All measures to improve the reliability of conventional DC transmission are still effective for improving the reliability of UHV DC transmission, and should be further strengthened. Mainly include: reduce the failure rate of components; adopt reasonable structural design, such as modularization, open type, etc .; widely adopt the concept of redundancy, such as parallel redundancy of control and protection systems, water cooling systems, and serial redundancy of thyristors; Strengthen equipment status monitoring and equipment self-checking functions.

In view of the problems existing in conventional DC projects, such as the station power system that once caused the DC system pole or bipolar outage, the converter body protection relay, the DC protection system unit failure and other weak links, in the UHV DC transmission system Measures will be taken to improve during design and construction. In addition, the training of operation and maintenance personnel will be strengthened, and the spare parts for wearing parts will be appropriately increased.

To improve the reliability of the UHV DC transmission project, the design principles can also ensure that each pole and each converter of each pole are independent of each other to the greatest extent, to avoid failure transmission between each other. In addition to the main circuit, its independence also needs to be considered: valve hall layout, power supply system, water supply system, cable trench, control and protection system, etc.

What is the reliability index of UHV DC transmission?

The voltage of the southwest hydropower UHV DC transmission project planned to be constructed in China is ± 800 kV. The main wiring method is different from the existing DC projects in China. Each pole uses two 12-pulse converters in series. If a 12-pulse converter fails, the sound converter can still operate with any converter of the same pole-to-end converter station, so the probability of unipolar outage will be significantly reduced, considering the first This UHV DC project lacks experience, and the feasibility study report initially proposed the same reliability indicators as the Three Gorges-Shanghai DC project. After the technology is mature, the number of outages is expected to be reduced to less than 2 times / (per pole • year). The probability of a bipolar outage will also drop significantly, and can be controlled at 0.05 times / year. In addition, due to the improvement of system research level, equipment manufacturing technology, construction and operation level, due to the increase in the number of DC projects and the accumulation of relevant experience, the average converter failure rate is expected to be controlled at 2 times / (per converter • year) . Overall, UHV DC projects will be more reliable than conventional DC.

What is the actual operating index of China's DC transmission?

As of the end of 2005, China has completed five ± 500 kV HVDC transmission projects. They are: Gezhouba-Nanqiao DC transmission project, Tianshengqiao-Guangdong DC transmission project, Three Gorges-Changzhou DC transmission project, Three Gorges-Guangdong DC transmission project and Guizhou-Guangdong I return DC transmission project, with a total conversion capacity of 24 million kilowatts The total length of the DC line reaches 4741 kilometers. China has built the first back-to-back DC project-Lingbao back converter station, with a voltage of 120 kV and a capacity of 360,000 kW. In the past 3 years, the energy availability rate of all projects has exceeded 80%; especially for the Sanchang and Sanguang projects, the energy availability rate has been above 90%. The number of single-pole trips is generally specified in the contract as 5 times / year or 6 times / year. The actual number of occurrences of each project does not exceed the contract requirements; in particular, the Sanchang and Sanguang DC projects reach within the first year of commissioning In accordance with the requirements of the contract, this is also rare in the history of DC transmission in the world, and the failures are caused by auxiliary system problems. With the improvement of DC transmission technology, the reliability index of DC transmission is expected to be further improved.

What are the specific indicators of the reliability of DC transmission systems?

The reliability index of the DC transmission system totals more than 10 items. Here we only introduce the four main reliability indexes of the number of outages, derated equivalent outage hours, energy availability, and energy utilization.

Number of outages: including the number of forced outages caused by system or equipment failures. For the commonly used bipolar DC transmission system, it can be divided into unipolar outage, and bipolar outage due to the simultaneous shutdown of two poles due to the same reason. For DC transmission systems with multiple independent converters for each pole, the number of outages can also be counted as converter outages. Different outages represent different levels of disturbance to the system.

