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.
SWER in Alaska
We are looking at SWER to use to provide power to Alaska's widely dispersed rural villages. Unfortunately, the National Electric Safety Code does not allow single wire earth return for AC or DC over extended periods. In the next few days we can make comments to the committee that is working on Alaska's adoption of the 2008 code. We will argue for the use of SWER.
One of the questions asks is how do you find a grounded conductor fault to a high resistance ground like in the desert or snow? Have you experienced any safety problems as a result of lack of sensitivity to this type of fault? Our single phase with ground return people argue that if a conductor earths it is likely to touch a ground wire and cause a solid fault. Of course with long spans you can get a number of clashing faults that can cause conductor failure
How have you dealt with these arguments?
Is there a source of information, published, that delineates the miles of SWER in varying countries? When working with code people numbers count.
Any sources that are obtainable would be appreciated.
1) Regarding your comments on the NESC I understand the issues and confirm that low resistance remote end ground faults sometimes don't clear readily. There are several ways round this problem.
a) Turn it into a more standard configuration as per the North American system i.e. a suspended earthed neutral strung beneath the line so on the way down the conductor ends up resting one way or another on a fairly solid earth and the protection sees this more easily.
b) a variation on the above where the suspended neutral is only run in areas of either high ground resistance or more densely populated areas
c) have some sort of ground fault detecting autoreclosers to sectionalize the line
d) There is some research around that I have not got to the bottom of on new relays that look for an "arcing signature" that can be used to detect high resistance remote end ground faults. If you google this you should turn it up. The important thing here is that high resistance remote end ground faults are not just a SWER issue they also apply to conventional distribution where the lines are long, the fault levels are low and the ground resistance is high.
2) There is a huge amount of SWER in Australia, I attach a paper I gave a couple of years back at the local ESA conference that contains the statistics, it also contains further thoughts on earth fault protection.
My firm opinion is that SWER would be ideal for a place like Alaska to reduce costs i.e. long distances with low population densities. It also will improve reliability as clashing is non existent.
Rural Electrification for Remote Areas
How to design a distribution system for a remote area which is very far away from grid. i.e., the only source of power generation is renewable energy
I suggest you search "The Mini Grid Design Manual" published by ESMAP, which you can find by googling it. It is a very large document about 18 Mb so make sure you have a good internet connection when you download it
Rural Electrification for Water Pumping
I am a student in my final year at University. We are working on a senior design project that involves pumping water over 4km from the base of a mountain to the top where a community lives that was built after a hurricane destroyed the residents' homes. Part of the project involves supplying power to all the water pumps along the line. We are working with 240V over 4km. As the project is in Nicaragua working with a community with very little funds, we were thinking maybe a SWER type system would be appropriate. Do you think this is a situation where we should look into SWER further? Also, where can we find more information on the components necessary for such a system? We would like to price it out and figure out feasibility to bring down to the community in January. Thank you! Julie Class of 2007 Mechanical Engineering
Sounds like a good project! At 4 kilometres you are really getting to the limits of a feasible 110 volt or 230 volt supply so you may have to consider using a medium voltage system i.e. stepping the voltage up and back down again at the pump motors using transformers. You may be able to get away with 230 volts over 4 km if your loads/motor sizes are very small e.g. 1/4 horse power. Get someone in the electrical department to do some voltage drop calculations for you after you have decided the motor sizes
The real issue is the size of the pump motors and the motor starting current. In the States there is a lot of single phase supplies to rural areas and so there are specialist single phase pumps that go up to about 10-15 hp. Beyond 10-15 hp the electric motors are more or less all three phase. If all the pump motors are small then motor starting / availability of single phase motors is not an issue so the project could be done with single phase (with stepped up/ step down voltage) , and the availability of electric motors/pumps locally in Nicaragua would not be an issue. If the pump motor sizes are beyond about 10-15 horsepower you will have to go to three phase and carefully investigate the availability of larger electric motors locally
My understanding is that Nicaragua basically follows North American practice for single phase medium voltage distribution i.e. an overhead phase wire with an understrung neutral, there is not a huge cost differential between the use of North American single phase distribution costs and SWER costs and for a small medium voltage installation it would be much better to stick with the status quo i.e. North American phase plus understrung neutral than go to a new system such as SWER which may cause installation difficulties because local people are not familiar with it. If you go to the USDA web site you can download their drawings for rural electrification and you will see what I mean.