DC Microgrids

Benefits, Designs, Perspectives, and Challenges of DC Microgrids

1. Introduction

In recent years, a new paradigm for electrical distribution networks has arisen. Instead of traditional AC networks, which are particularly associated with micro- and mini-grids, the usage of DC networks found to be a preferable alternative when numerous factors were considered. One of the features is concerned with distributed generation (DG) and renewable energies. Other considerations include the usage of energy storage devices, the stability of electric networks, and energy-efficient loads. Because of these advantages, DC networks are currently regarded highly appealing, particularly in the context of modern smart power distribution systems.

Because of the numerous benefits that can be obtained with these microgrids, as well as the previously mentioned shift in loads and utilization of storage systems, they may be employed in a variety of applications. Small dwellings, buildings, computer centers, and agricultural fields are examples of these applications. In recent years, new realities have emerged, such as charging infrastructures for electric cars, which is an excellent illustration of a prospective use of this type of microgrid. In reality, electric car rapid chargers require a DC power source, which is why suggestions for DC microgrids have begun to emerge. Prosumers began to play an important role in the framework of electricity distribution Low Voltage (LV). 

Once significant savings can be realized, their incorporation in LVDC systems is considered as logical. Another consideration is the influence of DC microgrids on social welfare. This type of microgrid, particularly in the context of off-grid systems, has the potential to significantly improve the quality of life for many people, particularly those living in remote towns and villages.

Taking current technology development and the future realities of electrical generators and loads into account, DC microgrids began to emerge as an essential alternative to AC infrastructures. AC and DC infrastructures are projected to coexist as complimentary options in the very near future. In this context, this study discusses the prospects for DC microgrids in the near future.

Several issues linked with DC infrastructures must be overcome. The defining and standardization of these networks is a key issue. On the other hand, there are several factors that must be established, such as the introduction of new standards and/or laws.

Under the above mentioned elements, this article will discuss the technology underlying DC microgrids, which may be employed in a variety of future applications. Many of these applications have yet to be realized. This paper will also describe novel applications in which this type of microgrid may be successfully deployed in this context. Some applications and accompanying solutions are the authors' visions. Finally, we will discuss the problems that must be overcome in order to utilize this technology. With this in mind, we hope to give insight into the future of DC microgrids.

The format of this work is divided into six pieces, the first of which is this introduction. The usual architectures and configurations for DC microgrids that have already been developed are discussed in the second part. In the third part, the advantages of using a DC microgrid over regular AC grids are discussed. Section four discusses the potential uses of DC microgrids. Some of the suggested applications and solutions are the writers' imaginations. Section five focuses on the upcoming issues that are critical to the development of DC microgrids. Finally, in section six, the paper's conclusions are offered.

2. Architectures and Configuration

One issue that has yet to be standardized is the sort of architecture that should be used or is best suited to a particular application. In actuality, there are various alternative topologies for establishing a DC microgrid. These various structures are as follows:

• Topology with a single bus. Because it is made up of a single DC bus, this topology is the simplest. As a result, all generators, storage systems, and loads will be linked to a single point (bus). Figure 1 depicts two common instances of this architecture, one with an electrical grid connection and the other with an operation in islanded mode. Aside from its simplicity, this topology is distinguished by low maintenance needs and inexpensive expenses.

Figure 1 shows a diagram of a typical single bus

 topology: (a) linked to the grid; and (b) in island mode.

• Radial topology is a kind of topology. This topology may be thought of as an extension of the single bus topology. As seen in Figure 2, this architecture includes many DC buses, each of which is utilized to connect generators, storage systems, and loads. There are two common configurations: series and parallel. In the first arrangement, two or more DC microgrids can be linked in series (Figure 2a), while the other is linked in parallel (Figure 2b). This architecture is still simple and allows for varying voltage levels. Furthermore, this topology improves dependability. One issue that may arise is considerable volatility during the islanding mode.

Figure 2 depicts a schematic of a typical radial topology: (a) series configuration and (b) parallel arrangement.