Equivalent downtime for derating: The DC transmission system is completely or partially shut down or some functions are damaged, so that the transmission capacity is lower than the rated power is called derated operation. The equivalent downtime for derating is: multiplying the duration of the derating operation by a factor, which is the ratio of the transmission loss capacity of the derating operation to the system's maximum continuous transmission capacity.

Energy availability: a measure of the degree of energy transmission limitation due to forced and planned outage of converter station equipment and transmission lines (including cables), mathematically defined as the transmission capacity multiplied by the corresponding in various states of the DC transmission system within the statistical time The sum of the duration and the maximum allowable continuous transmission capacity multiplied by the percentage of the statistical time.

Energy utilization: refers to the ratio of the energy delivered by the DC transmission system within the statistical time to the rated transmission capacity multiplied by the statistical time.

What is the periodic statistics and evaluation of the reliability index of the DC transmission system?

The DC transmission system is a complex, self-contained engineering system that in most cases undertakes large-capacity, long-distance transmission and networking tasks. Therefore, it is necessary to set some reliability indexes of the DC transmission system to measure the reliability of the DC transmission system to achieve its design requirements and functions, and to evaluate the operation performance of the DC transmission system. The reliability of the DC system directly reflects the level of the system design, equipment manufacturing, engineering construction and operation of the DC system. Through the reliability analysis of the DC system, specific measures to improve the reliability of the project can be proposed, and reasonable index requirements for the new project can be proposed. The International Power Grid Conference specially established a DC transmission system reliability working group to conduct comprehensive statistics and evaluation of the reliability of all DC transmission projects in the world every two years.

How to determine the width of the corridor of the UHV DC transmission line and the scope of house demolition when the line is adjacent to the residential house?

The corridor width of the UHV DC transmission line is mainly determined by two factors: 1. The requirement of ensuring the electrical clearance when the wire is at maximum wind deviation; 2. The meeting of electromagnetic environment indicators (including electric field strength, ion current density, radio interference and audible noise) Limit requirements. According to the characteristics of line erection, the impact is most severe in the center of the gear span. Studies have shown that for UHV DC projects, when the line is adjacent to residential houses, demolition measures are taken to ensure that the electrical clearance and environmental impact of the project after completion of the project meet the requirements of the national regulations. Usually, the feasibility study in the early stage of project construction should calculate the indicators of electric field strength, ion current density, radio interference and audible noise. Only when these indicators meet the relevant national regulations, the project can meet the approval conditions.

How to choose the wire type of UHV DC transmission line?

In the UHV DC transmission project, the choice of line conductor type must meet the requirements of environmental protection in addition to the long-distance safe transmission of electrical energy. Among them, the requirements of line electromagnetic environment limits have become the most important factor in wire selection. At the same time, economically speaking, the choice of line conductor type is also directly related to project construction investment and operating costs. Therefore, in addition to meeting the requirements of economic current density and long-term allowable current carrying capacity, the study of the UHV DC conductor cross-section and split type also needs to take into account the electromagnetic environment limits, construction investment, and operating losses. Methods, calculations of the field strength of the wire surface and the halo voltage at different altitudes, as well as the analysis of the electric field strength, ion current density, audible noise, and radio interference to determine the final wire split type and sub-wire cross section. For ± 800 kV UHV DC projects, in order to meet the requirements of environmental impact limits, especially the requirements of audible noise, the conductor structure of 6 × 720 square millimeters and above should be used.

How is the number of insulators of DC transmission lines determined?

Due to the electrostatic adsorption of the DC line, the pollution level of the DC line is higher than that of the AC line under the same conditions. The number of insulators required is also greater than that of the AC line. Its insulation level is mainly determined by the pollution discharge characteristics of the insulator string. Therefore, at present, there are two main methods for selecting the number of insulators: 1. Use the insulator pollution tolerance method according to the artificial pollution test of insulators, measure the pollution flashover voltage of insulators under different salt densities to determine the number of insulators. 2. According to the operating experience, the creepage specific distance method is adopted. In general, the creepage specific distance of DC lines is twice that of AC lines. Of the two methods, the former is intuitive, but it requires a large number of tests and test data, and the results of test tests are widely dispersed. The latter is simple and easy to implement, but the accuracy is poor. In practice, the two are usually combined.

