Electrical substation

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Elements of a substation
A: Primary power lines' side B: Secondary power lines' side
1. Primary power lines 2. Ground wire 3. Overhead lines
4. Transformer for measurement of electric voltage
5. Disconnect switch 6. Circuit breaker
7. Current transformer
8. Lightning arrester
9. Main transformer
10. Control building
11. Security fence
12. Secondary power lines Electrical substation model (side-view).PNG
Elements of a substation
A: Primary power lines' side B: Secondary power lines' side
1. Primary power lines 2. Ground wire 3. Overhead lines
4. Transformer for measurement of electric voltage
5. Disconnect switch 6. Circuit breaker
7. Current transformer
8. Lightning arrester
9. Main transformer
10. Control building
11. Security fence
12. Secondary power lines
A 50 Hz electrical substation in Melbourne, Australia. This is showing three of the five 220 kV/66 kV transformers, as well as high-voltage transformer fire barriers, each with a capacity of 150 MVA. This substation is constructed using steel lattice structures to support strain bus wires and apparatus. Melbourne Terminal Station.JPG
A 50 Hz electrical substation in Melbourne, Australia. This is showing three of the five 220 kV/66 kV transformers, as well as high-voltage transformer fire barriers, each with a capacity of 150 MVA. This substation is constructed using steel lattice structures to support strain bus wires and apparatus.
A 115 kV to 41.6/12.47 kV 5 MVA 60 Hz substation with circuit switcher, regulators, reclosers and control building at Warren, Minnesota. This substation shows elements of low-profile construction; apparatus is mounted on individual columns. Electrical Substation.JPG
A 115 kV to 41.6/12.47 kV 5 MVA 60 Hz substation with circuit switcher, regulators, reclosers and control building at Warren, Minnesota. This substation shows elements of low-profile construction; apparatus is mounted on individual columns.

A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages.

Electricity generation process of generating electrical power

Electricity generation is the process of generating electric power from sources of primary energy. For electric utilities in the electric power industry, it is the first stage in the delivery of electricity to end users, the other stages being transmission, distribution, energy storage and recovery, using the pumped-storage method.

Electric power transmission bulk movement of electrical energy from a generating site to an electrical substation

Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines which facilitate this movement are known as a transmission network. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution. The combined transmission and distribution network is known as the "power grid" in North America, or just "the grid". In the United Kingdom, India, Malaysia and New Zealand, the network is known as the "National Grid".

Electric power distribution Final stage of electricity delivery to individual consumers in a power grid

Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmission voltage to medium voltage ranging between 2 kV and 35 kV with the use of transformers. 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 used by lighting, industrial equipment or household appliances. Often several customers are supplied from one transformer through secondary distribution lines. Commercial and residential customers are connected to the secondary distribution lines through service drops. Customers demanding a much larger amount of power may be connected directly to the primary distribution level or the subtransmission level.


Substations may be owned and operated by an electrical utility, or may be owned by a large industrial or commercial customer. Generally substations are unattended, relying on SCADA for remote supervision and control.

Supervisory Control and Data Acquisition (SCADA) is a control system architecture that uses computers, networked data communications and graphical user interfaces for high-level process supervisory management, but uses other peripheral devices such as programmable logic controller (PLC) and discrete PID controllers to interface with the process plant or machinery. The use of SCADA has been also considered for management and operations of project-driven-process in construction.

The word substation comes from the days before the distribution system became a grid. As central generation stations became larger, smaller generating plants were converted to distribution stations, receiving their energy supply from a larger plant instead of using their own generators. The first substations were connected to only one power station, where the generators were housed, and were subsidiaries of that power station.

Power station facility generating electric power

A power station, also referred to as a power plant or powerhouse and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Most power stations contain one or more generators, a rotating machine that converts mechanical power into electrical power. The relative motion between a magnetic field and a conductor creates an electrical current. The energy source harnessed to turn the generator varies widely. Most power stations in the world burn fossil fuels such as coal, oil, and natural gas to generate electricity. Others use nuclear power, but there is an increasing use of cleaner renewable sources such as solar, wind, wave and hydroelectric.

220 kV/110 kV/20 kV station in Germany Umspannwerk-Birkenfeld Transformatoren 220kV-110kV-20kV.jpg
220 kV/110 kV/20 kV station in Germany


Substations may be described by their voltage class, their applications within the power system, the method used to insulate most connections, and by the style and materials of the structures used. These categories are not disjointed; for example, to solve a particular problem, a transmission substation may include significant distribution functions.

