The electricity transmission line is realized by boosting the power generated by the generator with a transformer and then connecting it to the power transmission line through control equipment such as a circuit breaker. The transmission lines are divided into overhead transmission lines and cable lines.
Overhead transmission lines mainly refer to power transmission lines that are erected on the ground and fixed with transmission insulators to pole towers standing on the ground to transmit electrical energy. Overhead lines are a widely used method of power transmission.
Cable line is an electrical signal transmission system consisting of power transmission cables and auxiliary equipment, such as cable lugs, cable connectors, cable wire terminals. Cable lines are often used in special situations such as urban underground power grids, power generation station outlets, internal power supply of industrial and mining enterprises, and power transmission under seawater across the river.
Path selection and survey are the key to the entire line design. The rationality of the scheme plays an important role in the economic, technical indicators and construction and operating conditions of the line. In order to shorten the length of the route reasonably, reduce the investment of the route and ensure the safety, reliability and convenient operation of the route, it is sometimes necessary to determine the best scheme for a route for many times. The route survey is a comprehensive test of the professional level, patience and responsibility of the designers.
In the stage of project route selection, the designer shall fully search and investigate the above-ground, underground, under construction and proposed engineering facilities along the line according to the actual situation of each project, and carry out multi-path scheme comparison and selection, so as to select the scheme with short length, less corners, less crossing and better terrain conditions as far as possible. Considering the compensation cost and civil work, try to avoid trees, houses and cash crop planting areas.
In the survey work, both the economic rationality of pole positions and the possibility of establishing key pole positions (such as corners, intersections, and special locations where pole towers must be established, etc.) are taken into account. The repeated measurement and comparison shall be carried out in some special sections, so that the position of the pole and tower can avoid the traffic difficult area as much as possible, so as to create better construction conditions for the assembly of towers and tight lines.
Different tower types are different in terms of cost, land occupation, construction, transportation, and operation safety. The cost of the tower project accounts for about 30% to 40% of the entire project. The reasonable choice of tower type is the key.
For the new construction project, if funds permit, only 1-2 kinds of linear cement poles are generally selected, and angle steel towers are selected as far as possible. The material preparation is simple and clear, the construction operation is convenient, and the safety level of the line is improved. For the lines with multiple circuits of the same tower and built along the planned road, the steel tube tower with less floor space is generally used for the tower. But if the steel towers are used for large corner towers, the top of the pole will be easily deformed due to structural reasons, and the foundation construction cost will be doubled than that of the angle steel tower. So the straight tower uses steel pipe tower and the corner tower uses angle steel tower, which is more reasonable and can meet the requirements of environment, investment and safety.
In view of the hidden danger caused by insufficient distance to ground after more than ten years of operation of many old lines, choosing the appropriate selection of higher pole towers and reducing the horizontal distance in the design of new lines can increase the distance between the conductor and the ground. In the line elevation project, a Y-shaped steel pipe tower with a small footprint and easy installation is designed. The construction period can be shortened from 3-5 days of the traditional tower to 1 day, which can reduce the construction outage time.
The tower foundation is an important part of the transmission line structure, and its cost, construction period and labor consumption account for a large proportion in the entire line project. The construction period accounts for about half of the entire construction period, the transportation volume accounts for 60% of the entire project, and the cost accounts for about 20% to 35% of the entire project. The tower foundation selection, design and construction directly affect the construction of the line project. According to the actual geological conditions of the project, it is important to optimize the design of each foundation tower section by section, especially for the bearing tower.
Looking at the transmission construction projects in recent years, each project has its own characteristics. It is impossible to guarantee the design quality and meet the needs of power grid development if the design is separated from the actual project. Only by combining the actual situation, adjusting measures to local conditions, optimizing schemes, tackling scientific and technological problems, and constantly exploring and innovating can we meet the requirements of building a strong power grid.