Analysis of Influencing Factors of Transmission Line Operation Safety and Prevention Measures

In recent years, with the accelerated construction of China's power grid, the transmission lines of various voltage levels have developed rapidly. Taking the national power grid system as an example, as of the end of 2013, the AC transmission lines of voltage levels of 110 (66) kV and above have reached 755,000 km, and DC transmission. The number of lines is 14,400 km, including 105,400 km for 500 kV lines and 12,700 km for 750 kV lines. The length of the UHV lines under construction is more than 10,000 km. Due to the wide coverage of transmission lines, especially the transmission distances of super and UHV lines are long. The meteorological and geographical environment is complex, and the probability of encountering extreme weather is greater. In particular, some important transmission channels are often densely arranged with multiple super-high-voltage lines. In extreme weather conditions, multiple return lines may trip at the same time.

The main faults affecting the safe operation of transmission lines include lightning strikes, ice damage, mountain fires, wind deviations, pollution flashes, and bird damage. Over the years, through the implementation of a series of prevention measures against the above-mentioned faults, some of them have shown a significant downward trend. However, with the changes in the operating environment and influencing factors, some line faults have a growing trend, and some hidden dangers have not yet caused the failure, but need to be analyzed early to prevent possible harm. From the statistical analysis in recent years, the line faults caused by mountain fires and ice damage have a growing trend. Lightning strike faults are still one of the main faults of line operation. In some areas where wind speeds occur frequently, wind bias discharge faults still occur from time to time. The possible impact of large-scale haze weather on transmission lines also needs to be analyzed and prevented. Preventing grid accidents caused by line faults, especially large-scale power outages, is still the top priority of line operation and maintenance. Therefore, it is necessary to analyze the characteristics, mechanism and laws of the main faults in combination with the operation of the line, and to explore related prevention measures.

1 Fault Type 1.1 Mountain Fire In recent years, mountain fires have occurred frequently in some areas, posing a serious threat to the safe operation of the line. For example, in the Southern Power Grid during the period of 2010-012010-03, there were 68 times and 60 times of mountain fire trips respectively, of which the single-phase fault ratio was 70.3%, and the single-phase tripping re-closing success rate caused by the mountain fire occurred. 52%, and the resulting multi-phase trip reclosing success rate is only 36.7%. Also as in January 2014, affected by dry and rainy weather, Hunan, Hubei, Jiangxi, Sichuan, Fujian and other places frequently hit a mountain fire, 500kV and A total of 223 hill fires occurred in the transmission line corridors of the above voltages, resulting in 67 trips or emergency shutdowns, and 63 reclosings. One of the mountain fires near the UHV DC corridor was as shown. The causes of frequent mountain fire failures in various transmission lines: 1) With the development of the power grid, the number of routes passing through dense vegetation areas increases; 2) the mountain fires caused by vegetation environment, climate and customs during the dry season in winter and the festival season in spring. The probability of occurrence has increased.

The discharge of the line gap under the mountain fire condition has a large dispersibility, mainly due to the large dispersion of the parameters such as the temperature of the mountain fire, the concentration of the soot, the height of the pyrotechnic mixture area, and the conductivity of the soot particles. At present, there are various mechanism analysis models for the transmission of mountain fires on transmission lines, including thermal free model, air density model, conductivity model, flow injection model, etc. Most of the above models analyze the mechanism of mountain fire discharge from one aspect. In fact, mountain fires are often the result of a combination of the above factors. From the large number of experiments on simulating mountain fire discharge at home and abroad, the high temperature of mountain fires leads to a decrease in air density in the line gap, and a large number of soot particles with a particle size of 0.2 to 5 are generated during combustion. The main components include organic carbon and elemental carbon. And potassium ions, etc., the smoke particles have high conductivity, the flue is easy to trigger electron collapse under the action of high electric field, and the decrease of high temperature and air density promotes the development of electron collapse and flow, and the breakdown voltage of the gap decreases. Lead to breakdown of the line gap. The fault of mountain fire generally has the following characteristics: the success rate of reclosing is low, and when the transmission line trips, the reclosing time is generally about 1 s, because the high temperature of the mountain fire and the concentration of soot will last for a certain period of time, more than tens of s or more. The insulation strength of the line gap is greatly reduced during a period of time, so the line operating voltage or the operating overvoltage generated by the reclosing may break the gap again. If the smoke becomes thin after the fire passes, the gap insulation strength can be quickly recovered, and the strong delivery success rate is high.

