Insulators are installed between conductors at different potentials or between conductors and grounding components that can withstand voltage and mechanical stress. The main function is to achieve electrical insulation and mechanical fixation. According to different installation methods, insulators can be divided into suspension insulators and post insulators; according to the different insulating materials used, they can be divided into porcelain insulators, glass insulators and composite insulators; according to different voltage levels, they can be divided into low-voltage insulators and high-voltage insulators and ultra high voltage insulators; according to the different environmental conditions used, pollution-resistant insulators used in polluted areas are derived; according to the types of voltage used, DC insulators are derived.
How to choose insulators on different conditions?
1) For the (3~20) kV outdoor post insulators and wall bushings of power plants and substations, when there is ice and snow, products with a higher voltage level should be used. For (3~6) kV, products with two-level voltage increase can also be used.
2) When verifying the mechanical strength of post insulators, the short-circuit electromotive force of the bus bar acting on the center of gravity of the bus bar section should be converted to the top of the insulator. When verifying the mechanical strength of post insulators installed non-vertically 35kV and above, the weight of the insulator, combined effect of busbar weight and electrodynamic forces should be considered. For post insulators, in addition to checking the bending mechanical strength, the torsional mechanical strength should also be checked.
3) Outdoor support insulators should adopt post insulators. When the outdoor post insulator needs to be inverted, the hanging post insulator can be used. Indoor post insulators generally adopt multi-edge post insulators with joint cementing.
4) The power distribution device in the house should adopt the aluminum conductor wall bushing. For the busbar type wall bushing, the size of the busbar allowed to pass through the window should be checked.
5) The type of suspension insulator and the number of pieces per string can be selected according to the following conditions:
(a) Select according to the highest system voltage and creepage distance. The effective creepage ratio distance of the insulator string shall not be less than the value listed in the specification, and anti-pollution insulators should be used in areas with polluted air, and the same anti-pollution measures shall be adopted.
(b) Select according to internal overvoltage. The voltage of 220kV and below is selected according to the internal voltage multiple and the power frequency wet flash voltage of the insulator string; the voltage of 330kV and above is selected according to the operating overvoltage protection level of the arrester and the 50% discharge voltage of the positive polarity operation of the insulator string.
(c) Select according to atmospheric overvoltage. Determined by the impact protection level of the arrester, and not lower than the corresponding corresponding value of the disconnector and the post insulator; the aging of the insulator should be considered when selecting the suspension insulator, and the zero-value insulator to be reserved for each string of insulators is: 35~220kV tension string 2 pieces, 1 piece of suspension string; 2~3 pieces of 330kV and above tension string, 1~2 pieces of hanging piece.
6) In areas with an altitude of (1000~4000) m, when it is necessary to increase the number of insulators to strengthen insulation, the number of pieces of strain insulator strings should be corrected according to the formula.
7) In areas with clean air and no obvious pollution, the number of insulators in the suspension insulator string can be less than the same type of insulator in the tension insulator string. The insulator sheets of the suspension insulator strings in polluted areas shall be the same as the tension insulator strings. Insulator strings with a voltage of 330kV and above should be equipped with voltage equalization and shielding devices to improve the voltage distribution of the insulator strings and prevent corona from connecting fittings.