EMFs.info

Electric and magnetic fields and health

index/glossary | EMFs At A Glance | EMF The Facts (pdf)
  • What are EMFs
    • Terminology – an introduction
    • Electric fields
    • Magnetic fields
    • Units for measuring EMFs
    • Measuring and calculating EMFs
      • “EMF Commercial”
    • Adding fields together
    • Radiofrequencies
    • Screening EMFs
  • Sources
    • Overhead power lines
      • Fields from specific power lines
        • 400 kV
        • 400 kV – specific cases
        • 275 kV
        • 132 kV
        • 66 kV
        • 33 kV
        • 11 kV
        • 400 V/230 V
        • Replacing a 132 kV line with a 400 kV line
      • Summaries of fields from all power lines
      • Factors affecting the field from a power line
        • Voltage
        • Current
        • Clearance
        • Height above ground
        • Conductor bundle
        • Phasing
        • Balance between circuits
        • Balance within circuit
        • Ground resistivity
        • Two parallel lines
      • Calculating and measuring fields from power lines
        • Geometries of power lines
        • Raw data
        • On-line calculator
      • Fields from power lines – more detail on the physics
        • Field lines from a power line
        • The direction of the field from a power line
        • Power law variations in the field from a power line
      • Statistics of power line fields
    • Underground power cables
      • Different types of underground cable
      • Fields from cables in tunnels
      • Gas Insulated Lines (GIL)
      • Underground cables with multiple conductors
      • Effect of height on fields from underground cables
      • Screening fields from underground cables
    • Low-voltage distribution
      • UK distribution wiring
      • USA distribution wiring
    • House wiring
    • Substations
      • National Grid substations
        • Static Var Compensators
      • Sealing-end compounds
      • Distribution substations
      • Final distribution substations
        • Indoor substations
    • Transport
      • EMFs from electric trains (UK)
      • EMFs from cars
    • Appliances
    • Electricity meters
      • Smart meters
      • Traditional meters
    • Occupational exposures
      • Live-line work
      • Static Var Compensators
      • Occupational exposures on pylons
    • Field levels and exposures
      • Personal exposure
      • Other factors that vary with magnetic fields
      • Fields greater than 0.2 or 0.4 µT
    • Screening EMFs
      • Screening fields from underground cables
      • EMF Reduction Devices
  • Known effects
    • Induced currents and fields
    • Microshocks
      • Control of microshocks in the UK
      • Microshocks from bicycles
      • Bees and microshocks
    • EMFs and medical devices
      • Standards relating to pacemakers and other AIMDs
    • Effects of EMFs on equipment
  • Research
    • Types of research
    • Epidemiology
    • Animal and laboratory experiments
    • Mechanisms
    • Specific studies
      • UKCCS
      • CCRG
      • French Geocap study
      • CEGB cohort
      • Imperial College study
  • Current evidence on health
    • Childhood leukaemia
      • Survival from childhood leukaemia
      • Childhood leukaemia and Downs
      • Childhood leukaemia and night-time exposure
      • The “contact current” hypothesis
    • Other health effects
    • Scientific review bodies
      • WHO
      • IARC
    • Electric fields and ions
    • Comparing EMFs to other issues
  • Exposure limits for people
    • Limits in the UK
    • Limits in the EU
    • Limits in the USA
    • Limits in the rest of the world
    • Limits from specific organisations
      • ICNIRP 1998
      • ICNIRP 2010
      • NRPB 1993
      • NRPB 2004
      • EU 2004
      • EU 2013
  • Policy
    • UK policy
      • Power lines and property – UK
    • Compliance with exposure limits
    • European EMF policy
    • Precaution
    • SAGE
      • SAGE First Interim Assessment
        • Government response to SAGE First Interim Assessment
      • SAGE Second Interim Assessment
        • Government response to SAGE Second Interim Assessment
        • SAGE Second Interim Assessment – the full list of recommendations
  • Finding out more
    • EMF measurement and commercial services
    • Links
    • Literature
    • Contact us
  • Static fields
    • Static fields – the expert view
You are here: Home / Sources / Overhead power lines / Factors affecting the field from a power line / Phasing / Optimum phasing of triangular phase arrangements

Optimum phasing of triangular phase arrangements

This page discusses (at some length!) how the concept of "optimum phasing" works out for overhead lines where the conductor bundles (or phases) are arranged in a triangle rather than a straight line.  See our main page on phasing including details of the UK policy to adopt optimum phasing.

photomontage of t-pylonThis is prompted by the introduction by National Grid of the T-pylon, an example of exactly this triangular arrangement, but the principles hold true for any other triangular arrangement.

