Critical flashover voltage of insulator string

[sarumacs]

Hello everyone,

I want to know the CFO of a 34.5kV medium voltage line for Insulation coordination studies in the ending substation. The line is using a string of 3 units of ANSI 52-4 ceramic disc insulators with a positive polarity CFO of 125kV.

The question is: Should I add the CFO values of each string?, should I use the value of the individual insulator unit?

I've seen several videos of lab testing of insulators strings and the flashover always occurs along the entire length of the string, between the first metal part of the first insulator disc and the last metal part of the insulator string. This makes me incline for the adding calculation, but I don't think the CFO-to-number of disc relation would be linear.

 

[7anoter4]

According to:

http://constellation.uqac.ca/662/1/18260499.pdf

ch.2.4.2.2 Influence of insulator length:
"The flashover performance of short string of insulators covered with ice was studied by Farzaneh et al [13], Phan et al [50] and Kannus et al [42] independently. It was found that the flashover voltage of a short insulator string increased in a more or less linear fashion with the increase in insulator string length, up to 1 m"

 

[sarumacs]

Thankyou 7anoter4,

Indeed is linear to the length of the string, I kept searching and I found a voltage gradient to use for the calculations.