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 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).
- In some systems substations can feed energy back into the national grid.
Influence of supply system
The electric supply system has a considerable influence on the feasibility of
energy recovery:
AC systems are generally better fitted for recovery and obtain much higher
recovery rates. Under DC regeneration is only occasionally or partly possible.
15 kV 16 2/3 Hz systems are very well fitted for regenerative braking for two
reasons:
- Catenary voltage (15 kV) is high which allows for a transmission at low
losses over relatively big distances (Compared to DC systems operated at 3 kV
or less).
- Due to the particular frequency of 16,7 Hz systems, railways are usually
forced to produce their own energy. They can therefore easily force their
entire network to make use of the same electric phase. The catenaries of the
various contact lines can then be electrically inter-connected without the
danger of causing short circuits. Consequently the probability of a vehicle
being in traction when another one is braking is very elevated.
For these two reasons recovery rates in 16,7 Hz systems are usually high.
Feeding back into the national grid
This is in principle possible in AC networks. However, in 16,7 Hz systems
there is usually no need for a feedback into the superior 3-phase grid since the
chances for using the recovered energy in the railway grid are very
high. |