Power frequency single-phase AC electric locomotive is a high-power single-phase load, which will inevitably affect the symmetry of three-phase power system. Therefore, the important task of the traction substation is not only to convert the high-voltage electricity sent by the power system into the voltage required by the electric locomotive, but also to minimize the adverse impact of the single-phase load of the electric locomotive on the power system by adopting different types of transformers and their connection methods.
In order to realize the above functions, China's railway traction substations not only use ordinary single-phase power transformers and three-phase power transformers, but also use some special transformers, mainly three-phase and two-phase balance transformers, including Scott wiring transformers, impedance matching and non-impedance matching balance transformers, and LeBron wiring transformers used in Taiwan Province Province. In foreign countries, such as Japan, wooden bridge wiring and balanced transformers with improved wooden bridge wiring are also adopted.
According to different types of transformers, traction substations are usually divided into: single-phase traction substations (including pure single-phase substations, single-phase V-junction substations and three-phase V-junction substations); Three-phase substation; Three-phase and single-phase substations (including Scott wiring substation and impedance matching and non-impedance matching wiring substation).
The rated voltage of the primary side of the traction transformer is 1 10 kV or 220 kV, and the rated voltage of the secondary side is 55 kV or 27.5 kV, which is 10% higher than the rated voltage of the traction network.
Traction transformer has a large capacity, generally tens of kilovolts A or tens of millivolts A. For example, the capacity of 20,000 kilovolts A is 20 millivolts A. At present, there are nine commonly used rated capacity grades (MVA); Namely 10, (12.5), 16, 20, 25, 3 1.5, 40, 50, 63, etc. The capacity of Scott connection transformer in Shenmu-Shuozhou coal transportation trunk line reaches 75mv·a, and the above capacity grades are designed according to the standard series of R 10, that is, the multiple of10/10 is increased. For example: 25kVA× (1010) = 25kVA×1.258 9 = 31.4725kVA ≈ 31.5kVA.
Because of the large capacity and large number of traction transformers, the capacity utilization rate of transformers has become an important economic index for traction substation operation. The so-called capacity utilization ratio refers to the ratio of rated output capacity of transformer to rated capacity, that is, r = rated output easy/rated capacity. The low utilization rate of transformer capacity not only wastes capital investment, but also increases operating cost. Because traction power supply not only costs electricity, but also charges basic electricity according to transformer capacity. The basic electricity charge per KVA is 20 yuan (RMB).
In order to improve the reliability of traction power supply, two transformers are generally installed in traction substation, which is called redundant configuration. Each transformer can withstand full load. During normal operation, one machine works and the other machine is used as a backup for maintenance or failure.
At present, there are several types of traction transformer wiring widely used, namely, single-phase transformer wiring, three-phase transformer wiring, Scott wiring and impedance matching and non-impedance matching balance transformers.