Brief introduction of UHVDC transmission lines

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

A: As far as its basic structure is concerned, DC transmission lines can be divided into three types: overhead lines, cable lines and overhead cable hybrid lines. Overhead DC lines are widely used in power grid construction because of their simple structure, low line cost, high corridor utilization rate, low operating loss and convenient maintenance, which can meet the requirements of large capacity and long distance transmission. Therefore, DC overhead lines are usually used for DC transmission lines, and cables are only considered when overhead lines are limited.

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

Answer: Compared with AC overhead lines, there is no obvious difference in mechanical structure design and calculation. But in electrical aspect, it has many different characteristics and needs special research. For the construction of UHVDC transmission lines, special attention should be paid to the following three aspects:

1. Corona effect. Under normal operation, DC transmission lines are allowed to have a certain degree of corona discharge, which will lead to corona loss, electric field effect, radio interference and audible noise, resulting in operational loss and environmental impact of DC transmission. Due to the high voltage of UHV project, if it is not designed properly, its corona effect may be greater than that of UHV project. By studying the corona characteristics of UHV DC, the conductor type, insulator string and hardware assembly type are reasonably selected to reduce corona effect, operation loss and environmental impact.

2. Insulation coordination. The insulation coordination of DC transmission project has great influence on the investment and operation level of the project. Due to the electrostatic dust collection effect of DC transmission, the pollution accumulation and pollution flashover characteristics of insulators are very different from those of AC transmission, and the pollution discharge caused by this is more serious than that of AC transmission. Reasonable selection of insulation coordination of DC transmission lines is very important to improve the operation level. Because UHVDC transmission is the first case 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 insulation coordination of UHVDC transmission.

3. Electromagnetic environmental impact. The adoption of UHVDC transmission is undoubtedly of great significance to realize the optimal allocation of resources in a wider range, improve the utilization rate of transmission corridors and protect the environment. However, compared with EHV project, UHVDC transmission project has the characteristics of high single-circuit line voltage, large conductor, high tower and wide corridor. Its electromagnetic environment is different from that of 500 kV DC line, and its environmental impact is bound to be concerned by all sectors of society. At the same time, the electromagnetic environment of UHVDC project is closely related to the type of conductor and the height of overhead line. Therefore, it is of great significance to study the influence of UHVDC transmission on the electromagnetic environment of engineering construction to meet the requirements of environmental protection and reduce costs.

Q: What is the "electrostatic cleaning effect" of DC?

A: Under DC voltage, charged particles in the air will be attracted to the insulator surface by the constant electric field force, which is the "electrostatic dust collection effect" of DC. Because of its function, under the same environmental conditions, the surface pollution of DC insulators can be more than twice as large as that under AC voltage. With the increase of pollution, the insulation level decreases. Under certain weather conditions, insulators are prone to pollution flashover. Therefore, due to this technical feature, the external insulation characteristics of DC transmission lines are more complicated than those of AC transmission lines.

Q: What problems need to be solved in the insulation coordination design of DC transmission lines?

Answer: The insulation coordination design of DC transmission line is to solve all kinds of gap discharges that may occur in transmission line towers and span centers, including insulation selection and mutual coordination between conductors to towers, conductors to lightning wires, conductors to ground, and conductors with different poles. Its specific contents include: selecting insulator types and determining the number of insulator strings, determining the air gap at the tower head and the distance between poles and wires. Different insulation levels are selected according to different engineering and atmospheric conditions to meet the reasonable insulation level of DC transmission lines.

Q: How is the number of insulators on DC transmission lines determined?

Answer: Due to the electrostatic adsorption of DC lines, under the same conditions, the pollution degree of DC lines is higher than that of AC lines, and the number of insulators required is also higher than that of AC lines. Its insulation level mainly depends on 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 pollution lightning voltage of insulators under different salt densities is measured by insulator pollution tolerance method, so as to determine the number of insulators. 2. According to the operation experience, the creepage ratio method is adopted. Generally speaking, the creepage ratio of DC line is twice that of AC line. Of the two methods, the former is intuitive, but it needs a lot of test data and the test results are scattered. The latter is simple, but the accuracy is poor. In practical application, the two are usually combined.

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

Answer: In the UHVDC transmission project, the selection of line conductor type must meet the requirements of environmental protection besides long-distance safe transmission of electric energy. Among them, the requirement of electromagnetic environment limit value of transmission line becomes the most important factor in conductor selection. At the same time, economically speaking, the choice of line conductor type is also directly related to the construction investment and operation cost. Therefore, the research on the section and split type of UHVDC conductor should not only meet the requirements of economic current density and long-term allowable current carrying capacity, but also comprehensively consider the electromagnetic environment limit, construction investment and operation loss. By calculating and studying the surface field strength and corona discharge voltage of conductor under different structural modes and altitude, and analyzing the electric field strength, ion current density, audible noise and radio interference, the final split type and split type of conductor can be determined. For the ±800kV DC transmission project, in order to meet the requirements of environmental impact limits, especially the requirements of audible noise, the conductor structure of 6×720 mm2 and above should be adopted.

Q: When the line is adjacent to the house, how to determine the corridor width of the UHVDC transmission line and the scope of house demolition?

Answer: The corridor width of UHVDC transmission lines is mainly determined by two factors: 1. Requirements for ensuring the electrical gap when the conductor deviates from the maximum wind force; 2. Meet the requirements of the limits of electromagnetic environment indicators (including electric field intensity, ion current density, radio interference and audible noise). According to the characteristics of line erection, the influence is the most serious in the middle span. The research shows that for UHVDC project, when the line is close to the house, the electrical gap and environmental impact after the project is completed can meet the national requirements by taking demolition measures. Usually, when the feasibility study is carried out in the early stage of engineering construction, the electric field intensity, ion current density, radio interference, audible noise and other indicators should be calculated. Only when these indicators meet the relevant national regulations can the project meet the examination and approval conditions.

The above content is taken from the introduction of UHVDC transmission lines by State Grid, which should be regarded as authoritative. I hope my answer is helpful to you. I hope it can be adopted. O (∩ _ ∩) O。