A 35/10 kV substation in Cakovec, Croatia Industrijska zona Cakovec-zapad - trafostanica.jpg
A 35/10 kV substation in Čakovec, Croatia
substation in Russia Switchgear 110 kV.jpg
substation in Russia

Transmission substation

A transmission substation connects two or more transmission lines. [2] The simplest case is where all transmission lines have the same voltage. In such cases, substation contains high-voltage switches that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station may have transformers to convert between two transmission voltages, voltage control/power factor correction devices such as capacitors, reactors or static VAR compensators and equipment such as phase shifting transformers to control power flow between two adjacent power systems.

Transformer electrical artefact that transfers energy through electromagnetic induction

A transformer is a static electrical device that transfers electrical energy between two or more circuits. A varying current in one coil of the transformer produces a varying magnetic flux, which, in turn, induces a varying electromotive force across a second coil wound around the same core. Electrical energy can be transferred between the two coils, without a metallic connection between the two circuits. Faraday's law of induction discovered in 1831 described the induced voltage effect in any coil due to changing magnetic flux encircled by the coil.

In a power system, voltage at various buses tends to increase or decrease during its daily operation. To ensure constant voltage to consumers, various techniques are utilized.

A static VAR compensator is a set of electrical devices for providing fast-acting reactive power on high-voltage electricity transmission networks. SVCs are part of the Flexible AC transmission system device family, regulating voltage, power factor, harmonics and stabilizing the system. A static VAR compensator has no significant moving parts. Prior to the invention of the SVC, power factor compensation was the preserve of large rotating machines such as synchronous condensers or switched capacitor banks.

minimal HV station in Germany Dulmen, Umspannstation -- 2014 -- 0005.jpg
minimal HV station in Germany

Transmission substations can range from simple to complex. A small "switching station" may be little more than a bus plus some circuit breakers. The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, many circuit breakers, and a large amount of protection and control equipment (voltage and current transformers, relays and SCADA systems). Modern substations may be implemented using international standards such as IEC Standard 61850.

Busbar Strip inside switchgear for local high current distribution

In electric power distribution, a busbar is a metallic strip or bar, typically housed inside switchgear, panel boards, and busway enclosures for local high current power distribution. They are also used to connect high voltage equipment at electrical switchyards, and low voltage equipment in battery banks. They are generally uninsulated, and have sufficient stiffness to be supported in air by insulated pillars. These features allow sufficient cooling of the conductors, and the ability to tap in at various points without creating a new joint.

Instrument transformers are high accuracy class electrical devices used to isolate or transform voltage or current levels. The most common usage of instrument transformers is to operate instruments or metering from high voltage or high current circuits, safely isolating secondary control circuitry from the high voltages or currents. The primary winding of the transformer is connected to the high voltage or high current circuit, and the meter or relay is connected to the secondary circuit.

Digital protective relay

In utility and industrial electric power transmission and distribution systems, a digital protective relay is a computer-based system with software-based protection algorithms for the detection of electrical faults. Such relays are also termed as microprocessor type protective relays. They are functional replacements for electro-mechanical protective relays and may include many protection functions in one unit, as well as providing metering, communication, and self-test functions.

Distribution substation

A distribution substation in Scarborough, Ontario disguised as a house, complete with a driveway, front walk and a mown lawn and shrubs in the front yard. A warning notice can be clearly seen on the "front door". Disguises for substations are common in many cities. SubstationHouseScarborough.jpg
A distribution substation in Scarborough, Ontario disguised as a house, complete with a driveway, front walk and a mown lawn and shrubs in the front yard. A warning notice can be clearly seen on the "front door". Disguises for substations are common in many cities.

A distribution substation transfers power from the transmission system to the distribution system of an area. [2] It is uneconomical to directly connect electricity consumers to the main transmission network, unless they use large amounts of power, so the distribution station reduces voltage to a level suitable for local distribution.

The input for a distribution substation is typically at least two transmission or sub-transmission lines. Input voltage may be, for example, 115 kV, or whatever is common in the area. The output is a number of feeders. Distribution voltages are typically medium voltage, between 2.4 kV and 33 kV, depending on the size of the area served and the practices of the local utility. The feeders run along streets overhead (or underground, in some cases) and power the distribution transformers at or near the customer premises.

