LV distribution systems: single phase or 3 phase? 7

[grounded7]

Hi,

Its a pretty basic question (embarassingly enough) but I am somehow confused about the phases on 415V distribution systems (typical neighbourhood voltage) in the UK.

Some references quote that voltage is stepped down from 11kV (3 phase) to 415V (3 phase) at the end of the street and this is then distributed as seperate 1-phase supplies, one phase per street or groups of streets, so that each house in the street gets a single phase supply.

While others state, that the 11kV (3 phase) is stepped down to 415V (single phase) and this is then distributed as it is, which I take is equivalent to 240 V (3 phase) supply.

Neither of these statements might be correct. The more I probe in, the more confused I get. Could someone kindly explain this stage of distribution to me?

Thanks.

 

[waross]

When only single phase is required a 240 Volt transformer may be used.
When three phase is required, a 240/415 Volt transformer may be used or three 240 Volt single phase transformers may be connected in wye for 240/415 Volts.
As the secondary system is four wire, 240/415 volts, 240 Volts single phase services may be connected to the 240/415 Volt distribution.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

It is most likely that the first statement is correct. You could use a single phase transformer to step down from 11 kV to 240 V and serve all customers on two 11 kV phases. If the load is small (only a few houses), this might be the most economical option.

 

[grounded7]

Thanks Bill and jghrist. Just to probe in a bit more, is the wiring in a house divided into sub-phases?

When you see the switching board in your house, there are a number of 'phases' (as people generally refer to them) that control a certain area of wiring in the house (and trip if something goes wrong). E.g. the kitchen has a seperate 'trip-switch', the TV room has another and so on.

Are these 'wiring phases' in the house different from the ones we talk about in t&d systems?

 

[Marmite]

There are anomalies for historic reasons, but generally in the UK there are distribution substations every 300m or so in suburban residential areas where the voltage is transformed from 11kV to 415V 3 phase. The transformer feeds a 4 or 5 way LV fuseboard to which LV underground cables (3 phase + neutral) are connected. Each LV cable would head off down a different street and may serve a number of streets. The domestic services are single phase services 240V phase to neutral tapped off the distribution LV main in the street and distributed as evenly as possible over the three phases to balance voltage drop on the main.
Regards
Marmite

 

[ScottyUK]

One thing you might be missing is that 11kV 3ph transformers to 415V 3ph+N. 415V exists between any two lines, but only 240V exists between any line and neutral. Domestic services are tapped off from one of the lines and the neutral, which is a single phase supply. Utilities normally try to ensure that the current drawn from each line is roughly equal by connecting a similar number of properties to each line.


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If we learn from our mistakes I'm getting a great education!

 

[grounded7]

Thank you all for the input. ScottyUK, you mentioned that domestic supplies are taken from one of the lines. Does it mean that a house is fed in by a 240V 1ph supply? And if the utilities try to keep current equal in all the lines; would it mean that if the requirement of a house is greater than the standard current (say x A), this house would be fed in by an additional phase? making it a 240V 2ph supply?

 

[Marmite]

Most domestic properties are more than amply serviced by a single phase 240V supply. However in the event that this is inadequate, the utility may install a "bottom side loop" which is a further fuse unit and meter fed from the same service cable, an additional single phase service fed from a different phase to the original service, or a 3 phase service with 2 or 3 phases metered depending on the load. The normal domestic cut out (main fuse) is rated at 100A and would be fitted with an 80A or 100A cartridge fuse.
Is there a specific problem or scenario you need an answer to, or advice on, or are you looking for generalised answers?
Regards
Marmite

 

[ccjersey]

""""
would it mean that if the requirement of a house is greater than the standard current (say x A), this house would be fed in by an additional phase? making it a 240V 2ph supply?
"""""

No. It would be possible to supply the house with 415/240 single phase, but that isn't a 2 phase supply.

Part of your confusion comes from the difference between "lines" and "phases" and the fact that many people use the two terms interchangably and incorrectly.

They can be the same thing, for example the utility normally distributes power with 3 lines/3 phases + a neutral conductor. Each line is a different phase.

Or they can be different. In the supply to a single phase panel board, there will be two "lines" or conductors but only a single phase. The tranformer serving the property is connected to a single utility phase and the neutral,or a different transformer could be connected to 2 lines/phases, but it would still supply single phase power.

Two conductors can only supply single phase power. 3 phase has 3 conductors, no exceptions.

Now to confuse things a bit, 3 phase can be derived from 2 lines/phases and a neutral by connecting transformers in an open Y/open delta arrangement. Notice that 3 conductors are still required to supply 3 phase power. Open delta is not used as often here in the US as it was in the past. Instead the vast majority of 3 phase service is supplied with 3 lines/phases and the utility neutral is used only for single phase transformers.

