HFT, the FB converter is used to
provide suitable input voltage for the high-frequency transformer. The
output ports of FB-HFTs in the same phase are then connected together
to another FB converter, forming an LVDC terminal. The abovementioned HB-SMs, FB-HFTs, and an additional FB converter in the
same phase make up the compact interlinking converters, providing an
2. The topology and control structure of the proposed hybrid
AC/DC microgrids
2.1. The topology of the proposed hybrid microgrids
Fig. 6 shows the topology of the proposed five-terminal MV/LV
hybrid AC/DC microgrids, which is established based on MMC. MMC is
composed of 24 half-bridge (HB) submodules (SMs). Each HB-SM in­
cludes two IGBTs, two parallel-connected diodes, and one capacitor, as
shown in Fig. 3. The SM capacitor voltages are defined as vSMxjk (x = u, l;
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Table 2
Detailed comparisons between the proposed and the existing hybrid microgrids.
Topologies N ¼ 4
Items
Fig. 1 [20]
CHB-PET-based
Fig. 2 [21]
CHB-based
Fig. 3 [22]
MMC-PET-based
Fig. 4 [23]
MMC-based
Fig. 5 [24]
MMC-based
Fig. 6
Proposed
MVDC microgrid
No. of the original LV ports
No. of the LV microgrids
Additional power conversion
Operation capability under unbalanced power
IGBTs in the interlinking converters
No
1
2
Yes
High
144 + 4
No
2
2
No
Medium
144
Yes
1
1
Yes
High
240 +4
Yes
24
24
No
Low
240 + 4x
(x: No. of LVAC)
Yes
2
2
No
Medium
240
Yes
3
3
One LV DC/AC
High
156 þ 14
Fig. 7. The hierarchical control structure of the proposed hybrid microgrids.
hybrid microgrids, there are one MVDC terminal, one LVAC terminal,
and two LVDC terminals. It has an improved operation capability under
unbalanced power distribution. In addition, there are 156 IGBTs in the
compact interlinking converters (including MMC, FB-HFT, and FB con­
verter), and the number of IGBTs can be greatly reduced.
Based on the above analysis, the characteristics and advantages of
the proposed five-terminal MV/LV hybrid AC/DC microgrids are sum­
marized as follows.
LVDC terminal. Two LVDC terminals, T3 and T4, are connected to the
LVDC microgrids directly. The other LVDC terminal is converted into the
LVAC terminal, T5, through a DC/AC converter and then connected to an
LVAC microgrid. It is noted that the two LVDC microgrids are designed
with two different DC voltages, providing higher flexibility for power
interchange. Each LVDC terminal provides the LVDC voltage according
to the requirement of the LVDC or LVAC microgrids.
By the above interconnection scheme, the MVAC grid, MVDC
microgrid, LVAC microgrid, and two LVDC microgrids can be integrated,
and the microgrids are supposed to operate at either the grid-connected
mode or the voltage source mode. It is noted that the proposed inter­
connection scheme can be applied to both the single-phase and threephase LVAC microgrid. The energy control with a single-phase micro­
grid is more challenging due to the higher voltage ripples in the DC side.
Therefore, this paper takes the single-phase LVAC microgrid as an
example.
(1) The proposed five-terminal hybrid microgrids provide higher
flexibility for the power interchange system: The MVDC terminal
can realize the power exchange with the MVDC microgrid,
absorbing power from the MVDC microgrid or releasing power to
the MVDC loads such as mining site and DC ship. Two different
LVDC microgrids can be integrated with different DC bus volt­
ages. It can meet the variable DC voltage demands from different
types of DC loads, and the power supply reliability can be
improved in case of failures in the LDVC microgrids. In addition,
the LVAC microgrid can also be integrated, obtaining higher
compatibility for the conventional LVAC power systems.
(2) Compared with the conventional hybrid microgrids, the MVAC
grid can be directly connected. The bulky and expensive gridfrequency transformer is not necessary. Compared with the
MMC-based hybrid microgrids [22–24], the number of power
switches can be reduced, and the system cost can be lowered.
(3) Compared with the conventional hybrid microgrids and the
multilevel converter PET-based hybrid microgrids [20,22], the
LVDC and LVAC microgrids are naturally isolated through the FBHFTs, and they can be controlled independently. Therefore, a
more compact and efficient system can be obtained.
(4) When the system operates under unbalanced power distribution
in LVDC and LVAC microgrids, low circulating currents are
injected to balance the active power. Therefore, the operation
capability under unbalanced power distribution can be improved.
2.2. The characteristics of the proposed hybrid microgrids
The conventional hybrid microgrids are usually designed based on
the grid-frequency transformers, which are applied to connect the
MVAC grid with the low-voltage microgrids. These transformers are
usually bulky and volume-occupying, and the price could be high for
high-power applications. The multilevel converter (CHB converter or
MMC)-based hybrid microgrids usually lack interconnection flexibility
or have a limited operation capability under unbalanced power distri­
bution in microgrids, especially among the LVDC and LVAC microgrids.
The advantage and disadvantages of the existing hybrid microgrids are
demonstrated in Table 1. More detailed comparisons with the proposed
hybrid microgrids are shown in Table 2. For the CHB-PET-based hybrid
microgrids in [20], and the CHB-based hybrid microgrids in [21], there
is no MVDC terminal, and there are two low-voltage microgrids. In
addition, there are 144 IGBTs in the CHB and DAB converters. For the
MMC-PET-based hybrid microgrids [22] and the MMC-based hybrid
microgrids in [23] and [24], there is an MVDC terminal in the system. In
addition, there are 240 IGBTs in the MMC and DAB converters. How­
ever, the 24 low-voltage microgrids in [23] lead to a limited operation
capability under unbalanced power distribution. For the proposed
2.3. The control challenges in the proposed system
Although the proposed five-terminal hybrid microgrids have the
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Applied Energy 306 (2022) 118140
Fig. 8. The overall diagram of the converters control.
above advantages, the following control challenges still exist, especially
under unbalanced power distribution in LVDC and LVAC microgrids.
control method is proposed in this paper. To obtain fast dynamic
response and symmetric MVAC currents, the power management layer is
designed with feedforward power regulation and references genera­
tions. To realize balancing control in MMC, the circulating currents are
injected to balance arm energy, and the modulation reference in each
SM is adjusted to balance the individual capacitor voltages. To realize
different modes of operation, system controllers for the microgrids are
designed individually.
(1) The system is supposed to have a fast dynamic response under
power step or power reversal changes in microgrids, and sym­
metric MVAC currents are expected in the MVAC grid, even under
unbalanced MVAC grid voltages.
(2) The arm energy and HB-SM capacitor voltages in MMC are sup­
posed to maintain balanced, and the voltages in the microgrids
are supposed to stabilize at the rated values, even under unbal­
anced power distribution in the LVDC and the LVAC microgrids.
(3) The MVDC microgrid, LVDC microgrids, and LVAC microgrid
should be able to work at different operation modes, either the
grid-connected mode or the voltage-source mode.
2.4. The hierarchical control structure
The hierarchical structure of the proposed control method is shown
in Fig. 7, which includes the power management and the converters
control.
The power management in this paper features the operation mode
selection (the grid-connected operation mode or voltage source operation
Therefore, to realize the above control targets, a hierarchical energy
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Applied Energy 306 (2022) 118140
mode) and the control reference calculation. It is designed to provide a
faster dynamic response through feedforward control in the proposed
five-terminal hybrid microgrids. Firstly, it receives information from the
load dispatching center, including the operation modes and the trans­
mitted power references of the MVDC microgrid, two LVDC microgrids,
and LVAC microgrid (P*MVDC, P*LVDC1, P*LVDC2, and P*LVAC). Then, to
obtain a faster response and avoid voltage and current surge during the
operation condition changes in microgrids, references of the AC output
currents (i*gj) and the DC component of the circulating currents
(i*dca, i*dcb, i*dcc) are calculated based on the power distribution in the
hybrid microgrids.
The converters control includes the power control in the MMC, FBHFTs, FB converters, and DAB converters. For the MMC control, it re­
ceives AC output current references and DC circulating current refer­
ences. In addition, the internal arm energy and SM capacitor voltage also
need to be controlled in the MMC. For MVDC microgrid and LVDC
microgrids, the interlinking converters are controlled to generate the
specific power (P*MVDC, P*LVDC1, P*LVDC2) or rated DC voltage according
to their references (V*MVDC, V*LVDC1, V*LVDC2). For LVAC microgrid, the
interlinking converters of the DC side are controlled to generate the DC
voltage according to their references (V*LVAC_DC), and the DC/AC con­
verter is controlled to generate the specific AC power (P*LVAC) or rated
voltage according to its reference value (V*LVAC).
fluctuation during power step.
