During our travels by car we all noticed the presence of strange disk -shaped devices attached to electric pylons. These devices, called insulatorsin addition to having a purely function mechanics supporting the cables to which they are connected, they have a second one, in fact they need electrically isolate the naked conductor who supportseparating it from the trellis that is located to a different electric potential. It is therefore easy to guess that without them, if the cables were connected directly to the trellis, the current would close to the ground through the pylons themselves, frustrating the task of the power lines, namely to transport electricity to our homes or industries.
How is an insulator for electric pylons
There are mainly two types of insulators; Depending on the type of support attack, they are divided into:
- Rigid insulators;
- Suspension insulators.
The former are formed by a only fixed element rigidly to support. Suspension ones, instead they are formed by one series of repeated single elementsmechanically connected to each other to form a more or less long chain. They are the latter who are used on the pylons that support high voltage lines.
The single element of suspension insulators is called in technical isolating jargon A hood and pivot Precisely because of its typical shape: they consist of a malleable cast iron hood on which the insulating element is fixed, typically in timing glass with a high dielectric rigidity. Each insulator has a higher cavity, called orbitand a lower pivot which is grafted in the orbit of the isolator below so as to form one chain of insulators. The two extremes of the chain are connected, respectively, to the support and the conductor.
The main insulating materials used in insulators are the timing glassthe ceramic and the synthetic resins – especially in the latest generation rigid insulators.
Because they have that characteristic disk shape
Regardless of the type of insulator, the thing that certainly jumps to the eye is the geometry with which they are made. This “disk” form “or” mushroom “serves maximize the equivalent distance Between the trellis and the conductor, while limiting the length of the insulator itself. In other words, this geometry serves to extend the way of surface escape electric charges. In this way, in the event of dispersion to the ground, the electrical charge that “runs away” from the conductor must make a longer and tortuous road than the road that would travel if the insulator was a simple insulating cylinder. This “trick” does nothing but increase the electrical resistance Between the conductor and the trellis, making the discharge more difficult. In addition, the more the tension increases, More insulating material must be interposed between the trellis and the conductor but even longer the escape path must be. For this reason, the insulators used for high voltages are longer than those used for media or low voltage. The use of this geometry also serves:
- prevent the accumulation of Water, salt and dust that worsen the insulating properties on the surface of the insulator;
- favor the Natural cleaning with rain.
How many “discs” are used to form an insulator
As mentioned in the previous paragraph, the length of the insulator depends on two variables: the operating voltage of the line and the presence of unfavorable environmental conditions. Each ceramic or glass element has one Dielectric resistance about 10-15 kVdepending on the type and environmental conditions. If a line operates at 132 kV (132,000 V), for example, they are theoretically needed among the 10-12 discs To ensure electrical isolation but because of the presence of unfavorable environmental conditions (such as salt, smog, humidity, etc …) are more mounted. So really, the actual number of discs does not depend only on the operating voltage, but also on environmental categorythat is, from the level of pollution present in the air. These categories are defined by the sector rules and the designer who must size the line must take into account.
What happens if an insulator breaks
The insulators are designed for lastbut obviously they are not eternal. They can deteriorate and break due to:
- lightning;
- mechanical stress (wind, ice);
- thermal stresses;
- chemical pollution (acid rains, smog);
- vandalism.
The insulators thanks to their modularity are able to guarantee a certain degree of redundancy. If one of the discs breaks, the overall insulating effect is compromised only partially without completely prejudiced it. However, if more discs are damaged, the insulating effect is less and less increasing by the risk of partial download or total towards the trellisresulting in a land failure that would make the line protection intervene, causing a blackout. For this reason, the managers of the network (like Terna in Italy) carry out Periodic checks Using:
- drones with HD video cameras;
- Thermolet to detect dispersions or hot points;
- Automatic systems with IA for predictive analyzes of faults.
All this to ensure the continuity of the service, intervening in advance before the failure happens.
