Catenary feed seems like it can’t possibly work, yet it does

-July 18, 2017

As current levels in server racks and similar fixed-in-place installations routinely reach hundreds of amps and higher, the feed cables and connectors demand careful attention. Basic use of Ohm’s law demonstrates that heavy-gauge cables are needed to ensure acceptably low IR drop, minimal ohmic losses, and overall reliability. Of course, if there are cables, there are also connectors and transitional elements so these, too, have to be both hefty and secure. With proper attention to design details, material and component selection, and installation details, it all works consistently and reliably.

Then there’s the catenary feed used on railway lines (Figure 1), developed in the late 1800s. In concept, it’s straightforward: a wire is strung between poles, and it naturally takes on the classic catenary shape defined by the hyperbolic-cosine trigonometric function (an equation that was derived in 1691 by Gottfried Leibniz, Christiaan Huygens, and Johann Bernoulli in response to a challenge by Jakob Bernoulli). This catenary wire (also called a messenger wire) has closely spaced drops which support the actual contact wire.

Figure 1 The catenary system used to supply power to electric locomotives and coaches is simple in concept, but has many subtleties in implementation due to the high voltages and currents, as well as the high-speed sliding-motion contact that is inherent in the system. Source: Pinterest

Some systems use hydraulic tensioners to provide tension on the catenary wire and thus maintain its shape despite changes in temperature. However, many catenary systems instead use suspended deadweights of 5000 to 10,000 pounds (about 2500 to 5000 kg) at the support poles, since the tension provided by weights is independent of temperature, unlike the operation with the hydraulic solution. (Note that there are some catenary systems, primarily in tight urban areas, which use closer pole spacings and higher tension on the catenary wire, so it very closely approximates a straight line. This eliminates the need for drops and the separate contact wire.)

This catenary/contact wire arrangement provides supply access, but how does that power reach the load of the locomotive or individual coaches? To pick off the power, a top-mounted pantograph assembly with a cross-wire contact area made of special carbon-based material reaches up from the engine or coach contact wire. It then pulls the current from the overhead supply as it travels underneath it. Current-return path is through the rails acting as ground, so touching the rail presents no danger.

How much current is being transferred “on the fly”? Using my unique back-of-the-envelope calculation pad (Figure 2), I did some rough estimates using the approximate numbers for the Amtrak Northeast Corridor line, not taking in account any losses or inefficiencies:

  • Locomotive rated power is 8500 horsepower, so let’s roughly estimate that at 10,000 horsepower, or about 7.5 MW (1 horsepower is about 750 W)
  • Overhead supply voltage is 25,000 V
  • Power = voltage × current, so the current is about 7.5 MW/25 kV = 300 A.

Figure 2 This custom-made pad encourages making rough order-of-magnitude estimates, rather than using excess, often unneeded, and possibly misleading numerical precision.

Here is where I reach the “it can’t work” zone in my mind, even though I know it does work. That’s a lot of current to pass through a sliding contact traveling at up to 100 mph (160 km/hr), and even much faster on the European TGV or the Japanese Shinkansen bullet trains. Yet it works, and has been field-proven over time, distance, and speed to work well.

This is not the first time I have thought “that can’t possibly work” yet it does. Are there any power-related things you have looked at and had the same feelings about, even when you know it actually works, works well, and is widely used?

  1. Overhead catenary system preliminary description, Aalborg High Class Transit System, May 2014
  2. Signs of Construction: Overhead Catenary System and Messenger Wire Power Light Rail, RTD FasTracks
  3. Catenary vs Third Rail, Trains Magazine forum
  4. Rigid Catenary (or Overhead Contact System), RailSystem
  5. Overhead line, Wikipedia
  6. Catenary, Wikipedia
  7. Amtrak's New 8600-Horsepower Locomotive Reports to Work Tomorrow, Popular Mechanics, February 6, 2014
  8. Pantograph, Steel Rails Advocate
  9. 45 years of Amtrak locomotives, USA Today, November 23, 2016

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