All maintenance works carried out on overhead lines are intended to verify their technical condition. Areas at risk of possible failure are searched for and the current technical condition of the tower structure and phase and lightning conductors are checked for any hazard to the health and life of humans and animals within the line’s range of impact. What problems related to the maintenance of overhead lines occur most frequently?
Rheological processes - degradation of the overhead line progressing over time
Every overhead power line has a specified period of trouble-free operation. Unfortunately, it is difficult to determine precisely due to the changing conditions to which the entire line infrastructure is subjected. A conductor suspended in a span elongates over time under the influence of its own weight and rheological processes (deformation processes of solids and liquids under external conditions). The result is a gradual reduction in its distance from the ground, which can eventually lead to failure. This process is determined by a number of factors.
How to minimise the impact of rheological processes?
The type of wire (quality of materials, design, mechanical and electrical properties, etc.), installation method (span length, height of support structures), climatic conditions and the amount of energy to be transmitted are of great importance. Minimisation of this phenomenon is based on calculations (still at the design stage) estimating the wire behaviour in specific climatic conditions. Taking into account all the data of the designed overhead line enables the correct selection of line parameters, installation and subsequent operation.
Wind, ice and temperature - effects on LV power lines
These are typical seasonal hazards for overhead power lines in the Polish climate zone. Wire vibrations under the influence of wind may cause damage to the working wires or lightning protection wires.
Icing means both ice, rime and wet snow deposited on the wires. The theory is that the sediment forms a cylinder around the wire with walls of equal thickness. This situation is assumed in the calculations when designing the line and determining the wire spacing. However, it is sometimes different in the reality. The phenomenon of "dancing wires" is caused by the vibrations of an unevenly iced line. This causes the wires of different phases to come too close together, resulting in an electric arc.
High temperatures, on the other hand, increase the wires sag and can also cause them to melt and eventually break.
How to minimise the impact of wind, ice and high temperatures?
Vibration dampers or anti-vibration weights are used to prevent damage to the wires.
The only way to prevent this is to increase the distance between the wires.
By taking care of the green belt area (no tall trees or shrubs) immediately below the line, the occurrence of a short circuit due to an increased cable sag can be avoided.
Buildings near overhead lines
The problems related to the location of buildings near energy infrastructure are twofold.
- Construction of overhead lines close to existing buildings. The presence of roads, railways, buildings etc. often forces requirement to dodge, for example, protected areas. From a technical point of view, such a location will not pose a problem, provided that the provisions of the standards governing the conditions for construction of power lines in such places are complied with.
- Appearance of buildings near an already existing overhead line. The prerequisite is that safe distances are maintained between the line and the new building. It is necessary to comply with both power line standards as well as technical and building regulations.
Random accidents - unforeseeable
Unfortunately, in the electricity industry, failures occur as a result of hidden material defects or naturally occurring phenomena (tectonic movements of the earth, unusual climatic conditions occurring incidentally in a given area).
How to minimise the risk of failures caused by random accidents?
It is not possible to protect overhead lines for every eventuality, so being aware of the existence of unforeseen situations puts an obligation on power line designers to plan the line in such a way as to limit the extent of damage as much as possible in the event of failure due to random events. One way is to place sturdy towers (pylons), so-called dead-end (section) structures in order to prevent a domino effect in the event of breakdown and avalanche destruction of the through towers. In addition, the EN 50341 standard assumes in the design stage the simulation of possible failures and loads higher than the actual ones. The overhead power line shall be constructed to this standard, taking into account the necessary safety margin.
