Questions for transmission line design 2

[blueshadow007]

I am a civil structural engineer and interested in switching to unility engineer. Have read ASCE 10 -97 for transmission structures. am reading 1724E-200(transmission line design manual). The former is purely about structure analysis. The latter in addition to structure also includes non traditionally structure staff, like vertical/horizontal clearance, etc. all used for structure spotting. In a real work setting who will does this structure spotting, civil engineer or electrical engineer?
Is it a must for a structure engineer to command this knowledge?

Thanks

 

[transmissiontowers]

It depends on the company and the tools available. PLS-CADD is the gold standard program for line design since they have a huge market share. If it is a new line in a new Right-of-Way, there is an Optimum spotting feature in PLS-CADD that will spot structures. If the ROW is old and you are adding a new line, you pretty much are stuck with the current structure locations because the swing of the conductors needs to have fixed points that are adjacent so you don't swing into the adjacent line.

In my company, the T-Line engineers are all EE's but I support them with structural things that they can't handle like difficult tower member replacement.

Take a look at the big consultant firms like Burns & McDonnell, Black & Veatch, Power Engineers, plus a couple of others I have forgotten if you want to work in the utility industry but not at one utility. We are an aging workforce and we need some young blood to take the place of the retiring engineers.

I will complete my 38th year next month.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[MikeDB]

As transmissiontowers said, it varies a lot from one company to the next. I have worked for a utility that used all civil engineers in the transmission line group and each designed all aspects of a project. I have worked for a utility were I was the only civil engineer in the electric group, but I still handled complete projects, not just the structural aspects. I worked at a small consulting firm where I was the transmission department and I did everything from survey to final inspection.

I would have to say that there are more opportunities and more job openings for a civil engineer that can handle all aspects of the transmission line project rather than just the structural aspects. The broader your experience the better versed you are in the entire project.

There are some firms, usually large consultants or large investor owned utilities, that have a position for a structural engineer who deals only with structure and foundation design for transmission and substation structures.

If you will search the topic on this site, I think there are other threads along this line. Best of luck in your endeavors.

Congrats (or condolences) to transmissiontower for 38 years. I'm working on 34, but have been part-time for the last 5.

 

[blueshadow007]

Thank both of you for sharing your experience very much.
One of my friend told me that in his company they only do the spotting and don't do the strucctural
modeling and analysis of tower. The tower design are done by fabricators. For the job itself, it's all about familiarity of PLs-CADD. Sounds like as long as you have good understanding of basic concept, everybody can do the job. What then makes a structural engineer stand out?

I have a difficulty to understand the dashed line for uplift in the attachment.What does this dsahed line means?

Thanks

 

[transmissiontowers]

The structural aspect comes in handy when you are evaluating an existing line by adding a second circuit or a second wire to a single conductor to increase the electrical capacity. If the lattice tower needs to have members replaced or additional bracing done to increase the compression capacity, the structural engineer is equipped to know what changes to make while the EE has no clue on what to do.

AFA your picture, the dashed line is the conductor position when it gets cold or has ice on it. With aluminum wire, as the amount of power transmitted goes up, the wire gets hot because of the resistance. We rate our ACSS lines to run continuously at 250°C with an emergency rating of 300°C. As the wire heats up, it gets longer and the sag from the attachment point increases. There are cases where a long span can have a sag increase of 20 feet from the normal 60°F position. If there is no power on the line and it gets cold, it shrinks (gets shorter) and the position can be the dashed line in your picture. On a hot day the structure at the low point holds the wire up. On a very cold day, the structure holds the wire down.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[stevenal]

Apparently the dashed line is the conductor position when cold and not being held down by the structure (4 in this case) being checked for uplift.

 

[MikeDB]

The dashed line represents the extension of the parabolic (or catenary) curve to determine the low point of the conductor sag curve. The weight span or vertical span is determined by the distance between the low points on the curves. Any time the vertical span is negative, it means you have uplift or a vertical force in the opposite direction of gravity. Uplift can happen at any temperature, but the uplift force increases as the temperature decreases.

For level spans the low point is always between the supports, but as the incline increases the low point moves to the structure on the down hill side. If the incline is steep enough the low point can be beyond the structure.

Prior to the advent of computerized sag curves, the designer would use a sag template to mark the low points of the sag curve and measure between them to determine the vertical span. When multiplied by the weight of the conductor would provide the weight of conductor that the structure must support. This is important to determine insulator swing.

One should be required to understand the principals of line design before attempting computerized design. That is what makes an engineer stand out from the technicians and draftsman. One understands why something needs to be done, the other just does what he is told. The devil is in the details.

 

[blueshadow007]

"The dashed line represents the extension of the parabolic (or catenary) curve to determine the low point of the conductor sag curve. The weight span or vertical span is determined by the distance between the low points on the curves. Any time the vertical span is negative, it means you have uplift or a vertical force in the opposite direction of gravity. Uplift can happen at any temperature, but the uplift force increases as the temperature decreases."

Agree this in general.

The importance of finding vertical span is: 1.to get the downward or upward forces on the structure and foundation so that a structural analysis/design can proceed. 2. to check insulator swing to maitain proper clearance.

In order to get worst loading scenarion for structure 2, it is assumed that for cold curve conditon structure 3 is detached from conductors, which changes the so called "adjacent spans" in common sense.

Same idea shall be applied to each structure. But for structure 3, how come the tributary vertical span shown totally shifted away from structure 3 ?
seems it uses the distance between sag points of cold and hot curves. doesn't make sense?

"For level spans the low point is always between the supports, but as the incline increases the low point moves to the structure on the down hill side. If the incline is steep enough the low point can be beyond the structure."

Is "Low point beyond the (next adjacent) structure" possibly happenning or just a assumption for conservatives?

For structure 4, the uplift(vertical span) shown seems to be distance between sag low points of hot ahead conductor and hot(?)back conductor. Why not cold curves?

 

[bacon4life]

Looking at just structure 3, it is clear that the span on the left is pulling the structure up and the span on the right is pulling it down. The vertical span is the sum of these loads.

When looking at structure 2, the vertical loads at 2 will be the same whether the wire is being pulled down by the left side of structure 3, or if it was connected to the entire theoretical dashed curve.

All of the curves shown in figure 10-6 are cold curves.