Voltages used in "neighborhood" power distribution

Hi:
Does anyone here know the voltages used in typical surburban sub-division power distribution where you have transformers on pads serving 1 to 8 homes?
I "thought" I heard some local utility repair men tell me that it was 38kv here (East Central Virginia, semi-rural.)
Is that "reasonable" or do I need to get my hearing checked?
I have watched these repairmen do some splicing of these cables. They were shielded and seem to have a "semi-conductor" plastic around the center conductor with some kind of white plactic to the insulating braid. The shield was covered with a black plastic and the cable can be exposed to the element since it goes up poles to transition to overhead transmission.
thanks
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On Fri, 20 Aug 2010 09:31:23 -0400, "John Gilmer"

I suppose it is really up to your utility but they use 13,200v line to neutral here. (22,860Y) The other tap on the transformers is for 7,620v
http://gfretwell.com/electrical/50_kva_label.jpg
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snipped-for-privacy@aol.com wrote:

In my area served by Los Angeles Dept. of P & L, the underground transformers are still 5000V to 240/120V. The one serving my home blew a few months ago. The linemen said there were still a few spares around, but they had a hard time locating one. They can "work hot" on anything 5KV or below.
The sub-division, just north of UCLA, is about fifty years old.
--
Virg Wall, P.E.

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Probably 4800/8360 Wye system volts. We use that in our rural distribution but is losing to 27.6kV for higher loads over long distances.
wrote: In my area served by Los Angeles Dept. of P & L, the underground transformers are still 5000V to 240/120V. The one serving my home blew a few months ago. The linemen said there were still a few spares around, but they had a hard time locating one. They can "work hot" on anything 5KV or below.
The sub-division, just north of UCLA, is about fifty years old.
--
Virg Wall, P.E.



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http://gedigitalenergy.com/products/brochures/ResidentialSinglePhasePadMounted.pdf
Well, at least I know that 38 kV isn't "crazy!"
"Next time" is talk with the "underground linemen" I will ask a few more questions.
It's a lot more difficult to start and maintain a conversation with a crew that's looking up into the air than a crew with two guys basically sitting in a ditch.