• Topology might be ring or loop. In this configuration, all generators, storage systems, and loads will be connected in a loop to the same DC bus to allow supply from both sides (Figure 3). As a result, this topology is more dependable than earlier designs since, in the event of a malfunction in the DC bus, it is feasible to operate in a single bus configuration, and the primary disadvantage with this topology is its increased complexity.

Figure 3 shows a schematic of a typical ring topology.

• Mesh structure. This topology is distinguished by the ability to combine an integrate ring (or rings) with radial topologies using a mesh configuration derived in this manner (Figure 4). It has a complicated structure that provides for more dependability and versatility when compared to prior versions.

Figure 4 shows a schematic of a common mesh topology.

• Topology with interconnections. Previous topologies were distinguished by a single link to the alternating current main grid. Thus, in order to increase system stability, it is also possible to link it to alternative AC grids (two or more), indicating that this topology is defined by interconnections. Figure 5 depicts an example of this type of architecture, in which the DC microgrid is linked to two AC grid supplies.

Figure 5 is a typical example of an interconnected topology.

Table 1 summarizes the DC microgrid topologies and the essential references connected with each of the structures.

Table 1 summarizes the DC microgrid topologies and references.

In terms of configurations, the following are the most common ones that have been used, tested, and studied:

• The configuration is unipolar. This is the simplest design because it is made up of only two wires. All generators, loads, and storage devices will be linked to the same poles in this setup. Figure 6 depicts two typical examples of this architecture, one connected to the grid and the other operating in island mode.

Figure 6: A schematic of a typical unipolar configuration: (a) linked to the grid; and (b) in island mode.

• The configuration is bipolar. This design is more complicated since it is made up of three wires (a positive pole, a neutral pole, and a negative pole). There are several ways to link the generators, loads, and storage system in this design. They can, in fact, be linked to different poles (positive and neutral, neutral and positive, or the three poles). This equipment can also be linked to other voltages, particularly between the positive and negative poles or between one of the poles and the neutral pole. Figure 7 shows two examples of this sort of arrangement, one connected to the grid and the other running in island mode.

Figure 7: A diagram of a typical bipolar configuration: (a) linked to the grid; and (b) in island mode.

Table 2 provides an overview of the DC microgrid configurations and the essential references related with each of the structures.

Table 2 summarizes the DC microgrid setups and references.

When comparing the two arrangements, the bipolar DC microgrid demonstrated various benefits, including a greater number of voltage levels (two instead of one), enhanced efficiency, and a higher-quality power supply. Another key characteristic is dependability, which is better in the bipolar architecture because if one of the wires fails, the load may be provided by the remaining two healthy lines. Another advantage is that the maximum voltage to ground may be reduced by using the link between the positive and negative poles to create extended voltages. However, there are several drawbacks to the bipolar design. 

The biggest downside is the need for extra wires and the possibility of voltage imbalance between the bipolar terminals. This imbalance can be generated by connecting different loads to each of the terminals. Another source of the imbalance might be generation. Typically, generators such as PV employ a DC/DC converter that only connects to a single pole, contributing to voltage imbalance. However, there are a number of measures that may be implemented to reduce or remove the imbalance. 

3. Benefits

AC distribution systems are used in traditional electrical infrastructures. This form of network, however, is not the most efficient or adaptable in the context of distributed renewable DC generating and storage systems. Furthermore, DC networks are capable of directly powering the majority of electronic loads. Electronic loads often require a DC voltage source, and they typically employ a rectifier to connect to the AC network. However, the installation of these rectifiers diminishes the load's efficiency and raises its cost. DC infrastructure networks have already been effectively established in a number of specialized applications in this manner. According to certain research, the utilization of these infrastructures results in efficiency gains ranging from 12% to 18%.

Another feature in which DC microgrids may be superior to AC networks is dependability. Indeed, as previously indicated, when using a bipolar construction, even if one of the poles fails, the grid may still be operated, at least partially. The following are a few of the benefits connected with this type of network over AC networks:

• The fact that the dispersed generators mostly generate DC electricity. Thus, the direct connection of these generators to the grid without the requirement for a new converter (DC/AC) enables for system efficiency to be improved.

• Storage systems are important in the context of decentralized and renewable energy sources. As with generators, these storage technologies (such as batteries) often create and receive DC power, allowing for improved global system efficiency through direct integration in the distribution system.

