A Smart Grid Paradigm for Smart Cities: DC Microgrids
1. Introduction
DC microgrids have grown in popularity in recent years as the complexity with which they can combine diverse energy storage technologies such as batteries and supercapacitors has increased, as has the usage of solar photovoltaic (PV) and fuel cell power, among other DC loads. Another element contributing to DC microgrids' rising appeal is their ability to accommodate a wide range of DC loads. DC microgrids do not require harmonics, frequency control, synchronization, reactive power regulation, or other controls. Voltage management and power distribution become more difficult when several dispersed generators, such as solar, wind, or fuel cell generators, are integrated with loads and energy storage devices on a single DC bus.
2. The Pros and Cons of a DC Microgrid
2.1. Sustainability and tenability
The carbon footprint of direct current (DC) microgrids is lower than that of alternating current (AC) microgrids. The ability of DC microgrids to function independently from the main power grid is an environmentally favorable characteristic. Owners have more flexibility in accomplishing sustainability goals when they control all aspects of energy generation, distribution, and usage in their properties. Instead of purchasing energy from distant power plants that are likely to use fossil fuels to create electricity, building owners with this level of control can generate their own DC power from renewable sources such as solar panels and wind turbines.
DC microgrids enable decentralized power generation, which can lower power plant emissions by up to 6%. Because DC power is being generated and consumed, a DC microgrid can save an extra 10% on energy expenditures by eliminating the requirement for wasteful AC/DC and DC/AC conversions.
2.2. Credibility
A DC microgrid is an important power layer for supplying dependable power to buildings and infrastructure. DC microgrids are distinctive in that they can "island," or function independently of the main power grid, while still satisfying local demand. This is a backup system for when a natural disaster or severe weather disrupts the main power grid. In the case of a blackout, DC microgrids may function independently of the grid, making them more dependable than traditional AC systems. Despite the extensive usage of air conditioning, there are still miles and miles of unprotected open space. However, given to its smaller scale, a DC microgrid may be more resistant to the environment and natural calamities.
2.3. Incorporation
DC microgrids are the most reliable and efficient solution to power a smart building. "Smart buildings" have evolved as businesses become more conscious of the need of providing secure, comfortable, and productive work environments for their employees. Sensors, lighting, displays, and other Internet of Things (IoT) enabled devices are used in smart buildings to improve occupant comfort, safety, and efficiency. A Power over Ethernet (PoE) network may be established to power and connect each of these devices. Because power over Ethernet only requires a low DC voltage, a DC microgrid is the most efficient approach to generate power because no AC/DC conversions are required inside the building. DC microgrids can also be utilized to link to other systems.
DC microgrids can also be utilized to link to a smart building's network. Connecting power plants to a smart building platform enables facility managers to more efficiently distribute electricity to devices while reducing waste. The facilities manager, who is in charge of ensuring that the building and company have enough energy, may monitor the DC microgrid.
There are several variables leading to the growing popularity of microgrids. It is advantageous to use both conventional and renewable energy sources. Second, having them in place protects you against service disruptions caused by things like a faulty central power supply or a natural disaster. As a result, they are a feasible solution in extremely harsh regions. Finally, microgrids can greatly reduce expenses. The use of alternating current (AC) distribution systems is standard practice for traditional power grids. However, this is not the most efficient or adaptive network structure for distributed renewable DC generating and storage systems. Furthermore, most electronic loads may be powered directly from DC networks. Rectifiers are often employed by electronic loads to convert alternating current (AC) from the power grid to direct current (DC).
Rectifiers are often employed by electronic loads to convert alternating current (AC) from the power grid to direct current (DC). Including these rectifiers boosts the costs and reduces load efficiency. Several application kinds have made use of the DC network backbone. According to some research, employing such technologies may increase an organization's production by up to 18%.It is not impossible that DC microgrids would prove to be more reliable than AC ones. As previously stated, the bipolar structure can nevertheless contribute to grid operation. The table below illustrates the several advantages of this network design over AC networks.
Direct current (DC) is the primary kind of power provided by distributed generators. The growing use of battery banks and supercapacitors in DC microgrids illustrates the importance of storage systems in relation to distributed and renewable energy generating. These generators can assist increase system efficiency by obviating the requirement for a fresh DC/AC converter. The efficiency of the distribution system is increased since various storage technologies, like batteries, may be directly connected into it and generally produce and receive DC power. This is due to the fact that electronic loads are frequently powered by DC voltage sources, allowing them to be directly connected to the generators that resemble distribution grids.
More rectifiers are required to produce the direct current voltage required by the majority of loads due to the anticipated increase in the number of electric cars connected to the grid [79]. The ability to directly connect electric vehicles to the grid is excellent news for system efficiency since it eliminates the power quality problems that AC systems suffer from. This is excellent news for system effectiveness. In a DC microgrid, the skin effect is avoided since the whole current flow passes along the distribution cable rather than being concentrated at one location, hence there is no need to synchronize with the utility grid or reactive power. Reactive power and grid synchronization are not necessary either. Because of the large capacity to operate in island mode, losses may be reduced by employing shorter cable runs, and reliability can be increased.
