Principle:

The energy put into accelerating a train and into moving it uphill is “stored” in the train as kinetic and potential energy. In vehicles with electric traction motors (this includes electric, diesel-electric and hybrid stock) a great part of this energy can be reconverted into electric energy by using the motors as generators when braking. The electric energy is transmitted “backwards” along the conversion chain and fed back into the catenary. This is known as regenerative braking and widely used in railways.

Braking and safety

Braking safety requires installation of additional brakes besides regenerative brakes, for two reasons:

• Braking power of 3-phase AC motors is of the same order as power installed for traction. Additional braking power is therefore indispensable and provided by mechanical (e.g. disk brakes) or other dissipative brakes. Typically brakes are blended, i.e. when the driver brakes, first the regenerative brakes are applied, if more power is needed (especially in unforeseen situations) additional brakes are applied.
• If the contact between pantograph and catenary is interrupted, regenerative braking is impossible.

Use of recovered energy

The energy recovered by dynamic braking is used for different purposes:

• on-board purposes (auxiliaries or comfort functions). On-board demand is usually far too low to consume all the energy supplied.
• energy is fed back into catenary to be used by other trains motoring close enough (in a section of track supplied by the same substation).
• If DC substations are equipped with thyristor inverter units, they can feed back energy into the national grid.

Influence of supply system

The electric supply system has a considerable influence on the feasibility of energy recovery. In DC systems, the catenary can be interconnected over great distances (since in contrast to AC systems, no phase shifts can occur). This would in principle allow for a long-distance transmission of recovered energy. However, given the low voltage of these systems (1,5 or 3 kV), transmission losses strongly limit the feasible feeding distances. Therefore the probability of having trains braking and trains accelerating close enough to each other to allow for an effective transmission is rather small.