It is generally considered that 500 kV is the most economical voltage level at which to transmit large quantities of electrical energy over long distances. The best material for insulating overhead lines has been found to be porcelain, as its insulating qualities remain practically the same when exposed to all weather conditions. It has low tensile strength but considerable compressive strength, and so most types of insulator are designed to utilize the porcelain in compression.
We will first discuss Puncture Voltage, Creepage Distance and Flash Over Voltage to understand the causes of failure of Insulators.
It is the shortest distance through air between the electrodes of the insulator. For a pin type insulator as shown in Figure below, the double headed red arrow line is flashover distance.
The voltage at which the air around insulator breaks down and flashover takes place shorting the insulator is called Flash Over Voltage.
The voltage at which the insulator breaks down and current flows through the inside of insulator is called Puncture Voltage.
The creepage length is the shortest distance between two metallic end fittings of insulator along the surface of insulator. In the string of insulators for creepage length calculation the metallic portion between two consecutive insulator discs is not taken into account.
The corrugation below the insulator is for the purpose of obtaining longer creepage path between the pin and cap. The corrugation increases the creepage length so consequently increasing resistance to the insulator leakage current. The leakage current that flows through the surface of insulators should be as little as possible.
The Creepage Distance required in clean air may be 15 mm per kV (line voltage). In the polluted air depending on the level of pollution of air the required creepage distance increases.
To clearly understand the Creepage Length / Distance, suppose we drop a water droplet at the top of insulator, then the path which will be followed by the water droplet to come down at the bottom of Insulator will be a zig zag path through many discs which is nothing but the Creepage Distance.
Electrical failure follows a puncture through the porcelain or by ‘flash-over’ round its surface, which produces an arc short-circuiting the line. As puncture destroys the insulator, it is more serious than flash-over. Therefore Safety Factor is defined for an Insulator. Safety factor of an Insulator is defined as the ration of Puncture Voltage to the Flash Over Voltage.
Safety Factor = Puncture Voltage / Flash Over Voltage
For pin type insulator the value of Safety Factor is about 10 which mean that Puncture Voltage is 10 times that of Flash Over Voltage. It is expected that Flash over to take place first as Insulator Puncture damages the Insulator and after puncture of insulator it needs to be replaced.
Insulators are designed with a puncture voltage of about twelve times and a flash-over voltage of about six times the working voltage. Failures occurring in practice are usually due to lightning or to deposits of soot or sea salt on the insulator surface.
Lightning affects the design of the transmission line rather than that of the insulators. Often no permanent damage is done by lightning flash-over. The problem of deposits on the surface of the insulators is a serious one and has not yet been completely solved, although many suggestions have been made for improving the standard types. For use near the sea, anti-deposit insulators have long, recessed, protected surfaces.
For industrial areas, types with open exposed surfaces which can be cleaned by wind and rain have proved the best. For testing purposes, a percentage of the finished insulators are selected at random and tested for flash-over voltage both dry and in rain produced artificially by a watering pot, impulse flash-over voltage, mechanical strength and electrical puncture.