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In UK, it's 11kV. However a suburban pad transformer will normally do 100 or more homes. Only in rural areas would it drop as low as 1 to 8 homes, and that would normally be pole mounted. Again, normally fed at 11kV, but there are some directly fed from 33kV.
I have a length of 11kV single core underground cable (normally 3 of them are used for a 3-phase supply). It's 150mm Al cross sectional area. The Al core has a thin black semi-conducting layer around it. The bulk of the insulator is a thick layer of orange plastic, apparently XLPE (Cross-Linked Polyethylene), surrouded by another semi-conducting layer and earthed copper braid, and then the outer tough PVC sheath (red in this case).
--
Andrew Gabriel
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With that many homes (100 or more) on one padmount transformer you must have severe voltage drops into the homes. Running 220v cables to the home at the end must incurr about 100 meters or more in length.
Ours, in the cities, with normal load density run up to about 13 homes on one transformer. The homes at the far ends get heavier wire to avoid voltage drop problems.
Does the centre conductor has strands of steel inside it? Ours has that for overhead distribution on HV lines. ACSR = aluminum conductor, steel reinforced. Not sure about U/G cables.
Translations: "centre" "center" (US); "aluminum" = "aluminium" (UK)
LOL
In plumbing they call XLPE it "PEX" = polyethylene cross linked.
When they terminat thos concentric/sheilded cables they built stress cones at the ends to avoid termination impedance transition shock. They flare every layer (except the conductor) out to 2.5 times it size before discontinuing the layer. This takes a lot of insualting tape, semi-con tape and braided conductor materials.
In UK, it's 11kV. However a suburban pad transformer will normally do 100 or more homes. Only in rural areas would it drop as low as 1 to 8 homes, and that would normally be pole mounted. Again, normally fed at 11kV, but there are some directly fed from 33kV.
I have a length of 11kV single core underground cable (normally 3 of them are used for a 3-phase supply). It's 150mm Al cross sectional area. The Al core has a thin black semi-conducting layer around it. The bulk of the insulator is a thick layer of orange plastic, apparently XLPE (Cross-Linked Polyethylene), surrouded by another semi-conducting layer and earthed copper braid, and then the outer tough PVC sheath (red in this case).
--
Andrew Gabriel
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Sorry, I meant 1000 metres instead of 100.
In UK, it's 11kV. However a suburban pad transformer will normally do 100 or more homes. Only in rural areas would it drop as low as 1 to 8 homes, and that would normally be pole mounted. Again, normally fed at 11kV, but there are some directly fed from 33kV.
I have a length of 11kV single core underground cable (normally 3 of them are used for a 3-phase supply). It's 150mm Al cross sectional area. The Al core has a thin black semi-conducting layer around it. The bulk of the insulator is a thick layer of orange plastic, apparently XLPE (Cross-Linked Polyethylene), surrouded by another semi-conducting layer and earthed copper braid, and then the outer tough PVC sheath (red in this case).
--
Andrew Gabriel
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--------------- Stress cones are not used to avoid "termination impedance shock" but, rather to control the voltage gradient in the region of the junction. The flaring is one way to do this. There are other ways.
"In cable installation, shielded power cables require electrical stress control when terminated. When the insulation shield is removed from a cable, high potential gradients are concentrated at the cutback point, causing high electrical stress. Electric field enhancement at these points can produce local discharges that could lead to either flashover along the insulation surface or dielectric breakdown causing cable failure. Cable terminations are designed to eliminate the stress concentration at the screen termination to avoid the break-down of the cable. In other words, the electrical field has to be controlled in a cable termination."
www.idc-online.com/.../electrical_engineering/Stresscontrolledhighvoltage.pdf
--
Don Kelly
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Sure sounds like an echo.

---------------
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I am not sure what you imply or mean by "electrostatic stresses". I only know where stress cones are not used the "end impedance change" of the complete cable (sheild and conductor and semi-con)changes too rapidly and tends to break down. This implies, to me, a standing wave or similar is a problem. Theory behind it is not studied or understood by myself.
It sounds lke you think it would be from disturbances, consisting of higher frequencies from lightning or switching transients. Either way, I feel there will be a tuned resonant frequency of the cable and loads that will be the one to break down the termination as a weak spot.
Am I too far off base to your knowledge? Wasn't really my forte.

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----------------------- A bit far off base. Salmon Egg might have some comments which might help.
What I looked at as the basic consideration of the use of stress cones is not due to any impedance changes. What is of concern is that where there is an abrupt change in the geometry there will be a change from a uniform E field to a non-uniform field, near the point of change, which can produce excess stress on part of the insulation. This can lead to internal breakdown of the insulation or surface flashover-both not desirable. The reference I gave shows distributions of stress as contours (and also in colour gradients) where equipotential contours that are close together indicate higher E field stress (as in maps-close contours indicate steep slopes).
This is present in the DC situation as well as AC so standing waves are not a concern. Hence "tuned resonance" is really not of concern.
Where I may have confused you is my going on to consideration of surges. In the case of no "flaring" there may well be a change in characteristic impedance at the junction and this could be a problem with reflections. Flaring would affect this characteristic impedance in the transition region. However, I suggest, and I haven't analyzed this, that the dimensions involved are such a small part of a wave length of any incoming surge that the flaring is not going to be effective in reducing the overvoltages due to reflections. This could be an interesting analytical study for a grad student -considerable related information is available in references on the behaviour of exponential cones in audio systems. I suspect that the end result of such a study might lead to a conclusion that, for surges, it isn't of importance. Note also the flaring typically means a conic section rising to a maximum diameter at some point and then decreasing to a smaller diameter. This makes sense in consideration of the electrostatic field stresses but not in terms of impedance matching (an analog would be using a transformer to step up from 20 to 40V and then using another transformer to step down from 40 to 30V rather than stepping directly from 20 to 30V. Ideally the same but practically of no purpose).
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wrote:

