As the reserves of nonrenewable sources of energy are falling down in the nature we need to look for some other alternatives in place of these sources. Some of the renewable sources of energy are wind, solar, small hydro, etc., but these sources are distributed, hence if we could find a way to combine all these energy sources then loads with high power demand can be served easily. The configuration in which various distributed sources are combined to serve together is simply a microgrid.

Technically, a microgrid is a localized grouping of electricity sources and loads that normally operate connected to the centralized grid, but can disconnect and function autonomously as physical and/or economic conditions dictate.

The microgrid is a logical evolution of simple distribution networks and can accommodate a high density of various distributed generation sources. A typical microgrid power system consists of generators, wind turbines, solar photovoltaic (PV) arrays, and other renewable technologies, such as geothermal generation, main grid connection/interconnection switch, and energy-storage devices, such as flywheels and batteries for long and short-term storage. The typical range of microgrid rating is 500 kW–15 MW.

Why Microgrids?

Some of the various reasons for interest in microgrids include:

• Rising cost and burdens of transmission and distribution (T&D) infrastructure: Building new T&D infrastructure has become difficult in some areas due to permission issues, public resistance, and the difficulty, cost, or both of upgrading or building new infrastructures.
• Integration of renewable and storage technologies: The greatest obstacle to the integration of renewables has been the risk to grid stability. Some renewable energy sources, most notably wind and solar, are intermittent by nature, which increase the voltage and frequency fluctuations on the grids. The risk is greatest when there is a high penetration of renewables. However, technology advancements in grid stabilization and energy storage have addressed these concerns in microgrids. Nowadays, the challenge of renewable energy generation peaking not matching up with the demand peaks can be resolved using energy-storage modules. An energystorage module is a packaged solution that stores energy for use at a later time. The energy is usually stored in batteries for specific energy demands or to effectively optimize cost. The modules can be stored as electrical energy and supplied to the designated loads as a primary or supplementary source. Moreover, it provides a stable and continuous power supply, regardless of the supply source status. Voltage and frequency can also be improved by using storage modules.
• Power quality and reliability: The need for high reliability and good power quality has increased as more customers install microprocessors-based devices and sensitive end-use machines.
• Public policy: In a full reversal from the past, public policy today is favouring distributed generation that offers improved efficiency, lower emissions, enhanced power system security, and other benefits of national interest. Policies supporting this include tax credits, renewable portfolio standards, emission restrictions, grants, and so on.
• More knowledgeable energy users: Energy users are becoming more aware of alternative power approaches and are more willing to consider on-site generation options than in the past. Many are interested in combined heat and power (CHP) as well as reliability enhancements.

Microgrid Topology

Microgrids can operate in parallel with or isolated from the utility grid during emergency conditions or planned events. This type of distribution grid structure offers potential for improvement in power supply efficiency and reliability of power supply in comparison with the traditional and passive distribution grids. But what is a optimal topology for this kind of power distribution network? The well-known approaches include a radial, a normally open-loop, or a meshed structure for the distribution systems, which constitute the possible solution for microgrid optimal topology as shown in Figure 1. The factors determining the selection of optimal microgrid network topology include:

• Size, type, and location of distributed energy resources (DERs) and loads
• Power quality and reliability targets (the minimum power quality that a microgrid has to provide to its customers affects the selection of an optimal network structure)
• Economic constraints/available budget
• Investment cost (primary equipment, protection and control, communication)
• Operating and maintenance costs, including cost of power losses and energy not supplied due to interruptions
• Technical constraints (e.g., protection system, voltage profile, and physical equipment dimensions)
• Voltage level (usually medium-voltage networks are open-loop networks and low-voltage networks are radial, with normally open-loop topologies in some exceptional cases).

A schematic structure of a radial microgrid consisting of fuel cells and photovoltaic generators as the DERs is shown in Figure 2. The PCC (point of common coupling) is the point where the microgrid is coupled to the main utility grid using the STS (static transfer switch).

Microgrid Control and Operation

Microgrid control

Microgrids are comprised of different components, such as:

• Distributed generators, such as microturbines, fuel cells, PV, diesel generators, etc.
• Energy-storage devices, such as batteries, flywheels, supercapacitors
• Flexible loads, such as heating, ventilation, air-conditioning, and lighting
• Reconfigurable feeders, tap-changing transformers, and reactive power compensation.

These components can be controlled in a continuous or discrete way in order to keep a microgrid running in utilityconnected or islanded operating modes, as well as to guarantee a seamless transition between two modes. Typical control tasks include:

• Spinning reserve management to cope with the emergency conditions
• Generation, energy storage, and demand management for both steady-state and dynamic microgrid control
• Network configuration management
• Coordination with distributed generation and feeder overload protection relays.

A major control challenge related to microgrids is coordinating a variety of microsources in the microgrid to facilitate their parallel operations without loss of voltage and frequency stability, and doing so in the most economical way. Techniques and approaches vary, resulting in a continued wide divergence as to what type of microgrid control system will work best. There are several control approaches applicable to microgrids that have been evaluated by a scientific community and tested in the field at several pilots. They vary between fully decentralized and fully centralized approaches, with various hierarchical/hybrid schemes in between.