3. The proposed hierarchical energy control method
Req = R + Rarm /2, Leq = L + Larm /2
3.2. Modeling and control of MMC
1) MMC modeling
To realize the precise control of MMC, the simplified control model
of MMC is analyzed, which can be divided into the AC control loop and
the DC control loop. The AC control loop controls the output current,
and the DC control loop controls the circulating current [25].
For the AC control loop, the control model can be expressed as
⎧
dij
⎪
⎪
⎨ Req ij + Leq = uj − vgj
dt
(4)
⎪
u − uuj
⎪
⎩ ij = iuj − ilj , uj = lj
2
where ij (j = a, b, c) is the output current of MMC; iuj and ilj are the arm
current of the upper and lower arms; vgj is the grid voltage; uuj and ulj are
the output voltages of the upper and lower arms; uj is the equivalent
output voltage of MMC. Leq and Req are the equivalent inductance and
the equivalent resistance in the AC control loop, which can be expressed
as
For the DC control loop, the control model can be expressed as
⎧
dicirj
⎪
⎪
= Vdc − ulj − uuj = − 2ucirj
⎨ 2Rarm icirj + 2Larm
dt
(6)
⎪
(iuj + ilj )
⎪
⎩
,
icirj =
2
The power management (power regulation and reference genera­
tion) in Fig. 7 is discussed in Section 3.1. The overall diagram of the
converters control is shown in Fig. 8. It introduces the control principles
of the interlinking converters, including the MMC, the DAB converter in
the MVDC microgrid, the FB-HFTs, and FB converters in the LVDC
microgrids, the FB-HFTs, FB converter, and DC/AC converters in the
LVAC microgrids. More details are as follow.
where icirj is the circulating current; Vdc is the DC voltage of the MMC;
ucirj is the voltage reference for the circulating current controller.
With the above control model, the MMC controller can be designed.
3.1. Power management
2) MMC control
To realize the power regulation and obtain a faster dynamic response
in the hybrid microgrids, the power references of each interlinking
converter need to be calculated. Based on the information from the
central load dispatching center, the operation mode and transferred
active and reactive power references of each interlinking converter can
be obtained.
For MMC, the active power can be calculated by the operation con­
dition of each microgrid, while the reactive power references will be
provided by the MVAC grid. With the positive active power defined as
from MMC to the MVAC grid, or from MMC to the microgrids, the power
references can be expressed as
P*MVAC = − P*MVDC − P*LVDC1 − P*LVDC2 − P*LVAC
(5)
The overall control diagram of the MMC is shown in Fig. 8(a) and
Fig. 8(b), which includes the AC output current control and the DC
circulating current control.
For the AC output current control, references of the active and
reactive currents are calculated firstly, based on the power references
and the sum of all capacitor voltages. In the proposed five-terminal
hybrid microgrids, the reactive current reference of MMC is deter­
mined by the requirement of the MVAC grid. The active current refer­
ence is decided by both the system power regulation and the sum
capacitor voltages in MMC. It is noted that the reference calculated by
system power regulation works as the feedforward control component.
In Fig. 8(a), the sum of all capacitor voltages in MMC are realized by
absorbing or releasing active currents from the MVAC grid. Therefore,
the active and the reactive current references can be expressed as
(
)
⎧
)
2P*
KiAC1_MMC (
⎪
⎪ i*d = MVAC + KpAC1_MMC +
vSMsum − 24v*SM
⎪
⎨
3Vg
s
(7)
⎪
⎪
2Q*MVAC
⎪
*
⎩
iq =
3Vg
(1)
Supposing the AC output currents of MMC are symmetric, the active
AC power flow in each phase can be expressed as
/
(2)
P*ACa = P*ACb = P*ACc = P*MVAC 3
With the calculated power references and active power flow in each
microgrid, the DC component of the circulating currents can be derived
as
)/
(
P*
P*
⎧ *
idca = − MVAC − MVDC + P*LVDC1
Vdc
⎪
⎪
3
3
⎪
⎪
⎪
⎪
⎪
)/
(
⎨
P*
P*
(3)
Vdc
i*dcb = − MVAC − MVDC + P*LVDC2
⎪
3
3
⎪
⎪
⎪
⎪
⎪
)/
(
⎪
⎩ *
P*
P*
Vdc
idcc = − MVAC − MVDC + P*LVAC
3
3
where KpAC1_MMC and KiAC1_MMC are the control parameters of the
proportional-integral (PI) controller; vSMujk and vSMljk are the capacitor
voltages in the upper arm and the lower arm; v*SM is the capacitor
voltage reference; vSMsum is the sum of all capacitor voltages in MMC.
j=a,b,c;
∑
vSMsum =
(
vSMujk + vSMljk
)
(8)
k=1,..N;
Then, the AC output current controller is designed to control the AC
Thus, the references of MMC can be calculated to reduce the power
7
Q. Xiao et al.
Applied Energy 306 (2022) 118140
⎧ *
icirj
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎨
i*cirj
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎩
dc
1st
(
)
)
KiDC1_MMC (vSMsum
KpDC1_MMC +
− vSMuj − vSMlj
s
3
(
)
)
KiDC2_MMC (
= KpDC2 MMC +
0 − vSMlj + vSMuj cosθj
s
(
)
)
KiDC3_MMC ( *
i*dc = KpDC3 MMC +
Vdc − Vdc
s
=
(11)
where Vdc and V*dc are the DC voltage of MMC and its reference value;
vSMuj and vSMlj are the sum capacitor voltage in the upper and lower
arms; θj is the phase angle of each phase; KpDC1_MMC, KiDC1_MMC,
KpDC2_MMC, KiDC2_MMC, KpDC3_MMC, and KiDC3_MMC are the PI control pa­
rameters of the arm voltage controller.
⎧
j=a,b,c;
∑
⎪
⎪
vSMuj =
vSMujk
⎪
⎨
Fig. 9. The arm output voltage reference calculation and modulation scheme.
k=1,..N;
j=a,b,c;
⎪
∑
⎪
⎪
⎩ vSMlj =
vSMljk
(12)
k=1,..N;
With the calculated circulating current references, the capacitor
voltages can be balanced between each arm. In addition, the capacitor
voltage balancing within each arm can be realized by modulation
reference modification in each SM. The circulating current control is
designed to track the circulating current reference and suppress the
inherent second-order circulating current in MMC. Therefore, the PIR
circulating current controller can be designed based on [26].
)
(
ucirj = i*cirj − icirj ×
(
)
KiDC4_MMC
KR1_MMC ω0 s
2KR2_MMC ω0 s
KpDC4_MMC +
+ 2
+ 2
2
2
s
s + ω0 s + ω0 s + 2ω0 s + (2ω0 )
where ω0 is the grid angular frequency. In this paper, the grid frequency
is 50 Hz, and ω0 = 100π; KpDC4_MMC, KiDC4_MMC, KR1_MMC, and KR2_MMC
are the PI and resonant control parameters of the circulating current
controller.
Fig. 10. Transmission power model of DAB.
3) Arm output voltage reference calculation and modulation
output current of MMC according to their references. Decoupled PI
controllers are applied in the dq coordinate, and the final output voltage
references in the dq coordinate can be expressed as
(
)
⎧
)
KiAC2_MMC ( *
⎪
u
=
K
+
id − id − ωLeq iq + vgd
d
pAC2_MMC
⎪
⎨
s
(9)
(
)
)
⎪
⎪
KiAC2_MMC ( *
⎩u = K
iq − iq + ωLeq id + vgq
q
pAC2 MMC +
s
The modulation scheme is designed to realize arm output voltage
generation and SM capacitor voltage balancing. The detailed modula­
tion scheme is shown in Fig. 9. The carrier phase-shifted (CPS) pulse
width modulation (PWM) technique is applied in this paper.
With the calculated voltage references uj and ucirj, the arm output
voltage references in the upper and lower arms can be derived as
⎧
Vdc
⎪
⎪
− uj + ucirj
⎨ uuj =
2
(14)
⎪
⎪
⎩ ulj = Vdc + uj + ucirj
2
where ud and uq are the AC output voltage references of the MMC;
KpAC2_MMC and KiAC2_MMC are the control parameters of the AC output
current controller; id and iq are d-axis and q-axis components of the AC
output currents in MMC; i*d and i*q are the reference values of id and iq;
vgd and vgq are d-axis and q-axis components of the MVAC grid voltages.
