Frequently Asked Questions (FAQ)
The frequently asked questions below are based on emails received. I have edited out or changed names in some cases. I have also heavily edited some emails for clarity. While I am happy to answer queries on the use of SWER and on rural electrification in general, sometimes replies can be delayed.
Alternatives to SWER for Rural Electrification
What are the other types of power distribution network topologies available apart from SWER. Can we use two phase distribution systems I mean primary distribution with 2 wires line to line and secondary with local neutral so that we can obtain more power. Of course load balancing has to be done on the primary distribution side, please comment
The main power distribution network topologies include;
On the primary side
1) 3 wire, 3 phase (European/Asian system)
2) 3 wire, 3 phase with underun neutral wire (North American System), transformers can be connected phase to phase or phase to neutral
3) 2 wire, 1 phase (European system also used in Asia, generally only used in rural situations as 3 phase motors can not be readily connected)
4) 2 wire, 1 phase with underun neutral wire (North American System)
5) 1 wire, 1 phase with underun neutral wire (North American System), developed for the remote areas of North America to enable rural electrification
6) 1 wire, Single Wire Earth Return (New Zealand, Australian System) developed for reticulating mountainous areas in New Zealand but now used in Australia for rural electrification in the outback
On the secondary side
7) 4 wire star 400/230 Volt (IEC European System but also used in Asia), 3 phase wires and 1 neutral wire
8) 3 wire delta 120 Volt (I am not sure how common this is in North America)
9) 3 wire, +230,0,-230 Volt (European and North American) 2 phase wires and 1 neutral wire, means that using a single phase primary supply you can connect either 230 phase to neutral or -230 to +230 (460 Volts) for motors etc.
10) 2 wire 230 Volt, phase to neutral (European)
11) 2 wire 110 Volt, phase to neutral (North American)
Please note that the North Americans have secondary voltages other than 230 Volts but 230 volts is the standard voltage for European/Asia etc. systems as defined by the IEC.
SWER for use in Japan
I saw your website http://www.ruralpower.org/ on single wire earth return (SWER) system and I'm interested in the system.
Now, we are considering installation of the system in the rural areas, but, unfortunately, there is no information in our country because we don't have such system.
I would like to know several matters as below, and I would appreciate if you could give us some information.
Specification of each type of material used for SWER systems
(transformer, conductor, insulator, etc.)
General manufacture of SWER transformers
General installation cost of SWER system,
especially cost comparison to single-phase 2-wire and/or general 3-phase
With regards to your questions above;
1) Specification of each material used for SWER system (transformer, conductor, insulator, etc.)
In general the SWER material used is the same as normal distribution material, given that the limitation for SWER is 10-20 amps only small conductors are used and generally aluminium or galvanised (zinc covered) steel or small ACSR to cope with the large spans.
Normal Insulators are used generally 24 kV voltage class for 12.7 kV SWER and 33 kV voltage class for 19.1 kV SWER.
Distribution transformers and isolating transformers are different from normal transformers, you can download the distribution transformer specification from the "Standards Australia" website the standard number is AS 2558-1982 "Transformers for use on single wire earth return distribution systems"
2) General manufacture of SWER transformers
Please see the Standards Australia web site noted above for a specification for SWER isolating and distribution transformers.
3) General installation cost of SWER system, especially cost comparison to single-phase 2-wire and/or general 3-phase
Please see the paper in the downloads section, however please note that for SWER the line current is generally a maximum of 10-20 Amps which means that a SWER feeder working at 19.1 kV line to ground can supply about 382 kVA. If each household uses 1 kVA this means that the SWER feeder can only supply 382 houses.
You can see from the above calculation that SWER is best suited to
a) rural electrification where the load is low
b) large single point, single phase loads e.g. water pumping for irrigated agriculture
c) small villages located some distance from the existing grid supply
The reason why the earth return current is generally limited to 10-20 Amps is that larger earth return currents may interfere with telephone circuits.
SWER for use in Norway
My name is Vegard and I am studying electrical engineering at University of Cape Town. I am doing a project on Single Wire Earth Return (SWER) and how to introduce this technology for use in Norway. As far as I know we do not use this technology in Norway, but I think it would be a perfect country for SWER. There are many remote rural areas in Norway, with low load density. I think it can be compared with New Zealand, and I know they use SWER there.
