Single-wire earth return
Single-wire earth return is principally used for rural electrification, but also finds use for larger isolated loads such as water pumps.
It has been applied around the world, such as in the Canadian province of Saskatchewan; Brazil; Africa; and portions of the United States' Upper Midwest and Alaska (Bethel).
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.
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.
The resistance may be high enough to affect self-resetting circuit breakers, which usually reset due to a difference in voltage between line and neutral.
With dry, high-resistance soils, the reduced difference in voltage between line and neutral may prevent breakers from resetting.
[6] 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.
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.
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.
The transformer secondary should also be protected by a standard high-rupture capacity (HRC) fuse or low voltage circuit breaker.
Most fire safety hazards in electrical distribution are from aging equipment: corroded lines, broken insulators, etc.
[4] SWER avoids lines clashing in wind, a substantial fire-safety feature,[4] but a problem surfaced in the official investigation into the Black Saturday bushfires in Victoria, Australia.
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.
[4] This can present a danger in fire-prone areas where a conductor may snap and current may arc through trees or dry grass.
SWER's high line voltage and low current also permits the use of low-cost galvanized steel wire (historically, No.
If the poles also carry optical fiber cable for telecommunications (metal conductors may not be used), capital expenditures by the power company may be further reduced.
Since the bulk of the transmission line has low resistance attachments to earth, excessive ground currents from shorts and geomagnetic storms are more rare than in conventional metallic-return systems.
This configuration causes most ground currents to cancel, reducing shock hazards and interference with communication lines.
To combat this, some installations have automatic variable transformers at the customer site to keep the received voltage within legal specifications.
SWER systems are forbidden for national electric regulation RETIE (REGLAMENTO DE INSTALACIONES ELECTRICAS).
In 1981 a high-power 8.5 mile prototype SWER line was successfully installed from a diesel plant in Bethel to Napakiak in Alaska, United States.
The A-framed poles could be carried on lightweight snow machines, and could be installed with hand tools on permafrost without extensive digging.
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.
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.
Examples of HVDC systems with single wire earth return include the Baltic Cable and Kontek.