The microgrid's generators and customers are connected through an alternating current (AC) bus system. Renewable energy resources and traditional power generating methods, such engine-based generators, are frequently used in AC microgrids. A battery energy storage system (BESS) and an alternating current (AC) bus are used to coordinate these dispersed power units. DC electricity is produced by renewable energy sources like solar cells, wind turbines, etc. Through power electronics-based converters, AC power may be produced from this output. They are adaptable since they can run alone or in combination with more conventional energy producing methods.
Flexible AC microgrid is made possible by the ease of using a transformer with motors and other alternating current loads. Running this equipment is perfect since the microgrid's dependable AC power doesn't require an inverter for AC loads. The dependability of AC loads was improved by efficient and inexpensive power protection systems. However, there are disadvantages to AC microgrids, including as the decreased conversion efficiency. An adapter's price (especially a DC-AC one) may mount up rapidly. It is possible for regulation to become unstable in terms of frequency, voltage, and other areas. Power fluctuations can harm equipment that works best with a consistent current. Comparing the transmission efficiency to DC equivalents, it is similarly less.
Although converting to DC microgrids may have certain advantages, it also has some significant disadvantages. The requirement for additional costs is a significant issue that might hinder the adoption of this technology. A new point of view is necessary since it might be challenging to persuade people and investors to change their opinions. However, it has been stated that using a DC load converter or similar adaptor is required for DC microgrids. However, it is now possible to use this kind of AC load in DC outlets due to the increasing usage of electronic loads. We can cut costs and get more supporters if we let people use their electronics with both AC and DC power outlets.
A rectifier converter, also known as an AC to DC converter, is normally required whenever alternating current (AC) powered appliances or gadgets are plugged. Since the rectifier enables DC power to be supplied into the circuit, the AC devices can still operate. This information determines the voltage of the DC microgrid that will power residences (and maybe other applications). For instance, the AC voltage of many electronic gadgets ranges from 100 to 240 VRMS. As a result, it is recommended that the DC microgrid voltage be increased over the threshold. Bipolar DC microgrids, however, can be particularly interesting in terms of safety since they permit a reduction in the pole(s)' voltage level, making it feasible to utilize voltage levels between 50 and 120 V.
In terms of their underlying operating concept, DC microgrids and AC microgrids are similar. The use of a direct current (DC) bus network rather than an alternating current (AC) one to connect the dispersed generators and loads is one of the most obvious differences. Typically, these DC buses run on 350–400 V of energy. The primary DC bus can be separated into several lower-voltage buses to accommodate the low-voltage needs of electronic loads [83]. Solar modules, which normally run between 20 and 45 V, may be connected to low-voltage power sources like DC-type microgrids more easily by using high-voltage gain DC-DC converters. As a result, it is now possible to connect power sources with different voltages.
Depending on how much power or voltage increase they offer, these converters may be divided into many categories. They are ideal for powering more complex electrical devices because they convert energy more effectively.
Cost-effective converter systems that may provide benefits in addition to the monetary savings provided by renewable energy sources are required. Transmission efficiency is highest when there is no reactive current in the circuit. A DC microgrid can improve the dependability of energy provision to rural areas. Because of the low current needs, smaller cables may be utilized even at higher voltages. Using efficient, simple, and basic methods to control factors such as frequency and power consumption, as well as synchronization, harmonics, and reactive power.
However, due to immature power protection technologies, DC microgrids may be insecure, particularly in places with sensitive electrical demands. One possible barrier to widespread adoption of such systems is their relatively high entry cost. DC microgrids are less common than their AC equivalents. They are less likely to function together when there are greater AC loads. Voltage drop issues are more likely to develop without reactive power sources, especially in larger systems. Switching from AC to DC is a more complicated and expensive operation due to the present infrastructure that must be updated. The first table compares an AC Microgrid to a DC Microgrid.
Table 1 shows a comparison between AC and DC microgrids.
3. DC Microgrid Infrastructure Topology in Smart Cities
A microgrid would be complete with DC (direct current) power generators such as solar panels if it has DC (direct current) infrastructure, DC (direct current) end devices, and DC (direct current) battery storage. According to the IEEE 2030 standard, a smart grid infrastructure is the combination of multiple domains, one of which is the DC grid, which is a vital component of smart cities. Because of the age and fragility of the electricity infrastructure, problems are unavoidable. There is a major threat to the future supply of energy to buildings and their occupants, and the moment to prepare has arrived.
DC microgrids are the logical choice for the future of global power generation due to their various environmental, economic, and social benefits.
3.1. A Single Bus Topology
The single-bus topology is the most basic microgrid design. Various converters will link all of the system's sources, loads, storage devices, and so on to a central bus. Figure 3 depicts the fundamental design of a single bus DC microgrid.
Figure 3 depicts the fundamental design of a single bus DC microgrid.
Storage devices can also be connected directly. Despite its strong construction, bus voltage variations caused by changes in battery current and SOC make this design unstable. Because of the converters, voltage control may be simplified and made more adaptive, increasing the overall efficiency of the system. The battery is linked to the DC microgrid through a PEC, as indicated in Figure 1. However, there are difficulties, such as creating control circuits or powering many units from a single network bus.
