Electricity is widely used in modern life, both in production and in everyday life. The generation of electricity and its consumption in the overwhelming majority of cases does not occur in one place, and the distance between these two points is quite significant. The main means of delivering electricity to the right place are various power lines.
The construction of a power electric line for significant capacity is a very costly undertaking. One of the means to reduce the payback period of capital costs is to increase the operating voltage: as it increases with a constant power, the operating current decreases and, accordingly, losses decrease.
Power lines can be implemented on the basis of cables or as overhead power lines (LEP). The latter are advantageous in that air, as a good natural dielectric, allows the wires to be effectively separated, which again saves costs.
Corona discharge in power lines
Losses for conversion to Joule heat directly in phase conductors are not the only loss mechanism in transmission lines. In addition to them, there are losses for the so-called. corona discharge. The acoustic effect of its presence is clearly audible, especially at high humidity, crackling, and at night, corona discharge manifests itself as a glow (corona) around the sharp edges of metal items. An example of this phenomenon is shown in Figure 1.
The corona discharge is based on the effect of air breakdown as an insulator, which occurs at an electric field strength of at least 30 kV / cm. In this case, the tension naturally grows in the area of the sharp edge. The result of breakdown is ionization of air molecules with the appearance of free charges. The latter interact with the electric field and are intensively accelerated in it. When it collides with the next molecule, its secondary ionization occurs, and then the process develops like an avalanche.
Due to the fact that with distance from the wire, the field strength decreases rapidly (in proportion to the square of the distance), the considered mechanism:
- has a limited scope;
- always “tied” to an energized metal object;
- most intense in the area of sharp edges.
When leaving the ionization region, the recombination of free charge carriers begins, which is accompanied by the release of their accumulated energy in the form of a glow and a click.
Varieties of coronal discharges
The ionization process can begin both at the cathode, which generates an avalanche of electrons, and at the anode, which becomes a source of positive charges. The movement of charges created during breakdown always occurs from one electrode towards the other.
In this case, due to the greater mobility of electrons, determined by a lower mass, a large uniformity of their distribution in the core, and the corona, as a result, has a uniform glow.
For positive charges, the conditions for corona formation are usually localized, as a result of which they acquire the form of a cord or spark channel.
The second electrode may not generate a corona.
Crown suppression
Regardless of the type of corona, its appearance means the appearance of an additional current, i.e. growth of losses. To reduce them, it is most expedient to reduce the field strength below the breakdown one. The easiest way is to eliminate sharp edges on the current-carrying elements of power lines. This is most important when designing insulators, because in them the smoothness of the lines of details is naturally disturbed. An example is shown in Figure 2.
A more costly and structurally complex, but at the same time more effective way to radically solve the problem is to switch to wires from the so-called. split structure. An example of their design is shown in Figure 3. In this case, the goal is achieved by the fact that an increase in the number of wires naturally decreases the electric field strength below the critical one.