Undergrounding of Power Lines 5

[ajeebmjeeb]

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

 

[Dumbo2929]

Transmission undergrounding is not a "standard" thing. Each installation has its own issues that can affect cable size, type, conduit size, type backfill material, plus a whole host of other issues. Some of us make our living doing these such designs, and they can be complicated. Just putting conduits in the ground for future, can be like putting money in the ground, because if the issues are not designed, the conduit system may be not useable in the future. I recommend working with an electric power engineer.

 

[wareagle]

This has to be in the North East. That's the only place that
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.

 

[Dumbo2929]

If you mean the Northeast US, there is no 94.5kV. I believe this is an international voltage, not in the US.

 

[davidbeach]

I'd look at single conductor per conduit. Easier pull, less likely that a single conductor ground fault will escalate into a phase-phase fault, wider conductor spacing will help reduce the capacitive reactance.

 

[jghrist]

You are going to have to get the customer to be more specific in his requirements. As Dumbo2929 said, there are no standard solutions. If you in the bidding stage, you could make a minimal estimate of how much money to throw in the ground, say 3-6" (150 mm) PVC conduits. If you need to do more that plow in some money, you need more data.

 

[ajeebmjeeb]

Thanks! everyone for your feedback just to shed more light the project is in the west coast and the power company itself does not have much standards. This request by client is to address the future construction of a substation and extension of Transmission Lines they just want to have this provision available. Our client want to keep the "ugly" power lines out of sight. I was able to dig up the following site for some information.

http://www.cpuc.ca.gov/PUBLISHED/GENERAL_ORDER/52591.htm

Wareagle, thanks for your feedback it was the most relevant.

 

[wareagle]

ajeebmjeeb

http://www.cpuc.ca.gov/PUBLISHED/GENERAL_ORDER/52591.htm

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.

 

[wareagle]

ajeebmjeeb
Check this site for some man hole dwgs and specs. You will need a big one.

http://www.tampaelectric.com/TEBSGeneralRulesSpecifications.cfm

 

[weh3]

DavidBeach:

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

 

[Dumbo2929]

Most UG transmission lines are in single conduits due to their size (for solid dielectric cables). Conduits and fittings need to be non-metallic so there is no inductive heating.

 

[wareagle]

ajeebmjeeb

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.

 

[BJC]

PG&E has installed an undergroudn 115 KV line. Google up the "Potrero-Hunters Point Transmission Line" and the "Jefferson-Martin 230 kV Transmission Line Project" a 230 KV project.
Both are in the San Francisco peninusla. Technologically they are not difficlut. They are very expensive. Millions of Dollars per mile.

 

[EEJaime]

ajeebmjeeb,

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.

 

[Dumbo2929]

This is one of those things that I do every day and people tend to think is very simple (dig a hole, bury some conduit). I have seen countless installations where people have installed future, spare conduits without the proper planning & engineering, only to find out when it is time to use the conduit, they no longer meet their needs and are either dug up, replaced or abandoned.

 

[tinfoil]

BTW, make sur ethat you put it extra conduits, as one or more may collapse between now and whenever, and a cable failure may render a pipe unusable.

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.

 

[cgrodzinski]

Sorry for being so late but you may still need it. As Dumbo2929 posted this is not a “standard thing” but some information would help. Please consider it as a friendly suggestion and not as a guideline because this type of installation is always!! “custom design”. You need a lot of planning.

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.

 

[benlanz]

I recommend you contact an engineering firm such as Black and Veatch or Power Engineers who do this kind of work all the time. You can not possibly build provisions for an underground HV cable correctly without experience. If you want contact information I would be happy to help.

Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants