Single wire earth return
Encyclopedia
Single wire earth return (SWER) or single wire ground return is a single-wire transmission line
Single-wire transmission line
A single-wire transmission line is a method of supplying electrical power through a single electrical conductor.-History:In 1729, the English physicist Stephen Gray noticed the phenomenon of electrical conductivity...

 for supplying single-phase
Single-phase electric power
In electrical engineering, single-phase electric power refers to 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 are mostly lighting and heating, with few large electric motors...

 electrical power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

 from an electrical grid to remote areas at low cost. Its distinguishing feature is that the earth
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

 (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
Since the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding of equipment and structures...

) to act as a return path. It is principally used for rural electrification
Rural electrification
Rural electrification is the process of bringing electrical power to rural and remote areas. Electricity is used not only for lighting and household purposes, but it also allows for mechanization of many farming operations, such as threshing, milking, and hoisting grain for storage; in areas...

, but also finds use for larger isolated loads such as water pumps, and light rail
Light rail
Light rail or light rail transit is a form of urban rail public transportation that generally has a lower capacity and lower speed than heavy rail and metro systems, but higher capacity and higher speed than traditional street-running tram systems...

. Single wire earth return is also used for HVDC over submarine power cable
Submarine power cable
Submarine power cables are major transmission cables for carrying electric power below the surface of the water. These are called "submarine" because they usually carry electric power beneath salt water but it is also possible to use submarine power cables beneath fresh water...

s.

Description

SWER is a good 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.

Power is supplied to the SWER line by an isolating transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

 of up to 300kVA
Volt-ampere
A volt-ampere is the unit used for the apparent power in an electrical circuit, equal to the product of root-mean-square voltage and RMS current. In direct current circuits, this product is equal to the real power in watts...

. This transformer isolates the grid from ground or earth, and changes the grid voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 (typically 22 kilovolts line to line) to the SWER voltage (typically 12.7 or 19.1 kilovolts 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 earth
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

 through an earth stake. From the earth stake, the current eventually finds its way back to the main step-down transformer at the head of the line, completing the circuit
Electrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...

. SWER is therefore a practical example of a phantom loop.

In areas with high-resistance soil, the resistance of the soil wastes energy. The wasted energy turns into heat at the grounding rod, and can burn up the rod. 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, 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
Split phase
A split-phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial applications. It is the AC equivalent of the original Edison 3-wire direct current system...

 (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 obsoleted 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

The 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 No. 8 galvanized steel fence wire. More modern installations use specially-designed AS1222.1 high-carbon steel, aluminum-clad wires. Aluminum clad wires corrode in coastal areas, but are otherwise more suitable.
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 because polymers are prone to damage from ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

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

Reinforced concrete
Reinforced concrete
Reinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...

 poles have been traditionally used in SWER lines because of their low cost, low maintenance, and resistance to water damage, termites and fungi
Fungus
A fungus is a member of a large 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, Fungi, which is separate from plants, animals, and bacteria...

. 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 12M to permit safe passage of giraffes.
If an area is prone to lightning, modern designs place lightning ground straps in the poles when the poles 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.

History

Lloyd Mandeno OBE
Order of the British Empire
The Most Excellent Order of the British Empire is an order of chivalry established on 4 June 1917 by George V of the United Kingdom. The Order comprises five classes in civil and military divisions...

 (1888-1973) fully developed SWER in New Zealand 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 have now been installed in Australia 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 in the Canadian
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

 province of Saskatchewan
Saskatchewan
Saskatchewan is a prairie province in Canada, which has an area of . Saskatchewan is bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, and on the south by the U.S. states of Montana and North Dakota....

, Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

, Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

, portions of the United States' Upper Midwest
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 there are no uniformly agreed-upon boundaries, the region is most commonly used to refer to the states of Minnesota, Wisconsin, and...

, and SWER interties have been proposed for Alaska and prototyped.

Safety

SWER's safety is assured because transformers isolate the ground from both the generator and user. Most electrical systems use a metallic neutral connected directly to the generator or a shared ground. Certain groups claim that stray voltage
Stray voltage
Stray voltage describes the occurrence of electrical potential between two objects that ideally should not have any voltage difference between them. Small voltages are often measured between two grounded objects in distant locations, due to normal current flow in the power system...

s from SWER can injure livestock.

Grounding is critical. Significant currents on the order of 8 ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

s flow through the ground near the earth points. A good-quality earth connection is needed to prevent risk of electric shock
Electric shock
Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....

 due to earth potential rise
Earth potential rise
In electrical engineering, earth potential rise also called ground potential rise 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...

 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. SWER systems are designed to limit the voltage 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 (reclosers). 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.

The official investigation
2009 Victorian Bushfires Royal Commission
The 2009 Victorian Bushfires Royal Commission is an Australian Royal Commission which concluded on 31 July 2010, it investigated the nature of circumstances surrounding the Black Saturday bushfires.-Precluding events:...

 into the Black Saturday bushfires in Victoria, Australia, disclosed that a broken SWER conductor that comes in contact with a return path entry point with resistance similar to the circuit's normal load (such as a tree) can cause large amounts of current to flow to ground without a fault indication. This presents 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.

Cost advantage

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 70% less than 3-wire three-phase systems. Maintenance costs are roughly 50% of an equivalent 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 seven poles per kilometre, with spans of 100 to 150 metres. SWER’s high line voltage and low current also permits the use of low-cost galvanized steel wire (historically, No. 8 fence wire). Steel’s greater strength permits spans of 400 metres or more, reducing the number of poles to 2.5 per kilometre.

If the poles also carry optical fiber cable
Optical fiber cable
An optical fiber cable is a cable containing one or more optical fibers. The optical fiber elements are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable will be deployed....

 for telecommunication
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

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

Reliability strengths

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 brush fires.

