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earth return

Single-wire earth return

Single wire earth return (SWER) or single wire ground return is a single-wire transmission line for supplying single-phase electrical power from an electrical grid to remote areas at low cost. It is principally used for rural electrification, but also finds use for larger isolated loads such as water pumps, and light rail. Single wire earth return is also used for HVDC over submarine power cables.

Description

SWER is a 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 of up to 300 kVA. This isolates the grid from ground or earth, and changes the grid voltage (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, visiting a number of termination points. At each termination point, such as a customer's premises, current flows from the line, through the primary coil of a step-down transformer, to 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. SWER is therefore a practical example of a phantom loop.

The secondary winding of the local transformer will supply the customer with either single ended single phase (N-0) or split phase (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 (most likely 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).

History

At the end of the 19th century, Nikola Tesla demonstrated that only a single wire was necessary for power systems, with no need for a wired return conductor (using the Earth instead). Lloyd Mandeno 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 Saskatchewan, Brazil, Africa, portions of the United States' Upper Midwest, and SWER interties have been proposed for Alaska and prototyped.

Characteristics

Safety

SWER violates common wisdom about electrical safety, because it lacks a traditional metallic return to a neutral shared by the generator. SWER’s safety is instead assured because transformers isolate the ground from both the generator and user. However, certain groups claim that stray voltages from SWER can injure livestock.

Grounding is critical because of the significant currents on the order of 8 amperes that flow through the ground near the earth points, so a good-quality earth connection is needed to prevent risk of electric shock due to earth potential rise 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.

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.

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 be 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 m to 150 m. SWER’s high line voltage and low current permits the use of low-cost galvanized steel wire. Steel’s greater strength permits spans of 400 m or more, reducing the number of poles to 2.5/km.

Reinforced concrete poles have been traditionally used in SWER lines because of their low cost, low maintenance, and resistance to water damage, termites and fungus. Local labor can produce them in most areas, further lowering costs.

If the cable contains optic fibre, or carries telephone service, this can further amortize the capital costs.

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 line can't clash in the wind, and the bulk of the transmission line has low resistance attachments to earth, excessive ground currents from shorts and geomagnetic storms are far 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.

When used with distributed generation, SWER is substantially more efficient than when it is operated as a single-ended system. For example, some rural installations can offset line losses and charging currents with local solar power, wind power, small hydro or other local generation. This can be an excellent value for the electrical distributor, because it reduces the need for more lines. (Kashem and Ledwich)

After some years of experience, the inventor (Mandeno, below) advocated a capacitor 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, 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.

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 wirelines.

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 moderate amounts of three-phase are needed, it can be economically synthesized from two-phase 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.

Use in interties

In 1981 a high-power 8.5 mile prototype SWER intertie was successfully installed from a coal plant in Bethel, Alaska to Napakiak, Alaska. It operates at 80 kV, and has special lightweight fiberglass poles that form 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.

The phase conductor also carries a bundle of optical fibres within the steel armor wire, so the system supplies telecommunications as well as power.

Researchers at the University of Alaska Fairbanks estimate that a network of such interties, combined with coastal wind turbines, could substantially reduce Alaska’s dependence on increasingly expensive diesel fuel for power generation. Alaska’s state economic energy screening survey advocated further study of this option, in order 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 systems. These include Tunisia, which has made its own version of the single wire earth return; notably MALT. MALT is used within the country to reduce costs and simplify installation; an approach which is working and has made Tunisia's electricity network one of the best amongst the developing nations.

Use for HVDC systems

Many HVDC systems using submarine power cables are (or were until their expansion to bipolar schemes) single wire earth return systems. In order to avoid electrochemical corrosion, the ground electrodes of such systems are situated apart from the converter stations and not in the proximity of the transmission cable. The electrodes can be situated in the sea or on land. As cathodes, bare copper wires can be used in the sea or on land. As anodes, graphite rods dug in the ground, or titanium grids in the sea are used. In order to avoid electrochemical corrosion (and passivation of titanium surfaces) the current density at the surface of the electrodes may be only small and therefore large electrodes are required.

The advantage of such schemes is saving money for a second conductor, because the saltwater is an excellent conductor. Some ecologists claim bad influences of electrochemical reactions, but they do not occur on very large underwater electrodes.

Examples of HVDC systems with single wire earth return

References

External articles and links

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