Company Name: Henan Xinchaoda Cable Co., Ltd.
Contact: Manager Guo
Address: Xibaishui Village, Xitao Town, Wuzhi County
Marketing Center: 1812, Building B, Building 3, Shenglong Plaza, Zhengdong New District, Zhengzhou
Fault caused by wind
The environment in which power transmission lines operate is relatively complicated. A large part of the power transmission line is in a complicated mountainous terrain. At the same time, the power transmission line is long, and the conditions of the route section can be met, such as mountains, sand dunes, and branch lines. If there is a breeze, the local transmission line will indirectly shake under the effect of wind load, which will cause the onset of wind partial flashover. At the same time, when a wind load occurs, a certain requirement will be formed about the tower with a long service life, which will break the balance of the original tower or form the collapse of the tower. Local power transmission lines are located near the trees. When these trees grow from time to time, the safety interval between them and the transmission lines is broken. Once a strong wind occurs, it will cause grounding problems or short circuits. Therefore, the impact of the breeze on the transmission line is very large, and the results are very serious. Moreover, once the transmission line is out of order due to the wind disaster, it is difficult to lose the treatment in a short time, which will cause the loss caused by the fault from time to time Extension.
The thundercloud discharge caused the overvoltage in the power fragmentation is called thunderbolt overvoltage. Because its electromagnetic energy comes from the system, it is also called internal overvoltage. Because thundercloud discharge occurs in the atmosphere, it is also called atmospheric overvoltage.
There are two rare overvoltages in overhead transmission lines: one is the induced overvoltage on the overhead line, that is, the lightning strike occurs near the overhead line, and the overvoltage occurs on the transmission line through electromagnetic induction; the second is direct strike Lightning overvoltage refers to the overvoltage that occurs when lightning strikes on a lightning conductor or a conductor indirectly.
Lightning strikes often cause flashover discharges of insulators, causing flashover discharge traces on the surface of insulators. Generally speaking, after the insulator is struck by lightning, there are melting marks on the iron parts, the appearance of the porcelain insulator is burnt, and the glass insulator has a mesh crack on the outer surface.
After the lightning flashover occurs, because the air insulation is a self-healing insulation, the breakdown of the air insulation strength quickly recovers, and the original conductive channel becomes the insulation medium. Therefore, when the reclosing action is performed, the normal recombination work is performed.
Of course, lightning strikes can also cause eternal problems. Generally, there are three conditions: porcelain insulators are scattered, lightning conductors are disconnected, and conductors are disconnected.
Due to the large lightning current, a lightning strike can form an insulator flashover or insulator burst. The burn marks left on the wire by lightning strikes and pollution flashes are characterized by: the burn marks left by pollution flashes are concentrated, and burn marks are left only on the wire clip or the wire close to the wire clip. The area is small but the marks are deep and burn. The damage is heavy. Lightning strike burns are often large and scattered, and burns are relatively light.
Lightning strikes and pollution flashes can both form wire burns inside the wire clip. This kind of burn wire contamination flashes in wire clips are higher than lightning strikes. Lightning strike flashover can also burn the suspension head of the lightning conductor, the ground bolt connection of the ground wire of the grounding down conductor, and the connection of the cable wedge clamp, leaving a clear burn mark. Lightning activity is a complex process of atmospheric activity. Lightning damage is an important factor affecting the safety of transmission lines, and lightning strike trips have consistently ranked first in line faults for many years. With the development of superstitious technology from time to time, lightning protection methods and measures have emerged and improved from time to time. The lightning protection task of the transmission line should be combined with the actual situation of the line, and the relevant measures should be taken from the cause of the lightning strike and trip, tailored, and targeted measures should be taken to ensure the safe operation of the transmission line.
3. Line icing problems
At present, line icing can stop classification from many perspectives. Under normal conditions, according to its hazard level and requirements for power fragmentation operation, maintenance, design, and scientific research, the wire icing is divided into hoarfrost, fog, rain, and mixed. In the fourth category, fog is a rare icing method for transmission lines in high-altitude and high-altitude mountain areas in summer. The icing accident on the transmission line is related to the annual average rainy days and annual average foggy days in various places. Generally speaking, the effect of the annual average rainy days is more serious than the annual average foggy days.