Since the bulk of the transmission line has low resistance attachments to earth, excessive ground currents from shorts and geomagnetic storm
Geomagnetic storm
A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in the interplanetary medium. A geomagnetic storm is a major component of space weather and provides the input for many other components of space weather...

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

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.

SWER combined with distributed generation
Distributed generation
Distributed generation, also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from many small energy sources....

 is substantially more efficient than a single-ended system. For example, some rural installations can offset line losses and charging currents with local solar power
Solar power
Solar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar radiation, along with secondary solar-powered resources such as wind and wave power, hydroelectricity and biomass, account for most of the available...

, wind power
Wind power
Wind power is the conversion of wind energy into a useful form of energy, such as using wind turbines to make electricity, windmills for mechanical power, windpumps for water pumping or drainage, or sails to propel ships....

, small hydro
Small hydro
Small hydro is the development of hydroelectric power on a scale serving a small community or industrial plant. The definition of a small hydro project varies but a generating capacity of up to 10 megawatts is generally accepted as the upper limit of what can be termed small hydro. This may be...

 or other local generation. This can be an excellent value for the electrical distributor, because it reduces the need for more lines.

After some years of experience, the inventor advocated a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

 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
Power factor
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...

, reduce losses and improve voltage performance due to reactive power flow. Though theoretically sound, this is not standard practice.

Networks and circuits

As demand grows, a well-designed SWER line can be substantially upgraded 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
Two phase
Two-phase electrical power was an early 20th century polyphase alternating current electric power distribution system. Two circuits were used, with voltage phases differing by 90 degrees. Usually circuits used four wires, two for each phase. Less frequently, three wires were used, with a common...

 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
Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...

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

Regulatory issues

Many national electrical regulations (notably the U.S.) require a metallic return line from the load to the generator. In these jurisdictions, each SWER line must be approved by exception.

Rural electrification in Alaska

In 1981 a high-power 8.5 mile prototype SWER line was successfully installed from a coal plant in Bethel
Bethel, Alaska
Bethel is a city located near the west coast of the U.S. state of Alaska, west of Anchorage. Accessible only by air and river, Bethel is the main port on the Kuskokwim River and is an administrative and transportation hub for the 56 villages in the Yukon-Kuskokwim Delta.Bethel is the largest...

 to Napakiak
Napakiak, Alaska
Napakiak is a city in Bethel Census Area, Alaska, United States. At the 2000 census the population was 353.-Geography:Napakiak is located at ....

 in Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...

, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. It operates at 80 kV, and has special lightweight fiberglass poles forming an A-frame. The poles can be carried on lightweight snow machines, and most poles can be installed with hand tools on permafrost without extensive digging. Erection of “anchoring” poles still required heavy machinery, but the cost savings were dramatic.

Researchers at the University of Alaska Fairbanks
University of Alaska Fairbanks
The University of Alaska Fairbanks, located in Fairbanks, Alaska, USA, is the flagship campus of the University of Alaska System, and is abbreviated as Alaska or UAF....

, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 estimate that a network of such lines, combined with coastal wind turbine
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...

s, could substantially reduce rural Alaska’s dependence on increasingly expensive diesel fuel 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.

Use by developing nations

At present, certain developing nations have adopted SWER systems as their mains electricity
Mains electricity
Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...

 systems, notably Laos
Laos
Laos Lao: ສາທາລະນະລັດ ປະຊາທິປະໄຕ ປະຊາຊົນລາວ Sathalanalat Paxathipatai Paxaxon Lao, officially the Lao People's Democratic Republic, is a landlocked country in Southeast Asia, bordered by Burma and China to the northwest, Vietnam to the east, Cambodia to the south and Thailand to the west...

, South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...

 and Mozambique
Mozambique
Mozambique, officially the Republic of Mozambique , is a country in southeastern Africa bordered by the Indian Ocean to the east, Tanzania to the north, Malawi and Zambia to the northwest, Zimbabwe to the west and Swaziland and South Africa to the southwest...

. SWER is also used extensively in Brazil where it is termed “Redes Monofilares com Retorno por Terra” or “MRT”. There are detailed standards and drawings available in Brazilian Portuguese that would be transferable to other Portuguese speaking countries such as Angola and Mozambique.

Use for HVDC systems

Many high-voltage direct current systems using submarine power cable
Submarine power cable
Submarine power cables are major transmission cables for carrying electric power below the surface of the water. These are called "submarine" because they usually carry electric power beneath salt water but it is also possible to use submarine power cables beneath fresh water...

s 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
Passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...

 of titanium surfaces) the current density at the surface of the electrodes must be small, and therefore large electrodes are required.

The advantage of such schemes is eliminating the cost of a second conductor, since salt water is an excellent conductor. Some ecologists claim that electrochemical reactions caused by the earth return can affect wildlife. However, these reactions do not occur on very large underwater electrodes.

Examples of HVDC systems with single wire earth return

  • Baltic Cable
    Baltic Cable
    The Baltic Cable is a HVDC power line running beneath the Baltic Sea that interconnects the electric power grids of Germany and Sweden.The Baltic Cable uses a transmission voltage of 450 kV – the highest operating voltage for energy transmission in Germany...

  • Kontek
    Kontek
    The Kontek is a 170 kilometer long, monopolar 400kV DC-cable for the interconnection of the German power grid with the electricity grid of the Danish island Sealand. The name comes from "continent" and the name of the former Danish power transmission company "Elkraft", which operated the power grid...

  • Basslink
    Basslink
    Basslink is a high-voltage direct current cable link crossing Bass Strait, connecting the Loy Yang Power Station, Victoria on the Australian mainland to the George Town substation in northern Tasmania...


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