A multi-criteria approach to the analysis of modified 2x25kV bi-voltage systems using higher negative voltages

In bi-voltage systems, the positive voltage is normally established by the standards while the negative voltage can be chosen by the designer. Often the negative voltage is set to the same value used for the positive one in order to simplify the design, the operation and the maintenance. However, using a higher negative voltage can significantly enhance the capacity of the power supply system. This paper analyzes the benefits of using higher negative voltages from a technical point of view. To perform this analysis, a multi-criteria approach is used to find the most efficient negative voltages in relation to their cost. A. Introduction In the last years, railway consumptions have significantly increased to allow higher operations speeds, denser traffics and increasingly consuming auxiliary services (air conditioning, lighting, information systems, etc.). Thus, railway electrifications are progressively migrating to higher feeding voltages and new lines are very often electrified in AC, especially for high-speed lines. In addition, existing AC-fed lines are also being migrated to bi-voltage systems, such as 2x25kV, as a way to increase feeding voltages. However, in some important railway corridors, in some cases, it is simply not enough. In every case the possible solutions for upgrading the electrical capacity of the railways lines are different and normally include: (i) the addition of substations and/or (ii) the reinforcement of the catenaries with additional conductors. For a given frequency, UIC standards do not consider different nominal voltages and thus increasing the transmission voltage is not an option. A solution for using higher voltages consists on using bi-voltage systems (such as 2x25kV system), where electrical power is transmitted in a higher voltage and afterwards reduced nearby the trains by means of autotransformers. In such systems, the positive feeding voltage is established by UIC standards but negative can be chosen freely. Normally, the negative voltage is set to the same value used for the positive one in order to simplify the design, the operation and the maintenance. However, using higher negative voltages can significantly enhance the capacity of the power supply system. This paper analyzes the benefits of using higher negative voltages from a technical and economical point of view. To perform this analysis, a multi-criteria approach is used to find the most efficient negative voltages in relation to their cost. In the section B the AC power supply system in railways in described. Then, section C presents the equivalent model used to represent bi-voltage systems (f.i. 2x25kV) as if they ware monovoltage systems (f.i. 1x25kV). After that, section D discusses the advantages of using higher transmission voltages. Compatibility with other different power systems is discussed en section E. Finally, in section F the conclusions of this work are presented. B. Description of AC electrifications The general structure of an AC power-supply system [2] is described in Figure 1: Sector 3-R Sector 1-R Sector 2-L Sector 2-R Sector 3-L Sector 1-L Three-phase high-voltage network Traction substation 1 Traction substation 2 Traction substation 3 Figure 1. General structure of the power supply system The railway electrical system is divided in electrically-isolated single-phased sectors, which are fed from the three-phase network through a traction substation. Normally each substation has 2 transformers, each of whom is connected between two of the three phases (in the in the three phase network) and feeds one sector. It should be noted that topology can be modified in case of failures to guarantee the operation. For instance, if one of the transformers of a substation fails, the other takes on the corresponding sector. Each of these sectors can use either mono-voltage system or bi-voltage system [1, 3]. In monovoltage systems, the feeding conductors are set to the specified voltage level (see Figure 2). In bi-voltage systems, a higher voltage is set between feeding conductors [5]. This voltage is reduced by using autotransformers distributed along the catenary (see Figure 3). Typically distances are between autotransformers are in the range 10÷15 km. High-Voltage network Positive