I just realised I don't really understand earth ground!

[NiHaoMike]

Some apartments in the US are wired 3 phase (120/208), though each unit generally can only access 1 or 2 of the phases.

Any reason why most residential neighborhoods aren't wired like that? I see that as helping balance out the load. (So a house with an unusually large or small load would likely be spread out over 2 phases instead of being concentrated on one phase.)

Also, if you took a 3 phase motor and wired each stator coil to a separate H bridge (so you have 6 wires from the motor going into 3 H bridges), would that be 6 phase or split 3 phase?

[G7PSK]

Also, if you took a 3 phase motor and wired each stator coil to a separate H bridge (so you have 6 wires from the motor going into 3 H bridges), would that be 6 phase or split 3 phase?

3 phase 6 wire used where reversing is required or for starting purposes as star delta connection. starting is on star and switch to delta when up to speed helps to stop the lights going out during starting on line with big motors.

[ptricks]

It'd be much more interesting to look into the origins and reasoning behind that center tapped pole transformer system.
So OK, the US street distribution is several KV, to minimize resistive power loss in the distribution system, for a given amount of copper used. But then there's all the copper required in the transformers... I wonder how that works out, overall? Did they actually need less copper in the end, with that system? As opposed to the more direct system like in Australia, where the street level distribution is the same as the household voltage - three phase wires plus a multiply grounded neutral return, and premises directly tapping the street wires, for one, two or three phases as required.

In the US and many other countries, copper is not used for transmission lines, it isn't strong enough, it stretches over time and causes line sags, it hasn't been used in a very long time. Steel coated aluminum is used now for transmission lines, it is strong enough to not break and cost is low.  One item to remember is that the USA was, it isn't now, one of the countries to pioneer technology when it was invented. The USA was one of the first to have a large electricity roll out and they made a lot of mistakes. Other countries that provided service later could benefit from not having to make those mistakes.  Lots of areas were wired for single phase because it was the cheapest way to wire at the time. Later when industry wanted 3 phase it tended to cluster in areas that were lots of industries but few homes.

With the US system, I presume the street distribution does actually have three high voltage phases (120 deg apart), and if premises need true three phase power for rotational machinery, they can have a true 3-phase street transformer installed. The output of that would be star config, with grounded center point, I suppose? Of course in such installations there's no question, those really are three different phases, and electricians call them so.

In the US most lines coming to the home are single phase because 3 phase isn't needed or 50 years ago the area was single phase and cost prohibits changing that. 3 phase terminates at the distribution sub station and then one of the phase is sent to a different section of homes.  3-phase power can be extremely expensive to have added to a site because it often means stringing new lines all the way back to the sub station.  The nearest 3 phase connection that I could tap is about 5 miles away, at over $20k per mile that is an install cost of $100k.

Then with premises that only have small power requirements, they get a pole transformer with only one or two outputs. With only one  HV source phase, so of course the two outputs are just that phase. Except electricians don't care about terminology precision, so they call the two opposed polarity outputs 'phases'.

Electricians in the US do not call the single phase (split phase) output two phases because they know that would confuse itself with the distribution 2 and 3 phase installs.

[ptricks]

Some apartments in the US are wired 3 phase (120/208), though each unit generally can only access 1 or 2 of the phases.

Any reason why most residential neighborhoods aren't wired like that? I see that as helping balance out the load. (So a house with an unusually large or small load would likely be spread out over 2 phases instead of being concentrated on one phase.)

It really depends on what the load average is for the area. Sometimes all you can get is single phase and rather than string new lines all the way from the sub station it is cheaper to just upgrade the single phase line capacity. Load calculations have some pretty high margins, 300A allotted for each home when only 200A is possible because of the mains breaker of the home limit,  to prevent overload so spreading out the load over lots of single phases really isn't needed near the homes. Homes are supplied with lines all the way up to 35KV @ 2-500A which are fed by 50KV@10KA so balancing the load isn't too much of an issue except when areas sustain a heavy build up of new homes or business and the capacity wasn't designed for that increase.  A typical single phase line can supply 360 homes easily.

[SeanB]

Originally power distribution was done with DC voltage, and the substation in the street provided 100VDC on 2 power rails from motor generators running either on steam or from a higher voltage DC supply fed in from a larger power station. Thus one side had +100V and another -100V as a supply, with higher loads being supplied by using the 2 100V lines to give 200VDC as power. The wiring was done to reduce the size of the common conductor, as it normally only has to handle the imbalance current between houses, not the whole street load, which are handled by the 2 line wires, at half the current.

