Low/Medium Voltage Direct Current Microgrids: Comparison
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Please note this is a comparison between Version 2 by Peter Tang and Version 1 by Maria Fotopoulou.

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). 

  • DC microgrid
  • architectures
  • DC applications
  • DC MG interfaces

1. Introduction

In electrical microgrids (MG), as in all sectors of modern technology and applications, 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 penetrated the market over the past few decades, improving the energy mix that covers the electricity demand [2,3][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 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][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.
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Figure 1. The transition from AC MGs to DC MGs.

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][9].
  • 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. [
  • 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][14]
  • 15][9].
  • .
Zonal configuration.

4.6. Synopsis and Comparison among the Topologies

Table 2 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 2. Evaluation of topologies (Data from [18,19,20,35,45][12][13][14][18][29]).

Features

Single-Bus [35]

[18]

Radial [19]

[13]

Ring [18]

[12]

Mesh [20,45]

[14][29]

Interconnected [20]

[14]

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

Standards

Insufficient

  • 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][10][11]. Furthermore, since DC MGs operate only with active power, there is no need for reactive power control, in contrast with their AC counterparts [18][12].
  • Lack of expertise
    No need13]. 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.
  • 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, 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: The existing grids are most commonly AC-based. The AC technology 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.
    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
  • 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][9][12]).
 

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

Low

Medium

Medium

High

Synchronization

Not required

Required

Easy integration of remote RES

Frequency regulation

No frequency

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 [49,50,51,52,53,54][33][34][35][36][37][38]. 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][39][40]:
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Figure 9. Hierarchical control.
  • 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.
  • 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][39]. It is also responsible for the seamless reconnection of the MG to the main grid.

No

Yes

Yes

Yes

Yes

Capability for continuous supply from utility

No

Constant, equal to 50 or 60 Hz

Skin effect

No

Yes

Sufficient

Protection

Underdeveloped, expensive

Fully developed, not expensive

Expertise

Low

High

Construction cost

High

Low
  • ]
  • [
  • 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][13].
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:

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][15]. 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][16]. 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][17].
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Figure 2. Interfaces between the DC MG and the main grid.

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][13]. 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][18]. 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 [36,37,38][19][20][21], 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.
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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][13].
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.
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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.
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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][22] study the power sharing capability of an extensive radial DC MG with high DER penetration, while the authors of [40][23] 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][12]. The ring configuration appears in a number of studies over the past few years. For example, the authors of [41,42,43][24][25][26] have studied protection schemes, fault detection and reconfiguration on DC MGs with ring configuration, especially due to their capability for advanced fault management.
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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[27][28][29],44,45], DC MGs with mesh configurations are presented. More specifically, in [45][29] 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).
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Figure 7. Mesh configuration.

4.5. Interconnected

Nevertheless, the aforementioned types of configurations have one common disadvantage. 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][14]. 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 [46[30][31][32],47,48], all of who apply this configuration in relation to ships.
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Figure 8.

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
  • 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.

6. 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 observed, as presented in [74,75,76,77][41][42][43][44]. 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, including [78,79][45][46]. More specifically, the authors of [78][45] 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 (both utilizing DC current) favor the adoption of a DC power system for data centers. According to [80][47], 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][48] 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 installed 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 building. 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 [82,83,84][49][50][51]. In fact, according to the research of [83][50], 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 efficient 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][52] 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][53] 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][54][55].

6.7. Industrial Applications

MVDC MGs have applications in some industrial systems, 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 field of copper or zinc electro-winning installations, electric arc furnaces, etc. [79][46]. 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][46][56].

6.8. Synopsis of Applications of DC MGs

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][57][58]. 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.

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