Single wire earth return
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Single wire earth return (SWER) or single wire ground return is a method of supplying single-phase electrical power to remote areas with low installation costs. It is used for rural electrification. SWER is a reasonable choice for a distribution system when conventional return current wiring would cost more than SWER's isolation transformers and inefficiency. Power engineers experienced with both SWER and conventional power lines rate SWER as equally safe, more reliable, less costly, but less efficient than conventional lines (ref. Stone Power, L. Mandeno, below).
Image:Swer.gif Power is supplied to the SWER line by a single, large step-down transformer that isolates the grid from the powered ground, and changes the grid Voltage (typically 22 kilovolts) to the SWER voltage (typically 12.7 or 19.1 kilovolts).
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 typically supplies the customer with split phase power in the region's standard appliance voltages. In most countries this is 240-0 volts or 240-0-240 volts, with the 0 volt line connected to a safety earth that does not normally carry an operating current. Other voltages may be supplied.
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History
SWER was invented in New Zealand for rural electrification around 1925 by Lloyd Mandeno. Although he called it "Earth Working Single Wire Lines" it was often called "Mandeno's Clotheslines."
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 done. The Australian standards are widely used and cited. It has been applied in Saskatchewan, Brazil and Africa, and SWER interties have been proposed for Alaska and prototyped.
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, grounding is critical. Significant currents (of the order of 8 Amperes) flow through the ground near the earth point, So, a good-quality earth connection is needed to prevent risk of electric shock near this point.
Separate grounds for power and safety are also needed. Separation of the grounds assures that the capacity of the safety ground cannot be overloaded by power transmission.
A good earth connection is normally 6 m of copper-clad steel stake driven vertically into the ground, and bonded to the transformer wire. A good ground resistance is 5-10 Ohms.
Other standard safety features include automatic reclosing circuit breakers reclosers. Most faults (over current) are transient, so since the network is rural, most fault reclosers should automatically reset. Each service site needs a standard high-rupture capacity (HRC) fuse to prevent transformer overloads. A surge arrestor (spark gap) is common, especially in lightning-prone areas.
Bare-wire or ground-return telecommunications can also be compromised by ground-return power if the grounding area is closer than 100 m or sinks more than 10 A of current. Modern radio and optic fibre channels are unaffected.
Reliability: A Strength
SWER can be used in a grid, but is usually arranged in a linear layout to save cost. 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, it does not clash with other lines, 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, grounding faults from geomagnetic storms do not happen in the single line distribution (as they do in conventional metallic-return systems).
Power quality: A Weakness
SWER lines tend to be long, with high resistances, so the voltage drop along the line is often a problem, causing poor power quality. Variations in demand cause variation in the delivered voltage. To combat this, some installations have automatic variable transformers at the customer site to keep delivered voltages 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 PV, 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 reactivity of the transformers, wire and earth return. The plan was to improve the power factor, reducing resistive losses and power transients due to reflected power. Though theoretically sound, this is not standard practice.
Low Cost: The Main Advantage
SWER's main advantage is its low cost. It is often used in sparsely populated areas where the cost of building an isolated transmission line cannot be justified. Capital savings are roughly 50% compared to two-wire single-phase systems. They can be 70% less than 3-wire three-phase systems. An important source of savings is that there is no need to regulate the tension of a second or third wire to match the first. Maintenance costs are roughly 50%.
SWER also reduces the largest cost of a distribution network, the number of poles. Conventional copper or aluminum wiring has high current capacity, but can require seven poles per kilometre, with runs 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 runs of 400 m or more, reducing the number of poles to 2.5/km.
Reinforced concrete poles are traditional in SWER because of their low cost, low maintenance, and resistance to water damage, termites and fungus.
If the cable contains optic fibre [1], or carries RF phone line service, this can further amortize the capital costs.
Upgrading a SWER line
If demand grows, a well-designed line can be upgraded without new poles. The first step may be to replace steel with a more expensive copper-clad or aluminum-clad steel wire.
If more capacity is needed, two wire 19 kV service provides two SWER lines, but 90 degrees out of phase. This requires more insulators and wire, but doubles the power. This configuration causes most ground currents to cancel, reducing shock hazards, and interference with communication wirelines.
Conventional two phase service is also possible: Though less reliable, it is more efficient.
Customer equipment installed before the upgrade will all be two-phased, and adapt to either upgrade. If moderate amounts of three-phase is needed, it can be economically synthesized from two-phase with on-site equipment.
Areas prone to energy theft may need a centralized neutral so tamper-resistant electricity meters can be used.
Transformers need not be permanently installed. it is perfectly possible to place maximum-indicating thermometers in them, and install larger ones as needed, moving the smaller ones to other posts.
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 optic fibres within the steel armor wire[2], so the system supplies telecommunications as well as power.
Researchers at the University of 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. [3] 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.[4]
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.
External links
References
Please use these lightly to avoid traffic on the servers and long-haul backbones, some of which are quite remote from the U.S. and Europe.
- Rural Power Supply Especially in Back Country Areas, L Mandeno, B.E., M.I.E.E. Proceedings of the New Zealand Institute of Engineers, 1947, Volume 33, Page 234. 7 megabyte scanned PDF
- Manual for Single Wire Earth Return Power Systems From the Network Power Standards Branch of the Australian Government. Includes dimensioned mechanical drawings and parts lists.
- AS2558-2006, Transformers for use on Single Wire Earth-Return Distribution Systems- An Australian standard
- Stone Power AB discusses low cost networks
- 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.
Power Delivery, IEEE Transactions on Volume 19, Issue 3, July 2004 Page(s): 1002 - 1011 [5]