Traction sub-station supply from Grid

[Stirrick]

I've been reading up on electrical supplies for Traction systems (Railway networks), but am really struggling to clarify one major point.

The Grid supply to a Traction Sub-station comprises 2 x Phase conductors (the actual Phases used being rotated over the entire rail network to iron-out load imbalances).
But I can't find a definitive statement as to why 2 x Phases are used, rather than a single Phase-Neutral connection.

The Traction (LV) supply from the Sub-station is a single Phase-Neutral quantity. So it would seem simpler to repeat this on the Grid (HV) supply side.

The only arguments I've found, so far, are that: (1) using 2 Phase conductors increases the Power delivered by 41% compared to 1 x Phase-Neutral, (2) using 2 Phase conductors reduces the load imbalance on the Grid side, and (3) using 2 Phase quantities helps maintain constant power in the system. But mainly the point just seems to get glossed over. Which makes me suspect that the reason is not particularly well understood.

Any ideas ?

 

[ZeroSeq]

To address your points:

1. Using a single phase, line-line supply vs. a single phase, line-neutral supply increases the power supplied to the load by 1.732x (square root of 3) for the same number of conductors. A three phase (LLL) supply further increases this by another 1.732x with an increase of 50% more conductors.
2. This won't alleviate the load imbalance, but the loads can be balanced through design of the feeders from the supply transformer.
3. A three phase supply is the only way to provide a constant rotating field for induction motors. Single phase motors (whether it be LL or LN supply) still need to "create" the rotating field through shaded poles, external capacitors, etc.