For the circulating current control, their references can be calculated
based on the power regulation layer control and the arm capacitor
voltages.
i*cirj = i*cirj
dc
+ i*dcj + i*cirj
1st
+ i*dc
(13)
For individual voltage balancing, the SM modulation references are
modified with an additional adjustment to the arm modulation refer­
ences. According to [27], the final modulation signals can be expressed
as
⎧
uxjk = uxj + sgn(ixj )⋅Kpind (v*SM − vSMxjk )
⎪
⎨
(15)
⎪
⎩ v* = Vdc (x = u, l; j = a, b, c; k = 1, 2, 3, 4)
SM
4
(10)
where i*cirj_dc is the DC component reference used to balance the
capacitor voltages between each phase; i*cirj_1st is the reference of the
fundamental frequency component used to balance the capacitor volt­
ages between the upper and lower arms; i*dcj is the feedforward
component reference generated by power regulation in Fig. 7; i*dc is the
DC component reference used to control the DC voltage of MMC. They
can be further expressed as
where uxj is the arm modulation reference; uxjk is the final modulation
reference of each SM; Kpind is the control parameter of the proportional
controller; ixj is the arm current. The sgn function can be described as
⎧
⎨ 1, ixj > 0
0, ixj = 0
sgn(ixj ) =
(16)
⎩
− 1, ixj < 0
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Applied Energy 306 (2022) 118140
In this paper, the dual active bridge (DAB) converter is used as the
DC/DC converter for the MVDC microgrid, and the single phase-shift
control method is applied to the DAB converter.
When the MVDC microgrid operates in grid-connected mode, the
input and output voltages of the interlinking DAB converter are fixed
and clamped by the MVDC bus of MMC and the MVDC grid. Therefore,
the main control target of this operation mode is to control the active
power flow between the input port and the output port. For a conven­
tional DAB controller, the transferred active power can be illustrated in
Fig. 10. In Fig. 10, nT is the transformation ratio; VHV is the input DC
voltage of DAB; VLV is the output DC voltage of DAB; fs is the switching
frequency of DAB; Lp is the power transferring inductor of DAB; PDAB is
the transferred power of DAB; D is the phase shift angle. More detailed
information can be found in [28]. It is noted that the maximum trans­
ferred power of the MVDC microgrid can be expressed as
Fig. 11. The control principle of DAB.
PDABmax =
nT VHV VLV
8fs Lp
(17)
Equation (17) indicates that the maximum transferred power of DAB
depends on the power transferring inductor. Therefore, the power
transferring inductor should be carefully designed to meet the power
transmission requirement of the DAB converter. Usually, when the DAB
reaches its rated power, the phase shift angle D should be designed at
about 0.3 to 0.45 [29].
According to Fig. 10, the transferred power of DAB is decided by the
phase-shift angle D, and the phase shift angle is positive correlation or
negative correlation under each quarter operation range. In this paper,
only the shadowed area in Fig. 10 is utilized [30], where D ∈ [-0.5, 0.5].
Therefore, the phase shift angle is in positive correlation to the trans­
ferred power of DAB. The PI controller can thus be applied to control the
transferred active power in DAB. With the feedforward component of
the phase-shift angle, the MVDC microgrid acquires a faster dynamic
response and smaller fluctuation during the power step. The phase-shift
angle can be derived by the following control equation.
(
)
(
)
)
Ki1_MVDC ( *
DMVDC = FMVDC P*MVDC + Kp1_MVDC +
PMVDC − PMVDC
s
(18)
where DMVDC is the phase shift angle of the MVDC microgrid;
FMVDC(P*MVDC) is the power function of transferred power to describe
Fig. 10; Kp1_MVDC and Ki1_MVDC are the control parameters of the trans­
ferred power controller in the MVDC microgrid.
Therefore, the DAB converter in the MVDC microgrid can be
controlled under the grid-connected operation mode.
Fig. 12. The operation region of the proposed hybrid microgrids under
different MVAC power. (a) The operation region under hMVAC = 1. (b) The
operation region under hMVAC = 0.5. (c) The operation region under hMVAC = 0.
(d) The operation region under hMVAC = -0.5. (e) The operation region under
hMVAC = -1.
2) Voltage source operation mode
When the MVDC microgrid works in the voltage source operation
mode, the control target of the interlinking converter is the MVDC
output voltage. Under this condition, the phase shift angle can be
calculated based on the MVDC output voltage.
(
)
)
Ki2 MVDC ( *
DMVDC = Kp2 MVDC +
(19)
VMVDC − VMVDC
s
When the MVDC output voltage is lower than its reference value, the
phase shift angle of DAB increases. When the MVDC output voltage is
higher than its reference value, the phase shift angle decreases. With the
derived phase shift angle, the MVDC output voltage can be controlled.
Based on the above analysis, the output signal of DAB can be shown
in Fig. 11, where Ts is the switching period; uHV is the input voltage
waveform of the transformer; uLV is the output voltage waveform of the
transformer; D is the phase shift angle.
Fig. 13. The current waveform of the upper arm in phase a.
3.3. MVDC microgrid control
The MVDC microgrid is designed to operate under two modes, the
grid-connected operation mode, and the voltage source operation mode.
In this paper, the two operation modes are discussed individually.
1) MVDC grid-connected operation mode
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Applied Energy 306 (2022) 118140
Table 3
Simulation parameters.
Table 4
Control parameters of the proposed hierarchical control method.
Table 5
Simulation scenarios under normal operation.
Scenarios
1
2
3
4
5
Time
0 s ~ 0.5 s
0.5 s ~ 1.0 s
1.0 s ~ 1.5 s
1.5 s ~ 2.0 s
2.0 s ~ 2.5 s
MVAC
grid voltage
Balanced
Balanced
Balanced
0.5 sag in phase C
0.5 sag in phase C
MVDC
LVDC-1
LVDC-2
grid-connected
voltage source
grid-connected
300 kW
300 kW
300 kW
300 kW
− 300 kW
50 kW
100 kW
100 kW
100 kW
100 kW
50 kW
50 kW
100 kW
100 kW
100 kW
3.4. LVDC microgrid-1 and LVDC microgrid-2 control
LVAC
grid-connected
100 kW
100 kW
100 kW
100 kW
100 kW
connected to HB SM, and they are parallel-connected to a FB con­
verter of the LVDC microgrid. Therefore, the rated power of each FBHFT is one-eighth of the rated power in the LVDC microgrid. Corre­
spondingly, the power transferring inductor can be selected based on the
The control principle of the LVDC microgrids is similar to that in the
MVDC microgrid. However, it is noticed that there are eight FB-HFTs
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Applied Energy 306 (2022) 118140
Fig. 14. Simulation results of the modular multilevel converter from scenario 1 to scenario 2 with a single-phase LVAC microgrid.
rated power of each FB-HFT. In addition, to make sure the equal power
distribution in these FB-HFTs and the connected HB SMs, their switching
signals are assigned with the same PWM signal of 50% duty cycle.
Then, the switching signals of the output FB converter can be derived
based on the calculated phase shift angle in different operation modes.
microgrid-2; FLVDC1,2(P*LVDC1,2/8) is the power function of transferred
power in LVDC microgrid-1 or LVDC microgrid-2; Kp1_LVDC and Ki1_LVDC
are the control parameters of the transferred power controller in LVDC
microgrids; PLVDC1,2 and P*LVDC1,2 are the transferred power in LVDC
microgrid-1 or LVDC microgrid-2 and its reference value.
Based on equation (20), the phase shift angle of the output FB con­
verter in LVDC microgrid-1 or LVDC microgrid-2 can be calculated.
1) LVDC grid-connected operation mode
When the LVDC microgrid works in grid-connected operation mode,
the phase shift angle can be calculated based on the transferred active
power reference.
( *
) (
)
)
PLVDC1,2
Ki1 LVDC ( *
DLVDC1,2 =FLVDC1,2
+ Kp1 LVDC +
PLVDC1,2 − PLVDC1,2
8
s
(20)
2) Voltage source operation mode
When the LVDC microgrid works in the voltage source operation
mode, the control target of the interlinking converter is the LVDC output
voltage. Under this condition, the phase shift angle can be calculated
based on the LVDC output voltage.
(
)
)
Ki2_LVDC ( *
DLVDC1,2 = Kp2_LVDC +
(21)
VMVDC1,2 − VMVDC1,2
s
where DLVDC1,2 is the phase shift angle of LVDC microgrid-1 or LVDC
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Applied Energy 306 (2022) 118140
Fig. 15. Simulation results of the LVDC microgrids and single-phase LVAC microgrid from scenario 1 to scenario 2.
where VLVDC1,2 and V*LVDC1,2 are the output voltage in LVDC-1 or LVDC2 microgrid and its reference value; Kp2_LVDC and Ki2_LVDC are the control
parameters of the output voltage controller in LVDC microgrids.
The control principle under voltage source operation mode is similar
to that in the MVDC microgrid, and it will not be further discussed.
