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An electrical grid is an interconnected network for delivering electricity from suppliers to consumers. It consists of three main components: 1) power stations that produce electricity from combustible fuels (coal, natural gas, biomass) or non-combustible fuels (wind, solar, nuclear, hydro power); 2) transmission lines that carry electricity from power plants to demand centers; and 3) transformers that reduce voltage so distribution lines carry power for final delivery.
In the power industry, electrical grid is a term used for an electricity network which includes the following three distinct operations:
The term grid usually refers to a network, and should not be taken to imply a particular physical layout or breadth. Grid may also be used to refer to an entire continent's electrical network, a regional transmission network or may be used to describe a subnetwork such as a local utility's transmission grid or distribution grid.
|The examples and perspective in this article may not represent a worldwide view of the subject. (September 2011)|
Since its inception in the Industrial Age, the electrical grid has evolved from an insular system that serviced a particular geographic area to a wider, expansive network that incorporated multiple areas. At one point, all energy was produced near the device or service requiring that energy. In the early 19th century, electricity was a novel invention that competed with steam, hydraulics, direct heating and cooling, light, and most notably gas. During this period, gas production and delivery had become the first centralized element in the modern energy industry. It was first produced on customer’s premises but later evolved into large gasifiers that enjoyed economies of scale. Virtually every city in the U.S. and Europe had town gas piped through their municipalities as it was a dominant form of household energy use. By the mid-19th century, electric arc lighting soon became advantageous compared to volatile gas lamps since gas lamps produced poor light, tremendous wasted heat which made rooms hot and smoky, and noxious elements in the form of hydrogen and carbon monoxide. Modeling after the gas lighting industry, Thomas Edison invented the first electric utility system which supplied energy through virtual mains to light filtration as opposed to gas burners. With this, electric utilities also took advantage of economies of scale and moved to centralized power generation, distribution, and system management.
During the 20th century, institutional arrangement of electric utilities changed. At the beginning, electric utilities were isolated systems without connection to other utilities and serviced a specific service territory. In the 1920s, utilities joined together establishing a wider utility grid as joint-operations saw the benefits of sharing peak load coverage and backup power. Also, electric utilities were easily financed by Wall Street private investors who backed many of their ventures. In 1934, with the passage of the Public Utility Holding Company Act (USA), electric utilities were recognized as public goods of importance along with gas, water, and telephone companies and thereby were given outlined restrictions and regulatory oversight of their operations. This ushered in the Golden Age of Regulation for more than 60 years. However, with the successful deregulation of airlines and telecommunication industries in late 1970s, the Energy Policy Act (EPAct) of 1992 advocated deregulation of electric utilities by creating wholesale electric markets. It required transmission line owners to allow electric generation companies open access to their network.
With deregulation, a more complex environment occurred as opposed to the traditional vertically-integrated monopoly that oversees the entire grid’s operations. Newer participants entered the market including Independent Power Providers (IPPs) who decided and constructed the new facility; Transmission Companies (TRANSCOs) who constructed and owned the transmission equipment; retailers who signed up end-use customers, procured their electric service, and billed them; integrated energy companies (combined IPPs and retailers); and Independent System Operation (ISO) who managed the grid being indifferent to market outcomes. Also, day-to-day to long term operations altered. Infrastructure additions which were long-term planning now became an investment analysis with IPPs that decided construction of a new power plant under economic considerations (taxes, labor and material costs) and ability to obtain financing. Load and supply management that fell under mid-term planning became risk management as private utilities had to manage a portfolio of end customers and assets with the company’s risk preference. Day-ahead scheduling and real time grid management in the short-term planning which involves forecasting demand and dispatch schedule became asset management as power plants and grid equipment were assets to be scheduled and dispatched. Here, the ISO sets dispatch schedule at the market clearing price where the supply bids of generating units equilibriated with demand bids of retailers.
Many engineers argue the unfortunate disadvantages that stem from deregulation. Where under regulated monopolies, long distance energy lines were used for emergencies as backup in case of generation outages, now, particularly in North America, the majority of domestic generation is sold over ever-increasing distances on the wholesale market before delivery to customers. Consequently, the power grid witnesses fluctuating power flows that impact system stability and reliability. To reduce system failure, the power flow of a transmission line must operate below the transmission line’s capacity. Yet now, companies are continually operating near capacity. Additionally, as utilities exchange power to other utilities, power flows along all paths of connection. Therefore, any change in one point of generation and transmission affects the load on all other points. Oftentimes, this is unanticipated and uncontrolled. Usually, a longer line’s capacity is less than a shorter line’s capacity. If not, power-supply instability occurs resulting in transmission lines that break or sag. Such phase and voltage fluctuations cause system interruptions as witnessed in the Northeast Blackout of 1965 (which involved a circuit breaker to trip) and 2003 (which involved a sagging line on a tree that rippled in magnitude). Furthermore, IPPs add new generating units at random locations determined by economics that extend the distance to main consuming areas adversely affecting power supply. Also, utilities, because of competitive information needs, do not publicize needed data to predict and react to system stress such as with energy flows and blackout statistics. Overall, the economics of the electrical grid do not align sufficiently with the physics of the grid. Experts advocate for fundamental changes to avoid serious consequences in the near future.