The icing of the wire will occur in the severe winter or early spring. When the temperature drops to -5 ° C to 0 ° C and the wind speed is 3 to 15 m / s, a small amount of supercooled water in the air hits the wire, causing the liquid After the cooling water is deformed, it forms a fog under the guide. If it encounters heavy fog or light rain and snow, it will form a rain grate on the wire; if the temperature continues to drop, freezing rain and snow will rapidly grow on the ice surface with a high bonding strength, forming an ice layer. If the temperature continues to fall, fog will accumulate on the outer layer of the original ice layer. This process will cause the appearance of the wire to form a composite layer of rain, mixed, and mist.
The hazards of icing on the line include overload, galloping and deicing leap, insulator ice flash, which can cause pole tower deformation, inverted tower, broken wire strands, damage to metal fittings and insulators, insulator flashover and other accidents.
Transmission lines not only accept static loads such as their own weight and icing, but also dynamic loads due to wind. Under certain conditions, the icing wire is subject to steady-state transverse wind, which can cause large low-frequency vibrations, that is, galloping. At this time, the de-icing and leap of the wire will also cause the wire to dance. Conductor galloping is an important factor for the safe operation of transmission lines.
When the wire is evenly covered with ice, although its load surface is increased, its shape still adheres to an average circle. Therefore, the frequency of wire vibration caused by a certain wind force is lower than that of the bare wire, and the amplitude is lower than that of the bare wire. The line time is small, and the frequency drop can be reduced below the ineffective operating range of the vibration-proof installation.
When the ice on the wire is uneven, due to the asymmetry of its cross section, aerodynamic fluctuations will occur when the wire is blown by the wind. Under the corresponding wind force, the wire will have low frequency (0.1 to 3 Hz) and large amplitude ( Up to 10m). The galloping of the wires will cause differential frequency loads, which will lead to serious accidents such as damage to the hardware, broken wires, short circuits between phases, line trips, and tilting or collapse of the tower.
When the single wire is covered with ice, due to the small change in stiffness, the wire is liable to change greatly under fair icing, which makes the ice coating close to a circle. When the split wire is covered with ice, due to the action of the distance rod, the relative of each sub-wire The change of stiffness is much greater than that of a single wire. Under fair icing, the change of the wire is extremely large, which cannot prevent the asymmetry of the wire icing, and the wire icing is more likely to form an airfoil section. Thus, with regard to split wires, the lift and torque that are encouraged by the wind are much greater than single wires.
The relative change stiffness of large-section wire is larger than that of small section, and the change angle is smaller under fair icing. The icing of wire is more likely to form the airfoil section. Under the encouragement of wind, the lift and change that occur are larger. Therefore, split wires and large-section wires are more likely to gallop.
When the temperature of the ice-coated wire is reduced, or due to the effect of natural wind, or due to the impact of artificial vibration, the de-icing or uneven de-icing will occur. Uneven de-icing of the wires can also cause severe mechanical or electrical accidents on the lines. As the amount of ice covered by the wire increases, the tension increases correspondingly, and the sagging also decreases. When the large section or the entire gear is de-iced, the elastic energy storage of the wire quickly changes to the kinetic and potential energy of the wire, causing the wire to move upward. Leaping, and then dancing, caused violent swing of adjacent overhang strings, and the tension of the wires at both ends also changed significantly.
4. Line pollution flash
Transmission line insulators require reliable operation under atmospheric overvoltage, external overvoltage, and temporary operating voltage. However, the pollution accumulated on the surface of the insulator under the influence of climatic conditions such as fog, dew, drizzle, melting ice, and snow will greatly reduce the electrical strength of the insulator, thereby causing a pollution flashover accident under the operating voltage of the transmission line.
Pollution flashover The basic cause of pollution flashover is pollution. After being exposed to moisture, the soluble substances contained in the pollution layer gradually dissolve in water, called electrolytes, and form a thin conductive film on the surface of the insulator. The conductivity of this conductive film depends on the chemical composition of the dirt and the level of wetting. When wetting and saturation, the external resistance of the stained layer can even be reduced by several orders of magnitude, and the leakage current of the insulator also increases correspondingly. First, a partial drying area is formed somewhere near the iron feet. Due to being dried, the resistivity of the sides of the area increased greatly, forcing the current flowing through the exterior of the area to be transferred to the wet films on both sides parallel to the area, so that the current density flowing through these wet films increased. , Slowing down the drying process of these wet films. When this is carried out, an annular drying belt is quickly formed around the iron feet. The drying belt has a very large resistance, which increases the shared voltage dramatically. When the field strength added to the drying zone exceeds a critical value, a partial discharge occurs on the surface. (Because this discharge scene has a non-fluctuation and intermittent nature, we call it a flicker discharge.) Therefore, most of the leakage current flows through the channel of the leakage current flicker discharge. The current density at the outer surface near the outer end of the scintillation discharge channel is larger than that on both sides, which promotes the drying area to expand outward (radial). On the other hand, the existence of the flashing discharge channel is equivalent to short-circuiting the drying belt, so that the leakage current flowing through the drying belts on both sides is reduced to a very small extent, and the drying effect in these areas is very weak. Gradually wet the appearance of these areas, and increase the surface conductance. In the past, shunting the scintillation current channel will increase the current in the scintillation discharge channel, so that the scintillation discharge can be extinguished. Therefore, the current in the original channel is transferred to both sides. Wet zone, re-dry the zone, and trigger a new flashing discharge in the zone. In this way, the path of the flicker discharge is shifted in the radial direction, and the overall trend gradually increases the width of the annular drying belt, and the length of the flicker discharge also increases.
If the leakage interval of the dirty insulator (crawl distance for short) is longer and the resistance of the other series wetting parts is larger, the electric current of the flashing steel in the drying zone will be smaller and the discharge channel will be a thin blue-violet line. When the length of this flashing discharge is added to a certain level, the voltage shared on the discharge channel (which is equal to the total voltage minus the voltage drop across the wetting band) is lacking to sustain such a long flashing discharge, and the flashing discharge is extinguished. Since the drying belt has expanded to a large radius at this time, the total leakage current from the iron feet to the iron cap is limited to a small value by the high resistance of the drying belt, the drying effect is greatly weakened and it is simply terminated. During this period, the small water droplets in the atmosphere gradually wet the drying zone and the leakage current increased. The above cycle was repeated again and again. In this way, the whole process is called drying and wetting, arc extinguishing and reignition, and the process alternates intermittently. Such a process can continue for several hours in the fog without forming the entire surface flashover of the insulator.
If the contamination is severe or the creepage distance of the insulator is small. The smaller the total leakage resistance of the wetting zone is, the larger the flashing discharge current flowing through the drying zone is. The discharge channel is yellow-white braided and thick. The temperature in the channel also increases to the level of heat release and decreases. The arc discharge of the volt-ampere characteristic reduces the required field strength of the channel, and the voltage shared on the flashing discharge channel is sufficient to maintain a long part of the arc without extinguishing, which spreads to the flashover of the entire insulator.
Similarly, the drying belt can be presented around the iron cap, and the above process can also be presented.
At this time, the condition of an insulator. As for a series of insulators, the basic process of pollution flashover is as described above. The difference is that the voltage spread across the insulators is not only constant, but also not caused by the insulators themselves. The shape is determined by the shape of the whole string of insulators at that time. In practice, the atmospheric environment of each insulator in the whole string of insulators is opposite, and the processes of pollution, drying, and discharge of each insulator are almost the same. It's just that the processes are tidy in time and strong in level. The mutual influence constitutes a rather complicated process.
Pollution insulators generally do not have pollution flashover in heavy rain. After the insulator is completely wetted by rainwater, rainwater forms a continuous conductive layer, and the leakage current increases a lot, which reduces the flashover voltage along the surface. In addition, the heavy rain washed away a part of the contamination, which had a condensing effect on the conductive film on the outside of the insulator. On the other hand, under heavy rain, it was difficult to form a drying zone to cause part of the arc.
5. External damage to the line
Defects caused by external forces are mainly caused by illegal tasks, theft and destruction of electrical equipment, house barriers, tree barriers, cross-road crossings, burning of crops under power transmission lines, fires in mountains and forests, and floating objects (such as flying kites, balloons, and white residue). form.
In view of the secondary causes of external force destruction, it is necessary to stop detailed fault analysis and propose invalid and feasible prevention and control measures to ensure the safe operation of transmission lines.
The secondary reasons for the external damage to the transmission line are as follows:
6. Birds' minor hazards to the line
With the enhancement of human understanding of the maintenance of the natural ecological environment, the number of birds breeding has gradually increased, and the range of activities has gradually expanded, posing great harm to transmission lines. Statistical materials in recent years indicate that line faults caused by bird activities are second only to lightning damage and external force damage, and once accounted for the third largest number of line faults.
7. Other problems
In addition to the above, there are also problems such as floods and heavy rains, and body defects.
During the thunder and drought season and season, the base of the pole tower was washed away, which caused the collapse of the base slope, cracks in the foundation, settlement, or more serious problems with the tower.
Due to the main defects of the line, such as the technical results, electrical interval results, and data quality, the line fault will also be formed under the influence of long-term wind vibration and temperature changes.