Unraveling the Mystery of Electrical Phases

[deakn]

Hi,
I am struggling with finding info on Electrical Phases. I understand that Single Phase is simply one ac signal, and I understand that 3 phase is 3 independent ac signals, all of which are exactly 120 degrees out of phase. My struggle comes in when I think about 220 volt dryer hookup in my house. There are 3 legs...2 'hots' and a ground. The 2 hots are each 110 volts.
1. But are they in phase or out of phase? I think they are out of phase in order to get 220 volts between the hot legs, right?

2. Are they 180 degrees out of phase?

3.What makes them 180 degress out of phase?

 

[vk6kro]

What you have in your house is nothing to do with the 3 phase high voltage power distribution system.

Out in the street somewhere there is a transformer that converts the voltage between two phases of the high voltage power supply to a centre-tapped 220 volt output from a single winding.

The opposite ends of this are out of phase with each other by 180 degrees. When one end swings positive relative to the centre tap, the other swings negative.

Other appliances and lighting in your house use the centre tap (as a neutral) and one of the outside legs (as an active) of this winding to get 110 volts.

This system is fairly unique to North America. Most other countries bring one of three phases or all three phases into the house at about 230 volts. High power (at about 400 volts) is available between the phases. Three phase motors are more efficient than single phase ones and self starting, too

 

[sophiecentaur]

I think this is mostly a terminology problem.
Two wires will just have an alternating voltage (say 50Hz) between them. You might call this a 'single phase system'. Another two wires may have another 50Hz AC between them. These two signals may not be in phase with each other; there is a 'phase difference'. You can have three pairs of wires with 120 degrees of phase difference between them. Let's make the amplitudes of the voltage on all three pairs the same. If you take the appropriate wire of each pair and join them together, you will have a 'three phase' system. The common point is called the Neutral. Somewhere in the system, this is usually connected to an 'Earth' . If you take the three pairs and connect them to equal loads then very little current (zero) will flow into the Earth connection because the neutral currents will add up (in phase and amplitude) to zero at the common point. There will be a voltage between any two of the phases which is √3 times the voltage on one pair. This is the Vector Difference between the two so-called Phasors. The three 'live', physical connections are referred to as 'phases' - which could be viewed as an unfortunate choice of name.

If you have just two pairs carrying AC and the signals are in phase, you can connect one of each pair to a common point and you will get Zero Volts difference, connecting one way round and twice the voltage, connecting the other way. Again, this is the vector difference between the two signals.

The US domestic system (above) seems very bizarre, to me, but it is just the consequence of the way the system grew and the fact that someone chose 110V as the operating voltage. Cables would need to be so thick for supplying large loads that 'they' had to introduce this strange 'two phase' system which can deliver four times the power with the same cable thickness by supplying twice the voltage as an option.
(Power = V²/R).

It has caused a lot of confusion - especially to non US observers! - but it serves a purpose and allows a lower (and a bit safer) operating voltage for most domestic equipment.

It is the three phase system that is used, worldwide, for generation, distribution and large motors and it is this system which is most elegant and useful. The other system just gives you twice the volts.

 

[deakn]

Out in the street somewhere there is a transformer that converts the voltage between two phases of the high voltage power supply to a centre-tapped 220 volt output from a single winding.

The opposite ends of this are out of phase with each other by 180 degrees.

These two phases from the high voltage power supply are they 120 degrees out of phase of each other before they hit the transformer? what changes them to 180 degrees out of phase?

 

[sophiecentaur]

The voltage between two wires can't be "out of phase" it is just an alternating voltage between them. You can only compare the phases of two signals. If you refer the voltages of two of the three phase supply to Earth, then they will be 120 degrees out of phase. But, using one as a reference, the other will just have an alternating voltage. The magnitude of that voltage is given by the trigonometry of the Phasors.
It's a matter of terminology.

 

[Okefenokee]

A typical house in the US uses this setup:

One of the power company's three-phase legs comes to an individual house. Power companies try to distribute the legs equally so that they each have the same load but that's a different story. If I remember correctly, a leg may be around 4kV at distribution level.