In addition to transforming voltage, distribution substations also isolate faults in either the transmission or distribution systems. Distribution substations are typically the points of voltage regulation, although on long distribution circuits (of several miles/kilometers), voltage regulation equipment may also be installed along the line.

The downtown areas of large cities feature complicated distribution substations, with high-voltage switching, and switching and backup systems on the low-voltage side. More typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side.

Collector substation

In distributed generation projects such as a wind farm, a collector substation may be required. It resembles a distribution substation although power flow is in the opposite direction, from many wind turbines up into the transmission grid. Usually for economy of construction the collector system operates around 35 kV, and the collector substation steps up voltage to a transmission voltage for the grid. The collector substation can also provide power factor correction if it is needed, metering, and control of the wind farm. In some special cases a collector substation can also contain an HVDC converter station.

Collector substations also exist where multiple thermal or hydroelectric power plants of comparable output power are in proximity. Examples for such substations are Brauweiler in Germany and Hradec in the Czech Republic, where power is collected from nearby lignite-fired power plants. If no transformers are required for increasing the voltage to transmission level, the substation is a switching station.

Converter substations

Converter substations may be associated with HVDC converter plants, traction current, or interconnected non-synchronous networks. These stations contain power electronic devices to change the frequency of current, or else convert from alternating to direct current or the reverse. Formerly rotary converters changed frequency to interconnect two systems; nowadays such substations are rare.

Switching station

A switching station is a substation without transformers and operating only at a single voltage level. Switching stations are sometimes used as collector and distribution stations. Sometimes they are used for switching the current to back-up lines or for parallelizing circuits in case of failure. An example is the switching stations for the HVDC Inga–Shaba transmission line.

A switching station may also be known as a switchyard, and these are commonly located directly adjacent to or nearby a power station. In this case the generators from the power station supply their power into the yard onto the Generator Bus on one side of the yard, and the transmission lines take their power from a Feeder Bus on the other side of the yard.

An important function performed by a substation is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be planned or unplanned. A transmission line or other component may need to be de-energized for maintenance or for new construction, for example, adding or removing a transmission line or a transformer. To maintain reliability of supply, companies aim at keeping the system up and running while performing maintenance. All work to be performed, from routine testing to adding entirely new substations, should be done while keeping the whole system running.

Unplanned switching events are caused by a fault in a transmission line or any other component, for example:

The function of the switching station is to isolate the faulty portion of the system in the shortest possible time. De-energizing faulty equipment protects it from further damage, and isolating a fault helps keep the rest of the electrical grid operating with stability. [4]


Electrified railways also use substations, often distribution substations. In some cases a conversion of the current type takes place, commonly with rectifiers for direct current (DC) trains, or rotary converters for trains using alternating current (AC) at frequencies other than that of the public grid. Sometimes they are also transmission substations or collector substations if the railway network also operates its own grid and generators to supply the other stations.

Mobile substation

A mobile substation is a substation on wheels, containing a transformer, breakers and buswork mounted on a self-contained semi-trailer, meant to be pulled by a truck. They are designed to be compact for travel on public roads, and are used for temporary backup in times of natural disaster or war. Mobile substations are usually rated much lower than permanent installations, and may be built in several units to meet road travel limitations. [5]


The Adelard-Godbout substation in Old Montreal is Canada's oldest substation, in continuous operation since 1901. It has a facade in clay brick with grey stone ornaments, to blend into its downtown environment. Poste Adelard-Godbout 06.JPG
The Adélard-Godbout substation in Old Montreal is Canada's oldest substation, in continuous operation since 1901. It has a facade in clay brick with grey stone ornaments, to blend into its downtown environment.
Substation in a castle-like building from the 1910s, serves as distribution point next to the Lesna dam, one of several hydroelectrical stations at Bobr river. Lesna, Elektrownia wodna Lesna - fotopolska.eu (129510).jpg
Substation in a castle-like building from the 1910s, serves as distribution point next to the Lésna dam, one of several hydroelectrical stations at Bóbr river.
15 kV/400 V distribution tower in Poland Lesna-Podlaska-trafo-180902.jpg
15 kV/400 V distribution tower in Poland

Elements of a substation

Substations generally have switching, protection and control equipment, and transformers. In a large substation, circuit breakers are used to interrupt any short circuits or overload currents that may occur on the network. Smaller distribution stations may use recloser circuit breakers or fuses for protection of distribution circuits. Substations themselves do not usually have generators, although a power plant may have a substation nearby. Other devices such as capacitors and voltage regulators may also be located at a substation.

