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The wind causes problems
Transmission lines operating environment is relatively complex, a big local transmission line in complex mountainous terrain, a long transmission lines at the same time, through the road conditions can be met, mountains, sand dunes, such as regional traffic around, so that once have the breeze weather, is the local transmission line is indirect shaking under the action of wind load, attack, thus cause the onset of monsoon flashover.At the same time in the wind load attack, the use of a longer period of time tower will form a certain coercive force, break the original tower tower equilibrium or form tower collapse.Local transmission lines are located close to the left of the trees. When these trees grow from time to time, the safe interval between them and the transmission lines is broken. Once strong winds break out, grounding faults or short circuit will occur.Therefore, the influence of breeze on the transmission line is very great, and the resulting results are very serious. Moreover, once the fault of the transmission line is caused by the wind disaster, it is difficult to deal with it in a short time, and the loss caused by the fault will expand from time to time.
2. Lightning tripping
Thunder cloud discharge causes overvoltage to call thunder and lightning overvoltage in power fragmentary, because its electromagnetism energy comes from system interior, call again internal overvoltage, because thunder cloud discharge happens again in atmosphere, call again atmospheric overvoltage.
There are two rare overvoltages in overhead transmission lines: one is the induced overvoltage on overhead lines, that is, the overvoltage occurred when lightning strike occurred near the left of overhead lines and passed through electromagnetic induction on the transmission line.The second kind is direct lightning overvoltage, that is, lightning indirectly hit the lightning conductor or conductor when the overvoltage occurs.
Lightning tripping often causes flashover discharge of insulators, which results in flashover discharge marks on insulators.Generally speaking, after the insulator is struck and discharged by lightning, there are melting marks on the iron parts, burns on the surface of the porcelain insulator and scattered cracks on the surface of the vitreous body of the glass insulator.
After lightning strike flashover occurs, because the air insulation is self-restoring insulation, the air insulation strength of the breakdown is rapidly restored, and the original conductive channel becomes the insulating medium again. Therefore, when the re-closing action is taken, the ordinary redecomposing work is performed.
Of course, lightning can also cause permanent problems, there are three common conditions: porcelain insulator scattered, lightning conductor broken wire, wire broken wire.
Because the lightning current is large, a lightning strike can form insulator flashover or insulator burst.The burn marks left by lightning strike and contamination flashover on wires are characterized by: burn marks left by contamination flashover are concentrated, and even burn marks left only on wires clamped online or close to wire clamps, with small area but deep trace and heavy burn damage.Lightning burns tend to be large and scattered, and burn levels are relatively light.
Both lightning strike and fouling flash can cause wire burns inside the wire clip, and the fouling flash of burned wire inside the wire clip is higher than that of lightning strike.Lightning flashover can also burn the suspension head of lightning conductor, grounding bolt connection of grounding wire of grounding lead and wedge clamp connection of pull wire, and leave distinct burn marks.Lightning activity is a complex process of atmospheric activity.Lightning damage is an important factor that affects the safety of transmission lines.With the development of superstitious technology, lightning protection methods and measures emerge and improve from time to time.The lightning protection task of transmission line should be combined with the practical situation of the line, and the corresponding measures should be taken according to the reason of lightning strike and tripping to ensure the safe operation of transmission line.
3. Circuit icing problem
At present, the line ice can stop classification from many Angle, normal conditions, according to the damage level and power spare operation, maintenance, design and scientific research request, conductor ice coating can be divided into rime, the rime, sleet, and four categories, such as mixing Song rime is rare summer cold high altitude mountain transmission line a way of ice.The icing accident of transmission line is related to the number of days of uniform rain rime and rime.Generally speaking, the influence of annual uniform rime days is more serious than the annual uniform rime days.
Secondary attack conductor ice in the winter or early spring season, when the temperature dropped to - 5 ℃ ~ 0 ℃, when 3 ~ 15 m/s wind speed, small met conductor in cooling water in the air, make the liquid cooling water attacks rely on under the guide form rime after deformation.In case of fog or rain plus snow, it is formed on the wire rime;If the temperature continues to fall, freezing rain and snow in the adhesion of high strength on the rapid growth of rime ice, ice.If the temperature continues to fall, rime will accumulate on the outer layer of the original ice.This process will lead to the appearance of wire formation of rime - rime mixed rime - rime composite ice.