The leg is connected to center-tapped transformer at the house. The primary side is a single coil that connects to the distribution voltage. The secondary side of the transformer is a center-tapped coil and the tap is connected to the house's ground which ties it to 0V. The center-tapped coil is arranged so that the ends of the secondary coil are +120V on one side and -120V on the other side with respect to the grounded center-tap. Note, I don't mean that the voltages are literally positive or negative, instead they are out of phase by 180 degrees.

Smaller appliances can connect across one of the split 120V legs to ground. Larger appliances can connect across both 120V legs, and because they are out of phase by 180 degrees, they will get 240V.

Go look inside the circuit breaker box (carefully, after opening the main switch outside your house at the main box under the meter). You'll see that the box has three buses. The center bus is the 0V ground an the sides are the +/-120V legs. Look at how the big appliance lines and small appliance lines are connected.

ETA: Scratch that last part about looking inside the breaker box. Don't try this at home kids. (Are you a kid?)

 

[mheslep]

A typical house in the US uses this setup:
...

Nice summary.

 

[berkeman]

Nice summary.

Agreed. Lots of good info in this thread, so I highlighted it. It's in Engineering Highlights now. Thanks folks.

 

[sophiecentaur]

Fame at last chaps!

 

[vk6kro]

Good move.

Variations on this question come up all the time.

 

[deakn]

The voltage between two wires can't be "out of phase" it is just an alternating voltage between them. You can only compare the phases of two signals.

Everyone else is saying they are out of phase by 180 degrees... are you saying this becuase they are only 180 degrees out of phase relative to the center tap?

 

[deakn]

Out in the street somewhere there is a transformer that converts the voltage between two phases of the high voltage power supply to a centre-tapped 220 volt output from a single winding.

Here you say 2 phases of the high voltage power supply are connected to the transformer and further down the post someone quotes that one leg is attached to the transformer (assuming the other side is hooked up to earth). Which one is correct or can it be done either way? what would be the point of tapping off 2 phases of the highvoltage instead of just the one?

 

[vk6kro]

It doesn't make much difference. In the US, as I understand it, the high voltage distribution system does not include a neutral. In other countries it does.

You can take the voltage between phases to the primary of the transformer or you can take the voltage from one phase to neutral to the primary of the transformer.

[PLAIN]http://dl.dropbox.com/u/4222062/US%20power%20system.PNG

 

[sophiecentaur]

If you connect between two phases you will get 'root three' times the voltage.
But either the so called Star or Delta connections can be used. In the UK, it is more common, afaik, to have a three phase HV transformer to produce three low voltage , single phase supplies with a common Neutral. These are then distributed around a neighbourhood, houses being supplied with phase 1,2,3,1,2,3,1,2,3, alternately as you go down the road. It is discouraged to connect electrical supplies from a house to the adjacent house as this will involve the existence of 400V between points in the same premises and this is a potential hazard if you are not prepared for it.

 

[sophiecentaur]

Everyone else is saying they are out of phase by 180 degrees... are you saying this becuase they are only 180 degrees out of phase relative to the center tap?

I am, perhaps, being a bit picky but you can only have a phase difference between TWO signals. A signal must be the PD between two points. If you consider the phase of the voltages between the centre tap and one leg then the other leg, then they will be in antiphase. I think that's what you mean and I agree.

If you consider the potentials of the two legs relative to some other reference, then there could be a different value of phase difference - but this is not a likely scenario unless you happen to be a power engineer waving a phase meter around in a substation and connecting, willy-nilly to all possible terminals in sight!

 

[Okefenokee]

It doesn't make much difference. In the US, as I understand it, the high voltage distribution system does not include a neutral. In other countries it does.

That's exactly it. By the way, you hooked the house up with a delta source in your drawing. I'm pretty sure that delta is used for long distance transmission, because you only need three wires, but residential lines are in a wye configuration for safety (wye can be grounded). There's a delta-wye transformation somewhere in between long distance transmission and residential distribution.

