Fundamentals of Microgrids

Development and Implementation

Stephen A. Roosa, Stephen A. Roosa

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000168655
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Microgrids provide opportunities to develop new electrical networks targeted for the needs of communities. The fourth industrial revolution is associated with the global trend toward decentralizing energy grids. Within this context, microgrids are seen as a solution to how renewable electricity can be supplied to local areas. The Fundamentals of Microgrids: Development and Implementation provides an in-depth examination of microgrid energy sources, applications, technologies, and policies. This book considers the fundamental configurations and applications for microgrids and examines their use as a means of meeting international sustainability goals.

It focuses on questions and issues associated with microgrid topologies, development, implementation and regulatory issues. Distributed energy resources are defined, stand-a-lone generation systems are described and examples of typical microgrid configurations are provided. The key components of developing a business model for microgrid development are also considered.

Features:

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• Describes what microgrids are and details the basics of how they work while considering benefits of microgrids and their disadvantages.

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• Provides answers to the fundamental questions energy managers and other professionals want to know about the basics of microgrids.

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• Details the applications for microgrids and demystifies the types of microgrid architectures that are successful.

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• Includes real-world examples of functioning microgrids which provide models for the development of microgrids in the future.

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• Discusses the key considerations that must be addressed to develop a business case for microgrid development.

1 Introduction to Microgrids

Microgrids are an exciting way to provide electricity to serve local needs and solve supply problems. They offer new ways to provide reliable and resilient electrical power. With the expanding use of decentralized energy resources, the role of microgrids in power supply systems is increasing as more are being developed.

Microgrids often consist of a number of small power supply systems which makes them more flexible than a single electrical power source. They can generate electricity from fossil fuels and renewable energy resources. They are categorized by their size, the types of customers they serve, the types of generation systems they have, and the regions in which they operate. They can be configured to produce either direct current, alternating current, or both. Some are semiautonomous and provide both heat and power. Their purposes vary from those of conventional electrical plants.

The categories of the energy we use for electrical generation are divided unevenly into nonrenewable sources; carbon-based energy sources such as coal, oil, and oil shale; and renewable sources such as wind power, solar, geothermal, and gravitational water sources. A conventional power station, also referred to as a power plant, powerhouse, generating station, or generating plant, is an industrial facility for the generation of electric power. Most power stations contain one or more generators, rotating machines that convert mechanical power into electrical power. Conventional power stations typically use fossil fuel-fired generators, most notably coal, natural gas, and nuclear power, using the Rankin cycle. However, there are many others that use renewable technologies such as hydroelectric dams and large-scale solar power stations. Such stations are centralized and require electric energy to be transmitted over long distances [1].

Renewable energy sources can be categorized as sustainable and inexhaustible, while most nonrenewable energy sources are potentially unsustainable and likely exhaustible. Microgrids are alternatives to conventional power stations for generating power. Technologies are converging that enable microgrids to be seen as a new and viable solution to providing locally generated electrical power. They are the next step in the evolution of supplying and delivering electricity.

WHAT ARE MICROGRIDS?

Today, large power plants commonly use coal, gas, hydroelectric, and nuclear energy. They are centralized. Electricity is transmitted over long distances. Like central power stations, microgrids generate electricity. Being small-scale versions of the larger electrical grids, they are sometimes referred to as mini-grids. Microgrids have become more common with the growth of renewable electrical generation. While microgrids share common characteristics with utility grids, they are not identical. The electricity generated by microgrids is primarily consumed by local users within the boundaries of the microgrid. They are often custom configured to solve local electrical generation problems and may use combined heat and power systems.

As there does not seem to be a universal definition as to what a microgrid is, a number of various definitions are offered. According the U.S. Department of Energy, a microgrid is a “group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid” [2]. A microgrid can connect and disconnect from the grid to enable it to operate in either grid-connected or island modes. When thus configured, seamless transfer of electric power is important for system operation. The International Council on Large Electric Systems (CIGRR, Fr.) C6.22 Working Group defines microgrids as “electricity distribution systems containing loads and distributed energy resources (such as distributed generators, storage devices, or controllable loads) that can be operated in a controlled, coordinated way either while connected to the main power network or while islanded” [2]. Its definition is qualified by the presence and type of electrical generator, storage devices (electrical, pressure, gravitational, flywheel, and heat storage technologies), and controlled loads [2]. Examples of controlled electrical loads include automatically dimmable lighting or pumping systems. Yet another definition: “Microgrids are modern, small-scale versions of the centralized electricity system. They achieve specific local goals, such as reliability, carbon emission reduction, diversification of energy sources, and cost reduction, established by the community being served. Like the bulk power grid, smart microgrids generate, distribute, and regulate the flow of electricity to consumers, but do so locally” [3].