(
DLVAC =
KpDC
LVAC
+
KiDC
)
LVAC
s
(
*
VDC
LVAC
− VDC
)
LVAC
(22)
where DLVDC is the phase shift angle of the FB converter in the LVAC
microgrid; KpDC_LVAC and KiDC_LVAC are the control parameters of the DC
bus voltage controller in the LVAC microgrid; VDC_LVAC and V*DC_LVAC are
the DC bus voltage in the LVAC microgrid and its reference value.
Then, the DC/AC converter can be controlled according to its oper­
ation mode.
3.5. LVAC microgrid control
For the LVAC microgrid, there is a single-phase DC/AC interlinking
converter connected to the FB output converter. For the FB-HFTs and FB
converter, the control target is to regulate the DC output voltage to the
rated value. The control method and the switching signal distribution
are the same as the LVDC microgrids, where the FB-HFTs share the same
PWM signal with a 50% duty cycle to obtain equal power among the FBHFTs and their connected HB SMs.
The phase shift angle of the FB converter is calculated based on the
DC bus voltage in the LVAC microgrid interlinking converter.
1) LVAC grid-connected operation mode
As mentioned before, the LVAC grid can be categorized into the
single-phase AC grid or the three-phase AC grid. Therefore, when the
LVAC microgrid operates in grid-connected mode, the control strategy
should be designed individually. This paper mainly focuses on the
single-phase AC grid application. However, to keep the uniform control
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Applied Energy 306 (2022) 118140
Fig. 16. Simulation results of the modular multilevel converter from scenario 2 to scenario 3 with a single-phase LVAC microgrid.
structure and simplify the controller design process, this paper uses the
decoupled PI controller in the virtual dq frame to control the DC/AC
controller.
Firstly, the active power and reactive power required by the AC
microgrid are used to calculate the active and reactive current references
as
⎧
3P*LVAC
*
⎪
⎪
⎪ idLVAC = 2V ,
⎨
g
⎪
⎪
3Q*LVAC
⎪
⎩ i*
qLVAC =
2Vg
The AC output current can be further transformed into virtual dq
frame as
{
idLVAC = iαLVAC cos(ωt) + iβLVAC sin(ωt)
(25)
iqLVAC = − iαLVAC sin(ωt) + iβLVAC cos(ωt)
With the calculated output currents in the virtual dq frame, the
conventional decoupled PI controllers are used to control the active and
reactive currents, which can be expressed as
(
)
⎧
)
KiAC_LVAC ( *
⎪
idLVAC − idLVAC − ωLac iqLVAC + vdLVAC
⎪ udLVAC = KpAC_LVAC +
⎨
s
(
)
)
⎪
⎪
KiAC_LVAC ( *
⎩u
iqLVAC − iqLVAC + ωLac idLVAC + vqLVAC
qLVAC = KpAC LVAC +
s
(26)
(23)
Then, the single-phase AC output current can be transformed into αβ
frame as
(
)
T
iαLVAC = iLVAC (t), iβLVAC = iLVAC t −
(24)
4
where udLVAC and uqLVAC are output voltage references of the DC/AC
converter on the virtual d-axis and q-axis; KpAC_LVAC and KiAC_LVAC are the
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Applied Energy 306 (2022) 118140
Fig. 17. Simulation results of the LVDC microgrids and single-phase LVAC microgrid from scenario 2 to scenario 3.
control parameters of the AC output current controller in LVAC micro­
grid; Lac is the AC filter inductor in the LVAC microgrid; idLVAC and iqLVAC
are virtual d-axis and q-axis components of the AC output currents in
LVAC microgrid; i*dLVAC and i*qLVAC are the reference values of idLVAC
and iqLVAC; vdLVAC and vqLVAC are the LVAC grid voltages on the virtual daxis and q-axis, which can also be calculated based on the trans­
formation for virtual dq frame as shown in (24) and (25).
Next, the output voltage references in the virtual dq frame are
transformed back into αβ frame
{
uαLVAC = udLVAC cos(ωt) − uqLVAC sin(ωt)
(27)
uβLVAC = udLVAC sin(ωt) + uqLVAC cos(ωt)
If the LVAC microgrid operates in the voltage source mode, the
output voltage of the DC/AC converter is fixed. In this circumstance,
only a modulation scheme is required in the single-phase DC/AC con­
verter. Therefore, with the output voltage reference uLVAC, the final
PWM signals of the single-phase DC/AC converter can be obtained.
4. Discussion
In this section, the operation region of the proposed hybrid micro­
grids is analyzed, and it is compared with the other MMC-based topol­
ogies. Then, the design principle of MMC is discussed
The voltage reference in the α-axis, uαLVAC, will be the final modu­
lation reference of the single-phase DC/AC converter, as shown in Fig. 8.
More detailed information can be found in [31], and it will not be
further discussed in this paper.
2) Voltage source operation mode
4.1. Operation region analysis of the proposed hybrid microgrids
In the proposed five-terminal hybrid microgrids, each microgrid is
designed based on its rated power. Supposing the rated MVAC power is
PratedMVAC; the rated MVDC power is PratedMVDC; the rated LVDC-1 power
14
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Applied Energy 306 (2022) 118140
Fig. 18. Simulation results of the modular multilevel converter from scenario 3 to scenario 4 with a single-phase LVAC microgrid.
is PratedLVDC1; the rated LVDC-2 power is PratedLVDC2; the rated LVAC
power is PratedLVAC.
Here, the maximum system power is defined as Pmax, the rated power
of each microgrid in this paper can be configured as
{
⃒ rated ⃒ ⃒ rated ⃒
⃒ = ⃒P
⃒ = HMV Pmax
⃒P
⃒ rated ⃒MVAC⃒ rated MVDC
⃒ ⃒ rated ⃒
(28)
⃒P
⃒ = ⃒P
⃒ = ⃒P
⃒ = HLV Pmax
LVDC1
LVDC2
PMVDC = − PMVAC − PLVDC1 − PLVDC2 − PLVAC
(30)
The above operation equation can be further expressed as
hMVDC Pmax = − (hMVAC + hLVDC1 + hLVDC2 + hLVAC )Pmax
(31)
where hMVDC is the operating power coefficient of the MVDC microgrid;
hMVAC is the operating power coefficient of the MVAC grid; hLVDC1 is the
operating power coefficient of LVDC microgrid-1; hLVDC2 is the operating
power coefficient of LVDC microgrid-2; hLVAC is the operating power
coefficient of LVAC microgrid.
It is noted that the power flow in each microgrid can be either pos­
itive or negative, and the above parameters meet the following
constraint
{
∀(hMVDC , hMVAC ) ∈ [ − HMV , HMV ]
(32)
∀(hLVDC1 , hLVDC2 , hLVAC ) ∈ [ − HLV , HLV ]
LVAC
where HMV and HLV are the rated power coefficients for the mediumvoltage and low-voltage microgrids. They are supposed to meet the
following constraints
{
HMV ∈ [0, 1]
(29)
HLV ∈ [0, 1]
Usually, HMV and HLV are selected based on the operation data of the
microgrids. In this paper, they are designed as HMV = 1 and HLV = 0.5 for
simplicity. Then, the operating power of the MVDC microgrid should be
15
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Applied Energy 306 (2022) 118140
Fig. 19. Simulation results of the LVDC microgrids and single-phase LVAC microgrid from scenario 3 to scenario 4.
When the MVAC grid operates at the different power conditions,
equation (31) can be further expressed as
hMVDC = − (hMVAC + hLVDC1 + hLVDC2 + hLVAC )
microgrid is zero, hMVAC = 0, the operation region of the low-voltage
microgrids reaches its maximum boundary.
(33)
4.2. Operation capability comparisons under unbalanced power
distribution
Based on the rated power of MVDC, LVDC, and LVAC microgrid,
hMVDC, hLVDC1, hLVDC2, and hMVAC should meet hMVDC ∈ [-1, 1] and
∀(hLVDC1, hLVDC2, hMVAC) ∈ [-0.5, 0.5]. The operation region of the lowvoltage microgrids under different MVAC power can be shown in
Fig. 12.
As shown in Fig. 12, if the operating powers of the MVAC grid are
opposite, the low-voltage microgrids have the same operation region but
in the opposite direction. When the absolute operating power of the
MVAC grid is equal to its rated value, hMVAC = 1 or hMVAC = -1, the LV
microgrids have the smallest operation region. When the absolute
operating power of the MVAC microgrid is half of its rated value, hMVAC
= 0.5 or hMVAC = -0.5, the operation region of the low-voltage micro­
grids grows higher. When the absolute operating power of the MVAC
The operation ability under unbalanced distributed power is
compared between the proposed hybrid microgrids and two other
topologies.
