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Direct current (DC) microgrids (MG) constitute a research field that has gained great attention over the past few years, challenging the well-established dominance of their alternating current (AC) counterparts in Low Voltage (LV) (up to 1.5 kV) as well as Medium Voltage (MV) applications (up to 50 kV). The main reasons behind this change are: (i) the ascending amalgamation of Renewable Energy Sources (RES) and Battery Energy Storage Systems (BESS), which predominantly supply DC power to the energy mix that meets electrical power demand and (ii) the ascending use of electronic loads and other DC-powered devices by the end-users. In this sense, DC distribution provides a more efficient interface between the majority of Distributed Energy Resources (DER) and part of the total load of a MG. The early adopters of DC MGs include mostly buildings with high RES production, ships, data centers, electric vehicle (EV) charging stations and traction systems. However, the lack of expertise and the insufficient standards’ framework inhibit their wider spread. This review paper presents the state of the art of LV and MV DC MGs in terms of advantages/disadvantages over their AC counterparts, their interface with the AC main grid, topologies, control, applications, ancillary services and standardization issues. Overall, the aim of this review is to highlight the possibilities provided by DC MG architectures as well as the necessity for a solid/inclusive regulatory framework, which is their main weakness.
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energies
Review
State of the Art of Low and Medium Voltage Direct Current
(DC) Microgrids
Maria Fotopoulou , Dimitrios Rakopoulos * , Dimitrios Trigkas, Fotis Stergiopoulos, Orestis Blanas
and Spyros Voutetakis


Citation: Fotopoulou, M.;
Rakopoulos, D.; Trigkas, D.;
Stergiopoulos, F.; Blanas, O.;
Voutetakis, S. State of the Art of Low
and Medium Voltage Direct Current
(DC) Microgrids. Energies 2021,14,
5595. https://doi.org/10.3390/
en14185595
Academic Editors: Aditya Shekhar
and Laura Ramírez Elizondo
Received: 4 August 2021
Accepted: 30 August 2021
Published: 7 September 2021
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4.0/).
Chemical Process and Energy Resources Institute, Centre for Research and Technology Hellas, Thermi,
GR-57001 Thessaloniki, Greece; fotopoulou@certh.gr (M.F.); dtrigkas@certh.gr (D.T.); foster@certh.gr (F.S.);
blanas@certh.gr (O.B.); paris@certh.gr (S.V.)
*Correspondence: rakopoulos@certh.gr; Tel.: +30-210-6899-689
Abstract:
Direct current (DC) microgrids (MG) constitute a research field that has gained great
attention over the past few years, challenging the well-established dominance of their alternating
current (AC) counterparts in Low Voltage (LV) (up to 1.5 kV) as well as Medium Voltage (MV)
applications (up to 50 kV). The main reasons behind this change are: (i) the ascending amalgamation
of Renewable Energy Sources (RES) and Battery Energy Storage Systems (BESS), which predominantly
supply DC power to the energy mix that meets electrical power demand and (ii) the ascending use
of electronic loads and other DC-powered devices by the end-users. In this sense, DC distribution
provides a more efficient interface between the majority of Distributed Energy Resources (DER) and
part of the total load of a MG. The early adopters of DC MGs include mostly buildings with high RES
production, ships, data centers, electric vehicle (EV) charging stations and traction systems. However,
the lack of expertise and the insufficient standards’ framework inhibit their wider spread. This review
paper presents the state of the art of LV and MV DC MGs in terms of advantages/disadvantages
over their AC counterparts, their interface with the AC main grid, topologies, control, applications,
ancillary services and standardization issues. Overall, the aim of this review is to highlight the
possibilities provided by DC MG architectures as well as the necessity for a solid/inclusive regulatory
framework, which is their main weakness.
Keywords: DC microgrid; architectures; applications; ancillary services; standards
1. Introduction
In electrical microgrids (MG), as in all sectors of modern technology and applica-
tions, the need for sustainability in terms of reducing the energy footprint is considered
to be a major priority. In fact, according to the European Union (EU) targets of 2020, the
greenhouse gas emissions need to be reduced by at least 55% by 2030, compared to 1990
levels [
1
]. In order for such goals to be achieved, the reduction in fossil fuel-based energy
production is required. As an alternative, Renewable Energy Sources (RES) have proven
to be a solution with minimal environmental impact of vital importance. Photovoltaic
(PV) systems, wind generators (WG), biomass and geothermal installations have pene-
trated the market over the past few decades, improving the energy mix that covers the
electricity demand [
2
,
3
]. Nevertheless, a major drawback of many RES, is the intermittent
production, due to the sources that they utilize. In order for the production to meet the
demand curve, the utilization of Energy Storage Systems (ESS) is considered to be an
effective solution. ESS typically include Battery Energy Storage Systems (BESS), flywheels,
compressed air systems, etc. [
4
]. The most common, widely utilized ESS technology is
the BESS, with advantages such as high controllability, fast response and geographical
independence [
5
]. Subsequently, for sustainability-related reasons, the combination of
distributed RES (especially PV systems and WGs) with BESS has created a new field of
Energies 2021,14, 5595. https://doi.org/10.3390/en14185595 https://www.mdpi.com/journal/energies
Energies 2021,14, 5595 2 of 26
research and development, promoting the decarbonization, autonomy and cost efficiency
of MGs [6].
However, the increasing integration of RES and ESS in the current energy mix does
not only result to the rise of eco-friendly energy supply, but also to the rise of proliferation
of DC systems, i.e., DC generation and DC storage units. In fact, some of the most widely
utilized RES and ESS, such as PV and BESS, originally produce DC power (either as
current or voltage sources), which is then converted to AC power through DC/AC power
electronics converters in order to be injected to the AC distribution grid. Additionally,
the same phenomenon is observed on the side of energy demand. More specifically, DC
loads including electric vehicles (EVs), Light Emitting Diode (LED) systems, DC motors,
data centers and other battery-based devices have penetrated the market following an
ascending curve [
7
,
8
]. Yet, these devices too incorporate special converters that convert
AC into DC power in order to function. Also, one should also have in mind that most
of the electronics loads and devices are based on DC power. Taking the above facts into
consideration, it is evident that traditional AC distribution needs to cope with the new
developments, and innovative DC distribution challenges its dominance, as presented in
Figure 1.
Energies 2021, 14, x FOR PEER REVIEW 2 of 26
has created a new field of research and development, promoting the decarbonization,
autonomy and cost efficiency of MGs [6].
However, the increasing integration of RES and ESS in the current energy mix does
not only result to the rise of eco-friendly energy supply, but also to the rise of proliferation
of DC systems, i.e., DC generation and DC storage units. In fact, some of the most widely
utilized RES and ESS, such as PV and BESS, originally produce DC power (either as
current or voltage sources), which is then converted to AC power through DC/AC power
electronics converters in order to be injected to the AC distribution grid. Additionally, the
same phenomenon is observed on the side of energy demand. More specifically, DC loads
including electric vehicles (EVs), Light Emitting Diode (LED) systems, DC motors, data
centers and other battery-based devices have penetrated the market following an
ascending curve [7,8]. Yet, these devices too incorporate special converters that convert
AC into DC power in order to function. Also, one should also have in mind that most of
the electronics loads and devices are based on DC power. Taking the above facts into
consideration, it is evident that traditional AC distribution needs to cope with the new
developments, and innovative DC distribution challenges its dominance, as presented in
Figure 1.
Figure 1. The transition from AC MGs to DC MGs.
Up until recently, DC power systems have been utilized mostly in High Voltage (HV)
applications, predominantly for the purpose of power transmission over long distances,
due to the low-power losses, high-power quality and cost efficiency that the High Voltage
Direct Current (HVDC) transmission has to offer in such applications. This has been a
milestone of DC power systems, as presented in the work of [9–13]. Nevertheless, since
the necessary high-voltage, high-power and efficient power electronics and DC cables
have only been developed during the past decades, the advantages of DC power systems
have not been fully exploited in most sectors, especially at the Low Voltage (LV)
(indicatively up to 1.5 kV) and Medium Voltage (MV) levels (indicatively up to 50 kV),
leaving the dominance of power systems to AC architectures. Yet, the modern challenges
for efficient integration of RES in the LV and MV levels have brought DC power to the
spotlight. Low Voltage Direct Current (LVDC) as well as Medium Voltage Direct Current
(MVDC) MGs constitute a modern field of research and development and have found
applications in a variety of fields. However, there are still obstacles that the DC MGs have
to overcome in order to be widely adopted, most significant of which is the lack of a
sufficient standards’ framework [14].
This review paper aims to present the state of the art of LV and MV DC MGs,
including their advantages/disadvantages (Section 2), their implementation methods (i.e.,
Figure 1. The transition from AC MGs to DC MGs.
Up until recently, DC power systems have been utilized mostly in High Voltage (HV)
applications, predominantly for the purpose of power transmission over long distances,
due to the low-power losses, high-power quality and cost efficiency that the High Voltage
Direct Current (HVDC) transmission has to offer in such applications. This has been a
milestone of DC power systems, as presented in the work of [
9
13
]. Nevertheless, since the
necessary high-voltage, high-power and efficient power electronics and DC cables have
only been developed during the past decades, the advantages of DC power systems have
not been fully exploited in most sectors, especially at the Low Voltage (LV) (indicatively
up to 1.5 kV) and Medium Voltage (MV) levels (indicatively up to 50 kV), leaving the
dominance of power systems to AC architectures. Yet, the modern challenges for efficient
integration of RES in the LV and MV levels have brought DC power to the spotlight. Low
Voltage Direct Current (LVDC) as well as Medium Voltage Direct Current (MVDC) MGs
constitute a modern field of research and development and have found applications in a
variety of fields. However, there are still obstacles that the DC MGs have to overcome in
order to be widely adopted, most significant of which is the lack of a sufficient standards’
framework [14].