When AC was being introduced, the motor generator sets were replaced by transformers and mercury arc rectifiers, still keeping the 2 power rails so as not to have to replace all the infrastructure in a district when upgrading. When finally the switch was made to AC distribution the mercury arc rectifiers were simply removed and the transformers were simply switched to supply the power rails, as all these substations either had 3 sets of gear if they had a 3 phase supply, or had 2 sets if single phase. The load output was simply rearranged to use a single phase, as again it was desired to not have to rewire the streets and houses, with the AC voltage being chosen to work with older equipment in houses at the time.

Thus the 2 phase is cast in stone. In the UK and elsewhere they had chosen a higher voltage in the beginning, typically 200-250VDC and had thus only a single wire pair along the street, as the insulation of the time was problematic at higher voltages, and winding the motors and transformers for higher voltage was more difficult, so they just supplied full current ( same current as the US on the lines as well, but capable of higher power to loads) to the lines. When they changed to AC it was merely a wiring change.

[G7PSK]

The UK was lucky with the early decisions on power distribution, it started with Ferranti then in the 30's they started on closing all the small local town plants and building the national grid then we really lucked out with free demolition by the luftwaffe so we had to build all new.https://www.eevblog.com/forum/Smileys/default/smiley_laughing.gif
I walked trough our village looking at the power distribution as that is overhead lines and noted that it is pretty well every other house is on another phase in order to keep things balanced. Four wires plus earth strung on poles so fairly easy to spot which line the connection is.

[IanB]

But then there's all the copper required in the transformers...

With the typical "pole pig" transformer used in the USA the secondary is not copper wire but rolled up aluminium sheet. There are actually only just a few turns on the secondary side, maybe a dozen or so (but a lot of cross sectional area per turn). When people say the the mains supply has effectively zero impedance they are not kidding... 

[SeanB]

Most have copper windings, though you do get a lot of the smaller units using aluminium strip as wiring. The cores though are run at a very low power level, along with having a large cross section of the wiring, mostly to reduce the off load losses in the core to as low a level as possible, as the transformer is always running at low power levels with only short duration of full power operation during peak hour. Often you find it only has a 20C rise over ambient, with possibly only a 40C rise at full load. oil cooling on all of them, though the oil is more there as an insulator, but it keeps hot spots from forming in the inner windings. Bigger ones that are doing distribution down from the grid though often have forced cooling, as the power levels are high enough that they have oil pumps and thermostatically controlled fans to cool the unit when it reaches full load. Those that are that big have a conservatory tank with a pipe connecting it with a flow switch, which trips out the transformer if there is a sudden flow of oil into the tank, which means the transformer is arcing over internally.

[c4757p]

Curious question relevant to the US using so many damn transformers: What is the typical efficiency of one of those? How does the power loss in the transformer compare to the power saved by the higher voltage lines?

[IanB]

Here is a video showing how those "little" pole mounted transformers are made. Fascinating for anyone who likes power electronics:



It's a "little" transformer, because those things are still huge and need a fork lift to move them when you see them on the ground instead of stuck up a wooden pole...

[G7PSK]

Is that number on the outside the rating, some had 25 and others 50, so I wondered if that was KVA.

[IanB]

I don't know, but I would guess the "25" is 25 kVA and the "14.4" is 14.4 kV.

[NiHaoMike]

3 phase 6 wire used where reversing is required or for starting purposes as star delta connection. starting is on star and switch to delta when up to speed helps to stop the lights going out during starting on line with big motors.

I was thinking more along the lines of using an industrial 3 phase motor in a homemade EV. You'll almost surely wire it for delta (with paralleled windings if applicable) in order to reduce battery voltage requirements.

But an EV requires a lot of power, so those DIY inverters out there need a bank of IGBTs on each phase. So you decide to "open up" the delta and connect each of the three windings to its own pair of IGBT banks, in order to ensure current sharing. You now have 3 windings connected to their own H bridges.

And now you realize that rather than create 3 sine waves 120 degrees out of phase, you create 6 sine waves 60 degrees out of phase, so that each winding of the motor is connected across two phases 180 degrees out of phase. Now your battery voltage requirements go down a bit (or you can get a little more horsepower for the same voltage).

Would that (6 waves 60 degrees out of phase) be called 6 phase or split 3 phase? If it's the latter, what would true 6 phase look like?

[G7PSK]

The windings will only give you 3 pairs of poles or 6 pairs depending on the speed rating however you use the connections as one set of windings will be on top of the other.