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The structure, or "topology" of a grid can vary considerably. The physical layout is often forced by what land is available and its geology. The logical topology can vary depending on the constraints of budget, requirements for system reliability, and the load and generation characteristics.
The cheapest and simplest topology for a distribution or transmission grid is a radial structure. This is a tree shape where power from a large supply radiates out into progressively lower voltage lines until the destination homes and businesses are reached.
Most transmission grids require the reliability that more complex mesh networks provide. If one were to imagine running redundant lines between limbs/branches of a tree that could be turned in case any particular limb of the tree were severed, then this image approximates how a mesh system operates. The expense of mesh topologies restrict their application to transmission and medium voltage distribution grids. Redundancy allows line failures to occur and power is simply rerouted while workmen repair the damaged and deactivated line.
Other topologies used are looped systems found in Europe and tied ring networks.
|The examples and perspective in this article may not represent a worldwide view of the subject. (February 2010)|
In cities and towns of North America, the grid tends to follow the classic radially fed design. A substation receives its power from the transmission network, the power is stepped down with a transformer and sent to a bus from which feeders fan out in all directions across the countryside. These feeders carry three-phase power, and tend to follow the major streets near the substation. As the distance from the substation grows, the fanout continues as smaller laterals spread out to cover areas missed by the feeders. This tree-like structure grows outward from the substation, but for reliability reasons, usually contains at least one unused backup connection to a nearby substation. This connection can be enabled in case of an emergency, so that a portion of a substation's service territory can be alternatively fed by another substation.
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Transmission networks are more complex with redundant pathways. For example, see the map of the United States' (right) high-voltage transmission network.
A wide area synchronous grid or "interconnection" is a group of distribution areas all operating with alternating current (AC) frequencies synchronized (so that peaks occur at the same time). This allows transmission of AC power throughout the area, connecting a large number of electricity generators and consumers and potentially enabling more efficient electricity markets and redundant generation. Interconnection maps are shown of North America (right) and Europe (below left).
In a synchronous grid all the generators run not only at the same speed but also at the same phase, each generator maintained by a local governor that regulates the driving torque by controlling the steam supply to the turbine driving it. Generation and consumption must be balanced across the entire grid, because energy is consumed almost instantaneously as it is produced. Energy is stored in the immediate short term by the rotational kinetic energy of the generators.
A large failure in one part of the grid - unless quickly compensated for - can cause current to re-route itself to flow from the remaining generators to consumers over transmission lines of insufficient capacity, causing further failures. One downside to a widely connected grid is thus the possibility of cascading failure and widespread power outage. A central authority is usually designated to facilitate communication and develop protocols to maintain a stable grid. For example, the North American Electric Reliability Corporation gained binding powers in the United States in 2006, and has advisory powers in the applicable parts of Canada and Mexico. The U.S. government has also designated National Interest Electric Transmission Corridors, where it believes transmission bottlenecks have developed.
High-voltage direct current lines or variable frequency transformers can be used to connect two alternating current interconnection networks which are not synchronized with each other. This provides the benefit of interconnection without the need to synchronize an even wider area. For example, compare the wide area synchronous grid map of Europe (above left) with the map of HVDC lines (below right).
This redundancy is limited. Existing national or regional grids simply provide the interconnection of facilities to utilize whatever redundancy is available. The exact stage of development at which the supply structure becomes a grid is arbitrary. Similarly, the term national grid is something of an anachronism in many parts of the world, as transmission cables now frequently cross national boundaries. The terms distribution grid for local connections and transmission grid for long-distance transmissions are therefore preferred, but national grid is often still used for the overall structure.
Despite the novel institutional arrangements and network designs of the electrical grid, its power delivery infrastructures suffer aging across the developed world. Four contributing factors to the current state of the electric grid and its consequences include:
As the 21st century progresses, the electric utility industry seeks to take advantage of novel approaches to meet growing energy demand. Utilities are under pressure to evolve their classic topologies to accommodate distributed generation. As generation becomes more common from rooftop solar and wind generators, the differences between distribution and transmission grids will continue to blur. Also, demand response is a grid management technique where retail or wholesale customers are requested either electronically or manually to reduce their load. Currently, transmission grid operators use demand response to request load reduction from major energy users such as industrial plants.
