According to IEC: 60050 – 471 “The insulator is a device intended for electrical insulation and mechanical fixing of equipment or conductors which are subject to electric potential differences”
1. Electrical isolation
2. Mechanical support
3. Protect internal components from the effects of external conditions
In transmission and distribution of electricity.
1. Transmission line insulators
a. Disc insulators
b. Long rod insulators
c. Line post insulators
2. Substation insulators
a. Solidcore insulators
b. Hollow insulators
3. Traction insulators
Stay arm, Bracket, 9 ton, operating rod etc.
4. Distribution insulators
Pin, shackle, guy strain etc.
Basic Insulation Level or Standard Insulation Level
To ensure that the transmission & distribution network and the substation equipment perform uninterrupted under normally expected overvoltage conditions.
a. Temporary over voltages (TOV) and
b. Transient over voltages – Slow Front Over voltages(SFO), Fast Front Over voltages (FFO) & Very Fast Front Over voltages (VFFO)
According to IEC 60071-1 it is “The selection of the standard or rated insulation level of equipment in relation to the operating voltages and over-voltages which can appear on the system for which the equipment is intended and taking into account the service environment and the characteristics of the available preventing and protective devices.”
It consists of the selection of the most economical set of standard rated withstand voltages of the insulation sufficient to prove that all the required withstand voltages are met. With increasing system voltage, the need to reduce the amount of insulation in the system, by proper co-ordination of the insulating levels become more critical.
a. System voltage & Basic insulation level – Determines the height / length of the insulators.
b. Pollution level – Defines the creepage requirements.
c. Mechanical strength – Determines the core dimensions and the weight of the insulator.
d. Others include shed profile, end fixing methods, glaze colour, material of end fittings etc.
The factors that affect the pollution performance are
1. Creepage distance (or specific creepage)
2. Profile shape – Alternate / Standard / Under-rib
3. Average diameter of the insulator
4. Overall length
5. Arcing distance
6. Material of insulator
The choice and performance of insulators for polluted environments is very often expressed solely in terms of the creepage distance necessary to withstand the polluted conditions under the system voltage.
However the use of creepage distance alone to establish orders of merit does not take into account other factors which depend on the creepage distance available per unit length of the insulator. This point is worth being borne in mind when choosing insulators, notably for applications where insulator length is a minor constraint.
Conversely, if insulator length or height is a major constraint, increasing the creepage distance in the available space may not give the full improvement in performance expected due to reduced profile efficiency.
Additionally, for polymer materials, such an increase of creepage or reduction of shed spacing may result in aggravated ageing effects.
Similarly, increasing the shed diameter to increase creepage distance may also not yield a favorable result as experience has shown that pollution performance tends to decrease with increasing diameter.
Hence, it is important that all factors that affect the pollution performance be considered and analyzed prior to specifying the type of insulator and creepage distance.