Here you say 2 phases of the high voltage power supply are connected to the transformer and further down the post someone quotes that one leg is attached to the transformer (assuming the other side is hooked up to earth).

Delta has no ground. The hot wires serve as both the send and return path of current. If the loads are balanced, there is no return current at all. Note that we're talking about current in the frequency domain here. If the loads become unbalanced, the voltages become unbalanced as the currents return through neighboring wires. In wye, the current always go out through hot wires and returns through the neutral. That means that each leg can draw whatever current it wants without disturbing the other legs.

The advantage of delta is that it only requires three wires as opposed to wye's 4 wires. The advantage of wye is that it can survive being unbalanced. Wye can be safer too because you can ground it. That's why both are used.

Deakn, maybe you could look at a diagram for a delta-wye transformer to get an idea of delta and wye are related. A "leg" is simply a sampling of one voltage difference.

Let's circle back around to your original question. I think there was a bit of initial confusion because the US system is totally different from European systems. The three-phase system is not important inside a US home. You won't have to deal with any three-phase power unless you work at an industrial park that uses lots of juice. You were right about the phases of the two buses being out of phase by 180 degrees. Vk's drawing shows how the transformer accomplishes that.

 

[deakn]

In that picture drawing can i get more info explained on how the two phases that are taking from the powerplant that are 120 degrees out of phase relative to neutral are then changed to 180 degrees out of phase

 

[vk6kro]

The voltage between phases goes right back to the generator at the power station, where three voltages are produced by a rotating machine.
They are then connected in a delta pattern for transmission. They pass through transformers to get different voltages, but always retain their original phase timing.

There is a very real, normal, (and deadly) AC voltage between any two phases and each phase is displaced 120 degrees from the other phase.

The voltage applied to the primary of the transformer is a perfectly normal AC voltage.

The other phase to phase voltages are the same, but occur 1/3rd of a cycle earlier or later.

A neutral may be introduced at one of the transformers or at the generating machine, but it does not affect the voltage between phases.

 

[sophiecentaur]

In that picture drawing can i get more info explained on how the two phases that are taking from the powerplant that are 120 degrees out of phase relative to neutral are then changed to 180 degrees out of phase

you seem set on not understanding this. The phase difference between the two supply phases is IRRELEVANT. There is no 'phase' difference between two of the company wires. There is just an alternating voltage. One signal, which cannot have a phase difference with itself.

 

[zgozvrm]

This is confusing to many people, even those who have been working with single- and three-phase systems for several years. The thing to remember is that a simple single-phase transformer (with one primary coil and one secondary coil) takes one sine wave (on the primary) and duplicates it on the secondary (typically, at a smaller amplitude). The primary and secondary waveforms are in phase with each other since the secondary is an induced copy of the primary (the secondary increases as the primary increases and it decreases as the primary decreases).

If you were to place another identical secondary coil (one with the same number of turns of wire in it) into the transformer so that it induces voltage from the same primary, it too would be in phase with the primary, and therefore with the first secondary. So now we would have 2 coils, each producing the same voltage, in phase with each other. Let's assume these coils are both inducing a voltage of 120 VAC each.

Now, if you were to reverse the leads of only one of the secondary coils, it would be 180 degrees out of phase with respect to the other. Both coils are still producing the same voltage, only now one coil's voltage would be increasing as the other decreases.

In either case, if you connect the coils end-to-end, their voltages add; in the case where they are in phase with each other the voltage measured across the 2 coils would be 120VAC + 120VAC = 240VAC. In the case where the coils are out of phase with each other (and therefore, so are their voltages), the voltage measured across the 2 coils would cancel out (120VAC + -120VAC = 0VAC).

