Last time we’ve spoken about the irregularity in the distribution of voltage applied across the separate parts of insulating string and its effect on insulation turnover. If we consider glass suspension string units with the specific number of insulators maintaining for a certain voltage class of transmission line, we can estimate the value of voltage drop on each insulator. Of course, the number of disks varies as it has different creepage distance in each particular case. For example, on the (Fig.1) you may see the HVTL 330 kV, string with bundle conductors: insulators of U70BL and U120B type, creepage distance 407mm (manufactured by “YuAIZ” AO, part of Global Insulator Group).
The number of insulators on the HV 330 kV transmission line may vary from 16 to 20 pieces (+/-4 pieces); HVTL 220 kV– 11-14 pieces (+/- 3 pieces), HVTL 110 kV – 6-7 pieces, HVTL 35 kV – 3-4 pieces.
If the average spacing of disk type insulator is 146 mm and the number of insulators vary from 3 to 4 pieces, than string clearance for the same applied voltage may vary up to half of the meter (438 – 584 mm). Thus, nominal load and voltage overload at the separate insulator will depend on the string design and the number of insulators in it.
Load imbalance is considered in different designs and even described in Soviet Union inventions. Thus, for the invention SU 1781706 А1 on the (Fig. 2) several designs were offered using glass parts with growing creepage distance from cross-arm to conductor. Disks with the same diameter had extended protrusion of ribs (see Fig. 3). For parts of sting with practically high level of creepage current, creepage distance was increased correspondently. The set of typical strings of insulators with extending protrusion of ribs is possible within the range manufactured by “YuAIZ” AO, part of Global Insulator Group, producing items with the same diameter of disk and ribbing from standard to superfog type.