Hazards of icing on the line include over-load, ice-covered dance and de-icing prance, insulator ice flashover, which may lead to tower deformation, tower down, wire breakage, damage of fittings and insulators, insulator flashover and other accidents.
Transmission lines not only accept static load such as dead weight and icing, but also dynamic load caused by wind.Under certain conditions, the ice-covered wire is affected by the steady transverse wind, which can cause large and low-frequency vibration, namely dancing.At this point, the wire deicing prance will also make the wire burst dancing.Wire movement is an important factor to coerce transmission line to run safely.
When the wire is coated with ice on average, although its load surface increases, it still insists on an average circular shape. Therefore, the frequency of wire vibration caused by a certain wind force is lower than that of the bare wire, while the amplitude is smaller than that of the bare wire, and the frequency drop can be lower than the ineffective operation range of anti-vibration installation.
When the icing on the wire is uneven, due to the asymmetry of its section, aerodynamic undulations will occur when the wire is blown by wind. Under the action of corresponding wind force, the wire will have low-frequency (0.1 ~ 3Hz) and high-amplitude (up to more than 10m) dancing.Wire dancing will cause differential frequency load, resulting in metal damage, wire broken strand, interphase short circuit, line trip and tower tilt or collapse and other serious accidents.
When a single conductor is icing, due to the small stiffness of change, the conductor is prone to make great changes under the effect of fair icing, so that the icing is close to the circle;However, when the conductor is divided for icing, due to the action of distance bar, the relative change stiffness of each sub-conductor is much greater than that of a single conductor. Under the action of fair icing, the change of the conductor is extremely huge, which cannot prevent the asymmetry of icing of the conductor, and icing of the conductor is more likely to form an airfoil section.Thus, in the case of split conductors, the wind-induced lift and torque are much greater than in the case of a single conductor.
The relative change stiffness of a large cross-section wire is larger than that of a small cross-section, and the change Angle is smaller under the action of fair icing. The ice-covered wire is more likely to form an airfoil section, and the lift force and change are larger under the action of wind encouragement.Therefore, split wires and large cross-section wires are more likely to dance.
The unevenly or synchronously deicing of the ice-covered wire may occur under the influence of temperature drop, natural wind force or artificial vibration.Uneven deicing of conductors can also cause serious mechanical or electrical accidents.Since the conductor ice quantity added, correspondingly increases tension and clear, sag has also fallen, when long or the whole file to take off the ice, because the wire elastic energy-storage quickly transformed into kinetic energy and potential energy of the wire, provoked wire candle, to dance, violent swings in the adjacent trailer string, also was a significant change on both ends of the wire tension.
4. Line pollution flashover
Transmission line insulators are required to operate reliably under atmospheric overvoltage, external overvoltage and temporary operating voltage.However, under the action of fog, dew, drizzle, melting ice, melting snow and other climatic conditions, the pollution accumulated on the surface of insulators will greatly reduce the electrical strength of insulators, thus causing pollution flashover accidents of transmission lines under operating voltage.
The basic cause of pollution flashover pollution flashover is pollution. After the pollution insulator is wet, the soluble material contained in the pollution layer is gradually dissolved in water, which is called electrolyte. It forms a thin conductive film on the surface of the insulator.The conductivity of this conductive film depends on the chemical composition of the filth and the wetting level.At wetting and saturation, the external resistance of the pollution layer can even be reduced by several orders of magnitude, and the leakage current of the insulator increases correspondingly.First it's in a part of the drying area near the iron foot.As a result of drying, the resistivity of the side surface of the region increases greatly, forcing the current flowing through the surface of the region to transfer to the wet film on both sides parallel to the region, increasing the current density flowing through the wet film and slowing down the drying process of the wet film.As this develops, a circular drying belt is quickly formed around the iron foot.The drying belt has a high resistance, which makes the voltage sharing surge.When the intensity of the field added to the drying belt exceeds the critical value, part of the discharge along the surface occurs there.(due to the unfluctuating, intermittent nature of this discharge scene, we call it scintillation discharge) so, a large part of the leakage current flows through the leakage current scintillation discharge channel.At the outer end of the scintillation discharge channel, the current density near the wetting surface on the left side is higher than that on both sides, which promotes the drying area to expand outwards (radially).Flashing, on the other hand, the existence of the plasma channel, is equal to the drying with short circuit, reduce the leakage current of flows on both sides of the drying zone is small, the area of the drying effect is very weak, the moisture in the atmosphere has gradually make the appearance of these regions wetting, exterior conductivity increases, the past of flashing current form distributary channel, increase the flashing current of plasma channel, so that to make shine discharge extinguished as a result, the original channel on either side of the current transfer to wetting area, make the dry again, and trigger new flash discharge in the region.In this way, the way of scintillation discharge faces radial and transverse transfer, and the width of ring drying belt is gradually increased, and the length of scintillation discharge is also increased.
