SWER (Single-Wire Earth Return)

[Adamito]

Hi,

I've just learned how a SWER system works, but there's still a few things I don't understand. How can you get a decent efficiency with all your current going trough earth?
The ground must be a huge resistance for the current that has to go through many kilometers through ground to come back. It just doesn't make sense to me.

Thanks for your help,

Adam

 

[davidbeach]

Small loads, long runs, savings in conductor costs make up for other problems.

 

[ausgen]

Yes, efficiency loss is a problem although running at HV helps. As davidbeach noted, on a network level, these losses aren't enough to justify the additional 1000's (10s of 1000's in my part of the world) of kms of conductor. Ultimately, the customer bears most of the technical drawback through a supply that fluctuates with changing earth mass conditions (very cold, very dry, etc). Once again though, my choices are to put up with it or spend the capital (on site generation) myself.

I look after around 20 SWER sites and aside from a couple of intermittent issues, my experience is that the system works as well as can be expected.

 

[roydm]

Ausgen,
I remember seeing one of these in the backblocks of New Zealand where it might have been 10 miles between customers. Can you give us a loss comparison between earth return and conventional 2 wire system in your experience.
What proportion of the voltage drop is in the wire, compared to earth return? How would you allow for voltage sag with load to give the customer a steady supply voltage?
Thanks
Roy

 

[mgtrp]

With reference to use of SWER in NZ, where there is quite a lot used in rural NZ in the central-lower North Island thanks to one Lloyd Mandeno (usually supported by old rail way irons), code requires that the earth connections have a resistance of no more than 5 Ohms and current is limited to 8A. Therefore volt drop through the earth return path will be no more than 80V and as voltage is usually 11 kV phase to ground, this is fairly small in context.

The limitation with regards to current is more to do with concerns over safety and electromagnetic interference when running current through earth than volt-drop or efficiency. NZ's HVDC link will run up to 2000A via earth return when in monopole operation.

http://www.energysafety.govt.nz/upload/33453/nzecp41_1993.pdf

 

[ausgen]

I'm an end user so aside from the varying volts it appears like any other two wire system. I would have suspected that it would be less than 10 Ohms though.

2000 A on a single return would be interesting - these must be the SWER isolation poles I see with the fencing around the base to stop animals (and people!) being exposed to a step potential problems at the base of the pole.

 

[itsmoked]

Thanks for the info mgtrp.

And welcome to Eng-Tips!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

The 2000A was something very different than SWER. It is an HVDC (High Voltage DC) link that can, as can many of them, work with one of two conductors out of service. It isn't a desired mode of operation, but it is better than not being able to operate at all on the loss of part of a converter station or one conductor.

 

[bacon4life]

It doesn't take that many feet of small wire to get up to 5 ohms.

On HVDC, in some locations it is actually more efficient to use the ground as a return instead of a conductor. As David mentioned there is more than just efficency at stake though.

 

[SteveFehr]

The conductivity of earth may be low, but the cross-section is HUGE, so overall resistance is small- practically 0, with all the resistance centered around the ground rod and very little difference if it's 100km or 1000km. You really couldn't ask for a better conductor for long distances.