 

[waross]

I disagree with most of what you say, ccjersy.
If your service is 415/240V you have two of three phases. Single phase would be 480/240V

Part of your confusion comes from the difference between "lines" and "phases" and the fact that many people use the two terms interchangably and incorrectly.

Your statements are proof of this.

Two conductors can only supply single phase power. 3 phase has 3 conductors, no exceptions.

There are probably several million three phase panels in North America using four wires.

Most utilities have a limit to the maximum size of single phase service that they will allow. If you require more power than is allowed with single phase you must install a three phase service.
In some high density areas, true single phase may not be available. If there is a 120/208 V grid, two phases may be used instead of true single phase. The difference is most noticeable if 240V appliances are used and you are waiting for a hot water tank to recover or for your clothes to dry. This system is popular in apartment buildings and millions of people are unaware of the difference between 208V and 240V.
If you are trouble shooting there is also a difference in the expected neutral current between 120/240V and 120/208V.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Bill,

Europe, not North America. The 415 would be phase-phase voltage in a wye system, where the phase-neutral voltage is 240. Where you have 120/208 in your post, they are correct above to have 240/415.

Probably best to let North Americans deal with questions about services in North America and let Europeans deal with questions about services in Europe. Too many differences, some blatant and others quite subtle, between how it is done on each side of the puddle to get good answers from the other side.

 

[ScottyUK]

Over here I would consider that a 'line' is one of the live conductors, but not a neutral.
'Line voltage' is the voltage between two lines.
'Phase voltage' is the voltage between line and neutral.

I'm willing to be corrected if my brain has slipped into neutral.

We don't do strange tricks like corner-grounded delta, centre-tapped windings, open delta transformer banks (or not that I've seen at least), and so on. All our public LV systems have a solidly grounded neutral, and in the UK the connections to the systems are as described by Marmite. I admit to not having seen a bottom side loop - maybe not used by my utility (NEEB)?


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If we learn from our mistakes I'm getting a great education!

 

[ccjersey]

Bill,
With all due respect, 3 phase has to have 3 lines, true? It may also have a neutral, but that is optional.

In the case of utility distribution, the utility side neutral is not used for 3 phase Y services. To the best of my knowledge, the most common connection is delta on the utility side and Y on the customer side. Even if a Y/Y 3 phase transformer is used, the high side neutral is still not connected is it?

Utility distribution neutral is used for a 3 phase connection with the open Y/open delta 3 phase and for single phase services connected line/phase to neutral.

My understanding of 2 phase power is that the electrical displacement between the voltages of the phases is 90 degrees vs 120 degrees for 3 phase.

http://en.wikipedia.org/wiki/Two-phase_electric_power

I am way out of my depth here and only posted because I thought I could understand the original poster's confusion, experiencing a lot of it myself.

 

[davidbeach]

Utility service transformers are often wye-wye, and when so the neutral on the high side is generally connected.

 

[EvilJeb]

ccjersey,

I'm pretty sure your original post was mostly correct; it just had a few slip-ups. Bill took issue with what you said about numbers of conductors, but you were totally correct that a service with only two conductors will always be only a single-phase service, even if it is derived from two phases on the transmission/distribution side.


For the benefit of the original poster, who was not sure what is meant by phase, it generally means a separate wire on which the alternating-voltage waveform reaches its peak at a different time. When I was still a child, I envisioned AC voltage as a square wave, but actually it is a sine wave. Look it up if you don't know the difference, and recognize that a sine wave varies continuously, repeating the same wave form 50 times per second (or 60 times over here in the USA). Because the available voltage crosses through zero 100 times each second, there is no power available from a single-phase supply at those times. Hence, a phase can be added, another wire with a voltage that varies at the same frequency, but crosses the zero point at a different time.

Years ago, there were two-phase systems with the phases 90 degrees apart. One wire would be at maximum voltage (positive or negative) while the other one was at zero. There are very few of these systems still in existence. All modern power distribution is done with three phases, evenly spaced 120 degrees apart.

Your main power line in the street has 3 hot or phase conductors and one neutral (grounded) conductor. If you take a voltage reading from phase to phase, it is 415 volts, phase to neutral it is 240. Your service should be derived from a single phase conductor and the neutral conductor, in which case it is a single-phase service. The switches on your panel board would be known as "circuits" here in the United States. If they call them "phases" there in the UK, it is technically an incorrect use of the term "phase."

And, if your house requires more power than a single phase service can provide, it would normally be done by bringing power in from all three phases, and the neutral, for a 4-wire, 3-phase service. You possibly could have a 3-wire service using only two of the phases and the neutral, but I think most people would say you couldn't really call that a two-phase service. True, if you looked at voltage to the neutral wire, you would have two separate 240 volt wires with the peak voltage occurring at different times. But it would be difficult to make use of that supply to drive a motor or piece of equipment, because the two are separated by 120 degrees, so there is no easy way to fill in the missing one that is 120 degrees separated from both. Two-phase equipment isn't built like that, so you would still only be able to run single-phase equipment from your service, although you would have two voltages to choose from, 240 or 415.

