Overhead SWER Lines

[sabrepoint]

Hello

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 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 conducive 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.

Thanks!
Graeme

 

[cuky2000]

Check with the World Bank Rural Electrification.

There are several projects in Latin America, New Zeeland and other part of the world using SWER for 11, 22 and possible 33 kV.

Good luck.

 

[cuky2000]

Enclose is the link with the point of contact in the Rural Power Department at the World Bank.

http://www.ruralpower.org/swer_070_contact_us.htm

 

[sabrepoint]

Thanks cuky2000.
I'll follow this up.

Incidentially the 2nd-to-last line should read "... requirement with 22 kV 3-wire system (12.7 kV to ground)."

 

[RalphChristie]

First of all, you can contact power authorities in other countries which are also using SWER technology. Countries include Australia, Brazil, Canada, India and South Africa. Australia is by far the greatest user of SWER. One of the advantages of SWER is longer span lenghts (less structures and materials) but it is also one disadvantage, because you can't convert to three phase without major changes.

In Australia SWER voltages are 12.7kV and 19.1kV phase to ground (three phase 22kV and 33kV) Higher voltage SWER-lines is thus an option.

Some comments on higher SWER-voltages:
The higher the voltage, the more costly are the main substations. A balance between line costs and substation costs must be considered if an overall economy is to be achieved. Voltage regulation will improve with higher voltage, but this alone is usually not sufficient to justify higher voltages, unless long lines are involved. The Australians claim that it is sometimes more beneficial to use a larger size conductor and a lower voltage to satisfy other requirements, such a telephone coordination. Also, according to them, the higher the the voltage, the higher the charging current will be, which may cause overloading of the primary system, increase line and isolating transformer losses and require an unnecessarily large isolating transformer to supply it. Due to the higher charging currents, higher voltage systems causes more telephone interference than lower voltage systems. (electromagnetic induction)
However, if the load current is much greater than the charging current, the higher voltage can be more beneficial.

Hope it helps

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[sabrepoint]

My thanks to both of you. I have followed some of the suggestions up.
Cheers
Graeme

 

[Jonturner]

Graeme
Certainly feasable. In Ergon Energy, Queensland we have around 4,000km 11kV SWER, 16,000km 12.7kV and 44,000km of 19.1kV SWER. Some 180deg duplex operating. All this is isolated SWER with 19.1kV the perfered choice for the reasons you indicate. Australia has around 191,000km SWER I believe. Our largest schemes around 400km with 2x200kVA isolating transformers, SWER voltage regulators, shunt reactors, 25kVA customer subs (some 3x25kVA capacity). Issues with unbalance on 3ph network but otherwise working ok.
Jon Turner

 

[sabrepoint]

Thanks Jon

After the posting I was surprised to discover so much 19.1 kV SWER in Queensland (and NT too I think). Then I found some in the bottom of NZ as well! So I spent some time talking to Otago Power who were very helpful. Their system is actually 22 kV; some sourced from 11 kV but most from 33 kV zone subs.

I've got enough info for now I think, as I'm doing an economic study at present. Further down the track we may need to do a technical analysis of options, but this could be some years away I gather.

Thanks again
Graeme

 

[keewee]

Graeme we had a SWER on Banks Peninsula which I think was the last one of its kind in the Canterbury region. It was replaced some years ago. I don't know if its a joke but I heard that Telecom subsidised the replacement.