Electrical power grid

An electrical grid is an interconnected network for electricity delivery from producers to consumers. Electrical grids vary in size and can cover whole countries or continents. It consists of:Kaplan, S. M. (2009). Smart Grid. Electrical Power Transmission: Background and Policy Issues. The Capital.Net, Government Series. Pp. 1-42.

* power stations: often located near energy and away from heavily populated areas
* electrical substations to step voltage up or down
* electric power transmission to carry power long distances
* electric power distribution to individual customers, where voltage is stepped down again to the required service voltage(s).

Grids are nearly always synchronous, meaning all distribution areas operate with three phase alternating current (AC) frequencies synchronized (so that voltage swings occur at almost 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.

The combined transmission and distribution network is part of electricity delivery, known as the "power grid" in North America, or just "the grid." In the United Kingdom, India, Tanzania, Myanmar, Malaysia and New Zealand, the network is known as the National Grid.

Although electrical grids are widespread, , 1.4 billion people worldwide were not connected to an electricity grid. As electrification increases, the number of people with access to grid electricity is growing. About 840 million people (mostly in Africa) had no access to grid electricity in 2017, down from 1.2 billion in 2010.

Electrical grids can be prone to malicious intrusion or attack; thus, there is a need for electric grid security. Also as electric grids modernize and introduce computer technology, cyber threats start to become a security risk. Particular concerns relate to the more complex computer systems needed to manage grids.

History

Early electric energy was produced near the device or service requiring that energy. In the 1880s, electricity competed with steam, hydraulics, and especially coal gas. Coal gas was first produced on customer's premises but later evolved into gasification plants that enjoyed economies of scale. In the industrialized world, cities had networks of piped gas, used for lighting. But gas lamps produced poor light, wasted heat, made rooms hot and smokey, and gave off hydrogen and carbon monoxide. They also posed a fire hazard. In the 1880s electric lighting soon became advantageous compared to gas lighting.

Electric utility companies established central stations to take advantage of economies of scale and moved to centralized power generation, distribution, and system management.Borberly, A. and Kreider, J. F. (2001). Distributed Generation: The Power Paradigm for the New Millennium. CRC Press, Boca Raton, FL. 400 pgs. After the war of the currents was settled in favor of AC power, with long-distance power transmission it became possible to interconnect stations to balance the loads and improve load factors. Historically, transmission and distribution lines were owned by the same company, but starting in the 1990s, many countries have liberalized the regulation of the electricity market in ways that have led to the separation of the electricity transmission business from the distribution business.

In the United Kingdom, Charles Merz, of the Merz & McLellan consulting partnership, built the Neptune Bank Power Station near Newcastle upon Tyne in 1901, and by 1912 had developed into the largest integrated power system in Europe. Merz was appointed head of a parliamentary committee and his findings led to the Williamson Report of 1918, which in turn created the Electricity (Supply) Act 1919. The bill was the first step towards an integrated electricity system. The Electricity (Supply) Act 1926 led to the setting up of the National Grid. The Central Electricity Board standardized the nation's electricity supply and established the first synchronized AC grid, running at 132 kilovolts and 50 hertz. This started operating as a national system, the National Grid, in 1938.

In the United States in the 1920s, utilities formed joint-operations to share peak load coverage and backup power. In 1934, with the passage of the Public Utility Holding Company Act (USA), electric utilities were recognized as public goods of importance and were given outlined restrictions and regulatory oversight of their operations. The Energy Policy Act of 1992 required transmission line owners to allow electric generation companies open access to their networkMazer, A. (2007). Electric Power Planning for Regulated and Deregulated Markets. John, Wiley, and Sons, Inc., Hoboken, NJ. 313pgs. and led to a restructuring of how the electric industry operated in an effort to create competition in power generation. No longer were electric utilities built as vertical monopolies, where generation, transmission and distribution were handled by a single company. Now, the three stages could be split among various companies, in an effort to provide fair access to high voltage transmission.. (2001). Glover J. D., Sarma M. S., Overbye T. J. (2010) Power System and Analysis 5th Edition. Cengage Learning. Pg 10. The Energy Policy Act of 2005 allowed incentives and loan guarantees for alternative energy production and advance innovative technologies that avoided greenhouse emissions.

