Insulator (electricity)

An insulator is a component of the electronics, which has a high mechanical strength, but only a very low electrical conductivity. Insulators are used wherever insulated electrical wires attached, must be kept or made ​​without there must be a substantial flow of current through the fastener.

Insulators can be found on pylons, substations, antennas or as insulating bushings on power transformers, large capacitors, shielding enclosures or as spark plugs.

Electrical cables shall have insulation with an electrically insulating coating.

• 3.1 Overhead Lines 3.1.1 Low voltage and medium voltage
• 3.1.2 High and extra high voltage

• 3.2.1 transmission towers
• 3.2.2 Wire Antennas

• 5.1 Optical Isolator
• 5.2 " isolator valves " in the high-frequency technology
• 5.3 Frequency-selective insulators

Basics

Materials

Insulators made ​​of materials with as low electrical conductivity, for example:

• Special ceramics with high alumina moieties
• Steatite
• Porcelain
• Glass
• Glass-fiber -reinforced plastics in the interior
• Hydrophobic plastics outdoors. The hydrophobicity prevents the formation of drops of conductive traces which may otherwise lead to a pollution layer over.

Insulators Outdoors used must be permanently resistant to ultraviolet radiation.

Insulation

The higher the resistivity of the insulator body, the better its insulating property in principle. The glass commonly used for insulators has a resistivity 1010-1014 OMEGA.m. Porcelain or certain plastics have partially an even higher insulating properties.

Leakage

Insulators outdoors subject to adverse environmental conditions such as rain, snow, large temperature changes, dust and direct sunlight. During temperature changes have an insignificant effect on the properties of an insulator, weather conditions such as heavy rain, sleet or snow deposition can significantly reduce the electrical insulation capability. Although the material of the insulator itself remains virtually non-conductive, but due to dirt and moisture may form on the surface of the component is a sufficiently conductive film, which impairs the function of the insulator. Then create a so-called leakage current along its surface, which causes only a transmission loss in a favorable case, in the worst but also to a creeping and consequently to a flashover, arcing, and thus can cause a short circuit or ground fault.

In order to keep leakage current as small as possible, it is desirable to extend the creepage distance on the outside of the insulator. Therefore insulators are not manufactured with " straight " surface, but with bell -shaped shields or ribs that increase the creepage distance. Roof shaped beveled pool umbrellas in the rain for dry - and thus poor conductive - sections on its underside.

Designs

Insulators for voltages above 1 kV are manufactured primarily as a complete assembly of ceramic materials (long rod insulator ). The support function is performed by the central stalk, the screens or ribs serve the creep path. The fixing metal fittings at both ends are attached with solid cements. In the novel silicone insulators, the shank is made of high strength glass fiber reinforced plastic, the panels made ​​of silicon are applied with an adhesive joint.

Older lines are often equipped even with cap insulators. These insulators are not manufactured as a whole, but composed of individual metal-glass insulating elements that are hung each other in the required number of pieces. For the same basic elements as insulators for all nominal voltages and applications can make, so this technique is very economical. Since the elements are reciprocally movably mounted, they form flexible isolators, the lateral forces, more able than long-rod insulators. However, in contrast to the long-rod insulator, they are not fail-safe, so run the risk of over-voltages ( for example, when there is lightning in the head rope) penetrate and mechanical damage. Therefore, they are no longer used in Central Europe for new construction and renovations.

The conductors are attached to the insulator with special terminals, whose design depends on the purpose of use: For supporting masts only the weight of the rope ladder section must be worn while the significantly higher pulling force must be absorbed by the terminal when tension towers.

Insulators for mounting overhead lines ( overhead lines ) do not differ fundamentally from those for overhead lines, but must be designed for the specific mechanical loads on the upper line. Insulators for busbars must carry the heavy track. Often an existing protective covering serves as insulation for insulated mounting as with the Berlin S -Bahn.

High voltage insulators are often equipped with a spark gap as surge to keep the high-energy arc from the insulator in case of overvoltage (lightning ) and bring him by certain design to be extinguished.

Applications

Overhead lines

Low-voltage and medium-voltage

Insulators for overhead lines previously used telephone and low voltage overhead lines up to 1 kV rated voltage button-shaped ceramic bodies are mostly used at the cap the wire is attached with a special sling. You are standing cemented to metal hooks or gehanft (that is screwed with a hemp insert), with which they are mounted on poles or walls.

Also ceramic Isoliereier be used in this area, which are used exclusively under compressive stresses in bracing. Since the ability of insulation Isoliereiern due to the short creepage distances is small, they only come for the low voltage range considered. To increase the isolation voltage you can use multiple Isoliereier succession.

Usually ribbed insulators made ​​of glass or ceramic are used for medium voltage (range 1 kV to 30 kV ) that are either on the traverses of the masts and can hang underneath. Standing insulators enable lower pole heights and offer their design more safety from falling of the conductor ( the insulator break it falls on the traverse). Hanging insulators can avoid lateral forces ( eg due to wind) by lateral deflection and thus less subject to bending. In addition, the live conductors provide for a vertical arrangement is not such a big threat to birds that land on the traverse.