Substations may be on the surface in fenced enclosures, underground, or located in special-purpose buildings. High-rise buildings may have several indoor substations. Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions.

A grounding (earthing) system must be designed. The total ground potential rise, and the gradients in potential during a fault (called touch and step potentials), [6] must be calculated to protect passers-by during a short-circuit in the transmission system. Earth faults at a substation can cause a ground potential rise. Currents flowing in the Earth's surface during a fault can cause metal objects to have a significantly different voltage than the ground under a person's feet; this touch potential presents a hazard of electrocution. Where a substation has a metallic fence, it must be properly grounded to protect people from this hazard.

The main issues facing a power engineer are reliability and cost. A good design attempts to strike a balance between these two, to achieve reliability without excessive cost. The design should also allow expansion of the station, when required. [7]

Location selection

Selection of the location of a substation must consider many factors. Sufficient land area is required for installation of equipment with necessary clearances for electrical safety, and for access to maintain large apparatus such as transformers.

Where land is costly, such as in urban areas, gas insulated switchgear may save money overall. Substations located in coastal areas affected by flooding and tropical storms may often require an elevated structure to keep equipment sensitive to surges hardened against these elements. [8] The site must have room for expansion due to load growth or planned transmission additions. Environmental effects of the substation must be considered, such as drainage, noise and road traffic effects.

The substation site must be reasonably central to the distribution area to be served. The site must be secure from intrusion by passers-by, both to protect people from injury by electric shock or arcs, and to protect the electrical system from misoperation due to vandalism.

Design diagrams

Tottenham Substation, set in wild parkland in North London. LEB substation, Tottenham Marshes.JPG
Tottenham Substation, set in wild parkland in North London.

The first step in planning a substation layout is the preparation of a one-line diagram, which shows in simplified form the switching and protection arrangement required, as well as the incoming supply lines and outgoing feeders or transmission lines. It is a usual practice by many electrical utilities to prepare one-line diagrams with principal elements (lines, switches, circuit breakers, transformers) arranged on the page similarly to the way the apparatus would be laid out in the actual station. [2]

In a common design, incoming lines have a disconnect switch and a circuit breaker. In some cases, the lines will not have both, with either a switch or a circuit breaker being all that is considered necessary. A disconnect switch is used to provide isolation, since it cannot interrupt load current. A circuit breaker is used as a protection device to interrupt fault currents automatically, and may be used to switch loads on and off, or to cut off a line when power is flowing in the 'wrong' direction. When a large fault current flows through the circuit breaker, this is detected through the use of current transformers. The magnitude of the current transformer outputs may be used to trip the circuit breaker resulting in a disconnection of the load supplied by the circuit break from the feeding point. This seeks to isolate the fault point from the rest of the system, and allow the rest of the system to continue operating with minimal impact. Both switches and circuit breakers may be operated locally (within the substation) or remotely from a supervisory control center.

With overhead transmission lines, the propagation of lightning and switching surges can cause insulation failures into substation equipment. Line entrance surge arrestors are used to protect substation equipment accordingly. Insulation Coordination studies are carried out extensively to ensure equipment failure (and associated outages) is minimal.

Once past the switching components, the lines of a given voltage connect to one or more buses. These are sets of busbars, usually in multiples of three, since three-phase electrical power distribution is largely universal around the world.

The arrangement of switches, circuit breakers, and buses used affects the cost and reliability of the substation. For important substations a ring bus, double bus, or so-called "breaker and a half" setup can be used, so that the failure of any one circuit breaker does not interrupt power to other circuits, and so that parts of the substation may be de-energized for maintenance and repairs. Substations feeding only a single industrial load may have minimal switching provisions, especially for small installations. [7]

This single-line diagram illustrates the breaker and a half concept often used in switchyards. Breaker and a half.svg
This single-line diagram illustrates the breaker and a half concept often used in switchyards.

Once having established buses for the various voltage levels, transformers may be connected between the voltage levels. These will again have a circuit breaker, much like transmission lines, in case a transformer has a fault (commonly called a "short circuit").

Along with this, a substation always has control circuitry needed to command the various circuit breakers to open in case of the failure of some component.