• The fact that electronic loads are often powered by a direct current voltage source, allowing them to be directly linked to the distribution grid. Most loads require a rectifier to supply the necessary DC voltage source.

• The expected rise of electric cars need a link to electrical grids in order to charge their batteries. Thus, the ability to directly connect an electric car to the grid in order to eliminate the rectifier can increase overall system efficiency.

• The decrease of power quality issues that commonly afflict AC grids. Indeed, voltage sags and swells, flickering, harmonics, and imbalances, which are common in AC grids, may be avoided in these DC microgrids.

• The absence of constraints for synchronization with the utility grid and reactive power;

• The absence of skin effect, which would result in a complete dispersion of current across the distribution cable. As a result, losses will be reduced or smaller section cables will be used.

• The potential to increase dependability due to a large capacity to operate in island mode.

Although DC microgrids have several advantages, there are also disadvantages to switching to this type of infrastructure. One of the key issues is the necessity for additional fees, which might stymie the transition to this sort of technology. Another key component is a shift in mindset, since convincing employees and investors to change is not always simple. Many individuals, on the other hand, argue that it will be essential to convert AC loads to DC loads or to include an additional adaptor for usage in DC microgrids. However, the majority of loads nowadays are electronic loads, which means that this type of AC load may be utilized directly in DC sockets.

This will save money and persuade customers to make the switch because they can use their equipment in either AC or DC connections. AC electronic loads are often linked to the socket through a rectifier converter (AC to DC). Typically, this converter is a single-phase H-bridge diode rectifier (Figure 8a). Taking this into account, the AC equipment can still be used because the rectifier allows for operation with a DC input voltage. Because there is no negative voltage, just two diodes are utilized in this scenario, as seen in Figure 8b. This is a crucial consideration when selecting the DC microgrid voltage level for residential (and other) applications.

Many electrical devices, for example, require an AC voltage of 100 to 240 VRMS. Taking this into account, the voltage level of the DC microgrid should be greater than that value. In the security context, however, the usage of bipolar DC microgrids might be quite intriguing since they allow for a reduction in the voltage level of the pole(s), because in this case,  voltage levels of ± 50 to ±120 V can be used.

Figure 8: A typical rectifier utilized in AC electronic loads: (a) connection to an AC socket and (b) connection to a DC socket.

Another factor to consider is the preservation of the DC infrastructure, which is more complex than the one employed in the AC infrastructure. This is an area that still need investigation.

Table 3 provides a review of the benefits and drawbacks of AC and DC microgrids.

Table 3 summarizes the benefits and drawbacks of AC and DC microgrids.

4. Perspectives

As previously noted, there are several applications for DC microgrids, and their deployment might be a significant advantage over traditional AC grids or microgrids. However, only a few DC microgrid applications have been realized thus far. Furthermore, some of the applications that have previously been developed were part of an experiment or were integrated into a research project. One of the uses of DC microgrids that has already been mentioned is data centers, but there are many additional applications where DC microgrids might be a valuable asset.

Electric car charging station infrastructures are one application area where DC microgrids are likely to be used. 

There are several points of view about this. One of the options is to employ a bipolar DC microgrid, as depicted in Figure 9. It is possible to see in this figure that this is a case in which a bipolar configuration is very well adapted. Other options, like the usage of a unipolar DC microgrid, were also presented. One critical feature of this microgrid is whether or not the type of application can simply incorporate a storage system. Storage systems in these infrastructures can be quite essential since they allow for the mitigation of potential load surges. Renewable energy sources such as PV generators may also be simply integrated.

Figure 9 depicts one of the potential applications for a DC microgrid in electric car charging infrastructure.

Interconnecting a Medium-Voltage DC grid with a Low-Voltage DC grid is another conceivable approach. This will be particularly relevant in places near residential customers where there are some renewable energy production parks of sufficient size to be linked to a Medium-Voltage DC grid. Figure 10 depicts a proposed infrastructure plan of this type. Because home users are linked to an LV grid, a transformer is necessary, as indicated in this image. However, in this specific scenario, adopting solid state transformers (SST) is highly desired. Another consideration is the LV grid arrangement, which might be advantageous in this instance.

