Braking System in Train

Braking System in Train:

Braking System in Train has number of advantages over mechanical braking. It reduces wear and tear on rails and wheels, substantially increasing their life. It is smooth, fast and can be precisely and automatically controlled. On the other hand, mechanical brakes require frequent maintenance, particularly when subjected to heavy duty as in suburban trains requiring frequent stops and in hilly regions while going down a gradient. Further, regenerative braking can help in saving energy. Thus present practice is to use electric braking for stopping and decelerating. Electric braking torque is available only for speeds greater than zero; it cannot be used for holding the train at zero speed against gradients, as would be needed at stations. Therefore, even when the electrical Braking System in Train is used, mechanical brakes are provided for holding the train on a gradient.

Because of the large inertia, a running train will have large energy stored in the form of kinetic energy. When mechanical brakes or dynamic braking is used, the stored kinetic energy is converted into heat; and is wasted. When regenerative braking is used, major proportion of this energy is returned to the mains. In case of main line trains, where the distance between stations is large, the energy saved forms only a small proportion of the energy spent in running. Thus, mostly dynamic braking is used and regenerative braking is used only when the drive has inherent capability for regenerative braking (i.e. when no additional equipment is required to get regenerative braking). In suburban trains, where distance between stations is small, the energy saved by regenerative braking forms a substantial proportion of the energy spent in running. Hence regenerative braking is generally used.

In hilly region, when a train moves down-gradient, in addition to kinetic energy, its potential energy is also available for regeneration. Thus, large amount of energy is saved if regenerative braking is used. If dynamic braking is used, dissipation of such a large amount of energy can be a problem. In case of mechanical brakes wear and tear will be rather large. Because of these reasons, trains for hilly regions are usually equipped with regenerative braking capability.

Main problem faced with regenerative braking is of over voltages. When the current is forced against the supply voltage for regeneration, line voltage may rise 10 to 15% above the normal. Therefore, with regenerative braking, voltage rating of the line and associated equipment, has to be 10 to 15% higher than without regenerative braking.

An important requirement for regenerative braking is that the supply should have the ability to absorb regenerated energy. When this is not assured, dynamic braking is used.

In case of buses and trolleys also, because of the frequent stops, substantial amount of energy can be regenerated, that is why it is now common to use regenerative braking. The mechanical brakes are also common because of simplicity.

In case of battery and solar powered vehicles, regenerative braking increases the range of vehicle, hence it is desirable.

Power Factor and Harmonics:

Many traction systems employ ac to dc thyristor converters which have the drawback of low power factor at low output voltage and generation of harmonics in source current and voltage.

Locomotives with capacities as large as 5 MW (and higher) are powered from a single phase supply, resulting into large voltage drop and high transmission line losses during acceleration. If the power factor is allowed to fall low, the voltage drop and transmission line losses will be too high and transmission line and substation equipment may be over loaded. Thus, measures are taken to ensure that the power factor does not fall below 0.8.

Harmonics in source current and voltage have a number of undesirable effects such as malfunction of electronic equipment connected to the line, excitation of system resonances, overloading of capacitors, decrease in efficiency owing to increase in losses due to harmonic currents and skin effect, saturation of transformer and interference with telecommunication equipment. As far as traction application is concerned two most undesirable effects of harmonics are maloperation of signals and interference with telephone lines which run by the side of the track.

The circuit which operates signals is called track circuit and its supply consists of low voltage dc or ac. As already mentioned, one of the running rail is used as earth for the main power circuit. Usually, same running rail is used as the return line for track circuit. As the power and track circuit share the same return conductor (earthed running rail), power circuit harmonics can cause maloperation of the signals, which can be dangerous. Further, high frequency harmonics in the source current produce electromagnetic radiation, which generates noise in telecommunication lines.

Problem of low power factor and source current harmonics is solved in two steps. First, those converters are used which have good power factor and lower harmonics. Secondly static var compensator is used to maintain power factor above 0.8 and filters to reduce harmonics to the extent that the possibility of maloperation of signals is completely eliminated and the noise in the telecommunication lines is reduced to tolerable level.

Updated: January 14, 2019 — 10:39 pm