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I want to ask about efficiency and economy of electric power distribution in USA.

I live in Europe and American standards are so weird for me. I was very surprised when I saw this:

See inside main breaker box

Question 1: Every house has separate transformer? 7.2kV overhead transmission lines are present over all streets at small towns and villages? Seriously?


Here where I live - we have 230V and few hundreds of houses are powered from single 3-phase 15000/230V or 30000/230V transformer. Transformers like this have 99% or similar efficiency.

Im sure, that smaller transformer (like American household transformer) wastes much more power, and in addition output voltage is less stable than voltage from high power transformer. By less stable I mean voltage changes with load changes.

Question 2: Is "American standard" much less efficient than European?


I was wondering if 400kVA transformer is cheaper than 20 20kVA household transformers. Other thing is 7.2kV lines everywhere - they need bigger insulators (more expensive).

Question 3: Is "American standard" less cost efficient than European?

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I think you had better narrow your last two questions down to two specific areas for comparison. Population density and company density in a given area plays a huge role in how the last mile is delivered. I don't think you can sensibly compare "american" and "european" last mile electrical distribution since it will vary - in both places - according to the exact location where service is provided. In my neighborhood, for instance, we have a pad mount transformer servicing 8-10 homes, so the entire neighborhood has 5-8 transformers. At the farm there's a pole transformer for each house. etc. –  Adam Davis Jun 13 at 16:33
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3 Answers 3

up vote 8 down vote accepted

I can't definitively answer the questions about efficiency, but

Question 1: Every house has separate transformer?

No. One pole transformer handles about 200 kVA and typically serves about 5 to 10 homes.

7.2kV overhead transmission lines are present over all streets at small towns and villages?

Yes.

Seriously?

Yes.

The reasons are probably mostly historical, but there are advantages.

Since the "low" voltage (240 VAC) distribution connections are very short, the I2R losses are low.

The failure of a single transformer affects a smaller number of customers. Repair is very quick.

A lot of the USA is very rural, with a low population density. Smaller transformers are a better fit for those customers.

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A typical cylindrical pole transformer is 25-75kVA, handling between 1 and 3 homes (100A 240V service each). You can get pole mount transformers up to 500kVA. Pad mount transformers can be much, much larger. For those interested in how pole mount transformers are constructed, here's a video of the assembly process: youtube.com/watch?v=tUO3o5JTGhQ –  Adam Davis Jun 13 at 16:30
    
Thanks @AdamDavis. Maybe my question was too broad or I should start this discussion on some forum. I'm electrican by education and I was just wondering about sanity of all these pole transformers everywhere. Now I see that it maybe makes sense (in low population denisity areas). Im also wondering about 3-phase system availability in USA, but this is for another question. –  Kamil Jun 13 at 17:01
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Question 1: Every house has separate transformer? 7.2kV overhead transmission lines are present over all streets at small towns and villages?

No and yes. It depends on population density. The closer together houses are, the greater the number served from a single transformer. In rural areas, yes. With houses spaced far apart, it is more economical to use a transformer per house.

Look, the power companies have standards as to how much the delivered voltage can vary. If you look at the distribution lines as being in two parts, high and low voltage, each part can be tailored as to the gauge of the wire being used. The heavier the gauge, the less the power lost in distribution and the better the performance at the customer. The heavier the wire, the more it costs. And yes, 20 small transformers cost more than one big one, but for wide separation of loads, the extra cost of the heavier wire to distribute the lower voltage may greater.

Note that, for the European system, distributing 220 to several hundred homes takes a lot of copper between the transformer and the homes if voltage is not to vary too much with load. The up side of this is that less copper is needed for the HV portions.

Because line losses scale with the square of the current, keeping currents low for as much of the distribution chain as possible makes sense. That is, if the wire sizes are identical between two systems, and the lengths are the same, then if one system operates at half the current (and twice the voltage) of the other, the high current system will lose 4 times as much power as the low current system, and in order for the losses to be equal the high current system will need wires with 4 times as much copper. In the case of 7.2 kV and 230 lines, the voltage (and current) ratio is about 32 : 1, so the line losses for transmitting the same amount of power for the same distance over the same size wires is more than 1000 : 1.

And part of it is historical, just as the choice of more or less 110 volts (110? 117? 120?) is also strongly tied to the history of the power distribution industry.

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Nice answer, but you have little mistake there. I mean "distributing 220 to several hundred homes takes a lot of copper between the transformer and the homes". In overhead lines there is no copper - aluminium is used as conductor (probably because of cost and weight/conductivity ratio). –  Kamil Jun 13 at 12:52
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Copper is still used in parts of the world especially for LV, and there is plenty of old copper overhead still in existence, just maybe not where you live. :) –  Raggles Jun 13 at 18:24
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And the same argument applies regardless of the metal used as a conductor. –  RBerteig Jun 13 at 18:27
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In the USA, 220-240VAC is normally provided by a wye-transformer, giving two 110-120VAC legs 180 degrees out of sync. Its input is a single phase 7-8kV (either the only line, or one of three phases available). Typically, three phases are used for the main distribution and commercial/industrial areas, with one phase on branches (most residential areas). I can't ever recall seeing two phases running down the road, but I suppose it's possible. They will be 120 degrees out of phase, of course, making for an unusual voltage and waveform if you try to use the two phases directly.

(I would have done this as a comment, but I don't have enough rep yet)

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It isn't a "wye transformer", it's just a single-phase transformer with a center tap on the secondary. –  Dave Tweed Jun 13 at 23:48
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