1-s2.0-S0306261921014161-main

Applied Energy 306 (2022) 118140
Available online 20 November 2021
0306-2619/© 2021 Elsevier Ltd. All rights reserved.
Novel modular multilevel converter-based ve-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
b
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
HIGHLIGHTS
•The proposed topology has a more exible 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.
ARTICLE INFO
Keywords:
Multi-terminal microgrids
Hybrid AC/DC microgrids
Modular multilevel converter
Unbalanced power distribution
Energy control
ABSTRACT
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 ve-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 signicantly 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).
Contents lists available at ScienceDirect
Applied Energy
journal homepage: www.elsevier.com/locate/apenergy
https://doi.org/10.1016/j.apenergy.2021.118140
Received 29 July 2021; Received in revised form 8 October 2021; Accepted 24 October 2021

Applied Energy 306 (2022) 118140
2
1. Introduction
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
(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-efcient 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 low-
voltage applications, where the AC terminal is connected to the low-
voltage 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 efciency 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.
1.2. 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 exible 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 exibility 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 inuenced by the power rating of the CHB converter, and the two
microgrids controllers can be designed independently. However, it lacks
the interconnection exibility 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 gure, 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
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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 microgrid-
1 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
efciency 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 exibility and improve the
system operation capability under unbalanced power distribution in
LVAC and LVDC microgrids, this paper proposes the novel MMC-based
ve-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.
(1) The proposed hybrid microgrids can improve the interconnection
exibility of the power system. It can realize exible 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 signicantly 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. 3. The MMC-based hybrid AC/DC microgrids in [22].
Fig. 4. The MMC-based multi-terminal hybrid AC/DC microgrids in [23].
Fig. 5. The MMC-based four-terminal hybrid AC/DC microgrids in [24].
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Applied Energy 306 (2022) 118140
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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 ve-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.
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 ve-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 dened as v
SMxjk
(x =u, l;
j =a, b, c; k =1, 2, 3, 4). Four SMs and an arm inductor L
arm
are included
in each arm. There are two arms in each phase, dened 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, T
1
, can be directly connected to the MVAC grid through a lter
inductor L, without grid-frequency transformers. The DC terminal of
MMC is connected to a DAB converter, providing an MVDC terminal T
2
and exible 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 above-
mentioned HB-SMs, FB-HFTs, and an additional FB converter in the
same phase make up the compact interlinking converters, providing an
Fig. 6. The proposed MMC-based ve-terminal MV/LV hybrid AC/DC microgrids.
Table 1
Advantages and disadvantages of the existing multilevel converter-based hybrid microgrids.
Items
Topologies
Advantages Disadvantages
Fig. 1 [20]
CHB-PET-based
1. No control challenge under unbalanced power distribution. 1. Only two microgrids (one LVDC, one LVAC microgrid) are integrated;
2. No MVDC terminal.
Fig. 2 [21]
CHB-based
1. Realize interconnection between two different LVDC microgrids. 1. Limited operation capability under unbalanced power distribution;
2. No MVDC terminal;
3. Limited to voltage source operation mode.
Fig. 3 [22]
MMC-PET-based
1. No control challenge under unbalanced power distribution;
2. Provide the MVDC terminal.
1. Only one LVAC microgrid is integrated.
Fig. 4 [23]
MMC-based
1. A number of microgrids can be integrated;
2. Provide the MVDC terminal.
1. Highly limited operation capability under unbalanced power distribution.
Fig. 5 [24]
MMC-based
1. improved operation capability under unbalanced power distribution;
2. MVDC microgrid is integrated
1. High circulating current injection under unbalanced power distribution;
2. Limited to MVDC and LVDC voltage source operation mode.
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Applied Energy 306 (2022) 118140
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LVDC terminal. Two LVDC terminals, T
3
and T
4
, are connected to the
LVDC microgrids directly. The other LVDC terminal is converted into the
LVAC terminal, T
5
, 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 exibility 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 three-
phase 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.
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 exibility
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
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 ve-terminal MV/LV hybrid AC/DC microgrids are sum-
marized as follows.
(1) The proposed ve-terminal hybrid microgrids provide higher
exibility 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 grid-
frequency 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 FB-
HFTs, and they can be controlled independently. Therefore, a
more compact and efcient 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.
2.3. The control challenges in the proposed system
Although the proposed ve-terminal hybrid microgrids have the
Table 2
Detailed comparisons between the proposed and the existing hybrid microgrids.
Topologies N ¼4
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 No Yes Yes Yes Yes
No. of the original LV ports 1 2 1 24 2 3
No. of the LV microgrids 2 2 1 24 2 3
Additional power conversion Yes No Yes No No One LV DC/AC
Operation capability under unbalanced power High Medium High Low Medium High
IGBTs in the interlinking converters 144 +4 144 240 +4 240 +4x
(x: No. of LVAC)
240 156 þ14
Fig. 7. The hierarchical control structure of the proposed hybrid microgrids.
Q. Xiao et al.