Out in the suburbs it is more like 2 - 4 houses on a transformer with a 13kv primary and a neutral. (wye distribution) They give each one about 12.5kva (25KVA transformer feeds 2 houses, 50kva feeds 4) There are 20 transformers on my leg, about 55-60 houses, all fed from one primary.
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Complete opposite. With a 1-2MVA transformer, nothing I can do at home with varying the load makes the slighest difference to the voltage. Variations are very much less than in the US. OK, I'd expect 4 times less due to twice the voltage and half the current, but it actually seems much better than even that. Maybe we use thicker street conductors, I don't know?

[1000m] Probably 500m max.

If you look at a tiny village here with overhead supply, you might find a pole mount transformer in the middle with a street mains heading in each direction down the road. With reasonably modern installs (like last 25 years), the same thick conductors run right along the road feeding the taps right to the last house.

We don't use the edison system - our street distribution is all 3-phase 240/415V Wye (or Star as it's known here). I don't know the construction of overhead cables. From the ground, there are two distinct styles - older use 4 separate conductors (sometimes the neutral is uninsulated, sometimes all uninsulated, sometimes all insulated). Newer use a thick bundle of 4 insulated cables twisted quite tightly together.
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Andrew Gabriel
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**1** You must be referring to another form of transformer, usually called a "substation or distribution transformer" other than what most called the "street transformer". Street distribution transformers are the last transformation in voltage feed your house with 120/240vac (in N.America) and I have never seen one at 1-2MVA. The poles won't hold them up. It doesn't work well in distribution systems for residential. I did note some confusion here and this is why I asked.
- You don't run this low voltage 1000m to a house as the copper would be too expensive. - You don't supply a residence with that high of a fault capacity using a trnasformer and copper that big. Explosions result.
**2** ACSR has nothing to do with an Edison system. Houses do not use 240v/415wye feeds in North America. Where are you located?
**3** The cable bundle you refer to is called "quadriplex"
Thanx
writes:

**1** Complete opposite. With a 1-2MVA transformer, nothing I can do at home with varying the load makes the slighest difference to the voltage. Variations are very much less than in the US. OK, I'd expect 4 times less due to twice the voltage and half the current, but it actually seems much better than even that. Maybe we use thicker street conductors, I don't know?

[1000m] Probably 500m max.

If you look at a tiny village here with overhead supply, you might find a pole mount transformer in the middle with a street mains heading in each direction down the road. With reasonably modern installs (like last 25 years), the same thick conductors run right along the road feeding the taps right to the last house.

**2** We don't use the edison system - our street distribution is all 3-phase 240/415V Wye (or Star as it's known here). I don't know the construction of overhead cables. From the ground, there are two distinct styles - older use 4 separate conductors (sometimes the neutral is uninsulated, sometimes all uninsulated, sometimes all insulated). Newer use a thick bundle of 4 insulated cables twisted quite tightly together.
--
Andrew Gabriel
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If you read back through the thread, you'll see this branch was talking about pad mount transformers in the UK. I think that's where your confusion is.
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Andrew Gabriel
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There are many different voltage levels and schemes in effect. it depends on where you are, North America, UK or elsewhere as well as load density (roughly houses per km along the line) and single/three phase distribution. Different strokes for different people or conditions. Beyond the technical factors there are historic factors which which have generated a lot of inertia, attitude problems as well as economic problems in making changeover to a world wide uniform approach. Pity Japan with both 50 and 60 Hz systems -cheapest solution is an asynchronous back to back DC link. :)
--
Don Kelly
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Whoever accomplishes that will rule the civilized world. ITER (the closest to that) does not sound so hopeful. This post is outlandish and done. RR
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writes:

Ah, yes, Randy or proteusiiv No capitals or" I am Proteus"?
You can't be Proteus(n) - you make marginally more sense than he/she/it does. However sensible engineering doesn't compete with BS in ruling the world.
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