Energies 2021,14, 5595 3 of 26
This review paper aims to present the state of the art of LV and MV DC MGs, including
their advantages/disadvantages (Section 2), their implementation methods (i.e., possible
interfaces with the main AC grid—Section 3, topologies—Section 4, control—Section 5),
their most popular applications (Section 6), the ancillary services that they may provide
to the main grid (Section 7) and their main obstacles as regards the market penetration
(Section 8). Although, as revealed in the following sections, there is a number of relevant
review papers on DC MGs, none of them addresses all of these issues, but rather discuss
only a part of them. The purpose of this review is to provide an overall framework of the
DC MG capabilities, addressing all their main aspects and highlighting their importance in
future grids and power distribution applications.
2. Advantages and Disadvantages of DC MGs
The benefits of DC MG infrastructures, compared to their AC counterparts include:
Easier integration of RES and ESS and reduction in primary energy consumption:
A high proportion of RES and ESS produce DC power that would be more efficiently
integrated in a DC MG than in an AC MG. Examples include PV, BESS and fuel-
cell systems. In a DC MG, the use of these sources’ supply does not need to be
converted from DC to AC. On the contrary, instead of DC/AC converters, DC/DC
converters need to be implemented, which are more efficient and smaller, resulting in
the reduction in primary energy consumption [15].
More effective integration of DC loads:
Distributing DC power to DC loads (e.g.,
from popular electronic devices to EVs) instead of converting it from AC to DC can
lead to energy and cost savings from the aspect of the consumer. By skipping the
AC/DC conversion phase, losses are reduced, resulting in lower costs of energy. This
modification could lead to substantial savings considering DC loads such as EVs, LED
lights, data centers, electronic equipment, etc. [15].
Easy enhancement of power quality and control of the MG:
In DC MGs there are no
harmonic oscillations or phase unbalances, which occur in AC MGs and undermine the
power quality. Instead, the DC systems provide a “firewall” that prevents disturbances
propagating from one network to another, improving the MGs’ robustness [
16
,
17
].
Furthermore, since DC MGs operate only with active power, there is no need for
reactive power control, in contrast with their AC counterparts [18].
No need for synchronization:
In DC systems there is no need to synchronize the
grid-connected RES with the main AC grid. This can further reduce the operational
complexity of the system [
19
]. On the other hand, in AC MGs the frequency needs to
be regulated in order to be constantly kept equal to 50 or 60 Hz giving rise to stability
issues.
No skin effect:
In DC systems there is no skin effect. This allows the current’s
flow through the entire distribution cable, not just the outer edges. As a result, DC
distribution reduces losses and provides the possibility to use smaller cables for the
same flow of current [19].
On the other hand, DC technologies have not been researched as much as their
AC counterparts. This is attributed to the fact that the entire concept of electrical energy
production, transmission and distribution has been built on AC technology, which provided
the means to progress and develop more efficient, reliable and cost effective equipment.
This means that the implementation of DC solutions has certain drawbacks such as:
Lack of specific standards:
In order for a system, such as the DC MG to be widely
implemented, the definition of certain parameters, such as the voltage levels, need
to be specified. Due to the fact that DC applications are not as widespread as AC
applications, there is a general lack of standardized values regarding their function.
This issue needs to be addressed, in order for the DC MGs to enter the worldwide
market [20].
Protection issues:
In the case of DC power, there are protection issues that are not only
related to the lack of standards but also to the specific nature of DC current. Specifically,
Energies 2021,14, 5595 4 of 26
breaking a functioning DC circuit is considered to be more difficult, compared to its
AC counterpart, because there is no natural zero crossing of the current, to minimize
the arc effect. Major research efforts are undertaken for the development of switchgear
that can accommodate the secure disruption of DC voltages in the order of kVs, with
low cost, to enable the development of grid infrastructures [18,19].
Lack of expertise
: The existing grids are most commonly AC-based. The AC technol-
ogy is proven and mature, whereas DC technology is in a process to be established.
This means that few specialists, grid developers and system operators have studied
DC MGs extensively.
Construction cost:
The overall cost regarding the construction of AC MGs is lower
than the respective cost for DC MGs. This occurs because the development of DC
technologies, e.g., dedicated power converters, terminals, etc., is more recent and the
innovation is integrated into the overall cost.
The comparison between DC and AC MGs is briefly presented in Table 1. Overall,
the implementation of DC MGs appears to be a key driver in paving the way towards
sustainability, efficiency and mitigation of the anthropogenic climate change. For their
proper incorporation in the traditional AC grid and their establishment in the worldwide
market, further research needs to be conducted for their proper design and function in
terms of interface, topology and control.
Table 1. Comparison between DC and AC MGs (Data from [15,18]).
DC MG AC MG
Integration of RES and ESS Effective Not effective
Reduction in primary energy
consumption Yes No
Integration of DC loads Effective Not effective
Power quality and control of
the MG Easy Complicated
Synchronization Not required Required
Frequency regulation No frequency
Constant, equal to 50 or 60 Hz
Skin effect No Yes
Standards Insufficient Sufficient
Protection Underdeveloped, expensive Fully developed, not
expensive
Expertise Low High
Construction cost High Low
3. Interface with the AC Grid
The interconnection of the DC MG with the conventional AC power network is an
issue of interest that has led to the emergence of a new research area over the past few years.
In fact, several classifications have emerged regarding the interface of the conventional
Medium Voltage Alternating Current (MVAC) grid with the DC grid [
21
]. To begin, it is
important to state two fundamental types of interfaces, as presented in Figure 2: (a) those
based on standard, separate converters, i.e., (1) and (2) and (b) those based on the concept
of the Solid State Transformer (SST), i.e., (3), (4) and (5). An SST is an advanced, multi-stage
power electronics device that enables the connection of grids with different voltage and
frequency levels [
22
]. Its special configuration has advantages that do not exist in typical
power transformers. In fact, the SST can provide DC ports that facilitate the integration of
BESS, DC RES, DC loads and enables the implementation of power quality features, such
as advanced control schemes [23].
Energies 2021,14, 5595 5 of 26
Energies 2021, 14, x FOR PEER REVIEW 5 of 26
Figure 2. Interfaces between the DC MG and the main grid.
As regards the interfaces that do not include a SST, there are two main configurations,
i.e., (1) and (2) as presented in Figure 2 (top part). The first one establishes a LVDC
distribution line, utilizing a distribution transformer and an AC/DC converter. The
distribution transformer provides the LVDC MG with galvanic isolation. Furthermore, it
provides the capability for the implementation of a LVAC line, coupled with the MVAC
grid (with galvanic isolation too, due to the distribution transformer). The second one
establishes a MVDC distribution line, utilizing only an AC/DC converter. In this case,
there is no galvanic isolation nor the capability to provide a LVAC distribution line.
However, this interface may be suitable for the implementation of a MVDC MG cluster
that connects two remote AC grids, enhancing their stability. The above configurations
have been researched by the authors of [24,25], considering RES/BESS integration in the
LV level, but also by the authors of [26], considering WG integration in the MV level.
Nevertheless, such interfaces seem, at present, outdated when compared to the SST
interfaces. As seen in Figure 2 (bottom part), these can be divided in two main categories,
the two-stage SST and the three-stage SST. As regards the two-stage SST, there are two
main configurations, i.e., (3) and (4). The first one provides a MVDC link while the second
one provides a LVDC link, both utilizing an AC/DC conversion stage as well as a DC/AC
conversion stage. The main difference is the point where the voltage level is transformed.
In the first case, the voltage is transformed from MV to LV in the (second) DC/AC stage,
which means that there is no galvanic isolation in the side of the MVDC MG. On the other
hand, in the second case, the voltage is transformed in the (first) AC/DC stage, which
means that the produced LVDC line has galvanic isolation. It is also noted that in both
cases the SST provides the capability for implementation of a LVAC line that has galvanic
isolation (due to the transformation of the voltage level) and is decoupled from the main
MVAC grid (due to the mediation of the DC MG), which inhibits the propagation of
disturbances. Researchers have studied both configurations, as presented in the work of
[27,28].
Figure 2. Interfaces between the DC MG and the main grid.
As regards the interfaces that do not include a SST, there are two main configurations,
i.e., (1) and (2) as presented in Figure 2(top part). The first one establishes a LVDC
distribution line, utilizing a distribution transformer and an AC/DC converter. The
distribution transformer provides the LVDC MG with galvanic isolation. Furthermore, it
provides the capability for the implementation of a LVAC line, coupled with the MVAC
grid (with galvanic isolation too, due to the distribution transformer). The second one
establishes a MVDC distribution line, utilizing only an AC/DC converter. In this case, there
is no galvanic isolation nor the capability to provide a LVAC distribution line. However, this
interface may be suitable for the implementation of a MVDC MG cluster that connects two
remote AC grids, enhancing their stability. The above configurations have been researched
by the authors of [
24
,
25
], considering RES/BESS integration in the LV level, but also by the
authors of [26], considering WG integration in the MV level.
Nevertheless, such interfaces seem, at present, outdated when compared to the SST
interfaces. As seen in Figure 2(bottom part), these can be divided in two main categories,
the two-stage SST and the three-stage SST. As regards the two-stage SST, there are two
main configurations, i.e., (3) and (4). The first one provides a MVDC link while the second
one provides a LVDC link, both utilizing an AC/DC conversion stage as well as a DC/AC
conversion stage. The main difference is the point where the voltage level is transformed.