If the leakage interval of filth insulator (abbreviation creep distance) is longer, the resistance that other series wets local is bigger, the current of scintillation steel electricity in drying belt is lesser, discharge channel assumes blue-purple fine line shape.When the length of the scintillation discharge is added at a certain level, the voltage Shared to the discharge channel (which is equal to the total voltage minus the pressure drop on the wetting band) is insufficient to sustain such a long scintillation discharge, which is extinguished.As the drying belt has been extended to a larger radius at this time from the iron foot to the iron cap between the total leakage current is limited by the high resistance of the drying belt to a small value, the drying effect is greatly weakened, is simply terminated.In this period, small and medium-sized water droplets in the atmosphere have gradually drying with wetting, leakage current increased, and repeated above - mentioned cycle.In this way, the whole process is called drying and wetting, arc extinguishing and reburning, intermittent alternating process, such a process in the fog can continue for several hours without forming the appearance of the whole insulator flashover along the surface.
If filth is serious, or the crawl distance of insulator is lesser.Wetting with total leakage resistance is smaller through the dry belt of discharge current is larger, with yellowish-white discharge channel have been woven, and coarser, the temperature is increased to the level of the thermal dissociation channels become drop volt-ampere characteristics of arc discharge, channel required field intensity decreases, share to the flashing voltage is enough to maintain a long part of the discharge channel on arc not extinguish, carry out to the whole along the surface of insulator flashover.
Likewise, a drying band can also be present around the iron cap, and the above process can also be present.
In this case, the condition of an insulator, as for a string of insulators, the fundamental process of pollution flashover is described above. The difference is that the voltage scattered on each insulator is not only not constant, but also determined by the shape of the insulator itself at the time, but by the shape of the whole string of insulators at the time.In practice, on the contrary, the atmospheric environment of each insulator in the whole string of insulators is the same.However, each process is orderly in time and strong or weak in level.They interact to form a rather complex process.
Pollution flashover usually does not occur in dirty insulators in heavy rain. After the insulator surface is completely soaked by rain, rain constitutes a continuous conductive layer, and leakage current increases a lot, which reduces flashover voltage along the surface.And the heavy rain washed away a part of the pollution, on the insulator surface of the conductive film has a concentrated effect;On the other hand, in heavy rain, it is difficult to form a drying belt to cause part of the arc.
5. Fault caused by external damage to the circuit
The damage caused by external force is mainly caused by illegal operation, theft and destruction of power equipment, building barriers, tree barriers, interweaving and crossing highways, burning crops under transmission lines, forest fires and floating objects (such as flying kites, balloons and white residues).
Aiming at the secondary causes of external damage, it is necessary to stop detailed fault analysis and put forward effective preventive measures to ensure the safe operation of transmission lines.
The secondary causes of transmission line external damage are as follows:
6. Minor damage to the wiring caused by birds
With the enhancement of human's understanding of the maintenance of natural ecological environment, the breeding number of birds gradually increases, and the range of activities is increasingly expanded, which has caused great harm to power transmission lines.Statistical data in recent years indicate that the circuit fault caused by bird activities is only second to lightning damage and external damage, and once occupied the third place in the total number of circuit fault.
7. Other problems
In addition to the above there are floods and rainstorms, body defects and other defects.
Thunder drought season, season flood scour tower roots, thus causing root slope collapse, foundation cracks, settlement or more serious inverted tower bar inverted trouble.
Due to the circuit such as process performance, electrical interval performance, data quality and other reasons for the body defects, for a long time under the influence of wind vibration, temperature changes will also form a circuit fault.