The mountain fire discharge distance is long. Generally speaking, the discharge in the air gap mostly occurs between the shortest path of the charged body and the grounding body. However, since the insulation strength of the channel formed by the flame of the mountain fire and the smoke is greatly reduced, the discharge is often not Occurs along the wire-to-column gap or insulator string, but along a longer pyrotechnic channel.

The specific climate, geography and vegetation environment have established the development trend of mountain fires. If the wind is strong when the mountain fire occurs, the air convection can greatly increase the height and intensity of the flame. The wind direction toward the line and the slope of the wind easily lead to the penetration of the wire and the ground. Pyrotechnic channels, the height of the trees under the line can reduce the gap distance, the burning dust particles such as reeds and straws are tip-shaped, and the electric field is easily distorted in the high electric field and stimulates the discharge. These are all important factors affecting the development trend of mountain fire discharge.

In view of the mountain fire failure, it is necessary to start from the channel planning, patrol prevention, monitoring, early warning, pre-judgment, emergency operation and other measures to reduce the impact of mountain fire on the safe operation of the line. Specifically, it is in the following four aspects: avoiding dense arrangement of multiple lines in the mountain fire-prone and multi-channel, and reducing the probability that multiple fires cause multiple lines to trip at the same time. 2) Special inspection and key control are carried out on the vulnerable sections and dangerous points of the mountain fire to clean the surrounding flammable plants. 3) Establish a monitoring system, early detection, early warning and pre-judgment, when the mountain fire occurs and spread, quantitatively measure the flame height, flame temperature, soot particle concentration, etc. through special instruments and meters, analyze the probability of discharge occurrence, and provide for the selection of operational countermeasures. Technical basis. 4) Develop a response plan that combines the status of the mountain fire with the development trend and adopts different coping styles. For mountain fires with only shallow surface flames and no dense vegetation above the line, the smoke can be kept running normally; the mountain fires with only shallow surface flames, low vegetation under the line, and strong smoke and dust, DC lines It can be operated at a reduced pressure; for mountain fires with high flame, high-density vegetation under the line, and easy formation of high-concentration soot channels, the load can be actively transferred and the operation mode can be changed.

1.2 Ice damage Since 2005, snow and ice disasters have had a major impact on the safe operation of the power grid. From 2006-01 to 2007-06, the national network system caused tripping of 500kV lines by ice coatings, accounting for 8.84% of the total number of trips. The 500kV line was unplanned to stop 4 times, accounting for 11.11% of the total number of outages. In particular, the large-scale and continuous ice-covered in 2008 caused a large range of ice flashing, dancing, towering and disconnection of transmission lines. According to statistics, Due to ice coating, the State Grid system caused more than 140,000 towers of 10~110kV lines, and more than 1,500 towers of 220kV and above lines, and 707 substations were shut down. In recent years, ice-covered disasters have repeatedly plagued the power grid, and it is still one of the main threats affecting the safe operation of power grids. In 2014, affected by the continuous low temperature rain and snow weather, 211 35kV and above transmission lines in Chongqing, Hunan, Hubei, Jiangxi and other places were covered with ice, some lines flashed and danced, and some lines appeared inverted towers and broken lines. Lines and other phenomena, as shown.

