A Brief Analysis Of Factors Affecting The Safety Of Transmission Lines

The factors that affect the safety of transmission lines are generally divided into two categories, namely, the wear and aging caused by the accumulation of daily time and the sudden failure caused by external factors. These two types of factors can cause fault tripping, interrupt the power supply operation, and can make the power grid paralyzed in serious cases. Among them, the time accumulation factor can be artificially controlled, causing a very small probability of failure, and the external environmental weather (including lightning strikes, typhoons, wildfires, ice damage, pollution, temperature and humidity, air pressure, bird damage, etc.) factors are high in uncontroll ability, causing a high probability of failure and great damage.

1. Lightning strikes

Lightning weather is common in the rainy season and has great randomness. A large number of transmission lines set up in the field have the characteristics of large distance and height difference of the tower, which makes the lightning resistance level of the line relatively poor and easy to be hit by lightning. In harsh terrain, there are many single-circuit lines, and there is no shielding and shunt protection provided by parallel lines, which increases the incidence of lightning accidents. Lightning trip accounts for the largest proportion of transmission line faults, and the lower the voltage level, the higher the probability of lightning failure.

When lightning discharge, cumulonimbus layer charge accumulates, forming a powerful electrostatic high-voltage electric field, colliding and neutralizing with a large number of negative charges carried by the ground, releasing high energy, voltage up to millions of volts, current up to thousands of amps, resulting in high overvoltage wave gradient and amplitude, causing line insulation flash over trip. Flash over tripping is a transient failure, the success rate of reclosing action is high, and the conductive path can be restored, but in serious cases, it will lead to line breakage, insulator breakage, or intrusion into the substation along the line, and high temperature fuse the wire, threatening to destroy the insulation strength of power equipment.

In order to reduce the probability of lightning accidents in transmission lines, it is necessary to prevent flash over, arc construction, direct strike and interruption of power supply in the line of defense. Adopt the following protection measures: ① set up lightning arrester; ② Add coupling ground wire; ③ Reduce the impact grounding resistance of the tower; ④ the neutral point is not directly grounded system; ⑤ Strengthen the insulation level of the line; ⑥ Install automatic reclosing; 7 Installation of tubular arrester; ⑧ Use unbalanced insulation, double loop network power supply and other methods.

2. Typhoon

Typhoon weather accompanied by strong wind and rain phenomenon, transmission lines will occur conductor dancing, wind deviation, line breaking, tower falling and other accidents. With the increase of the design wind speed of the power system on the transmission line, the probability of the accident of the line breaking and tower falling is reduced, but the probability of the accident caused by the violent swing of the wire, wind bias discharge and the damage of floating objects is high.

The wind speed is large, the transmission line dancing amplitude is large, reducing the air gap between the wire and the tower, the wire phase and the phase, the air gap distribution is uneven, so that the voltage strength in the gap is reduced to different degrees, can not withstand the voltage value of operation, breakdown and discharge.

If there is a storm at the same time, the rain will reduce the air resistance value, the wire is prone to short circuit, so that the discharge probability increases, this process occurs in high frequency, repeated long duration, randomness, large amplitude, will cause flash over, breakdown, tripping and other faults, and line tripping after the coincidence rate is low, the human action is weak, the recovery of power supply time is long. In addition, it will also cause mechanical failures of insulator strings, fittings, cross arms and other supports, auxiliary materials and equipment, resulting in huge economic losses.

For vibration lines with low wind speed, most lines can be controlled by installing anti-vibration devices. When encountering strong wind speed, the following measures can be adopted: (1) Add anti-vibration hammer and install a heavy hammer device to increase the vertical load carrying capacity of the transmission line and reduce the swing amplitude of the line; ② Install damping wire, wire protection, increase the number of split wires, strengthen the wire, and ensure the safe distance between the wire and the tower; (3) The suspension mode of V-type insulator string is adopted to enhance the insulator’s resistance to the phenomenon of transverse drift in the wind and reduce the wind deviation swing Angle of the insulator string; (4) When designing the tower and erecting the transmission line, consider leaving a large distance between the wires, moving the hanging point down, or lengthening and widening the cross arm.

3. Mountain fires

The wildfires caused by high temperature, human factors and other factors make the air continue to be in a state of high temperature and low humidity for a long time, and the transmission lines located in the mountain terrain, across the forest vegetation and the earth or between the phase lines, due to the rise of air particles thermal ionization, the increase of charged charge, the acceleration of particle movement, and the formation of conductive channels, the line flashover trip occurs. 90%uFF0C is the main reason for the fire trip, but also includes high temperature burn loss of line insulation, wire discharge to the tower.

