Page 52 - 360.revista de Alta Velocidad - Nº 5

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

1. Introduction

The electric traction offers, against diesel traction, advantages such as the possibility of
building vehicles of great power and speed, better efficiency from the point of view of energy
consumption, and less environmental impact. Undoubtedly this traction is the traction of the
present and the future in the railways, occupying the first place in the railway systems of the
developed countries with the only important exception of the United States of America. In
developing countries this type of traction is also the one that tends to be installed in all of its
major railroads.
On the other hand electric traction requires large economic investments in its own facilities
(electric power lines, substations and electric power transmission lines for train power), so it
requires important economic studies. In any case in railway lines with high traffic speed and
high traffic density, the use of electric traction is always necessary.
Traction Power System (TPS) can be distinguished between Direct Current (DC) and Alternating
Current (AC) systems with different nominal voltages and power frequencies. Currently the most
commonly used TPS is based on 25 kV nominal voltage and 50 Hz power frequency (industrial
frequency). Such systems have inherent advantages like simple substation design and low
transmission losses compared to DC systems.
1.1 The use of the AC system in High-Speed railways

It can be said that the speed of circulation () at which a railway line is designed conditions the
electrification system to be used. It is evident that as this speed increases the power demanded
by the train is also greater. This is justified considering that in the general formula of drag resis-
tance the term representing the aerodynamic drag is proportional to the square of the velocity:

The term A + Bv represents the rolling resistance while Cv is the term corresponding to the
2
aerodynamic drag. R is expressed in [kN] and v in [kph].
a
On the other hand, in High-Speed railway traction, the fundamental equation of the dynamics
applied to a train (with mass M) and characterized by an acceleration can be written as:

In this equation Fj [kN] represents the total effort on the wheels of the locomotive with all its
motors; Mg sin α represents the gravity component; i is the slope expressed in [‰]; k represents
the coefficient of inertia of the rotating masses. It is dimensionless, slightly higher than 1. The
term kM, therefore, represents a fictitious mass referenced to the wheels of the locomotive.
In summary a train of mass M and drag resistance R has an acceleration γ on a line of profile ί.
a
The motor vehicle must develop in its wheels a total effort Fj which is calculated by equation
(2) for each speed and slope.

50 360.revista de alta velocidad