Notices by Garry Keenor (25kv@mas.to)

@babe @SarraceniaWilds "He was nicknamed Bum because as a child he would frequently hang around the fire station in Key West."

that is NOT an explanation

there's a special circle of hell reserved for organisations that configure their "manage cookies" webpage with a dozen "legitimate interest" toggles sprinkled on a long-scroll dialog and defaulted to on

So to sum up, for most railways, AC electrification is the cheaper, more efficient option.

There are exceptions, such as underground railways without room for overhead line, where use of a third rail close to the ground means voltage must be low for safety reasons. For a given voltage, DC contains more power than AC, so third rail systems are always DC.

As always, you can find out more about AC and DC electrification by downloading my book from https://ocs4rail.com/downloads. Sections 8 and 11 refer.

You can also buy the physical book here: https://www.thepwi.org/product/overhead-line-electrification-for-railways

Finally, you can read my other #RailwaysExplained threads here: https://mas.to/@25kV/110644720069392183

/thread ends

The other key exception is extension of an existing DC system, where it is usually better to continue with the existing configuration than attempt a lengthy conversion - unless you are at power capacity, in which case you may have no choice.

Welcome to another #RailwaysExplained thread... a while back I asked for ideas and @DiegoBeghin suggested something on AC versus DC electrification. So here goes!

HOW IT WORKS - AC and DC electrification

Trains can be supplied with either Alternating Current (AC) or Direct Current (DC) electricity. If you don't understand the difference, check out https://learn.sparkfun.com/tutorials/alternating-current-ac-vs-direct-current-dc/all

#railways #OLE #OCS #OverheadLine

If you look at railway electrification worldwide you will see lots of AC and DC routes - often in the same country. To understand why that is, and what the advantages and disadvantages are, we need to look at how the earliest electric railways developed.

Electric railways have their genesis in the late 19th century with trams. Cities were getting clogged with horse- and cable-drawn trams and were desperate for a simpler, cleaner, dung-free system. So they turned to the new-fangled electricity.

Obviously you cannot supply AC electricity to a DC motor and expect it to rotate. You first have to rectify the current – that’s the process of AC to DC conversion. These days rectifiers are made using solid state electronics, and can comfortably fit under the floor of a rail vehicle. Back in the 1890s though, the only option was the Mercury Arc Rectifier, a large, sensitive and vulnerable glass bottle containing, um, mercury. Yikes.

Additionally, the DC motor was easy to control, via a series of resistors and contactors; all readily available Victorian technology. Now all you have to do is decide how to supply electricity to the tram.

So you’re a late Victorian engineer working out how to configure your electric tram. The first thing you need to do is to pick a motor to use in your rail vehicle. In the 1890s, this meant the Series DC motor. This motor was well-understood, easy to build and crucially, had the right speed-torque characteristic for its purpose: high torque from standstill, dropping away as speed builds up.

So the very earliest electric railways used DC overhead line or third rail. This kept the the on-vehicle equipment simple and reliable. Rectification was done back at the substation, since all electrical power is generated as AC.

So why aren’t all electrified railways DC? Because AC supply has several significant advantages over DC.

This places practical limits on the voltage you can use with DC. Anyone with high school physics will tell you that power is proportional to voltage^2, so doubling the voltage roughly quadruples the power available. This is important when trying to move heavy trains. The highest DC voltage used for rail electrification is 3000V.

For one thing AC current is much easier to interrupt because the voltage crosses zero multiple times a second. Why would you want to do that? Because faults frequently occur in railway electrification systems, and you must break that fault rapidly to prevent damage to the system and danger to people.

DC electrification has a number of other disadvantages; because voltage is lower but power needs to be high, and power = volts x amps, the current in DC systems needs to be much higher. Putting high currents through a wire without excessive losses due to volt drop and wire heating means using much larger conductors, so DC OLE is heavier and more expensive.

AC circuit breakers are easier to design for high voltages, removing this limitation and enabling more power. The electrification pioneers knew this, & wanted AC OLE - but that meant on-train rectification. A few pioneer railways successfully ran AC systems – in the UK the Lancaster - Heysham (1908) and London Victoria - London Bridge (1909) systems both ran at 6.7kV 25Hz AC. But the reliability of Series DC motor control meant AC traction did not make headway in the UK until after World War 2.

There is one final problem with using DC current: stray current corrosion. Whenever a DC current passes between two materials with a different atomic composition, electrons are stripped from one material and deposited on the other – its how a battery works. However, it is impossible to keep DC traction current entirely within the railway, since we use the running rails as part of the circuit and they are placed on the ground.

The volt drop problem also means that substations must be much closer together – typically every 10 miles for 1500V DC versus every 40 miles for 25kV AC. They are also more complex, needing to house rectifiers. Substations are a major cost for any new electrification build. Taken together, these factors mean the capital cost of DC electrification is significantly more than for AC electrification.

Despite all these disadvantages, DC electrification was reliable and cheaper to operate than steam; and so between the start of the 20th century and World War 2, DC electrification spread across the globe, using the Series DC motor, contactor/resistor control, and the multiple unit control system pioneered by Frank J. Sprague. 1500V DC was the most common, but some countries used 3000V DC. For this reason many developed countries have a DC electrification network to this day.

DC return current will therefore enter any adjacent conductive materials such as water pipes, building foundations and other important services. Where that stray current leaves the service, it will corrode it. Managing DC stray currents can be a major headache for any DC electric railway. AC electrification does not suffer from this problem.

The 1950s saw renewed interest in AC OLE, driven by the emergence of reliable control mechanisms and more compact rectifiers. This meant that long-distance high voltage AC transmission was now feasible, and with it intercity electrification. Experiments with 50Hz AC traction were made in Germany in 1940. The French picked these up after hostilities ended, and as the 1950s progressed the 1500V DC standard was dropped across Europe in favour of 15kV and 25kV at AC industrial frequency.