All measures to improve the reliability of conventional DC transmission are still effective to improve the reliability of UHVDC transmission and should be further strengthened. It mainly includes: reducing the failure rate of components; Adopt reasonable structural design, such as modularization and openness; The concept of redundancy is widely used, such as parallel redundancy of control and protection system, series redundancy of water cooling system and thyristor. Strengthen equipment status monitoring and equipment self-inspection functions.
In view of the problems existing in conventional DC projects, such as weak links such as station power system, protective relay of converter transformer and unit failure of DC protection system, measures will be taken to improve the design and construction of UHVDC transmission system. In addition, the training of operation and maintenance personnel will be strengthened, and spare parts for wearing parts will be appropriately increased.
To improve the reliability of UHVDC transmission project, we can also ensure that each pole and each converter of each pole are independent of each other to avoid fault transmission between them. In addition to the main circuit, its independence 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 UHVDC transmission?
The proposed UHVDC transmission project of Southwest Hydropower Station in China has a voltage of ±800kV, and its main connection mode is different from the existing DC project in China, with two 12 pulse converters connected in series at each pole. If a 12 pulsating converter fails, a sound converter can still operate with any converter in the same pole-to-end converter station, so the probability of single-pole outage will be significantly reduced. Considering the lack of experience of the first UHV DC project, the feasibility study report initially puts forward the same reliability index as the Three Gorges-Shanghai DC project. After the technology is mature, it is estimated that the number of outages can be reduced to less than 2 times/(every polar year). The probability of bipolar outage will also be greatly reduced 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, the increase of DC projects and the accumulation of relevant experience, the average failure rate of converter is expected to be controlled at 2 times per converter year. Generally speaking, UHVDC project will be more reliable than conventional DC.
What are the specific indicators of the reliability of DC transmission system?
The total reliability index of DC transmission system exceeds 10. Here, only four main reliability indicators are introduced: outage times, derated equivalent outage hours, energy availability and energy utilization rate. Number of downtime: including the number of forced downtime caused by system or equipment failure. For the commonly used bipolar DC transmission system, it can be divided into unipolar outage and bipolar outage caused by simultaneous outage of two poles for the same reason. For DC transmission systems with multiple independent converters at each pole, the number of outages can also be counted as converter outages. Different power outages represent different levels of interference to the system.
Derated equivalent outage hours: due to total or partial outage or some functional damage, the transmission capacity of DC transmission system is lower than the rated power, which is called derated operation.
The equivalent outage hour of derating is: the duration of derating operation multiplied by a coefficient, which is the ratio of transmission loss capacity of derating operation to the maximum sustainable transmission capacity of the system.
Energy availability rate: a measure of the limit degree of energy transmission caused by the forced and planned outage of converter station equipment and transmission lines (including cables), which is mathematically defined as the percentage of transmissible capacity multiplied by the sum of corresponding duration and maximum allowable continuous transmission capacity multiplied by the statistical time in various States of DC transmission system.
Energy utilization ratio: refers to the ratio of energy delivered by DC transmission system to rated transmission capacity multiplied by statistical time.
Why should the reliability index of DC transmission system be regularly counted and evaluated?
HVDC transmission system is a complex self-contained engineering system, which undertakes large-capacity and long-distance transmission and networking tasks in most cases. Therefore, it is necessary to set some reliability indexes of DC transmission system to measure the reliability of DC transmission system to meet the design requirements and functions, and to evaluate the operational performance of DC transmission system. The reliability of DC system directly reflects the system design, equipment manufacturing, engineering construction and operation level of DC system. Through the reliability analysis of DC system, we can put forward specific measures to improve the reliability of the project, and also put forward reasonable index requirements for new projects. A working group on the reliability of DC transmission systems was established at the International Grid Congress, and the reliability of all DC transmission projects in the world was comprehensively counted and evaluated every two years.
When the line is adjacent to the house, how to determine the corridor width of UHVDC transmission line and the scope of house demolition?
The corridor width of UHVDC transmission lines is mainly determined by two factors: 1. When the conductor is biased by the maximum wind force, ensure the requirements of electrical gap; 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.
How to choose the conductor type of UHVDC transmission line?
In the UHVDC transmission project, the selection of line conductor type must meet the requirements of environmental protection in addition to the 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 ±800kV DC transmission project, in order to meet the requirements of environmental impact limit, especially audible noise.