For the hybrid microgrids in Fig. 4, each SM in MMC is connected to a
DC microgrid through a DAB converter. In this topology, the main re­
striction for unbalanced power distribution is the modulation differ­
ences (or the maximum transferred power) between two SMs in the same
arm. Supposing the amplitude of the MVAC grid is Iac, and the active
power flows among the three phases are asymmetry (which means the
injected circulating currents should be 0). Taking two SMs in the upper
arm in phase a as examples, the arm current can be expressed as
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Applied Energy 306 (2022) 118140
Fig. 20. Simulation results of the modular multilevel converter from scenario 4 to scenario 5 with a single-phase LVAC microgrid.
iua =
iga
Iac
= −
cosθ
2
2
Usually, the modulation references should be within the linear
modulation region. Therefore, the maximum transferred power between
two SMs can be expressed as
(
)
0.5Vdc − Vg Iac
(37)
ΔPmaxMG = max(PSMm − PSMn ) =
2
(34)
Supposing the voltage drops across the inductors and the circulating
current control references are small enough to be neglected, the arm
modulation references will be approximted to
1 vga
u*ua = −
2 Vdc
(35)
where Vg is the amplitude of the MVAC grid voltage.
The maximum transferred power can be further expressed as
If the microgrids connected to the mth and nth SM consume the largest
and the smallest active power, the average active power flow in SMm and
SMn can be expressed as
( ∫
)/
⎧
udc
⎪
⎪ PSMm =
T
u*uam iua dt
⎪
⎨
4
(36)
( ∫
)/
⎪
⎪
udc
⎪
⎩ PSMn =
T
u*uan iua dt
4
ΔPmaxMG1 = max(PSMm − PSMn ) =
(1 − m)Vdc Iac
4
(38)
where m is the modulation index in MMC.
For the hybrid microgrids in Fig. 5, the restriction for the unbalanced
power distribution is the circulating current injection. Supposing the
maximum injected circulating current is Icir_max, the maximum power
exchange between the upper and lower arms can be expressed as
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Applied Energy 306 (2022) 118140
Fig. 21. Simulation results of the LVDC microgrids and single-phase LVAC microgrid from scenario 4 to scenario 5.
∫
ΔPmaxarm =
(vga Icir
max cosθ)dt
T
=
Vg Icir
2
max
=
mVdc Icir
4
max
twice the rated AC output current, which means the maximum ampli­
tude of the circulating current can be higher than Iac.
(39)
For the proposed hybrid microgrids, the maximum transferred power
between two LVDC microgrids can be expressed as
ΔPmaxMG2 = 3ΔPmaxarm
3Vg Icir
=
2
max
3mVdc Icir
=
4
max
Icir
max , icirb
= − Icir
max , icirc
=0
(40)
ΔPmaxMG3 = 2Vdc Icir
max
max
> ΔPmaxMG2 =
> ΔPmaxMG1 =
3mVdc Icir
4
(1 − m)Vdc Iac
4
max
(44)
Therefore, the proposed hybrid microgrids have improved operation
capability under unbalanced power distribution in the AC/DC
microgrids.
(41)
On this condition, the maximum transferred power should be
ΔPmaxMG3 = 2Vdc Icir
(43)
> Iac
The modulation index m is usually selected from 0.85 to 1.
Comparing the equations in (38, 40), and (42), it is obvious that
For the proposed hybrid microgrids in Fig. 6, if the maximum
circulating current injection is Icir_max, the maximum transferred power
between two low-voltage microgrids appears when the circulating cur­
rent injection references are
icira = Icir
max
(42)
According to [32], the rated current of the power module is about
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Fig. 22. Simulation results of the modular multilevel converter from scenario 3 to additional 10 kW negative-sequence active power in the three-phase
LVAC microgrid.
4.3. Design of the MMC
expressed as
With the confirmed rated power in each microgrid, the MMC can be
designed accordingly. Taking the upper arm in phase a as an example,
the arm current should be
i*ua = (−
i*ua =
i*a *
+i
2 cira
(45)
P*MVAC P*MVDC
−
+ P*LVDC1 )/Vdc
3
3
(47)
Considering Vg < 0.5Vdc, the upper arm current waveform in phase a
can be shown in Fig. 13. Therefore, it can be concluded that the
maximum arm current should be about
When the system is in stable operation condition, the circulating
current only includes the DC component in equation (3)
i*cira = i*dca = (−
1
1
1
0.5
cosωt +
+
+
)Pmax
3Vg
3Vdc 3Vdc Vdc
i*ua
max
1
7
=(
+
)Pmax
3Vg 6Vdc
(48)
The capacity of the arm current should be designed based on this
result. In addition, the maximum power flow of each SM is expressed as
(46)
The maximum power flow in phase a should include the condition
when hMVAC = -1, hMVADC = -1, hLVDC1 = 0.5. Supposing only active
power are transferred in the MVAC grid, the arm current can be
P*SM
max
=
Pmax Vdc 1
7
+
)
(
3Vg 6Vdc
N
With the above analysis, the MMC can be designed.
19
(49)
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Fig. 23. Simulation results of the LVDC microgrids and three-phase LVAC microgrid from scenario 3 to additional 10 kW negative-sequence active power in the
LVAC microgrid.
[24].
Table 6
Simulation scenarios under system fault.
Fault type
Fault time
Fault conditions
Figure number
Fault 1
Fault 2
Fault 3
0.5 s
0.5 s
0.5 s
MVDC microgrid fault
LVDC microgrid-1 fault
LVAC microgrid fault
Fig. 24 and Fig. 25
Fig. 26 and Fig. 27
Fig. 28 and Fig. 29
5.1. Operation of the proposed scheme with single-phase LVAC microgid
To verify the effectiveness of the proposed hybrid microgrids and the
hierarchical energy control method, a series of simulations are con­
ducted, and the simulation scenarios are listed in Table 5.
The simulation results of the proposed hybrid microgrids from
operation scenario 1 to operation scenario 2 are shown in Fig. 14 and
Fig. 15. The active power of LVDC-microgrid-1 increases from 50 kW to
100 kW at 0.5 s under this operation condition.
Fig. 14 shows the waveforms of the MMC. The MVAC grid voltages
are shown in Fig. 14(a), whose amplitudes are about 2450 V. The am­
plitudes of MVAC output currents increase from about 136 A to about
150 A at 0.5 s, as shown in Fig. 14(b). The circulating currents of MMC
are shown in Fig. 14(c). From 0.3 s to 0.5 s, the active power of LVDC
5. Simulation results
To verify the effectiveness of the proposed hybrid microgrids and
energy control method, the five-terminal MV/LV hybrid AC/DC micro­
grids are built in the MATLAB/SIMULINK simulation environment. The
simulation model is established based on the proposed topology in
Fig. 6. The circuit parameters are given in Table 3, and the control pa­
rameters are given in Table 4. These parameters are selected based on
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Fig. 24. Simulation results of the modular multilevel converter under fault type 1.
microgrid-1 and LVDC microgrid-2 are the same, and the circulating
currents in phase a and phase b are the same. When the power step oc­
curs, the active powers in LVDC microgrid-1 and LVAC microgrid are the
same, and the circulating currents in phase a and phase c are the same.
The three-phase SM capacitor voltages are shown in Fig. 14(d) to Fig. 14
(f), where the average values remain stable at about 1500 V. The voltage
and current of the DC side are shown in Fig. 14(g) and Fig. 14(h), which
are about 6 kV and 50 A.
Fig. 15 shows the waveforms of the microgrids. The MVDC microgrid
voltage and current are 5 kV and 60 A, as shown in Fig. 15(a) and Fig. 15
(b). The voltage of LVDC microgrid-1 is stable at 1 kV, and its current
increase from 50 A to 100 A at 0.5 s, as shown in Fig. 15(c) and Fig. 15
(d). The voltage and current of LVDC microgrid-2 are stable at 750 V and
about 66.7 A, as shown in Fig. 15(e) and Fig. 15(f). Fig. 15(g) and Fig. 15
(h) show the DC side voltage and current of the LVAC microgrid, where
the voltage stabilizes at about 750 V and the current stabilizes at about
133.3 A. Fig. 15(i) and Fig. 15(j) show the AC voltage and output current
of the LVAC microgrid, where the amplitude of the voltage is about 311
V and the amplitude of the current is about 643 A. It indicates that the
active power of the LVAC microgrid remains at about 100 kW during the
whole operation condition. The above results verify the effectiveness of
the proposed hybrid microgrids and the hierarchical control method
under power increase in the voltage source operation mode in LVDC
microgrid-1.
The simulation results of the proposed hybrid microgrids from
operation scenario 2 to operation scenario 3 are shown in Fig. 16 and
Fig. 17. The active power of LVDC-microgrid-2 increases from 50 kW to
100 kW at 1.0 s under this operation condition.
The waveforms of MMC are shown in Fig. 16. The MVAC grid volt­
ages are shown in Fig. 16(a), the amplitudes of which are about 2450 V.