With everything interconnected, and open competition occurring in a free market economy, it starts to make sense to allow and even encourage distributed generation (DG). Smaller generators, usually not owned by the utility, can be brought on-line to help supply the need for power. The smaller generation facility might be a home-owner with excess power from their solar panel or wind turbine. It might be a small office with a diesel generator. These resources can be brought on-line either at the utility's behest, or by owner of the generation in an effort to sell electricity. Many small generators are allowed to sell electricity back to the grid for the same price they would pay to buy it. Furthermore, numerous efforts are underway to develop a "smart grid". In the U.S., the Energy Policy Act of 2005 and Title XIII of the Energy Independence and Security Act of 2007 are providing funding to encourage smart grid development. The hope is to enable utilities to better predict their needs, and in some cases involve consumers in some form of time-of-use based tariff. Funds have also been allocated to develop more robust energy control technologies.
Various planned and proposed systems to dramatically increase transmission capacity are known as super, or mega grids. The promised benefits include enabling the renewable energy industry to sell electricity to distant markets, the ability to increase usage of intermittent energy sources by balancing them across vast geological regions, and the removal of congestion that prevents electricity markets from flourishing. Local opposition to siting new lines and the significant cost of these projects are major obstacles to super grids. One study for a European super grid estimates that as much as 750 GW of extra transmission capacity would be required- capacity that would be accommodated in increments of 5 GW HVDC lines. A recent proposal by Transcanada priced a 1,600-km, 3-GW HVDC line at $3 billion USD and would require a corridor 60 meters wide. In India, a recent 6 GW, 1,850-km proposal was priced at $790 million and would require a 69 meter wide right of way. With 750 GW of new HVDC transmission capacity required for a European super grid, the land and money needed for new transmission lines would be considerable.
As deregulation continues further, utilities are driven to sell their assets as the energy market follows in line with the gas market in use of the futures and spot markets and other financial arrangements. Even globalization with foreign purchases are taking place. Recently, U.K’s National Grid, the largest private electric utility in the world, bought New England’s electric system for $3.2 billion. See the SEC filing dated March 15, 2000 Here Also, Scottish Power purchased Pacific Energy for $12.8 billion. Domestically, local electric and gas firms begin to merge operations as they see advantage of joint affiliation especially with the reduced cost of joint-metering. Technological advances will take place in the competitive wholesale electric markets such examples already being utilized include fuel cells used in space flight, aeroderivative gas turbines used in jet aircrafts, solar engineering and photovoltaic systems, off-shore wind farms, and the communication advances spawned by the digital world particularly with microprocessing which aids in monitoring and dispatching.
Electricity is expected to see growing demand in the future. The Information Revolution is highly reliant on electric power. Other growth areas include emerging new electricity-exclusive technologies, developments in space conditioning, industrial process, and transportation (for example hybrid vehicles, locomotives).
As mentioned above, the electrical grid is expected to evolve to a new grid paradigm--smart grid, an enhancement of the 20th century electrical grid. The traditional electrical grids are generally used to carry power from a few central generators to a large number of users or customers. In contrast, the new emerging smart grid uses two-way ﬂows of electricity and information to create an automated and distributed advanced energy delivery network.
Many research projects have been conducted to explore the concept of smart grid. According to a newest survey on smart grid, the research is mainly focused on three systems in smart grid- the infrastructure system, the management system, and the protection system.
The infrastructure system is the energy, information, and communication infrastructure underlying of the smart grid that supports 1) advanced electricity generation, delivery, and consumption; 2) advanced information metering, monitoring, and management; and 3) advanced communication technologies. In the transition from the conventional power grid to smart grid, we will replace a physical infrastructure with a digital one. The needs and changes present the power industry with one of the biggest challenges it has ever faced.
The management system is the subsystem in smart grid that provides advanced management and control services. Most of the existing works aim to improve energy efﬁciency, demand proﬁle, utility, cost, and emission, based on the infrastructure by using optimization, machine learning, and game theory. Within the advanced infrastructure framework of smart grid, more and more new management services and applications are expected to emerge and eventually revolutionize consumers' daily lives.
The protection system is the subsystem in smart grid that provides advanced grid reliability analysis, failure protection, and security and privacy protection services. We must note that the advanced infrastructure used in smart grid on one hand empowers us to realize more powerful mechanisms to defend against attacks and handle failures, but on the other hand, opens up many new vulnerabilities. For example, NIST pointed out that the major benefit provided by smart grid, the ability to get richer data to and from customer smart meters and other electric devices, is also its Achilles' heel from a privacy viewpoint. The obvious privacy concern is that the energy use information stored at the meter acts as an information rich side channel. This information can be mined and retrieved by interested parties to reveal personal information such as individual's habits, behaviors, activities, and even beliefs.
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