By Paul Grad, engineering writer
Single wire earth return (SWER) lines have proved ideally suited for supplying electric power to remote, sparsely populated areas of Australia, which include some of the lowest customer density networks in the world. Lately, utilities with SWER lines have undertaken system upgrades to ensure the aging and capacity constrained SWER networks continue operating satisfactorily.
SWER is a single wire transmission line which supplies single-phase electric power from an electrical grid, with all equipment grounded to earth and with the earth used as the return path for the load currents. SWER lines have been chosen for rural areas where the loads were small and spread over a wide area, mainly due to the high cost of conventional return current wiring for those areas. Power is supplied to the SWER line by means of an isolating transformer of up to 300kVA. The transformer isolates the grid from the earth and changes the grid voltage – typically 22kV or 33kV line to line – to the SWER voltage – typically 12.7kV or 19.1kV line to earth. The SWER voltage levels are convenient because they are the phase-to-ground voltages of a 22kV and 33kV system, respectively, and allow using standard equipment.
SWER’s main advantage is its low cost. Capital costs can be some 50 per cent of an equivalent two-wire single-phase line, and 70 per cent less than three-wire three-phase systems. The lower costs are due to savings in wiring, fewer pole-top fittings, and fewer switching and protection devices. SWER lines are also simpler to design, allowing for speedier construction. Maintenance costs are also about 50 per cent lower than those for an equivalent conventional line. SWER lines also require only 2.5 poles per kilometre, while conventional two-wire or three-wire distribution lines require seven poles per kilometre.
One of the main costs in SWER systems is in the isolating transformer. This is why SWER is seldom economic for extensions less than 3km.
The pioneer of the development of SWER for rural electrification was the New Zealand engineer Lloyd Mandeno. He outlined the technology in a seminal paper titled “Rural power supply, especially in back country areas”, in 1947. The technology was further developed in Australia. Many other countries followed suit, establishing SWER lines for rural electrification, including South Africa, Brazil, India, Laos and Vietnam. A SWER line was installed in Alaska, in the US.
Presently, Australia operates a total of about 200,000km of SWER lines, at 19.1kV and 12.7kV.
New South Wales and the ACT operate about 30,000km of conventional 22kV single and three phase lines, about 110,000km conventional 11kV or lower voltage single and three phase lines, and 30,000km of SWER lines. For Victoria the corresponding figures are 60,000km, 1600km, and 30,000km. Queensland has no 22kV lines, but has 62,000km conventional 11kV or lower voltage single and three phase lines, and 64,000km SWER lines. South Australia has no 22kV lines, but has 16,000km conventional 11kV or lower single and three phase lines, and 31,000km SWER lines. Western Australia operates 15,000km conventional 22kV single and three phase lines, 1700km conventional 11kV or lower single and three phase lines, and 40,000km SWER lines. Tasmania operates 11,000km conventional 22kV single and three phase lines, 3000km conventional 11kV or lower single and three phase lines, and 600km SWER lines.
There are, however, also disadvantages associated with the SWER option, such as stability problems with the voltage as the load increases, restricted load capacity, and inability to provide a three-phase supply. Another issue is a certain risk of bushfires. On the one hand, there is a lower fire risk in SWER because it avoids lines clashing in wind. On the other hand, a broken SWER conductor can short to ground across a resistance closed to the circuit’s normal load, as for example, a tree. This could cause large currents to flow through trees or dry grass.
SWER lines have been seen as safe due to isolation of the ground from both the generator and user. Grounding is critical, and currents as high as 8A or higher can flow through the ground near the earth points. A good-quality earth connection is also needed to prevent risk of electric shock due to earth potential rise near this point.
In Victoria, in response to the 2009 Black Saturday bushfires, the Victorian Bushfire Royal Commission, which was established to look into the causes and possible prevention of bushfires, determined that the electricity infrastructure had caused several of those fires. Accordingly, the Commission recommended the progressive replacement of all SWER lines in Victoria with aerial bundled cable, underground cabling or other technology that delivers greatly reduced bushfire risk. This replacement should be completed within 10 years in the areas of highest bushfire risk. The Commission also recommended that distribution businesses should inspect, at least every three years, all SWER lines and all 22kV distribution feeders in areas of high bushfire risk.
To comply with those recommendations, Citipower-Powercor has upgraded powerline safety in its network with new generation technology to reduce the likelihood of bushfires. The upgrade included the installation of 179 remote Automatic Circuit Reclosers (ACRs) in high bushfire risk areas within Powercor’s distribution network.
ACRs are circuit breakers which can automatically close the breaker – and automatically restore power to the line – after it has been opened due to a fault. Installation of the ACRs will enable to remotely adjust the devices on Total Fire Ban days, reducing the likelihood of fires from SWER systems. Thus the bushfire risk can be reduced without the need to send out a service truck as with older generation, manual ACRs. As of July 2013, 610 ACRs have been installed.
Also, Rapid Earth Fault Current Limiters (REFCLs) have been installed in selected substations.
Powercor’s total line length is about 84,000km, serving more than 730,000 customers. About 32,000 customers are supplied via SWER rural grid.
Voltage stability in SWER lines, especially at fairly high loads, can also be a significant problem, leading to customer complaints.
Several measures have been applied to SWER lines throughout Australia to improve voltage stability. For example, Ergon Energy has installed low voltage regulators (LVRs) in its network. Initially the utility installed six LVRs from MicroPlanet for performance testing. The devices performed well, keeping the output voltage at the chosen 245V set point. After the testing stage, Ergon Energy purchased and installed 1000 MicroPlanet LVRs. Ergon Energy set stringent performance specifications for MicroPlanet to meet. The units had to be able to withstand the harsh environment in many areas of Queensland, throughout Ergon Energy’s vast service area. The 1000 LVRs were deployed between late 2008 and late 2012.
The utility has also been looking at battery storage to further improve the SWER network. A tender assessment process for a number of 25kW/100kWh battery storage units is under way.
The utility is also proceeding with traditional augmentation options such as isolating transformer upgrades, voltage conversion and SWER splitting.
Ergon maintains some 65,000km of SWER lines in regional Queensland, representing about 40 per cent of the utility’s distribution network.
Essential Energy operates 30,000km of SWER lines and 116,000km of conventional lines.
The length of SWER lines has hardly increased over the past few decades, and the basic technology has changed little since the paper by Mandeno. There has, however, been a significant advance in the technology of ancillary equipment, largely overcoming traditional limitations of SWER lines. In spite of the technical challenges of installing and running SWER lines, and the complexities posed by increasing use of renewable energy, people in the utilities generally believe that SWER is here to stay.
