110 kV poled line tips 2

[sabrepoint]

Hi All
I've some experience in 11 and 33 kV feeder design but not much on 110 kV, so I would appreciate any advice on a number of issues. I'm to assist in the survey and design of a single-cct 110 kV line to be erected some 70 km across a mix of steep hill country (mainly clear), winding road and flat land. The intention is to use polymer post insulators on single poles in the main. Line connects from a small power station to a substation near a city. Some sectors are to be conjoint with 11 kV under. Design to be to AS/NZS 4676:2000 (Structural design requirements for utility service poles) which I understand to be somewhat similar to an IEC standard (can't locate number - may be IEC 826?). Some of the points I'd appreciate comments on any of ...
1) Acceptable polymer post insulator types/brands.
2) Some conductors (around 350 to 400 A) have been mentioned ... Wolf 37/2.59 ACSR, Chafer AAC, Pearl AAAC. I'm yet to look at sags/tensions but can run these through a model. Any technical advantages of one type over another? What is most commonly used? Economic considerations?
3) Corona loss likely to be a problem?
4) We will look to erect at somewhere around 20 % UTS max to avoid vibration issues on longer spans.
5) Overhead earth wire for 1 km each end.
6) AAAC creep. I have no info on this ... any reference available?
7) Conductor separation formula.
8) Prestressed concrete or pine poles?
9) Snow is likely. I can deal with ice during the loading calc process, but how does snow differ? Can I treat snow in a similar way to radial ice? (sorry, I live in a warm climate and have rarely seen snow!)
I've a few ideas already but there are many engineers with far greater experience than I, so would appreciate any help!
Thanks, Graeme

 

[jghrist]

Some thoughts on your numbered points:
2) ACSR is most common because of strength. AAC not commonly used for transmission. The stronger the conductor, the longer the spans possible. This makes ACSR the economic choice usually, but there may be local circumstances to make your economics different. Wolf seems marginal for size. An evaluation of losses may favor a larger conductor.
3) Corona is not normally a concern at 110 kV as long as the conductor diameter is at least that of the conductors you are considering.
4) 20% UTS max is conservative. You could consider higher tensions and possibly use vibration dampers. Damper manufacturers should be able to give you some guidance.
5) If you are in a lightning prone area, full length shielding (with good grounding) will provide much improved reliability.
6) Your sag-tension software, e.g. SAG10 by Alcoa, should include creep information on AAAC.
7) If there are no AS/NZS standards that cover spacing, you can get guidance from the American RUS Design Manual for High Voltage Transmission Lines available at

http://www.usda.gov/rus/electric/bulletins.htm

8) Either is acceptable. Choice is largely economic and highly dependent on local availability. Concrete poles get very expensive if you have to ship them far.
9) Don't worry about snow. It's light compared to ice and will blow off before it produces any extra wind loading.

 

[sabrepoint]

Thanks jghrist.
2) I'm not sure why Wolf etc is suggested ... I think came out of study some years ago so I guess I stick with something around that size or AAAC equiv (yes, I don't think AAC is appropriate cross-country, but will do calcs to check).
4) Sorry, 20 % at 10 deg C no wind ... may equate to some 33 % wind loaded (Pearl AAAC). Will look at dampers too - thanks.
5) Well I haven't seen anything on lightning so will check further. There are other lines in vicinity so there may be some outage info available.
7) Lots of good info here! For many years national legislation covered line design in NZ ... was easy to apply but unduly conservative. Then Govt partially allowed utilities to do what they liked - "light-handed legislation" (as long as was safe!). Now the AS/NZS is issued which provides guidance, which is much better than nothing in my opinion. It's a pity we've not had ready access to such material as these USDA bulletins in the past rather than each utility doing its own thing.
9) Apparently the snow falls are light and have never caused problems with other lines, but the brief I have is to allow for it above 200 m. I'll probably treat it like ice for loading purposes.
Thanks again. Most helpful!

 

[jbartos]

Suggestion: Visit

http://standards.ieee.org/catalog/olis/td.html

for a list of IEEE Stds pertaining to transmission and distribution

 

[gordonl]

I'm not a transmission designer by any stretch, but as a utility customer I cringe at the thought of an unshielded line. Even an area with moderate activity would be severly affected. There are other options worth considering besides shielding, but from my point of view this is the most important factor in line design.

 

[sabrepoint]

Thank you all. gordonl, I don't yet know reliability required for this line ... it's to act mainly to take the rural load off the existing city supplies so maybe customer has lesser-reliability criteria. Shielding for 1 km out from subs seems pretty common here for 110 kV lines and I gather provides acceptable protection for lesser-import lines. In this particular job, while it might be ideal from a protection perspective, adding a wire over would really add to construction difficulties so frankly I hope we can get away without one! Thanks.

 

[jbartos]

Suggestion: Visit

http://www.optimalsoftware.com/polecad.htm

http://tdworld.com/ar/power_transmission_line_conductor/

http://www.epri.com/destinations/dynamic/dilbert.asp?product_id=2979

etc. for more info

 

[ItAintMe]

Couple of comments. You've got some pretty good comments up there already.

We use a shield wire on all our 132kV lines for the added reliability. There was considerable debate over the added cost when we first started doing these voltages in about the 1960s, but it proved itself - lightning outage rates are lower than calculated (a relatively high tree canopy helps shield the lines anyway). Don't forget - if you don't have a shield wire, you may have to increase post insulator separations anyway, so poles may not be much shorter. We often just use a steel angle iron raiser to hold the wire. In fact, I have a feeling that corona/interference compliance may have ended up as the determining influence on minimum separation distance.

Smaller conductors than around 20mm may cause corona loss, but will most likely cause interference with TV or radio reception, especially in marginal areas. We ended up using Uranus (AAC) for suburban areas as about the smallest we could use at 132kV and comply with AS/NZS interference levels.

Concrete poles may give rise to some problems with step and touch potential hazards at ground level - especially if you don't have a shield wire. At the 11kV too - you might find you need a stand off insulator to maintain insulation co-ordination! We found we needed 11kV insulators to hold LV ABC conductors attached to a 132kV concrete pole line for this reason.

Insulator makers are generally OK so far as I know (US/European/Japanese suppliers). Haven't tried any Chinese or Korean stuff yet. Our preference is for silicon rubber rather than EPDM or others. The south africans (SEKOM) did field trials and found that you had to be pretty careful to get decent performance out of cyclo-aliphatic resins, but americans seem to love them. maybe it depends on your weather/climate?

The main advantage of AAAC over ACSR is lower weight for the same strength. If you are running steep hilly areas, that might be a consideration, although I have no personal experience in mountainous conditions. A good use of greased conductors should help with corrosion. We've had a couple of problems with joints slippiing on ACSR, but and it was a construction problem and only one contract. Of course, AAAC is likely to blow out more than the heavier ACSR, so if easement widths are an issue, that could be worth considering.

Don't worry - 110kV isn't really all that more sophisticated than 33kV design!

 

[sabrepoint]

Thanks for everything, all.

ItAintMe (Australia?) you've raised a couple of points ...
* You mention possible increase of insulator separations if no shield wire. Reason please?
* Maybe close to residential areas we will investigate different (larger) conductor to limit corona.
* Step/Touch potentials ... I will do some research on this, but can you briefly comment more on benefits of shield wire to reduce these?

Thanks again for your time.
Graeme