In the first case, the voltage is transformed from MV to LV in the (second) DC/AC stage,
which means that there is no galvanic isolation in the side of the MVDC MG. On the
other hand, in the second case, the voltage is transformed in the (first) AC/DC stage,
which means that the produced LVDC line has galvanic isolation. It is also noted that
in both cases the SST provides the capability for implementation of a LVAC line that has
galvanic isolation (due to the transformation of the voltage level) and is decoupled from
the main MVAC grid (due to the mediation of the DC MG), which inhibits the propagation
of disturbances. Researchers have studied both configurations, as presented in the work
of [27,28].
Energies 2021,14, 5595 6 of 26
Yet, the most popular out of all configurations is the three-stage SST, presented in
Figure 2. This configuration is considered to be more sophisticated and technologically
advanced than the previous configurations [
29
]. It includes an AC/DC conversion stage, a
DC/DC conversion stage and a DC/AC conversion stage. Therefore, it provides all three
possible links and voltage levels, i.e., MVDC, LVDC and LVAC. Its complex composition
has the unique advantage of providing links for all types of inputs/outputs. Yet, since it
provides a MVDC link, the transformer needs to be located after the MV AC/DC converter.
This means that galvanic isolation is only provided for the LVDC and LVAC lines. The
design of a three-stage SST has gained a lot of attention over the past few years, due to
the advantages it provides. Research and development work is conducted regarding the
optimization of its operation, the improvement of its efficiency, the enhancement of its
reliability and, of course, the reduction in its cost and size. In this context, special attention
is paid to the Dual Active Bridge (DAB) (i.e., a bidirectional DC/DC converter with identical
full bridges on its primary and secondary side and a high frequency transformer being the
set of power electronics responsible for converting MVDC to LVDC in a three-stage SST),
the minimization of the core’s volume, losses and cost, and the optimal incorporation of
the three-stage SST in modern applications [
30
,
31
]. Such applications include a wide range
of scales, from smart buildings up to entire distribution systems [3234].
All interfaces presented above provide the capability for the incorporation of a DC
MG in the MVAC grid. However, some of them have characteristics that others lack, and no
configuration meets all evaluation criteria to the highest level. Table 2summarizes the basic
criteria and their potential of fulfilment for each configuration that has been reviewed. It is
noted that the configurations without SST require lower maintenance and have a lower cost
than the SST configurations. The features that give them a comparable advantage make
them a “safe” option with well-tested and established expectations. On the other hand, the
SST configurations exceed in electronics technology (scalability, modularity, controllability)
and also provide the capability for decoupled LVAC distribution. Especially, the three-stage
SST is the only configuration able to facilitate the integration of MVDC, LVDC and LVAC
MGs, including a variety of flexible capabilities. These attributes are the means required for
achieving the goals of the future grids, i.e., smart grids, DC MGs, AC/DC hybrid MGs, etc.
Table 2. Evaluation of interfaces (Data from [21,29]).
Without SST
Configurations (1) and (2)
With SST
Configurations (3), (4) and (5)
(1) (2) (3) (4) (5)
Integration of MVDC MG No Yes Yes No Yes
Integration of LVDC MG Yes No No Yes Yes
Advanced power electronics
(scalability, modularity,
controllability)
No No Yes Yes Yes
Galvanic isolation Yes No No Yes Yes, but not for
MVDC MG
Cost Low Low High High High
Maintenance requirements Low Low Medium Medium High
Capability for LVAC
distribution Yes (coupled) No Yes (decoupled) Yes (decoupled) Yes (decoupled)
4. Topologies of DC MGs
As regards the topologies of a DC MG, five (5) main types can be distinguished: (a)
single-bus, (b) radial, (c) ring, (d) mesh and (e) interconnected [
19
]. This section aims to
analyze and compare the aforementioned types of topologies.
Energies 2021,14, 5595 7 of 26
4.1. Single-Bus
The general concept of the single-bus configuration is presented in Figure 3[
35
]. In
this type of configuration, the main characteristic is that there is only one DC bus and one
point of connection between the components of the system, i.e., loads, generation units,
storage units and the interface with the AC distribution network. Its main highlight is
its simplicity, its low cost and low maintenance requirements. However, when it comes
to flexibility in terms of fault management, the single-bus configuration has very limited
options. This type of configuration appears in various studies, as for example in the work
of [
36
38
], where single-bus DC MGs constitute test grids for the implementation of DC
MG control schemes, demonstrating the single-bus capability for simple integration of RES
and efficient operation.
Energies 2021, 14, x FOR PEER REVIEW 7 of 26
4. Topologies of DC MGs
As regards the topologies of a DC MG, five (5) main types can be distinguished: (a)
single-bus, (b) radial, (c) ring, (d) mesh and (e) interconnected [19]. This section aims to
analyze and compare the aforementioned types of topologies.
4.1. Single-Bus
The general concept of the single-bus configuration is presented in Figure 3 [35]. In
this type of configuration, the main characteristic is that there is only one DC bus and one
point of connection between the components of the system, i.e., loads, generation units,
storage units and the interface with the AC distribution network. Its main highlight is its
simplicity, its low cost and low maintenance requirements. However, when it comes to
flexibility in terms of fault management, the single-bus configuration has very limited
options. This type of configuration appears in various studies, as for example in the work
of [36–38], where single-bus DC MGs constitute test grids for the implementation of DC
MG control schemes, demonstrating the single-bus capability for simple integration of
RES and efficient operation.
Figure 3. Single-bus configuration.
4.2. Radial
The general concept of a radial configuration includes a number of DC buses
connected with each other without forming loops, having only one way of connection
between the MG’s interface and each component of the MG. The radial configuration is
divided in two main sub-categories, i.e., a series configuration and a parallel configuration
[19].
The general topology of a series configuration is presented in Figure 4. It is noted that
there are two (or more) DC buses, each of which directly serves a combination of load,
generation, storage and supply units. The first DC bus is directly connected to the interface
between the DC MG and the main grid. However, the second DC bus is only connected
to the first DC bus, through a DC power cable, with the appropriate switching and
protection devices. In this way, if a fault occurs to the DC grid, the faulty part can be
isolated, giving to the rest of the grid the possibility to operate normally. Obviously, the
proposed configuration can be extended to more than two buses, according to the
system’s requirements.
Figure 3. Single-bus configuration.
4.2. Radial
The general concept of a radial configuration includes a number of DC buses connected
with each other without forming loops, having only one way of connection between the
MG’s interface and each component of the MG. The radial configuration is divided in two
main sub-categories, i.e., a series configuration and a parallel configuration [19].
The general topology of a series configuration is presented in Figure 4. It is noted
that there are two (or more) DC buses, each of which directly serves a combination of
load, generation, storage and supply units. The first DC bus is directly connected to the
interface between the DC MG and the main grid. However, the second DC bus is only
connected to the first DC bus, through a DC power cable, with the appropriate switching
and protection devices. In this way, if a fault occurs to the DC grid, the faulty part can be
isolated, giving to the rest of the grid the possibility to operate normally. Obviously, the
proposed configuration can be extended to more than two buses, according to the system’s
requirements.
Energies 2021,14, 5595 8 of 26
Energies 2021, 14, x FOR PEER REVIEW 8 of 26
Figure 4. Radial series configuration.
On the other hand, the topology of the parallel configuration is presented in Figure
5. In this case, the DC buses are not connected with each other. Instead, both of them are
connected, through power cables, to the power electronics converter interfacing the DC
MG and the main AC grid. In this way, if a fault occurs on one bus of the DC MG, then
the other bus of the grid will remain connected to the main grid, maintaining the ability
of safe and normal operation. For this reason, this solution is considered to be more
reliable than the series configuration. The parallel configuration can also be extended to a
higher number of buses, depending on the system’s requirements. When it comes to
parallel configurations including more than two DC buses, an advantage over its series
counterpart is the ability to share power between buses even in the instance of a fault that
would isolate one or more DC buses. This feature highlights the power sharing capability
of the parallel configuration.
Figure 5. Radial parallel configuration.
Overall, the radial configuration constitutes a simple and cost-effective solution, with
low maintenance requirements but limited flexibility/fault management options. It has
been extensively researched in many applications of DC MGs, from single smart buildings
up to district level, mostly due to its simplicity. For example, the authors of [39] study the
power sharing capability of an extensive radial DC MG with high DER penetration, while
Figure 4. Radial series configuration.
On the other hand, the topology of the parallel configuration is presented in
Figure 5
.
In this case, the DC buses are not connected with each other. Instead, both of them are
connected, through power cables, to the power electronics converter interfacing the DC
MG and the main AC grid. In this way, if a fault occurs on one bus of the DC MG, then
the other bus of the grid will remain connected to the main grid, maintaining the ability of
safe and normal operation. For this reason, this solution is considered to be more reliable
than the series configuration. The parallel configuration can also be extended to a higher
number of buses, depending on the system’s requirements. When it comes to parallel
configurations including more than two DC buses, an advantage over its series counterpart
is the ability to share power between buses even in the instance of a fault that would isolate
one or more DC buses. This feature highlights the power sharing capability of the parallel
configuration.
Energies 2021, 14, x FOR PEER REVIEW 8 of 26
Figure 4. Radial series configuration.
On the other hand, the topology of the parallel configuration is presented in Figure
5. In this case, the DC buses are not connected with each other. Instead, both of them are
connected, through power cables, to the power electronics converter interfacing the DC
MG and the main AC grid. In this way, if a fault occurs on one bus of the DC MG, then
the other bus of the grid will remain connected to the main grid, maintaining the ability
of safe and normal operation. For this reason, this solution is considered to be more
reliable than the series configuration. The parallel configuration can also be extended to a
higher number of buses, depending on the system’s requirements. When it comes to
parallel configurations including more than two DC buses, an advantage over its series
counterpart is the ability to share power between buses even in the instance of a fault that
would isolate one or more DC buses. This feature highlights the power sharing capability
of the parallel configuration.
