Undergrounding of Power Lines
Undergrounding of Power Lines
(OP)
On one of my roadway design project we are being requested to incorporate future provisions for undergrounding 94.5 KV transmission lines. What does that mean? How do we size the trench and size the conduits for this almost 3 miles of length of roadway. Where can we find standards for such design. What about the Vaults? How many and what type.
Since I am a Civil Engineer by profession any help in this regard will be appreciated. Thanks
Since I am a Civil Engineer by profession any help in this regard will be appreciated. Thanks






RE: Undergrounding of Power Lines
RE: Undergrounding of Power Lines
puts transmission lines underground so the cable will fail and take half of the country out.
"On one of my roadway design project we are being requested to incorporate future provisions for undergrounding 94.5 KV transmission lines. What does that mean?"
It sound like your roadway is displacing a transmission line and the "they" want it underground.
"How do we size the trench and size the conduits for this almost 3 miles of length of roadway. Where can we find standards for such design."
I would check with the utility and see what they use. The utility would have a standard for their installations.
" What about the Vaults? How many and what type."
I don't know how long a piece of 94.5 kv cable can put on a real but it not very long. An estimate would be a pull box/splice box every 400 ft. How many conduits and what size depends on the cable diameter. Maybe 5" and 6" C and 9 conduits. Check utility requirements.
I have some info on 69 kv 500 kcm is 2.3" diameter.
Area of 5"C = 19.62 inches Area of 3 #500 = 12.45 inches.
5" is not large enough. Better make it 6 inches.
RE: Undergrounding of Power Lines
RE: Undergrounding of Power Lines
RE: Undergrounding of Power Lines
RE: Undergrounding of Power Lines
http:/
Wareagle, thanks for your feedback it was the most relevant.
RE: Undergrounding of Power Lines
http:/
I tried this site but got an error message. It maybe be a temp problem.
You might do a search on precast manhole. I know they are available.
RE: Undergrounding of Power Lines
Check this site for some man hole dwgs and specs. You will need a big one.
htt
RE: Undergrounding of Power Lines
If you put a single conductor in each conduit, will you not get severe inductive heating effects, or is that somehow not so with transmission lines?
WEH
RE: Undergrounding of Power Lines
RE: Undergrounding of Power Lines
Another possible solution to your problem is to lay the conduits above ground on concrete supports
similar to the concrete bumpers in parking lots. The supports would need to be wide enough for the number of conduits required. You would still need to install pull/splice boxes for the cable and for taps within the complex. If the conduit is galvanized steel you may have to
account for solar heating when sizing the cable. I don't think PVC conduits like sunlight.
Maybe someone else can address this.
RE: Undergrounding of Power Lines
Both are in the San Francisco peninusla. Technologically they are not difficlut. They are very expensive. Millions of Dollars per mile.
RE: Undergrounding of Power Lines
I was able to get into the document you provided at the link you provided and General Order 128 does give you the basics, but I agree with some of the others that you really need to consult with someone whom specializes in this type of installation. The depth of the conduit bank, the relative arrangement of the ducts, the material in which they will be buried, (concrete encased-sand backfill, earth), and the resistivity of the soil, will impact the heating, current carrying capacity, etc..., of the future installation.
These installations at this voltage level will assuredly be at one conductor per duct. But you need to consider many other factors also: wether you need to include provisions for future circuits, the sizing and spacing of manholes, the types of connections to be used, the grounding requirements in the manholes, wether you need to tee from a manhole for installing pad mounted circuit interrupter switches, (are there any on the overhead line now?), etc....
Even if the system is not going to be installed at this time, these factors will definitely impact the design of the conduit pathway system.
RE: Undergrounding of Power Lines
RE: Undergrounding of Power Lines
So provide for AT LEAST four pipes if three are required. I think that I'd put in six! The incremental cost of an extra conduit vs the overall digging costs approaches zero when you round off.
RE: Undergrounding of Power Lines
First of all the voltage is "strange" as I have not met such level unless 94.5kV is 161kV system level (line-to-line). At this voltage level cables are usually single conductor type.
If it is, then you have to know how many circuits will be installed in parallel. In N.A. there is a practice to use so called duct banks as a mean of installation. As a rule of thumb you can assume that you need min. 4 conduits per circuit (for different purposes than collapsing). If there are 2 circuits running together you would need a duct bank 3high x 3 wide (9 conduits total). Each conduit would be 6” diameter if the cable conductor size is not bigger that 2000kcmil. The whole duct bank would be approx. 3’ x 3’ size. Please remember that these lines are meant to carry quite large load, their conductor size could be significant and they generate a lot of heat. I would suggest installing a fiber optic sensor to monitor cable temperature. A duct bank should be cover with a backfill which must be tested BEFORE design stage of a project.
As there are limitations on the cable pulling force and size of a cable reel you should include manholes to splice sections of a circuit. Every pulling length depends on conductor size and route design. For 3 mile circuit there will be approximate 7 – 8 manholes for jointing cable sections. Each manhole size for this purpose could be quite large – approx. 7’x7’x25’.
These are just a few very basic pieces of information. As I mentioned at the beginning it is always custom design project and you should get a help from an electrical engineer specialized (!) in u/g transmission (!) lines.
RE: Undergrounding of Power Lines
Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants