Identifying overhead power lines (esp. UK)

[Guineafowl]

Has anyone got a good link to a guide to UK power lines? I hate looking up and not knowing what's there.

For example, my house has several poles leading to it, strung with one live 240V and one neutral, the latter being tapped to Earth at each pole.

What if there are two lines, strung side-by-side - is that the same? What about three - two phases and a neutral? Why do you never see four conductors, as you'd expect for 415V three phases and a neutral?

My main interest is the low voltage, domestic/commercial supplies, rather than great big pylons. And it's more about what each wire actually IS, rather than the voltage level.

 

[scottdave]

It does seem hard to find UK specific information about power transmission networks. I did come across this website, which has links to the various power distribution companies in UK. Perhaps you could find information on one of their websites. http://www2.nationalgrid.com/UK/Our-company/electricity/Distribution-Network-Operator-Companies/
I also took a look at www.IEEE.org but did not readily find anything that looked like what you want.

 

[PhanthomJay]

I don't know about U.K., but here in US the main line distribution from a substation is typically three phase 13 kV phase to phase voltage plus a neutral (4 wire system). A single phase is then tapped from the mainline onto the side streets plus a neutral ( about 8 kV phase to ground , 2 wire system) to a pole mounted transformer which steps the voltage down to 120/240 volts which supplies the home or homes via a 3 wire system (120 volts live wire to neutral, 240 volts between 2 live wires.

 

[scottdave]

Has anyone got a good link to a guide to UK power lines? ... What if there are two lines, strung side-by-side - is that the same? What about three - two phases and a neutral? Why do you never see four conductors, as you'd expect for 415V three phases and a neutral?

Again, I do not know the specific answer, but I could offer a possible answer. First: 4 wires is typically a Y configured 3-phase. This is commonly seen in the United States. Sometimes the neutral wire is a smaller wire, because if the loads are close to being balanced, there will not be much current flowing through the neutral.
And 3 wires could be used for 3 phase in Delta configuration. The difference is: in Delta, you use two of the wires (line to line) to get one of the phases, and delta does not have a neutral wire. The advantage is that you only need 3 wires. If you are going long distances, then there is a cost savings in material, as well as the physical loading on the poles.

It is possible to configure inside the transformer to convert between Y and Delta configuration. You could have a generator in Y configuration, with the neutral connected to Earth, then go to a transformer to convert to Delta. 3 wires go over transmission, then another transformer to convert back to Y (with the neutral connected to Earth again).
Here are a couple of links that may help. http://www.ecmweb.com/content/basics-delta-wye-transformers
and
https://en.wikipedia.org/wiki/Delta-wye_transformer

If you only see 2 conductors, then I'm not sure - perhaps a single phase (hot and neutral), is one possibility. Or maybe they are not even power lines (communication perhaps?). Do you have any pictures - especially of the attachment at the pole?

 

[Guineafowl]

I don't know about U.K., but here in US the main line distribution from a substation is typically three phase 13 kV phase to phase voltage plus a neutral (4 wire system). A single phase is then tapped from the mainline onto the side streets plus a neutral ( about 8 kV phase to ground , 2 wire system) to a pole mounted transformer which steps the voltage down to 120/240 volts which supplies the home or homes via a 3 wire system (120 volts live wire to neutral, 240 volts between 2 live wires.

Yes, I've always wondered why the US uses a split phase system, but there's already been a discussion about that. Seems like you go for lower voltage, but have (I think) less safe plugs/sockets etc. UK and Europe go for higher voltage but safer kit.

Where outdoor power tools are used, possibly in wet environments, there do exist portable 110V isolation transformers with a centre tap and dedicated tools, so that a potential shock from 240V to Earth is stepped down to only 55V, with 110V phase-to-phase to run the tool.

 

[Guineafowl]

Again, I do not know the specific answer, but I could offer a possible answer. First: 4 wires is typically a Y configured 3-phase. This is commonly seen in the United States. Sometimes the neutral wire is a smaller wire, because if the loads are close to being balanced, there will not be much current flowing through the neutral.
And 3 wires could be used for 3 phase in Delta configuration. The difference is: in Delta, you use two of the wires (line to line) to get one of the phases, and delta does not have a neutral wire. The advantage is that you only need 3 wires. If you are going long distances, then there is a cost savings in material, as well as the physical loading on the poles.