I searched the internet for information and I found your web page (ruralpower.org). Now I was wondering if you might have some more information about SWER or suggestion on where I can find info (web pages, books, etc) on the topic? I am especially interested in challenges on how to implement this technology in Norway, both on the technology side and political/bureaucracy/juridical side. I hope you can help me with the technology challenges!
Should be an interesting project!.
Please find attached a paper (see downloads section) on SWER. The bibliography has further references that you can examine for more information. If you are studying at the University of Cape Town you should see if anyone can introduce you to the people at ESKOM distribution in Johannesburg as there are extensive South African ESKOM standards and drawings for SWER.
The particular things that you have to focus on in a developed country include a) ensuring adequate earthing b) maximum allowable earth return current, this is limited to 8 amps in New Zealand to avoid voltage rise across poor earths and to avoid interference with copper telephone circuits c) earth fault protection implications d) sometimes there are also issues with aviation regulations as SWER lines are hard for helicopter pilots and glider pilots to see and to minimise the cost of the line they are sometimes strung from "hill top to hill top".
You should also refer to the SWER drawings etc. that can be downloaded from
SWER Systems in India
I am an Electrical Engineer working with Jammu and Kashmir (India) State Power Development Department. Power Development Department is the sole agency responsible for Transmission and Distribution of Electricity in Jammu and Kashmir State. Our Sub Transmission Voltage is 33kV and 11kV and distribution voltage is 380V/220V. Recently we executed a High Voltage Distribution System Pilot Project in one of our residential colonies near Srinagar - the capital city of J&K State. We simply tapped two conductors of an existing three phase 11kV line passing near the colony and supplied 10 no single phase 11kV/230-0-230 V Transformers with one phase and remaining 10 nos with other phase. The neutral terminal of the transformers was grounded through 25X5 mm MS strip. The idea is to give quality power to the consumers and prevent theft of electricity by illegally connecting service lines to the bare LT conductors.
The 33/11kV Sub Station wherefrom the 11kV line is emanating is 10kM away. We did not use any isolating transformer.
Is this the correct way of installing SWER?.
What tends to happen with what you are doing is that the current will flow back through the ground and up through the earth connection on the supplying transformer in the zone substation. So there must be a good earth/ground to ensure a high quality electricity supply and also for safety reasons. The other thing that will happen is that the earth fault protection at the substation will trip when the earth return current reaches the setting on the earth fault relay.
The voltage 11 kV phase to ground will be 6.35 kV so the secondary voltage will be below normal as I presume your single phase transformers are normally connected 11 kV phase to phase. However if you connect across the 230-230 you will get 460/sqrt(3) volts.
Ideally you should use an isolating transformer to stop the earth fault relay at the substation from tripping, this is an extra cost.
SWER has been used in India, and there was a conference on it once, I forget which institute supported this you should search Google with "India and SWER"
The REC in Delhi also has some information on SWER that you may be able to download.
The main reason that SWER is not so prevalent in India is that you generally have a high population density which is more suitable for conventional electrification (I understand that Kashmir is not so crowded however !)
SWER for Hydro Mini Grids
My name is Kazuhiro. I am a Japanese volunteer working at Department of Energy in Fiji as a hydro Engineer. I am thinking to implement several micro hydro projects for rural village using Japanese funding.
We have many potential micro hydro sites in Fiji.
I am very interested in your product.
I would like to ask you couple of question about your product.
Could you kindly reply my questions as below?
1. Is your system able to transfer the electricity as far as 6km distance? Because it is normally said that as micro hydro project 1.5km is maximum distance to be able to transfer power at low voltage?.
2. How much is the per km cost?
3. If we transfer 20kW from the power house site to the adjacent village the distance between them will be 6km, how much will the total cost be?
1) Generally SWER is limited to about 150 to 200 kVA at 12.7 kV line to earth as the earth return current has to be below 10 amps otherwise their can be interference with telephone lines.