Early electrical substations required manual switching or adjustment of equipment, and manual collection of data for load, energy consumption, and abnormal events. As the complexity of distribution networks grew, it became economically necessary to automate supervision and control of substations from a centrally attended point, to allow overall coordination in case of emergencies and to reduce operating costs. Early efforts to remote control substations used dedicated communication wires, often run alongside power circuits. Power-line carrier, microwave radio, fiber optic cables as well as dedicated wired remote control circuits have all been applied to Supervisory Control and Data Acquisition (SCADA) for substations. The development of the microprocessor made for an exponential increase in the number of points that could be economically controlled and monitored. Today, standardized communication protocols such as DNP3, IEC 61850 and Modbus, to list a few, are used to allow multiple intelligent electronic devices to communicate with each other and supervisory control centers. Distributed automatic control at substations is one element of the so-called smart grid.


Switches, circuit breakers, transformers and other apparatus may be interconnected by air-insulated bare conductors strung on support structures. The air space required increases with system voltage and with the lightning surge voltage rating. For medium-voltage distribution substations, metal-enclosed switch gear may be used and no live conductors exposed at all. For higher voltages, gas-insulated switch gear reduces the space required around live bus. Instead of bare conductors, bus and apparatus are built into pressurized tubular containers filled with sulfur hexafluoride (SF6) gas. This gas has a higher insulating value than air, allowing the dimensions of the apparatus to be reduced. In addition to air or SF6 gas, apparatus will use other insulation materials such as transformer oil, paper, porcelain, and polymer insulators.


Outdoor, above-ground substation structures include wood pole, lattice metal tower, and tubular metal structures, although other variants are available. Where space is plentiful and appearance of the station is not a factor, steel lattice towers provide low-cost supports for transmission lines and apparatus. Low-profile substations may be specified in suburban areas where appearance is more critical. Indoor substations may be gas-insulated switchgear (at high voltages), or metal-enclosed or metal-clad switchgear at lower voltages. Urban and suburban indoor substations may be finished on the outside so as to blend in with other buildings in the area.

A compact substation is generally an outdoor substation built in a metal enclosure, in which each item of the electrical equipment is located very near to each other to create a relatively smaller footprint size of the substation.

See also

References and further reading

  1. "Joint Consultation Paper: Western Metropolitan Melbourne Transmission Connection and Subtransmission Capacity". Jemena. Powercor Australia, Jemena, Australian Energy Market Operator. Retrieved 4 February 2016.
  2. 1 2 3 Stockton, Blaine. "Design Guide for Rural Substations" (PDF). USDA Rural Development. United States Department of Agriculture. Retrieved 4 February 2016.
  3. Steinberg, Neil. "Lights On but Nobody Home: Behind the Fake Buildings that Power Chicago" . Retrieved 14 December 2013.
  4. "Transformer Fire Video". metacafe. User Eagle Eye. Retrieved 4 February 2016.
  5. Boyd, Dan; Rampaul, Glen. "Mobile Substations" (PDF). IEEE Winnipeg PES Chapter. IEEE Power and Energy Society. Retrieved 11 October 2017.
  6. John, Alvin. "EE35T - Substation Design and Layout". The University Of The West Indies at St. Augustine, Trinidad And Tobago. Retrieved 4 February 2016.
  7. 1 2 Donald G. Fink, H. Wayne Beatty Standard Handbook for Electrical Engineers Eleventh Edition, McGraw Hill 1978 ISBN   0-07-020974-X Chapter 17 Substation Design
  8. Baker, Joseph W,. "Eliminating Hurricane Induced Storm Surge Damage To Electric Utilities Via In-place Elevation Of Substation Structures And Equipment" (PDF). DIS-TRAN Packaged Substations. Crest Industries. Archived from the original (PDF) on 5 February 2016. Retrieved 4 February 2016.
  1. R. M. S. de Oliveira and C. L. S. S. Sobrinho (2009). "Computational Environment for Simulating Lightning Strokes in a Power Substation by Finite-Difference Time-Domain Method". IEEE Transactions on Electromagnetic Compatibility. 51 (4): 995–1000. doi:10.1109/TEMC.2009.2028879.