Microgrid operation

A microgrid can operate in either the grid-connected or islanded mode, and may experience mode transition. In the grid-connected operation mode, the microgrid is connected to the main grid through the point of common coupling and operates in parallel with the main grid to deliver power to the load. From the distribution system perspective, a gridconnected microgrid can be treated as an individual controllable entity, acting like a generator to the supply power or a load to consume power. On the other hand, the microgrid operating in the islanded mode is independent of or isolated from the main grid, due either to disturbance in the main grid or to its geographical isolation, such as a remote island. In the islanded mode, the microgrid operation, such as the voltage and frequency control and regulation, is implemented in a stand-alone system. The transition between the two modes can occur during the operation. For example, when a fault occurs in the main grid, the microgrid is resynchronized and connected back to the main grid via the common coupling point. Some types of microgrids, such as institutional/campus, industrial/commercial, community/utility, and military base microgrids, operate in the grid-connected mode most of the time, and only switch to the islanded mode when disturbance happens to the main grid. Some other types of microgrids, that is, remote off-grid and weakly gridconnected microgrids (such as military forward installations and remote mining industrial sites), operate in the islanded mode all the time, or very often.

Vehicle to Grid Technology

The microgrid may consist of many types of distributed energy resources but the latest trend in the DERs are the electric vehicles which can feed power from the battery packs to the grid or can pull power back from the grid to recharge the battery packs according to the requirements. When the electric vehicles feed power to the grid it is known as vehicle-to-grid (V2G) interaction, while the reverse flow of power for the purpose of charging the batteries is known as grid-to-vehicle (G2V) interaction.

The scheme of the power system with V2G is shown as Figure 3. The power from thermal power plant or wind power station is transmitted to the consumption areas via transmission systems and distribution systems. When electric vehicles are connected to the power grid, they can receive signals of the grid operator and power is fed into the electric grid by two modes—one is vehicles joined to the distribution systems in homes, the other is that vehicles are aggregated and joined to the transmission systems in aggregations. The application of V2G will be beneficial to both grid operators and vehicle owners. In addition, it will bring advantages to the environment in the future.

Benefit to the grid operator

The stored battery energy can be used to serve a portion of the local demand on a feeder thus contributing to peak shaving. Secondary advantages of peak shaving include reducing transmission congestion, line losses, delay transmission investments and reduce stressed operation of a power system. In a deregulated market, load serving entities purchase electric energy through long-term contracts with generation companies and short-run spot electricity markets. Peak shaving applications of electric vehicles reduces the cost of electricity during peak periods. Moreover, the price of services from electric vehicle is more competitive than conventional generations and electric vehicles offer the power system with a flexible controllable load.

Benefit to the vehicle owner

Energy is stored in electric vehicles during the night—when the price is low—and is withdrawn during peak-time—when the price is high, electric vehicles act like pumped-storage units. So, vehicle owners can gain revenue from the difference of prices and compensate part of the initial investment.

Benefit to the environment

Electric vehicles release almost no air pollutants at the place where they are operated. In addition, it is generally easier to build pollution control systems into centralized power stations than retrofit enormous numbers of cars. Another advantage is that electric vehicles typically have less noise pollution than internal combustion engine vehicles, whether it is in idle or in motion. Electric vehicles emit no tailpipe CO2 or pollutants, such as NOx, NMHC, CO, and PM at the point of use.

Technical Challenges

Some of the technical challenges that must be overcome to achieve stable, economic, and secure microgrid operational status must deal with several aspects.

• Intermittent renewable generation: One of the major incentives to deploy microgrids is to facilitate the integration of renewable generation in the distribution system. The power output of renewable generation (such as solar and wind) is significantly influenced by the season and weather, and these are characterized as intermittent power resources. That is, the power output of these resources can vary abruptly and frequently and impose challenges on maintaining microgrid stability, especially in the islanded mode.
• Low grid inertia: A microgrid may include both conventional and modern DERs. Conventional distributed generators, such as diesel generators, usually synchronous generators that directly connect to the grid. Modern distribution grids use the most renewable resources and energy-storage devices that are connected to the grid indirectly through PE interface. These DERs either have a low inertia or are inertialess, and bring dynamic problems to microgrid operation.
• Coordination among distributed energy resources: A microgrid may have various types of DERs, such as diesel generator, microturbine, fuel cell, CHP, energy storage device, and so on. These DERs usually have different operation characteristics in their generation capacity, startup/shutdown time, ramping rate, operation cost/ efficiency, energy storage charging/discharging rate, and intertemporary control limitations. The microgrid operation should consider the characteristics of different components and provide appropriate control strategies.

Conclusion

The microgrid is the cluster of various DERs which is subjected to combine the power from various DERs using various control and operation strategies so as to achieve a higher power level with no compromise with the power quality. The microgrids may have many configurations and are connected according to the geographical location and power demand of the loads. Microgrids can play a significant role in eliminating the energy crisis state in our country but some plans and strategies need to be framed first so as to overcome technical and economic challenges arising with the adoption of the microgrid technology.

Mr Pankaj Verma, Scholar (M E Power System), Electrical and Instrumentation Engineering Department, Thapar University, Patiala, India. Email: bluepankaj123@gmail.com; and Dr Prasenjit Basak, Member IEEE, Assistant Professor, Electrical and Instrumentation Engineering Department, Thapar University, Patiala, India. Email: prasenjit@thapar.edu