Classification of insulators

INSULATOR TYPES

Insulators are widely used in overhead transmission lines to provide mechanical support and electrical protection, as well as in distribution lines and substations. In transmission and distribution (T&D) systems, overhead conductors are usually supported by towers and poles, which are both grounded. Hence, an insulator must be deployed between the tower or pole body and current-carrying conductor to prevent the flow of current from the conductor to the earth via the grounded supporting towers or poles.

Depending on voltage considerations, different kinds of insulators are used in power systems. A look at some types of insulators and the conditions under which they are deployed.

Pin-type insulators

These insulators are used for the transmission and distribution of electric power at voltages up to 33 kV. A single-pin insulator is deployed to transmit voltages up to 11 kV but higher voltages require two-, three- or four-piece pin insulators. Pin-type insulators are secured with steel or lead bolts onto transmission poles and generally used for straight-running transmission lines. However, beyond voltages of 33 kV, these types of insulators become bulky and uneconomical.

Line post insulators

Line post insulators are used in medium voltage overhead distribution lines for the purpose of fixing conductors to tower bodies. They are used in distribution systems within cities and are commonly installed on metal, concrete and wooden structures to horizontally or vertically support line conductors. These insulators can also be used to support high voltage conductor jumpers or leads.

Suspension insulators

Suspension-type insulators, which consist of a number of porcelain discs connected by metal links in the form of a string, are deployed for higher voltages. The conductor is suspended at the bottom end of the string and the other end is secured to the cross-arm of the tower. Each unit or disc is designed for low voltage, usually 11 kV, and the appropriate number of discs, depending on the working voltage, are connected in a series. Suspension-type insulators are generally used with steel towers. As the conductors run below the earthed cross-arm of the tower, this arrangement provides partial protection from lightning.

Strain insulators

These insulators are designed for handling mechanical stresses at angle positions where there is a change in the direction of the line or at the termination point of the line. In the case of high voltage lines that have longer spans and greater mechanical loading, suspension insulator strings are arranged in a horizontal position, and these are referred to as strain insulators. However, in situations when a single string is not sufficient for taking the load, two or more strings can be deployed in parallel for higher conductor tensions.

Shackle insulators

Shackle or spool insulators are generally deployed in low voltage distribution networks, and can be used in horizontal or vertical positions. Like strain-type insulators, they are also deployed on sharp curves, end poles and section poles.

Post insulators

Post insulators are similar to pin-type insulators but they are more suitable for higher voltage applications. These insulators can be mounted on supporting structures, both horizontally and vertically.

Cap and pin insulators

Cap and pin insulators are generally deployed on overhead T&D lines to evacuate bulk power over long distances. They are also used for substation busbar high-level strained connections.

Stay insulators

Stay insulators, also called egg insulators, are primarily used to prevent stay wires from getting energised from accidentally broken live wires. Hence, they function to provide insulation between stay clamps and transmission poles.

Hollow insulators

Hollow insulators are employed by substation equipment manufacturers to house post-type current transformers, voltage transformers, cable bushings, circuit breaker supports with central operating rods and interrupting chamber assemblies, isolator supports, etc. However, such insulators need to be of high mechanical strength as they are used in applications that are subject to sudden pressures, like circuit breakers or surge arresters.