The condition of power lines is significantly affected by the type of supports used. For 100 years, a wooden support remained one of the main structures of overhead lines (OHL).
Only in the 60s of the last century they began to do it with protective impregnation. Then instructions were given on the use of antiseptics, but they were poorly executed, which led to the decay of the supports. The ubiquitous transition to reinforced concrete poles did not solve all the problems, since they showed flaws that were not inherent in wood products:
- brittleness upon impact;
- low bending strength;
- significant weight;
- the presence of leakage currents.
Benefits
A wooden pedestal will never be debited due to the following advantages:
- Low cost.
- Light weight.
- When a wooden support falls, whose weight is 3 times less than reinforced concrete, it hangs on wires without the "domino" effect characteristic of heavy posts.
- Indispensable in areas with increased seismic activity.
- They better withstand wind loads.
- High dielectric characteristics.
- Long service life with proper manufacturing (up to 40 years).
- No special maintenance required.
disadvantages
Along with the advantages, a wooden support has disadvantages.
- Impregnating compounds may contain harmful substances that are in the air of the working area (fuel oil, kerosene, creosote, etc.). Oil-based antiseptics are especially harmful. In addition, they have an increased fire hazard.
- Logs must be made with the required diameters and taper.
- High-quality products are obtained during winter cabin and drying under a canopy for 6 months. Here it is necessary to treat the logs with an antiseptic so that they do not decay.
Materials
The wooden transmission line support is made from coniferous trees, where the resin is a natural preservative and antiseptic. Northern pine, which has high strength and elasticity, is in greatest demand. The problems that reinforced concrete pillars have due to their fragility never create wooden supports (photo below - loading of finished products).
Shale or creosote oil is used for impregnation, as well as mixtures containing copper, chromium, and arsenic. Additionally, with antiseptics, supports are treated with fire protection agents (flame retardants). This allows you to bury the pillars directly into the ground, without concrete stepsons, which increases their service life.
The highest absorbency is possessed by pine products. If spruce and larch are used for supports, it is much more difficult to impregnate them.
Impregnation allows you to bury the pillars directly into the ground. Only here it is necessary to additionally protect the ends with a protective paste or covers. It is important to dry the wooden support before impregnating for up to 3 months. The fastening to the stepsons of reinforced concrete leads to the splitting of wood under the bandages of wire rod.
Important! For the manufacture of pillars, the lower part of the tree (butt) is used, where there are fewer branches and a more uniform structure.
Dimensions and Strength
The length of the supports is 3.5-13 m. Depending on the diameters in the upper (d) and lower (D) parts, they are as follows:
- lungs: d = 140 mm; D = 160-220 mm;
- medium: d = 160 mm; D = 180-235 mm;
- durable: d = 195-210 mm; D = 210-260 mm.
An important indicator is the strength at the bottom of the rack. For a log diameter of 190 mm, the maximum bending moment is 55 kNm, and for 240 mm - 95 kNm.
Criteria for choosing a wooden support
- The northern pine of the winter cabin is used as the material.
- At the top of the column, a thickness of at least 16 cm.
- An aqueous solution of CCA is used as an impregnation.
- The entire support or the lower part is impregnated at the factory with a pressure of 12-14 atm.
- Technological holes are made before impregnation.
- The depth of impregnation is 85% of the outer layer of wood - sapwood (up to 40 mm).
- The impregnation process is completed if the color of the support is gray-green. If it is brown or brown, it means that the reaction is not completely over. The border should be visible on the cut of the log.
- Supports are sold in classes C 1 and C 3 with a complete set of sizes.
Features of support impregnation
Poles are installed in the ground without stepsons. The ends are impregnated more than the side surface. During operation, up to 90% of the protective composition is washed out of them. To prevent this, the end is covered from the top with a galvanized roof of 250x250 mm in size, and the bottom is closed with a flat material that does not allow moisture to pass through.
According to GOST 20022.0-93, the wood under the supports is impregnated with the protective agent XM-11 in terms of dry salt in an amount of 13-15 kg / m 3 . When purchasing logs for poles, you should find out under what conditions they were made, since in some technical specifications this quantity is underestimated by 2 times. Not all manufacturers correctly withstand the technology of manufacturing products. This requires the organization of quality control, although a specialist can determine it visually.
Support Technology
The process includes 4 important steps.
