Electrification System for Rail Industry

The electric railway needs a power supply that the trains can access at all times. Transmission of power is always along the track by means of an overhead wire (catenary) or at ground level, using an extra, third rail laid close to the running rails. AC systems always use overhead wires, DC systems can use either an overhead wire or a third rail, both are common.

 

The railway electrification system supplies electrical energy to the trains at different voltages which include 600V DC, 750V DC, 1500V DC, 3000V DC, 15KV AC 16⅔Hz & 25KV AC 50Hz. The lower voltages are often used with third or fourth rail systems and voltages above 1000V are normally limited to overhead wiring for safety reasons. 750 volt DC third rail supply has been used extensively in UK for trains running up to 145 km/h, which is about its limit for speed.

 

25KV AC is the preferred system for high-speed and long distance railways, commonly adopted in Europe, India and United States. DC system is the preferred option for shorter lines, urban systems and tramways. 1500V DC is commonly used in Netherlands, Japan, India, France, New Zealand and United States, 3000V DC is commonly used in Belgium, Italy, Spain, Poland and South Africa.

 

While use of a third rail does not require the use of DC, in practice all third-rail systems use DC because it can carry 41% more power than an AC system operating at the same peak voltage. Third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems. DC systems are limited to relatively low voltages, and this can limit the size and speed of trains and the amount of air-conditioning the trains can provide. This may be a factor favoring overhead wires and high voltage AC, even for urban usage. In practice the top speed of trains on third-rail systems is limited to 100mph (160km/h) because above that speed reliable contact between the shoe and the rail cannot be maintained. Besides the third rail, fourth rail system may be adopted, as found in the London Underground. The additional rail carries the electrical return that on third rail and overhead networks is provided by the running rails. On the London Underground, a top contact third rail is beside the track, energized at +420V DC and a top contact fourth rail is located centrally between the running rails at -210V DC, which combine to provide a traction voltage of 630V DC.