Shanghai Metro uses overhead catenary

[daveklepper]

Seattle Light Rail also uses 1500V overhead catenary, but streetcar runs at 750V trolley wire, ditto Tacoma streetcar.   And some day they probably will connect!

[John WR]

That does make some sense.  I assume it allows them to be closer to the ceiling.  

[beaulieu]

John WR

I've certainly heard of third rails but never one suspended from above.  

It is becoming a lot more common in locations where clearance is a problem. Like spans on electrified drawbridges and in tunnels used for HSR. These look nothing like the ones used at ground level, rather they look like a very straight and level piece of cantenary contact wire, and typically they maintain the stagger of the adjoining contact wire.

[John WR]

tomikawaTT

there is at least one subway line in Tokyo that uses pantographs (of standard design) to contact an overhead third rail suspended from the tunnel roof on insulators.

I've certainly heard of third rails but never one suspended from above.  

[tomikawaTT]

Can't say about Shanghai, but there is at least one subway line in Tokyo that uses pantographs (of standard design) to contact an overhead third rail suspended from the tunnel roof on insulators.

The biggest advantage for overhead distribution is the safety factor of removing the 'hot' conductor from ground level - especially useful where the tunnel might be subject to minor flooding.  Also a lot safer for track workers, who don't have to kill power or worry about killing themselves.  The pans and hot rail DO require slightly more tunnel clearance, and put a definite limit on vertical rolling stock dimensions.

Chuck

[John WR]

PS.  You may recall from high school physics Ohm's law:  I = E / R where I = current in Amperes, E = electromotive force in Volts and R = Resistance in Ohms.  

As you increase the current in an electric wire you increase the heat produced according to the formula  H = I squared R.  Notice the heat increases according to the square of the current.  30 Amps produces 4 times as much heat as 15 Amps.  Also notice the value that does not appear:  E or Volts.  As voltage increases heat does not increase.  

For total power the formula is W = EI where W = Watts.  So for a given amount of power you can increase E the Voltage and decrease I the Amperes.  That allows you to use a smaller wire size without overheating.  

I suspect that is why in Shanghai they are using 1500 Volts.  

[John WR]

Generally speaking a catenary is preferred to a third rail and shoe for high speed trains.  For example, the Newark Light Rail which runs in a tunnel for a good part of its route used a catenary.  Obviously there has to be room for the overhead wiring and pantograph.  

With the Newark Light Rail in the tunnels there is not a true catenary, that is a power wire suspended from a second wire  allowed to fall into the shape of a geometric catenary.  Rather there is a power wire suspended from a second wire that drops a short distance from the ceiling of the tunnel. a

As far as the 1500 volts DC, for a given amount of power (number of watts) high voltage electricity requires a smaller wire than lower voltage with higher amperage.  However, that is just for transmission.  Today with electronic equipment it is common to change and change and change electricity so the motors may operate on AC which is produced by an electronic inverter.   

I don't know how much of a mathematical background you have.  If you take AC from a generator and run it into an oscilloscope it looks like a sine wave.  Then if you take the bottom half of that sine wave and flip it up so both parts are above the axis the total power will be the area under the wave.  The top of the wave is the maximum power and the wire must be able to handle that power.  Now if you draw a straight line that touches the tops of the sine wave you have a graph of a DC current and the total power is the area under the line.  You can see that the area is greater.  And the same size wire can be used to transmit that greater amount of power.  When you are building an electrically operated mass transit system and will need many miles of copper using the smallest possible wire to transmit the power you need becomes important.  

[aegrotatio]

I recently learned that the Shanghai Metro uses overhead catenary at 1500 volts DC in almost all of its system. I wasn't aware of this when I travelled it earlier this century, and I appreciate that the Boston MBTA Blue Line uses overhead catenary for parts of its route for sea-spray corrosion resistance, but I still find it an odd thing for any underground line to use overhead lines.

What are the advantages that Shanghai Metro uses so that they are using overhead 1500 DC instead of third-rail power? Surely the overhead lines are more expensive, aren't they?

Thank you.