Ducts for secondary (12 KV) running 4

[WWTEng]

Ducts for secondary (12 KV) running at 90 to each other. At the point of intersection, how much of a separation is recommended and what should the infill be? I saw numbers of 2' to 4' of "fluidized thermal break" but I can't understand why and how this is determined.

 

[cuky2000]

WWTEng, here is our though regarding your post”
1)Why “low thermal resistivity backfill”? Low thermal resistivity (RHO) material is used to maximize the ampacity of electrical feeder in a duct bank. Either thermal fluidized backfill (TFB) concrete with low strength or low RHO sand thermal backfill have been used. Reasonable soil thermal resistivity (RHO) usually used in the NEC for US soil is around 90oC-cm/W. We have been used in UG application of thermal sand with <5% moisture content less than 60oC-cm/W with <5% moisture content. Soil usually is compacted at 95% of modified Proctor Compaction test per ASTM D68.

2)How this is determined? By computation or by testing the existing soil. It is usually easier and accurate to measure the RHO than to do the computations. Methods are given by ASTM 5334-08) and IEEE1992.

3)How much of a separation is recommended for a 90o crossing duct banks?: It is hard to say without know the details and target ampacity. Considering that the duct bank concrete is hydroscopic material (absorbs and retains moisture) many applications can be used virtually without separation.

4)Options to solve this issue:
a) Recommended: Perform cable ampacity calc. by qualified EE using thermal and electrical model. Evaluate the economics to oversize the cable vs. the thermal backfill.
Fast but Risky & expensive option: Oversize the cable, separate the duct bank couple feet and fill the duct bank crossing with a monolithic structural concrete.

 

[7anoter4]

Regardless what cuky2000 said -and I agree totally with him, of course-in my opinion, if the crossing ducts contain the same power system then a minimum 1.5" clearance is required -from construction point of view. However, our experience is 2”-3" clearance. If the system are different-for instance one duct bank for supplied company and the
other of utility -then 12"[one feet] clearance it could be required.
Usually each utility provides its own instructions. See-for instance [Eversource Electricity Massachusetts]:

https://www.eversource.com/Content/docs/default-source/ema---pdfs/c1100.pdf?sfvrsn=0

CONSTRUCTION STANDARD ELECTRIC OPERATIONS ORGANIZATION
7.7 Clearance– The minimum clearance between an NSTAR conduit or ductbank and any other subsurface structure or utility (EXCEPT Steam Lines) shall be 12 inches unless otherwise approved by NSTAR. Steam lines shall maintain a 10 ft. minimum from NSTAR ductbank or lines.
NESC/2007 Rule 320. Location. B. Separation from other underground installations:
"A conduit that crosses over another subsurface structure shall have a separation sufficient to limit the likelihood of damage to either structure. These separations should be determined by the parties involved."
“2. Separations between supply and communication conduit systems. a. 75 mm (3 in) of concrete.
EXCEPTION: Lesser separations may be used where the parties concur.”
In my opinion, you have, at first, to build the ducts as usual structural construction, in such a manner to resist to the presumed burden and to keep a minimum clearance required by structural consideration.
The ampacity derating will be the job of a qualified EE-as cuky said.

 

[WWTEng]

Thank you all for the feedback. This is very informative.

I had a feeling that the separation between the ducts is driven more by magnetic fields created by power cables and there inference with each other, and not so much by the heat generated. Is that about right?

I am not EE so if I sound ignorant, it probably am.

 

[cuky2000]

WWTEng, we welcome your questions and interest interfacing with the electrical discipline. Ultimate we all benefit collaborating in a common areas.

The separation between the ducts is not driven by magnetic fields created by power cables. In fact the EMF in 3 phase configuration tends to cancel. Some of the EMF will create between conductors and duct banks hysteresis and eddy current losses in form of a heat.

The dominant factor for continuous current sizing is the thermal Joule effect (I^2R Watts). This heat has to be dissipated through thermal barriers such as conductor insulation, air inside a duct, duct wall, the soil surrounding, concrete of multi duct banks, etc.

Hope this help

 

[waross]

Crossing at right angles the magnetic fields tend to ignore each other. The primary concern is heat.
As an interesting piece of trivia:
If fibre optic cables are run with the cables, the temperature profile of the ducts and the temperature of the crossing area may be determined quite accurately.
This effect is used to detect and locate leaks on both oil and gas lines. The fibre buried run near the pipeline. An oil leak will show up as a hot spot and a gas leak will show up as a cool spot.

https://en.wikipedia.org/wiki/Leak_detection[/URL]]

https://en.wikipedia.org/wiki/Leak_detection

Fibre-optic leak detection
At least two fibre-optic leak detection methods are being commercialized: Distributed Temperature Sensing (DTS) and Distributed Acoustic Sensing (DAS). The DTS method involves the installation of a fibre-optic cable along the length of pipeline being monitored. The substances to be measured come into contact with the cable when a leak occurs, changing the temperature of the cable and changing the reflection of the laser beam pulse, signalling a leak. The location is known by measuring the time delay between when the laser pulse was emitted and when the reflection is detected. This only works if the substance is at a temperature different from the ambient environment. In addition, the distributed fibre-optical temperature-sensing technique offers the possibility to measure temperature along the pipeline. Scanning the entire length of the fibre, the temperature profile along the fibre is determined, leading to leak detection.[5][16]

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cuky2000]

Hi Waross -
DTS is a great technology that could eliminate the guess and the conservatism in the UG lines due to the uncertainties on the type of soil and heat dissipation of the cable since it is possible to detect the critical hot spots.
Not sure if the industry is ready for deploy DTS in MV applications. Economics and culture of the electrical community will be a great challenge for acceptance and deploy this technology.
For HV applications DTS is an option and could be justified in some instance to increase the system reliability and monitoring smart grid. This could be used in not only in UG but also in limited OH applications.
Recently we did specified DTS with FO cable embedded in the HV cable for a submarine T. Line for an HVDC project. and other 345 kV HV project.
I do not remember seen any DTS deployment in MV project in service.

 

[7anoter4]

In my opinion, no one has to be concern about the crossing duct heat emission. See for instance:
Derating Cable Ampacity for Short Segments in Buried Conduit

http://www.eng-tips.com/viewthread.cfm?qid=304441

IEEE Transactions "Ampacity Derating Factors for Cables Buried in Short Segments of Conduit"
"The results show that once the length of conduit exceeds about 20 times its outer diameter,
then the ampacity of the circuit must be reduced to the value that it would have if the entire length
were buried in the conduit.
Factors that result in lower cable ampacities, such as high soil thermal resistivity and deeper burial depths lead to larger derating factors."
NEC allows 3fts length or 10% of the entire run to be neglected.
If the duct of is of 4" and the concrete between ducts is 3" the maximum of 4*20=80 inches is maximum length permissible [it will be a row of 80/7=11 ducts].
The low thermal resistivity backfill is very good but it is expensive and not practical for large surfaces.
You may use this above the intersection if you are still concern about heating.
DTS system it seems to me to be very interesting but not in the time of the design process of the duct banks.
In my opinion, it is more important to design a suitable support for upper duct bank not to damage the lower duct bank.
In my experience, even at an intersection of three duct banks -in a power station-it did not arise any electrical problem.