Arcing Horns - Background

Background

High voltage equipment, particularly that which is installed outside, such as overhead power lines, is commonly subject to transient overvoltages, which may be caused by phenomena such as lightning strikes, faults on other equipment, or switching surges during circuit re-energisation. Overvoltage events such as these are unpredictable, and in general cannot be completely prevented. Line terminations, at which a transmission line connects to a busbar or transformer bushing, are at greatest risk to overvoltage due to the change in characteristic impedance at this point.

An electrical insulator serves to provide physical separation of conducting parts, and under normal operating conditions is continuously subject to a high electric field which occupies the air surrounding the equipment. Overvoltage events may cause the electric field to exceed the dielectric strength of air and result in the formation of an arc between the conducting parts and over the surface of the insulator. This is called flashover. Contamination of the surface of the insulator reduces the breakdown strength and increases the tendency to flash over. On an electrical transmission system, protective relays are expected to detect the formation of the arc and automatically open circuit breakers to discharge the circuit and extinguish the arc. Under a worst case, this process may take as long as several seconds, during which time the insulator surface would be in close contact with the highly energetic plasma of the arc. This is very damaging to an insulator, and may shatter brittle glass or ceramic disks, resulting in its complete failure.