For the transmission lines in the mountain area, considering the topography and economic factors, usually two or more lines are set up with the tower. In case of a mountain fire, it is easy to cause multiple lines on the same transmission corridor to fail and trip at the same time, and because the mountain fire is easy to large-scale extension and long duration, it will produce a lot of high temperature smoke and fog, and the reclosing rate of the line is low. Until the fire is reduced, the smoke is dispersed, the air environment and the temperature of the wire are reduced, it is difficult to take the way of artificial recovery, so the power supply of the line is interrupted for a long time, and at the same time, the affected lines are more than one, endangering the safety of the power grid operation.

In view of the phenomenon of mountain fire, we should start from the fire prevention measures to reduce the incidence of mountain fire: ① to control the height, number and type of trees near the base; (2) Establish emergency response measures, leaving enough length and width of the safe passage; ③ The line protection system is reliable and sensitive; ④ Establish real-time monitoring system, and improve the monitoring effect; ⑤ By season, time segment and weather conditions, increase line inspection to eliminate hidden dangers.

4. Ice disaster

Ice disasters include freezing rain, frost, freezing and snow accumulation. Abnormal natural environment climate, hot and cold alternations, will make the transmission line and insulator string covered with ice, line, tower bearing weight increase, wire wind area, sag increase, lead to unstable continuous vibration of the wire, dancing, serious breakage, tower and other accidents. Insulator string If the snow freezes, the melting ice water will cause the insulator to short circuit, ice flash trip.

According to statistics, ice flash is more likely to form when ice pollution is combined. Therefore, in order to prevent ice flash, it is necessary to increase the pollution cleaning of lines and insulator strings to improve cleanliness; Increase the creepage distance to prevent the snow from freezing due to the small distance between insulators and umbrellas; The arrangement of V-type insulator string and inverted V insulator string is adopted, and the series spacing of double string should be increased to improve the flashover voltage. In the case of ice dancing, ice melting technology should be adopted to accelerate the melting speed of ice covering or enhanced interphase spacer should be adopted to increase the interphase distance of wire and reduce the breakdown and discharge phenomenon caused by small interphase distance of wire due to large swing and amplitude of wire.

5. Other

1) Degree of pollution. The transmission line span is long, affected by atmospheric pollutants, salinity, smoke, dust, etc., which will lead to accumulation of pollution on the surface of the wire. Due to the large electric field around the high-voltage transmission line, corona phenomenon is easy to occur, resulting in a large number of charged particles, which are attached to pollution, increasing the conductivity of charged particles, and easy to discharge. Therefore, the transmission line should be cleaned in time.

2) Temperature. High temperature enhances the surface temperature of the wire, making the heat on the original high-voltage transmission line difficult to dissipate, poor heat dissipation, resulting in a continuous high temperature of the line, resulting in the increase of the arc of the wire, reducing the safety distance between the wire and the ground and the cross-crossing object, resulting in discharge, resulting in line tripping. Therefore, the distance between the wire and the earth and the wire phase can be appropriately increased.

3) Humidity. With the increase of water molecules, attached to the surface of the line, the charge around the wire absorbs electrons to form negative ions, weakens the ionization ability, increases the breakdown voltage, enhances the local electric field, and easily produces corona, resulting in power loss and voltage drop. Therefore, when selecting wire insulation materials, attention should be paid to the hydrophobicity and hydrophobic mobility of the materials.

4) Air pressure. With the increase of altitude, the air is gradually thin, the density is reduced, and the air pressure is decreased, so that the breakdown voltage between the transmission wire and the earth, the conductor phase and the phase is reduced, and the breakdown is easy to occur. Therefore, in the higher elevation of the terrain, the height of the cable should be reduced.

5) Bird damage. Birds like to hold dead branches in their mouths to build nests on the tower. If the branches are placed close to or fall into the power line, it is easy to short-circuit. When large birds fly or play with each other, they can easily touch multiple wires, resulting in short circuit or grounding trip accidents. Therefore, specific signs can be made on the line or the tower to make birds afraid and stay away from the transmission line.

Severe environmental climate and other factors on the transmission line destructive power, will cause transmission facilities failure or damage, resulting in power outage, line change loss, affect the line operating conditions, transmission capacity and transmission capacity, interrupt the power supply, serious lead to grid disconnection operation, bringing huge economic losses. From the perspective of safety, this paper analyzes the safety risks and causes of transmission lines caused by lightning strikes, typhoons, mountain fires, ice disasters, pollution degree, temperature, humidity, air pressure and bird damage, as well as prevention and control measures under different conditions and factors of transmission lines, with a view to strengthening mechanism research, rational design and scientific and effective prevention and control by relevant power systems, departments and technicians. To ensure the transmission line safe and reliable stable operation.

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