Figure 5. Radial parallel configuration.
Overall, the radial configuration constitutes a simple and cost-effective solution, with
low maintenance requirements but limited flexibility/fault management options. It has
been extensively researched in many applications of DC MGs, from single smart buildings
up to district level, mostly due to its simplicity. For example, the authors of [39] study the
power sharing capability of an extensive radial DC MG with high DER penetration, while
Figure 5. Radial parallel configuration.
Energies 2021,14, 5595 9 of 26
Overall, the radial configuration constitutes a simple and cost-effective solution, with
low maintenance requirements but limited flexibility/fault management options. It has
been extensively researched in many applications of DC MGs, from single smart buildings
up to district level, mostly due to its simplicity. For example, the authors of [
39
] study the
power sharing capability of an extensive radial DC MG with high DER penetration, while
the authors of [
40
] study the radial DC MG configuration as part of a hybrid AC/DC MG
consisting of RES, i.e., PV panels and WGs, EVs, DC and AC equipment.
4.3. Ring
In spite of the advantages of radial distribution described above, there are certain
limitations that pose a challenge in terms of flexibility and fault management. In order to
overcome the limitations of the radial configuration, a more complex topology, i.e., the ring
configuration, has been introduced. The main concept of the ring configuration is presented
in Figure 6. The proposed solution includes the placement of all loads, generation and
storage units, interconnected along one single ring. For safety reasons, protection switches
are located before and after the integration of each bus. This means that each component
has two possible ways of connection with the interface between the DC MG and the main
grid, i.e., through the line on its left-hand side and through the line on its right-hand side.
The ring configuration provides the DC MG with flexibility, meaning that in case a fault
occurs, the respective switches isolate it, allowing all units to maintain their functionality,
except for the faulty one [
18
]. The ring configuration appears in a number of studies over
the past few years. For example, the authors of [
41
43
] have studied protection schemes,
fault detection and reconfiguration on DC MGs with ring configuration, especially due to
their capability for advanced fault management.
Energies 2021, 14, x FOR PEER REVIEW 9 of 26
the authors of [40] study the radial DC MG configuration as part of a hybrid AC/DC MG
consisting of RES, i.e., PV panels and WGs, EVs, DC and AC equipment.
4.3. Ring
In spite of the advantages of radial distribution described above, there are certain
limitations that pose a challenge in terms of flexibility and fault management. In order to
overcome the limitations of the radial configuration, a more complex topology, i.e., the
ring configuration, has been introduced. The main concept of the ring configuration is
presented in Figure 6. The proposed solution includes the placement of all loads,
generation and storage units, interconnected along one single ring. For safety reasons,
protection switches are located before and after the integration of each bus. This means
that each component has two possible ways of connection with the interface between the
DC MG and the main grid, i.e., through the line on its left-hand side and through the line
on its right-hand side. The ring configuration provides the DC MG with flexibility,
meaning that in case a fault occurs, the respective switches isolate it, allowing all units to
maintain their functionality, except for the faulty one [18]. The ring configuration appears
in a number of studies over the past few years. For example, the authors of [41–43] have
studied protection schemes, fault detection and reconfiguration on DC MGs with ring
configuration, especially due to their capability for advanced fault management.
Figure 6. Ring configuration.
4.4. Mesh
The radial and ring configuration can be combined in a mesh configuration, as
presented in Figure 7. The mesh configuration constitutes a complex topology that has
partly the simplicity of the radial configuration and partly the flexibility of the ring
configuration. Although its deployment is quite rare, several researchers have studied the
capabilities that it provides. For instance, in [33,44,45], DC MGs with mesh configurations
are presented. More specifically, in [45] an interesting aspect regarding the architecture of
MGs is developed. In fact, the authors study mesh configurations of AC MGs, DC MGs,
hybrid AC/DC MGs but also bilayer MGs. The latter constitute a new design for future
grids, where each node is allowed to be universal, meaning that it can include two buses
(AC and DC).
Figure 6. Ring configuration.
4.4. Mesh
The radial and ring configuration can be combined in a mesh configuration, as pre-
sented in Figure 7. The mesh configuration constitutes a complex topology that has partly
the simplicity of the radial configuration and partly the flexibility of the ring configuration.
Although its deployment is quite rare, several researchers have studied the capabilities that
Energies 2021,14, 5595 10 of 26
it provides. For instance, in [
33
,
44
,
45
], DC MGs with mesh configurations are presented.
More specifically, in [
45
] an interesting aspect regarding the architecture of MGs is devel-
oped. In fact, the authors study mesh configurations of AC MGs, DC MGs, hybrid AC/DC
MGs but also bilayer MGs. The latter constitute a new design for future grids, where each
node is allowed to be universal, meaning that it can include two buses (AC and DC).
Figure 7. Mesh configuration.
4.5. Interconnected
Nevertheless, the aforementioned types of configurations have one common disad-
vantage. Due to their single connection to the main grid, if a fault occurs on the main grid,
there is no possible way for the DC MG to absorb power. In order to tackle this issue, the
interconnected configurations are formed, including more than one interface between the
MG and the main power supply, which renders them by far more reliable in terms of fault
management than all of the topologies previously described. Obviously, the increased
flexibility they possess is reflected in the cost of increased complexity [
20
]. Although there
are many ways to implement more than one connection between the MG and the main
power supply, the most popular is the zonal configuration, presented in Figure 8. In this
case, the DC MG is divided into a number of zones, each of which interacts with the rest
of the MG through two buses, one at each side. The MG contains more than one interface
with the utility grid. This configuration is completed by a number of switches that enable a
variety of energy mixes as well as a number of solutions, in terms of reconfiguration, in
case a fault occurs. This configuration is characterized by symmetry and reliability but
also by complexity. Research on zonal configuration has been conducted by the authors
of [4648], all of who apply this configuration in relation to ships.
Energies 2021,14, 5595 11 of 26
Energies 2021, 14, x FOR PEER REVIEW 11 of 26
Figure 8. Zonal configuration.
4.6. Synopsis and Comparison among the Topologies
Table 3 summarizes all described features along with the classification of each
topology. Ιt can be stated that the flexibility of a topology, in terms of fault management
and acquisition of power supply, is inversely proportional to its simplicity and cost
effectiveness, as one could expect. Consequently, the selection of topology at the stage of
design of a DC MG needs to be made according to its needs and available means of
development.
Table 3. Evaluation of topologies (Data from [18–20,35,45])
Features Single-Bus [35] Radial [19] Ring [18] Mesh [20,45] Interconnected [20]
Cost Very low Low Medium Medium High
Simplicity Very high High Medium Medium Low
Maintenance
requirements Very low Low Medium Medium High
Fault management
capability Very low Low Medium Medium High
Easy integration of
remote RES No Yes Yes Yes Yes
Capability for
continuous supply from
utility
No No No No Yes
Reconfiguration No No Yes Yes Yes
Main field Buildings, small districts Districts with RES Districts with RES Districts with RES Ships
5. Control of DC MGs
Apart from the selection regarding the interface with the main grid and the design of
the topology for the connection of all the components included in the DC MG, it is
essential to determine the control strategies according to which the total system will
operate. The field of control strategies regarding these special structures has gained
attention over the past few decades, which has led to the rise of advanced control
Figure 8. Zonal configuration.
4.6. Synopsis and Comparison among the Topologies
Table 3summarizes all described features along with the classification of each topol-
ogy. It can be stated that the flexibility of a topology, in terms of fault management and
acquisition of power supply, is inversely proportional to its simplicity and cost effective-
ness, as one could expect. Consequently, the selection of topology at the stage of design of
a DC MG needs to be made according to its needs and available means of development.
Table 3. Evaluation of topologies (Data from [1820,35,45]).
Features Single-Bus [35] Radial [19] Ring [18] Mesh [20,45] Interconnected [20]
Cost Very low Low Medium Medium High
Simplicity Very high High Medium Medium Low
Maintenance
requirements Very low Low Medium Medium High
Fault management
capability Very low Low Medium Medium High
Easy integration of
remote RES No Yes Yes Yes Yes
Capability for
continuous supply
from utility
No No No No Yes
Reconfiguration No No Yes Yes Yes
Main field Buildings, small
districts Districts with RES Districts with RES Districts with
RES Ships
Energies 2021,14, 5595 12 of 26
5. Control of DC MGs
Apart from the selection regarding the interface with the main grid and the design of
the topology for the connection of all the components included in the DC MG, it is essential
to determine the control strategies according to which the total system will operate. The
field of control strategies regarding these special structures has gained attention over the
past few decades, which has led to the rise of advanced control algorithms, creating a
mixture of different approaches presented in the worldwide literature [
49
54
]. Overall,
control strategies deal with stability and protection issues, power balance, smooth transition
regarding transient occurrences (e.g., black start), synchronization, optimization of various
objectives (e.g., cost), market participation, etc. Three levels of hierarchical control of DC
MGs can be distinguished, as presented in Figure 9[55,56]:
1. Primary control:
This is the lowest hierarchical control level and has the fastest
response. It deals with the primary voltage regulation, the load sharing among
the distributed generation of the MG and safety/protection issues. The respective
DC/DC power converters of the MG undertake the above tasks.
2. Secondary control:
While the primary control level is responsible for the primary
voltage regulation, the secondary control level is responsible for the regulation of volt-
age fluctuations/deviations [
55
]. It is also responsible for the seamless reconnection
of the MG to the main grid.
3. Tertiary control:
This is the highest hierarchical control level. It sets the power flow
between the DC MG and the main grid. It is also known as an energy management
system (EMS) and communicates with the distribution system operator (DSO). In
this sense, the DSO, or even the transmission system operator (TSO), may decide the
power exchange with the MG.
