The basic properties of insulators include electrical, mechanical and thermal properties. In addition, there are environmental resistance and aging resistance.
(1)Electrical performance: The destructive discharge occurring along the insulation surface is called flashover. Flashover characteristic is the main electrical performance of insulators. For different voltage levels, insulator withstand voltage requirements are different, its indicators include power frequency dry voltage resistance, wet voltage resistance, lightning impact voltage resistance, electric impact cut-off voltage resistance, operating impact voltage resistance. In order to avoid breakdown in operation, the breakdown voltage of the insulator is higher than the flashover voltage. In the factory test, can be porcelain insulator after a spark test, that is, high pressure to make the insulation surface frequently spark, maintenance time, see whether it is broken down. Some insulators also need to pass corona test, radio interference test, partial discharge test and dielectric loss test.
At altitude, the electrical strength of insulators decreases due to the decrease of air density, so their withstand voltage should be increased when converted to standard atmospheric conditions. The flashover voltage of dirty insulators is much lower than that of dry and wet flashover voltage when they are damp. Therefore, insulation should be strengthened or pollution-resistant insulators should be adopted in dirty areas. The specific creepage distance (the ratio of creepage distance to rated voltage) should be higher than that of normal insulators. Compared with AC insulators, DC insulators have poor electric field distribution, adsorption of dirt particles and electrolytic effect, and lower flashover voltage. Therefore, special structural design and larger creepage distance are generally required.
(2) Mechanical properties: insulators are often affected by the gravity and tension of wires, wind, ice weight, insulator weight, wire vibration, equipment operation mechanical force, short-circuit electric power, earthquake and other mechanical forces in operation. The relevant standards have strict requirements for mechanical properties.
(3) Thermal performance: outdoor insulators are required to have the ability to withstand rapid temperature changes. Porcelain insulators, for example, require several cycles of heat and cold without cracking. The temperature rise of the parts and insulation parts and the allowable short-time current value shall comply with the relevant standards because of the current passing through the insulation casing.
