T-line Design

[james001]

I'm working on the pole line design with (2) 69KV and 35kV underbuilt, does anyone have exp or know any website so I can access to get some additonal info. All I have is the standard RUS guidelines which I could follow. My thinking is 2-69kV circuits back to back and 35kV circuit approx 9 below the bottom 69kV circuit. I believe the 35kV circuit can be on crossarm (to save on pole height). What are other advantages/disadvantages between crossarm vs. horizontal post insulators on 35kV ckt. The conductor will be 795ACSR. What other factors should I keep in mind when design such a pole. It is very crowded, and usually with this design, I expect that the span is very short with 200' or less.

Any help is appreciated.

jt

 

[jimbobpe]

What other factors/information are you looking for? Pole height, sag (795 vs. other conductors?), right-of-way (where the conductors will overhang?), safety of linemen, etc. are factors you should consider.

Is the lower voltage (35KV) distribution?

 

[MikeDB]

I don't know much about your situation, but I think I would put a 69kV circuit on each side of the pole with the 35kV below as you indicated. 69kV post insulators could be used or davit arms if you want greater seperation for maintenance. The 35kV on a crossarm might provide more climbing space and shorter pole, so I would lean that way. The spacing from the bottom 69kV phase to the 35kV would be dictated by diferential ice, diferential temperature, or galloping conductor clearances.

Determining the most economical span length is always a balancing act. Increasing line tension may reduce phase spacing and pole height/class, but add to the cost of line angles. The 200' sounds short, but for wood poles you are probably real close.

RUS guidelines and standards may be conservative, but I have not much better. I think it is a good resource. BPA used to have a great transmission line manual, but I don't know if it is still available.

 

[james001]

Thank you MikeDB for your input. Also is the rule of thumb when you try to sag the conductors, you want the sag to be about 1%-1.5% of the avg span at 60F with no wind. It means that I can stretch my 795ACSR to 5000# max under NESC heavy load with Grade B construction. When I run my short calc, I found that 200' span with (2) 795ACSR (5000#/conductor) and (1) 397ACSR (3000#/conductor) underbuilt, the pole height and class is 70-H1. Is the H1 pole class very easy to obtain?? How about H2 pole class, I heard it is extremely hard to buy, is that true?

Thx

jt

 

[MikeDB]

I have not used a 1% rule on conductors -- that sounds like a fiber optic cable guideline. I try to stay below the limits indicated in the NESC and also try to keep tension low enough to avoid the use of vibration dampers and to facilitate guying.

If you are using SYP, I don't think 70-H1 or H2 will be a problem. The taller and larger class poles are harder to come by, more so with DF or WRC. I remember trying buy some 115-H4 WRC in the early 80s. They said they could do it, but never did find them in the forest, so we bought steel poles.