Use of composite insulators to prevent contamination flashover of transmission lines

21-01-2022

The selected insulators for overhead transmission lines not only meet the electrical performance of the power grid, but also meet the requirements of its mechanical performance. If pollution flashover occurs in the power grid, it will cause power outages and other accidents in the power grid. The selection of appropriate insulators plays a vital role in avoiding flashover accidents in transmission lines and ensuring the normal operation of the power system.

Insulators for transmission lines need to operate normally and reliably under normal and extreme conditions such as atmospheric overvoltage, internal overvoltage and long-term operating voltage. However, if the dirty solids, liquids and gases deposited on the surface of the insulator, together with fog, dew, rain, melting ice, melting snow, etc. are under the action of various meteorological conditions, the original electrical strength of the insulator will be greatly reduced, thereby making the transmission of electricity. Lines may flashover when the voltage is too high, or dirty flashover may occur under normal operating voltage, resulting in power outages. According to relevant statistics, the number of accidents caused by the pollution flashover of insulators in transmission lines currently ranks second in the total number of accidents in China's power grid, second only to lightning strike disaster accidents, and the loss caused by pollution flashover accidents is approximately 10 times the accident.

The losses caused by insulator pollution flashover accidents to the power grid are catastrophic. Selecting appropriate composite insulators is one of the effective measures to prevent pollution flashovers in transmission lines, which can ensure stable and safe operation of power systems at lower costs. .

1. Mechanism of insulator pollution flashover

Contamination flashover discharge is an intricate process involving electrical, thermal and chemical phenomena. It can be roughly divided into the following four stages: fouling of the insulator surface, wetting of the insulator surface, generation of partial discharge, development of partial discharge and lead to flashover.

1.1 Contamination on the surface of the insulator

The outer insulating surface of power transmission and transformation equipment will inevitably fall on various pollutants such as smoke and dust in the atmosphere. The more serious the air pollution is, the more serious the pollution of insulators will be. Generally speaking, the accumulation of contamination on the surface of insulators is a slowly changing process. The amount of contamination on the surface of insulators accumulates gradually in dry and rainy seasons, but the attack of sea breeze in coastal areas can rapidly build up contamination.

1.2. Wetting of the insulator surface

The wetting process of the insulator surface is closely related to meteorological conditions. Heavy fog, condensation, drizzle, sleet, sticky snow, melted snow, melted ice, smog, and rain are extremely unfavorable meteorological conditions for dirty insulators. When the relative humidity is high, the surface of the polluted insulator is easy to wet, so pollution flashover is prone to occur.

In dry weather conditions, the contamination on the surface of the insulator does not significantly reduce the flashover voltage on the surface of the insulator. Dirty and wetness are two basic conditions that constitute a contamination flashover, and both are indispensable.

Dirty and wet conditions may also build up at the same time, such as sea splashing under sea breezes, wet precipitation, acid rain and acid fog. These are extremely unfavorable meteorological conditions that can lead to contamination flashovers.

1.3. Partial discharge generation

Under humid weather conditions, after the surface of the dirty insulator is wet, the soluble substances in the dirt will gradually dissolve in water, and a water film with certain conductivity will be formed on the surface of the insulator. The soluble substances in the filth determine the conductivity of the water film, and the insoluble substances in the filth can absorb water. This water film constitutes a conductive channel along the surface of the insulator, so that leakage current flows along the surface of the insulator. The magnitude of the leakage current depends not only on the degree of dirt on the surface of the insulator and the composition of the dirt, but also on the degree of wetting of the dirt.

1.4. Development of partial discharges leading to flashovers

If the insulator is seriously dirty, the surface of the insulator is fully damp, and the leakage distance of the insulator is small, these factors determine that the resistance of the wet layer of the insulator is small, so that a strong partial discharge phenomenon will occur. Under such conditions, the discharge across the dry zone takes the form of arcing. The leakage current pulse value corresponding to this type of discharge is large, up to tens or hundreds of milliamps. The stronger the local arc, the larger the corresponding leakage current value. The occurrence and development of this intermittent pulse-like discharge phenomenon are also random and unstable. Under certain conditions, the local arc will gradually extend along the surface and finally complete the flashover. Once the dry area of ​​all insulators is bridged by the arc, the leakage current will be determined by the resistance of the wet layer of the insulator string. At this time, the leakage current will increase greatly, and the strong discharge may develop into a flashover of the entire string of insulators.

2. Anti-pollution flashover measures for transmission lines

The large-scale pollution flashover accidents in 6 provinces and 1 city in the middle and lower reaches of the Yangtze River in 1996 and Liaoning, North China and Henan power grids in 2001 remind people again that the external insulation configuration of power transmission and transformation equipment established by large-scale manual cleaning cannot meet the safety of modern large power grids Due to the needs of operation, it is difficult to prevent the occurrence of large-scale pollution flashover accidents. To realize the effective anti-pollution flashover design of my country's power grid, it is necessary to improve the overall level of external insulation of the power grid, so as to achieve "insulation in place, leaving a margin".

2.1. Reduce the amount of contamination

Clean the insulator as shown in Figure 1. In order to improve the anti-fouling flashover ability of porcelain insulators, the cleaning of the insulators should be done well, and the porcelain insulators should be cleaned frequently. However, cleaning is only an auxiliary method for the line anti-pollution flashover work, not the main method. To fundamentally improve the anti-pollution flashover capability of the line is a reliable guarantee to prevent the pollution flashover accident of the transmission line.

2.2. Adjust the creepage distance

For line insulation, adjusting the creepage distance is to increase the number of insulators or replace the pollution-resistant composite insulators. Increasing the number of insulating sheets has the problem of affecting the length of the wire, especially the tensile string. The increase of the wire leads to the increase of the sag across the span, and the increase of the number of the suspension string is also limited by the size of the tower window. The ideal way is to replace the standard insulator with a pollution-resistant insulator, or change it to a V-string. The latter inclined installation can save the size of the tower. Moreover, it is generally believed that the pollution flashover voltage of the standard insulator in the V-shaped string can be 25%~30% higher than that of the hanging string, but it is necessary to change the hanging point of the tower, which is relatively difficult to implement.

2.3. Using anti-fouling flashover paint

For the tension tower, the pollution-resistant insulator with the same height as the original ordinary insulator and the largest creep distance has been selected during the initial adjustment. If the adjustment continues, it will be technically difficult and the workload will be large. Approach to rubber RTV long-lasting anti-fouling flashover coatings. RTV coating is a solid coating that is considered to have excellent performance at home and abroad. According to the artificial pollution test done in the laboratory, the pollution flashover voltage of insulators with RTV is more than twice that of non-RTV, and the water-enhancing effect of RTV can migrate to the dirt surface.

2.4. Using composite insulators

The composite insulator is relatively slender and has a large shape factor. The surface of the silicone rubber is hydrophobic, so that it has a high wet flashover and pollution flashover voltage, which can effectively prevent the occurrence of pollution flashover accidents. . According to the operation experience, the composite insulator does not need to measure the zero value and clean, which greatly reduces the maintenance workload during the operation of the line.