Single-wire earth return (SWER) or single-wire ground return is a single-wire transmission line which supplies

single-phase electric power In electrical engineering, single-phase electric power (abbreviated 1φ) is the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads ...

from an

electrical 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 ...

to remote areas at lowest cost. Its distinguishing feature is that the

earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

(or sometimes a body of water) is used as the return path for the current, to avoid the need for a second wire (or ''

neutral wire Ground and neutral are circuit conductors used in alternating current electrical systems. The ground circuit is connected to earth, and neutral circuit is usually connected to ground. As the neutral point of an electrical supply system is often ...

'') to act as a return path. Single-wire earth return is principally used for

rural electrification Rural electrification is the process of bringing electrical power to rural and remote areas. Rural communities are suffering from colossal market failures as the national grids fall short of their demand for electricity. As of 2017, over 1 billion ...

, but also finds use for larger isolated loads such as water pumps. It is also used for

high-voltage direct current A high-voltage direct current (HVDC) electric power transmission system (also called a power superhighway or an electrical superhighway) uses direct current (DC) for electric power transmission, in contrast with the more common alternating curre ...

over

submarine power cable A submarine power cable is a transmission cable for carrying electric power below the surface of the water.light rail, uses a very similar system. It uses resistors to earth to reduce hazards from rail voltages, but the primary return currents are through the rails.

History

Lloyd Mandeno Lloyd Mandeno (3 October 1888 – 30 December 1973) was a New Zealand electrical engineer, inventor and local politician. He was born in Rangiaowhia, Waikato, New Zealand, on 3 October 1888. He is credited with nine hydroelectric installatio ...

, OBE (1888–1973) fully developed SWER in

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...

around 1925 for rural electrification. Although he termed it "Earth Working Single Wire Line", it was often called "Mandeno’s Clothesline". More than 200,000 kilometres (100,000 miles) have now been installed in

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

and New Zealand. It is considered safe, reliable and low-cost, provided that safety features and earthing are correctly installed. The Australian standards are widely used and cited. It has been applied around the world, such as in the

Canadian province Within the geographical areas of Canada, the ten provinces and three territories are sub-national administrative divisions under the jurisdiction of the Canadian Constitution. In the 1867 Canadian Confederation, three provinces of British North ...

Saskatchewan Saskatchewan ( ; ) is a Provinces and territories of Canada, province in Western Canada, western Canada, bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, to the northeast by Nunavut, and on t ...

;

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

;

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

; and portions of the United States'

Upper Midwest The Upper Midwest is a region in the northern portion of the U.S. Census Bureau's Midwestern United States. It is largely a sub-region of the Midwest. Although the exact boundaries are not uniformly agreed-upon, the region is defined as referring ...

and

Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S., ...

(

Bethel Bethel ( he, בֵּית אֵל, translit=Bēṯ 'Ēl, "House of El" or "House of God",Bleeker and Widegren, 1988, p. 257. also transliterated ''Beth El'', ''Beth-El'', ''Beit El''; el, Βαιθήλ; la, Bethel) was an ancient Israelite sanct ...

Operating principle

SWER is a viable choice for a distribution system when conventional return current wiring would cost more than SWER's isolation transformers and small power losses. Power engineers experienced with both SWER and conventional power lines rate SWER as equally safe, more reliable, less costly, but with slightly lower efficiency than conventional lines. SWER can cause fires when maintenance is poor, and bushfire is a risk. Power is supplied to the SWER line by an isolating

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

of up to 300 kVA. This transformer isolates the grid from ground or earth, and changes the grid

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...

(typically 22 or 33 kV line-to-line) to the SWER voltage (typically 12.7 or 19.1 kV line-to-earth). The SWER line is a single conductor that may stretch for tens or even hundreds of kilometres, with a number of distribution transformers along its length. At each transformer, such as a customer's premises, current flows from the line, through the primary coil of a step-down isolation transformer, to

through an earth stake. From the earth stake, the current eventually finds its way back to the main step-up transformer at the head of the line, completing the circuit. SWER is therefore a practical example of a

phantom loop In telecommunication and electrical engineering, a phantom circuit is an electrical circuit derived from suitably arranged wires with one or more conductive paths being a circuit in itself and at the same time acting as one conductor of another circ ...

