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Three-phase electric power systems
Three-phase electric power systems used for high voltage AC transmission lines (50 kV and above).

As the need for increased transmission line capacity have forced power utility and owner to maximize power density in existing transmission corridors.  Many experienced have already been done on this, one way to get to this goal (increased capacity), is the use of more than three phases.

Theoretical and experimental studies have considered up to 36 phases systems. Up to 36 phases, the six-phase and twelve phase systems have been chosen the most attractive alternatives to replace for the three-phase systems.

Example of phase to phase voltages for multiple phase systems up to 36 phases are shown in below, comparing with the corresponding phase to ground voltages.

Comparison between phase to phase voltages with phases to ground for multiple phase systems in kV:

Phase to ground VoltagePhase to phase voltage
N36122436
8013880412114
133230133693523
1993451991035235
2895002891497550
43375043322411375

What are the key points of multiple phase lines?

  1. In six-phase systems, phase to phase voltages are equal to phase to ground voltages.
  2. For the orders higher than six, phase to phase voltages between adjacent phases are lower than phase to ground voltages. This is the reason that always in multiple phase systems, the phase to ground voltages are taken as reference.
  3. Whereas in three phase systems phase to phase voltages are always equal, phase to phase voltages in multiple phase systems depend on the selected conductor combination.

 Properties of multiple phase systems

Comparing with three phase system, we get the following characteristics of multiple phase system.

  1. Lower voltage for the same power can be transmitted.
  2. Lower phase to phase and phase to ground clearances and shorter towers with narrow right of way
  3. Lower phase to phase spacing
  4. Lower voltage gradients, resulting smaller conductor size to used.
  5. With increasing number of phases, the probability of flashovers’ between phases increases. Therefore special attention must be paid to the design of insulation
  6. Power transmission with more than three phases, for example six-phases, requires corresponding transformers and circuit breakers, switchgear and bus bars for six poles as well, what is more expensive and results in difficult protection (relaying) when interconnected with three phase lines. Therefore, the use of multiple phase transmission is presently restricted to some few lines.

About Author //

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electrowsky - Electrical engineer interested in electrical science and coding.

8 Comments


  1. Wayne Masters
    Sep 27, 2015

    I’m interested in costing the doubling of the power lines in BC Canada to six phase as an alternative to building another dam.
    posted a drawing of an axial flux motor for six phase power at the Facebook website link above.


  2. krishnaprasad
    Mar 28, 2015

    Very good portal for conveying the knowledge to and from others.


  3. Abdulwahid
    Dec 22, 2014

    good write up


  4. endau
    Oct 12, 2013

    I hope those young electrician wont mess up with the phase sequence if we use more than 3 phase.


  5. sinnadurai
    Mar 24, 2012

    Please give safe working distances(indoor/outdoor) for each rated voltage in ELV/LV/MV/HV/EHV

  6. Asif Eqbal
    Asif Eqbal
    Mar 07, 2012

    Great article very useful


  7. b s sastry
    Jan 03, 2012

    Good information.
    Thanks


  8. Ibrahim Seder
    Nov 30, 2011

    Grate !!! I found it very useful,thanks

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