Question about Pylons (AC Mains Distribution Towers)

[Jimmy87]

Hi,

Please could someone explain in detail the arrangements of transmission cables in a UK pylon? In the image below there seems to be 3 sets of two wires on each side. As I understand it UK power stations produce 3-phase electricity so does each side come from two power stations? So this pylon would trace back to 4 power stations (12 cables total)? Also, I thought there would have to be at least one common neutral return wire to complete the circuit? Also why are they arranged in pairs? So I was expecting each set from a power station to come as 4 transmission lines (one for each phase and a neutral return).

Any explanations and insights are much appreciated.

Image source: https://commons.wikimedia.org/wiki/File:Electric_Pylon_-_geograph.org.uk_-_368353.jpg

 

[Baluncore]

Each phase is suspended from a long string of insulators, so it is a very high voltage AC transmission line.

Each close pair of wires forms one phase conductor. The two close parallel wires are used to reduce corona discharge from the thin wires at very high voltage.

The black bumps on each wire, on each side of the insulator are vibration dampers designed to stop wind induced vibration fatigue at the insulator support point.

The three conductor pairs on each side of the pylon make a three phase circuit, there are two circuits on that pylon, one 3PH circuit on each side. One generator will drive three phases through a 3PH transformer. The same three phases will be transformed down again at a sub-station for lower voltage distribution.

At the very top of the pylon there is an Earth wire to attract and conduct lightning strikes to ground via the pylon.

 

[Baluncore]

Also, I thought there would have to be at least one common neutral return wire to complete the circuit?

The elegant thing about 3PH AC is that only three conductors are needed to form a return circuit. The voltages and hopefully the currents on the three wires are separated in phase by 120°. The total current on the three conductors at any instant is zero, so the radiated magnetic fields cancel and are greatly reduced. The voltage on the three 3PH wires also averages zero, so it reduces the radiated electric field.
https://en.wikipedia.org/wiki/Three-phase_electric_power#Generation_and_distribution

 

[anorlunda]

@Baluncore covered all the important points very well.

One additional point. There are two circuits shown as @Baluncore said. The geographical origin and destination of the two are not necessarily the same. For example, coming from two power stations as @Jimmy87 guessed, but also perhaps delivering to two cities.

 

[Jimmy87]

The elegant thing about 3PH AC is that only three conductors are needed to form a return circuit. The voltages and hopefully the currents on the three wires are separated in phase by 120°. The total current on the three conductors at any instant is zero, so the radiated magnetic fields cancel and are greatly reduced. The voltage on the three 3PH wires also averages zero, so it reduces the radiated electric field.
https://en.wikipedia.org/wiki/Three-phase_electric_power#Generation_and_distribution

Thanks for lots of great detail from all - much appreciated. Could you explain why a return wire is not needed? An A.C. circuit still needs to be complete so if three wires go out (one for each phase) how can this form a complete circuit if there is no neutral return wire? Thanks.

 

[Baluncore]

Could you explain why a return wire is not needed?

The voltages and currents on the three wires are phase shifted AC. The voltage and current on each wire passes through zero twice for each cycle. When one wire is at zero volts, a second wire is positive (sourcing current), while the third wire is negative (sinking an identical current). In effect the three wires continuously take it in turn to play the part of the supply and the return wires of the circuit.

An A.C. circuit still needs to be complete so if three wires go out (one for each phase) how can this form a complete circuit if there is no neutral return wire?

If you imagine a phase wire at each point of a star or a Y, a neutral would be at the central point which would be near Earth potential. But there is no need for a neutral wire in a 3PH transmission line, because the sum of all phase currents is zero, there is nothing left to be conducted on a neutral.

 

[anorlunda]

This is a case where pictures are easier than words. Please watch this video.
At 2:30 into the video, they show that if there was a return wire in three phase, it would carry not current, and therefore it can be removed without having any effect.

