Electrical Insulators are components of electrical power systems through which electric current cannot flow. This is possible due to a physical property of such material known as resistivity, which restricts the flow of electrons through it. Insulators have higher resistivity than conductors and semiconductors.
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However, there is no perfect electrical insulator. This is because all insulators have a small amount of mobile charges that can conduct electricity. Moreover, all insulators become conductive on the application of sufficient voltage. This is known as an electrical breakdown of insulation and the voltage at which the insulator becomes conductive is known as the breakdown voltage of an electrical insulator.
FUNCTIONS OF ELECTRICAL INSULATORS
Electrical Insulators used in power sectors have basically two functions:
- To provide insulation so that no current can bypass them thereby increasing the circuit integrity and safety.
- To provide support to the suspended conductors or live electrical elements.
INSULATING MATERIALS
Insulators used for high-voltage power transmission use glass, porcelain, or composite polymer material.
Glass
Glass has very good dielectric strength but it attracts condensation which is not ideal. Further, it is difficult to cast various shapes without inducing internal strain.
Porcelain
Porcelain however has a good dielectric strength of 4-10KV/mm and is made from clay, quartz or alumina, and feldspar. It is covered with a glaze to shed water. Porcelain rich in alumina is used where high mechanical strength is required.
Polymer Composite
Polymer composite insulators are composed of a central rod made from fiber-reinforced plastic and an outer weather shed made of silicone rubber or ethylene propylene diene monomer rubber (EPDM). These are less costly, lighter in weight, and have an exceptional hydrophobic property. They are mostly used in polluted areas.
TYPES OF INSULATORS
Pin insulator:
These insulators are mounted on a pin fixed on the cross-arm of a pole. The insulator has a groove through which the conductor passes. The conductor is tied to the insulator by an annealed wire of the same material as the conductor. They are generally used for transmission of power upto a voltage level of 33KV.
Post insulators:
These insulators are mounted on a post as the name suggests. They are mostly used in HV switchyards as a support to the conductors to maintain sufficient electrical clearance (Ph-G). It has a palm on the upper side to which conductors are bolted using different clamps.
String insulator:
For voltages greater than 33 kV, it is a usual practice to use String type insulators, consisting of a number of glass or porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the top end is secured to the cross-arm of the tower or in tension-type with one end of the string attached to the tower’s cross-arm and another to the conductor in tension. The number of disc units used depends on the voltage level and the mechanical strength needed to support the conductor’s weight.
Standard disk insulators are 25cm in diameter and 15cm long. They have a dry flashover voltage rating of 72KV can support the load of 70-120KN and are rated at an operating voltage of 10-12KV.
Strain insulator:
A dead end or anchor pole is used for low voltage distribution, where a straight section of line ends, or makes an angle in another direction. These poles therefore have to withstand the lateral stress of the long straight section of wire. For supporting this lateral load, strain insulators are mostly used. For low-voltage lines (less than 11 kV), shackle insulators are also used as strain insulators.
Bushings:
These are hollow electrical insulators through which electrical lead/conductor is passed safely without touching the outer case as in a transformer, or circuit breaker.
Factors that Influence the performance of insulators
Temperature
An increase in temperature impacts the insulators and results in poor performance. As insulators have negative temperature co-efficient, the resistance that an insulator provides to current decreases with an increase in temperature. Some types of insulator materials like polymer can degrade due to high heat.
Moisture:
Moisture also functions to decrease the dielectric strength of insulators. High glaze is used in electrical insulators to reduce the formation and retention of moisture in the surface of the insulator. Moisture mixed with pollutants that often get deposited in the insulators can form a conducting part which often leads to a flashover of the insulator.
Mechanical Stress:
Insulators used in high voltage transmission lines undergo mechanical stress because of various tensile and compressive forces of the line acting on them. It is therefore important that the insulators withstand these stresses efficiently. Mechanical stress can cause deformation in the insulator which results in structural dis-integrity and loss of insulating properties.
Because of repetitive electromechanical stress on the insulating material, fatigue develops in the insulator gradually losing the capability to withstand the stress, often leading to a failure of the insulator.
Failure of Electrical insulators:
Excessive voltage often causes electrical breakdown of insulators and can occur in one of two ways as discussed below:
Puncture of insulator:
The puncture of an insulator is caused by an electric arc inside the insulator because of the conduction of the material of an electrical insulator. The heat from the arc causes irreparable damage to the insulator. The voltage at which a puncture arc is formed is called puncture voltage.
Flashover of insulator:
Flashover arc results from the breakdown of air insulation around the insulator along its surface. The arc is formed outside of the insulator. Insulators are basically designed to withstand the flashover arc. However, it can cause serious damage to the insulator. The voltage at which flashover is caused is called flashover voltage.
In general, high-voltage insulators are designed to have lower flash-over voltage than puncture voltage. This is done to avoid damage.