When you use a center-tapped transformer (like the one drawn in vk6kro's post), you are basically splitting the secondary coil into 2 equal parts (there are an equal number of turns of wire on either side of the center tap), and in effect, you have 2 coils producing the same sine wave at half the amplitude (voltage). The confusion stems from the fact that the center tap is your ground reference. Looking at the left half of the split coil, you have the right side of the coil grounded. But, looking at the right half of the split coil, you have the left side of the coil grounded. This is just like swapping the leads of one of two secondary coils in relation to the first ... it changes your perspective.

 

[adrianace]

So the way i see it is by looking at two secondary coils beside each other with opposite polarities. So current travels from left to right in the first and right to left in the second.

Left coil left side is the bottom of the sine wave, and right coil right side is the top of the sine wave giving the difference of 240volts.

so its the phase differnece on a single sine wave. (Or signal.)

 

[sophiecentaur]

so its the phase differnece on a single sine wave. (Or signal.)

There is a 180° phase difference between the two sine waves. How can there be any confusion about that? Just because they are generated from the same original makes no difference.

 

[adrianace]

The confusion is in youir terminology using two sine waves.

Isnt it one sine wave . Unless your counting the primary coil as one sine wave and the secondary as another sine wave, both are the same sine wave appearing in two different coils. In which case the sine waves appear in phase with each other.

It seems its the reversed polarity of the two secondary coils that's the important part to understanding North american residential single phase transformers.

 

[cabraham]

That's exactly it. By the way, you hooked the house up with a delta source in your drawing. I'm pretty sure that delta is used for long distance transmission, because you only need three wires, but residential lines are in a wye configuration for safety (wye can be grounded). There's a delta-wye transformation somewhere in between long distance transmission and residential distribution.



Delta has no ground. The hot wires serve as both the send and return path of current. If the loads are balanced, there is no return current at all. Note that we're talking about current in the frequency domain here. If the loads become unbalanced, the voltages become unbalanced as the currents return through neighboring wires. In wye, the current always go out through hot wires and returns through the neutral. That means that each leg can draw whatever current it wants without disturbing the other legs.

The advantage of delta is that it only requires three wires as opposed to wye's 4 wires. The advantage of wye is that it can survive being unbalanced. Wye can be safer too because you can ground it. That's why both are used.

Deakn, maybe you could look at a diagram for a delta-wye transformer to get an idea of delta and wye are related. A "leg" is simply a sampling of one voltage difference.

Let's circle back around to your original question. I think there was a bit of initial confusion because the US system is totally different from European systems. The three-phase system is not important inside a US home. You won't have to deal with any three-phase power unless you work at an industrial park that uses lots of juice. You were right about the phases of the two buses being out of phase by 180 degrees. Vk's drawing shows how the transformer accomplishes that.

I'm not sure who told you this, but please observe the following. A delta is, of course, a 3 wire system. But the voltages stay balanced even if the currents are very unbalanced. A wye with only 3 wires can remain balanced voltage-wise even when currents are very unbalanced. The key is to use at least 1 delta winding in each transformer.

If a primary is wye, with a secondary delta, should the secondary load currents become very unbalanced, the secondary voltages remain balanced even if the primary wye has no neutral connection. The only case where wye connection requires 4 wires is the wye-wye connection w/o a delta. If both primary & secondary are wye, and the load currents are unbalanced, then the voltages will unbalance without a neutral connection. With a neutral, the voltages stay balanced. The neutral carries the unbalanced current. An exception to this rule involves the 3 legged E core type of transformer known as "3 phase core type". This xfmr can have a wye-wye pri-sec with no delta at all, and maintain balanced voltages with unbalanced currents, without a 4th wire. The reference books detail the reasons for this.

Neutral connections to Earth are for lightning stroke and safety reasons. Neutrals do not carry the unbalanced phase currents except for a very small amount due to current division among parallel paths..

Again, if just 1 winding of the transformer is delta, the other windings in wye stay balanced w/o a 4th wire even if loads are unbalanced. A good power reference book details all this. To summarize, 3 phase power transmission requires only 3 wires under all load conditions unbalanced or balanced, unless a wye-wye xfmr is used w/o a delta & w/o a 3 legged "E" type of core. In such a case, a 3rd winding connected in delta is needed, or a neutral (4th) wire must be used.