 
 
 

Phasing for linear arrays

First, a recap on how optimum phasing works for traditional lattice pylons, where the phases for each circuit are roughly in a straight line, or a "linear array".

"Untransposed" phasing has the same order of the three phases on the two sides of the pylon (the two circuits), and the magnetic fields are in the same direction and reinforce each other:

animation of untransposed phasing

Note: to illustrate the principles as clearly as possible, we've simplified a bit - we've considered circuits that are exactly vertical rather than sloping slightly because of the taper of the pylon, and we're considering the field level with centre of the conductors rather than at ground level.  Both of those factors introduce some elliptical polarisation rather than the linear polarisation we show here.  You can see this in more detail on a separate page.

"Transposed" phasing has the order of the phases in the opposite direction on the two sides.  The fields are in opposite directions (exactly so in this simplified diagram, very nearly so for real-life pylons which tend to have a slight taper) and there is partial cancellation:

animation of transposed phasing

This produces a lower overall field to the sides of the line, so we call it "optimum" phasing.

Phasing for triangular arrays

Linear arrays (as above) produce linear fields.  The key difference is that triangular arrays produce fields that rotate instead of just oscillating backwards and forwards in a straight line - "elliptical polarisation".  For any triangle, the field goes round an ellipse.  For the phases in an equilateral triangle, which is what we will consider here for simplicity, the field goes round an exact circle.

With two triangular arrays, suppose we fix the order of the phases on one side (the left side for convenience).  There are then, mathematically, six different possible orders of the phasing on the other side.  They each produce a different field as shown here:

graph of all six phasing options for T-pylon

Note: the purpose of this graph is to show how changing the phasing changes the field.  So the absolute values of field aren't so important.  But in fact, this graph is calculated for the same conditions we use to illustrate all the other factors affecting the field from power lines - 12 m ground clearance and 500 A per circuit, fairly typical of normal conditions for UK power lines.

We will return to this graph later and make sense of all the various different curves.

The magnetic field from each circuit rotates in a circle, but just as for the linear arrays, the worst case is untransposed phasing, with the same order of phases on the two sides.  Then the two fields rotate in the same way, reinforcing each other:

animation of t-pylon with unstransposed phasing

(please note that these diagrams are designed to show the principles at work and are not to scale - the resultant field to the sides is actually a lot smaller because of the way fields fall with distance, so we've amplified it so you can see what is happening more clearly.)

But now consider transposed phasing (which was the best option for linear arrays):

animation of t-pylon with transposed phasing

The horizontal components of the field are behaving exactly as they did for the linear array - they are opposing each other and partly cancel each other (you are closer to one circuit than the other, so the field from that circuit is stronger, so the cancellation isn't perfect).  But the vertical components are reinforcing each other.  This means transposed phasing is not actually the optimum phasing for triangular arrays.

The best phasing would be if we could have the two fields exactly opposite each other (180 degrees apart) and rotating the same way:

animation of t-pylon with phases 180 degrees apart

Unfortunately, you can't achieve this with a T-pylon or similar - you'd need conductors on one circuit positioned half way between where they actually are - the triangle pointing up instead of down.