It actually is possible to derive a true three-phase service from the above-described two phases of a 3-phase system, but to do so, you have to put in a bank of two transformers, Y-connected on the primary, open-delta on the secondary, meaning there's an empty spot where the third transformer would be connected in a true delta configuration. If you do this, you have three phases available at the three corners of the open-delta, but you are still limited in total power to what your two phases can supply, and you would put an unbalanced load on the true three-phase supply that your system is derived from (because the third phase isn't being used).

 

[ScottyUK]

Nice summary Jeb.

FWIW - I've never heard of outgoing circuits from a DB being described as 'phases'. 'Circuits' sounds familiar enough.


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If we learn from our mistakes I'm getting a great education!

 

[grounded7]

Thanks a lot for taking it step by step, EvilJeb.

 

[waross]

Two phase, two phases.
The old two phase systems were as described by EvilJeb.
In North America it is common to feed single phase sub panels with two phases of a three phase service. Two places where you will run into this are panels in small shops in malls and in any apartment building that requires more than a 400 Amp single phase service.
Hi David. Your comments are accepted. I was not aware of the use of more than one service in the UK. This would be a code violation in most instances in North America.
I have had some exposure to single phase UK systems when installing 120:240V single phase generators that were configured in the UK.
Three phase;
North America uses 120:208 Volt systems with a grounded neutral.
When a 120 Volt circuit is taken from the 120:208 Volt system, the grounded line is still referred to as the neutral by way of it being the neutral of the three phase parent system.
The UK uses 240:415 Volt systems with a grounded neutral.
When a 240 Volt circuit is taken from the 240:415 Volt system, the grounded line is still referred to as the neutral by way of it being the neutral of the three phase parent system.
The systems are pretty similar except that UK voltages are double North American voltages.
Single phase systems may get a little confusing.
In North America, we use 120:240 Volt, center tapped transformers. The neutral point is grounded resulting in voltages to ground of;
L1, 120 Volts to ground.
Center tap, zero Volts to ground.
L2, 120 Volts to ground.
L1 to L2, 240 Volts.
I was told by the engineer who sold us the gen sets, and have seen references to the practice in the Eng-Tips forums, that 120 Volts is occasionally used in the UK to power hand tools in large shops.
The 120 Volts may be supplied by a 120:240 Volt, center tapped transformer.
When this is done, one of the 240 Volt lines is still grounded, giving voltages of;
L1, zero volts to ground.
Center tap, 120 Volts to ground.
L2, 240 Volts to ground.
L1 to L2, 240 Volts.
(This ground connection would be a code violation in North America.)
This may be the main source of confusion between UK and North American systems.
But, I may have been misinformed. Scotty I will defer to your comments.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ccjersey]

And I defer to everyone's!

I think we all know what we mean, but to put it on the screen, as Bill says a difference of an "s" changes the meaning.

A little too early in the morning after a long night working on a recalcitrant pump.

 

[ScottyUK]

Hi Bill,

Almost all right!

Over here portable tools - typically stuff which uses less than 3kW, so that covers pretty much any drill, breaker, saw, etc - are supplied from a 110V centre-tapped-earth supply. Normally this is derived from a 240/110V isolation transformer: the 110V secondary winding is centre-tapped, and the centre tap is connected solidly to earth. This results in a maximum shock potential of 55V to earth. For areas where there are a large number of users there are also 3-phase isolation transformers which supply 110V from two phases and therefore have a maximum potential of 63.5V to earth. Both solutions result in a much lower probability of a user receiving a fatal electric shock in the event of the cable being damaged or the tool developing a fault compared with using a 240V or 415V equivalent, especially if the transformer output is equipped with an instantaeous RCD (GFCI).

For locations with particularly high risk we also use 24V AC systems. These are typically used for lighting in wet locations where a shock is likely to be more severe or high risk activities such as vessel entry where a rescue or first aid to a shock victim would be unusually difficult. 24V systems suffer badly from volt-drop in the cables so they tend to be used for with fairly low wattage lighting. For that reason they are usually reserved for applications where they are really necessary and elsewhere we use 110V lighting where a decent sized lamp can be employed.

FWIW we no longer have 415/240V over here, not officially at least. UK nominal voltages are now 400/230V in blissful harmony with our European neighbours. My domestic supply is still stubbornly high at nearly 250V though, so draw your own conclusions just how harmonised things really are!


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If we learn from our mistakes I'm getting a great education!