2) SWER is used in Australia for lines that go up to 100 km so 6 km is no problem
2) Please note that SWER is single phase and some micro hydro generators use 3 phase synchronous generators or converted 3 phase induction motors (except for very small schemes) So to use SWER in this situation you would have to have a three phase to single phase converter or use a single phase generator.
4) This problem has been reviewed in detail for micro hydro projects in Nepal please see the book entitled "New Designs for Rural Electrification, Private-Sector Experiences in Nepal" by Allen Inversin
5)I attach a recent paper I gave on SWER which has costs in it (please see the downloads section for this paper)
Upgrading SWER lines
I was referred to your SWER Web site from an Eng-Tips responder to my question as follows
"In New Zealand we have quite a lot of SWER (single-wire earth return) line in various remote areas. In fact I think a NZ power board engineer developed the concept in the 1930s. As far as I know it's all 11 kV line-to-ground. Under the old regulations such a system was limited to 8 A (telecom interference considerations) - not sure limit at present (will check) but probably not too different.
Because the line is near full capacity with more load likely, I have been asked to investigate replacement of the 20 km SWER line with a 3-phase 11 kV line (11 kV line-to-line is the standard distribution voltage in NZ). I need to check the route in detail, but I do know it's real difficult (which I guess was a reason for 1-wire in the first place 50 years ago!). Some spans are close to 1 km long and much is routed adjacent to and across a river gorge with steep, bush covered faces. Changing it to 3-wire will be a nightmare as there are limitations on line placement (the dreaded Resource Management Act here is not exactly condusive to development - getting contiguous landowner, river crossing and tree-cutting permissions may be impossible).
Anyway, I have another idea - what if we retained the line, but re-insulated and ran at say 22 kV SWER? Some power companies in NZ have recently changed their 3-wire systems from 11 kV to 22 kV, so I believe 22 kV is becoming more of a ‘standard' voltage here. So to my questions: Does anyone in the group have experience in near-22 kV or higher SWER lines? Do they exist at all? What problems do you foresee in the use of such a high voltage? Feasible or not?
Considerations may include 22/0.23 kV non-standard transformers, non-standard isolating transformer, additional step/touch potentials, higher insulation requirement than 22 kV 3-wire system (12.7 kV to ground).
Any and all comments most appreciated."
Do you have any comments? I would really value your input if you can spare the time.
You can indeed uprate to 22 kV (which is IEC voltage class 24 kV) or 12.7 kV phase to earth for a SWER system. You will have to
1) Upgrade your isolating transformer so it becomes a combined step up transformer and isolating transformer i.e. 12.7 kV line to ground on the secondary side.
2) Upgrade the supply transformer(s) on the line so they become 12.7 kV line to ground on the primary side and 230/0/230 on the secondary side
3) Upgrade the insulators on the line and check the clearance requirements, generally the critical clearance will be the mid span clearance to ground at about 40 to 50 degrees C with no wind.
4) You may have to use the opportunity to replace the conductor if it is not in good condition, as many of the earlier lines were built with No 8 wire. Look at the series of conductors that have been specifically designed for SWER 3/2.75 GZ etc. You should also review pole and cross arm strength and condition.
In Australia they also use 33 kV or 19.1 kV line to earth. Give us a ring/email if you need more info.
SWER Transformer Sizing
How do I work out the size of the SWER distribution transformer?
To calculate the single phase transformer size you must multiply the amount of customers within about a 600 to 1000 metre radius of the transformer by 600 to 1000 VA for newly electrified customers in rural areas e.g. if there are 60 households in a small village that has never had electricity before, then you would multiply 1000 VA x 60 houses = 60,000 VA or 60 kVA. The standard transformer sizes are approximately 50 kVA, 63 kVA and 100 kVA therefore you would probably install a 63 kVA transformer. In New Zealand the average household has an ADMD (after diversity maximum demand) of about 3000 VA, however a newly electrified village would have a lower demand as each household would probably only have lighting, TV etc for the first 5-10 years after this and as the economy improves they would buy freezers, air conditioners, washing machines etc and the load would increase. When the load increased you would install either a larger transformer or another transformer.
The demand per household is an important parameter, you can either estimate this from the connected load e.g. 3 lights and 1 TV or measure the demand with a clip on ammeter or data logger at the last similar village you electrified
SWER in Western Australia
I was very interested to see your construction methods for your SWER installations. I am interested in where you are actually located. We use quite a lot of SWER also. What type of earthing do you use for the H.V. return, and what value of resistance do you allow?