 

[Jimmy87]

The voltages and currents on the three wires are phase shifted AC. The voltage and current on each wire passes through zero twice for each cycle. When one wire is at zero volts, a second wire is positive (sourcing current), while the third wire is negative (sinking an identical current). In effect the three wires continuously take it in turn to play the part of the supply and the return wires of the circuit.


If you imagine a phase wire at each point of a star or a Y, a neutral would be at the central point which would be near Earth potential. But there is no need for a neutral wire in a 3PH transmission line, because the sum of all phase currents is zero, there is nothing left to be conducted on a neutral.

Wow that's really interesting thanks.

 

[Jimmy87]

Each phase is suspended from a long string of insulators, so it is a very high voltage AC transmission line.

Each close pair of wires forms one phase conductor. The two close parallel wires are used to reduce corona discharge from the thin wires at very high voltage.

The black bumps on each wire, on each side of the insulator are vibration dampers designed to stop wind induced vibration fatigue at the insulator support point.

The three conductor pairs on each side of the pylon make a three phase circuit, there are two circuits on that pylon, one 3PH circuit on each side. One generator will drive three phases through a 3PH transformer. The same three phases will be transformed down again at a sub-station for lower voltage distribution.

At the very top of the pylon there is an Earth wire to attract and conduct lightning strikes to ground via the pylon.

Hi, saw another pylon arrangement the other day that got me thinking back to this post. This is a head on picture to pylon:

And this is to the side:

In the OP you said a pair of wires forms a single phase and are sent down as a pair to reduce corona discharge. What is happening here with each set going down as 3 wires with spacers? Would these be two sets of the same phase? Thanks

 

[Baluncore]

Each conductor is a bundle of 4 wires, all electrically connected together at every spacer and at the bottom of the insulator. Four wires makes the bundle appear to have a greater radius. A line will probably need 4 wires to reduce corona discharge if it is operating at or above 500 kV.

There are three of those conductor bundles for each 3 PH circuit. There are two circuits shown, one on each side of the pylon. Given that the pylon in the OP wiki example was located to the north of minor road, east of Efail Isaf, Llantwit Fardre, South Wales. UK.
You can Google Earth 51.553882°, -3.305573° Then street view and zoom in on the detail.

You can see the spacers between the pair, or bundle of wires;
https://en.wikipedia.org/wiki/Overhead_power_line#Bundle_conductors

The loops to prevent corona at the bottom of the insulator;
https://en.wikipedia.org/wiki/Corona_ring

and the vibration dampers;
https://en.wikipedia.org/wiki/Stockbridge_damper

 

[Jimmy87]

Each conductor is a bundle of 4 wires, all electrically connected together at every spacer and at the bottom of the insulator. Four wires makes the bundle appear to have a greater radius. A line will probably need 4 wires to reduce corona discharge if it is operating at or above 500 kV.

There are three of those conductor bundles for each 3 PH circuit. There are two circuits shown, one on each side of the pylon.Given that the pylon in the OP wiki example was located to the north of minor road, east of Efail Isaf, Llantwit Fardre, South Wales. UK.
You can Google Earth 51.553882°, -3.305573° Then street view and zoom in on the detail.

You can see the spacers between the pair, or bundle of wires;
https://en.wikipedia.org/wiki/Overhead_power_line#Bundle_conductors

The loops to prevent corona at the bottom of the insulator;
https://en.wikipedia.org/wiki/Corona_ring

and the vibration dampers;
https://en.wikipedia.org/wiki/Stockbridge_damper

View attachment 298979

Great thanks for such a detailed response. Just so I have understood correctly; So all four of those bundled wire sets form a single conductor of one phase? The three sets of four on one side form a complete 3 phase unit? I did some reading on corona discharge and it mentions increasing wire diameter reduces this effect so the 4 wire bundle effectively increases the wire diameter? Is the 4 wire bundle purely for his effect then - so if corona discharge wasn’t an issue they would all be single wires?

 

[Baluncore]

So all four of those bundled wire sets form a single conductor of one phase?

Correct.

The three sets of four on one side form a complete 3 phase unit?

Correct.