Switching from a single-bus architecture to a multi-bus design with different voltage levels improves the system's dependability and flexibility. To improve system efficiency, designers are exploring utilizing a multi-bus architecture DC microgrid, as shown in Figure 4, rather than a single bus architecture, as shown in Figure 3. There are several options for identifying which bus should be utilized to connect the load. During moments of surplus or deficit, each microgrid in a microgrid cluster can operate as an injector or absorber. The different elements of the system complement one another and can be operated individually if necessary.
Figure 4 depicts the fundamental design of a multi-bus DC microgrid.
3.2. Topology of Reconfigurability
Due to the unpredictability of RES, several various reconfigurable designs for DC systems have been developed. The DC systems are coupled to more typical AC systems to maintain system dependability. There are several methods to categorize the shared interface of the systems.
Furthermore, the multi-bus architecture depicted in Figure 4 is widely used because to its extensive coverage across a wide range of applications.
3.3. Radial Topology
This configuration is distinguished by an AC/DC terminal interface. There is only one way for power to reach the load. Figure 5 depicts the DC microgrid's radial arrangement. As previously stated, the setup may use zero, one, or more buses. The circumstances determine whether a radial arrangement is series or parallel. Parallel radial design has mainly superseded series radial architecture in recent years due to its better flexibility and simplicity of isolating defective areas. Using a central bus to distribute electricity adds a significant amount of complexity to the architecture. Because of the reduced distribution losses, this configuration is suited for low-voltage applications like as residential loads.
Figure 5 depicts the fundamental design of a radial topology-based DC microgrid.
3.4. Topology (Ring or Loop)
The availability of many pathways, both at the customer and grid interface, is the primary advantage of a ring topology versus a radial layout with a single channel for power transmission. The fundamental construction of a ring or loop topology-based DC microgrid is depicted in Figure 5. An integrated (Intelligent Electronic Devices) IED allows the system's problematic component to be disconnected at the switch. In the case of a blackout, the system's DC component is disconnected from the power supply.
3.5. Interconnected Topology
Interconnected systems ensure that the DC microgrid always has access to at least one AC source, increasing system reliability. A linked topology may be built using both mesh and zonal architectures. A mesh-like design for high-voltage DC systems is far more dependable than earlier arrangements in use. The impact of a malfunction is reduced in a mesh-type system since there are many AC feeds. This handshake process can be used to isolate the problematic bus. Many approaches for assessing DC systems with multiple outputs are covered in depth. The second architecture, a zonal layout, is commonly utilized on ships for integrated power systems. Zoning provides several alternatives for powering the load. Power can be delivered to the load either all at once from many buses or in sequence from a single bus. Because there may be several buses that can transport people, the bus selection method is utilized to restrict the alternatives.
Buses can swap cargoes with one another if necessary. A typical zonal DC microgrid configuration is shown in Figure 6. The system, which is separated into zones, includes energy sources, converters, loads, and energy storage devices. Power can be switched off to the damaged components while the remainder of the system continues to work correctly by employing several switches. Although this system is more dependable than mesh-based systems, it is more complicated to build. Since the beginning of the technological revolution, the usage of direct current (DC) electricity in ordinary consumer items has increased dramatically. DC systems are presently used for a wide range of modern applications, such as common loads, data servers, communication systems, and so on.
Maintaining a constant bus voltage despite fluctuations in load or generation is critical for these systems. The use of supercapacitors to smooth out DC bus voltage variations has risen in favor in recent years. Figure 6 depicts a supercapacitor-based DC microgrid design created to prevent the need of numerous output capacitors at diverse sources and storage devices. In this configuration, the bus voltage will stay steady independent of external conditions.
Figure 6: A zonal topology-based DC microgrid's basic design.
4. Network Topology Selection
To accommodate energy storage systems (ESS), distributed generation (DG), electric vehicles (EV), constant power loads (CPL), and renewable energy sources (RES), significant adjustments to the traditional distribution network are required. Many of the issues that afflict traditional distribution networks can be overcome by including DC or hybrid DC/AC microgrids into the networks. However, DC microgrid structure and topology design are important and constitute a considerable barrier to the planning process. Plans for the development, operation, and administration of a DC microgrid are developed. DC microgrids enable the new concept of a net-zero energy building (NZEB), which substantially reduces carbon emissions and fossil fuel use.
The architecture of vast DC microgrids must be considered for the successful integration of renewable energy sources on a large scale. It is aimed to create a hybrid ESS with an active topology to function with DC bus voltage. A DC dump load is placed in series with the DC bus to avoid overcharging the ESS and to maintain a constant bus voltage. A radial DC bus connects the components of a microgrid, such as lead-acid batteries, biodiesel-powered diesel generators, residential and business loads, and renewable energy sources. A hybrid DC/AC microgrid combines the finest features of DC and AC microgrids. This design connects DC and AC loads to independent but complimentary DC and AC grids to improve overall efficiency.
Another advantage is that charging stations for electric vehicles may be connected into the DC bus. As a result, establishing a topology for a DC microgrid is a critical planning phase with implications for both operation and control.
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