. In areas with high-resistance soil, the resistance wastes energy. Another issue is that the resistance may be high enough that insufficient current flows into the earth neutral, causing the grounding rod to float to higher voltages. Self-resetting circuit breakers usually reset because of a difference in voltage between line and neutral. Therefore, with dry, high-resistance soils, the reduced difference in voltage between line and neutral may prevent breakers from resetting. In Australia, locations with very dry soils need the grounding rods to be extra deep. Experience in Alaska shows that SWER needs to be grounded below

permafrost Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surface ...

, which is high-resistance. The secondary winding of the local transformer will supply the customer with either single ended single phase (N-0) or

split-phase A split-phase or single-phase three-wire system is a type of single-phase electric power distribution. It is the alternating current (AC) equivalent of the original Edison Machine Works three-wire direct-current system. Its primary advantage is t ...

(N-0-N) power in the region's standard appliance voltages, with the 0 volt line connected to a safety earth that does not normally carry an operating current. A large SWER line may feed as many as 80 distribution transformers. The transformers are usually rated at 5 kVA, 10 kVA, and 25 kVA. The load densities are usually below 0.5 kVA per kilometer (0.8 kVA per mile) of line. Any single customer's maximum demand will typically be less than 3.5 kVA, but larger loads up to the capacity of the distribution transformer can also be supplied. Some SWER systems in the USA are conventional distribution feeders that were built without a continuous neutral (some of which were obsolete transmission lines that were refitted for rural distribution service). The substation feeding such lines has a grounding rod on each pole within the substation; then on each branch from the line, the span between the pole next to and the pole carrying the transformer would have a grounded conductor (giving each transformer two grounding points for safety reasons).

Mechanical design

Proper mechanical design of a SWER line can lower its lifetime cost and increase its safety. Since the line is high voltage, with small currents, the conductor used in historic SWER lines was Number-8 galvanized steel fence wire. More modern installations use specially-designed AS1222.1

high-carbon steel Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states: * no minimum content is specified or required for chromium, coba ...

, aluminum-clad wires. Aluminum clad wires corrode in coastal areas, but are otherwise more suitable.Power to the People
Describes use of SWER in the rural electrification of Mozambique. Transmission & Distribution World, 2009. Accessed 2011-8-10 Because of the long spans and high mechanical tensions, vibration from wind can cause damage to the wires. Modern systems install spiral vibration dampers on the wires. Insulators are often

porcelain Porcelain () is a ceramic material made by heating substances, generally including materials such as kaolinite, in a kiln to temperatures between . The strength and translucence of porcelain, relative to other types of pottery, arises mainl ...

because polymers are prone to

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...

damage. Some utilities install higher-voltage insulators so the line can be easily upgraded to carry more power. For example, 12 kV lines may be insulated to 22 kV, or 19 kV lines to 33 kV.

Reinforced concrete Reinforced concrete (RC), also called reinforced cement concrete (RCC) and ferroconcrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having hig ...

poles have been traditionally used in SWER lines because of their low cost, low maintenance, and resistance to water damage,

termite Termites are small insects that live in colonies and have distinct castes (eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blattode ...

s and

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

. Local labor can produce them in most areas, further lowering costs. In New Zealand, metal poles are common (often being former rails from a railway line). Wooden poles are acceptable. In Mozambique, poles had to be at least high to permit safe passage of

giraffe The giraffe is a large African hoofed mammal belonging to the genus ''Giraffa''. It is the tallest living terrestrial animal and the largest ruminant on Earth. Traditionally, giraffes were thought to be one species, ''Giraffa camelopardalis ...

s beneath the lines. If an area is prone to lightning, modern designs place lightning ground straps in the poles when they are constructed, before erection. The straps and wiring can be arranged to be a low-cost lightning arrestor with rounded edges to avoid attracting a lightning strike.

