why not 3 phase?

[zigzagflux]

Conventional Transformer

Typical picture of a single phase transformer for residential service. The bushing is the ribbed insulator sticking out of the top cover. There is a conductor through the center which carries the current to the hot end of the primary winding. A bushing is designed to bring an energized conductor through a tank wall, insulating the conductor from the tank wall.

[jitter]

Since our country is essentially sitting on sand and not rock, almost all wiring except very high voltage distribution lines is underground, so we don't have transformers sitting on poles. What we do have are little houses in quarters that hold somewhat bigger transformers. High energy consuming plants will have their own transformers on the premises.

[AndrewT]

Sand and rock and clay and loess and any other ground conditions have little to nothing to do with the choice the electricity transmission company make when locating their cables.

It is quite simply cheaper to hang the transmission lines from poles/towers than to bury them.

A pole mounted transformer can be supported by almost any ground condition. There are exceptions, as you may expect. peat/ quicksand (water flowing up through sand)/bog/marsh etc would be quite difficult terrain for any cable laying/stringing operation.

[zigzagflux]

US underground residential service is also quite common. It was not discussed earlier, because the point being made was that most of these transformers are single bushing single phase. The average Joe cannot drive around and see this to be the fact with underground service, so I brought up overhead.

But the same condition applies. We use small pad mount dead fronted units with a single bushing for the energized end of the primary. It is common the neutral end of the winding is not even brought out of the tank, but is internally grounded to the tank. Makes for difficulty in testing these units. There is then a grounding pad provided on the tank. Lowest cost solution with minimal components.

The primary cable is special, called URD (Underground Residential Distribution). It is a concentric neutral cable: Underground Copper 15kv

The neutral can be either fully rated (equal to the ampacity of the inner conductor) or 1/3 rated for networks in 3 phase systems. In our neck of the woods, aluminum conductor is still in high use, due to cost (again). The purpose in using this type of cable is it provides both phase conductor and return in the same jacketed single cable. Easy install, low material cost, error-free connection. Cable is rated for direct burial.

Cheap cheap cheap, but quite reliable. The utility stocks these cables in copious amounts, and standardizes on transformer sizes, so only has to stock a few types of spares. Something pops, no problem, swap it out and move on.

Regarding grounding, Andrew is correct, the composition of the earth has little effect on what the utility does. The utility system uses the earth as a constant multipoint reference, not as a conductor. A neutral is carried through in all the systems I have discussed, and is grounded with rods at regular intervals, as well as at the transformer. So while there is the potential for ground currents to flow, and they do, it is in relatively small amounts. The earth is a good conductor only because there is so much of it, not because it has low resistivity; it does not when compared to copper and aluminum. Broken neutrals will cause regulation issues, but the multipoint grounding method minimizes the effect to a degree.

The IEEE Red and Green books have documented equations that describe quantitatively how a system is grounded. It can be effectively grounded or high impedance grounded, and is related to the ratio between positive and zero sequence impedances. In general, grounded systems are preferred, with more antiquated high impedance systems being obsoleted. The older system in my area was 26.4 kV distribution for commercial use. It was grounded wye back at the substation, but the neutral was never carried through. By the time you were a few miles away, this grounded system was effectively a high impedance grounded system due to the earth as the only return conductor. In the few cases where single phase was needed from this system, a two bushing unit was required. This system is almost completely phased out now for a 24.9 kV grounded system, with neutral carried. Now the utility can use one system and serve both 3 and single phase circuits more cost effectively. Regulation is also simpler and cheaper with a neutral available. Ground fault protection/relaying is simpler and more effective when you have a low impedance return path.

[zigzagflux]

Originally Posted by jitter

What we do have are little houses in quarters that hold somewhat bigger transformers. High energy consuming plants will have their own transformers on the premises.

Love the cute little conservator (oil expansion tank). US only sees those guys on the big substation and GSU xfmrs. Do these houses only hold one or multiple units? The use of network systems is common in urban areas, held in vaults under the sidewalk, but not really for suburban or rural areas. Seems like if you're going to build a house, might as well put a bunch of equipment inside, not just one transformer.

[jitter]

Originally Posted by AndrewT

Sand and rock and clay and loess and any other ground conditions have little to nothing to do with the choice the electricity transmission company make when locating their cables.

Here it's probably been the government's decision to put everything but power lines of 50 kV and higher (predominantly 150 kV and 380 kV) underground. That includes remote areas (not that "remote" really exists in this densely populated country). AFAIK, it has been like that since before I was born when utility companies were still run by the government.
I mentioned the sand as it makes putting things underground relatively easy.

It is quite simply cheaper to hang the transmission lines from poles/towers than to bury them.

Yes it is. It looks messy, though. There's another advantage now that copper is expensive, it's hard to steal. Overhead wires from the railways getting stolen is not unheard of anymore.

[jitter]

Originally Posted by zigzagflux

Love the cute little conservator (oil expansion tank). US only sees those guys on the big substation and GSU xfmrs. Do these houses only hold one or multiple units? The use of network systems is common in urban areas, held in vaults under the sidewalk, but not really for suburban or rural areas. Seems like if you're going to build a house, might as well put a bunch of equipment inside, not just one transformer.

Litterally translated we would call them "tranfsformer houses" but ofthen they're just concrete blocks. They come in all shapes and sizes. So I guess some hold one, others more.
Some are located close to a house, others are in the middle of the street. They used to be built from masonry, nowadays they're often hideous concrete blocks but sometimes there's someone with a creative solution for that.
The conservator is probably only found on older transformers of this size.

First pic is of an older one in a city, this is big enough to hold a few. The second one is in a rural area (not entirely sure, I don't see the high voltage flash symbol on the door). The last two pics are about as small as they get. I included the one with the fireman for scale. Something similar is on the premises of the company I work for (reflow ovens and solder machines use a bit of electricity).
Edit: the last pic show one of the smallest ones you can often find in the countryside near a farm, usually they're made from metal.