*2.1. Configuration and Design of the 2* × *25 kV System*

The 25 kV AC single phase is the most common electrification system for new high speed lines in the variant 2 × 25 kV autotransformer feed. This electrification system is commonly used with the industrial frequency (50 or 60 Hz) but it may be used for the railway frequency as well [43].

The 25 kV electrification schemes are single-phase electrification systems that are powered from the three phase industrial grid. This poses some problems due to the unbalance that a high power single-phase load creates on the grid's voltage. The degree of unbalance on the grid is limited to reduce the possibility of damage to generators and electric motors and it is measured though the unbalance coefficient, which is defined as follows:

$$K = \frac{V\_i}{V\_d} \tag{1}$$

where *Vi* is the negative sequence voltage and *Vd* is the positive sequence voltage. In practice, the unbalance coefficient is measured through the following Equation (2):

$$K = \frac{P\_{sp}}{P\_{sc}}\tag{2}$$

where *Psp* is the power of the single-phase load and *Psc* is the three-phase short circuit power of the connection node. International standards and transmission system operators set the maximum degree of unbalance between 1 and 2% that means that the load has to be connected to a high-voltage node with sufficiently high short-circuit-power. This later is not a problem where a high-power grid is available, but can be difficult on islands characterised by weak networks. The railway line is powered by several substations that are connected to different phase pairs in order to further reduce load unbalance on the industrial grid. Each substation is built with redundant apparatuses, such as two transformers and two connections to the industrial grid, in order to prevent the line grinding to a halt, in case of failure of any of these components. The connection between two different phase-couples of two successive ESS (energy storage system) implies that a neutral section has to be built when the power supply switches from one ESS to the next because of the voltage difference between two out-of-phase circuits. Usually, neutral sections are placed at the substations and halfway between them. Each neutral section is signalled to the engine driver to allow them to disconnect all the train loads (traction motors included) before entering the neutral section itself (Figure 1).

**Figure 1.** Power flow in an ideal 2 × 25 kV autotransformer system.

The neutral sections can be energised from either end when needed. International standards set the tolerance of the voltage in the system once the nominal value is chosen. The tolerances and the maximum duration allowed for a nominal voltage of 25 kV are reported in Table 1.


**Table 1.** The 25 kV system operating voltages [44].

Low traffic lines are usually fed directly from the transformer through the over-head line and the rail as return conductor. High traffic lines and high-speed lines are usually fed with an autotransformer system because it allows building the feeding stations further apart and, consequently, to contain the costs of building high-voltage lines to connect substations to the grid. Each electrical substation has two power transformers that supply the system from the high-voltage industrial network. The distance between substations is usually between 40 and 60 km. Autotransformers are installed regularly every 10 to 15 km between two consecutive substations [45].

The railway traction supply system has to be protected against faults so circuit breakers and switches are vital to isolate the faulted section and keep the system running. The heart of the protection system is a distance protection relay whose shape varies depending on the manufacturer and it is programmed to allow heavy loads without causing a trip. The same relay has an integrated overcurrent protection to clear close-in faults as fast as possible and a close-onto-fault protection [46].