On my site the SWER constructions refer to Lao and other countries where we used SWER. The constructions are generally modelled on the New Zealand NT/NSW/SA/Queensland constructions. I am told that the WA SWER runs a suspended earth/neutral which means it has some resemblance to the American suspended neutral/earth system (from the papers I have read anyway not from having a look). My interpretation of this is that in WA the earthing is extra difficult so you have to use multiple earths along the suspended earth/neutral. If you refer to the The Electricity Authority of New South Wales authored " High Voltage Earth Return for Rural Areas" it states the following (published in 1978 out dated now and you should refer to the latest Australian standards, regulations and legislation)
Maximum Earth Resistance under worst conditions
H.V. Isolating Substation Earthing System
25 kVA substation - 5 ohms
50 kVA substation - 5 ohms
100 kVA substation - 10 ohms
LV Earth of Isolating Substation - 30 ohms
Whole of system for Isolating Substation be 10 ohms or less
H.V. Distribution Substation Earthing System
5 kVA substation - 30 ohms
10 kVA substation - 25 ohms
100 kVA substation - 10 ohms
LV Earth of Isolating Substation - 30 ohms
Whole of system for Distribution Substation be 10 ohms or less.
Obviously the lower the earth resistance the better, one of the principle issues that has arisen in eastern states is that the earth electrodes can burn out in dry summers when the soil dries out, one answer is to drive your earth rods deeper. A technically correct answer is that you have to limit voltage rise across your earth to a maximum amount at full load/maximum earth return current in the worst case earthing scenario, the voltage rise limit would be defined in the AS wiring regulation or other AS standards, regulations and legislation. I think there also may be further information on earthing in the AS standard on the manufacture of SWER transformers.
The download on the site refers to some PSS/Adept power system modelling that a colleague did in WA so reading that my give you further insights
SWER Installation Statistics
Can you please send me data/reports on installations that you have successfully completed and which have proved sustainable? We want to develop a rural electrification proposal for Africa.
Looking forward to hearing from you,
SWER is a mainstream technology in developed countries, especially Australia.
What has effectively happened in New Zealand is although we are a small country the ratios such as households/sq km and demand per household are getting beyond SWER as an optimal technology for grid connected rural electrification. This is due to demand growth over time e.g. each household now has a peak demand of between 2400 VA and 3000 VA whereas in developing countries initial demand can be as low as 200 VA per household
This is not the case in Australia especially in the outback where the load densities are low and SWER is definitely main stream. SWER is used in South Africa but it is best suited as a first step in grid based rural electrification, when the demand/household is low. Once load develops intermediate poles can be put in and a second wire strung for more conventional two wire single phase distribution.
Please see the paper in the downloads section which gives statistics on the use of SWER in Australia. There is also a bibliography that you can refer to.
When should we consider using SWER
In what situations should I consider using SWER?
SWER is best suited to rural electrification where the load is over about 10 km from the existing grid with a maximum load of about 380 kVA or where the population density is low. For example:
a) a SWER line to a village or collection of villages with up to 700 households. The village(s) should be more than 10 km or so from the existing grid( assuming each household has a maximum demand of about 500 Watts) or
b) a SWER line to a village or collection of villages with 500-600 households and a couple of large single point loads e.g. water pumps powered by electric motors.
c) Where the population density is low e.g. where there are 2 households or less per kilometre of line
For all the above examples you should do some preliminary costing of SWER vs conventional 2 wire single phase to confirm the least cost solution. Take into account the capitalised cost of losses as an added refinement if you want an exact solution. The costs of SWER and 2 wire single phase distribution are included in the download section in a spreadsheet format. You can alter the spreadsheet to reflect the local cost of labour and materials, but the spreadsheet figures can be used as is to get a ballpark comparison of initial costs. The critical items that should be updated in the spreadsheet to reflect local costs are 1) conductor costs, 2) pole costs, 3) distribution transformer costs and 4) labour costs i.e. dont waste your time by updating the costs for minor items like nuts and bolts.