I did some reading on corona discharge and it mentions increasing wire diameter reduces this effect so the 4 wire bundle effectively increases the wire diameter?

That is right. For higher voltages use 6 or 8 wires in the bundle.

Is the 4 wire bundle purely for his effect then - so if corona discharge wasn’t an issue they would all be single wires?

The 4 wire bundle, with the wide spacer, is there only to reduce corona discharge.
Since 4 wires are used they can be thinner and still carry the same current as one thick wire.

Can you please post the latitude and longitude of the transmission line pictures in post #9.

 

[anorlunda]

I did some reading on corona discharge and it mentions increasing wire diameter reduces this effect so the 4 wire bundle effectively increases the wire diameter?

Yes. In fact, if the material in that bundle could be arranged to make a hollow cylinder, that would be good electrically. But unfortunately, it would be bad mechanically, easier to break or blow down compared to cables.

Just for curiosity, the circular arrangement can in theory be extended to the whole transmission line. The picture below shows two prototypes, one for a 12 phase and the other a 6 phase transmission. Those ideas were explored in the 70s, but discarded because the improved electrical performance was not enough to offset the increased costs. The interesting point is that, the more phases used, the more the circular arrangement is preferred both in the bundles for each phase, and in the arrangement of the bundles.

In all real world engineering, trade offs are evaluated. In power transmission, electrical, mechanical, cost, construction, and materials are all part of the trade offs.

 

[Jimmy87]

Correct.Correct.That is right. For higher voltages use 6 or 8 wires in the bundle. The 4 wire bundle, with the wide spacer, is there only to reduce corona discharge.
Since 4 wires are used they can be thinner and still carry the same current as one thick wire.

Can you please post the latitude and longitude of the transmission line pictures in post #9.

Thanks! So it says corona discharge happens above 30kV so I guess that transmission cables in post 9 is well above that. In the UK the highest voltage lines are 400kV. Since you said they come in bigger bundles of 6 and 8 would the 4 bundle in post 9 be less than 400kV then? Coordinates (I think) are:

51.9640195, -0.2055259

Can you find out more about these particular lines from those coordinates then?

 

[rbelli1]

Here are some with 6 per phase.
https://www.tdworld.com/overhead-tr...ultrahigh-voltage-line-powers-up-across-china

I saw similar ones all around Shanghai when I was there.

BoB

 

[anorlunda]

Here are some with 6 per phase.
https://www.tdworld.com/overhead-tr...ultrahigh-voltage-line-powers-up-across-china

I saw similar ones all around Shanghai when I was there.

BoB

Good sight. The higher the voltage, the more corona becomes a problem. If I remember right, the first time bundled conductors were used in the USA was on the first 345KV line.

 

[berkeman]

Here are some with 6 per phase.
https://www.tdworld.com/overhead-tr...ultrahigh-voltage-line-powers-up-across-china

I saw similar ones all around Shanghai when I was there.

Why do they use DC for those transmission lines?

 

[anorlunda]

Why do they use DC for those transmission lines?

There's something wrong in that article. It talks about China's HVDC, but the picture with the article is of an AC line, not DC.

DC allows connecting areas non-synchronized, or with different frequencies. The power flow in the DC line can be controlled by computers instantaneously, thus offering control and stability advantages to the grid.

DC lines also do not generate reactive power (VARs). With AC lines the higher the voltage the more VARs. Therefore, DC is able to push to higher voltages than AC.

The disadvantage is the rectifiers/inverters to interface to AC at the terminations and every intermediate tap point of the DC line. That stuff (below) is not cheap. For scale, see the man on the floor. The picture is of the HVDC tie between the North and South islands of New Zealand.

 

[rbelli1]

There's something wrong in that article.

I didn't even read the article. I was only interested in showing an example of the larger lines. I guess I should have found a "correct" article to link to.

BoB

 

[berkeman]

I guess I should have found a "correct" article to link to.

Dang pop-science sources!

 

[DaveE]

There's something wrong in that article. It talks about China's HVDC, but the picture with the article is of an AC line, not DC.