Characteristics

Safety

SWER is promoted as safe due to isolation of the ground from both the generator and user. Most other electrical systems use a metallic neutral connected directly to the generator or a shared ground. Grounding is critical. Significant currents on the order of 8

ampere The ampere (, ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to elect ...

s flow through the ground near the earth points. A good-quality earth connection is needed to prevent risk of

electric shock Electrical injury is a physiological reaction caused by electric current passing through the body. The injury depends on the density of the current, tissue resistance and duration of contact. Very small currents may be imperceptible or produce ...

due to

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 poi ...

near this point. Separate grounds for power and safety are also used. Duplication of the ground points assures that the system is still safe if either of the grounds is damaged. A good earth connection is normally a 6 m stake of copper-clad steel driven vertically into the ground, and bonded to the transformer earth and tank. A good ground resistance is 5–10 ohms which can be measured using specialist earth test equipment. SWER systems are designed to limit the electric field in the earth to 20 volts per meter to avoid shocking people and animals that might be in the area. Other standard features include automatic reclosing circuit breakers (

recloser In electric power distribution, automatic circuit reclosers (ACRs) are a class of switchgear designed for use on overhead electricity distribution networks to detect and interrupt transient faults. Also known as reclosers or autoreclosers, ACRs a ...

s). Most faults (overcurrent) are transient. Since the network is rural, most of these faults will be cleared by the recloser. Each service site needs a rewirable drop out fuse for protection and switching of the transformer. The transformer secondary should also be protected by a standard high-rupture capacity (HRC) fuse or low voltage circuit breaker. A surge arrestor (spark gap) on the high voltage side is common, especially in lightning-prone areas. Most fire safety hazards in electrical distribution are from aging equipment: corroded lines, broken insulators, etc. The lower cost of SWER maintenance can reduce the cost of safe operation in these cases. SWER avoids lines clashing in wind, a substantial fire-safety feature, but a problem surfaced in the official investigation into the

Black Saturday bushfires The Black Saturday bushfires were a series of bushfires that either ignited or were already burning across the Australian state of Victoria on and around Saturday, 7 February 2009, and were among Australia's all-time worst bushfire disasters. T ...

Victoria, Australia Victoria is a state in southeastern Australia. It is the second-smallest state with a land area of , the second most populated state (after New South Wales) with a population of over 6.5 million, and the most densely populated state in Au ...

. These demonstrated that a broken SWER conductor can short to ground across a resistance similar to the circuit's normal load; in that particular case, a tree. This can cause large currents without a ground-fault indication. This can present a danger in fire-prone areas where a conductor may snap and current may arc through trees or dry grass. Bare-wire or ground-return telecommunications can be compromised by the ground-return current if the grounding area is closer than 100 m or sinks more than 10 A of current. Modern radio, optic fibre channels, and cell phone systems are unaffected. Many national electrical regulations (notably the U.S.) require a metallic return line from the load to the generator.''

National Electrical Code The National Electrical Code (NEC), or NFPA 70, is a regionally adoptable standard for the safe installation of electrical wiring and equipment in the United States. It is part of the National Fire Code series published by the National Fire Prote ...

'' (NEC) (2008). Quincy, Mass. (USA)
National Fire Protection Association
In these jurisdictions, each SWER line must be approved by exception.

Cost advantages

SWER's main advantage is its low cost. It is often used in sparsely populated areas where the cost of building an isolated distribution line cannot be justified. Capital costs are roughly 50% of an equivalent two-wire single-phase line. They can cost 30% of 3-wire three-phase systems. Maintenance costs are roughly 50% of an equivalent three phase line. SWER also reduces the largest cost of a distribution network: the number of poles. Conventional 2-wire or 3-wire distribution lines have a higher power transfer capacity, but can require 7 poles per kilometre (12 poles per mile), with spans of 100 to 150 metres (110 to 160 yards). SWER's high line voltage and low current also permits the use of low-cost

galvanized steel Galvanization or galvanizing ( also spelled galvanisation or galvanising) is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are submerged ...

wire (historically, No. 8 fence wire). Steel's greater strength permits spans of 400 metres (¼ mile) or more, reducing the number of poles to 2.5 per kilometre (4 per mile). If the poles also carry

optical fiber cable A fiber-optic cable, also known as an optical-fiber cable, is an assembly similar to an electrical cable, but containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with ...

for

telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...

s (metal conductors may not be used), capital expenditures by the power company may be further reduced.

