You have probably noticed that the power lines are fixed on poles on garlands of porcelain or ceramic plates. These plates are called insulators. They have both an insulating and a mechanical fastening role. Insulators for overhead power lines are different depending on the location, the place of application and the voltage of the line they hold. In this article, we will look at the types of electrical insulators and their purpose.
Content:
- Insulator characteristics
- Design
- Difference in material of execution
- Types by design and purpose
Insulator characteristics
An electrical insulator is a product designed to secure a wire, cable or busbar on the supporting structure of a power line and prevent its breakdown to earth. They come in different types and are made from dielectric materials such as porcelain, glass and polymers.
Since the electrical purpose of insulators is to insulate the conductor from the supporting structure, the main characteristics are:
- Dry discharge voltage is the voltage at which a spark discharge occurs over the surface in a dry state under normal environmental conditions.
- Wet discharge voltage is the same, but in the rain, if its jets hit the insulator at an angle of 45 degrees. In this case, the strength of the rain is 5 mm / min, the specific volumetric resistance of the water is 9500-10500 Ohm * cm (at 20 ° C). Since water conducts an electric current, the wet-discharge voltage is always lower than the dry-discharge voltage.
- Breakdown voltage is the voltage at which breakdown of the insulator body occurs between the rod and the cap (for suspended products). The rod and the cap are in this case the electrodes.
Design
Structurally, all electrical insulators differ in the way they are attached to the supporting structure and the cable is fastened. The main task of this product is to prevent electrical discharges, for this they are made in the form of plates or rods with ribs. These ribs are needed for the discharge to develop at an angle to the lines of force of the field. In the figure below you can see examples of typical products of different shapes and designs:
Difference in material of execution
To consider the classification of types and types of insulators, you must first figure out how they are distinguished. So, first of all, they are classified according to the material of manufacture:
- Porcelain.
- Glass.
- Polymeric.
Porcelain can be called classics, such were used before, even with outdoor wiring in houses. They are usually white in color, but they can be of other colors. These can be seen on different electrical installations. The advantage is that they can withstand high compressive loads and have good dielectric properties.
However, they break and break. Hence, it becomes necessary to regularly check their integrity, and often for this it is necessary to turn off the electrical installation and wipe off oil, dust and other contaminants from them. Their heavy weight is also a problem.
Glass ones, although they are afraid of shocks, but to control their integrity, a visual inspection is sufficient, which can be carried out without disconnecting the voltage. At present, in overhead power transmission lines, as suspended insulators, they displace ceramics, also because they weigh less, and are also cheaper in production.
Polymers are used indoors, rarely outdoors, as an exception. You can sometimes see support insulators made of polymers on overhead lines of 10 kV or other medium voltages, but rarely, or on non-critical lines. This is due to the fact that over time and under the influence of UV radiation, they age, the internal structure disintegrates and their electrical and mechanical characteristics deteriorate.
However, for equipment that is available for regular maintenance and repair, they are used frequently. For example, it can be busbar support insulators in transformer substations and switchgears.
Types by design and purpose
By design, there are three main types of insulators for overhead lines:
- pin;
- suspended linear;
- support and checkpoints.
Pins relate to linear insulators. They are used in power transmission lines up to 35 kV. Including on the lines of 0.4 kV. This type of execution is one-piece, it has a groove for fastening the wire and holes for installation on traverses, hooks, pins.
Interesting: on overhead lines from 6 to 10 kV, single-element insulators are used, and on 20-35 - from two elements.
Suspended are used on high-voltage overhead lines with a voltage of 35 kV and more. They are of two types, supporting (rod) and tensioning.
Tension poppet insulators work in tension and hold the line on the support, mounted at an angle. Structurally, they are made in the form of a porcelain or glass plate. A rod with an expanding head usually protrudes in the lower part. At the top there is a metal cover with a specially shaped hole so that the lower rod can be fixed in it. Thus, unification occurs and you can collect as many insulators as you need to achieve the required nominal breakdown voltages. Such a garland turns out to be flexible, it holds the power lines on the support.
Suspended rod insulators are installed on intermediate supports. They are made in the form of a support rod, at its ends there are metal parts for fastening to the support and wires. They are installed vertically and the wire lies on them - this is the main difference from the previous ones. They also differ in that the tension insulators can withstand more weight, so they can be used on supports located farther from each other.
Interesting: in critical areas and to increase the reliability of the installation of power lines, double strings of tension insulators can be used.
Support and bushing insulators are already stationary, and not linear. This type is so called because it is used inside power plants and transformer substations. Made from polymers or porcelain. The supporting ones are used for fastening busbars to grounded structures, for example, the case of transformers or inside the input and distribution switchboards.
The marking of insulators of all varieties is similar, usually it contains information about the type of product and the nominal line voltage, for example:
In order to lead the cable or bus through the wall, bushings are used. This type of products with a hollow body, in which the live part is located. To increase the insulating properties, it can additionally have an oil barrier or an oil-paper gasket. This type of insulators allows you to lay a line up to 110 kV. There are other types - without a conductor inside, just a dielectric hollow cylinder with a hole, which is put on the cable.
This is where we end our article. Now you know what insulators are for overhead power lines and where each version is used!
Related materials:
- How to install an electric pole on the site
- Retro style electrical wiring installation
- How to insulate wires
- Fittings for the installation of SIP cable
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