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High-voltage direct current

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Circuit breaker electrical switch designed to open when exposed to excess current

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Single-wire earth return technology to supply energy by single wire where earth will serve as return conductor

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Power engineering subfield of electrical engineering, which deals with power generation, conversion, storage, transport and forwarding in electrical networks and use of electrical energy

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HVDC converter station specialised type of electrical substation which converts direct current to alternating current or the reverse

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In an electric power system, switchgear is the combination of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is directly linked to the reliability of the electricity supply.

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Disconnector electromechanical switch

In electrical engineering, a disconnector, disconnect switch or isolator switch is used to ensure that an electrical circuit is completely de-energized for service or maintenance. Such switches are often found in electrical distribution and industrial applications, where machinery must have its source of driving power removed for adjustment or repair. High-voltage isolation switches are used in electrical substations to allow isolation of apparatus such as circuit breakers, transformers, and transmission lines, for maintenance. The disconnector is usually not intended for normal control of the circuit, but only for safety isolation. Disconnectors can be operated either manually or automatically.

This is an alphabetical list of articles pertaining specifically to electrical and electronics engineering. For a thematic list, please see List of electrical engineering topics. For a broad overview of engineering, see List of engineering topics. For biographies, see List of engineers.

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Arcing horns component of the electrical power system

Arcing horns are projecting conductors used to protect insulators or switch hardware on high voltage electric power transmission systems from damage during flashover. Overvoltages on transmission lines, due to atmospheric electricity, lightning strikes, or electrical faults, can cause arcs across insulators (flashovers) that can damage them. Alternately, atmospheric conditions or transients that occur during switching can cause an arc to form in the breaking path of a switch during its operation. Arcing horns provide a path for flashover to occur that bypasses the surface of the protected device. Horns are normally paired on either side of an insulator, one connected to the high voltage part and the other to ground, or at the breaking point of a switch contact. They are frequently to be seen on insulator strings on overhead lines, or protecting transformer bushings.

Earth potential rise

In electrical engineering, earth potential rise (EPR) also called ground potential rise (GPR) occurs when a large current flows to earth through an earth grid impedance. The potential relative to a distant point on the Earth is highest at the point where current enters the ground, and declines with distance from the source. Ground potential rise is a concern in the design of electrical substations because the high potential may be a hazard to people or equipment.

Electric power system

An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of an electric power system is the grid that provides power to an extended area. An electrical grid power system can be broadly divided into the generators that supply the power, the transmission system that carries the power from the generating centres to the load centres, and the distribution system that feeds the power to nearby homes and industries. Smaller power systems are also found in industry, hospitals, commercial buildings and homes. The majority of these systems rely upon three-phase AC power—the standard for large-scale power transmission and distribution across the modern world. Specialised power systems that do not always rely upon three-phase AC power are found in aircraft, electric rail systems, ocean liners and automobiles.

Surge arrester

A surge arrester is a device to protect electrical equipment from over-voltage transients caused by external (lightning) or internal (switching) events. Also called a surge protection device (SPD) or transient voltage surge suppressor (TVSS), this class of device is used to protect equipment in power transmission and distribution systems. The energy criterion for various insulation material can be compared by impulse ratio. A surge arrester should have a low impulse ratio, so that a surge incident on the surge arrester may be bypassed to the ground instead of passing through the apparatus.

Amtraks 25 Hz traction power system

Amtrak's 25 Hz traction power system is a traction power grid operated by Amtrak along the southern portion of its Northeast Corridor (NEC): the 225 route miles (362 km) between Washington, D.C. and New York City and the 104 route miles (167 km) between Philadelphia and Harrisburg, Pennsylvania. The Pennsylvania Railroad constructed it between 1915 and 1938. Amtrak inherited the system from Penn Central, the successor to Pennsylvania Railroad, in 1976 along with the Northeast Corridor. This is the reason for using 25 Hz, as opposed to 60 Hz – which is the standard for power transmission in North America. In addition to serving the NEC, the system provides power to New Jersey Transit Rail Operations (NJT), the Southeastern Pennsylvania Transportation Authority (SEPTA) and the Maryland Area Regional Commuter Train (MARC). Only about half of the system's electrical capacity is used by Amtrak. The remainder is sold to the commuter railroads who operate their trains along the corridor.

Low-voltage network

A low-voltage network or secondary network is a part of electric power distribution which carries electric energy from distribution transformers to electricity meters of end customers. Secondary networks are operated at a low voltage level, which is typically equal to the mains voltage of electric appliances.

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