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Power-User

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HV cable vs overhead power line

03/17/2022 7:31 AM

So, there's a project that an acquaintance is managing, requiring an overhead power line to supply power from the utility of ~1km in length, at (I think) 88kV or 66kV, thereabouts. Whilst the power line costs are apparently as expected, there are other services and infrastructure that must be moved as well as some operational interruptions, and this adds significant costs (and time) to the project. I asked as to why an HV cable was not proposed, as the distance will be shorter at about 400m only, with no services, roads, etc. to be moved and possibly less operational interruption, but was told that it would be too expensive (though seemingly no formal costing had been done).

Be this as it may, this got me thinking - what would the actual cost differences be? Unfortunately I have no idea of the power requirement, probably a few MVA. I'm just curious (the power line is being built) re. ball-park figures, since I've never been involved with cables at voltages higher than 22kV. There was a previous discussion on CR4, but it did not really help.

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Guru

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#1

Re: HV cable vs overhead power line

03/17/2022 8:26 AM

I can't quite understand from your description what the situation is, or why one way it's 1km and the other it's 400m, but ballpark rule of thumb is that underground cable installation is 8x to 10x overhead transmission line cost for the same circuit-miles.

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#2

Re: HV cable vs overhead power line

03/17/2022 8:27 AM

CR4 doesn't usually deal with <...costs...>, there are far too many variables, including the currency in which the things are to be displayed. Further, such deliberations are commercially sensitive; it is unwise to disclose these things in a global, anonymous, Engineering forum.

The best practice is to engage in a proper commercial tendering process at the facility locally, following which comparisons may be made between several on the basis of local specifications, practices and conditions.

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#3

Re: HV cable vs overhead power line

03/17/2022 10:28 AM

400m is almost half a km. I suspect your idea of a closer power grid connection is actually the same as what your power company is proposing.

As for what added expenses you are overlooking, that's hard to say from such sparse information. Common complications laypeople don't anticipate with grid distributions are the following:

  • Upstream transformers reserve capacities
    • Diversity of clients also using these transformers
    • Priority of these other clients
  • Existing "right of way" contracts
  • Disconnects for servicing
  • Alternate feed paths for reliability and servicing
  • Foliage, flooding, and a variety of other environmental complications
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Guru

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#4

Re: HV cable vs overhead power line

03/18/2022 1:18 AM

I installed about 1/2 mile of 40MVA @ 35kV insulated cable (Kerite) in upstate NY to replace overhead lines that were too unreliable for the industrial facility on the property. The cost benefit was demonstrated but not quantified for years before the project was executed, as is often the case.

These cables are still in service over 30 years later, some UV shielding was added about 15 years in, the concentric neutral wires were showing through the outer jacket in places.

So initial cost is not the only factor to consider. It looks like insulated cable might actually have an advantage in your case, so probably worth at least looking at it.

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#5

Re: HV cable vs overhead power line

03/18/2022 2:13 AM

One point often overlooked is the likelihood of the underground cable getting, ripped or graded over and if there is heavy vehicle movement over the soil when it gets wet then the soil can liquify and damage the underground cable.

We had underground cable crossings for haul roads on a mine site and all of the above happened in the years I was there. The underground cables were put in so we didn't have to go out and lower the aerials over the road crossings. Damage to the underground cables from rocks coming into contact with them as the rocks moved through the soil. These cables were 66KV and to replace a damaged one was way more expensive and time consuming than stringing a new aerial cable and needed more expertise.

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#6

Re: HV cable vs overhead power line

03/18/2022 7:20 AM

Obviously in SA and possibly an Eskom/City Power line being built. The simple cost are this: type of pole being used or is this a lattice tower? Poles: wood, concrete, steel mono pole. The pole type and height is determined by the clearance needed to ground, on the voltage at mid-span sag and this will be 7.8m. So this could be a 22m pole with 2.3m buried in the ground. A concrete pole will cost in SA between R19000.00 to R25000.00 per pole and this depends on the configuration required on the pole for stand off insulators, link switches, or a cross arm.

The insulators are around R2500.00 each, you need a minimum of three for an AC line.

The poles can be self supporting or stayed. So if they are stayed, that is extra for 7 core galvanised stay wire, turn buckles, stay anchors, and all the eyes and stay plates and bolts, so an average pole will cost around R37000.00 for a complete pole.

Now you need conductor which can be aluminium with a steel core or all aluminium alloy. The poles will be spaced around 45 - 85m part, depending on the route design and how many angels are in the route. So allowing R500K for conductor, another R500k for poles and basic line accessories, this pole route can be built in a week, if all materials are on site. Add labour cost and plant and you will have an approx cost of R2.5m

With the cable, one has the right of way to consider and the cost of this. The existing buried services that are in the ground need to be considered. The burial depth, the route length and how many cable joints are required, plus the terminations at each end. Joints and terminations are expensive along with the steel gantries to support the terminations. Cable jointing bays may be required to be build, but in SA this is probably direct buried.Trench excavation is slow and costly unless you use a Tesmec trencher, then it just expensive and faster. Back filling with sand, laying the cables which is for 1km, 3km of cables, back filling and reinstatement and that will cost around R2.5m - R3m for this short run. Longer runs of cable, the price sky rockets to around 8 times more expensive than OH.

Its easy them for them to steal the cable, even in the ground, as you know. It's harder and more costly to replace installed cable. With OH lines this is less costly and quicker to replace stolen conductor.

But, knowing Eskom/City Power they have a reason to be going OH rather than UG and that is usually because they are geared up for OH lines and not geared up for UG, and any faults that occur can be repaired easier and quicker on OH lines than on UG cables.

So in a quick guesstimate, OH line is more cost effective on this short run, quicker to build. UG has issues with theft and delays for the XPLE cable, is slightly more expensive to install and takes longer to install and takes much longer to have an operational line Using concrete or steel one piece self supporting monopoles is far quicker to build. Two piece poles take a bit longer.

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#7

Re: HV cable vs overhead power line

03/23/2022 3:29 PM

Why use insulated wires when you can use ACSR?

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#8
In reply to #7

Re: HV cable vs overhead power line

03/23/2022 3:53 PM

Hmm! Open conductor is good for making pollution and it is also prone to making corona when it rains, or is foggy or damp. Corona produces nitric acid and it falls on the ground. Then the conductor falls on the ground later on. Insulated conductors don't do that.

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Guru

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#9
In reply to #7

Re: HV cable vs overhead power line

03/23/2022 4:20 PM

And if you can afford it, its better to use HTLS as you get more amps for your money, and rather than using 50-60Hz use 16Hz and you will transmit 3 times further on the same diameter conductor with the same amount of losses.

Of course the higher the amps, the bigger bang one gets for ones bucks.

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