In France, electrification began in the 1900s, with 700 communes in 1919, and 36,528 in 1938. At the same time, these close networks began to interconnect: Paris in 1907 at 12 kV, the Pyrénées in 1923 at 150 kV, and finally almost all of the country interconnected by 1938 at 220 kV. In 1946, the grid was the world's most dense. That year the state nationalised the industry, by uniting the private companies as Électricité de France. The frequency was standardised at 50 Hz, and the 225 kV network replaced 110 kV and 120 kV. Since 1956, service voltage has been standardised at 220/380 V, replacing the previous 127/220 V. During the 1970s, the 400 kV network, the new European standard, was implemented.

In China, electrification began in the 1950s. In August 1961, the electrification of the Baoji-Fengzhou section of the Baocheng Railway was completed and delivered for operation, becoming China's first electrified railway. From 1958 to 1998, China's electrified railway reached 6,200 miles (10,000 kilometres). As of the end of 2017, this number has reached 54,000 miles (87,000 kilometres). In the current railway electrification system of China
State Grid Corporation of China
is an important power supplier. In 2019, it completed the power supply project of China's important electrified railways in its operating areas, such as Jingtong Railway, Haoji Railway, Zhengzhou–Wanzhou high-speed railway, et cetera, providing power supply guarantee for 110 traction stations, and its cumulative power line construction length reached 6,586 kilometres.

Components

Generation

Electricity generation is the process of generating electric power from sources of primary energy typically at power stations. Usually this is done with electromechanical generators driven by heat engines or the kinetic energy of water or wind. Other energy sources include solar photovoltaics and geothermal power.

The sum of the power outputs of generators on the grid is the production of the grid, typically measured in gigawatts (GW).

Transmission

Electric power transmission is the bulk movement of electrical energy from a generating site, via a web of interconnected lines, to an electrical substation, from which is connected to the distribution system. This networked system of connections is distinct from the local wiring between high-voltage substations and customers.

Because the power is often generated far from where it is consumed, the transmission system can cover great distances. For a given amount of power, transmission efficiency is greater at higher voltages and lower currents. Therefore, voltages are stepped up at the generating station, and stepped down at local substations for distribution to customers.

Most transmission is three-phase. Three phase, compared to single phase, can deliver much more power for a given amount of wire, since the neutral and ground wires are shared. Further, three-phase generators and motors are more efficient than their single-phase counterparts.

However, for conventional conductors one of the main losses are resistive losses which are a square law on current, and depend on distance. High voltage AC transmission lines can lose 1-4% per hundred miles. However, high-voltage direct current can have half the losses of AC. Over very long distances, these efficiencies can offset the additional cost of the required AC/DC converter stations at each end.

Transmission networks are complex with redundant pathways. The physical layout is often forced by what land is available and its geology. Most transmission grids offer the reliability that more complex ''mesh networks'' provide. Redundancy allows line failures to occur and power is simply rerouted while repairs are done.

Substations

Substations may perform many different functions but usually transform voltage from low to high (step up) and from high to low (step down). Between the generator and the final consumer, the voltage may be transformed several times.

The three main types of substations, by function, are:
* Step-up substation: these use transformers to raise the voltage coming from the generators and power plants so that power can be transmitted long distances more efficiently, with smaller currents.
* Step-down substation: these transformers lower the voltage coming from the transmission lines which can be used in industry or sent to a distribution substation.
* Distribution substation: these transform the voltage lower again for the distribution to end users.

Aside from transformers, other major components or functions of substations include:
* Circuit breakers: used to automatically break a circuit and isolate a fault in the system.
* Switches: to control the flow of electricity, and isolate equipment.
* The substation busbar: typically a set of three conductors, one for each phase of current. The substation is orgranized around the buses, and they are connected to incoming lines, transformers, protection equipment, switches, and the outgoing lines.
* Lightning arresters
* Capacitors for power factor correction

Electric power distribution

Distribution is the final stage in the delivery of power; it carries electricity from the transmission system to individual consumers. Substations connect to the transmission system and lower the transmission voltage to medium voltage ranging between and . Primary distribution lines carry this medium voltage power to distribution transformers located near the customer's premises. Distribution transformers again lower the voltage to the utilization voltage. Customers demanding a much larger amount of power may be connected directly to the primary distribution level or the subtransmission level.

Distribution networks are divided into two types, radial or network.

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.

Storage

''Grid energy storage'' (also called ''large-scale energy storage'') is a collection of methods used for energy storage on a large scale within an electrical power grid. Electrical energy is stored during times when electricity is plentiful and inexpensive (especially from intermittent power sources such as renewable electricity from wind power, tidal power and solar power) or when demand is low, and later returned to the grid when demand is high, and electricity prices tend to be higher.

, the largest form of grid energy storage is dammed