Claude

 

[sophiecentaur]

The confusion is in youir terminology using two sine waves.

Isnt it one sine wave . Unless your counting the primary coil as one sine wave and the secondary as another sine wave, both are the same sine wave appearing in two different coils. In which case the sine waves appear in phase with each other.

It seems its the reversed polarity of the two secondary coils that's the important part to understanding North american residential single phase transformers.

If you Earth one end of each winding then the voltages on the 'other ends' will either be in phase or 180° out of phase (giving 0 V or 240 V), depending upon which ends you choose.

Although the word "phase" can be commonly used to describe one of the three voltages on a three (/multiphase) system, it is strictly a term to describe the timing difference between two AC waveforms. The problem only occurs in Power Engineering discussions but it really needs to be spelled out when the term is used. People can spend dozens of posts talking at cross purposes if it's not. That pesky US 'two phase' system is to blame for most of the confusion.

 

[sophiecentaur]

I'm not sure who told you this, but please observe the following. A delta is, of course, a 3 wire system. But the voltages stay balanced even if the currents are very unbalanced. A wye with only 3 wires can remain balanced voltage-wise even when currents are very unbalanced. The key is to use at least 1 delta winding in each transformer.

If a primary is wye, with a secondary delta, should the secondary load currents become very unbalanced, the secondary voltages remain balanced even if the primary wye has no neutral connection. The only case where wye connection requires 4 wires is the wye-wye connection w/o a delta. If both primary & secondary are wye, and the load currents are unbalanced, then the voltages will unbalance without a neutral connection. With a neutral, the voltages stay balanced. The neutral carries the unbalanced current. An exception to this rule involves the 3 legged E core type of transformer known as "3 phase core type". This xfmr can have a wye-wye pri-sec with no delta at all, and maintain balanced voltages with unbalanced currents, without a 4th wire. The reference books detail the reasons for this.

Neutral connections to Earth are for lightning stroke and safety reasons. Neutrals do not carry the unbalanced phase currents except for a very small amount due to current division among parallel paths..

Again, if just 1 winding of the transformer is delta, the other windings in wye stay balanced w/o a 4th wire even if loads are unbalanced. A good power reference book details all this. To summarize, 3 phase power transmission requires only 3 wires under all load conditions unbalanced or balanced, unless a wye-wye xfmr is used w/o a delta & w/o a 3 legged "E" type of core. In such a case, a 3rd winding connected in delta is needed, or a neutral (4th) wire must be used.

Claude

If you want to distribute local low voltage (UK system) from a three phase transformer, how is this possible without a star secondary? If you distribute by connecting each (single phase) consumer across each arm of a delta transformer then everyone is working at a strange 'floating' voltage with no neutral. They would all get a 230V (UK) supply but you could be getting unneccessary high volts between adjacent house supplies.
Or are we, yet again, talking at trans-Atlantic cross purposes? It strikes me that we could be.

 

[cabraham]

If you want to distribute local low voltage (UK system) from a three phase transformer, how is this possible without a star secondary? If you distribute by connecting each (single phase) consumer across each arm of a delta transformer then everyone is working at a strange 'floating' voltage with no neutral. They would all get a 230V (UK) supply but you could be getting unneccessary high volts between adjacent house supplies.
Or are we, yet again, talking at trans-Atlantic cross purposes? It strikes me that we could be.

The secondary need not be wye, it could be delta. I don't know about UK, but here in US, the 3 legs of the delta are fed into 3 single phase transformers. The secondaries of said xfmrs are grounded. The delta can be grounded at the corner or center tap of one winding.

Yet some secondaries are wye connected, as it provides a neutral, ideal for grounding. But the neutral on the primary side, if primary is a wye, need not carry 4 wires, if a delta is present. That was my point. Wye neutrals are grounded for safety, not for carrying current unbalance. The delta provides that function.