The amplitudes of MVAC output currents increase from about 150 A to
about 163 A at 1.0 s in Fig. 16(b). The circulating currents of MMC are
shown in Fig. 16(c). From 0.8 s to 1.0 s, the active power flows of LVDC
microgrid-1 and LVAC microgrid are the same, and the circulating
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Fig. 25. Simulation results of the hybrid AC/DC microgrids under fault type 1.
currents in phase a and phase c are the same. After 1.0 s, the active
power in LVDC microgrid-2 becomes the same as the other microgrids,
and the circulating currents in the three phases are the same. The SM
capacitor voltages in three phases are shown in Fig. 16(d) to Fig. 16(f),
where the average values remain at about 1500 V. The DC side voltage
and current of MMC are shown in Fig. 16(g) and Fig. 16(h), and their
values are about 6 kV and 50 A.
The waveforms of the microgrids are shown in Fig. 17. The MVDC
microgrid voltage and current are 5 kV and 60 A as shown in Fig. 17(a)
and Fig. 17(b). The voltage of LVDC microgrid-1 is stable at 1 kV, and its
current is stable at 100 A, as shown in Fig. 17(c) and Fig. 17(d). The
voltage of LVDC microgrid-2 stabilizes at 750 V, and its current increases
from about 66.7 A to about 133.3 A, as shown in Fig. 17(e) and Fig. 17
(f). Fig. 17(g) and Fig. 17(h) show the DC side voltage and current of the
LVAC microgrid, where the voltage stabilizes at about 750 V and the
current current stabilizes at about 133.3 A. Fig. 17(i) and Fig. 17(j) show
the AC voltage and output current of the LVAC microgrid, where the
amplitude of the voltage is about 311 V and the amplitude of the current
is about 643 A. It indicates that the active power of the LVAC microgrid
remains at about 100 kW during the whole operation condition. The
above simulation results verify the effectiveness of the proposed hybrid
microgrids and the hierarchical control method under power increase in
grid-connected operation mode in LVDC microgrid-2.
The simulation results of the proposed hybrid microgrids from
operation scenario 3 to operation scenario 4 are shown in Fig. 18 and
Fig. 19. The grid voltage in phase c drops by about 50% at 1.5 s under
this operation condition.
Fig. 18 shows the waveforms of the MMC. The MVAC grid voltages
are shown in Fig. 18(a), and their amplitudes are about 2450 V. The
amplitudes of MVAC output currents increase from about 163 A to about
195.6 A at 1.5 s in Fig. 18(b), due to the voltage drop in phase c. The
circulating currents of MMC are shown in Fig. 18(c). From 1.3 s to 1.5 s,
the active powers in the LVDC and LVAC microgrids are the same, and
the circulating currents are also the same. At 1.5 s, the voltage drop
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Fig. 26. Simulation results of the modular multilevel converter under fault type 2.
occurs in phase c, and the circulating current in phase c becomes
different from those in other phases. The SM capacitor voltages in three
phases are shown in Fig. 18(d) to Fig. 18(f), where the average values
are about 1500 V, but they witness some fluctuation during the dynamic
transition process. The DC side voltage and current are about 6 kV and
50 A, as shown in Fig. 18(g) and Fig. 18(h).
The waveforms of the microgrids are shown in Fig. 19. The MVDC
microgrid voltage and current are about 5 kV and 60 A as shown in
Fig. 19(a) and Fig. 19(b). The voltage of LVDC microgrid-1 is stable at 1
kV, and its current is stable at 100 A, as shown in Fig. 19(c) and Fig. 19
(d). The voltage of LVDC microgrid-2 is stable at 750 V, and its current is
stable at about 133.3 A, as shown in Fig. 19(e) and Fig. 19(f). Fig. 19(g)
and Fig. 19(h) show the DC side voltage and current of the LVAC
microgrid, where the voltage stabilizes at about 750 V and the current
stabilizes at about 133.3 A. Fig. 19(i) and Fig. 19(j) show the AC voltage
and output current of the LVAC microgrid, where the amplitude of the
voltage is about 311 V and the amplitude of the current is about 643 A. It
indicates that the active power of the LVAC microgrid remains at about
100 kW during the whole operation condition. The above simulation
results verify the effectiveness of the proposed hybrid microgrids and the
hierarchical control method under unbalanced grid voltage operation
conditions.
The simulation results of the proposed hybrid microgrids from
operation scenario 4 to operation scenario 5 are shown in Fig. 20 and
Fig. 21. The operation power in MVDC reverse at 2.0 s under this
condition.
Fig. 20 shows the waveforms of the MMC. The MVAC grid voltages
are shown in Fig. 20(a), the amplitudes of which are about 2450 V. The
amplitudes of MVAC output currents decrease from about 195 A to 0 at
2.0 s in Fig. 20(b) due to the balanced power distribution between
MVDC, LVDC, and LVAC microgrids. The circulating currents of MMC
are shown in Fig. 20(c). From 1.8 s to 2.0 s, the circulating current in
phase c is higher than those in other phases due to the voltage sag in
phase c. At 2.0 s, the internal power flows between MVDC, LVDC, and
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Fig. 27. Simulation results of the hybrid AC/DC microgrids under fault type 2.
LVAC microgrids become balanced, and the transferred power in the
MVAC grid becomes 0. Therefore the circulating currents in three phases
become balanced again even under unbalanced MVAC grid voltages.
The SM capacitor voltages in three phases are shown in Fig. 20(d) to
Fig. 20(f), where the average values remain at about 1500 V. However,
the capacitor voltage ripples drop significantly due to the reduced
MVAC output currents. The DC side voltage is shown in Fig. 20(g), the
value of which is about 6 kV. The DC circulating current reverses from
50 A to − 50 A at 2.0 s, as shown in Fig. 20(h).
Fig. 21 shows the waveforms of the microgrids. The MVDC microgrid
voltage is 5 kV, and its current reverses from 60 A to − 60 A as shown in
Fig. 21(a) and Fig. 21(b). The voltage and current of LVDC microgrid-1
are stable at 1 kV and 100 A, as shown in Fig. 21(c) and Fig. 21(d). The
voltage of LVDC microgrid-2 is stable at 750 V, and its current is stable at
about 133.3 A, as shown in Fig. 21(e) and Fig. 21(f). Fig. 21(g) and
Fig. 21(h) show the DC side voltage and current of the LVAC microgrid,
where the voltage stabilizes at about 750 V and the current stabilizes at
about 133.3 A. Fig. 21(i) and Fig. 21(j) show the AC voltage and output
current of the LVAC microgrid, where the amplitude of the voltage is
about 311 V and the amplitude of the current is about 643 A. It indicates
that the active power of the LVAC microgrid remains at about 100 kW
during the whole operation condition. The above simulation results
verify the effectiveness of the proposed hybrid microgrids and the hi­
erarchical control method under power reverse operation conditions in
the MVDC microgrid.
5.2. Feasibility verification of the proposed scheme with three-phase
LVAC microgrid
To verify the feasibility of the proposed system interconnection
scheme and its hierarchical control method, the simulation is further
verified with the three-phase LVAC microgrid. It should be mentioned
that the control for the three-phase LVAC microgrid should be the same
control structure in the dq frame, as discussed in [33].
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Fig. 28. Simulation results of the modular multilevel converter under fault type 3.
The hybrid microgrids are integrated with a three-phase LVAC
microgrid. Firstly, before 0.5 s, the system operates in scenario 3 in
Table III. Then, at about 0.5 s, 10 kW additional negative-sequence
active power is injected in the three-phase LVAC microgrid. In this
operation condition, the grid voltage is balanced; the active power of the
MVDC microgrid is 300 kW; the active power of the two LVDC micro­
grids is 100 kW. The simulation results are shown in Fig. 22 and Fig. 23.
Fig. 22 shows the waveforms of the MMC. The MVAC grid voltages
are shown in Fig. 22(a), and their amplitudes are about 2450 V. The
amplitudes of MVAC output currents are about 163 A before 0.5 s, as
shown in Fig. 22(b). The amplitudes only increase slightly when the
negative-sequence active power is injected in the LVAC microgrid. The
circulating currents of MMC are shown in Fig. 22(c). Since the active
power of LVDC microgrid-1, LVDC microgrid-2, and LVAC microgrid are
almost the same, the circulating currents in the three phases are also
almost the same. The SM capacitor voltages in three phases are shown in
Fig. 22(d) to Fig. 22(f), where the average values remain stable at about
1500 V during the whole operation process. The voltage and current of
the DC side are shown in Fig. 22(g) and Fig. 22(h), which are about 6 kV
and 50 A.