The term microgrid today refers to a collection of multiple smaller energy resources of different types (including renewables such as solar arrays and wind turbines but also natural gas generators, combined heat and power systems, and energy storage) often owned by local prosumers, small businesses or small power operators interconnected to supply the locality with self-sustaining power [4]. Microgrids located in the U.S. and other Organization for Economic Co-operation and Development (OECD) countries have capacities in the range of hundreds of kilowatts (kW) or megawatts (MW) [4]. Utility-scale microgrids in the U.S. typically exceed 10 MW.

In many non-OECD countries, microgrids of this scale (sometimes called mini-grids) are the primary electrical power source for people in rural communities where access to the central grid is either too costly or unreliable, or is perhaps nonexistent [4]. The label microgrid in countries such as India is reserved for systems under 10 MW. Though India is striving for universal access to electrical power, other parts of the world are struggling. With 13% of the world’s population, Africa accounts for only 4% of the world’s energy demand, leaving roughly 600 million people with no access to electricity [5].

Standalone Power

Examples of technology options for electrical supply can be roughly viewed as two scales, the nature of which varies: 1) utility-scale—large, bulk power applications with better economics but experienced indirectly through purchases of premium electricity products with some renewable energy content; 2) distributed-scale—small, on-site applications experienced directly but somewhat limited due to economics and resource availability yet having greater opportunity for renewables [6]. Modern standalone electrical generation systems are usually considered to be distributed-scale applications.

A standalone power system (SAPS or SPS), also known as a remote area power supply (RAPS), is an off-the-grid, independent electricity system for locations that lack an extensive utility-scale electricity distribution system. The main categories of technological components for a SPS are: 1) components that supply power (generation equipment); 2) components that store energy for later use (energy storage equipment); and 3) components that convert one form of power to another (inverters) and those that control the flow of power in a system (power conditioning and control equipment) [7]. An SPS is often used to provide electricity for remote buildings, telecom sites, villages, or small-scale manufacturing (e.g., mining, resource processing) sites. In such situations, connecting to the electric grid via rural electrification programs is likely more expensive than installing an SPS. Since power is provided only to a local geographical area, transmission costs are minimal. Examples of an SPS include diesel or natural gas generators and solar photovoltaic systems. For owners, the most common reason to have an SPS is that a grid connection is not available. Having an SPS might also appeal to their desire for independent power, better align with their environmental values, or reduce their operation and fuel costs associated with generating electricity.

Typically, standalone power systems include one or more methods of electricity generation, energy storage, and regulation. They can be configured to act or be used as an uninterruptable power supply. They often take advantage of a combination of components and technologies to generate reliable power, reduce costs, and minimize inconvenience [8]. On-site generation (OSG), district/decentralized energy, or distributed generation systems generate or store electricity from a variety of small, grid-connected devices called a distributed energy resource (DER) or distributed energy resource systems (DERS) [1]. Some of the strategies for SPS that deploy distributed energy resources (DERs) include reducing the amount of electricity required for loads (via energy efficiency or load shedding) and using fossil fuel or renewable hybrid systems [8]. Many remote SPS that previously relied on diesel fuel are being augmented or replaced with renewable energy systems as prices for diesel fuel increase or become more volatile.

Distributed Energy Resources

Microgrids are designed to satisfy the demands of energy consumers within their limited service areas. Their local low- and medium-voltage electrical distribution systems contain a set of connected electrical loads and generation from DERS. Electrical switch gear, either manually or by automatic controls, is used to isolate the microgrid at the point of common coupling (PCC). For automatic or standby mode, the system operates as a single-utility service in the event of a power failure [9]. It can also operate in automatic peak-shaving mode for paralleling with the utility to reduce consumption of the power it supplies [9]. Manual-mode operation provides a wide latitude of control when conditions warrant using the system as a supervised station [9].

DER systems for microgrid applications may use fossil fuel or renewable energy sources, or combinations of both. Examples include thermoelectric generation, small hydro, micro combined heat and power (CHP), diesel, biomass, biogas, solar power, wind power, and geothermal power that connect to existing power distribution systems. DERs allow for infrastructure development in ways that protect against singular events and vulnerabilities [10]. Grid-connected devices for electricity storage can be classified as DER systems and are often called distributed energy storage systems (DESS). By means of an interface, DERs can be managed and coordinated within a smart grid. Distributed generation and storage enable the collection of energy from many sources while lowering environmental impacts and improving security of supply [1].

Within defined boundaries, microgrids provide a controllable electrical supply which maintains equilibrium between supply and loads. Many microgrids can connect and disconnect from the electric grid. This allows them to operate in either island (standalone, possibly emergency state) or grid-connected...