Main Types of Railway Electrification

The two most common ways of electrifying main line railways is by the 25kV AC single phase system at the industrial frequency of 50 Hz or by the 750V DC systems. Metros and light railways also use these systems but lower voltages are often adopted, particularly where some street running is involved. Other electrification systems do exist such as 15kV AC single phase operating at 16.6 Hz which is found in central Europe, 50 kV in Africa and some DC railways operating with voltages up to 3000V. These, however, are far less common than the two main types described above.

At first sight the 25kV AC and the 750V DC systems appear to be totally different, but in fact they do have many common features. This is because, until very recently, electric trains were usually powered by 750V DC traction motors. This electric power delivered to the trainset must, therefore, be suitably conditioned so that it can be applied to and used to control the speed and power output of the motor and, therefore, the speed and performance of the train.

On the DC system power is taken directly from the National Grid or railway power station at a voltage of 33kV or 11kV and is transformed down to a lower voltage at each substation placed along the route of the railway. These substations are usually spaced between three and seven kilometers apart and, after transforming the voltage down to a suitable level, it is rectified and supplied to the conductor rails.

On an AC system railway, however, the power is taken from the National Grid at usually 132kV or even higher. The power is again transmitted down at each feeder station to the normal voltage of 25kV. These feeder stations need only be spaced about 50 km apart and power is distributed around the network at this voltage by means of an overhead catenary system from which it is collected by the train through a sliding contact. The collected power is transformed down to a lower voltage and rectified by the equipment on board the train before application to the DC traction motors.

The essential difference between the AC and DC systems of electrification is that on the DC system the electric power is transformed and rectified to its DC form at fixed substations distributed along the railway whilst for the AC system the power is transformed and converted in a mobile, on board, substation. For a high capacity, high density, low speed railway with many trains spaced a few minutes apart, the DC system may prove to be the most economic. However, for high speed, high power Inter City type trains which run less frequently, the AC system is usually found to be most cost effective.

Ex.1. Answer the following questions: