The factors that affect the safety of transmission lines are roughly divided into two categories, namely, wear and aging that accumulate in daily time and sudden failures caused by external factors. Both types of factors can cause fault trips, interrupt power supply operation, and paralyze the power grid in severe cases. Among them, time accumulation factors can be controlled artificially, and the probability of causing failure is extremely small, while external environmental weather (including lightning strikes, typhoons, wildfires, ice damage, pollution, temperature and humidity, air pressure, bird damage, etc.) factors are uncontrollable. The probability of causing failure is extremely high and the destructive power is great.
1. a lightning strike
Thunder and lightning weather is common in the rainy season and has great randomness. A large number of transmission lines erected in the field have the characteristics of large transmission towers and large height differences, making the lines relatively poor in lightning resistance and prone to lightning strikes. In bad 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 tripping accounts for the largest proportion of transmission line faults, and the lower the voltage level, the higher the probability of lightning failure.
When lightning discharges, cumulonimbus layer charges accumulate to form a strong electrostatic high-voltage electric field, which collides with a large number of negative charges carried by the earth and neutralizes. The released energy is high, the voltage can reach several million volts, and the current can reach several thousand amperes. The overvoltage wave has a high steepness and large amplitude, which triggers a line insulation flashover trip. Flashover tripping is a transient fault. The success rate of reclosing action is high and the conductive path can be restored. However, in serious cases, it will cause line disconnection, breakage of insulators, or intrusion into substations along the line, high-temperature fuse wires, threatening to damage the insulation strength of power equipment .
In order to reduce the incidence of lightning accidents on transmission lines, it is necessary to prevent flashover, arc construction, direct strike, and line interruption of power supply. The following protective measures are adopted: ① erect lightning protection wires; ② add coupling ground wires; ③ reduce the impulse grounding resistance of the tower; ④ adopt neutral point indirect grounding system; ⑤ strengthen the line insulation level; ⑥ install automatic reclosing; ⑦ install pipes Type lightning arrester; ⑧Using unbalanced insulation, dual-circuit looped network power supply and other methods.
2. Typhoon
Typhoon weather is accompanied by violent storms and accidents such as wire galloping, wind deviation, disconnection, and tower falling will occur on transmission lines. Disconnections and towers are prone to unraveling from the north to the south. As the design wind speed value of the transmission line increases by the power system, the probability of disconnection and tower falling accidents decreases. However, the violent sway of the wire, the discharge of the wind, and the damage of floating objects cause accidents. The odds are high.
The wind speed is high, and the transmission line gallops greatly, which reduces the air gap between the conductor and the iron tower, and between the conductor phase and the phase. The air gap distribution is uneven, which reduces the voltage intensity in the gap to varying degrees and cannot withstand the operating voltage. Value, a breakdown discharge occurs.
If there is a storm at the same time, the rain will reduce the air resistance value, and the short circuit between the wires will increase the probability of discharge. The frequency of this process is high, the repeated duration is long, the randomness is large, and the amplitude is large, which will cause flashovers and shocks. Failures such as wear, tripping, etc., and the reclosing rate after the line tripping is low, the human effect is weak, and the power supply recovery time is long. In addition, it will also cause mechanical failure of insulator strings, fittings, cross arms and other supports, auxiliary materials and equipment, causing huge economic losses.
For vibration lines with low wind speeds, most of the lines can be controlled by installing anti-vibration devices. In case of strong wind speeds, the following measures can be taken: ①Add anti-vibration hammers and install heavy hammer devices to increase the vertical load bearing of the transmission line Ability to reduce the left and right swing of the line; ② Install damping wires, protective wires, increase the number of splits, strengthen the wires, and ensure the safe distance between the wires and the tower; ③ Use the V-shaped insulator string suspension mode to enhance the resistance of the insulators to wind The phenomenon of lateral drift reduces the wind yaw angle of the insulator string; ④When designing towers and erecting transmission lines, consider leaving a larger distance between the conductors, moving the hanging points down, or lengthening and widening the cross arm.
3. Wildfire
Forest fires caused by high temperature, man-made factors and other factors have kept the air in a state of high temperature and low humidity for a long time. It is located in mountainous terrain, between transmission lines across forest vegetation and the ground or between phase lines. Due to the heat of air particles Free rise, increased charged charge, faster particle movement, formation of conductive channels, and line flashover trips, which account for 90% of wildfire trips. uFF0C is the main cause of wildfire trips, including high temperature burns in line insulation and wires Discharge to the tower.
For transmission lines in mountainous areas, considering the topography and economic factors, two or more circuits are usually set up on the same tower. If a mountain fire occurs, it is easy to cause simultaneous failure and tripping of multiple circuits on the same transmission corridor. Easy to expand on a large scale and long lasting, it will produce a large amount of high temperature and dense smoke and fog, and the line reclosing rate is low. It can only be forced to be sent after the fire is reduced, the smoke dissipates, the air environment and the temperature of the wire are reduced, and it is difficult to take artificial recovery Therefore, the line interrupts power supply for a long time, and at the same time, many lines are affected, which endangers the safety of power grid operation.
In response to the phenomenon of wildfires, we should start with fire prevention measures to reduce the incidence of wildfires: ①Control the height, number, and type of trees near the tower base; ②Establish emergency response measures, leaving sufficient length and width of safety Channels; ③The line protection system is reliable and sensitive; ④Establish a real-time monitoring system and improve the monitoring effect; ⑤Increase line inspections according to seasons, time periods, and weather conditions to eliminate hidden dangers.
4. Ice disaster
Ice disasters include freezing rain, frost, icing, and snow accumulation. Abnormal weather conditions in the natural environment, alternating cold and heat, will cause icing of transmission lines and insulator strings, increase the load-bearing capacity of lines and towers, and increase the wind area and sag of the conductors, causing unstable continuous vibration and galloping of the conductors, which is serious Accidents such as disconnection and tower falling may occur. If the insulator string is snowed and frozen, the melting ice water will cause the insulator to short-circuit and cause an ice flash trip.
According to statistics, ice flash is more likely to form when ice and pollution are combined. Therefore, in order to prevent ice flash, it is necessary to increase the cleaning of lines and insulator strings to improve cleanliness; increase the creepage distance to prevent the small distance between the insulator umbrellas. Snow and icing occur; V-shaped insulator strings and inverted V insulator strings are arranged, and the distance between the double strings should be increased to increase the flashover voltage. In the case of wire icing and galloping, ice melting technology should be adopted to accelerate the melting speed of ice coating or use reinforced interphase spacers to increase the distance between the wires and reduce the impact caused by the small distance between the wires due to the large swing and large amplitude of the wires. Through discharge phenomenon.
5. Other
1) Degree of contamination. The transmission line has a long span and is affected by atmospheric pollutants, salinity, smoke, dust, etc., which will cause pollution to accumulate on the surface of the wire. Due to the large electric field around the high-voltage transmission line, corona is prone to generate a large number of charged particles, and the charged particles adhere Contamination, increase the conductivity of charged particles, easy to discharge. Therefore, the transmission line must be cleaned in time.
2) Temperature. The high temperature enhances the surface temperature of the conductor, making it difficult to dissipate the heat on the original high-voltage transmission line, and the heat dissipation is poor, resulting in a continuously high temperature of the line, causing the sag of the conductor to increase, and reducing the safety between the conductor and the ground and crossing objects. Distance, so that discharge occurs, causing the line to trip. Therefore, the distance between the wire and the ground and the phase of the wire can be appropriately increased.
3) Humidity. As water molecules increase and attach to the surface of the circuit, the charges around the wire absorb electrons to form negative ions, weaken the dissociation ability, increase the breakdown voltage, increase the local electric field, and easily produce corona, resulting in power loss and voltage drop. Therefore, when selecting the wire insulation material, attention should be paid to the hydrophobicity and hydrophobicity of the material.
4) Air pressure. As the altitude increases, the air gradually becomes thinner, the density decreases, and the air pressure drops, so that the breakdown voltage between the transmission wire and the ground, and the phase and phase of the wire is reduced, and breakdown is easy to occur. Therefore, the wire height should be lowered at higher altitudes.
5) Bird damage. Birds like to use dead branches in their mouths to build nests on iron towers. If the branches are placed close to or fall on the power line, short circuits are likely to occur. When large birds spread their wings and play with each other between wires, they are easy to touch multiple wires, causing a short circuit between phases or a line grounding trip accident. Therefore, specific signs can be made on lines or towers to make birds afraid and stay away from power transmission lines.
Factors such as harsh environment and climate have great destructive power to transmission lines, which can cause failure or damage to transmission facilities, cause power outages and line replacement losses, affect line operating conditions, transmission capacity and transmission capacity, interrupt power supply, and cause grid decommissioning in severe cases. Bring huge economic losses. This article analyzes the safety risks, causes and prevention measures of transmission lines under different conditions such as lightning strikes, typhoons, wildfires, ice disasters, pollution, temperature, humidity, air pressure, and bird damage from the perspective of safety. Power systems, departments, and related technical personnel should strengthen mechanism research, conduct reasonable design, scientifically and effectively prevent and control, so as to ensure the safe, reliable and stable operation of transmission lines.