Fig. 23 shows the waveforms of the microgrids. The MVDC microgrid
voltage and current are 5 kV and 60 A, as shown in Fig. 23(a) and Fig. 23
(b). The voltage of LVDC microgrid-1 is stable at 1 kV, and its current
stabilizes at about 100 A, as shown in Fig. 23(c) and Fig. 23(d). The
voltage and current of LVDC microgrid-2 are stable at 750 V and about
66.7 A, as shown in Fig. 23(e) and Fig. 23(f). Fig. 23(g) and Fig. 23(h)
show the DC side voltage and current of the LVAC microgrid. During the
whole operation process, the average value of the DC side voltage sta­
bilizes at about 750 V, and the average value of the DC current stabilizes
at about 133.3 A. However, when the negative-sequence active power is
injected, both the voltage and the current begin to fluctuate. Fig. 23(i)
shows the voltages of the LVAC microgrid, where the amplitudes are
about 311 V. Fig. 23(j) shows the output currents of the LVAC microgrid.
Before 0.5 s, the amplitudes of the currents are about 214 A. After 0.5 s,
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Fig. 29. Simulation results of the hybrid AC/DC microgrids under fault type 3.
the amplitude in phase a increases to about 237 A, and the amplitudes in
phase b and phase c decrease to about 202 A. The above results verify the
effectiveness of the feasibility of the proposed scheme with a three-phase
LVAC microgrid.
are shown in Fig. 24(a), and their amplitudes are about 2450 V. The
amplitudes of the output currents are about 163 A before the fault oc­
curs, as shown in Fig. 24(b). When the fault occurs, the amplitudes of
output currents decrease to about 81 A. The circulating currents of the
MMC are shown in Fig. 24(c). When the fault occurs, the circulating
current values decrease to 0 due to the disconnection of the MVDC
microgrid. The three-phase SM capacitor voltages are shown in Fig. 24
(d) to Fig. 24(f), where the average values remain stable at about 1500
V. However, when the fault occurs, the capacitor voltage ripples tend to
decrease due to the reduced active power exchange. The voltage of the
DC side is shown in Fig. 24(g), and the value stabilizes at about 6 kV. The
current of the DC side is shown in Fig. 24(h), which decreases from
about 50 A to 0 when the fault occurs.
The waveforms of the microgrids are shown in Fig. 25. The MVDC
microgrid voltage is shown in Fig. 25(a), and the value remains stable at
about 5 kV. The MVDC microgrid current is shown in Fig. 25(b), which
decreases from about 60 A to 0 when the fault occurs. The voltage of
5.3. Operation ability under system fault
The fault ride-through operation ability is important to improve the
system’s reliability. The internal SM fault will be solved by the internal
control strategy of the interlinking converter. Therefore, this paper only
discusses the external fault condition. The open circuit and short circuit
fault are two different common faults. It is noted that no matter the open
circuit fault or the short circuit fault, the microgrid will be disconnected
from the system. The fault operation conditions are listed in Table 6.
The simulation results under fault type 1 (MVDC microgrid fault) are
shown in Fig. 24 and Fig. 25.
Fig. 24 shows the waveforms of the MMC. The MVAC grid voltages
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LVDC microgrid-1 is stable at 1 kV, and its current stabilizes at about
100 A, as shown in Fig. 25(c) and Fig. 25(d). The voltage and current of
LVDC microgrid-2 are stable at 750 V and about 133.3 A, as shown in
Fig. 25(e) and Fig. 25(f). Fig. 25(g) and Fig. 25(h) show the DC side
voltage and current of the LVAC microgrid, where the voltage stabilizes
at about 750 V and the current stabilize at about 133.3 A. Fig. 25(i) and
Fig. 25(j) show the AC voltage and output current of the LVAC micro­
grid, where the amplitude of the voltage is about 311 V and the
amplitude of the current is about 643 A. It indicates that the active
power of the LVAC microgrid remains at about 100 kW. The above re­
sults verify the effectiveness of the operation ability of the proposed
scheme under MVDC microgrid fault.
The simulation results under fault type 2 (LVDC microgrid-1 fault)
are shown in Fig. 26 and Fig. 27.
Fig. 26 shows the waveforms of the MMC. The MVAC grid voltages
are shown in Fig. 26(a), and their amplitudes are about 2450 V. When
fault occurs, the amplitudes of the output currents decrease from about
163 A to about 136 A, as shown in Fig. 26(b). The circulating currents of
the MMC are shown in Fig. 26(c). When the fault occurs, the circulating
currents in phase a decrease due to the reduced active power. The threephase SM capacitor voltages are shown in Fig. 26(d) to Fig. 26(f), where
the average values remain stable at about 1500 V. However, when the
fault occurs, the capacitor voltages fluctuate in phase a. The voltage and
current of the DC side are shown in Fig. 26(g) and Fig. 26(h), which are
stable at about 6 kV and 50 A.
The waveforms of the microgrids are shown in Fig. 27. The MVDC
microgrid voltage and current are about 5 kV and 60 A, as shown in
Fig. 27(a) and Fig. 27(b). The voltage of LVDC microgrid-1 is stable at 1
kV, as shown in Fig. 27(c). However, its current decreases from about
100 A to 0 due to the disconnection of LVDC microgrid-1, as shown in
Fig. 27(d). The voltage and current of LVDC microgrid-2 are stable at
750 V and about 133.3 A, as shown in Fig. 27(e) and Fig. 27(f). Fig. 27(g)
and Fig. 27(h) show the DC side voltage and current of the LVAC
microgrid, where the voltage stabilizes at about 750 V and the current
stabilizes at about 133.3 A. Fig. 27(i) and Fig. 27(j) show the AC voltage
and output current of the LVAC microgrid, where the amplitude of the
voltage is about 311 V and the amplitude of the current is about 643 A. It
indicates that the active power of the LVAC microgrid remains at about
100 kW. The above results verify the effectiveness of the operation
ability of the proposed scheme under LVDC microgrid-1 fault.
The simulation results under fault type 3 (LVAC microgrid fault) are
shown in Fig. 28 and Fig. 29.
Fig. 28 shows the waveforms of the MMC. The MVAC grid voltages
are shown in Fig. 28(a), and their amplitudes are about 2450 V. When
the fault occurs, the amplitudes of the output currents decrease from
about 163 A to about 136 A, as shown in Fig. 28(b). The circulating
currents of the MMC are shown in Fig. 28(c). When the fault occurs, the
circulating current in phase c decreases due to the reduced active power.
The three-phase SM capacitor voltages are shown in Fig. 28(d) to Fig. 28
(f), where the average values remain stable at about 1500 V. However,
when the fault occurs, the capacitor voltages fluctuate in phase c. The
voltage and current of the DC side are shown in Fig. 28(g) and Fig. 28(h),
which are stable at about 6 kV and 50 A.
The waveforms of the microgrids are shown in Fig. 29. The MVDC
microgrid voltage and current are about 5 kV and 60 A, as shown in
Fig. 29(a) and Fig. 29(b). The voltage of LVDC microgrid-1 is stable at 1
kV, and its current stabilizes at about 100 A, as shown in Fig. 29(c) and
Fig. 29(d). The voltage and current of LVDC microgrid-2 are stable at
750 V and about 133.3 A, as shown in Fig. 29(e) and Fig. 29(f). Fig. 29(g)
and Fig. 29(h) show the DC side voltage and current of the LVAC
microgrid, where the voltage stabilizes at about 750 V, but the current
decreases from about 133.3 A to 0 when the fault occurs. Fig. 29(i) and
Fig. 29(j) show the AC voltage and output current of the LVAC micro­
grid, where the amplitude of the voltage is about 311 V, but the
amplitude of the current decreases from about 643 A to 0 when the fault
occurs. The above results verify the effectiveness of the operation ability
of the proposed scheme under LVAC microgrid fault.
6. Conclusion
This paper proposes the novel MMC-based five-terminal MV/LV
hybrid AC/DC microgrids. It realizes the flexible bidirectional power
interchange between the MVAC grid, MVDC microgrid, LVAC microgrid,
and two LVDC microgrids, providing mutual power support. To balance
the arm energy and capacitor voltage in the MMC and realize different
modes of operation in microgrids, a hierarchical energy control method
is proposed with improved operation capability under unbalanced
power distribution in MV/LV AC/DC microgrids. Based on the analysis
and simulation results in this paper, the main conclusions are drawn as
follows.
• The proposed hybrid microgrids have a simplified structure and a
greatly reduced number of power switches compared with the
existing MMC-based hybrid microgrids.
• The proposed hybrid microgrids possess a high operation capability
under unbalanced power distribution in the MV/LV AC/DC micro­
grids due to the reduced circulating current injection.
• In various conditions (including under power step, power reversal,
and unbalanced MVAC grid voltages), the proposed control method
can effectively balance the arm energy and the capacitor voltages in
the MMC, and meet the requirement of different operation modes in
microgrids.
