Power Supply of Electric Railroads

the conversion and transmission of electric power to electric rolling stock, accomplished by a special system comprising traction substations, a contact system, and connecting lines between the two. In the USSR the power supply system of an electric railroad is closely tied to the overall power system and is used to supply electricity to regional and nontraction railroad consumers.

Electricity arrives at the traction substations over three-phase, high-voltage power transmission lines and, after the required conversion, is transmitted over feed and negative booster lines to the contact system and thence to the rolling stock. The traction substations are connected to the power transmission lines in such a way that a fault in the latter will not cause more than one substation to lose power. Depending on the electrical equipment used in the rolling stock, the power may be supplied over DC systems, single-phase systems at the standard commercial frequency (in the USSR, 50 hertz), or a single-phase system at a lower frequency (such as 16 ⅔ or 25 Hz). The small track sections with a three-phase power supply to the rolling stock that were formerly operated abroad have not expanded and have been reequipped with a single conductor. The DC systems used in the USSR have a nominal voltage of 3 kilovolts (kV), and the AC systems at a frequency of 50 Hz have a nominal voltage of 25 kV. (Further electrification of railroads in most cases will be accomplished with an 25-kV AC system.) A 2 × 25-kV system will also be used, in which the contact system is supplied at a voltage of 25 kV from autotransformers located between traction substations; the autotransformers obtain power from the substations over a 50-kV line that connects a special supply conductor and the rails.

The power supply of electric railroads is characterized by the great disparity of equipment loads, the difficulty of protection against short-circuit currents, the nonsinusoidal nature and asymmetry of the currents (in AC systems), the influence on communications lines, and the recovery of power during regenerative braking by locomotives. In order to reduce fluctuations in the voltage supplied to the rolling stock and to improve the system’s power characteristics, transformers with load tap changing, compensating devices, and other apparatus are included in the system. The influence of AC power supply systems on communications lines is reduced by replacing overhead communications lines with cables and, where necessary, by installing negative booster transformers to ensure that all the current will flow along the rails (without leakage to ground) or through a special return conductor. When the contact system is supplied with direct current, smoothing devices are installed at the traction substations to reduce pulsations of the rectified voltage. When regenerative braking is used on locomotives in a DC system, special receivers are provided for power recovery.

Regional and nontraction railroad consumers are supplied with power over transmission lines at voltages of 10 and 35 kV; the lines may be mounted on special supports or on the supports of the contact system. Wide use is also made of special lines comprising two conductors suspended on the supports of the contact system with the rails acting as the third conductor in a three-phase system.

Uninterrupted railroad traffic is ensured by the high reliability of equipment used for power supply and by stationary and mobile reserve traction substations. The economy of Soviet railroads is increased by the use of contact systems with a bilateral supply from two neighboring traction substations. The aggregate of equipment used for power supply of electric railroads in the USSR represents a highly automated system. Day-to-day control is realized by power dispatchers, who perform the necessary switching and monitor the condition of the equipment by remote control or in cooperation with service personnel. Electrical engineering maintenance shops and mobile railroad-car shops for servicing the contact systems are provided to maintain and improve power supply equipment.

The methods used to select equipment parameters (the power ratings of transformers and rectifiers, the cross section of conductors used for contact systems, the power ratings of compensating devices, and the settings of protective systems and systems for automatic voltage regulation) differ substantially from those used for power supply systems of other facilities. Methods of probability theory and system simulation by computer modeling are used in calculating parameters that will provide the required reliability and economy for the power supply of electric railroads.