Regular inspections are the basis for preventing failures during the operation of overhead power lines
As part of the maintenance, an annual on-the-ground visual inspection of the overhead lines along their entire length is carried out. Many system parts are checked, as well as its surroundings:
- control joints and earthing conductors,
- tower structure (including completeness of lattice and corrosion protection),
- presence of foreign bodies on towers (e.g. tyres, tarpaulins, bird nests),
insulators (including assessment of the technical condition of insulator hardware),
- condition of lightning protection suspensions,
- lightning protection and phase conductors (with special attention to possible wire strand damage in the cable and the geometry of the phase wire bundle),
- area around and within the tower (checking for any pits and possible threats to the tower - for example the presence of vegetation),
- phase wires (whether they are hanging at an appropriate, i.e. normative/safe distance from the ground),
- trees under the wires as well as near the overhead line side (whether they are at a safe distance).
The average height of LV line towers varies between 50 and 60 m. Despite the high accuracy of from-the-ground maintenance, even an experienced electrician is not able to see all potential faults from ground level. Sometimes, even with binoculars, the worker cannot (while standing on the ground) verify the condition of the individual components that make up each insulator chain and lightning protection system. For this purpose from-the-tower maintenance are carried out, during which the electrician performs a detailed check of the entire tower structure with particular attention to the technical condition of the connection of the structure to the insulator, the insulator to the phase conductor and the tower structure through the lightning wire to the lightning conductor.
Traditional from-the-tower maintenance of LV overhead lines have one major drawback - to carry them out, the line in question has to be shut down for several days or even for a week or two. With the country's current demand for electricity (2021 was a record year in terms of electricity demand), it is very difficult or in some cases unrealistic to shut down overhead LV lines for an extended period of time. What then?
From-the-tower maintenance - live working (LW)
The inability to shut down overhead power lines for long periods has determined the specialist community to develop a method of performing from-the-tower maintenance of LV lines using live working technology (LW). In Q3 2021 Eltel Networks performed a maintenance of one of the most important LV lines in Poland - 2 x 400 kV lines Joachimów - Rogowiec 3 and Tucznawa - Rogowiec. The scope of work included a detailed check of 213 tower structures, the condition of insulator chains, lightning protection wires, with particular reference to bolt-and-bracket hangers. Numerous faults/damage of varying severity were diagnosed during the inspection, the most serious of which were:
- damaged stay wires
Steel melting at the connection between the shackle and the press-fit stay holder is a very high risk of the connection breaking and the lightning conductor falling onto the phase conductors.
Electrical discharges melted the double hinged connector and melted the pin in the shackle of the lightning stay wire. This condition significantly increases the risk of the switch breaking and the lightning conductor falling on the phase conductors, resulting in a long outage of the overhead LV power line.
- damaged lightning stockbridges
Damaged lightning stockbridge.
- damaged bolt-and-bracket hangers and extension couplings on five towers
A worn connection between the bolt-and-bracket hanger and the extension coupling - there is a high probability of the connection breaking and the phase wire falling to the ground, which can eventually lead to the breaking of the LV transmission tower.
The total number of items to be replaced was 30 sets. This inspection confirmed the need for LW (live working) from-the-tower maintenance, as it was possible to locate faults on as many as 8 transmission towers in a strategically important line for the national energy security. Thanks to the technology developed for live working and the involvement of specialists, it may have been possible to avoid blackout during the winter period.
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An electrician climbing the tower structure of 400 kV double-circuit overhead line Tucznawa - Rogowiec. Works carried out by Eltel Networks Energetyka SA under an operating agreement with PSE.
An electrician moving along a tower crossmember, under an active 400 kV conductor, to verify the condition of the bolt-and-bracket hanger and the extension coupling connecting the insulator to the tower structure. Strictly observed technical sheet of performing from-the-tower inspection in LW technology protects health and lives of Eltel Networks Energetyka SA employees.
Power line failures are inevitable. Due to the large number of possible causes, efforts should be made to eliminate all avoidable factors. Unfortunately, it is usually not possible to prevent extreme weather events, which pose one of the greatest dangers to electricity infrastructure. Line protection must be based on forecast weather phenomena analysed on the basis of mathematical models.
Author: Bogdan Sobolak