Page 54 - 360.revista de Alta Velocidad - Nº 5

Martínez Acevedo, José Conrado. Berrios Villalba, Antonio. Peregrín García, Eugenio.

phases, the use of alternating current in railways uses only one phase (single-phase alternating
current).

1.2 Electric power regenerated

Due to the use of AC systems it can be intuited that the energy efficiency of a high speed line is
much greater than in a DC line. This is mainly due to the existence of lower losses in the system.
On the other hand, the management of the electric power regenerated in the braking of the
trains has great incidence in the energetic improvement with respect to a line with DC system
DC. The braking process a train has to carry out, either to make a stop or to lower the existing
slopes along the path, or even to succeed in reaching the speed limits imposed, may lead to
important consequences in the final calculation of the energy required by an electric railroad
line. Indeed, energy is dissipated in the braking process, some of it is lost in friction brakes
(pneumatic brakes), that has no useful use, and some of it is dissipated in the dynamic brakes.
For the dynamic brakes in particular, in the case of having an electric traction train (or a
dieselelectric traction one), the braking process involves the generation of electricity. At
present, the electricity generated in this type of brake can have multiple destinations:
• Provided the train incorporates regenerative braking, energy is dissipated as heat into
electrical resistors provided on board (rheostat brake).

• Provided the train does not incorporate regenerative braking, energy is returned to the
catenary. In this case, if there is another train being fed from the same power sector
requiring energy, the train may consume the returned energy. This particular example
constitutes an optimal process from an energy standpoint. In the case there are no trains
demanding energy, two additional cases arise:

• In case of having a single phase AC power (for high-speed lines), the energy generated
is returned to the national grid and can be used by other consumers connected to it.
• In the case of having DC power, thus existing a rectifier group in the substation, energy
is dissipated in the resistors of the rheostat brake provided in the train.
That is, in DC electrifications, energy cannot be returned to the national grid taking into
account the current situation, since the substations are equipped with rectifier groups that do
not allow current flow into the grid. It also should be noted that the energy is regenerated to
the catenary when the train that is stopping has previously fed its auxiliary services (heating,
air conditioning).

2. Traction Power System (TPS)

A Traction Power System (TPS) is a system in which it is possible to absorb or generate
energy and distribute it to trains in an efficient and safe way. This system represents
in itself a power electrical system with its own characteristics. In most cases the TPS is
interconnected to the country’s general electrical system (Figure 1). It can also constitute
its own electrical system. In the first case the AC railway system will operate at industrial
frequency while in the second case they will operate at a special frequency (case of some
countries in Central Europe).

A common feature is that the electric energy, from its generation to its delivery to the
trains, goes through different stages of adaptation and transformation.

52 360.revista de alta velocidad