What problems should be solved for the insulation coordination design of DC transmission lines?

The insulation coordination design of the DC transmission line is to solve various possible gap discharges in the center of the line tower and the gear span, including wire-to-tower, wire-to-lightning conductor, wire-to-ground, and insulation selection and coordination between different pole conductors. The specific contents are: selecting insulator types and determining the number of insulator strings, determining the air gap of the tower head, and the spacing of the pole conductors for different projects and atmospheric conditions, so as to meet the reasonable insulation level of the DC transmission line.

What is the "static suction effect" of DC?

Under DC voltage, the charged particles in the air will be attracted to the surface of the insulator by the force of the electric field in a constant direction. This is the "electrostatic dust suction effect" of DC. Due to its effect, under the same environmental conditions, the surface area pollution of the DC insulator can be more than double that under the AC voltage. As the amount of pollution continues to increase, the insulation level decreases accordingly, and under certain weather conditions, pollution flashover of insulators is prone to occur. Therefore, due to the technical characteristics of the DC transmission line, compared with the AC transmission line, the external insulation characteristics are more complicated.

What key technical issues need to be studied for the construction of UHV DC transmission lines?

Compared with AC overhead lines, DC overhead lines have no significant differences in the design and calculation of mechanical structures. But in terms of electrical, it has many different characteristics and needs special research. For the construction of UHV DC transmission lines, special attention should be paid to the following three aspects of research:

1. Corona effect. Under normal operating conditions, the DC transmission line allows a certain amount of corona discharge to occur on the conductors, which will cause corona loss, electric field effects, radio interference, and audible noise, etc., resulting in DC transmission loss and environmental impact. Due to the high voltage of the UHV project, if it is not designed properly, its corona effect may be greater than that of the UHV project. Through research on the characteristics of UHV DC corona, reasonable selection of wire type, insulator string, and hardware assembly type can reduce corona effect, reduce operating loss and impact on the environment.

2. Insulation coordination. The insulation coordination of the DC transmission project has a great influence on the investment and operation level of the project. Due to the "electrostatic dust suction effect" of DC transmission, the dirt accumulation and flashover characteristics of insulators are very different from those of AC. The resulting pollution discharge is more serious than that of AC. Choosing the insulation coordination of DC lines for improving operation Level is very important. Since UHV DC 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 an in-depth study on the insulation coordination of UHV DC transmission.

3. Influence of electromagnetic environment. The use of UHV DC transmission is undoubtedly of great significance for achieving a wider range of optimal resource allocation, improving the utilization rate of transmission corridors and protecting the environment. However, compared with the UHV project, the UHV DC transmission project has the characteristics of high voltage, large conductors, high towers, and wide single-circuit line corridors. Its electromagnetic environment is somewhat different from that of ± 500 kV DC lines, which brings The environmental impact of it will inevitably attract the attention of all sectors of society. At the same time, the electromagnetic environment of the UHV DC project is closely related to the type of conductor and the height of the wiring. Therefore, a careful study of the electromagnetic environment impact of UHV DC transmission is crucial to meet the environmental protection requirements and reduce the cost of engineering construction.

What are the basic types of DC transmission lines?

In terms of its basic structure, DC transmission lines can be divided into three types: overhead lines, cable lines, and overhead-cable hybrid lines. DC overhead lines are used more and more in the construction of power grids due to their simple structure, low line cost, high corridor utilization rate, low operating loss, convenient maintenance, and meeting the requirements of large-capacity and long-distance transmission. Therefore, DC overhead lines are usually DC overhead lines, and cable lines should only be considered when the overhead lines are restricted.

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