Reliability

SWER can be used in a grid or loop, but is usually arranged in a linear or radial layout to save costs. In the customary linear form, a single-point failure in a SWER line causes all customers further down the line to lose power. However, since it has fewer components in the field, SWER has less to fail. For example, since there is only one line, winds can't cause lines to clash, removing a source of damage, as well as a source of rural bush fires. Since the bulk of the transmission line has low resistance attachments to earth, excessive ground currents from shorts and

geomagnetic storm A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field. The disturbance that d ...

s are more rare than in conventional metallic-return systems. So, SWER has fewer ground-fault circuit-breaker openings to interrupt service.

Upgradeability

A well-designed SWER line can be substantially upgraded as demand grows without new poles. The first step may be to replace the steel wire with more expensive copper-clad or aluminum-clad steel wire. It may be possible to increase the voltage. Some distant SWER lines now operate at voltages as high as 35 kV. Normally this requires changing the insulators and transformers, but no new poles are needed. If more capacity is needed, a second SWER line can be run on the same poles to provide two SWER lines 180 degrees out of phase. This requires more insulators and wire, but doubles the power without doubling the poles. Many standard SWER poles have several bolt holes to support this upgrade. This configuration causes most ground currents to cancel, reducing shock hazards and interference with communication lines. Two-phase service is also possible with a two-wire upgrade: Though less reliable, it is more efficient. As more power is needed, the lines can be upgraded to match the load, from single wire SWER to two wire, single phase and finally to three wire, three phase. This ensures a more efficient use of capital and makes the initial installation more affordable. Customer equipment installed before these upgrades will all be single phase, and can be reused after the upgrade. If small amounts of

three-phase power Three-phase electric power (abbreviated 3φ) is a common type of alternating current used in electricity generation, transmission, and distribution. It is a type of polyphase system employing three wires (or four including an optional neutral r ...

are needed, it can be economically synthesized from two-phase power with on-site equipment.

Power-quality weakness

SWER lines tend to be long, with high impedance, so the voltage drop along the line is often a problem, causing poor regulation. Variations in demand cause variation in the delivered voltage. To combat this, some installations have automatic variable transformers at the customer site to keep the received voltage within legal specifications. After some years of experience, the inventor advocated a

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

in series with the ground of the main isolation transformer to counteract the inductive reactance of the transformers, wire and earth return path. The plan was to improve the

power factor In electrical engineering, the power factor of an AC power system is defined as the ratio of the '' real power'' absorbed by the load to the ''apparent power'' flowing in the circuit. Real power is the average of the instantaneous product of v ...

, reduce losses and improve voltage performance due to

reactive power Reactive may refer to: *Generally, capable of having a reaction (disambiguation) *An adjective abbreviation denoting a bowling ball coverstock made of reactive resin *Reactivity (chemistry) *Reactive mind *Reactive programming See also *Reactanc ...

flow. Though theoretically sound, this is not standard practice. It does also allow the use of a DC test loop, to distinguish a legitimate variable load from (for example) a fallen tree, which would be a DC path to ground.

Use

In addition to New Zealand and Australia, single-wire earth return is used throughout the globe.

Alaska

In 1981 a high-power 8.5 mile prototype SWER line was successfully installed from a diesel plant in

to Napakiak in

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

. It operates at 80 kV, and was originally installed on special lightweight

fiberglass Fiberglass (American English) or fibreglass (Commonwealth English) is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet called a chopped strand mat, or woven into glass cloth ...

poles that formed an

A-frame An A-frame is a basic structure designed to bear a load in a lightweight economical manner. The simplest form of an A-frame is two similarly sized beams, arranged in an angle of 45 degrees or less, attached at the top, like an uppercase lett ...

