Big DC cabling

[Open Neutral]

In a forum about transit, the question of spacing of the Traction Power Stations came up. On WMATA, the newly upgraded (from 6MW) stations are 9MW, are fed from a dedicated 13.2KV (MD) or ~34KV [VA] PoCo feeder, and deliver 750 VDC. They are about a mile apart.

I started pondering the conductor size for such. (It got into the thread because at one point, the trackage is 200+ feet below the TPS.) Assuming 100% both of the accuracy of the term, and efficiency, that's 12E3 amps. My copper wire table does not seem to include that current.

My visual memory is about six cables bolted to the third rail, or the Wee-Z bond. They appeared to be roughly four aught size, but would seem to need to be far larger.

Anyone here with experience with the DC plant of such and able to comment?

(A Wee-Z bond is a inductor to pass the DC traction power from the rails back to the TDS, but block the audio frequencies used for signaling. The bond also seeks to balance the current through the two rails.)

 

[Ingenieur]

tps?

are you saying 12000 A are conveyed between stations?
or every station is tied to the 3rd rail and shares the load, like distributed generation?
the 3rd rail is continuous/common between all stations?
and each station is tied to the rail at one point?

 

[zbang]

The 3rd rails are sectioned, and generally, one TPS powers a single section, but sections can be isolated or tied together as needed for maintenance.

 

[Open Neutral]

As far as I know, in general, much if not all of the third rail is in parallel. There are non-TPS buildings called "Tie Breakers" at ~halfway between Traction Power Stations that [un]bridge sections if needed. (I was told those trip at 20KAmps.)

The system will run without every TPS; in ~2000 there was a large District blackout, and WMATA struggled on the DC coming across the third rail from VA and MD; slowly to be sure but moving.

The Tie Breakers and TPS's can be tripped off by the ROCC, and also trackside emergency trip stations.

 

[Ingenieur]

how many trains per mile?
guessing can't be more than 1 train per 10 miles
how many hp/train?

 

[Open Neutral]

how many trains per mile?
guessing can't be more than 1 train per 10 miles
how many hp/train?

So a 7000 series is 1.1E6 watts, but I think this is propulsion only, not including HVAC & lighting.

Rush hour, there's a Red Line train about every 7 minutes off peak, and 3-4 on peak.
There are live maps but I'm not finding one that works for me at present.

 

[Ingenieur]

So a 7000 series is 1.1E6 watts, but I think this is propulsion only, not including HVAC & lighting.

Rush hour, there's a Red Line train about every 7 minutes off peak, and 3-4 on peak.
There are live maps but I'm not finding one that works for me at present.

say 1.5 Mw/train
avg speed 40
8.5 trains/hr
~ 4.7 miles between trains, call it 4 or 1/4 train/mile
ignoring Z each tps supplies 1/4 of a train or 0.4 Mw, ~530 A
double for peak

obviously the closer stations to the train would supply a greater percentage

but say 100 miles, 25 trains and 100 tps it would seem to avg out
25 x 1.5 / 100 ~ 0.4 Mw/train

I guess when the train was in front of a tps it may peak close to the 1.5 Mw or 2000 A

the 9 Mw seems large?

 

[Open Neutral]

They have a real problem with lack of power with the 6MW TPS's, hence the upgrade. The newer 7000 trains draw more power than its predecessors. The TPS's transformers creep up well into the red temperature range during every rush hour; they take multiple hours to cool back down.

I know this was a major concern on the Obama 2009 Inauguration day, their all-time peak ridership of 1.2 million trips (2x a busy workday). That was a "Everything, everything movable rolls" event. But it was in the winter, so less HVAC load, and cooler air for the TPS's. (You also don't need a lot of interior heating when you have crushloads in every car....)

But back to my initial pondering; with 9MW @ 750V available, you gotta have conductors to carry that up to 200 ft. to trackside. Now, it's safe to assume that's 4.5/track, and I think it's more likely 4-way, i.e. Track 1 north of TPS, Track 2 south, etc. The Tie breaker own buildings are big enough that I assume the TBS's have them within, not at trackside. And it's DC, so no skin effect, but still, that's a lot of mm^2 copper.

 

[Open Neutral]

And on the # of trains per hour, I don't follow your math.
At 3 minutes between trains at rush hour, that's 20 per hour northbound, and another 20 southbound.
(Junction stations may be double that; there are three of those.)

 

[Open Neutral]

Here's a real-time train tracker

http://tracker.geops.ch/?z=13&s=1&x=-8570238.2918&y=4707625.9132&l=here

 

[Ingenieur]

And on the # of trains per hour, I don't follow your math.
At 3 minutes between trains at rush hour, that's 20 per hour northbound, and another 20 southbound.
(Junction stations may be double that; there are three of those.)

20 per hour
but over many miles
if avg speed is 40 mph, 3 min = 2 miles seperation
20 x 2 miles = 40 miles
or 1/2 trains per mile or per tps
or 1 per tps both directions