The best we can do is to have the two fields 120 degrees apart instead of the ideal 180 degrees (in this diagram, the field from the right-hand circuit is following the field from the left-hand circuit round but 120 degrees behind it):

animation of T-pylon with optimum phasing

To show the effect of these options, this graph shows the same six possibilities as before, but highlighting these three in particular:

  • untransposed, clearly the worst to the sides;
  • transposed, better but not the best;
  • and the optimum.

graph showing optimum phasing of T-pylon

Note that we are concerned with reducing the field to the sides of the line, where people are most likely to live or to spend time.  Close to the line, and in particular between the two circuits, the relative merits can actually reverse - this often happens with phasing and we explain why on a separate page.

Summary so far

  • The optimum phasing for T-pylons isn't transposed phasing, it's another arrangement with the fields 120 degrees apart
  • In any event, optimum phasing can never work as well for T-pylons as for traditional lattice towers

The next two drop-down boxes now go on to explain what happens to the phasing when you need to transition between different types of pylon.

Simultaneous phasing of lattice pylons and T-pylons

In some of the situations where the T-pylon is being proposed, the entire route is proposed as T-pylons.  But in some situations, there is a transition from traditional lattice pylons to T-pylons part way along the route.  You can only transition the phases from one to the other in one specific order, because of the need to preserve the clearances between the phases as they cross from one geometry to the other: top phase on the lattice has to go to inside top of the T-pylon and so on.  This is shown in this diagram:

perspective diagram of transition from lattice to T-pylon

Or, if it's easier to see the details, this end-on diagram:

diagram of transition from lattice to T-pylon

Unfortunately, this means that you cannot have optimum phasing for the lattice and the T-pylon simultaneously - you have to prioritise one or the other.

Which should you prioritise?  There are two reasons to prioritise the phasing of the lattice pylons.  The first is that they produce a higher field to start with.  This graph compares the fields of the two designs with untransposed phasing, that is, the "raw" field they each produce before any reduction by phasing:

graph comparing fields from lattice and T-pylon when both are untransposed

The second reason is that, as explained higher up this page, phasing is more effective for the lattice pylon than for the T-pylon.  Whatever phasing you chose for the T-pylon, it wouldn't produce as big a reduction to the sides of the line as transposed phasing does for the lattice pylon.

So the correct thing to do to get the best overall optimisation is usually to optimise the lattice section and let the phasing of the T-pylon be dictated by that. (There might be exceptions if the lattice section was very short and the T-pylon section very long.)

If we do that, what phasing do we end up with for the T-pylon?  You can see this from the diagrams of the transition above.  If the lattice is transposed phasing with BRY-YRB (top to bottom), the T-pylon will be RYB-YBR (left to right).  The field from this phasing is shown on this graph in comparison to the optimum phasing for the T-pylon alone:

graph showing fields from optimised phasing for T-pylon

Between the circuits, the situation is different, as we've said before.  But to the sides of the lines, which is where we're mainly concerned with, the phasing we end up with is actually not too bad.  It's the second lowest of the six options, and the further out you go, the closer it gets to the ultimate optimum.

More detail on how these two phasings compare

We said that the best phasing for the T-pylon is where the magnetic fields from the two circuits follow each other round 120 degrees apart:

animation of T-pylon with optimum phasing

There are two different ways of achieving this.  As you rotate clockwise round the triangle of the three phases, the order has to be R-B-Y.  But you can do this in two different ways:

diagram of two different 120 degree phasings for T-pylon

The one on the left is the one that is the optimum for the T-pylon on its own.  But the one on the right just so happens to be the phasing you get when you optimise the lattice pylons.

To complete this story and explain why these two aren't exactly the same, we now need to unpick the assumptions we've been making just a little bit more.  All this modelling of fields as rotating in circles is true when you are quite a bit further away from the conductors than the distance between them, so that you can treat the array of three phases as a single source and not have to worry about the contribution of the individual phases.  Obviously, the further away you get, the more true this becomes.  This is why the field for the  combined optimum phasing converges on the field for the T-pylon-alone optimum phasing, and if we extended the calculation to even larger distances, they would become exactly the same, because they are both producing fields that are 120 degrees apart.  And this means that the "rotating vectors" diagram - which shows what happens at large distances - is actually the same for these two options.  But when you are closer to the line itself, you cannot just rely on the circular-rotation approximation, you have to start taking account of the individual phases.  And, as can be seen from the diagram above, the relationship of the individual phases looks quite different when you're close to them.  That is why the T-pylon-alone optimum phasing produces lower fields than the combined optimisation.  But the combined optimisation is still better than any of the other options.