Energies 2021, 14, x FOR PEER REVIEW 12 of 26
algorithms, creating a mixture of different approaches presented in the worldwide
literature [49–54]. Overall, control strategies deal with stability and protection issues,
power balance, smooth transition regarding transient occurrences (e.g., black start),
synchronization, optimization of various objectives (e.g., cost), market participation, etc.
Three levels of hierarchical control of DC MGs can be distinguished, as presented in
Figure 9 [55,56]:
1. Primary control: This is the lowest hierarchical control level and has the fastest
response. It deals with the primary voltage regulation, the load sharing among the
distributed generation of the MG and safety/protection issues. The respective DC/DC
power converters of the MG undertake the above tasks.
2. Secondary control: While the primary control level is responsible for the primary
voltage regulation, the secondary control level is responsible for the regulation of
voltage fluctuations/deviations [55]. It is also responsible for the seamless
reconnection of the MG to the main grid.
3. Tertiary control: This is the highest hierarchical control level. It sets the power flow
between the DC MG and the main grid. It is also known as an energy management
system (EMS) and communicates with the distribution system operator (DSO). In this
sense, the DSO, or even the transmission system operator (TSO), may decide the
power exchange with the MG.
Figure 9. Hierarchical control.
The purpose of the primary/local control is to perform the current and voltage control
of the power converters connected to the distributed generation and storage units and to
be responsible for the load sharing among them, as presented in [55,56]. Α vastly used
local control system is the droop control method. The aim of droop control is to vary the
reference values (voltage, amplitude, frequency, etc.) depending on the active and reactive
power demand in order to share the load between the available devices. This control
strategy originates from the power sharing of synchronous AC generators in conventional
grids. In the case of DC structures, storage systems often incorporate this control system
in order to perform optimal power sharing [57,58]. Also, there are cases where the
control’s objective may be the extraction of maximum power provided by the sources. A
common example is the maximum power-point tracking (MPPT) mode, applied in wind
turbine or PV systems [59,60]. Additionally, the master-slave strategy is considered to be
widely adopted. This control strategy utilizes both voltage and current controllers in order
to perform power sharing between the converters. The assigned master unit contains a
voltage controller for the regulation of the voltage level and is responsible for the
Figure 9. Hierarchical control.
The purpose of the primary/local control is to perform the current and voltage control
of the power converters connected to the distributed generation and storage units and to
be responsible for the load sharing among them, as presented in [
55
,
56
]. A vastly used
local control system is the droop control method. The aim of droop control is to vary the
reference values (voltage, amplitude, frequency, etc.) depending on the active and reactive
power demand in order to share the load between the available devices. This control
strategy originates from the power sharing of synchronous AC generators in conventional
grids. In the case of DC structures, storage systems often incorporate this control system in
order to perform optimal power sharing [
57
,
58
]. Also, there are cases where the control’s
objective may be the extraction of maximum power provided by the sources. A common
Energies 2021,14, 5595 13 of 26
example is the maximum power-point tracking (MPPT) mode, applied in wind turbine
or PV systems [
59
,
60
]. Additionally, the master-slave strategy is considered to be widely
adopted. This control strategy utilizes both voltage and current controllers in order to
perform power sharing between the converters. The assigned master unit contains a voltage
controller for the regulation of the voltage level and is responsible for the specification of a
current reference of each slave unit. The slave units track the current reference given by the
master unit and adapt to it through current controllers. In this way, the master-slave control
performs current sharing with easy implementation, even with non-identical modules [
61
].
Altogether, the field of local control strategies in DC MGs has been extensively researched,
as presented in the work of [44,62,63].
As stated previously, above the primary control, in terms of hierarchy, there is the
secondary control. Secondary level strategies can be distinguished in two types, namely:
(a) centralized and (b) decentralized, depending on their location, as presented in
Figure 10
.
The main difference is that in centralized techniques control actions are taken at a central
point, whereas in decentralized techniques, which may or may not incorporate a communi-
cation network, control actions are taken at the local controller of each distributed power
supply unit [55,64].
Energies 2021, 14, x FOR PEER REVIEW 13 of 26
specification of a current reference of each slave unit. The slave units track the current
reference given by the master unit and adapt to it through current controllers. In this way,
the master-slave control performs current sharing with easy implementation, even with
non-identical modules [61]. Altogether, the field of local control strategies in DC MGs has
been extensively researched, as presented in the work of [44,62,63].
As stated previously, above the primary control, in terms of hierarchy, there is the
secondary control. Secondary level strategies can be distinguished in two types, namely:
a) centralized and b) decentralized, depending on their location, as presented in Figure
10. The main difference is that in centralized techniques control actions are taken at a
central point, whereas in decentralized techniques, which may or may not incorporate a
communication network, control actions are taken at the local controller of each
distributed power supply unit [55,64].
Figure 10. Secondary control strategies: centralized and decentralized (with/without
communication network).
As regards to centralized approaches, the management is performed from a central
controller. The central controller needs to be connected to a communication system. In this
way, the controller gathers information about the distributed generation and storage
Figure 10.
Secondary control strategies: centralized and decentralized (with/without communication
network).
Energies 2021,14, 5595 14 of 26
As regards to centralized approaches, the management is performed from a central
controller. The central controller needs to be connected to a communication system. In
this way, the controller gathers information about the distributed generation and storage
systems of the DC MG, such as active power measurements, voltage measurements, etc.
Also, information regarding market conditions and requests from the upper (tertiary)
control level are taken into account. Having processed the available data, the controller
performs the necessary actions and provides references to the primary control level. This
approach is suitable mostly for small scale DC MGs, where there exists a single owner of the
distributed generation and storage units, providing the controller with clear, single tasks,
thus avoiding conflicts of interest that would occur in the case of multiple owners [65,66].
Despite the efficiency of centralized control systems, the market is oriented towards
MGs of a larger scale, including more than one provider of power supply units. For
this purpose, decentralized control strategies are developed. These types of strategy
require the implementation of secondary control in each unit separately/locally. Two main
variations of this type of strategy are distinguished, i.e., with or without a communication
system. In general, the decentralized control approach offers simple integration, meaning
low communication requirements, facilitates the incorporation of multiple power supply
producers and provides the ability for plug-and-play connection of devices. However,
the operational pattern attained by this type of control is usually sub-optimal, since there
are sometimes conflicts of interest between the owners of the power supply units, which
means that some units may be competitive or have different kinds of goals with respect
to others. Also, in contrast with the centralized control, due to the lack of centralized
communication system, this DC MG operation cannot be attached to a larger and more
critical operation [6769].
The advantages and disadvantages of the two types of secondary level strategies are
summarized and compared to each other in Table 4.
Table 4. Comparison between centralized and decentralized control (Data from [56,65,67]).
Centralized Control Decentralized Control
Central communication and decision
point Yes No
Suitable scale Small Large
Purpose Simple objectives Multiple objectives,
conflicts of interest
Complexity (communication,
installation, general requirements) High Low
Implementation of advanced
algorithms Likely Unlikely
However, when the DC MG does not operate in isolation, but in a grid-tied mode, the
highest level of hierarchy is the tertiary control. Tertiary control is responsible for the en-
ergy management between the DC MG and the main grid, regulating the power exchanged
between the two sides as well as the rest of the operational values and actions of the DC
MG. This level of hierarchy establishes the reference values based on the requirements of
the overall system, taking into consideration the status of the DC MG and the market. This
includes the prices of energy, the state (e.g., state of charge) of the storage systems, the
forecasts of RES production, the energy demand and possible ancillary services that can
be offered by the DC MG. The data obtained can be utilized for the optimal schedule of
operation, taking into consideration objectives such as cost minimization, power quality
maximization, etc. In the literature, a great variety of hierarchical control architectures
(i.e., including tertiary, secondary and primary control) is observed, including the imple-
mentation of economic dispatch algorithms, reduction in CO
2
emissions, model predictive
Energies 2021,14, 5595 15 of 26
control (MPC) or even deep learning (DL) and deep reinforcement learning (DRL), for both
normal operation and treatment of special challenges [7073].
In conclusion, there is no standard way of control when it comes to DC MGs, mostly
due to the fact that they do not have a standard form. This leads to a number of possible
control architectures, depending on the size, the number of owners and the connection
to/isolation from the main grid. Considering that this is a field of innovation, no optimal
solution is provided for the control scheme of DC MGs in general. Each DC MG needs to
have its control architecture designed individually, based on its assets and requirements.
6. Applications of DC MGs
Having analyzed the possible advantages, types of interfaces, topologies and control
strategies, the aim of this section is to showcase modern examples of applications of DC
MGs.
6.1. Ships and Other Marine Applications
Ships constitute a special applications environment that provides the opportunity to
highlight the benefits of DC MGs. This is a case where the ways to supply the necessary
power are limited due to constraints imposed by the ship’s needs such as (a) constant power
availability (taking into consideration that the ship’s power system operates in isolation),
space and weight concerns (the installations responsible for ensuring that the ship has
always available power must be as compact as possible due to the limited space available on
ships) and (c) presence of pulse loads (the demand of which changes periodically, creating
the need for power systems that can keep up with the fast changes in load demand).
In the worldwide literature, several researches related to DC MGs on ships are ob-
served, as presented in [
74
77
]. The DC power system facilitates the integration of both
DC power supply (BESS, etc.) and DC power demand (radar, etc.) on the ship. The zonal
configuration is the predominant one in this type of applications, due to its flexibility. The
voltage level is usually higher than 1 kV.
