T & D - 5) What is the reason beyond three phase transmission?

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 Voltage	Phase to phase voltage
N	3	6	12	24	36
80	138	80	41	21	14
133	230	133	69	35	23
199	345	199	103	52	35
289	500	289	149	75	50
433	750	433	224	113	75

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.