Q: What are the basic types of DC transmission lines?

A: In terms of its basic structure, HVDC lines can be divided into three types: overhead lines, cable lines and overhead - cable hybrid lines. Dc overhead line has been used more and more in power grid construction because of its characteristics of simple structure, low cost, high utilization rate of corridor, small operation loss, convenient maintenance and meeting the requirements of large capacity and long distance transmission. Therefore, DC transmission lines usually use DC overhead lines, and cable lines are only considered when overhead lines are limited.

Q: What are the key technical issues to be studied in the construction of UHVDC transmission lines?

A: There is no significant difference in mechanical structure design and calculation between DC and AC overhead lines. But in the electrical field, there are many different characteristics that require special study. For the construction of UHVDC transmission lines, attention should be paid to the following three aspects:

1. Corona effect. A certain degree of corona discharge is allowed in the normal operation of THE HVDC transmission line, which will cause corona loss, electric field effect, radio interference and audible noise, leading to the operation loss and environmental impact of the HVDC transmission line. Due to the high voltage, the corona effect of UHV engineering may be larger than that of UHV engineering if it is not properly designed. Based on the research on the characteristics of UHV DC corona, reasonable selection of wire type, insulator string and metal assembly type can reduce corona effect, operation loss and impact on the environment.

2. Insulation coordination. The insulation coordination of HVDC transmission projects has great influence on the investment and operation level of projects. Due to the "electrostatic vacuuming effect" of DC power transmission, the pollution accumulation and flashover characteristics of insulators are very different from those of AC power transmission, and the pollution discharge caused by this is more serious than that of AC power transmission. It is very important to choose the insulation coordination of DC lines reasonably to improve the operation level. As the UHVDC transmission is the first in the world, the existing test data and research results at home and abroad are very limited, so it is necessary to conduct in-depth research on the insulation coordination of UHVDC transmission.

3. Electromagnetic environment impact. The use of UHVDC is of great significance for optimizing resource allocation in a wider range, improving the utilization rate of transmission corridors and protecting the environment. However, compared with UHV projects, UHV DC power transmission projects have the characteristics of high voltage, large wire, tower height, single circuit corridor width and so on. The electromagnetic environment of UHV DC power transmission projects is different from ±500 kV DC lines to some extent. The environmental impact brought about by this is bound to attract the attention of all walks of life. At the same time, the electromagnetic environment of UHVDC engineering is closely related to wire type and wire height. Therefore, it is very important for engineering construction to meet the requirements of environmental protection and reduce the cost to carefully study the electromagnetic environmental impact of UHVDC.

Q: what is the "electrostatic vacuuming effect" of DC?

A: Under dc voltage, charged particles in the air will be absorbed to the surface of the insulator under the action of constant direction electric field force, which is the "electrostatic vacuuming effect" of DC. Because of its effect, under the same environmental conditions, the amount of pollution on the surface of dc insulator is more than twice that of ac voltage. With the increase of pollution, the insulation level decreases, and pollution flashover of insulators is easy to occur under certain weather conditions. Therefore, due to the technical characteristics of DC transmission lines, compared with AC transmission lines, their external insulation characteristics are more complex.

Q: What problems should be solved in insulation coordination design of HVDC transmission lines?

A: Insulation coordination design of HVDC transmission line is to solve all kinds of possible gap discharge in the center of line pole tower and gear distance, including conductor to pole tower, conductor to lightning conductor, conductor to ground, and insulation selection and coordination between conductors of different poles. The specific contents are as follows: In order to meet the reasonable insulation level of HVDC transmission line, the insulator type and the number of insulator strings, the air clearance of tower head and the distance between pole conductors are selected according to different engineering and atmospheric conditions.

Q: How is the number of insulators for a DC transmission line determined?

A: Due to the electrostatic adsorption of DC lines, the pollution level of DC lines is higher than that of AC lines under the same conditions, and the number of insulators is also more than that of AC lines. The insulation level is mainly determined by the pollution discharge characteristics of insulator strings. Therefore, there are two main methods to select the number of insulators at present: 1. According to the artificial pollution test of insulators, the insulator pollution tolerance method is adopted to measure the pollution lightning pressure of insulators under different salt densities, so as to determine the number of insulators. 2. Creepage specific distance method is adopted according to operation experience. In general, the creepage specific distance of DC line is twice that of AC line. Of the two methods, the former is intuitive, but requires a large amount of test and detection data, and the results of test and detection are highly dispersed. The latter is simple and convenient, but less accurate. In practice, the two are usually combined.

Q: How to select the conductor type of UHVDC transmission line?

Answer: In the UHVDC transmission project, the selection of wire type must meet the requirements of environmental protection in addition to safe long-distance transmission of electric energy. Among them, the limit of line electromagnetic environment becomes the most important factor in wire selection. At the same time, economically speaking, the choice of wire type is also directly related to the project construction investment and operation cost. So UHV dc conductor cross section and division of research, in addition to meet the economic current density and allows the carrying capacity of the request for a long time, but also in consideration of the electromagnetic environment limits and under the condition of construction investment, operating loss, by way of different structure, the conductor surface field strength under different altitude, and the halo voltage computing research, The electric field intensity, ion current density, audible noise and radio interference are analyzed to determine the final wire splitting pattern and sub-wire cross section. For ±800 kV UHV DC engineering, in order to meet the environmental impact limit requirements, especially the requirements of audible noise, the wire structure of 6×720 square mm or above should be used.

Q: How to determine the corridor width of UHVDC transmission line and the demolition range of houses adjacent to the line?

Answer: The corridor width of UHVDC transmission line is mainly determined by two factors: 1. The requirement of ensuring electrical clearance when the conductor has the maximum wind deviation; 2. 2. Meet the requirements of electromagnetic environment indicators (including electric field intensity, ion current density, radio interference, and audible noise). According to the characteristics of the line erection, the most serious influence is in the center of the file distance. The research shows that for the UHV DC project, when the line is near the houses, the demolition measures can be taken to ensure that the electrical clearance and environmental impact of the project can meet the requirements of the national regulations. Usually, the indicators of electric field intensity, ion current density, radio interference and audible noise should be calculated when the feasibility study is carried out at the initial stage of engineering construction. Only when these indicators meet the relevant national regulations can the project be approved.