. Since then, the A frames have been removed and standard wooden power poles were installed. The A-framed poles could be carried on lightweight snow machines, and could be installed with hand tools on

without extensive digging. Erection of "anchoring" poles still required heavy machinery, but the cost savings were dramatic. Researchers at the

University of Alaska Fairbanks The University of Alaska Fairbanks (UAF or Alaska) is a public land-grant research university in College, Alaska, a suburb of Fairbanks. It is the flagship campus of the University of Alaska system. UAF was established in 1917 and opened for cla ...

estimate that a network of such lines, combined with coastal

wind turbine A wind turbine is a device that converts the kinetic energy of wind into electrical energy. Hundreds of thousands of large turbines, in installations known as wind farms, now generate over 650 gigawatts of power, with 60 GW added each year. ...

s, could substantially reduce rural Alaska's dependence on increasingly expensive

diesel fuel Diesel fuel , also called diesel oil, is any liquid fuel specifically designed for use in a diesel engine, a type of internal combustion engine in which fuel ignition takes place without a spark as a result of compression of the inlet air and t ...

for power generation. Alaska's state economic energy screening survey advocated further study of this option to use more of the state's underutilized power sources.

In developing nations

At present, certain developing nations have adopted SWER systems as their

mains electricity Mains electricity or utility power, power grid, domestic power, and wall power, or in some parts of Canada as hydro, is a general-purpose alternating-current (AC) electric power supply. It is the form of electrical power that is delivered to h ...

systems, notably

Laos Laos (, ''Lāo'' )), officially the Lao People's Democratic Republic ( Lao: ສາທາລະນະລັດ ປະຊາທິປະໄຕ ປະຊາຊົນລາວ, French: République démocratique populaire lao), is a socialist ...

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...

and

Mozambique Mozambique (), officially the Republic of Mozambique ( pt, Moçambique or , ; ny, Mozambiki; sw, Msumbiji; ts, Muzambhiki), is a country located in southeastern Africa bordered by the Indian Ocean to the east, Tanzania to the north, Malawi ...

. SWER is also used extensively in Brazil.

In HVDC systems

Many

systems (HVDC) using submarine power cables are single wire earth return systems. Bipolar systems with both positive and negative cables may also retain a seawater grounding electrode, used when one pole has failed. To avoid electrochemical corrosion, the ground electrodes of such systems are situated apart from the converter stations and not near the transmission cable. The electrodes can be situated in the sea or on land. Bare copper wires can be used for cathodes, and graphite rods buried in the ground, or titanium grids in the sea are used for anodes. To avoid electrochemical corrosion (and passivation of titanium surfaces) the current density at the surface of the electrodes must be small, and therefore large electrodes are required. Examples of HVDC systems with single wire earth return include the

Baltic Cable The Baltic Cable is a monopolar HVDC power line running beneath the Baltic Sea that interconnects the electric power grids of Germany and Sweden. Its maximum transmission power is 600 megawatts (MW). The Baltic Cable uses a transmission voltag ...

and

Kontek The Kontek HVDC is a long, monopolar 400 kV high-voltage direct current cable between Germany and the Danish island Zealand. Its name comes from "continent" and the name of the former Danish power transmission company "Elkraft", which o ...

Installations

The following table shows various installations of SWER systems

References

External links

Rural power.org
; Excellent site on this topic. Provides the PDF of Mandeno's article.
Manual for Single Wire Earth Return Power Systems
From the Network Power Standards Branch of the Australian Northern Territory Government. Includes dimensioned mechanical drawings and parts lists.
AS2558-2006 Transformers for use on single-wire earth-return distribution systems
- An Australian standard
Saskatchewan in Canada has operated SWER for more than fifty years
* Distributed generation as voltage support for single wire earth return systems, Kashem, M.A.; Ledwich, G.; IEEE Transactions on Power Delivery, Volume 19, Issue 3, July 2004 Page(s): 1002–101

{{Electricity generation Appropriate technology Electric power distribution