Summary

To sum up this whole story:

  • The optimum phasing for T-pylons isn't transposed phasing, it's another arrangement with the fields 120 degrees apart
  • In any event, optimum phasing can never work as well for T-pylons as for traditional lattice towers
  • When you have a line that is part lattice pylons and part T-pylons, the best thing to do is to optimise the phasing of the lattice section, and let that determine the phasing of the T-pylon section
  • When you do that, it turns out that the resulting phasing for the T-pylon is in fact almost as good as the true optimum
  • And, underlying all this, the T-pylon produces a lower field to start with.  All fields from all power lines comply with the relevant exposure limits in the UK.

Simultaneous phasing of lattice pylons and low-height pylons

photo of L9 low height towerThe discussion above was about what happens to the phasing when you transition from the traditional lattice pylon, with vertical circuits, to the T-pylon, with triangular arrays.  But there's another design of pylon with triangular arrays - the low-height pylon.

Compared to the T-pylon, the triangle is the other way up - two phases at the bottom and one at the top.  But the principle that you can only transition the phases from traditional lattice to the triangular array in one particular order still holds.  The order is shown in this diagram:

 lowheight-phase-transition-

The same thing happens as with the T-pylon: if we start with the traditional lattice having transposed phasing, we end up with the low-height lattice having the magnetic fields following each round 120 degrees apart, which we said was the best that could be achieved with triangular arrays:

animation of T-pylon with optimum phasing

There's one more sublety.  There are two different ways of having the magnetic fields 120 degrees apart.  They both deliver the same benefit of reduced fields at large distances, but they differ at closer distances. For the T-pylon, the one we end up with when we start with a transposed lattice was the less good one at closer distances - overall, the second best out of the six phasing options rather than the best.  But for the transition to the low-height pylon, the option we end up with turns out to be the best.

In other words, when transitioning from a traditional lattice to a low-height lattice pylon, both can have optimum phasing.

Unlike the T-pylon, which hasn't been built and connected to another pylon yet, low-height pylons do exist.  Here is how the transition looks in reality, first from side-on:

side-on photo of transition

Then standing at the traditional lattice and looking along the transition span to the low-height lattice:

lowheight-transition-photo-end

 

 

Other aspects of phasing

  • Our main page on phasing, including the UK policy on phasing
  • What happens between the circuits and why the optimum phasing can produce bigger fields there
  • The extent of phasing in the UK

Elliptical polarisation

This page explains how linear arrays of conductors produce mainly linearly polarised fields and triangular arrays produce mainly elliptically polarised fields.  But even linear arrays on traditional pylons end up producing fields that are to some extent elliptically polarised.

The field values

This page is mainly about how the phasing changes the field.  See more on the actual field values from traditional 400 kV lattice pylons and from alternatives including the T-pylon.

Try it for yourself

You can use our online calculator to experiment with varying the phasing of both T-pylons and traditional lattice pylons and see what effect it has on the magnetic field.

Latest news

  • New publication on cancer incidence from the UK electricity industry Cohort Study August 27, 2019
  • How has the reported risk for childhood leukaemia changed over time? February 11, 2019
  • Media stories about microshocks in children’s playground September 10, 2018
  • New studies on leukaemia and distance from power lines June 1, 2018
older news

Contact Us

To contact the electricity industry’s EMF Unit Public Information Line (UK only):
telephone 0845 7023270 or email [email protected].

See Contact us for more contact details including our privacy policy.

