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.