From the ice-flash mechanism, the flashover of the insulator under light ice-covered conditions is still a flashover formed by the combination of ice and dirt, which is closely related to the surface area of ​​the insulator, the ice conductivity, etc., often occurs during ice melting, and the discharge arc Developed along the surface of the insulator. The investigation shows that many of the insulators have some contamination on the surface of the insulator before ice coating. The conductivity of the supercooled water during the ice coating is related to the atmospheric environment and parameters at that time. The pollution affecting the ice flashing characteristics is actually the surface contamination and the surface. Superposition of wet deposition during ice. The conductive ions in the ice layer will migrate to the surface, causing the ion concentration of the ice surface to be the largest. When the ice is melted, the shape of the transmission line is severely covered with ice, causing the wire to discharge to the ground and the tower to fall into a high-conductivity water film, thereby reducing the ice flash. Voltage. The mechanism and process of ice flashing during light ice coating is similar to that of pollution flashover. Increasing the creepage distance and cleaning of line pollution is an effective measure to prevent light ice flashover. For example, increase the number of insulators and strings when the gap size of the tower head is allowed. Long, clean before the rain and snow. Under the condition of repeated ice, the insulator flashover is closely related to the shape of the icicle and the degree of bridging, and the discharge arc directly develops along the icicle and ice bridge. In general, the thickness of the ice coating increases with the altitude of the area where the line is located, and the duration of the cold freezing weather is the main influencing factor of the thickness of the ice coating. In order to improve the insulator ice flash voltage, different parasitic insulator interleaving methods can be used, and V-type strings and inverted V-strings should be used. Double-strings should increase the series spacing to prevent direct bridging of ice, and the test shows that it has 50% flashover voltage. A certain degree of improvement has been achieved.

The wire dancing is due to the eccentric ice coating changing the cross-section characteristics of the wire, and the vibration instability caused by the wind excitation. The test shows that as the thickness of the ice coating increases, the dancing threshold of the ice-covered wire decreases, and the probability of galloping occurs. Increase. In operation, it is also found that although the ice thickness of some lines is not prominent, the ice on the windward side is thick and the leeward surface is thin, and the unevenness is prominent. The combination of parameters such as wind speed, wind direction, eccentric ice coating and line can satisfy the dancing conditions. At the same time, the dancing of the wire can still occur.

In recent years, tripping faults caused by ice dancing on transmission lines have occurred many times, especially for compact lines, because the spacing between the three-phase wires is small, when the amplitude of the dancing is increased and the phase spacing is insufficient to withstand the operating voltage, Interphase discharge will occur. In response to this problem, measures taken in the past include the installation of phase separation bars, double pendulum defense devices, and the like. From the viewpoint of the operation effect, if the interphase spacers can effectively withstand the continuous dancing impact load without being damaged during the dancing, the phase discharge can be effectively prevented. However, in the operation, the spacer rod has been damaged many times during the dance, and some lines are even damaged in multiple stages. The phase-discharge is caused by the effect of suppressing the dancing spacer. Judging from the damaged parts of the phase-to-space spacers, the synthetic insulating rods are not damaged, and the damage is basically connected to the fittings. This is because the synthetic insulating rod has a stable post-buckling function. Hurricanes and tornadoes are strong winds formed by strong convection in cold and flat areas. The hurricane wind speed is distributed along the height. The tornado is a kind of airflow moving toward the center. Hurricane and tornado have the highest wind speed and strongest destructive force at 4060m from the ground. The maximum instantaneous wind speed of a typical hurricane or tornado can exceed 50m/s. The wind speed exceeds the line design standard. Once the transmission tower is exposed to hurricanes and tornadoes, the wind tower collapses. Accidents will inevitably occur.

In the wind-resistant design of transmission towers, based on the setting of the two important parameters of the meteorological design wind speed and the wind speed reference height, the specified values ​​in China's design regulations are lower than those in foreign countries. For example, the 110330kV tower design is basically resistant to wind. The wind speed is 70°% lower than that of Japan. The basic average wind speed of the 500750kV tower design is 48% lower than that of Japan. Therefore, the low wind-resistant defense benchmark of the transmission tower is also one of the important factors for the wind tower collapse.