Regarding the case you mentioned, if it is desirable to wye connect the secondary, then there are 3 options that will work without using the 4th wire to carry current. They are:

1) Use a delta primary. This assures that all 3 phase voltages stay balanced even when load currents are unbalanced. The wye secondary neutral is grounded for safety. The downstream single phase xfmr primary can be fed by 2 hot lines from the wye secondary. The single phase secondary gets grounded.

2) Use a wye for both primary & secondary, but add a 3rd winding connected in delta. The delta can be internal and no leads need be brought outside as the delta does not power any loads. Should the secondary wye load current go unbalanced, the delta will circulate current and the unbalanced currents will be carried in the 3 hot lines of the primary wye. No 4th wire is needed, except for safety.

3) Use a wye for both primary & secondary with a 3-legged E core, known as a "3 phase core type construction". If the secondary wye load is unbalanced, the primary wye maintains balanced voltage with just 3 wires. Again, the neutral is not needed to maintain balance, but only for safety.

Does this help?

Claude

 

[sophiecentaur]

Once you have grounded one corner of a delta system, the three cables are not the same; you are committed to one of them being grounded until the next transformer comes along. This is not relevant to distributing power at a local level as is done in the UK. You are making assumptions about the UK system.
Consumers do not have their own transformer, as in the US! They get low voltage power from a common three phase step down transformer at a local sub-station (feeding, perhaps a hundred homes). All consumers need to be connected in the same way and, as stated earlier, they are fed 1,2,3,1,2,3,1,2,3 all the way along the street from three cables and a neutral (from a wye secondary winding). I should be interested in any idea you could have as to how they could be fed from three windings on a delta connected secondary.
Heavy users get a HV three phase supply directly to their premises and they can do what they like with that, of course.

As I have said on several occasions, we are talking (even shouting, haha) at cross purposes.

As far as balance is concerned, is it not true to say that unbalanced power is coped with in a wye connection by current flowing back to the generator in the neutral and by asymmetrical voltages (wrt ground) on a delta system?

 

[cabraham]

Once you have grounded one corner of a delta system, the three cables are not the same; you are committed to one of them being grounded until the next transformer comes along. This is not relevant to distributing power at a local level as is done in the UK. You are making assumptions about the UK system.
Consumers do not have their own transformer, as in the US! They get low voltage power from a common three phase step down transformer at a local sub-station (feeding, perhaps a hundred homes). All consumers need to be connected in the same way and, as stated earlier, they are fed 1,2,3,1,2,3,1,2,3 all the way along the street from three cables and a neutral (from a wye secondary winding). I should be interested in any idea you could have as to how they could be fed from three windings on a delta connected secondary.
Heavy users get a HV three phase supply directly to their premises and they can do what they like with that, of course.

As I have said on several occasions, we are talking (even shouting, haha) at cross purposes.

As far as balance is concerned, is it not true to say that unbalanced power is coped with in a wye connection by current flowing back to the generator in the neutral and by asymmetrical voltages (wrt ground) on a delta system?

First let's address your quote in bold. I said that current flowing back to the generator in the neutral is a condition we wish to avoid. With a delta winding somewhere in the transformer, the unbalanced current does not return to the generator through the neutral. My point was that we wish to not use a neutral for current carrying, but only for safety. You misquoted me. Also, with a delta, the voltages stay balanced even when the currents are unbalanced, without neutral current at all.

Regarding the delta with a ground, I personally don't like it and would rather use a wye because the neutral is ideal for grounding. I did cover the case where both primary & secondary are wye connected. If a 3rd winding connected in delta is provided, the neutral on the wye primary does not carry unbalanced current back to the generator. The unbalanced currents circulate inside the delta tertiary winding. This is so well known I will not debate it.

Is that clear? Do I need to elaborate? BR.

Claude

 

[sophiecentaur]

You don't seem to be taking my point about supplying homes with single phases from the outputs of a three phase transformer. Are you implying it's not relevant? How could you connect people any other way?