1. Debarking
On a specialized machine, the bark with bast is removed. Only after this the trunk begins to dry. Sapwood should be minimized, as it is it that is well-infiltrated with an antiseptic. If the entire top layer is crushed, the durability of the support will be much reduced due to the fact that it will be more susceptible to decay. Then the wooden support, the dimensions of which correspond to the requirements, is sorted by purpose. Some manufacturers make drying without removing the bast, which makes it possible to prevent cracking of wood. Then the bast is removed, as it will interfere with the impregnation process.
2. Drying
Moisture removal is a long and energy-consuming process on which the quality of impregnation depends. Unfinished wood cannot be impregnated. Its humidity should be reached up to 28%. Drying is carried out naturally in stacks (2-5 months) or warm air in drying chambers, which circulates using fans (7-10 days).
3. Autoclave impregnation
A vacuum is created in the chamber, drawing out excess moisture. Then the logs are closed with an aqueous solution of an antiseptic, after which the pressure in the chamber rises to 14 atm. After the solution is drained, a vacuum is created there again. The best impregnating composition is a Finnish-made SSA antiseptic. Domestic analogues are made from industrial wastes and contain impurities that reduce the depth of processing and contribute to the leaching of the composition from wood.
4. Fixation
Impregnating composition contains harmful substances. Logs are aged for a while. In this case, the formation of insoluble antiseptic compounds in the structure of wood. The temperature of the medium must be positive. To speed up the process, the supports are autoclaved with superheated steam. Canadian manufacturers process logs with special compounds, thereby increasing the durability of products.
VL supports
Installation of wooden poles is carried out on an overhead line of the 3rd class, where the rated operating voltage is 1 kV or less. The most common are intermediate supports that support wires. In addition, they perceive wind loads, as well as the weight of the reinforcement and their own. On their own, they may not withstand the forces arising along the line if a break occurs. This load is perceived by anchor supports with the location of additional struts along the VL axis. Basically, they serve to create tension on the wire section. To absorb transverse loads, anchor supports are used with the struts or "legs" in the perpendicular direction.
There are also angular supports that perceive longitudinal and transverse loads. They are set to turn the lines.
Installation of wooden supports is carried out with precise marking of places, and assembly - with a tight fit of parts.
The gap where the notches are made should not be more than 4 mm. The interface is tightly fitted. The holes are drilled.
Maintenance and repair of wooden poles
Wooden power transmission towers are subject to periodic inspections and repairs. In the summer, at a depth of 30-50 cm, the depth of decay of the wood is checked. If the diameter of the log is 25 cm and the decay is more than 3 cm, it is considered unsuitable and should be replaced.
Overhaul of the lines, where most of the wooden poles are installed, is done at least 6 years later. The rest of the repair work is carried out in time, depending on the available resources.
The fire hazard of wooden poles requires laborious operations to reduce it. In the presence of wooden prefixes, a ditch 0.4 m deep breaks out around and grass with shrubs is removed.
Parts from supports are replaced with new ones when the line is running. It should be borne in mind that on parts of the structure the loads can exceed the calculated ones.
If the poles deviate from the vertical by an unacceptable amount, additional loads can cause a change in position and overlapping wires or touching parts. Displacements occur due to weakening of the foundation or termination of the base of the support, displacement of the soil, weakening of joints.
Dressing is done with steel cables mounted on a rack. The base is dug to a depth of 1.5 m and the support is straightened by a traction mechanism. Then the hole is poured and rammed.
When the stand warps due to a weakened connection with the bandage, it is straightened without shifting the stepson.
A bandage is installed on the rotten stand. Before this, the rot is removed and the pillar is covered with antiseptic paste.
Damaged parts are reinforced with temporary overlays made of wood or metal, using half-clamps, bolts and a retaining wire.
Details before being transported to the track are checked for compliance with design parameters.
To increase the service life of the racks, they should additionally be impregnated during operation with the diffusion method. Antiseptic bandages are installed on the underground and aboveground parts of the support and at the joints. An antiseptic paste is applied to the cracks and to the top of the racks with attachments.
Due to the fact that the mass of the wooden support is small, heavy equipment is rarely required for repairs.
The support that cannot be repaired is freed from all loads and replaced with a new one using special equipment.
Conclusion
A wooden support with impregnation is no worse than reinforced concrete, and in some cases even better, due to the mass of advantages. To make them more actively applied in practice, an industry standard is needed. This will allow to establish uniform requirements for all manufacturers so that the quality is guaranteed.