In order to improve the wind-resistance capability of the line, the route direction should be optimized in combination with climate, terrain and other conditions, and differentiated design should be implemented to determine the resistance-resistant section plan reasonably. High-strength materials should be used to improve the wind-resistant capacity of the transmission tower. In terms of operation and maintenance, patrols and seasonal special patrols should be strengthened to carry out maintenance and repair in a timely manner. At the same time, emergency recovery and reconstruction mechanisms after extreme weather disasters should be established to provide guarantee for rapid restoration of power supply.

1.4.3 Phase-to-side wind-bias failure of compact lines Compact line towers generally adopt V-type insulator string arrangement. The head gap can be ignored without considering the influence of wind deflection. However, due to the different periods of the conductors in the range of the wind, the phase-to-phase distance It will decrease, and phase-to-phase flashover will occur at the operating voltage or operating overvoltage.

The displacement of the wire is closely related to the length of the gear and the angle between the wind and the wire. Under the same wind force, the larger the angle, the more obvious the effect of the wind. The longer the gear distance, the larger the displacement of the wire. The displacement of the wires in different periods of swing is different from the dance in the state of ice coating. The motion track of the ice-covered dance is that the vertical displacement is greater than the horizontal displacement, while the simultaneous swing is the horizontal displacement greater than the vertical displacement. In order to prevent the phase-to-phase discharge caused by the large reduction of the phase distance under strong wind conditions, the targeted measure is to install the phase-to-phase spacers. After installing the spacers, the amplitude of the wind swing can be effectively controlled, so that the swings in different periods tend to be synchronized to avoid phase-to-phase flashover. In addition, when ice dancing occurs in winter, the phase spacing bar can also effectively control the dancing to prevent phase discharge.

1.5 Pollution flashing In recent years, through the investigation of pollution sources and the measurement of pollution (salt density, gray density), the preparation of pollution maps has been completed; the research and organization management of anti-pollution work has been strengthened; and the insulation has been completed by increasing the investment in technological transformation. Detection and replacement of low-zero porcelain insulators and self-explosive glass insulators; implementation of line cleaning, spraying RTV anti-fouling coatings, conducting charged water washing and cleaning of charged insulating agents, etc., pollution flashovers are effectively contained, many provinces The company realized the flashover and zero trip. In the middle of 20112013, the pollution flashover rate of 110kV and above transmission lines of the national network system has dropped below 1% of the total trip rate of the line.

In recent years, a large number of haze weather has occurred in many areas of China, which has raised concerns about whether large-scale pollution will be induced. In 1990 and 2004, the power grid had many large-scale pollution flashover accidents. After that, according to the saturated pollution degree of the line and the division of the pollution area, the design principle of the insulation configuration and the margin was left, and the external insulation modification method was adopted. Silicone rubber composite insulators are widely used in heavy pollution areas, which significantly reduces the phenomenon of pollution flashover.

From the form of smog, the fog is a small amount of tiny water droplets suspended in the air near the surface, and the atmospheric humidity is nearly saturated when the fog is formed. Niobium is composed of very fine dry dust particles of air and gaseous compounds such as sulfur dioxide and hydroxide. Fog and cockroaches can also occur together, that is, industrial, transportation, and domestic soot become foggy condensation nuclei, forming suspended droplets, and adsorbing gases such as sulfur dioxide and hydroxide which are easily soluble in water. Continue to accumulate under the influence of calm climate, and then form a large range of haze. The adverse effects of haze weather on transmission lines are clear. One is to have a wetting effect on the accumulated pollution on the insulator, and the other is to increase the fouling of the insulator by means of wet deposition.