It is possible to configure inside the transformer to convert between Y and Delta configuration. You could have a generator in Y configuration, with the neutral connected to Earth, then go to a transformer to convert to Delta. 3 wires go over transmission, then another transformer to convert back to Y (with the neutral connected to Earth again).
Here are a couple of links that may help. http://www.ecmweb.com/content/basics-delta-wye-transformers
and
https://en.wikipedia.org/wiki/Delta-wye_transformer

If you only see 2 conductors, then I'm not sure - perhaps a single phase (hot and neutral), is one possibility. Or maybe they are not even power lines (communication perhaps?). Do you have any pictures - especially of the attachment at the pole?

That's probably the answer - three wires are a delta supply to save costs on an extra conductor, which are then converted to 4-wire Y at the supply.

Two wires must be phase and neutral - we don't use the split-phase system here (see above) as everything domestic/light commercial is 240V. I don't have any particular pictures - I just like to know what's there as I travel about. I won't get into pylons (although they appear to have multiples of three conductors), as there's a danger of becoming a pylon-spotter. Yes, they do exist.

 

[Guineafowl]

Next question: Standard UK domestic supply is single phase 240V rms, or about 340V peak. (Yes, it should be 230V, but it isn't).

Often, a three-phase supply to a farm or light workshop has a phase tapped off for the house. But 3 phase supplies are rated at 415V rms. How does the maths (or math, but I think there's more than one of it) work for getting 240V rms from a phase of a 415V rms supply?

 

[scottdave]

Transformers are used to step down the voltage to usable levels. In the United States, they look like a cylinder, mounted on the pole. There are also transformers at ground levels, for underground lines.
This is the big advantage of alternating current vs direct current: the ability to easily step up voltages for transmission, then step down at the customer.

 

[Guineafowl]

Transformers are used to step down the voltage to usable levels. In the United States, they look like a cylinder, mounted on the pole. There are also transformers at ground levels, for underground lines.
This is the big advantage of alternating current vs direct current: the ability to easily step up voltages for transmission, then step down at the customer.

Yes. At the house I'm moving into, which is quite large (I'm just in an offshoot/annexe) and has three phase right into my kitchen. Each of the three phases is tapped off to a different part of the main house without a transformer - that's the confusing bit. Each phase must be 240V rms, so perhaps the supply is not 415V 3ph. How do you calculate the rms value of three phases, 240V rms each, 120deg apart?

EDIT: Just found it out - The rms value for 3ph is 1.73*Vph, so if each phase's rms value is 240V, then the rms for all three is 1.73*240 = 415.

 

[Asymptotic]

Next question: Standard UK domestic supply is single phase 240V rms, or about 340V peak. (Yes, it should be 230V, but it isn't).

Often, a three-phase supply to a farm or light workshop has a phase tapped off for the house. But 3 phase supplies are rated at 415V rms. How does the maths (or math, but I think there's more than one of it) work for getting 240V rms from a phase of a 415V rms supply?

Provided the three phase transformer has a wye secondary, and the center point is brought out as neutral,
415V line-to-line voltage (Ph1 to Ph2, Ph2 to Ph3, and Ph1 to Ph3).
Phase voltage is between Ph1 to neutral, Ph2 to neutral, and Ph3 to neutral.
Phase voltage = Line Voltage/Sqrt(3).

For 415V line voltage, phase voltage = 415/1.732 = 239.6 volts.
In the US, nominal 480V line voltage results in 277V phase voltage.

Edit: Turns out my reply was too late; you figured it out yourself :)

 

[scottdave]

So what it sounds like you are describing: Line to Line voltage is 415V (measure between one phase and another phase, you get 415 V_rms). Divide this by 1.73 (square root of 3) to get the Line to Neutral voltage of 240 Volts_rms.

 

[Guineafowl]

Edit: Turns out my reply was too late; you figured it out yourself

Yes, thanks, but your answer's better.