6.2. Transport Applications
Due to the high availability of DC motors and their ability to easily control their
speed, railways initially used DC current and continue to do so to this day. Even buses
are moving towards DC-based power for environmental reasons. Also, both motors and
auxiliary circuits inside urban transport vehicles use DC current. This results in the urban
transport system itself being, in many cases, a DC power system, drawing its power from
the main city power system. Because current power systems of cities are mostly AC,
AC/DC conversion is needed to power urban transports.
Research on DC transportation has been conducted by a number of researchers, includ-
ing [
78
,
79
]. More specifically, the authors of [
78
] demonstrated that a MVDC electrification
system for the Paris-Strasbourg line is at least on par with the current AC one in terms of
efficiency. It also allowed less installed power for substations, no phase shift between them,
and required less power electronics and no autotransformer purchases. It should be noted
that in transport applications the voltage level varies according to the application, yet some
indicative vastly used values are 750 V, 1500 V and 3000 V.
6.3. Data Centers
Data centers are extremely important facilities. In fact, their importance is rapidly
escalating as time passes and the need for high capacity of information storage increases.
Future data centers could require power levels up to a few MWs to operate. Most loads in
data centers are electronic in nature and operate on DC current, which means that an AC
system would lead to losses due to the necessary AC/DC conversion stage. Additionally,
AC/DC converters are more complex devices with higher volume and failure rates than
DC/DC converters. These problems of AC power systems supplying data centers along
with the trend of using renewable energy sources such as solar panels and battery storage
Energies 2021,14, 5595 16 of 26
(both utilizing DC current) favor the adoption of a DC power system for data centers.
According to [
80
], low voltage (380 V) powered DC data centers occupy 33% less floor
space, are more efficient and have a 36% lower lifetime cost than AC ones. The authors
of [
81
] claimed the optimal level of DC voltage to be 400 V (considered to be low voltage)
and ensured 7% energy savings along with the other benefits of DC, such as the non-
existence of harmonics.
6.4. Building Applications
Due to environmental and economic concerns, PV systems are quite commonly in-
stalled at commercial buildings (shopping centers, offices, etc.), residential buildings,
hospitals or even schools. Excess energy produced can be diverted to ESS that can supply
power back to the grid if necessary. As PV and energy storage systems are installed within
the building, they allow the minimization of transmission losses. Both these power sources
provide DC power and usually help to supply a large number of loads inside the build-
ing. Also, a proportion of the load of a building utilizes DC power. For example, Light
Emitting Diodes (LED), used for lighting with increased efficiency, operate on DC current.
Furthermore, chargers, small DC motors, power electronics and other loads of the average
building use DC current to operate. Due to the main grid being an AC one, these devices
are required to include AC/DC converters internally or, in the case of power sources such
as PV systems and BESS, they require an external DC-AC converter. However, by adopting
a DC power system for powering the building, the converters and their associated losses
can be omitted.
It should be noted that a building’s power system may incorporate both a DC and an
AC MG (the first of which may facilitate the connection of DC sources and loads while
the second one may facilitate the connection of AC loads to the main AC grid) or even be
entirely DC [
82
84
]. In fact, according to the research of [
83
], commercial buildings that
incorporate PV systems in combination with DC MGs use the PV generation 6%–8% more
efficiently than traditional AC systems do.
6.5. Lighting of Public Spaces and Roads
As part of public works and services, older lighting equipment is replaced with effi-
cient LED technology, lighting mostly public spaces, roads and highways. DC grids power
the LED lighting system, allowing a financial gain, since public lighting is a significant
public cost. This field of innovation has been approached by a number of researchers. For
example, in [
85
] the authors propose the incorporation of vacuum switches in low voltage
DC MGs that power LED-based road lighting, showcasing the efficiency and reliability
of the overall structure. Additionally, in [
86
] a model for public DC lighting system is
presented, including multiple DC MG clusters, each of which is equipped with LED lights,
PVs and storage.
6.6. Electric Vehicles and Charging Stations
Sales of EVs are increasing day by day around the world due to the need to protect
the environment from fossil fuel emissions. This has caused car manufacturers to focus on
producing EVs, which in the near future will partly replace traditional fossil fuel powered
vehicles. Since EVs need charging at regular intervals to operate and their batteries are
inherently DC powered, it would be more beneficial to have DC EV charging stations
instead of their AC counterparts. In fact, DC charging stations may charge EVs faster than
AC charging stations, due to increased efficiency, and may also efficiently incorporate PV
panels in order to be more friendly to the environment, as presented in the work of [
13
,
87
].
6.7. Industrial Applications
MVDC MGs have applications in some industries, because their processes require DC
power to operate. Such industries include those that process steel or chemicals or even
automotive manufacturing. For example, DC innovative solutions are observed in the
Energies 2021,14, 5595 17 of 26
field of copper or zinc electro-winning installations, electric arc furnaces, etc. [
79
]. Also,
there are cases where LVDC MGs are considered to be a solution, especially if the industry
includes automated production lines and/or RES power supply [79,88].
6.8. Synopsis of Applications of DC MGs
Table 5summarizes the main modern applications of DC MGs, along with their voltage
requirements and the purpose for which they have been developed (easier integration of
DC supply, DC load or both) [
19
,
89
,
90
]. It is noted that there are applications where the
voltage requirements have a wide range. For example, in the case of buildings, according
to various researchers, the optimal voltage range varies from 48 V up to 400 V, depending
on the application (where the lowest values are usually proposed for residential buildings,
while the highest values are usually proposed for commercial buildings) [
91
,
92
]. Also,
there are applications where the voltage level may vary in such a way that both LVDC and
MVDC architectures are acceptable, as in industry or transport, according to the size of
the application. Nevertheless, it should be noted that in AC applications, there are not
as many variations regarding voltage levels and other requirements, as voltage levels are
customized. This is attributed to the fact that the DC MGs are a relatively new trend that
lacks standardization and regulatory framework.
Table 5. Applications of DC MGs (Data from [19,7981]).
Application Usual Voltage (V) Voltage Level Developed for the Easier Integration
of DC Supply or DC Load
Marine (Ships) >1000 Both LV and MV Both (supply and load)
Transport
Mainly 750, 1000, 3000 [
79
]
Both LV and MV Load
Data centers 380–400 [80,81] Only LV Usually load but sometimes both
Buildings 48–400, depending on the
application Only LV Both (supply and load)
Lighting of public spaces 24 [19] Only LV Load
EV charging station <600 Only LV Load
Industry >600 Both LV and MV Usually load but sometimes both
7. Ancillary Services
Ancillary services are considered to be the specialty services and functions that fa-
cilitate and support the continuous flow of electricity so that the power supply does, in
all cases, meet the demand. They are provided by the DSO or procured from other stake-
holders and are essential for maintaining power quality, grid stability and security [
93
,
94
].
However, there is a lack of a general definition of these services because in every reg-
ulated zone they can be defined independently based on their own market rules. The
most common ancillary services found in the literature include loss compensation, black
start capability, control of voltage, frequency or reactive power, oscillation damping and
congestion management [
95
99
]. Further analysis about each of the categories of ancillary
services can be found in the works of [100106].
The implementation of DC infrastructures in the architecture of existing AC grids
can provide a number of possible functions that may improve the operation of the grid.
DC MGs links may be used to strengthen weak points in the power system, to control
power flow in the AC network and to provide increased controllability and flexibility to
its operator. Most common ancillary services provided by DC MGs and, generally, by DC
links, are presented in Figure 11 and can be summarized as follows [107]:
Energies 2021,14, 5595 18 of 26
Energies 2021, 14, x FOR PEER REVIEW 18 of 26
flow in the AC network and to provide increased controllability and flexibility to its
operator. Most common ancillary services provided by DC MGs and, generally, by DC
links, are presented in Figure 11 and can be summarized as follows [107]:
Figure 11. Ancillary services.
DC MGs as a firewall in the AC grid: A DC system between AC grids can act as a
“firewall” preventing disturbances spreading from one AC grid to another. When a
power imbalance occurs on one part of the AC grid, the DC MG may mitigate the
imbalance and prevent the disturbance from propagating to the rest of the network
[104].
Artificial inertia: Weak AC systems may suffer from frequency variations. This
results from the low ratio of rotating mass (inertia) related to synchronous machines.
In this case, the DC MGs can provide ancillary services by providing additional
inertia in order to strengthen the local stability [100].
Frequency stability: Frequency deviation in an AC grid results from imbalances
between produced and consumed power. DC MGs, which have zero frequency, can
mitigate the frequency deviation through their converters, thus restoring the
frequency stability [101].
Link grids of different frequency: A DC MG can act as a link between AC grids. The
capability of DC systems to offer the desired frequency set-points makes possible the
connection of AC grids with different frequencies [101].
Power oscillation dumping: Electromechanical oscillations of the rotors in the
synchronous machines may stress the main AC grid. These oscillations indicate an
operating working point close to the stability limit that wears down the governor
systems of the turbines. To reduce these oscillations and maintain a safe power
transfer, a control signal can be applied to the DC system, which is considered to be
a valuable ancillary service according to [105].
Black-start: Black-start is considered to be an important ancillary service. Due to the
interconnected power sources and storage means in a DC MG the restoration process
Figure 11. Ancillary services.
DC MGs as a firewall in the AC grid:
A DC system between AC grids can act as a
“firewall” preventing disturbances spreading from one AC grid to another. When a
power imbalance occurs on one part of the AC grid, the DC MG may mitigate the im-
balance and prevent the disturbance from propagating to the rest of the network [
104
].
Artificial inertia:
Weak AC systems may suffer from frequency variations. This results
from the low ratio of rotating mass (inertia) related to synchronous machines. In this
case, the DC MGs can provide ancillary services by providing additional inertia in
order to strengthen the local stability [100].
