Four phase commuter rail power (Northeast US)

[bacon4life]

Recently I was traveling in Pennsylvania, and was puzzled by the commuter train power. Above the tracks there were several circuits including what appeared to be 121 kV or 138 kV class transmission and 15 or 25 kV class distribution.

The puzzling part was that there was 4 separate conductors for the transmission. Does this configuration allow for 3 phases plus a spare for maintenance purposes? If so, is this common in any other overhead transmission applications?

 

[SDestimator]

I know nothing about commuter train power, but are you sure it isn't a standard 3 phase 4 wire system (3 hot phases and a neutral)?

 

[rbulsara]

They could be two single phase circuits. Very common thing for transmission lines for railways.

 

[EEJaime]

I was studying the commuter rail system here in LA and I noted that there are in fact two single phase overhead circuits. They are terminated individually at the system feed points where the switching is controlled by a central telemetry system. I believe that the circuits are selectable to balance load and to provide maintenance flexability. The contact shoes do appear to be able to be switched from one circuit to the other. I have personally not designed one of these systems, but I am meeting with the MTA's engineer in the next few weeks as we are doing some work on a few of their platforms-I'll see what information I can get from him.

 

[davidbeach]

The electrification of any systems in LA would be DC. That would be a whole different kettle of fish.

 

[rbulsara]

david:

AC single phase circuit feeds single phase stepdown single phase transformers that in turn feeds rectifiers to get DC for traction.

 

[alehman]

12.5kV and 25kV single phase is common. Some systems still use 25Hz. Maybe the transmission lines are there to deliver the 25Hz power to substations along the line?

 

[bacon4life]

I'll bite that the distribution class might be either multiple circuits of single phase AC or DC. However the 4 transmission conductors were mounted quite high above the tracks.

The picture is similar to the installation I saw. If the higher voltage has to be either transformed or rectified before being transfered to the train on the lower conductors, wouldn't 3 phase be more efficient?

 

[jghrist]

Looks like two DC transmission lines. They are on different structures. The railway catenary is the lower (25 kV?) line suspended by three suspension insulators.