The results of field test and simulation test show that the thicker the smog, the longer the time, the more the surface area of ​​the insulator increases, and the influence on the upper surface of the insulator is significantly greater than that of the lower surface. The results of the pollution flash test show that under the pollution condition, the pollution flashover voltage of the insulator decreases with the increase of the surface area, and the wetness caused by the contamination is the main reason for the decrease of the insulation performance of the insulator. In addition, the test results also show that the effect of haze on the breakdown voltage of the air gap is not obvious, and the discharge of the transmission line that occurs in severe haze weather is still the creeping discharge of the insulator string.

From the current operating situation, the level of external insulation of the line selected according to the saturated salt density can effectively resist the influence of the haze climate on the transmission line, and the line pollution flash rarely occurs in all areas under a wide range of haze weather. For the possible impact of smog, in the future, monitoring and analysis should be strengthened. Second, the principle of insulation configuration should be adhered to. Third, operation and maintenance should be strengthened for anti-pollution and weak points, especially for substations with insufficient insulation configuration. Some substation post insulators need to be coated with long-lasting RTV or regular cleaning to enhance anti-smudge ability.

In addition, the pollution of power equipment has an impact on rain flash, ice flash, and snow flash. Therefore, monitoring of pollution and pre-judgment analysis under various adverse weather conditions should be strengthened, and targeted protection can be adopted in insulation configuration. Rain flash, anti-ice flash, anti-snow flash insulation structure and parameters, can also be used for de-icing, snow removal and electrification cleaning, cleaning and maintenance operations when needed: 1) Limit the lines that cannot be tuned in the area with heavy pollution or the gap between the towers Use synthetic insulators; 2) Strengthen power-off cleaning and live cleaning in substations, or add climbing skirts or paint RTV coatings; 3) Strengthen salt density measurement and monitoring, combine operational experience and expected environmental pollution conditions, comprehensive inspection and adjustment Classification of dirty areas; strengthening the cleaning of line insulators and the detection and replacement of inferior insulators.

2 Preventive measures A variety of environmental factors affecting the safe operation of transmission lines will exist for a long time, and reducing line operation failures will also be a continuous and intensive work. In order to improve the line operation and maintenance level, it is necessary to further carry out the following discussion and research on the operation and maintenance system and the transportation inspection equipment. 2.1 Establish an operation and maintenance big data comprehensive analysis system combined with ice, wind, pollution, lightning activity, mountain fire-prone areas and other data and line data, geographic data, operational data, meteorological / micro-meteorological data, etc., to build transmission lines Global data system. Through data mining technology, acquire characteristics and rules, establish various fault analysis models, and implement advanced application functions such as line fault analysis and early warning, line equipment and operational status evaluation, and provide decision-making for extreme climate disaster prevention, disaster prevention and emergency repair, and design for differentiation. And provide technical basis for operation and maintenance.

2.2 R&D automatic detection device and mechanized maintenance equipment With the increase of voltage level and the increase of the number and length of lines, in order to improve the operation and maintenance efficiency, the inspection, monitoring, detection and overhaul technologies and equipment of transmission lines need to be further automated, mechanized, The direction of intelligence is developing.

In the aspect of transmission line condition monitoring and detection, the online monitoring and inspection platform is used to enhance the state awareness of the transmission line, and the intelligent transmission network is constructed by combining information, communication and computer technology. Online monitoring is an important technical means to realize the state awareness of transmission lines. At present, online monitoring of 11 types of projects has been carried out on the transmission line. However, the monitoring device has many faults in practical applications, which are reflected in the reliability and power supply of electronic devices. In the future, it is necessary to further develop on-line monitoring equipment with stable state, reliable performance, long operating life and good weather resistance. In terms of the inspection platform, the research and application of the UAV inspection line technology should be intensified, and the line inspection should be carried out by using the visible light detector, infrared imager, ultraviolet imager, airborne laser radar, etc. It is required to develop a long-distance drone inspection system with a remote control distance of 30km or more, and flight control functions such as flight stability control, GPS autonomous line navigation control, geographic matching automatic control, and line tower automatic tracking, so that the drone can be used. The inspection line is developing in full automation.