Frequency stability
: Frequency deviation in an AC grid results from imbalances
between produced and consumed power. DC MGs, which have zero frequency, can
mitigate the frequency deviation through their converters, thus restoring the frequency
stability [101].
Link grids of different frequency:
A DC MG can act as a link between AC grids. The
capability of DC systems to offer the desired frequency set-points makes possible the
connection of AC grids with different frequencies [101].
Power oscillation dumping:
Electromechanical oscillations of the rotors in the syn-
chronous machines may stress the main AC grid. These oscillations indicate an
operating working point close to the stability limit that wears down the governor sys-
tems of the turbines. To reduce these oscillations and maintain a safe power transfer, a
control signal can be applied to the DC system, which is considered to be a valuable
ancillary service according to [105].
Black-start: Black-start is considered to be an important ancillary service. Due to the
interconnected power sources and storage means in a DC MG the restoration process
(black-start) of an AC system can be fulfilled after a system power loss or blackout.
Additionally, Voltage Source Converter (VSC) transmission technology can follow the
cold load pickup as well as the pickup of the power production due to its smooth
control of both active and reactive power [95].
Energies 2021,14, 5595 19 of 26
Maintaining synchronization:
A DC MG system may provide ancillary services to
the AC system by maintaining synchronization. DC systems can support the power
flow in the AC grid, reducing the risk of falling out of step and losing synchronization,
as presented in [101,104].
Merchant links:
The coupling of electricity markets and growing commercial inter-
connections requires precise, controllable power flows for effective operation, in line
with the market-derived schedules. It is noted that, power scheduling on an hour or
minute basis is a common situation. The controllability of active power flow in the DC
systems guides the power flow in the AC system to fulfill prearranged commercial
deals [107].
In short, by their very nature, modern DC MGs can be means for providing function-
alities to the AC grid in a more controllable and efficient way. The integration of renewable
sources as well as the possibility of energy storage act as a framework for the provision of
specialized functions. The flexibility offered by a DC grid is evident in all areas of control of
an AC grid. However, there is not yet a network code on MV-LV DC grid connection rules.
A regulation on HVDC Connections and DC Connected Power Park Modules has been
established by the EU [
108
] and that could be a starting point for a guideline on connection
rules for MV and LV DC MGs.
8. Future Trends and Challenges
DC MGs are considered to be an innovative solution that shall facilitate the transition
towards “green” energy [
35
,
109
]. In this sense, they are expected to be widely adopted in
future applications related mainly to RES integration, EVs, data centers, etc.
However, in order to penetrate the market, they need to be further researched and
developed [
110
114
]. In fact, since the DC system components have not been researched as
much as their AC counterparts, it is natural that they still have parameters, such as cost,
that need to be further optimized. For example, it is essential to increase the efficiency and
power density of SSTs, DC/DC and AC/DC converters and reduce their cost.
Furthermore, it is critical to ensure the same level of safety of equipment and hu-
mans (workers, operators, etc.) as in the respective AC systems. Safety and reliability
issues include switches and protection schemes, which are not as developed as their AC
counterparts. Means to anticipate and overcome possible faults (fault detection and avoid-
ance techniques) in the DC MG need to be thoroughly investigated before the large-scale
deployment of such architectures [115117].
Also, it is expected that in order for DC MGs to be fully effective, compatible equip-
ment needs to be developed. In fact, one of the main reasons why the DC power systems
are researched and developed is the ascending amalgamation of DC devices in the pool
of loads that a distribution system feeds. Such devices include EVs, computers, power
electronics systems, DC motors, etc. However, currently, these devices are powered by
AC sources and have incorporated converters that convert the AC input to DC power in
order to be served. In DC MGs this conversion, which reduces the efficiency of the grid, is
not required. On the contrary, such DC loads are expected to be directly connected to DC
lines. In order for these connections to be achieved, it is highly important to promote the
development of DC-compatible equipment that is not yet available and suitable protection
devices.
Of course, in order for DC MGs to be established, the appropriate regulatory frame-
work needs to be developed. The lack of adequate standards is mostly observed in issues
of DC voltage levels. So far, the research community has not agreed on one specific DC
voltage level or even set clear limits between what is considered to be low, medium and
high voltage, in terms of standardization. This issue needs to be addressed because without
voltage standardization it is impossible to customize appliances, safety equipment and
devices that are directly connected to DC buses. In fact, it is inconvenient for manufacturers
to design DC products capable of operating on different voltage levels. In order to speed
Energies 2021,14, 5595 20 of 26
up the incorporation of DC technologies in the distribution grid, voltage standardization is
by far the highest priority.
The existing standards have mostly been developed by the International Electrotech-
nical Commission (IEC) and the Institute of Electrical and Electronics Engineers (IEEE).
A list of the most important contributions regarding standards and relative work that
deal specifically or partly with MV and LV DC applications (and, therefore, DC MGs),
developed by different entities/organizations, is presented in Table 6[
79
]. The presented
standards mostly refer to traction systems [
118
121
], ships [
122
,
123
], data centers [
124
],
safety issues [125128], LV and MV DC installations (in general) [129132].
Table 6. Current standards addressing DC MGs (Data from [79,118132]).
Serial Number Standard Description
Traction
1 IEC 60850 Railway applications, supply voltages of traction systems
2 IEC 60077-3 Railway applications, electric equipment for rolling stock and rules for DC circuit-breakers
3 IEC 61992-3 Railway applications, fixed installations, DC switchgear, DC disconnectors,
switch-disconnectors and earthing switches
4
IEEE Std 1653.6
Recommended practice for grounding of DC equipment enclosures in traction power
distribution facilities
Ships
5 IEEE 1709 Recommended practice for 1–35 kV Medium-Voltage DC power systems on ships
6 MIL-STD-1399 DC for submarines
Data centers
7 EN 300 132-3-1 Power supply interface for the input of telecommunications and datacom (ICT) equipment
Safety
8 IEC 61660-1 Calculation of short-circuit currents for DC installations in power plants and substations
9 IEC 60204-11 Safety of machinery for voltages above 1000 V AC or 1500 V DC and not exceeding 36 kV
10 IEC 60947-2 LVDC switchgear and controlgear
11 IEEE C37.14 Standard for DC power circuit breakers used in enclosures (voltage lower than 3200 V)
LV and MV DC installations
13 IEC TS 61936-2 Design of power installations exceeding 1.5 kV DC
14 IEC 60364-1
Fundamental principles, assessment of general characteristics and definitions for LV electrical
installations
15 IEEE 946-2020
Recommended practice regarding the design of DC power systems in stationary applications
16 IEEE 1547
Requirements for interconnecting distributed resources with electric power systems interfaces
As presented above, there is a variety of standards for different purposes that facilitate
the integration of DC MGs into future grids. Nevertheless, this list is far from complete.
There is a growing need for new standard developments, including all aspects of DC MGs,
such as standards on voltage levels, grounding, safety, specialized standards regarding
each application, etc. [
20
,
79
]. This need poses a great challenge that should be properly
addressed for the wider adoption of DC MGs by the worldwide market.
9. Conclusions
This review paper highlights the potential of LV and MV DC MGs in terms of sus-
tainability and efficiency and analyzes various aspects of their implementation, such as
interfaces with the main grid, topologies and control strategies. The most popular applica-
tion fields of DC MGs are presented, including ships, transport, data centers, buildings,
lighting installations, EV charging stations and modern industry. Furthermore, possible
ancillary services from the DC MGs to the main grid are examined. However, in order for
DC MGs to be widely adopted, an adequate and inclusive regulatory framework needs to
be developed, since the existing standards do not cover all aspects of their implementation.
The main contribution of this review paper is that, in contrast with the existing review
Energies 2021,14, 5595 21 of 26
papers on DC MGs, it addresses all of these issues, instead of only addressing a part of
them.
Author Contributions:
Conceptualization, M.F., F.S. and D.R.; methodology, M.F., F.S. and D.R.;
writing—original draft preparation, M.F., D.T., F.S. and O.B.; writing—review and editing, M.F., F.S.
and D.R.; visualization, M.F., D.T., F.S. and D.R.; supervision, F.S. and D.R.; project administration,
D.R.; funding acquisition, D.R. and S.V. All authors have read and agreed to the published version of
the manuscript.
Funding:
This research has received funding from the European Union’s Horizon 2020 research and
innovation programme, TIGON (Towards Intelligent DC-based hybrid Grids Optimizing the Network
performance), under grant agreement No 957769, https://cordis.europa.eu/project/id/957769.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data sharing not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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... Most of the electronics loads and devices, e.g., computers/laptops, light emitting diode (LED) lighting systems and battery chargers utilize dc power. Hence ac-dc PEC are required for their connection to ac DES [5]. The large number of conversion stages result in reduced efficiency and reliability of the overall DES [5]. ...
... Hence ac-dc PEC are required for their connection to ac DES [5]. The large number of conversion stages result in reduced efficiency and reliability of the overall DES [5]. If such devices are interfaced directly to a dc DES, then the conversion stages would be less and could also be substituted by a highly efficient dc-dc converter [4,7,8]. ...
... DC DES facilitate the integration of RES and energy storage systems (ESS), which would result in reduced CO 2 emissions, thus leading to an eco-friendly energy supply [4]. The dc DES do not have harmonic oscillators or phase unbalances resulting in improved power quality [5]. The dc DES do not require reactive power control as they operate on active power only [9]. ...