Figure 10 depicts a DC infrastructure vision in which a Medium-Voltage DC grid is interconnected with a Low-Voltage DC grid.

Residential areas and buildings are another application area where DC microgrids may play a key role in the future. DC microgrids are particularly useful in residential individual residences, as many of them now include solar generators. This has been an instance of success in the usage of renewable energies by consumers (usually referred to as prosumers). However, one element that has been confirmed is that many prosumers do not utilize the produced energy during peak hours. Some prosumers sell energy, although the prices are usually not the greatest. In this way, the utilization of storage systems is anticipated in the future.

This may be investigated in the context of second live batteries, which are expected to become accessible as a result of the widespread usage of electric cars. As a result, Figure 11 depicts a feasible construction for individual residential homes. A parallel construction with an AC network and a DC network may be the ideal solution, as it will allow the AC networks to be supplied without the need for an additional power electronic converter (DC to AC).

Figure 11: A possible arrangement for individual household electrical infrastructure: (a) solely a DC network and (b) AC and DC network architecture running in tandem.

Buildings (residential or commercial) are another element comparable to the one offered for individual residential residences. The same parallel infrastructure might be considered one of the most ideal in this circumstance. As depicted in Figure 12a, one perspective for one simplified design solely covers DC infrastructure linking to renewable energy generators, storage systems, and finally the electric car charging system. There is just one connectivity between the DC and AC infrastructures in this concept. However, the reliability and efficiency of the electrical distribution system may be increased by using several connections points rather than just one.

Furthermore, system dependability allows for power flow optimization and enhances the capability to deliver ancillary services to the AC network. Figure 12b depicts a proposed scheme for this system. Another viewpoint involves an internal DC infrastructure (just DC or parallel DC and AC). Again, as illustrated in Figure 13, the infrastructure can be made up of one or more links in the main DC infrastructure that will feed each dwelling. The bipolar layout is most appropriate for the primary DC infrastructure that will service each house.

Figure 12 depicts a possible arrangement of a building's electrical infrastructure in which only a DC infrastructure links to renewable energy generators, storage systems, and electric car charging systems: (a) with a single connectivity and (b) with many interconnections.

Figure 13: A possible configuration of the electrical infrastructure for buildings in which a DC infrastructure is also considered inside the residences: (a) with only one connector; (b) with many interconnections.

LV electrical networks are another area where parallel infrastructure may become highly important in the near future. This parallel structure makes sense since it is expected that almost all residences and buildings would integrate renewable energy sources and, ultimately, energy storage systems. This is especially significant in the context of renewable energy communities. Figure 14 depicts one perspective for this parallel infrastructure. This infrastructure will allow the system's efficiency and reliability to be improved.

Figure 14: A look at the parallel AC and DC infrastructure used in LV electrical networks.

DC microgrids are also projected to play a significant role in rural regions. This is strongly recommended, especially in the case of an isolated DC infrastructure with no adjacent AC infrastructure. In this situation, the DC microgrid can be made up of renewable energy sources (such as solar generators), fuel cells, storage systems, pumping systems, warehouses, and support buildings. Figure 15 depicts a typical installation that may be utilized in this type of rural application.

Figure 15 depicts an isolated DC infrastructure in a rural setting.

Other intriguing applications in which DC microgrids might be useful are listed below. Other writers have previously implemented or referenced to the applications. One of these sectors is transportation-related applications. Ships are one of the sectors where a significant surge is projected. Another point of view is the usage of these DC microgrids in conjunction with train supply. Data centers are examples of sites where DC microgrids have been employed successfully. Because of their success, DC microgrids are likely to power an increasing number of data centers in the future.

Table 4 provides a review of the probable uses of DC microgrids as well as the essential references related with each of the applications.

Table 4 summarizes the potential uses of DC microgrids and provides references.

This article provides some views into the future of DC electrical distribution networks. The realization of these techniques has the potential to significantly alter the existing electricity distribution system. The elimination of the AC electrical distribution system was not contemplated in almost any of these concepts; rather, their coexistence was discussed.