About this site

  • What this site covers and what it doesn’t
  • Industry policy
  • Sitemap

Specific questions

  • Affected by a new power line or substation?
  • Building or developing near a power line or substation?
  • EMF measurement and commercial services
  • Microshocks
  • Pacemakers and other medical devices
  • EMF policy in the UK
Site Authorship |Sitemap | Terms and Conditions | Privacy Policy | Cookies | Site Statistics
© 2021 EMFS.info
Navigation
  • What are EMFs
    • Terminology – an introduction
    • Electric fields
    • Magnetic fields
    • Units for measuring EMFs
    • Measuring and calculating EMFs
      • “EMF Commercial”
    • Adding fields together
    • Radiofrequencies
    • Screening EMFs
  • Sources
    • Overhead power lines
      • Fields from specific power lines
        • 400 kV
        • 400 kV – specific cases
        • 275 kV
        • 132 kV
        • 66 kV
        • 33 kV
        • 11 kV
        • 400 V/230 V
        • Replacing a 132 kV line with a 400 kV line
      • Summaries of fields from all power lines
      • Factors affecting the field from a power line
        • Voltage
        • Current
        • Clearance
        • Height above ground
        • Conductor bundle
        • Phasing
        • Balance between circuits
        • Balance within circuit
        • Ground resistivity
        • Two parallel lines
      • Calculating and measuring fields from power lines
        • Geometries of power lines
        • Raw data
        • On-line calculator
      • Fields from power lines – more detail on the physics
        • Field lines from a power line
        • The direction of the field from a power line
        • Power law variations in the field from a power line
      • Statistics of power line fields
    • Underground power cables
      • Different types of underground cable
      • Fields from cables in tunnels
      • Gas Insulated Lines (GIL)
      • Underground cables with multiple conductors
      • Effect of height on fields from underground cables
      • Screening fields from underground cables
    • Low-voltage distribution
      • UK distribution wiring
      • USA distribution wiring
    • House wiring
    • Substations
      • National Grid substations
        • Static Var Compensators
      • Sealing-end compounds
      • Distribution substations
      • Final distribution substations
        • Indoor substations
    • Transport
      • EMFs from electric trains (UK)
      • EMFs from cars
    • Appliances
    • Electricity meters
      • Smart meters
      • Traditional meters
    • Occupational exposures
      • Live-line work
      • Static Var Compensators
      • Occupational exposures on pylons
    • Field levels and exposures
      • Personal exposure
      • Other factors that vary with magnetic fields
      • Fields greater than 0.2 or 0.4 µT
    • Screening EMFs
      • Screening fields from underground cables
      • EMF Reduction Devices
  • Known effects
    • Induced currents and fields
    • Microshocks
      • Control of microshocks in the UK
      • Microshocks from bicycles
      • Bees and microshocks
    • EMFs and medical devices
      • Standards relating to pacemakers and other AIMDs
    • Effects of EMFs on equipment
  • Research
    • Types of research
    • Epidemiology
    • Animal and laboratory experiments
    • Mechanisms
    • Specific studies
      • UKCCS
      • CCRG
      • French Geocap study
      • CEGB cohort
      • Imperial College study
  • Current evidence on health
    • Childhood leukaemia
      • Survival from childhood leukaemia
      • Childhood leukaemia and Downs
      • Childhood leukaemia and night-time exposure
      • The “contact current” hypothesis
    • Other health effects
    • Scientific review bodies
      • WHO
      • IARC
    • Electric fields and ions
    • Comparing EMFs to other issues
  • Exposure limits for people
    • Limits in the UK
    • Limits in the EU
    • Limits in the USA
    • Limits in the rest of the world
    • Limits from specific organisations
      • ICNIRP 1998
      • ICNIRP 2010
      • NRPB 1993
      • NRPB 2004
      • EU 2004
      • EU 2013
  • Policy
    • UK policy
      • Power lines and property – UK
    • Compliance with exposure limits
    • European EMF policy
    • Precaution
    • SAGE
      • SAGE First Interim Assessment
        • Government response to SAGE First Interim Assessment
      • SAGE Second Interim Assessment
        • Government response to SAGE Second Interim Assessment
        • SAGE Second Interim Assessment – the full list of recommendations
  • Finding out more
    • EMF measurement and commercial services
    • Links
    • Literature
    • Contact us
  • Static fields
    • Static fields – the expert view