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The remarkable progress of power electronic converters (PEC) technology has led to their increased penetration in distributed energy systems (DES). Particularly, the direct current (dc) nanogrid-based DES embody a variety of sources and loads, connected through a central dc bus. Therefore, PECs are required to be employed as an interface. It would facilitate incorporation of the renewable energy sources and battery storage system into dc nanogrids. However, it is more challenging as the integration of multiple modules may cause instabilities in the overall system dynamics. Future dc nanogrids are envisioned to deploy ready-to-use commercial PEC, for which designers have no insight into their dynamic behavior. Furthermore, the high variability of the operating conditions constitute a new paradigm in dc nanogrids. It exacerbates the dynamic analysis using traditional techniques. Therefore, the current work proposes behavioral modeling to perform system level analysis of a dc nanogrid-based DES. It relies only on the data acquired via measurements performed at the input-output terminals only. To verify the accuracy of the model, large signal disturbances are applied. The matching of results for the switch model and its behavioral model verifies the effectiveness of the proposed model.
... Microgrids and smart grids have been proposed to integrate distributed generation to meet local energy demand and to connect to distribution networks without the costly expansion of the centralised utility grid. Microgrid grid structures and hardware topologies are very similar to the distribution topologies that are discussed in the next section and are often in the lower voltage ranges [110]. According to some sources [1,84,110], DC microgrids are set to take the dominance of AC microgrids, as the price decreases due to their ease of integration or renewable sources, which have better integration efficiency, DC load integration, no harmonic oscillations, no skin effect, and no synchronisation requirements [110]. ...
... Microgrid grid structures and hardware topologies are very similar to the distribution topologies that are discussed in the next section and are often in the lower voltage ranges [110]. According to some sources [1,84,110], DC microgrids are set to take the dominance of AC microgrids, as the price decreases due to their ease of integration or renewable sources, which have better integration efficiency, DC load integration, no harmonic oscillations, no skin effect, and no synchronisation requirements [110]. MVDC microgrids will be prominent in offshore oil rings [20], data centres [106], industrial applications, and EV charging stations. ...
... Microgrid grid structures and hardware topologies are very similar to the distribution topologies that are discussed in the next section and are often in the lower voltage ranges [110]. According to some sources [1,84,110], DC microgrids are set to take the dominance of AC microgrids, as the price decreases due to their ease of integration or renewable sources, which have better integration efficiency, DC load integration, no harmonic oscillations, no skin effect, and no synchronisation requirements [110]. MVDC microgrids will be prominent in offshore oil rings [20], data centres [106], industrial applications, and EV charging stations. ...
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... In the last decade, direct current (DC) power distribution research has become popular in commercial and residential facilities due to the developments in internally DC loadssuch as electronics and variable frequency drives, the widespread implementation of roof-top photovoltaic (PV) systems, and battery use [1][2][3]. DC distribution may be more energy efficient than AC distribution in data centers and commercial buildings [4], although implementing DC distribution in residential buildings necessitates further research and development in terms of distribution voltage level and load compatibility [5,6]. Compared to data centers and commercial facilities, the residential sector saves less electricity due to lower load power consumption and lower time of use coincidence between on-site renewable energy sources (RESs) power generation and demand [7,8], while ongoing research on DC houses and buildings in the residential sector depicted that DC power distribution is more efficient than AC alongside energy and cost savings; it is estimated that there are 14% and 5% total electricity savings in DC-powered residences with and without energy storage, respectively [9]. ...
... Different DC distribution topologies for a DC nanogrid are categorized in [3,20]; the unipolar topology is preferred in houses with low power consumption. In this study, the DC-RNG is defined as the internal network of a two-bedroom house; its structure and components are shown in Figure 1. ...
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... Similarly, AC-DC and DC-AC conversion are used in AC drives. Moreover, the DC loads such as electric vehicles (EVs), light-Emitting Diode systems (LED), DC motors, data centres, and other battery-based devices are penetrating the market with a rising curve [6]. These systems require AC-DC conversion systems, so there are multiple stages of AC-DC conversion, DC-DC converters act as a medium between the load and the source. ...
... for DC power at load has challenged the conventional AC distribution system, while th DC power systems are used for the transfer of DC power over long distances, mostly i high-voltage applications. The advantages of DC power systems have not been exploite in low voltage (LV) (1.5 kV) and medium voltage (MV) (50 kV), leaving the power system domain to AC architecture [6,7]. To solve this problem, the concept of a DC microgrid derived to meet local demands. ...
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High gain DC-DC converters are getting popular due to the increased use of renewable energy sources (RESs). Common ground between the input and output, low voltage stress across power switches and high voltage gain at lower duty ratios are desirable features required in any high gain DC-DC converter. DC-DC converters are widely used in DC microgrids to supply power to meet local demands. In this work, a high step-up DC-DC converter is proposed based on the voltage lift (VL) technique using a single power switch. The proposed converter has a voltage gain greater than a traditional boost converter (TBC) and Traditional quadratic boost converter (TQBC). The effect of inductor parasitic resistances on the voltage gain of the converter is discussed. The losses occurring in various components are calculated using PLECS software. To confirm the performance of the converter, a hardware prototype of 200 W is developed in the laboratory. The simulation and hardware results are presented to determine the performance of the converter in both open-loop and closed-loop conditions. In closed-loop operation, a PI controller is used to maintain a constant output voltage when the load or input voltage is changed
... With the unprecedented increased penetration of DC renewable energy sources (RES) (e.g., photovoltaics and fuel cells), DC energy storage systems (e.g., batteries and electric vehicles), and DC loads in the modern distribution networks, DC microgrids (MGs) have appeared as a promising way to ensure efficient and resilient electric networks [1,2]. Besides, they are immune to the inherent problems of AC MGs, such as reactive power control, DC/AC power conversion losses, synchronization, and inrush current [3,4]. Despite the advantages of DC MGs, a single MG may be subject to generation-demand power imbalance due to the volatility of RES and load uncertainty, which deteriorates the system reliability and stability [5]. ...
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A novel, fully distributed controller with a rapid convergence rate is developed to ensure the optimal loading dispatch for interconnected DC MGs. It comprises local and global-control levels, handling the economic load allocations in a finite-time manner, for distinct MGs and cluster of MGs, respectively. The local-control layer guarantees MG’s economic operation by matching the incremental costs (ICs) of all DGs, respecting the power equilibrium among generations and demands, DGs’ generation limits, as well as the transmission line losses. Furthermore, the economic operation of battery energy sources is considered, in the optimization problem, to strengthen the overall reliability and maximize energy arbitrage. The global controller adjusts MGs’ voltage references to determine the optimal exchanged power, between MGs, for reducing the global total generation cost (TGC). A rigorous analysis is developed to confirm the stable convergence of the developed controller. Extensive simulation case studies demonstrate the superiority of the proposed control system.
... The choice of such architecture topology presents several positive features, such as easy management, high quality of the energy, reduced number of power converters and no reactive power exchange, although currently incomplete standardization is defined as the main drawback [26]. In particular, an electrical architecture based on a common DC bus is widely used in microgrids, as reported in [27][28][29][30]. ...
Article
Background: Wave energy represents one of the most promising renewable energies due to its great theoretical potential. Nevertheless, the electrical compliance of grid-connected systems is a great issue nowadays, due to the highly stochastic nature of wave energy. Methods: In this paper, a Hybrid Energy Storage System (HESS) consisting of a Li-ion battery and a flywheel is coupled to a Wave Energy Converter (WEC) that operates in grid connected mode. The study is performed using real yearly wave power profiles relating to three different sites located along the European coasts. The Simultaneous Perturbation Stochastic Approximation (SPSA) principle is implemented as real-time power management strategy for HESS in wave energy conversion systems. Results: Obtained results demonstrate how the proposed HESS and the implementation of the SPSA power management coupled to a WEC allow a reduction of more than 80% of power oscillations at the Point of Common Coupling (PCC), while proving the robustness of the developed management strategy over the investigated sites. Moreover, the average energy penalty due to the HESS integration results slightly higher than 5% and battery solicitation is reduced by more than 64% with respect to the flywheel solicitation, contributing to extend its lifetime. Conclusions: HESS integration in renewable generation systems maximizes the WEC production while smoothing the power at the PCC. Specifically, flywheel-battery HESS together with the implemented power management strategy could provide a great flexibility in the view of increasing power production from waves, strongly mitigating the variability of this source while enhancing grid safety and stability.
... One of the basic reasons is the increasing incorporation of direct current (DC)-based renewable energy sources (RES) and storage (PVs and batteries) in the grid, for environmental purposes. Another reason is the ascending DC demand in various infrastructures such as data centers, electric vehicle (EV) charging stations, etc. [6][7][8]. However, the power switches in the inverter are vulnerable to failure for many reasons. ...
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Full-text available
An intelligent control strategy based on a membership cloud model in a high reliable off-grid microgrid with a reconfigurable inverter is proposed in this paper. The operating principle of the off-grid microgrid with the reconfigurable inverter is provided, which contains four operating modes. An open-circuit fault diagnosis for the inverter is presented first. The polarities of the midpoint voltages defined in the paper are used to recognize the faulty power switch. The reconfigurable inverter allows the power switches of different bridges to be reconfigured, when there are power switches faulty, to let the inverter operate in faulty state. The working principle of the reconfigurable inverter is given. The membership cloud model with two output channels is built to obtain the virtual impedance to suppress the circulating currents between inverters when the reconfigurable inverter is in faulty state. A pulse resetting method is presented. The general intelligent control strategy for the reconfigurable inverter is formed as the droop-virtual impedance-voltage-current-pulses resetting control. The validity of the intelligent control strategy of the system is verified by simulation.
... The choice of such architecture topology presents several positive features, such as easy management, high quality of the energy, reduced number of power converters and no reactive power exchange, although currently incomplete standardization is defined as the main drawback [26]. In particular, an electrical architecture based on a common DC bus is widely used in microgrids, as reported in [27][28][29][30]. ...
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