DC allows connecting areas non-synchronized, or with different frequencies. The power flow in the DC line can be controlled by computers instantaneously, thus offering control and stability advantages to the grid.

DC lines also do not generate reactive power (VARs). With AC lines the higher the voltage the more VARs. Therefore, DC is able to push to higher voltages than AC.

The disadvantage is the rectifiers/inverters to interface to AC at the terminations and every intermediate tap point of the DC line. That stuff (below) is not cheap. For scale, see the man on the floor. The picture is of the HVDC tie between the North and South islands of New Zealand.

View attachment 298993

Oh man, I want a tour of that facility lead by the chief EE and the systems designer. Maybe when the power is off, LOL.

Also, less transmission loss with HVDC because of no radiated power and lower current with really high voltage. I think this is the same as the comment above about VAR. This applies mostly to long transmission lines. The other type are short links between incompatible systems as mentioned.

 

[Baluncore]

Coordinates (I think) are: 51.9640195, -0.2055259
Can you find out more about these particular lines from those coordinates then?

Follow the lines on Google Earth. Use street view to read the name of the substation on the gate when you get there.
That is part of a National grid 400 kV, 32 km long link to Eaton Socon substation.
(52.214236°, -0.299694°) Lower voltage 132 kV is distributed from there.
Google 'Eaton Socon substation nationalgrid' for plenty of info about the new 400 kV transformers installed last year.

The southern end is at Wymondly Substation, a National Grid switchyard near Stevenage. (51.927200°, -0.250155°); Where the 400 kV lines run ESE and WSW.
That is shown on the page 3 map here;
https://www.nationalgrid.com/sites/default/files/documents/ETYS 2017 Appendix A.pdf

 

[Jimmy87]

Follow the lines on Google Earth. Use street view to read the name of the substation on the gate when you get there.
That is part of a National grid 400 kV, 32 km long link to Eaton Socon substation.
(52.214236°, -0.299694°) Lower voltage 132 kV is distributed from there.
Google 'Eaton Socon substation nationalgrid' for plenty of info about the new 400 kV transformers installed last year.

The southern end is at Wymondly Substation, a National Grid switchyard near Stevenage. (51.927200°, -0.250155°); Where the 400 kV lines run ESE and WSW.
That is shown on the page 3 map here;
https://www.nationalgrid.com/sites/default/files/documents/ETYS 2017 Appendix A.pdf

Excellent thanks, that’s really interesting! Could you explain how the corona discharge works. I have read up on it already and it says above 30kV the potential difference is high enough to cause this effect where the air breaks down and becomes conductive. Does each cable/phase cause this breakdown around its own cable or is it the potential difference between the difference phased bundles that causes it to break down?

 

[anorlunda]

It's time for you to do some of your own study, rather than just ask questions. I suggest you read this first, then ask questions after.

https://en.wikipedia.org/wiki/Corona_discharge

 

[pbuk]

Those ideas were explored in the 70s, but discarded because the improved electrical performance was not enough to offset the increased costs.
View attachment 298982

OMG I am sure I remember seeing pylons like those near Didcot power station in Oxfordshire in the late 60s/early 70s. I had forgotton about them until now.

 

[Baluncore]

Yes. In fact, if the material in that bundle could be arranged to make a hollow cylinder, that would be good electrically.

In those examples the parallel wires are carrying different phases of the balanced circuit, so it does not actually reduce corona discharge. It reduces the external E and M field losses from the transmission line.

A bundle of electrically connected wires make a single conductor with a greater virtual diameter, which reduces the corona discharge. The bundle has greater capacitance to other bundles, but with a lower self inductance than a single wire. Then since; Zo = √(L/C), it lowers the characteristic impedance of the transmission line.

When used for radio antennas, the bundle technique is called a “cage dipole”. A dipole was often slung between two masts of a ship, where the ends were tapered to reduce capacitance at the masts. The advantage of a cage at RF is that the diameter can be tapered along the dipole, which sharpens the tuning by reducing losses due to impedance changes along the line.