An interesting rabbit hole to follow. I remember a while back wondering why a generator's ouput was given in kVA, not kW, and ended up learning all about power factors. But I have a friend who glazes over when I talk of this stuff, but is always keen to follow rabbit holes about the gossip around the village - what so-and-so has said and whether X and Y are quarrelling about somesuch. Then it's my turn to glaze over.

Perhaps an ideal employment/career assessment would involve finding out just which types of rabbit hole you follow when given free access to Google.

 

[scottdave]

This link may help to visualize the relationship between the phases. http://www.ni.com/white-paper/8216/en/

 

[Guineafowl]

OK, then. The large pylons here generally have two groups of three conductors (which are themselves doubled up - skin effect?) and a single, central wire above. Is the single wire some sort of equipotential bond, or a lightning conductor?

Are the two groups of three the same supply, but in parallel? Or from a separate transformer?

 

[scottdave]

OK, then. The large pylons here generally have two groups of three conductors (which are themselves doubled up - skin effect?) and a single, central wire above. Is the single wire some sort of equipotential bond, or a lightning conductor?

The wire above is a shield wire, to protect from lightning strikes, as you had guessed. http://www.transmission-line.net/2011/04/shield-wires-on-transmission-line.html

 

[PhanthomJay]

Are the two groups of three the same supply, but in parallel? Or from a separate transformer?

In general , the two groups of three phases are 2 separate circuits installed on a common set of structures to save on the corridor width and reduce installation costs of a separate set of pylons. Each phase sometimes consists of 2 wires to reduce wire diameters and field effects, and save on i^2r heating losses.

 

[Guineafowl]

save on i^2r heating losses.

So, if you wish to carry 10A:

For a single conductor, of resistance R1: P = 100.R1
For two conductors, 5A each, and higher resistance R2: P=2(25.R2)

Doubling up the conductors nearly halves your power loss, discounting other effects?

 

[Baarken]

This link (https://www.electrical4u.com/advantages-of-bundled-conductors/) gives a good introduction to why bundled conductors are used (i.e. three conductors per phase instead of one bigger conductor).

The main reasons are:

• It reduces the line inductance and thereby increasing the power transfer capability.
• It reduces the corona effect, which is a phenomena that occurs at high voltage lines (see https://en.wikipedia.org/wiki/Corona_discharge).

 

[Baarken]

Also, when we are talking about two three-phase circuits on the same tower, we do not double the power transfer capability by introducing this extra circuit as one would might think. See for example this thread: https://www.physicsforums.com/threads/amperes-law-to-calculate-the-b-field-from-a-balanced-three-phase-system.923994 and from post #22. Because the lines are relatively close to each other, there are magnetic coupling between the two circuits and therefore increasing the inductance.

As seen in the 1st link in post#18 in this thread the the power transfer can be written as (ignoring resistance):

[itex]P = \frac{V_sV_r}{X}\sin(\delta)[/itex]​

Where [itex]V_s[/itex] is the sending voltage, [itex]V_r[/itex] the receiving voltage, [itex]\delta[/itex] is the phase angle between the two voltage nodes and [itex]X[/itex] is the reactance between the two nodes.

We can see that an increase in inductance ([itex]X = j\omega L[/itex], where L is inductance and if we ignore the capacitance) leads to a reduction in the power transfer capability from the equation above.

 

[PhanthomJay]

So, if you wish to carry 10A:

For a single conductor, of resistance R1: P = 100.R1
For two conductors, 5A each, and higher resistance R2: P=2(25.R2)

Doubling up the conductors nearly halves your power loss, discounting other effects?

In this example, power loss is about halved if R1 = R2. But R2 is most likely going to be greater than R1, because smaller wire is generally used , so the loss savings won't be as much.

 

[Tom.G]

As for bundling multiple conductors, don't forget the skin depth. For instance in the USA the largest commonly available wire is size 0000 AWG (American Wire Guage), also known as 4/0 or 4-ought. It turns out that the skin depth at our 60Hz line frequency is the radius of 4/0 wire. So with a larger wire diameter, the central portion conducts less current... leading to multiple parallel conductors spaced a bit away from each other.