Disc insulators are used in high voltage transmission and distribution lines which are designed for fulfilling the required electro mechanical strength. Disc insulators are used for providing insulation as well as support to the line conductors in suspension and tension systems, as required.
These disc insulators are mainly made of top grade wet procedure. They are the most used models to transmit and distribute the lines. They design may vary for suiting different kinds of polluted zones as well as mechanical strength according to the requirements of the customers.
The whole purpose of an insulator is mainly to insulate electrically charged parts of a machine or equipment from the other charged part of the uncharged metallic parts. At low utilisation voltage, this insulation even covers live conductor totally and also plays the role of a barrier and helps in keeping the live conductor unreachable from animals and humans. As far as high voltage overheads transmission as well as distribution is concerned, the transmission tower or pole support insulators as well as lines are used for insulating live conductors from transmission tower. The insulator used for distribution as well as transmission system is also needed for carrying out the compressive load or the large tensional.
The EHV or HV insulators may be divided broadly into 2 main types on the basis of material used. One is polymer and the other is ceramic insulator. Traditionally the ceramic insulators made of porcelain are mainly used in distribution as well as transmission lines.
Now composite or polymer insulators are being used increasingly in the high-voltage transmission system. The polymer insulator has fibre rod which is surrounded by the outer sheath of the polymer. Because of hydrophobic nature of the surface of the polymer insulator, the dry areas could be formed between the wet areas which results in discontinuities in the wet creepage paths. This phenomenon helps in improving the performance of polymer insulators in the coastal as well as polluted areas. The polymer insulator has a huge advantage that it’s very light as compared to the porcelain insulators. One big disadvantage of porcelain insulators is that they are capable of bearing big comprehensive force but lesser tensional force. The porcelain insulator surface is completely hydrophilic. This means that it has affinity for H2O. The polymer insulator ages faster as compared to the ceramic insulators.
Below are some definitions related to the insulators which an individual needs to know for understanding a few concepts.
Length of creepage- the length of creepage is considered to be shortest distance in between 2 metal end fittings of the insulator along with the insulator’s surface. In string of the insulator for the calculation of the length of the creepage, the metal portion between 2 consecutive insulation discs is not considered.
Corrugation below the insulators is meant for obtaining the longer creepage paths between the cap and the pin. Corrugation increases the length of the creepage thereby increasing the resistance to leakage current of the insulator consequently. This leakage current which flows across the surface of the insulator needs to be as low as possible.
The distance of creepage needed in the clean air could be 15mm for every kilovolt. In case of polluted air on the basis of the pollution level of air, the needed creepage distance also rises.
Flashover distance- it’s considered to be shortest distance across air between electrodes of the insulators. For the pin type of insulators, double headed arrow lines depict the flash over distance.
Flashover voltage- the voltage level at which air around the insulators break down and then flash over happens shorting the insulators.
Puncture voltage- the level of voltage wherein the insulators break down and the flow of current across the inside of the insulators.
An insulator might fail because of excessive electric stress, excess mechanical as well as thermal degradation or stress because of environmental chemical actions of the surface of the insulator. The electric failure may happen between the earth and the conductor through air or even through volume of the insulation materials. If an excess electrical stress happens, the insulator might fail to work when flashover happens across the air between the tower and the conductor. On the contrary, the insulators might be punctured through volume. The insulation material states that porcelain has a very high dielectric strength as compared to air. An insulator is designed in a way so that it would flash over just before it’s punctured. A failure because of flash over is usually temporary as well as self-restoring. But a failure because of insulation puncture is always permanent and insulator gets damaged and needs to be changed and replaced. The insulator which has some kind of internal defects such as impurities as well as voids reduces electric strength of the insulators.
The flashover might even cause damage to the glaze of the insulator that could be repaired. In the polluted regions, the contaminants are deposited on surface of the insulators which result in a reduction of flashover voltage of insulator in the wet conditions. For instance, if the frequency of power flash over volt of 33V pin insulator is about 95Kv, then in the wet conditions, flash over volt could be reduced to less than 80kv. The insulators are basically designed for withstanding flash over volt.
Other significant electric parameters of the insulators are electro mechanic failing load, lightning withstanding volt as well as witching impulses withstanding volt, etc. The HV line insulators requirements are based on the length of the creepage. Switching impulse withstanding volt is specifically much more significant in the case of additional high voltage, more than 300Kv and ultra-high volt lines.
The disc insulators have an added benefit of capacitive gradation. They are used in a number of commercial as well as industrial applications due to their useful features.
They are considered to be the best to support conductors
They help in maintaining the high voltage in higher loads
They are used in electric wiring as well as cables
Corrosion is less