In terms of maintenance equipment, helicopters and insulated buckets are a portable platform. Due to their high safety and convenient use, they are used in UHV AC and DC transmission lines in Europe and the United States. The application of the arm car in the 500kV AC and DC transmission line maintenance and helicopter in the UHV AC and DC transmission maintenance.

2.3 Research on integrated monitoring systems combining multiple monitoring methods In extreme climate and large-scale natural disasters, as well as in mountainous areas, plateaus, and permafrost regions, grid equipment monitoring methods are greatly limited. In order to dynamically detect the state of the line from the global, all-weather, and all-day time, it is necessary to study the satellite wide-area remote sensing telemetry technology.

Satellite remote sensing technology can carry out wide-area monitoring of terrain, geology, vegetation and transmission line conditions of transmission line corridors in a wide range of disaster weather, including tower damage detection, geological disaster monitoring, corridor fire monitoring, etc. Effective monitoring data.

For the important transmission channels with concentrated distribution of ultra-high voltage and ultra-high voltage transmission lines, a three-dimensional monitoring system combining multiple monitoring methods should be constructed.

2.4 Constructing an integrated operation and maintenance system for transmission lines The integrated operation and maintenance system is composed of a perceptual monitoring system, a fault analysis system, and an inspection and maintenance system. In the sense of the line state and monitoring, a ground-air stereo system is formed. It includes a patrol system consisting of drones, helicopters and robots, a monitoring system consisting of various online monitoring devices, various detection devices, detection systems consisting of instruments, and a wide-area sensing and monitoring system formed by satellite remote sensing telemetry. In terms of state analysis and fault warning, through the global data system of the transmission line, the information and fault characteristics are mined, and the fault analysis model is established to form a line fault identification and early warning system. In the aspect of line maintenance and operation and maintenance, through the research and development and application of mechanized and automated inspection and maintenance equipment, the fault is accurately determined, the hidden danger is timely responded and eliminated, and the emergency repair and rapid recovery capability is improved.

In short, with the help of information methods and methods, the sensory monitoring, state analysis, maintenance and maintenance form an interactive, holistic, efficient and intelligent operation and maintenance system.

3 Conclusions The main faults affecting the safe operation of transmission lines are lightning strikes, ice damage, mountain fires, wind bias, pollution flashes, bird damage, external damage, etc. In recent years, mountain fires, ice coatings, strong winds and other line faults have occurred frequently, seriously affecting For the safe operation of transmission lines, for the important transmission channels with concentrated distribution of ultra-high voltage and ultra-high voltage transmission lines, it is necessary to analyze the laws and characteristics and take targeted prevention measures.

For the failure of mountain fires, various measures such as channel planning, patrol prevention, monitoring, early warning, and emergency operation should be taken. The combination of civil air defense and technical defense should be adopted to reduce the impact on the safe operation of the line. For the phase-to-phase discharge of compact lines caused by galloping, reinforced interphase spacers should be developed. By using high-strength materials, increasing design strength, optimizing the connection structure, etc., the mechanical strength of the spacers can be improved, and the anti-dancing function of the spacers can be fully utilized. . For wind-induced partial discharge caused by strong winds, the inverted tower should optimize the path of the line, implement differentiated design, and use high-strength materials to improve the wind-resistant capacity of the transmission tower. In view of the focus of lightning protection and lightning protection for ultra-high voltage and ultra-high voltage lines, the anti-winding function of the line should be fully considered in the design stage, and the influence of micro-topography on the wrap-around should be fully considered.

Further research on operation and maintenance system and transportation inspection equipment, establish a comprehensive analysis system for operation and maintenance big data, develop automatic detection devices and mechanized maintenance equipment, study integrated monitoring systems combining multiple monitoring methods, and build transmission line integration. Operation and maintenance system.

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