 

[cabraham]

You can do it that way. If the secondary is wye with neutral grounded, then a hot wire & a neutral wire can be delivered to a home w/o a problem. Or a single phase xfmr can be used. The 2 hot wires from the wye (or hot & neutral) can be fed into a 1 phase xfmr. The 1 phase secondary can be grounded at either end of the winding, or center tapped like here in North America.

Different locales have their own methods of distribution and grounding. I was only pointing out that wye connected windings do not necessarily require a 4th wire to support unbalanced loads. Power reference manuals detail this along with the math. Does this help?

Claude

 

[sophiecentaur]

I see some of what you mean but I don't see why one should want three extra single phase transformers, co-sited with your three phase transformer, just to allow the use of a delta secondary. What they do on extended supply chain, with separate transformers on poles would clearly be very different. I assume that you do get what I'm saying about UK supplies.
I accept that you know what you're talking about on the unbalanced current thing. I shall have to read up on it. Power engineering is such a different world.

 

[cabraham]

The reason for the 3 single phase xfmrs, is not to allow for a delta, but for a center tapped 240V residential power. In N America it is common to have a 3 phase xfmr step down from 13 kV or so, to 2400V line-neutral, which is 4157V line-line if a Y is used on the secondary.

Then, 2 wires from the 2400/4157V 3 phase, 2 hots, or 1 hot plus 1 neutral, is fed into the primary of a 1 phase xfmr. The secondary is 240V center tapped. This center tap configuration means that each of the 2 outer hot wires is at a potential wrt Earth of just 120V rms.

We could, as you suggest, step the 13kV 3 phase down to 240V 3 phase and distribute to houses. But we don't have that center tap any more, which to me is a desirable thing. My understanding is that in Europe and other places, the house service is around 230V, with one side grounded, not center tapped. The hot wire has a potential of 230V rms wrt earth. This is IMHO less safe than the center tap system in N America.

I don't wish to start a war about which system is better, so let's just say they differ and let it go at that. BR.

Claude

 

[sophiecentaur]

The reason for the 3 single phase xfmrs, is not to allow for a delta, but for a center tapped 240V residential power. In N America it is common to have a 3 phase xfmr step down from 13 kV or so, to 2400V line-neutral, which is 4157V line-line if a Y is used on the secondary.

Then, 2 wires from the 2400/4157V 3 phase, 2 hots, or 1 hot plus 1 neutral, is fed into the primary of a 1 phase xfmr. The secondary is 240V center tapped. This center tap configuration means that each of the 2 outer hot wires is at a potential wrt Earth of just 120V rms.

We could, as you suggest, step the 13kV 3 phase down to 240V 3 phase and distribute to houses. But we don't have that center tap any more, which to me is a desirable thing. My understanding is that in Europe and other places, the house service is around 230V, with one side grounded, not center tapped. The hot wire has a potential of 230V rms wrt earth. This is IMHO less safe than the center tap system in N America.

I don't wish to start a war about which system is better, so let's just say they differ and let it go at that. BR.

Claude

Precisely. I get the impression that many (/most) houses in the US have their own transformer. Quite the opposite in the UK; virtually no one has their own transformer. Because we never used the 100V(ish) standard, we stuck with the same single (about twice) value of supply volts as cables can supply all domestic needs at 230V without being too thick. The original decisions were based on very different population densities in UK and US, I think.
Problem with using a centre tapped system in the UK model would be that you wouldn't be able to supply premises with three phase off the same transformer (not an uncommon requirement) as single phase domestic supplies - you'd get contention between the neutrals, each of which would be grounded somewhere. You are certainly right about the better inherent safety, though.

It's too late to change now, in any case. The only aggro these days seems to be between two sets of forum members who are unaware of the differences and make correct assertions about their own system that the other guys think are wrong because it's not the same on their side of the pond.

 

[mheslep]

Precisely. I get the impression that many (/most) houses in the US have their own transformer. ...

The US layout has at least several houses per xfmr. Lightning strike to a xfmr often takes out the street.