• The proposed hybrid microgrids and the hierarchical control method
possess the operation ability under different system faults (MVDC
microgrid fault, LVDC microgrid fault, and LVAC microgrid fault).
Despite the above advantages of proposed hybrid microgrids, the
wide application in practical engineering is still facing some challenges,
such as the massive data communication, measurement and control
signal transportation delay, optimization of system power flow, system
control stability, and fault ride-through operation strategies. These
challenges will be the main focus of our future research work.
CRediT authorship contribution statement
Qian Xiao: Conceptualization, Funding acquisition, Methodology,
Project administration, Writing –original draft. Yunfei Mu: Formal
analysis, Data curation, Writing – review & editing. Hongjie Jia: Data
curation, Writing – review & editing. Yu Jin: Investigation, Software,
Validation, Writing – review & editing. Xiaodan Yu: Resources, Visu­
alization, Writing – review & editing. Remus Teodoresc: Supervision,
Writing – review & editing. Josep M. Guerrero: Supervision, Writing –
review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation
of China (No. 52107121, U2066213), and China Postdoctoral Science
Foundation (No. 2020M680880).
References
[1] Najafzadeh M, Ahmadiahangar R, Husev O, Roasto I, Jalakas T, Blinov A. Recent
contributions, future prospects and limitations of interlinking converter control in
hybrid AC/DC microgrids. IEEE Access 2021;9:7960–84.
27
Q. Xiao et al.
Applied Energy 306 (2022) 118140
[18] Zou ZX, De Carne G, Buticchi G, Liserre M. Smart transformer-fed variable
frequency distribution grid. IEEE Trans Ind Electron 2018;65(1):749–59.
[19] Wang X, Liu J, Ouyang S, Xu T, Meng F, Song S, et al. Control and experiment of an
H-bridge-based three-phase three-stage modular power electronic transformer.
IEEE Trans Power Electron 2016;31(3):2002–11.
[20] Wang L, Zhang D, Wang Yi, Wu B, Athab HS. Power and voltage balance control of
a novel three-phase solid-state transformer using multilevel cascaded H-bridge
inverters for microgrid applications. IEEE Trans Ind Electron 2016;31(4):
3289–301.
[21] Jia H, Xiao Q, He J. An improved grid current and DC capacitor voltage balancing
method for three-terminal hybrid AC/DC microgrid. IEEE Trans Smart Grid 2019;
10(6):5876–88.
[22] Briz F, Lopez M, Rodriguez A, Arias M. Modular power electronic transformers:
modular multilevel converter versus cascaded H-bridge solutions. IEEE Ind
Electron Mag 2016;10(4):6–19.
[23] Lachichi A, Junyent-Ferre A, Green TC. Comparative optimization design of a
modular multilevel converter tapping cells and a 2L-VSC for hybrid LVAC/DC
microgrids. IEEE Trans Ind App 2019;55(3):3228–40.
[24] Xiao Q, Mu Y, Jia H, Jin Yu, Hou K, Yu X, et al. Modular multilevel converter based
multi-terminal hybrid AC/DC microgrid with improved energy control method.
Appl Energy 2021;282:116154. https://doi.org/10.1016/j.apenergy.2020.116154.
[25] Jin Y, Xiao Q, Jia H, Mu Y, Ji Y, Teodorescu R, et al. A dual-layer back-stepping
control method for Lyapunov stability in modular multilevel converter based
STATCOM. IEEE Trans Ind Electron; early access.
[26] Sharifabadi K, Harnefors L, Nee H, Norrga S, Teodorescu R. Design, control, and
application of modular multilevel converters for HVDC transmission systems. 1st
ed. Hoboken, NJ, USA: Wiley; 2016.
[27] Hagiwara M, Akagi H. Control and experiment of pulse width modulated modular
multilevel converters. IEEE Trans Power Electron 2009;24(7):1737–46.
[28] Chen L, Shao S, Xiao Q, Tarisciotti L, Wheeler PW, Dragicevic T. Model predictive
control for dual-active-bridge converters supplying pulsed power loads in naval DC
microgrids. IEEE Trans on Power Electron 2020;35(2):1957–66.
[29] Sha D, Zhang J, Liu K. Leakage inductor current peak optimization for dualtransformer current-fed dual active bridge DC-DC converter with wide input and
output voltage range. IEEE Trans on Power Electron 2020;35(6):6012–24.
[30] Xiao Q, Chen L, Jin Yu, Mu Y, Cupertino AF, Jia H, et al. An improved fault-tolerant
control scheme for cascaded H-bridge STATCOM with higher attainable balanced
line-to-line voltages. IEEE Trans Ind Electron 2021;68(4):2784–97.
[31] Riverso S, Tucci M, Vasquez JC, Guerrero JM, Ferrari-Trecate G. Stabilizing plugand-play regulators and secondary coordinated control for AC islanded microgrids
with bus-connected topology. Appl Energy 2018;210:914–24.
[32] Tu Q, Li Y, Liu W, Huang M, Zeng G, Du B, et al. Arm overcurrent protection and
coordination in MMC-HVDC. IEEE Power Energy Soc General Meeting (PESGM)
2018:1–5.
[33] Xiao Q, Wang J, Jin Yu, Chen L, Jia H, Dragicevic T, et al. A novel operation
scheme for modular multilevel converter with enhanced ride-through capability of
submodule faults. IEEE J Emerg Sel Topics Power Electron 2021;9(2):1258–68.
[2] Ge L, Xian Y, Wang Z, Gao B, Chi F, Sun K. A GWO-GRNN based model for shortterm load forecasting of regional distribution network. CSEE J Power Energy Syst;
early access.
[3] Zhang X, Wang B, Gamage D, Ukil A. Model predictive and iterative learning
control based hybrid control method for hybrid energy storage system. IEEE Trans
Sustain Energy; early access.
[4] Ma Z, Gao F, Gu X, Li N, Wu Q, Wang X, et al. Multilayer SOH equalization scheme
for MMC battery energy storage system. IEEE Trans Power Electron 2020;35(12):
13514–27.
[5] Som S, De S, Chakrabarti S, Sahoo S, Ghosh A. A robust controller for battery
energy storage system of an islanded AC microgrid. IEEE Trans Ind Informat; early
access.
[6] Xing X, Li X, Gao F, Qin C, Zhang C. Improved space vector modulation technique
for neutral-point voltage oscillation and common mode voltage reduction in threelevel inverter. IEEE Trans Power Electron 2019;34(9):8697–714.
[7] Sahoo S, Mishra S. A distributed finite-time secondary average voltage regulation
and current sharing controller for DC microgrids. IEEE Trans Smart Grid 2019;10
(1):282–92.
[8] Abhinav S, Modares H, Lewis FL, Davoudi A. Resilient cooperative control of DC
microgrids. IEEE Trans Smart Grid 2019;10(1):1083–5.
[9] Chen S, Li P, Ji H, Yu H, Yan J, Wu J, et al. Operational flexibility of active
distribution networks with the potential from data centers. Appl Energy 2021;293:
116935. https://doi.org/10.1016/j.apenergy.2021.116935.
[10] Mi Y, Zhang H, Fu Y, Wang C, Loh PC, Wang P. Intelligent power sharing of DC
isolated microgrid based on fuzzy sliding mode droop control. IEEE Trans Smart
Grid 2019;10(3):2396–406.
[11] Wang R, Sun Q, Ma D, Liu Z. The small-signal stability analysis of the droopcontrolled converter in electromagnetic timescale. IEEE Trans Sustain Energy
2019;10(3):1459–69.
[12] Yuan C, Haj-ahmed MA, Illindala MS. Protection strategies for medium voltage
direct current microgrid at a remote area mine site. IEEE Trans Ind Appl 2015;51
(4):2846–53.
[13] Khan MMS, Faruque MO, Newaz A. Fuzzy logic based energy storage management
system for MVDC power system of all electric ship. IEEE Trans Energy Convers
2017;32(2):798–809.
[14] Saleh K, Hooshyar A, Saadany E. Ultra-high-speed travelling-wave-based
protection scheme for medium-voltage dc microgrids. IEEE Trans Smart Grid 2019;
10(2):1440–51.
[15] Liu C, Li X, Zhi Y, Cai G. New breed of solid-state transformer mainly combing
hybrid cascaded multilevel converter with resonant DC-DC converters. Appl
Energy 2018;210:724–36.
[16] Liu W, Liu C, Lin Y, Bai K, Ma L. Interval multi-objective optimal scheduling for
redundant residential microgrid with VESS. IEEE Access 2019;7:87849–65.
[17] Xu T, Gao F, Hao T, Meng X, Ma Z, Zhang C, et al. Two-layer global synchronous
pulse width modulation method for attenuating circulating leakage current in PV
station. IEEE Trans Ind Electron 2018;65(10):8005–17.
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