Avoiding shocks when electrifying rail

We’ve got electronic mail and we’ve got electric cars. As one waits by the platform, choking on fumes and turning headphones up to drown out the noise, one wonders why are we still using century-old technology to run the trains?

Electric trains win out over diesel trains by almost every measure. They emit up to 60% less CO2 over similar journeys, and electric trains emit no pollution at the point of use, such as at the platform. They also pull ahead in terms of speed, acceleration, reliability, braking power and weight.

The only reason diesel trains have survived against this pressure, is that over two fifths of the UK’s rail network remains un-electrified, meaning only diesel trains are able to run these vast swathes of rail.

At the cost of engaging in the perpetual political melee, it’s not controversial to suppose that building infrastructure is invariably expensive. Not only must the infrastructure last the test of time, it must make best use of, often public, funds. To achieve this, it is critical that original equipment manufacturers (OEMs) select the right electrical components that are optimised in terms of application and cost.

How to stop a speeding train

A 150 tonnes locomotive travelling at 40 metres per second (around 90 miles per hour) carries 240 million joules of kinetic energy with it, equivalent to about 72kg of TNT. When coming to a stop at a platform, as trains often do, this huge amount of energy is simply discarded through the braking process.

This is where an important energy saving benefit of electrified trains becomes apparent. Instead of wasteful rheostatic, air or friction braking, regenerative braking captures the kinetic energy of the speeding train and turns it into electricity.

This captured energy can either be reintroduced to the grid or stored. Local storage uses batteries or supercapacitors to hold the charge until it’s required, which add costs to maintenance and construction, as well as increasing the weight of the train itself.

A better approach for large-scale electrified rail is reintroducing this power to the grid. In mature, well implemented systems, with trains consistently stopping and moving off, spare energy pushed onto the grid by braking vehicles is immediately co-opted for use elsewhere.

The issue of switching

Another challenge rears its head here. When switching between braking and driving, respectively pushing current to and pulling current from the grid, the switching of loads causes pulses of voltage on the grid. Rectifying and inverting circuits onboard the trains themselves also contribute to this phenomenon.

If unaccounted for, these transient pulses can interfere together to create dramatic voltage spikes in the network. These disturbances can have a wide array of negative effects, from simply blowing fuses and damaging voltage-sensitive equipment, to causing long, unshielded stretches of overhead lines to act like colossal radio transmitters.

These phenomena nibble away at the economical advantages of the electric train, but they can be well addressed with the correct equipment. Electromagnetic-compatibility devices take the incoming power signal, be it direct or alternating current, and strip out the unwanted higher frequencies. They then transmit clean and consistent voltage from their outputs.

Admittedly, clean and consistent aren’t adjectives often involved in discussion about the UK’s railways, but there’s no good reason that can’t change. If railways in Europe and eastern Asia are anything to go by, an electrified rail network is inevitable. What’s crucial is to do it right.