4. Economically, the main line inve

4. Economically, the main line investment and operating costs should be economically rational to achieve operational efficiency. Within that range. The economy mainly depends on the mode and number of bus structure, the capacity, number and structure of main transformer, the number of high-voltage circuit breakers, the area of distribution equipment and structure, etc. . There is often a contradiction between economy and reliability, therefore, to increase the main wiring, in the determination of the mode of the main line, to ensure the reliability of power transformation, or to ensure that the user's feeder equipment from different transformer, in particular, the single-bus non-segment is relatively simple, compared with the bus, with less equipment, lower distribution and higher wiring, simple and easy wiring. In order to increase the flexibility of equipment, it is necessary to increase the amount of equipment and investment. In addition, with the rapid economic development, the corresponding growth of railways, urban traffic, traction power increased. It is also common to increase the variety and expansion of feeders and other equipment. Therefore, the design of electrical mainlines should also be precise and long-term planning, leaving room for future construction, especially the design of Urban Rail Transit Substations, attention should also be given to the way in which locations are arranged and urban planning developed, as well as the way in which lines are placed between substations. Whatever is between the transformer and the line used in the substation. It is important to ensure the flexibility and reliability of the connection. The solution mainly adopts time line system, and different methods of pay line pulling are also used, so that the transformer can provide power for small users at least, or to ensure that the user's ground can get power from different transformers, the time line is also known as the bus, so in principle it is the circuit of electrical equipment.