High and very high voltage

Insulators for high voltage ( 30 kV to 150 kV ) are used only in a hanging version. The technique of fixing the conductors is not different from the technique used in the medium voltage range. Often double insulators are used. For traction current lines are the same types as used for three-phase lines.

Isolators for high voltages ( > 150 kV) are often produced as a chain of two or more insulators for high voltage ( insulator string ). Besides the classical insulating glass and porcelain are also increasingly high-strength plastics are used. In Germany basically double isolators are used for 380 kV lines. For very high static requirements, three or four parallel long-rod insulators or insulator chains may be used.

Insulators for high voltage direct current transmission in principle no different from the types used for AC voltage, their stress at the same voltage is even lower, since the pre-discharges (eg in wet weather ) are lower.

Antennas

Masts

Special requirements are placed on the insulators of radiating transmitting masts, because these must be able to carry voltages up to 300 kV and loads of up to 1000 tons at high transmit powers. One uses this to insulate the backstays Gurtbandisolatoren of steatite and isolation of the towers and masts hollow or solid body made ​​of steatite, on which the Aufliegekörper which carries the tower or mast is attached to exactly match.

The pole isolator must occur prior to installation in a pressing apparatus. The tower or mast is hydraulically lifted to the installation of the insulator and slowly deposited on the insulator.

Wire antennas

Guy ropes of masts and overhead power lines, but also wire antennas are insulated with egg-shaped insulators which have holes and grooves for receiving the ropes.

Through insulators

Insulator bushings cause the conductor inside along and isolate it from a metallic wall, through which they protrude. They are needed in the electricity grid for the sealed introduction to building, housing, underground cables, current and voltage transformers or transformers.

Smaller designs can be found on spark plugs or condensers with metal housing. Through insulators have to be mounted in a hole outside a flange or annular solderable metal surface. The inner conductor has conducted solder or screw terminals. Often in the insulator body concentric layers of metal foils are inserted which act as cylindrical capacitors and control the course of the electric field strength in the radial or axial direction.

Historic designs

In the early days of electric power transmission insulators were performed on operated with high voltage overhead lines with specially shaped oil grooves. These insulators, also referred to as oil - insulator, the oil was placed in specially shaped grooves which are guided in a circle around the insulator after assembly. It served to minimize undesired leakage current from the conductor to the grounded cable suspension due to environmental influences such as moisture (fog, rain). Also oil is lighter than water, so that the electrically non-conducting oil to a certain degree of contamination still remains on the surface and so is an electrically insulating barrier.

Oil insulators are no longer used because of the high maintenance costs, pollution problems and the availability of more efficient ways to prevent leakage currents today.

Insulator Museums

An insulator Museum is located in Lohr am Main in a listed former transformer station at the Haaggasse. In addition to a large part of the private collection of the owner, a skilled high-voltage electrician, there also individual loans from other insulators collectors are to be seen. Shown are the different sizes and types of insulators and their historical development. It shares the title of " Germany's smallest museum " with a few other candidates.

The museum Margaret hut in Grossdubrau (Saxony ) shows ceramic insulators and their production of kaolin. The Margarethe hut was a manufacturer of high voltage insulators in the GDR. The raw materials of the production came from the area, the finished product can today (May 2012) are tested in a high-voltage test facility.

Other insulator components

An optical isolator

Under an optical isolator is defined as a so-called optical diode, a kind of " valve " for light. This member can be polarized light to pass in one direction only: in particular, materials disposed in a magnetic field whose direction of polarization is rotated through 45 °, back to the other well, but in the same direction of rotation and not about. The rotated component can be removed by means of polarization filters.

" Isolator valves " in the high-frequency technology

In the high-frequency technology, an insulator is a device with two connection ports ( engl.: ports ), can pass through the electromagnetic waves only in one direction, while in the opposite direction, ideally, no power is transmitted. The finite in practice transfer ratio between forward and reverse direction is called isolation, usually measured in decibels.

Frequently such insulator is realized by means of a circulator, in which one of the three goals is provided with a terminating resistor. The signals are transmitted in this way between the remaining two goals in only one direction, in the other direction they will be redirected to the terminator where it is converted into heat.

Another type are the Faraday rotators as well as optical isolators work in principle, except that the spinning material is a ferrite and pass the polarization- limiting elements of slots.

Frequency selective insulators

A high -frequency double- line can - at a known wavelength - are supported with metal rods and grounded, although no insulating materials are used. The inner conductor of an air-filled coaxial cable for high transmission performance can be based on comparable manner by a λ/4-Topfkreis. This is achieved if the length of the support rods, the length precisely λ / 4, because then the vertical bracket as a trap circuit isolated (see line # theory special case λ / 4).